nadour et al2013

8/19/2019 Nadour Et Al2013

1/21

http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.procbio.2013.07.010mailto:[email protected]://crossmark.crossref.org/dialog/?doi=10.1016/j.procbio.2013.07.010&domain=pdfhttp://www.elsevier.com/locate/procbiohttp://www.sciencedirect.com/science/journal/13595113http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.procbio.2013.07.010

8/19/2019 Nadour Et Al2013

2/21

http://www.internationaloliveoil.org/

8/19/2019 Nadour Et Al2013

3/21

1534 S. Dermeche et al. / Process Biochemistry 48 (2013) 1532–1552

Washing

CONTINUOUS CENTRIFUGATION PROCESSDISCONTINUOUS PRESSING PROCESS

Crushing / Malaxing

Pressing

Decantation

Horizontal centrifugation

Vertical

centrifugation

Crushing / Malaxing Crushing / MalaxingWater

Pomace

OMWWOlive oil

PomaceLiquid

OMWWOlive oil

Horizontal centrifugation

Oil washing

TPOMW

Olive oil

Olive oilOlive oil + water

Water

Water

Small amount

of Water

Water

Olives

Fig. 1. Main processes for olive oil extraction.

components (olive oil, water and insoluble solids). Decanters, withminor modifications, can operate either as three-phase or two-phase systems. The continuous three-phase decanter process addswarm water at the centrifugation step (1.25–1.75 times morecompared to the press extraction), producing a larger quantityof OMWW (80–120L/100 kg olives) [19]. The three-phase sys-tem generates three fractions: a solid residue (olive cake) andtwo liquid phases (oil and OMWW). This system has numerous

advantages, such as complete automation, better oil quality, andsmaller area requirements, but also comes with disadvantagessuch as higher levels of water and energy consumption, a higherOMWW output, and more expensive installation [20]. The three-phase system, despite its high water consumption, is still the mostwidely employed method for producing virgin olive oil, especiallyincountriesthatproducelargeamountsofolivesinashortperiodof time [21]. This phenomenon, combined with the growing demandfor olive oil, may explain the recent increase in environmentalproblems associated with its production. The last few years haveeffectively seen units in Italy, Greece and Portugal introduce three-phase extraction systems. In Greece, 70% of all olive mills are nowof a three-phase centrifugal configuration, with the rest continu-ing to use the traditional system [16]. To minimize the volumeof OMWW and, therefore reduce phenol washing, the two-phase

extraction process (decanter) was developed during the 1990s.Using this technology, the olive paste is separated into two phases:oliveoilandwet pomace.Thewetpomacegeneratedusingthispro-cess is a semi-solid by-product (a combination of olive husk andOMWW) known as two-phase olive-mill waste (TPOMW), whichcan be reprocessed to further extract the oil and increase yields.Indeed,therearetwo typesofpomaceoil:(i)oilextractedusingsol-vents (traditional), and (ii) oilobtained through physical extraction

or centrifugation (second centrifugation). The two-phase systemshave been dubbed ‘ecological’ decanters because of the reductionin water consumption [20,22]. However, the resulting TPOMW(10L/100 kg olives) is difficult to manage because its pollutant loadis more concentrated. Over the last decade, the two-phase systemhas become dominant in Spain, where approximately 90% of olivemills use this technology. It is also widely used in Croatia but hasfailed to gain acceptance in other olive oil-producing countries,predominantly due to TPOMW management issues [23].

Another viable olive oil extraction method is the continuouscombined percolation-centrifugation process known as Sinolea[24]. Insteadof pressure andcentrifugation, thismethod uses selec-tivefiltration combined with centrifugation to separate theoil fromthe olive paste. However, this process is not widely applied dueto its high operational and maintenance costs and high energy

8/19/2019 Nadour Et Al2013

4/21

S. Dermeche et al. / Process Biochemistry 48 (2013) 1532–1552 1535

demand, and it is not completely efficient, leaving a large quantityof oil still in the olive paste. Therefore, the remaining olive paste(waste from the Sinolea method) has to be processed using thestandard modern method (industrial decanter) to increase the oilyields.

3. Chemical compositions of olive by-products

3.1. General composition

The main chemical characteristics of OMWW and the solidresidue from olive oil extraction processes are summarized inTable 1. In this review, the term “solid residue” or “olive pomace”refers to the pomace obtained from the different olive oil extrac-tion processes. The term “olive cake” is used specifically forpomace obtained from pressing processes and three-phase sys-tems, whereas “TPOWM” is a term specific to pomace fromtwo-phase systems.

OMWW is a mildly acidic, red-to-black colored, liquid of high conductivity. Its composition varies both qualitatively andquantitatively according to the olive variety, climate conditions,cultivation practices, the olive storage timeand the olive oilextrac-

tion process. Apart from water (83–92%), the main componentsof OMWW are phenolic compounds, sugars, and organic acids(Table1). OMWW also contains valuable resources such as mineralnutrients, especially potassium, which could potentially be reusedas a fertilizer.

As with OMWW, the chemical composition of olive pomacealso varies according to the olive species, culture conditions, originof the olives, and extraction process. Cellulose, hemicellulose andlignin are the main components; however, fat and protein are alsopresent in significant quantities [25]. The TPOMW from two-phaseextraction systems has very different characteristics compared tothe olive pomace from traditional press and three-phase systems.TPOMWisathicksludgethatcontainspiecesofstoneandpulpfromthe olive fruit and vegetation water. Its moisture content is in the

range 65–75%, compared to 22–25% in traditionally pressed olivepomace and 40–45% in three-phase systems [19,26]. Compared toOMWW, the dry matter content is high in olive cake (87.1–94.4%),and the ash content is more significant in olive cake and TPOMW(1.70–4% and 1.42–4%, respectively). Mineral analysis shows thatlike OMWW, the major elements in olive cake and TPOMW arepotassium, followed by calcium and sodium. The concentrations of these water-soluble salts arehigher in OMWW duetoitshighwatercontent. Note thatwater is also the main driverof total phenols andthe soluble sugar contents in OMWW and TPOMW.

3.2. Phenolic compounds

Phenolic compounds include many organic substances that all

possess an aromatic ring with one or more substituted hydroxylgroups and a functional side-chain. Natural phenolic compoundsinclude simple molecules, such as phenolic acids, and highly poly-merized compounds such as tannins. The most common forms of phenolic compounds are conjugated with various sugar molecules(mono-, di-, or oligosaccharides), organic acids and lipids (fats), oreven with other phenols linked to hydroxyl groups or, less fre-quently, to aromatic carbon atoms [27]. The differences in theseconjugated chemical structures account for the different chem-ical classifications and variability in their modes of action andhealth properties. Phenolic compounds can be grouped into classesaccording to the structuralcharacteristics of theircarbon skeletons.The main classes are phenolic acids and aldehydes, flavonoids, lig-nans, stilbenes, tannins and lignins [28]. Phenolics are responsible

for the brightly colored pigments of many fruits and vegetables.

They protect plants from disease and UV light and help preventdamage to seeds until they germinate. Phenolic compounds arequantitatively and qualitatively abundant in olive oil by-products.Table 2 and Fig. 2 show the main compounds described in the liter-ature andprovide examples of their structures. Themain families of phenolic compounds identified by authors in olive mill wastes arephenolic acids, secoiridoids and flavonoids. These compounds arevariably detected and concentrated in olive by-products accord-ing to their polarity, thus influencing their repartition in OMWW,TPOMW and pomace. The treatments applied to extract the oilfrom the olives and to treat the olive mill wastes, the olive varietyand its mode of culture also significantly impact the quantitativeand qualitative phenolic content of the OMWW and solid residues.This phenomenon could explain the sole detection of some phe-nolic compounds only in mature olive pulp or in specific oliveby-products.

3.2.1. Recovery of phenolic compoundsSeveral techniques have been used to recover phenolic com-

pounds from olive by-products, including enzymatic preparation,solvent extraction,membrane separation,centrifugation,and chro-matographic procedures. Khoufi et al. [29] demonstrated that theOMWW hydrolyzed with an enzymatic preparation from Aspergilusniger grown on wheat bran is a potential source of bioactive-freephenolic compounds (especially hydroxytyrosol) with promisingapplications. Another similar study [30] established that the enzy-maticpre-treatment of OMWWwith-glucosidase-rich Aspergillusniger and Trichoderma atroviride culture broths increased theamount of hydroxytyrosol released. However, similar tests witha Trametes trogii culture media found a high oxidation of phe-nolic compounds due to the high laccase activity of this strain.Solvent extraction is the most commonly employed technique toextract phenolic compounds, and ethyl acetate is the most effec-tive solvent for the treatment of OMWW under acidic conditions[31]. Zafra et al. [32] devised a simple and sensitive method forthe determination of 21 phenolic compounds in OMWW usingliquid–liquid microextractionwith ethyl acetate followed by a sily-

lationstep. The identification and quantification analysis werethenperformed using GC–MS. The disadvantages of organic solvents,such as flammability and toxicity, have recently been overcomeusingsupercritical fluids— notably, supercritical CO2. Thus, follow-ing the method described by Lafka et al. [33], phenolic compoundshave been extracted from TPOMW using a conventional liquid sol-vent and a CO2 supercritical fluid. Phenol extraction using liquidsolvents has been optimized using ethanol (5:1, v/w) at pH 2 for180 min. The phenolic content, estimated as caffeic acid equiva-lents on a dry basis (% w/w), varied from 0.43% to 1.29% usingsolvent extraction versus 0.76% using supercritical fluid extrac-tion (SFE)/CO2. An HPLC analysis of the ethanol extract showedthat the predominant phenolic compound was hydroxytyrosol;however, various phenolic acids and flavonoids were also identi-

fied. The extract obtained by SFE showed significant differences(P

8/19/2019 Nadour Et Al2013

5/21


Table 1

Chemical characteristics of OMWW and solid residues.

Parameters Olive oil by products References

OMWW Solid residue

Olive cake TPOMW

Pulp (%) 12–35 10–15 [212]Olive-stone (%) 15–45 12–18 [212]Dry matter (%) 6.33–7.19 87.1–94.4 [213–216]

Ash (%) 1 1.7–4 1.42–4 [33,212,217]pH 2.24–5.9 4.9–6.8 [198,218–220]Electrical conductivity (dS/m) 5.5–10 1.78–5.24 [215,220–222]Total carbon (%) 2–3.3 29.03–42.9 25.37 [37,102,217]Organic matter (%) 57.2–62.1 85 60.3–98.5 [213,216,218,223,224]Total organic carbon (g/L) 20.19–39.8 [37,224]Total suspended solids (g/L) 25–30 [221,225]Mineral suspended solids (g/L) 1.5–1.9 [37,221]Volatile suspended solids (g/L) 13.5–22.9 [37,221]Volatile solids (g/L) 41.9 [221]Mineral solids (g/L) 6.7 [221]Volatile acidity (g/L) 0.64 [221]Inorganic carbon (g/L) 0.2 [37]Total nitrogen (%) 0.63 0.2–0.3 0.25–1.85 [212,215,226]P (%) 0.19 0.03–0.06 0.03–0.14 [212,215,218,227]K (%) 0.44–5.24 0.1–0.2 0.63–2.9 [136,212,215,218,224]Na (%) 0.15 0.02–0.1 [215,218,228]

Ca (%) 0.42–1.15 0.23–1.2 [136,215,218,227]Mg (%) 0.11–0.18 0.05–0.17 [19,136,215,228]Fe (%) 0.26±0.03 0.0526–0.26 [136,218,222]Cu (%) 0.0021 0.0013–0.0138 [215,218,228]Mn (%) 0.0015 0.0013–0.0067 [215,218,228]Zn (%) 0.0057 0.0010–0.0027 [215,218,227]Lipids (%) 0.03–4.25 3.5–8.72 3.76–18.0 [212,215,217,220,222,229]Total phenols (%) 0.63–5.45 0.2–1.146 0.4–2.43 [136,212,217,230]Total sugars (%) 1.5–12.22 0.99–1.38 0.83–19.3 [215,217,224,230]Total proteins (%) 3.43–7.26 2.87–7.2 [19,217,231]Chemical oxygen demand (g/L) 30–320 [219,232]Biological o xygen d emand (g/L) 35–132 [232,233]Cellulose (%) 17.37–24.14 14.54 [217]Hemicellulose (%) 7.92–11.00 6.63 [217]Lignin (%) 0.21–14.18 8.54 [217]

Table 2a

(Part 1) Detection and concentration of themain phenolic compoundsof olive pulp and olive by-products (+,detected; −, notdetected; F, free; B, bound).

Phenolic compounds Mature olive pulp (%) OMWW (%) Solid residue (%) References

Phenolic acidsCinnamic acid − + + [332,1332, 120, 234–236]p-coumaric acid + + + (F0.021–0.030) [391,42, 47, 50, 1332, 234, 237, 238]Caffeic acid + + (0.0072–0.0097) +(F0.014–0.017, B0.006) [391,41, 47, 49, 50, 332, 1332, 234, 235, 237]Ferulic acid + + + [13,391 , 31, 47, 32, 332, 1332, 236, 237]Vanillic acid + +(0.0174–0.0198) +(F0.015–0.024, B0.002) [391,49, 50,1332 , 134, 234, 235, 237, 239-2412]Gallic acid + + + [32,1332 , 242, 243]Syringic acid + + + [50,1332 , 234–238]Sinapic acid + + + [13,1332 , 238, 243]Homovanillic acid + + + [224,237, 2412]4-hydroxyphenyl acetic acid − + − [13,31,43,238]

Secoiridoids and derivatives

Oleuropein +(0.006–0.60) + − [5,41,47,36,237,244–246]Demethyloleuropein +(0–0.2) + + [41,134, 2412, 246]Verbascoside +(0.0035–0.12) +(0.0075–0.0155) + [41,49, 131, 237, 2412, 244–247]Ligstroside + +(0.0087–0.0092) + [49,131, 237, 2412]Tyrosol + (0.04) +(0.0145–0.0208) + [5,391, 48, 49, 236,237, 240, 2412 , 247–249]Hydroxytyrosol +(0.02–0.55) +(0.0020–0.1224) + [5,391, 47, 49, 332, 239–2412, 245, 246, 248–250]3,4-Dihydroxyphenylethanol-elenolicacid dialdehyde

− + − [41,134,238]

FlavonoidsLuteolin +(0.035) +(0.0145–0.021) + [5,49, 332, 36, 131,240, 2412, 245]Luteolin 7-O-rutinoside + − − [5]Luteolin 7-O-glucoside +(0.01–0.05) +(0–0.0214) + [5,49, 131, 240, 2412, 245, 246]Luteolin 4-O-glucoside + + − [5,36,132]Rutin +(0.02–0.05) + +(F 0.017–0.082, B 0.0058) [5,36, 132, 1332 , 240, 2412 , 245, 246]

8/19/2019 Nadour Et Al2013

6/21


Table 2b

(Part 2) Detection and concentration of themain phenolic compoundsof olive pulp and olive by-products (+ detected,− notdetected; F free, B bound).

Phenolic compounds Mature olive pulp (%) OMWW (%) Solid residue (%) references

FlavonoidsHesperidin − − + [332,1332]Quercetin + + + [132,1332, 224]Quercetin 3-O-glucoside + − − [5]Quercetin 3-O-rutinoside + − − [246]Quercetin-3-O-rhamnoside + − − [237,245]

Apigenin +(0.013) − + [5,240,241]2

Apigenin 7-O- rutinoside + − − [5]Apigenin 7 -O-glucoside +(0.001–0.0068) − + [2412,245, 246]Cyanidin 3-O-rutinoside + − + [332,245, 246]Cyanidin 3-O-glucoside + − + [332,245, 246]

311:pomace obtained from three-phase olive extraction system.502, 592, 2442: pomace obtained from a two-phase olive extraction system.

induces a better penetration of solvent into cellular materials, thusimproving mass transport rates within the tissue and facilitatingthe transfer of components from the cell into the solvent, ulti-mately enhancing the extraction efficiency [35]. Gortzi et al. [36]investigated cloud point extraction using Genapol X-080 as a sur-factant for the separation of phenolic compounds from OMWW.They found that many of the individual phenolic compounds stud-

ied were recovered at a high yield. The total phenol recovery bysimple and successive cloud point extraction of OMWW with var-ious concentrations (2%, 5% and 20%, v/v) of Genapol X-080 wasup to 89.5%. This procedure offers an interesting alternative toliquid–liquid or liquid–solid solvent extractiondue to its simplicity,shorter extraction time, limited lab and equipment requirements,and its use of non-toxic extractants. In this same manner, mem-brane methods have been developed to avoid the use of solventsduring phenolic compound extraction and purification. For exam-ple, Garcia-Castello et al. [37] recently reported the recovery andconcentration of OMWW phenolic compounds using an integratedmembrane system based on membrane techniques where 78%of the initial phenolic compound content was recovered in themicrofiltered permeate solution. The microfiltration permeate was

then submitted to nanofiltration, after which a concentrated solu-tion containing approximately 0.5 g/L phenolic compounds, withhydroxytyrosol representing 56% of the total, was obtained byosmotic distillation of the nanofiltration permeate.

3.2.2. Phenolic compounds from liquid residue: OMWW During the olive oil extraction process, the different chemical

characteristics of water-soluble phenolic compounds lead todifferential partitioning between water and oil, and the majorfraction of these compounds shifts from olive pulp to OMWW. Thisphenomenon is the main obstacle to the biological detoxificationof OMWW. Although two-phase centrifugal decanters yield virginolive oils with a greater phenolic compound concentration thanthose obtained using three-phase systems, the majority of them

(approximately 98%) still remains in the TPOMW [38,39]. Asmentioned above, a medley of factors converge to determine theoccurrence of specific phenolic compounds in OMWW, includingthe olive cultivar, the ripeness of the fruit, the climate and agro-nomic conditions, the storage conditions prior to extraction, andthe processing techniques [31,40]. For example, Servili et al. [41]found strong differences in the phenolic compositions betweenOMWW and olive fruit. More remarkably, the phenolic compo-sition of OMWW varies strongly between studies. The OMWWphenolic fraction is characterized by a significant complexity, asdemonstrated by Bianco et al. [42], who identified 20 phenoliccompounds in OMWW using HPLC-MS-MS. The prevalent classesof hydrophilic phenols identified include phenyl alcohols, phenolicacids, secoiridoid derivatives, flavonoids (luteolin, luteolin-7-

glucoside), and lignans (Table 2). Visioli et al. [43] reported that

oleuropein, an ester of elenolic acid and hydroxytyrosol, is amajor phenolic compound in OMWW, whereas Allouche et al.[31] f ailed to identify this compound in their OMWW. One expla-nation could be that the OMWW was sampled late in the oliveharvest (mature olives), when the oleuropein had been degradedinto elenolic acid and hydroxytyrosol by an esterase during themechanical olive oil extraction process [43,44], which corresponds

with the high amount of hydroxytyrosol quantified in OMWW[42,45–49]. Among the phenolic compounds reported in Table 2,the following molecules have also been detected in OMWW:4-methylcatechol, 4-hydroxybenzoic acid, protocatechuic acid,vanillic acid, 3,4-dihydroxyphenylglycol, homovanillic alcohol, 4-hydroxy-3,5-dimethoxybenzoic acid, 3,4-dihydroxyphenylaceticacid, 2-(4-hydroxy-3-methoxy) phenylethanol, and 2-(3,4-dihydroxyphenyl)-1,2-ethandiol [13,48,50]. Moreover, the typicalbrownish-black color of this effluent has been explained by thepresence of polymeric phenols that display a lignin-like structureand constitute its most resistant fraction [51]. As detailed above,the intrinsic variability of the wide range of analytical factorsand methods used to extract and analyze the phenolic com-pounds could also explain the variability of phenols in OMWW.

Nonetheless, the variability in the phenolic contents of OMWWfrom various origins significantly complicates their treatments bybioconversion. The reason is that some consortia of microorgan-isms that can efficiently treat some OMWW can be inhibited byone another. The phenolic content is also related to the OMWWdilution ratio, with the objective being to reduce their toxicitybefore the microbial treatment. The putative valorization of somephenolic compounds also appears to be relatively uncertain andnot economically viable considering the low concentration of spe-cific species in OMWW and the cost and complexity of purificationtechniques.

3.2.3. Phenolic compounds in solid residues: olive pomaceOlive stones and seeds contain significant amounts of

phenolic compounds. Three glucosides, including salidro-side (tyrosol–glucose), nuezhenide (glucose–elenolicacid–glucose–tyrosol) and nuezhenide-oleoside, have beenidentified in olive stones [52]. Nuezhenide is found only in seedsas a predominant phenol, whereas verbascoside only appearsin significant quantities in the seeds and pulp [53]. Tyrosol andhydroxytyrosol have been detected in olive stones, whereasdecarboxymethyl oleuropein (3, 4 DHFEA-EDA) has been foundin the pulp, seeds and stones [54]. The most abundant phenoliccompounds in TPOMW are tyrosol and hydroxytyrosol [55],together with p-coumaric [39] and, to a lesser extent, vanillic acid[39]. Other minor compounds identified include verbascoside,rutin, caffeoylquinic acid, luteolin-4-glucoside, 11-methyl-oleoside, hydroxytyrosol-10-b-glucoside, luteolin-7-rutinoside,and oleoside (Table 2).

8/19/2019 Nadour Et Al2013

7/21


OH

R

COOH

R1

R2

COOH

R = H 4-hydroxyphenyl acetic acid

R = OH 3,4-dihydroxyphenyl acetic acid

R 1 = H - R 2 = H cinnamic acid

R 1 = OH - R 2 = H p-coumaric acid

R 1 =OH - R 2 = OH caffeic acidR 1 = OH - R 2 = OCH3 ferulic acid

R1

R2

OH

R3

R1

R2

COOH

R 1 = OH - R 2 =H - R 3 = H tyrosol

R 1 = OH - R 2 =OH - R 3 = H hydroxytyrosol

R 1 = OH - R 2 =OH - R 3 = OH 3,4-dihydroxyphenylglycol

R 1 = OH - R 2 =H 4-hydroxybenzoic acid

R 1 = OH - R 2 =OH protocatechuic acid

R 1 =OH - R 2 = OCH3 vanillic acid

OH

OH

O

O

O

MeOOC

OGlc

CH3

Oleuropein

O

O

ORha

OH

OH

O

OH

OH

O

OH

OH

Verbascoside

Fig. 2. Chemical structures of themain phenolic compoundsfrom olive oil by-products.Glc: Glucose; Rha: Rhamnose.

3.3. Carbohydrates: purification and structure

Dietary fiber (DF) is defined as the remnantsof edible plant cellsand consists of polysaccharides, lignin and associated substancesthat are resistant to digestion by human enzymes [56]. Olive cellwall polysaccharides recovered from olive mill by-products havebeen proposed as a source for several products, such as microcrys-

talline or powdered cellulose, gelling agents and fat replacements

[57–61]. However, even though these polysaccharides are presentin these by-products, their extraction and purification from solidor liquid residues rich in phenolic compounds and other organicmatter require costly polar solvents and processes. Table 3 showsthe sugar compositions of different fractions isolated from olivepulp during the ripening stage of the fruit and from the OMWWand solid residue. Cellulolic, hemicellulosic and pectic polysaccha-

rides are the main carbohydrates described in olive by-products.

8/19/2019 Nadour Et Al2013

8/21


Table 3

Sugar composition of differentfractionsisolatedfrom olive pulp in theripeningstage, OMWW and solid residue.

Fraction Origin Yield (%)a Monosaccharide composition (mol%) References

Total AIR Olive pulp 7.4–8.3 Glc (39%), Xyl (21%), Uronic acids (18%), Ara (16%), Gal (3%), Man (2%), Rha (2%) [70]4.7 Glc (34%), Uronic acids ( 23%), Xyl ( 17%), A ra (17%), Gal ( 5%), R ha ( 3%), M an (2%) [72]

OMWW 32–47 Ara (50.8–63.4%), Gal (24.9–26.4%), Glc (4.8–13.4%),Xyl(4.4–6.7%), Rha (1.5–1.8%), Man(0.7–0.8%), Fuc (0.2%)

[74]

4.3–5.1 GalA(36–45%),Ara(18–21%), Glc (14–17%), Gal (7–10%),Rha (2–7%), Man (4–6%), Xyl(4–5%)

[61]

2.7–2.8 n.d. [251]Solid residue 13.5 HexA ( 31%), G lc (29%), X yl (16%), Ara (14%), Gal (3%), Rha (3%), M an (2%), Fuc (1%) [75]13.5 Glc (40%), Xyl (28%), GalA (14%), Ara (11%) [57]4–5 Glc(36–37%),Xyl (30%), Uronic acids (14–15%), Ara (12–13%),Gal (3–4%),Rha (2%),Man

(1–2%)[74]

Soluble fraction OMWW 2.32–3.85 GalA (93.9%), Ara (6.1%) [61]Solid residue 23.3–28.6 Glc(15.6–53.9%), Xyl(3.2–46.3%),Ara (19.6–43.9%),Man (1.7–17.4%), Rha (6.5–13%), Gal

(2.6–9.7%)[81]

0.74–2.09 HexA(48–52%), Ara (15–22%), Glc (13–21%), Gal (5%), Rha (3–4%), Xyl (3–4%), Man (1–3%) [75]

Insoluble f raction OMWW 0.13–0.16 Rha (27.9%), Glc (21.6%), GalA(15.9%), Xyl (14.4%), Gal (8.6%), Ara (6.2%), Man (5.5%) [61]Solid residue 51.7–60.9 Xyl (12.5–86.5%), Glc (2.6–79.7%),Ara(2.2–20.2%),Gal(1.3-5.7%),Man (0.5–3.1%) [81]

3.25 Glc ( 37%), H exA ( 26%), X yl ( 26%), A ra ( 6%), G al ( 2%), M an (2%), Rha(2%),Fuc (1%) [75]

Pectin Olive pulpe n.d. Anhydro-uronic acids (2–83%), Gal (1–63%), Ara (2–56%), Glc (1–44%), Man (1–33%), Xyl(1–33%), Rha (0–3%)

[70]

50–91 Uronic acids (19–77%), Ara (19–75%), Gal (1–4%) [252]n.d. HexA ( 48–88%), Ara (9–45%), Glc (1–5%), Rha (1–3%), Gal (1–2%), Man (0–1%), Xyl (0–1%),

Fuc (0%)

[64]

2.2–15.9 Uronic acids (18–62%), Ara (17–39%), Glc (2–18%), Man (1–13%), Gal (6–10%), Xyl (1–6%),Rha (4–5%)

[72]

4.1–4.8 Ara (62–71%),GalA(17–25%), Gal (8%),Rha (2–3%),Xyl (1–2%),Glc (1%) [253]OMWW 2.32–3.85 GalA (93.9%), Ara (6.1%) [61]Solid residue 0.50 GalA ( 54%), Ara (26%), G al (9.5%), Glc (6%), R ha ( 2.3%), X yl (1.1%), Man (0.8%) [57]

1.1–9.0 Uronic acids (22–66%), Ara (15–57%), Gal (6–14%), Rha (2–6%), Glc (1–4%), Xyl (1–4%), Man(tr-2%)

[74]

Hemicellulose Olive p ulp n.d. Xyl ( 9–49%), A ra ( 11–42%), G lc ( 13–26%), A nhydro-uronic a cids ( 9–18%), G al ( 5–7%), M an(2–7%), Rha (1–7%)

[70]

n.d. Xyl ( 2–91%), A ra (1–76%), G lc (1–34%), H exA ( 4–24%), M an (0–18%), Gal (1–10%), Rha(0–3%), Fuc (0–1%)

[64]

22–78 Xyl(15–83%), Glc (1–43%), Ara (1–32%),GalA (tr-28%),4-O-methyl-GlcA (0–12%), Gal(tr-11%), Man (Tr-10%), Rha (1–8%), GlcA (0–4%), Fuc (0–1%)

[72]

n.d. Xyl(35–97.3%), Glc (0–42.3%),Ara (1.1–11.5%),Gal (0–9.2%),Man (0–1.4%), Fuc (0–0.7%),Rha (0–0.3%), Uronic acids (nd)

[68]

n.d. Glc (45–51%), X yl (34–36%), Ara (11–13%), G al ( 0–10%), M an (0%), Fuc (0%) [69]

a Expressedas % (w/w) of initial sample; n.d.: not determined; tr: trace amount.

Logically, soluble polysaccharides have been predominantly iden-tified in pulp and OMWW, whereas insoluble polysaccharides havebeen described in solid by-products. Note that the polysaccha-rides in OMWW are highly diluted. Numerous studies focusedon determining the DF content of olive fruits have identifiedpectin(arabinans,homogalacturonans and rhamnogalacturonans),hemicelluloses, cellulose and lignin [62–67]. Jiménez et al. [67]studied the hemicellulosic polysaccharides from “Manzanilla” and“Hojiblanca” olive varieties and reported the presence of a xylan,an arabinoxylan (>400kDa), and a xyloglucan (260kDa). Newanalytical methods have enabled a more accurate structural char-

acterization of the olive DF component. Vierhuis et al. [68,69]described a xyloglucan in olive fruit and revealed that this xyloglu-can is predominantly composed of two novel oligosaccharides: anoctasaccharide and a heptasaccharide. The octasaccharide has abackbone of four linked -(l,4) glucose (Glc) monomers, three of which are substituted at C-6 by a single xylose (Xyl) residue, andone of these Xyl residues, near the unsubstituted Glc monomer,is ramified at C-2 by an arabinose (Ara) unit. The heptasaccha-ride has a core in which two of the three Xyl residues near theunsubstituted Glc monomer are substituted at C-2 by a single Araresidue and a single Gal residue. In both oligosaccharides, some of the corresponding Ara residues are substituted with either one ortwo O-acetyl groups. Other investigators have thoroughly studiedthe structure of olive pectic polysaccharides. Arabinan-rich pectinsare reported to be one of the major classes of polysaccharides

found in olive pulp cell walls [63,70–73]. Ara occurs as side-chainslinked to the C-4 of the -(1,2)-linked L-rhamnose (L-Rha) unitsinterspersed in the -(1,4)-linked galacturonic acid (GalA) chain.Approximately 5–10% of the polysaccharides present in olive fruits,whicharewater-solublepecticmaterial,endupinvegetationwaterwhen the oil is extracted from the malaxed paste using a decanter[74].

Several authors have recommended cell wall isolation proto-cols for olive-related sources and by-products by using hot ethanolas a precipitant followed by an alkali, acid or solvent treatment[57,70,74]. The general protocol developed to isolate DF from olive

by-products is shown in Fig. 3. The DF-containing material, knownas alcohol-insoluble residue (AIR), is first extracted from OMWWand separated into two fractions based on their water solubilityaccording to Vierhuis et al. [74] and Galanakis et al. [60,61]. Theextraction is based on a thermal treatment with ethanol (EtOH)and acids prior to the precipitation of the AIR with boiling concen-trated EtOH. The insoluble material obtained is then washed withdifferent solvents (including chloroform, methanol, and acetone) toremove undesirable substances (lipids, phenolic compounds, etc.),then filtered and dried to obtain AIR. This efficient procedure illus-trates the complexity of DF extraction from OMWW and its highpolar and non-polar solvent consumption.

TheDF is predominantlycomposed of GalA, Araand Glcresiduesalong with minor proportions of galactose (Gal), mannose (Man),Xyl and Rha. There are differences in the sugar compositions of

8/19/2019 Nadour Et Al2013

9/21


Water-Insoluble AIR Water-Soluble AIR

Extraction of AIRs

Extraction of fractions from AIR

OLIVE OIL BY-PRODUCTS

(OMWW, olive pomace)

Alcohol Insoluble Residues (AIR)

Thermal treatment with solvent, acids and other

Extraction of AIR with water

Removal of undesirable substances with solvents (acetone, chloroform, etc.)

Filtration

Centrifugation

Fig. 3. Schematic principle of dietary fiber extraction process.

the samples (a higher Glc than Gal content) between the resultsobtainedbyVierhuisetal.[74] andGalanakisetal. [61], likelyduetothe different extraction procedures used. Vierhuis et al. [74] stud-ied the structural features of the soluble polysaccharides presentin olive oil by-products. A sugar composition analysis revealed thata substantial fraction of the pectic material present in OMWW iscomposed of arabinans and galactans. More information on thesestructures has been obtained by incubation with specific and well-characterized cell wall-degrading enzymes. Their monosaccharidecomposition is specific to a pectic origin. Ara f are predominantlypresent as terminal and (1,5)-linked residues, and approximately

13% is mono-substituted with branch points at C-2 or C-3. Most of the Gal p units appear to be present as (1,3)/(1,6)-linked galactanswith a high degree of branching (12.3%), whereas 20% of the Gal pis present as (1,4)-linked galactan. Note that a sugar compositionanalysis also revealed the presence of GalA. Therefore, either thecommercial enzyme preparations used had solubilized additionalpectic material with a high GalA content, or part ofthe pectic mate-rial had been partially debranched by previous treatments. Thesugar compositions of the insoluble and soluble fractions obtainedfrom the AIR were also determined by Galanakis et al. [61]. Theinsoluble fibers were rich in Rha, Glc, GalA and Xyl and weredeficient in Gal, Ara and Man, whereas the soluble fibers wereexclusively composed of GalA and Ara, thus validating the occur-rence of pectic polysaccharides. These composition results further

confirm those for compounds isolated from olives by other authors[57,70]. Cardoso et al. [75] isolated an arabinan (97% Ara and 3%hexuronic acid) from olive pomace by fractionation of a pecticpolysaccharide-rich extract obtained via a hot dilute acid treat-ment followed by a graded precipitation with ethanol. It was thenseparated from acidic pectic polysaccharides by anion-exchangechromatography, and size-exclusion chromatography determinedits molecular weight (an estimated 8.4kDa).

Olive pomacehas a high lignin, celluloseand hemicellulosecon-tent. Different studies have investigated the sugar content of AIR isolated from olive pomace and its soluble and insoluble fractions.The sugar composition of stones andseed husk hasalso been deter-mined for several varieties of olive fruit using a neutral detergentfiber (NDF) method [76,77]. The NDF method [77] was developed

to separate dry feed matter into a soluble fraction that is readily

digested and a fibrous fraction that is slowly and incompletelydigested. The NDF in plant material is primarily hemicellulose,cellulose and lignin; however, it also contains small amounts of protein and ash. The main components detected were cellulose inthe stone and hemicellulose in the seed. Coimbra et al. [78] iso-lated and characterized the cell wall polysaccharides of olive stoneand found that it contains 62% total carbohydrates that are rich inXyl (from hemicellulose) and Glc (from cellulose). A delignificationtreatment then made it possible to extract polysaccharides fromthe (olive stone) cell wall. Glucuronoxylans were the major non-cellulosic polysaccharide identified. To extract and recover all of these polysaccharide fractions, some processes have been devel-oped. High-pressure steaming followed by rapid decompression— a process known as steam explosion — has been touted as aneffective pretreatment [79] f or the further processing of waste cel-lulosic material, including olive stone [54,59]. This pre-treatmentleads to the autohydrolysis of hemicelluloses to water-solubleoligomers or to monosaccharides [80] and appears to be a promis-ing pre-treatment for converting low-value biomass (cellulose andhemicellulose components of lignocellulosic materials) into com-mercially useful products (food, fuels and chemicals). Under theseconditions, olive pomace is described as a good source of fer-mentable carbohydrates. Felizón et al. [81] used steam explosionto treat olive cakes. After fractionation, the main components of the water-soluble fraction were carbohydrates and, notably, Glcand Ara. The constitutive polymers were quantified in the insolu-blefraction,andasugarcompositionanalysisshowedthatcellulosewas associated with a high proportion of the xylans and otherAra- and Gal-rich polysaccharides. The results indicated that a solesteam explosion is not sufficient to obtain high levels of saccha-rification, and supplementary treatments, such as delignificationand enzymatic hydrolysis, were necessary to obtain high con-centrations of fermentable sugars. In these mixtures, the pentosecontent is high, which implies their specific fermentation in thecontext of biofuel production. Therefore, the potential competitionof solid by-products from the olive oil industry for the productionof fermentable sugars appears to be poor compared to other lig-

nocellulosic biomasses with higher fiber contents and lower ligninconcentrations.

4. Environmental impacts of OMWW

Untreated OMWW is a major ecological issue for olive oil-producingcountriesduetoitshightoxicorganicloads,lowpH,highCOD (up to 110 g/L) and high biological oxygen demand (BOD up to170g/L) [82]. OMWW can lead to serious environmental damage,ranging from coloring natural waters and toxicity to aquatic life topollution in surface and ground waters, altered soil quality, phyto-toxicity,andodornuisance.ThelargevolumesofOMWWproducedandthe brief periodof olive oilproduction aggravatetheseenviron-mental damages between November and March in Mediterraneanolive-growing countries where OMWW is discharged untreatedinto the environment.

4.1. Soil pollution andphytotoxicity

OMWW discharged directly into soil have detrimental effectsnot only on plant growth and microbial activity but also to thephysicochemical properties of soils [12]. The main obstacle forthe direct use of OMWW for irrigation is its high concentrationof phenolic compounds, which are phytotoxic and can inhibit plantseed germination. More, OMWW contains oil compounds that mayresult in increased soil hydrophobicity and decreased water reten-tion and infiltration rates [83]. It is important to highlight that

soils from different origins may have a different intrinsic buffering

8/19/2019 Nadour Et Al2013

10/21


capacity and, therefore, respond differently to the applied pertur-bations [84]. Regarding phytotoxicity, El Hajjouji et al. [85] usedthe Vicia faba micronucleus test to evaluate the genotoxicity of OMWW and found that OMWW was genotoxic at a 10% dilution.Thisgenotoxicitywasassociatedwiththegallicacidandoleuropeincontents of the OMWW. The impact of OMWW on soil proper-ties appears to be the result of opposite effects, depending on therelative amounts of beneficial andtoxic organic and inorganic com-pounds [84]. Mechrietal. [86] reportedthatagronomicapplicationsof OMWW have negative impacts on the functioning of arbuscularmycorrhizas between fungi and olive trees. This same conclusionwas published by Chartzoulakis et al. [87] when they studied theeffects of OMWW on soil properties, plant performance and theenvironment after 3 consecutive years of OMWW applications atincreasing rates from 252m3/ha(1year)to420m3/ha(for 2 years).ThestudyrevealedasignificantrateofavailableKandanincreaseof phenolic compounds with negative impacts on soil. More recently,Magdich et al. [88] also investigated the long-term impact of OMWW on olive trees by considering soil phenolic compound evo-lution, phytotoxicity progress and soil microbial counts at differentsoil layers. Three OMWW levels (50, 100 and 200 m3/ha per year)were applied over sixsuccessive years. The soil polyphenol contentincreased in accordance with yearly numbers and the quantity of OMWW applied. In addition, the microbial counts increased withthe OMWW quantities and spraying frequency. OMWW spreadingin the environment and on fields is currently employed, and thispractice has raised both controversies and debate regarding thepotential fertilizer and toxic compound content of this by-product.However, the main conclusions of investigators are that the directapplication of OMWW without treatment has a negative impact inthe long term. However, controlling the OMWW volume applied tosoil could reduce its negative impact and be beneficial withregardsto its fertilizer action.

4.2. Water pollution

OMWW discharged into fresh water reduces its oxygen avail-

ability, which upsets the entire balance of the ecosystem [89].In addition, the high concentration of reduced sugars can stimu-late microbial respiration, thus further lowering dissolved oxygenconcentrations. If discharged into waters with high phosphoruscontents, OMWW can lead to eutrophication. Eutrophication isthe response of an aquatic ecosystem to the addition of artificialor natural nutrients (such as phosphates or nitrates) through fer-tilizers or sewage. Negative environmental effects include plantgrowth and decay and, therefore, hypoxia, which induce reduc-tionsinotheranimalpopulations(fish).OMWW alsohassignificantimpactsonsurfacewatersbecausehighconcentrationsofdarkphe-nolic compounds can color natural waters (streams and rivers)[16]. Furthermore, the lipids from OMWW form an impenetra-ble film on the surface of the receiving water, which blocks out

sunlight and oxygen, thus inhibiting plant growth [89]. There arecases where OMWW is disposed of in sea-, river- or groundwa-ter. The construction of evaporation ponds/lagoons rarely meetsengineering criteria for stability and the safe accommodationof liquid wastes. Therefore, OMWW often overflow and affectneighboring systems (agricultural soils and surface- and ground-water). In most cases, the base of the pond is permeable, andthus, the probability for groundwater and deep soil contaminationis considered high [90]. On a longer time scale, the applica-tion of OMWW to soil may affect its infiltration capacity, withpossible negative effects on groundwater quality [91]. In this con-text, it has been reported that OMWW spreading on soil mightincrease phenolic compoundsin the groundwater duringthe activeperiod of olive mills [92]. Regarding the impact of OMWW as

a pollutant of marine environments, Danellakis et al. [93] have

demonstrated that direct OMWW disposal might induce pre-pathological alterations in marine organisms. Karaouzas et al. [94]investigated the toxic effects of OMWW on aquatic communi-ties.Biotic(macro-invertebrates) and abiotic (physicochemicalandhydro-morphological) data were monitored. The results revealedthe spatial and temporal structural deterioration of the aquaticcommunity due to OMWW pollution with the consequent reduc-tion of the river’s capacity for reducing the effects of pollutingsubstances through internal mechanisms of self-purification. Themost important factors affecting macro-invertebrate assemblageswere: the organic load of the wastewater (BOD and COD), sub-strate contamination (sewage bacteria) and the distance fromthe mill outlet. For centralized treatments of OMWW, conven-tional technologies such as upflow anaerobic sludge bed reactorsor membrane processes would have to be used. Nonetheless,these equipments require high construction costs and specializedpersonnel with an uncertain probability of payback. Moreover,considering that OMWW are seasonally operated, this consider-ation poses an additional seasonal maintenance constraint.

4.3. Air pollution

If OMWW is stored in open tanks and/or discharged onto landor into natural waters, it can undergo fermentation and emitmethane and other pungent gases such as hydrogen sulfide, cre-ating strong odor pollution [62]. Knowing that the Mediterraneanregion accounts for 95% of theglobal OMWW production, in touristand archaeological areas, the air pollution sometimes has negativeimpacts on economic activities. Pre-treatments to eliminate foulodors may include liming, which significantly reduces the organicload and precipitates the solids contained in the OMWW [89].

5. Applications andbioconversion of olive oil by-products

Olive oil extraction produces a solid residue and dark-coloredwastewater containing nutrients that can be further bioprocessedin parallel for disposal. Olive oil by-products currently have sev-

eral types of putative applications, as summarized below (Fig. 4).Note that the majority of these applications are not effective andare only described in the scientific literature as the agro-industrialby-products from the olive oil industry are unexploited. Thesestrategies can be considered the eco-design of an integrated pro-duction scheme. Only legislations and integrated studies followingwell-funded R&D programs will eventually attain the goal of usingolive oil by-products under optimized processes to successfullycreate a global olive refinery.

5.1. Agricultural uses

As explained above in the paragraph entitled Soil pollutionand phytotoxicity, the antimicrobial and phytotoxic properties of

OMWW indicate that it should not be directly applied to soils andcrops. However, its high water, organic matter and plant nutrientcontent make it a candidate for bioconversion as a valuable fertil-izer and soil conditioner. Consequently, its use after the removalof its phenolic components for soil fertilization could be con-sidered valuable [84,94]. However, in data from the literature,OMWW appears to have heterogeneous impacts on different bacte-rial groups involved in N cycling data, likely because their phenoliccontents are different [12,95–99]. Aerobic biological treatments,predominantly with yeasts andfilamentous fungi,have emerged assuitablebiofertilizationmethodsbecausetheyleadtoresidueswithlower toxicity, COD and phenolic contents (Table 4). Filamentouswhite-rot fungi such as Phanerochaete chrysosporium [100–103],Lentinula edodes [104,105], Pleurotus ostreatus [106] and Aspergilus

sp. [102,107,108] catabolize a wide range of simple aromatic

8/19/2019 Nadour Et Al2013

11/21


AGRONOMY

Edible fungi

Animal feeds

Composting

B IOENERGY

Bioethanol

Biohydrogen

Biomethane

VFA

Biodiesel

B IOMOLECULES

Biopolymers

Enzymes

Phenolic compounds

R ECOVERY OF

VALUABLE COMPOUNDS

BIOCONVERSION

Polysaccharides

Biophenols

OTHER USES

Cosmetic usesTextile dyeing Water decontamination

OLIVE OIL

BY-PRODUCTS

Fig. 4. Valorization opportunities for oil mill wastes.

Table 4

Dephenolization through microbial treatments of olive oil byproducts.

By-products Strain COD reduction (%) Phenol reduction (%) Toxicity reduction and effects References

Yeastsand yeast like fungiOMWW Candida holstii – 39 Improvement of barley germination [111]TPOMW Candida boidinii – 57.7 – [254]OMWW Geotrichum. candidum 51 46 – [108]OMWW Candida oleophila 55 83 Improvement of seed germination [112]OMWW Candida tropicalis 62.8 51.7 – [107]TPOMW Saccharomyces spp. – 61 – [254]OMWW Natural microflora – 75 Genotoxicity reduction [85]OMWW Geotrichum. candidum 75 – – [110]

OMWW Yarrowia lipolytica 20–40

8/19/2019 Nadour Et Al2013

12/21


(barley seed germination test), a lower phenolic content (44%removal), and lower COD (63% removal). The best germination rate(80%) on undiluted OMWW was obtained with a strain of Candidaholstii. AnotherCandida species (C. oleophila) isolated from OMWWremoved 50% of the organic load and 83% of the total phenoliccompounds from undiluted OMWW and increased the germina-tion index of these effluents by up to 32% compared to untreatedOMWW [112]. El Hajjouji et al. [85] studied the toxicity and geno-toxicity of OMWW on Vicia faba plants after biological treatmentwith microorganisms naturally present in OMWW and found thatthe genotoxicity and phytotoxicity of diluted OMWW had com-pletely disappeared post-treatment. This effect was explained bythe biodegradation of phenolic compounds (75.9%). Pinto et al.[113] tested cultures of two microalgae for OMWW treatmentunder heterotrophic conditions and demonstrated that they wereable to degrade up to 50% of their low-molecular-weight phenoliccompounds. OMWW treatment by photosynthetic microorgan-isms could be an interesting area of research in the futureconsidering the natural irradiance and temperatures of Mediter-ranean countries. Some photosynthetic microalgae are effectivelymixotrophic, meaning they are able to perform photosynthesisand catabolize exogenous organic nutrients. Therefore,with simplegrowth requirements, microalgae can sustainably generate lipids,proteins and carbohydrates on a large scale, offering promisingenvironmentally friendly alternatives to current consumer prod-ucts. The most prominent examples are third-generation biofuels.

Studies employing aerobic bacterial consortia, rather thanmicroalgae, fungi or yeasts, for OMWW detoxification haveachieved significant reductions of COD (up to 80%) and phytotox-ins [21,114–116]. These consortia had various origins but weregenerally isolated from agricultural soils and industrial/municipalwastes. Some data from the literature have focused on fermenta-tive or cofermentative (with cheese whey) dephenolization andthe color- removing ability of OMWW by pure bacterial culturesof Lactobacillus [117,118]. Other studies have successfully trialedaerobic bacterial strains such as Azotobacter vinelandii [119] andBacillus pumilis [120] f or phytotoxin removal and decreasing phe-

nolic compounds (50%), respectively, in OMWW. OMWW oftenrequires dilution prior to treatment by microbial aerobic or anaer-obic processes, and whereas dilution decreases the concentrationof toxic compounds in wastewater, it also increases the volume of this waste. Consequently, another approach adopted to minimizethe inhibitory effects of concentrated OMWW is the adaptationof microorganisms on OMWW [108,121]. Through an acclimatiza-tion process, Ergul et al. [121] reported that the fungus Trametesversicolor was able to achieve an up to 78% dephenolization onundiluted OMWW in shake-flask experiments and 70% in a con-tinuously stirred tank reactor (CSTR). The inoculum was preparedin two-fold-diluted OMWW, and Trametes versicolor was consecu-tively transferred to media containing 60, 70,80, 90 and 100% (v/v)OMWW. Aissametal. [108] also achieved dephenolization and COD

removal withCandidaboidinii,Geotrichumcandidum, Penicilliumsp.and Aspergillus niger strains isolated from OMWW.Composting is one of the most popular technologies for upcy-

cling OMWW into fertilizer. Organic waste composting is abio-oxidative process involving the mineralization and partialhumidification of theorganic matterto yield a stabilizedfinal prod-uct. Passing a phytotoxicity test is the principal prerequisite for acompost to be safely used on soil [122]. Tomati et al. [123] were thefirst to report enhanced plant soil system activity after the addi-tion of olive oil by-product composts. In an experimental OMWWcomposting process with barley straw, Zenjari et al. [124] f oundthat the degradation of phenolic compounds reached 95% afterthe maturation phase and that the toxicity disappeared completelyafter two months of composting. Another approach involving co-

composting with olive cake and OMWW has been reported as

efficient [125–127], and the resulting composts have been demon-strated to possess high agronomic value as soil additives [127,128].The bioconversion of olive pomace into compost by Trichodermaharzianium and Phanerochaete chrysosporium led to a product thatwas non-phytotoxic compared to controls [122]. Therefore, themicrobial bioconversion of olive by-products (diluted or not) toeliminate their toxicity appears to be an interesting mode of treatment, notably to produce fertilizers such as compost [189].However, to be economically viable, their dilution is not acceptablein terms of the volumes that have to be treated. Moreover, in thecase of OMWW, efficient processes should be used to concentratethe dry matter and to reduce the costs of treatment. Unfortunately,this solution is not fully satisfactory because the concentrationof dry matter is energetically expensive and increases the con-centration of toxic products and limits their bioconversion. Thiscontradiction remains the main drawback, limiting the treatmentof olive by-products by microorganisms.

5.2. Production of bioactive phenolic compounds

Numerous studies have focused on the biological proper-ties of phenolic compounds extracted from olive by-products[33,129–138]. Both residues (OMWW and TPOMW) appear to bean affordable and abundant source of biologically active phe-nolic substances that hold promising potential as antioxidant,anti-inflammatoryandantimicrobialagents.Nonetheless,thequal-itative and quantitative heterogeneity of phenolic compounds inthese by-products is often a difficulty, i.e., in terms of findingfeasible applications in this area. The antioxidant activities of OMWW and olive pomace have been studied and demonstratedby several antioxidant assays including DPPH radical-scavengingactivity, superoxide anion scavenging, LDL oxidation, and the pro-tection of catalase against hypochlorous acid [33,130–133]. Theanti-inflammatory properties of OMWW have also been estab-lished and measured using multiple assays, including leukotrieneB4 production by human neutrophils and the inhibition of tumornecrosis factor production [134,135]. In parallel to these therapeu-

ticactivities,OMWW exhibitsabroadspectrumoftoxicitiesagainstbacteria, fungi, plants, animals and human cells [129,132]. Obiedet al. [132] reported that the phenolic fraction of OMWW demon-strated antibacterial activities against several bacterial species(Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Pseu-domonas aeruginosa). Moreover, other studies have shown thatthe significant bactericidal and fungicidal activities of OMWWare predominantly tied to their phenolic monomer contents,such as hydroxytyrosol and tyrosol [136,137]. Furthermore, Schaf-fer et al. [138] described the cytoprotection of brain cells by ahydroxytyrosol-rich OMWW extract under different stresses orparadigms. Correlation analyses revealed that the observed cyto-protective effects were likely due to hydroxytyrosol. Consequently,the valorization of phenolic compounds for their use as bioactive

agentsimplies their fractionation and/orpurification to avoidsomeantagonist effects and, above all, to control their concentration.

5.3. Production of animal feedand DF for food

As stated earlier, DF componentshave been targeted as a poten-tial source of fermentable sugars and specific saccharides. Thereare a number of studies on the saccharification of olive stone, olivecake DF, olive seed husks and whole stones via various meth-ods (acidic hydrolysis, enzymatic hydrolysis and steam explosion)[59,76,139–141]. However, the feed and food areas appear to beinteresting fields for the valorization of native DF. Backed by well-documented studies, it is now accepted that DF plays a majorrole in many physiological processes and in the prevention of

several diseases. Diets containing substantial amounts of fibers,

8/19/2019 Nadour Et Al2013

13/21


which have a positive effect on health, and DF consumption havebeen related to a decreased incidence of several types of cancer[142]. The importance of fibers has led to the development of alarge potential market for fiber-enriched food products, ingredi-ents and gelling agents. OMWW and olive pomace contain pecticmaterials that could be transformed into a potential source of gelling material [57,58,75]. Nonetheless, their “contamination” byphenolic compounds and their high dilution in OMWW is a prob-lem for their valorization. In an effort to optimize the functionalproperties of these pectic polysaccharides, they were successfullyclarified by ultrafiltration with 25 and100 kDamembranesto elim-inate cations and phenols [143]. Fibers from several vegetableshave already been used as a fat replacement in meat products[56]. Cooking induces structural changes in meat proteins, whichresults in a reduction in their water-holding capacity, and thesefibers might improve this water-holding capacity by binding tothe released water. Galanakis et al. [60] have reported that water-soluble AIR material is able to reduce water loss and oil uptakein low-fat meatballs and, thus, could be utilized as a meatballadditive. Olive cake has also been utilized as animal feed [139].However, despite its high fiber and protein contents, it has a lownutritional value because its phenolic compounds act as inhibitorsof digestive enzymes. The impact of these phenolic compounds onthe microbiota was not described by the authors. Brozzoli et al.[144] recently investigated the potential of fungal-treated depittedolive pomace as an animal feed using selective lignin-degradingfungi and solid-state fermentation in an effort to improve its nutri-tional properties. Incubating depitted olive pomace (25%, w/w)mixed with various conventional feedstuffs, such as wheat bran,wheat middlings, barley grains, crimson clover, wheat flour shortsand field beans, with Pleurotus ostreatus and Pleurotus pulmonar-ius significantly increased its crude protein content by 7–29% andremoved 50–90% of its phenolic content after 6 weeks. Therefore,the utilization of olive by-products as a source of DF or animal feedappearstoactuallybeutopicandnotcompetitivewhenconsideringthe cost of treatments to eliminate their phenolic content.

5.4. Production of edible fungi

Edible fungi, especially Pleurotus or Lentinula species but also Agaricus bisporus, are able to grow using olive oil by-productsas a nutrient source [145–147]. Lakhtar et al. [105] selected and

cultivated a strain of Lentinula edodes on OMWW, enabling 65%decolorization and 75% dephenolization. Laccase production wasthe main lignolytic activity observed. Kalmis et al. [147] recentlysuggested cultivating the oyster mushroom Pleurotus ostreatus ona wheat straw substrate containing a mixture of tap water andOMWW (25% OMWW, v/v) as an environmentally friendly solutionfor commercial mushroom production.

5.5. Production of bioenergyand biofuels

OMWW is a candidate substrate for biohydrogen (bio-H2),biomethane and bioethanol productions because it contains sug-ars, volatile acids, polyalcohols and fats. Moreover, its low nitrogencontent makes it a good substrate for photofermentative bio-H2production because high NH4+ concentrations inhibit nitrogenasesynthesis and activity. Studies have tended to focus on biomethaneobtained through the anaerobic digestion of OMWW substrate[148,149]; however, work has also been performed in the areasof bioethanol generation [141,150,151] and bio-H2 production byphotofermentative processes [152–155] and dark fermentation[156,157]. With the actual development of bioenergy, the cost of plant biomass and fermentable organic matter will increase signif-

icantly in the near future and might compete with other industrialareas. As an example, the recent success of biofuels from first-generation biorefineries has been seriously discussed with regardsto its negative impact on plant production for food. The develop-ment of second- (the production of biofuels from cellulosic andhemicellulosic plantparts)and third-generation (the production of biofuels from microalgae) biorefineries is one of the proposed solu-tions forproducing bioenergy and valorizing no vegetable biomass.In this context, olive oil by-products are interesting substratesbecause they have no actualapplications and are an environmentalproblem. Nevertheless, the high dilution rate of liquid by-products(OMWW) from the olive oil industry is, once more, a problem.

5.5.1. Biohydrogen andbiomethane

A large number of microbial species with significant taxo-nomic and physiological differences can produce bio-H2 (seeTable 5) through single or combined metabolic pathways.Some biological processes use hydrogenases or nitrogenases ashydrogen-producing proteins. The activities of these enzymesare linked to the utilization of photosynthetic products [158].

Table 5

Physiological diversity and biohydrogen production pathways.

General metabolic reactions Microbial strains References

Single pathwaysHeterotrophicH 2 production from biomass

Dark fermentationC6H12O6 + 2H2O→2CH3COOH+2CO2 + 4H2C6H12O6 →CH3CH2COOH+CH3COOH+CO2 + H2C6H12O6 →CH3CH2CH2COOH+2CO2 + 2H2

Heterotrophic bacteria (Clostridiumsp.) [158,259]

H 2 production from CO

CO+H2O→H2 + CO2Photosynthetic bacteria [158]

Photo-heterotrophicBiophotolytic H 2 production

12H2O→12H2 + 6O2Direct biophotolysisIndirect biophotolysis

Greenmicro-algae (Chlamydomonas reinhardti) Cyanobacteria (Anabaena sp) [260][261,262]

Photoproductionof H 2 frombiomass

(Photo-decomposition of organic compounds).C6H12O6 + 6H2O→6CO2 + 12H2

Phototrophic bacteria(Rhodobacter sphaeroides)(Rhodopseudomonas palustris) [158,164,263]

Combined pathwaysStep I: Dark fermentationC6H12O6 + 2H2O→2CH3COOH+2CO2 + 4H2.

Facultative anaerobes [158,162]

Step II: Photofermentation2CH3COOH+4H2O→8H2 + 4CO2.

Photosynthetic bacteria (Rhodobacter sphaeroides)

8/19/2019 Nadour Et Al2013

14/21


Biophotolysis in microalgal or cyanobacterial cultures involvesthe light-driven decomposition of water (Table 5). Photosyntheticmicroorganisms can directly convert solar energy to bio-H2 fromorganic or inorganic substrates [158]. Bio-H2 production fromorganic substrates uses fermentative catabolic pathways, withacetate and butyrate yielding the highest amounts of hydrogen, incontrast to lactate and ethanol [159].

C6H12O6+2H2O → 2CH3COOH + 2CO2+4H2

C6H12O6→ CH3CH2CH2COOH + 2CO2+2H2

However, several studies emphasize that the any system forphotobiological H2 production using photosynthetic bacteria andolive oil by-products will clearly require enhancements due to thedark color and inhibitory effects of raw olive oil by-products. Toovercome these negative effects, experiments have trialed highdilution ratesof up to 1% (v/v) [152]. The process requires the addi-tion of nitrogen supplementation and adjustments of the OMWWpH. Nonetheless, dilution increases the volume of wastewater, andthis process has proven impracticable at a large scale unless theOMWW is diluted with other wastewaters such as cheese whey.

Anaerobic digestion can also convert organic substrates to CH4

and CO2 (biogas) through the concerted action of a mixture of microbes (consortia). The overall reaction may be written as:

Organicmatter + water → CH4+CO2+NH3

+H2S + newcells + heat

Effluents with high concentrations of fatty acids from ahydrogen-producing reactor can be fed into an anaerobic digester,producing methane as a secondary product. Thus, bio-H2 andbiomethane have been produced using both single- and two-stagefermentationsfrom olive oilby-products (Table 6a) with two-stagesystems emerging as an effective and useful solution for the treat-ment of non-diluted OMWW.

5.5.1.1. Biohydrogen production. Processes developed for bio-H2production are one- and two-stage fermentations. Fermentativebio-H2 production through the dark fermentation of water-diluted(1:4) TPOMW has been demonstrated as feasible, resulting in2.8–4.5 mmol bio-H2 per g of carbohydrates consumed [156,160].With a CSTR, one of the main factors dictating the bio-H2 yield isthe hydraulic retention time (HRT) [156,157]. HRT is the averageresidence time of a soluble compound in a constructed bioreactor.HRTs of between 27 and 33h have proven favorable in terms of thetotal acidification of the wastewater and propionate production,whereas a HRT of 14.5h favored both H2 and butyrate/acetate pro-duction [157]. It has been reported that the efficiency of hydrogenproduction per mass of carbohydrates consumed decreases withHRT [156].

Photobiological H2 production has attracted considerable atten-tion from solar energy-based biotechresearch as a potential sourceof renewable and pollution-free fuel. Bio-H2 production on diluted(1% and 20%) OMWW has been investigated using Rhodobacter sphaeroides in column photobioreactors [152]. The peak bio-H2production potential (HPP) was approximately 13.9 L/L OMWW. AcomparativestudywithseveraldilutedOMWW samples(4%)founda linear relationship between C/N molar ratios and bio-H2 produc-tion capacities. The highest HPP (20 L/L OMWW) was obtained fromthe OMWW samples with the highest organic content (predom-inantly acetic, aspartic, and glutamic acids) and the highest C/Nmolarratio [155]. Alaterstudyinvestigatedphotofermentativebio-H2 production from OMWW by a strain of Rhodobacter sphaeroidesunder iron and molybdenum supplementation [161] because both

molybdenumand iron arepart of thenitrogenase enzyme complex.

The iron-supplemented cultures on diluted OMWW (2%) yielded asignificant increase in bio-H2 production. Iron-supplemented cul-tures also yielded better wastewater treatment by achieving 48.1%degradation of theinitialCOD value compared to thecontrolreactorwith a 30.2% COD removal efficiency.

Eroglu et al. [162] reported a two-step process where darkfermentation by active sludge cultures was combined withphotofermentation by Rhodobacter sphaeroides in batch reac-tors. The highest HPP (29 L/L

OMWW) post-photofermentation was

obtained using OMWW (50%, v/v) from the dark fermentationstage as a substrate source. Bio-H2 and biomethane were alsoproduced in a two-stage process where bio-H2 was producedin stage one and methane in stage two [160]. In the first step,bio-H2-producing cultures obtained after the thermal pretreat-ment of anaerobic sludge were used for inoculation, and thereactor was fed with water-diluted (1:4) TPOMW. In the secondstep, methane production was performed under mesophilic con-ditions (35 ◦C) in a CSTR-type digester where the inocula werepre-adapted anaerobic mixed cultures fed with the effluent fromthe hydrogenogenic CSTR-type digester. TPOMW was found to beideal formesophilic and thermophilic (55◦C) methane productions[160], and it was demonstrated that thermophilic bio-H2 produc-tion was more efficient than the mesophilic process in terms of both production rate and yield. This effect was predominantlyattributed to the better performance of the hydrogenases due totheir lower affinity for hydrogen at higher temperatures [163]. Toimprove the efficiency of photobiological systems and overcomethe inhibitory effect of the dark color and antibacterial activitiesof the phenolic compounds without dilution, Eroglu et al. [153]engineered a two-stage process involving a clay pre-treatmentstepfollowedbyaphotofermentationthateffectivelydoubledphotofer-mentative bio-H2 production (31.5 L/L OMWW). More recently, aRhodopseudomonas palustris strain was successfully tested for bio-H2 production under a continuous light condition of 200 Em−2 s−1

(30 ◦C) on cylindrical (CPBR) and flat (FPBR) photobioreactorsusing diluted OMWW (25%, v/v) obtained after adsorption on bothdry- Azolla and activated carbon to remove COD and phenolic com-

pounds [164]. Two-stage fermentations have also been reportedusing microalgae and OMWW as the substrate [113,165], prom-pting Faraloni et al. [166] to propose bio-H2 photoproduction byChlamydomonas reinhardtii. The process is sustained by both pho-tosystem II-drivenwatersplittingand by thefermentation ofstoredcarbohydrates. Before microalgae cultivation, the OMWW waspretreated by “biofiltration” using Azolla caroliniana and granular-activated carbon to remove phenolic compounds. Chlamydomonasreinhardtiigrowthin diluted andpretreated OMWW thus generatesa source for bio-H2 production under sulfur-deprived conditions.

5.5.1.2. Methaneproduction. The processes developed for methaneproduction are two-stage anaerobic-aerobic pretreatments and

two-phase anaerobic digestion.Fungi have been used to pretreat olive oil by-productsprior to anaerobic digestion [167–169]. Pretreating OMWW with Aspergillusniger doubledthemethaneproductioninthesubsequentanaerobic digestion [170]. In other studies, anaerobic digestionafter pretreatment with the same strain eliminated over 60% of the COD, resulting in high methane yields [167,168]. Similarly,studies using Aspergillus terreus have demonstrated that aerobicpretreatment significantly reduces the concentration of phenoliccompounds and significantly increases methane production by upto 23% [116]. Yeasts, particularly Candida tropicalis, have also beenused under aerobic conditions to pretreat OMWW prior to anaero-bic co-digestion [171]. The combined system resulted in a reducedCOD and degraded the phenolic content of the OMWW in addition

to improving methane production.

8/19/2019 Nadour Et Al2013

15/21


Table 6a

(Part 1) Bioconversion of olive oil by-products into biohydrogen and bio-methane.

Biohydrogen and bio-methane production in a single and/or combined process

Olive mill by-products Single-stage fermentation References

Culture Microorganism Product Yield

OMWW Photofermentation Rhodobacter sphaeroides Bio-H2 16L/L [153]Diluted OMWW Photofermentation Rhodobacter sphaeroides Bio-H2 13.9L/L [152]Diluted OMWW Photofermentation Rhodobacter sphaeroides Bio-H2 0.05L/L [263]

Diluted OMWW Dark fermentation Anaerobic sludge Bio-methane 108 L/kg COD [177]OMWW and s laughterhouse w astewater Dark f ermentation Anaerobic s ludge Bio-methane 184 L/kg C OD [177]OMWW and winery residues Dark fermentation Anaerobic sludge Bio-methane 214 L/kg COD [177]

Two-stage fermentation process

Step 1 - Culture type andmicroorganism

Step 2 - Culture type andmicroorganism

Product Yield References

Diluted OMWW Anaerobic fermentation (activatedsludge) in CSTR

Aerobic culture (Pseudomonas sp)in SBR

Bio-H2 0.33L/L [157]

PHAs 0.089 g/g biomassOMWW Aerobic culture (Candida tropicalis) in

batch reactorAnaerobic co-digestion (activatedsludge) in fixed-bed reactor

Bio-methane 29 L/L [171]

Diluted TPOMW Anaerobic fermentation (activatedsludge) in CSTR

Methanogenesis in CSTR Bio-H2 0.23L/day [160]

Bio-methane L/L/dayOMWW and liquid cow manure Acidogenesis (indigenous microflora)

in CSTR

Methanogenesis in CSTR Bio-methane 0.91 L/L/day [178]

TPOMW Acidogenesis (anaerobic sludge) inCSTR

Methanogenesis in CSTR Bio-methane 261 L/kg COD [149]

OMWW Clay p retreatment Photofermentation ( Rhodobactersphaeroides)

Bio-H2 31.5L/L [153]

Diluted OMWW Filtration pretreatment Culture (Chlamydomonasreinhardtii) in PBR

Bio-H2 0.15L/L [166]

Recent studies have demonstrated that two-phase anaerobicdigestion offers an attractive alternativeto conventional one-phaseanaerobic digestion. In the first phase, complex organic materi-als, carbohydrates, proteins, lipids, amino acids and long-chainfatty acids are converted by acidogenic bacteria into intermedi-ates such as volatile fatty acids (VFA) and alcohols. In the secondphase, these metabolites are metabolized and biotransformed into

CH4 and CO2 by methanogens or archaea. Applied to TPOMW,this process provided well-stabilized effluents and a high methaneyield (0.268 L CH4/g COD eliminated) [172]. Fezzani and Ben Cheikh[173] studied two-phase anaerobic digestion for the treatment of an OMWW and olive cake mixture using two sequencing semi-continuous digesters operated at a mesophilic temperature. Thefirst stage (acidifier) was conducted at a HRT of 14 and 24 days,corresponding to organic loading rates ranging from 5.54 to 14gCOD/L per day. The second stage (methanizer) was run at a HRT of 18, 24 and 36 days, corresponding to organic loading rates rangingfrom 2.28 to 9.17 g COD/L/day. The VFA concentrations increasedwith an increasing HRT or increasing feed concentration. Methaneproductivity was doubled (32L/L OMWW) compared to a one-phasereactor [174]. This improvement in productivity can be explained

by the improved characteristics of the effluents produced by the

acidifier step. In fact, high VFA levels were easily metabolized andbioconverted into CH4 and CO2 in the methanizers after shortresidence times. Note that some research teams have trialeda chemical pretreatment before the anaerobic fermentation of the OMWW. Azbar et al. [175] demonstrated that the anaerobicbiodegradability of the OMWW associated with digester methaneproduction was significantly enhanced (80%) by a chemical pre-

treatment of acid cracking followed by coagulation–flocculationusing Al2SO4, FeSO4 and FeCl3.

Another recently studied approach is the co-digestion of OMWW with other substrates (wastes). This approach was appliedwith other waste substrates such as poultry manure, slaugh-terhouse wastewaters, winery residues and liquid cow manure,providing methane yields of up to 250.9 L/kg COD [176–178]. Amixture containing 20% OMWW and 80% liquid cow manure wasanaerobically digested using a two-stage process under mesophilicconditions with an HRT of 19 days [178], with thisprocess enablingmethane production at a steady-state rate of up to 0.91L/L/day.

5.5.2. Bioethanol productionThe high organic matter content of olive oil by-products makes

them a promising alternative resource for ethanol production by

Table 6b

(Part 2) Bioconversion of olive oil by-products into bio-ethanol.

Single-stage fermentation

Olive mill by-products Culture Microorganism Production (g/L) Yield Reference

TPOMW Simultaneous fermentation/saccharification Klyveromyces marxianus 14.5 – [264]

Two-stage fermentation process

Step 1 - Pretreatment Step 2–Culture of Microorganism Production (g/L) Yield

Olive cake Acid hydrolysis and heating Escherichia coli 8.1 0.44 g/g [141]TPOMW Enzymatic hydrolysis Saccharomyces cerevisiae 11.2 0.49 g/g [265]TPOMW Enzymatic hydrolysis Saccharomyces cerevisiae 3.2 0.42 g/g [151]Diluted OMWW Thermal processed and pretreament with Pleurotus sajor-caju Saccharomyces cerevisiae 14.2 – [150]

8/19/2019 Nadour Et Al2013

16/21


either bacteria or yeasts (Table 6b). The different types of polysac-charides in olive oil by-products can be bioconverted into ethanolvia a set of processes deployed in two separate steps. The firststep can be enzymatic hydrolysis or a physicochemical pretreat-ment for the release of reducing sugars, whereas the second stepis their bioconversion into ethanol by yeasts or bacteria. In anexperiment reported by Asli and Qatibi [141], the lignocellulosiccomponents of olive cake were pretreated with diluted sulfuricacid (160 ◦C), which was followed by precipitation and filtrationto eliminate fermentation inhibitors. The obtained hydrolysatescontained 18.1g/L of soluble sugars, which were fermented by arecombinant Escherichia coli strain to produce ethanol at a yieldof 0.45g/g of sugar. An enzymatic pre-treatment step using twocellulosic enzyme mixtures, Celluclast and Novozyme 188, com-bined with wet oxidation was applied to olive cake for ethanolproduction [151]. The glucose and xyloseconcentrations were highcompared to experiments run with only enzymes added. Ethanolwas produced after a subsequent fermentation step by Saccha-romyces cerevisiae and Thermoanaerobacter mathranii. The highestethanol production level was obtained when only an enzymaticpre-treatment step was applied. OMWW bioconverted by fungisuchas Pleurotussajor-cajualso appears to be a promisingsubstratefor bioethanol production [150], and pretreatment with Pleuro-tus sajor-caju has already been demonstrated to improve ethanolyields. Using half-diluted OMWW that was thermally processedand pretreated with Pleurotus sajor-caju, ethanol production was14.2g/L after 48h of yeast fermentation. This strain is also capableof removing phenolic compounds.

5.5.3. Biodiesel productionBiodieselis a promisingalternative to petroleumwith the added

advantagesof being non-toxic,biodegradable, and a renewable fuelsource. Researchers have recently turned to olive oil by-productexploitation and its bioconversion into biodiesel [179,180] andfound that a strain of Lipomyces starkeyi is able to proliferate inOMWW and convert it into lipids that are a suitable feedstock for

biodiesel production [179]. In contrast, Yücel et al. [180] investi-gated biodiesel production from olive cake oil using a lipase fromThermomyces lanuginosus immobilized on polyglutaraldehyde-activated olive cake powder. Under optimized conditions, themaximum biodiesel yield wasapproximately 93% (25◦C)after24h.

5.6. Products for industrial uses

OMWW and other olive by-products are potential sources of biomolecules for industry after their bioconversion.

5.6.1. Valuableproductsproduced throughbiohydrogen processes

Photobiological H2 production from OMWW by purple non-sulfur phototrophic bacteria generates valuable products such aspolyhydroxyalkanoates(PHAs) andcarotenoidpigments [152,157].Carotenoid pigments may have commercially viable uses asanti-cancer agents, food colorants and natural antioxidants or,alternatively,asasourceofprovitaminA.PHAsmayhaveimportantindustrial applications, particularly in the field of biodegradableplastics. These intracellular polyester granules are produced bysome bacteria to store carbon and energy through fatty acidmetabolism [179]. Furthermore, Ntaikou et al. [157] demonstratedthefeasibility of combined bio-H2 and biopolymerproductionfromOMWW using a two-stagesystem(Table6a). In this process, bio-H2and VFA were produced via anaerobic fermentation. The acidifiedwastewater was subsequently used as a substrate for aerobic PHAproduction using Pseudomonas putida.

5.6.2. Other biopolymers

Many teams of researchers have microbiologically treatedOMWW to produce microbial polysaccharides [180–184]. OMWWhave been proposed as a low-cost substrate forxanthan productionby Xanthomonas campestri. Culturing on diluted OMWW (30–40%)as a sole substrate source yields 4.4g/L of xanthan. Adding nitrogenand/or salts significantly increases xanthan yields, up to a maxi-mum of 7.7 g/L [185]. Different strains of Paenibacillus jamilaehavealso been demonstrated to grow and produce exopolysaccharidesusing OMWW as sole nutrient and energy sources, thus loweringthe toxicity of the waste. The highest yield of exopolysaccharideproduction was 5.1 g/L with 80%OMWW[182,183]. When TPOMW(aqueous extract of 20%, w/v) was used as substrate, the max-imal exopolysaccharide yield obtained was 2 g/L. Increasing theTPOMW concentration inhibited growth and exopolysaccharideproduction [186]. These Paenibacillus jamilae exopolysaccharideshave foundapplications as biofilters for heavymetal-contaminatedwaters [187]. Finally, it has beendemonstrated thatstrains of Pseu-domonas sp. can grow on OMWW as a sole carbon source and canaccumulate rhamnolipids [188].

5.6.3. EnzymeproductionIndustrial enzymes can be produced by yeasts and filamen-

tous fungi using olive oil by-products as substrate [183]. Themain enzymes obtained through the fungal treatment of oliveby-products are lipases, laccases, Mn-dependent peroxidases, andpectinases. Laccases and Mn-dependent peroxidases are producedfrom OMWW by Panus tigrinus and pectinases by Cryptococcusalbidus var . albidus [190], whereas lipases have been obtainedfrom Candida cylindraceaand Yarrowia lipolyticastrains [191–194].Microbial lipases have found applications in the dairy, pharmaceu-tical, detergent and other industries [195]. Olive oil cake has alsobeen successfully utilized for lipase production using thermostablefungal cultures of Rhizomucorpusillus and Rhizopus rhizopodiformis[196].

5.7. Other uses

OMWW has traditionally been used to make soaps such asMarseille soap. However, other authors have investigated non-biological applications for olive by-products, notably to valorizetheir high capacity of adsorption. In this manner, Meksi et al.[197] reported another approach for OMWW valorization as apossible resource for dyeing textile materials such as wool, andAkar et al. [198] described a high biosorption yield for olive cakefor removing RR198 dye from real-world wastewater. Activatedcarbon from olive stone is another potential application for remov-ing dyes [199], odors, tastes, and even contaminants such asarsenic [200] or aluminum [201] f or water purification and otherdecontamination processes [202,203]. Other studies have investi-gated the use of olive stone-based activated carbon as a biosorbent

of heavy metal ions [204–207]. More recently, Martin-Lara et al.[208] reported that acid-treated olive stone was a good biosorb-ent for lead removal from wastewater. Berrios et al. [209] testedolive stone-based activated carbon for the removal of methyleneblue from wastewater, and found that it also offers a viable low-cost alternative for removing organic compounds. However, themicropore structure of olive stone-based activated carbon meansit is only suitable for removing molecules smaller than methy-lene blue. Cosmetology could also be an interesting field for theapplication of olive by-products in the future. Olive stone hasrecently been effectively incorporated into cosmetics formulationsdue to its exfoliating properties(http://www.cosmoliva.com/).Thelast non-biological application for olive by-products described inthe literature is in chemistry as polyols, an attractive approach

for polyurethane production. A study recently reported the

http://www.cosmoliva.com/http://www.cosmoliva.com/

8/19/2019 Nadour Et Al2013

17/21

http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0205http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0200http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0195http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0190http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0185http://refhub.elsevier.com/S1359-5113(13)00391-7/sbref0185http://refhub.

nadour et al2013

Documents