Vol.:(0123456789)1 3

Chemistry Africa (2020) 3:189–197 https://doi.org/10.1007/s42250-019-00108-4

ORIGINAL ARTICLE

Shelf Life Prediction and Storage Stability of Deglet Nour Dates (Phoenix dactylifera L.): Microbiological and Organoleptic Properties

Mohamed Nabily1,5 · Abdelkader Nabili2 · Ahmed Namsi4 · Hatem Majdoub3  · Atf Azzouna5

Received: 1 July 2019 / Accepted: 20 November 2019 / Published online: 26 November 2019 © The Tunisian Chemical Society and Springer Nature Switzerland AG 2019

AbstractDeglet Nour dates (Phoenix dactylifera L.) were sorted out according to the UNECE DDP-08 standard concerning the market-ing and commercial quality control of dates. Desorption isotherms at 5, 30 and 40 °C and the Guggenheim–Anderson–deBoer (GAB) sorption model were used to estimate proper storage conditions and to predict shelf life. Physicochemical, organoleptic and microbiological analyses were performed initially on fresh and stored dates (18% db) packaged with polyethylene film at 5 °C and 85% relative humidity (RH). The moisture content affording the longest time period with minimum quality loss was predicted to be 17.44% on dry matter basis (db). The total heat of desorption needed for drying fruits was calculated to be 16.48 Kcal mol−1. Fruit was microbiologically stable after 154 days storage period. The sensorial analysis showed that the color, the odor and the taste did not change significantly during storage. Critical moisture content was predicted to be 26.5% (db). This value is smaller than what the UNECE DDP-08 codex, allows for commercial whole dates (30% db).

Keywords Deglet Nour dates · Isotherms · Shelf life prediction · Heat of desorption · Peer community · Organoleptic properties

1 Introduction

Date fruits were produced in hot arid regions of the world and marketed worldwide as a high value confectionery. Mid-dle East and North African countries furnish consumable date varieties which are highly appreciated by local people and traded to Europe according to the UNECE DDP–08 standard [1] concerning the marketing and commercial qual-ity control of dates. Conformity with relevant standards is an important requirement for date fruits trade. Nevertheless,

organoleptic quality and microbiological characteristics can be used to evaluate quality loss processes during storage and marketing.

The sensitivity of dates to alteration and the poor conser-vation at production sites poses big problems for contractors. Assuming the insect spoilage is under control, microbial attack and browning are most degrading factors, which can be counteracted. For that, many researchers tried to look for methods of postharvest conservation for high quality fruits, such as Deglet Nour dates. The moisture sorption properties of foods have been shown to be influenced by food composi-tion, treatment process, temperature, and relative humidity [2].Water activity (aw), based on the thermodynamic con-cept of water chemical potential, is an important factor to understand the control of the deterioration of reduced mois-ture and dry foods, drugs and biological systems as well as the total moisture content since it is related to physical and physicochemical modifications, microbiological stability, and both aqueous and lipid phase chemical reactions [3]. The relationship between water content and water activity at a given temperature is shown by the water vapour sorption isotherms which still needs to be determined experimentally. In general, the measurement requires bringing the material

* Hatem Majdoub hatemmajdoub.fsm@gmail.com

1 Institut Supérieur de l’Education et de la Formation Continue, Université Virtuelle de Tunis, Tunis, Tunisia

2 Faculty of Sciences of Gafsa, Research Unit of Materials, Environment and Energy, Gafsa University, Gafsa, Tunisia

3 Faculty of Sciences of Monastir, Laboratory of Interfaces and Advanced Materials, Monastir University, Monastir, Tunisia

4 Regional Research Center in Oasis Agriculture, Degache, Tunisia

5 Faculté des Sciences, Université de Tunis El Manar, Tunis, Tunisia

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to an equilibrium state corresponding to a point on sorption curve and measuring water content when aw is controlled.

Numerous models for predicting the relationship between equilibrium moisture content (aw) and temperature have been developed. Each of these models has relative success in reproducing equilibrium moisture content, depending on the water activity range or the type of foodstuff. The Gugen-heim–Anderson–de Boer (GAB) model has been applied most successfully to various foods [4, 5]. Adequately repre-senting the experimental data in the range of water activity of most practical interest in foods, the GAB model has been recommended as the fundamental equation for the charac-terisation of water sorption of food materials [6, 7].

In this work, water vapour desorption behaviour of Deglet Nour dates was studied. The specific objectives are to inves-tigate the suitability of the Guggenheim–Anderson–deBoer (GAB) equation in describing water sorption isotherms of Deglet dates, to predict and test proper storage conditions using microbial load, pH, colour, flavor, and texture as qual-ity parameters and to discuss the maximum water content tolerated (30% db) by the standard UNECE DDP–08.

2 Materials and Methods

2.1 Fruit Material

Date palm dates (Phoenix dactylifera L.) were obtained from the Regional Centre for Research in Oasis Agriculture (Degache, Tunisia). Fruits of the cultivar Deglet Nour were sorted out according to the UNECE DDP-08 standard. To avoid cells and tissues damage till physicochemical analyses, date fruits were rapidly cooled using air blast to − 40 °C according to the individual quick freezing (IQF) method. Then ultra-cooled sample was subsequent stored at − 18 °C. Typical morphological image for the experimental Deglet Nour dates is shown in Fig. 1.

2.2 Chemical Composition of Fresh Dates

The moisture content was measured gravimetrically by dry-ing 2 g of flesh in an air oven at 70 °C until constant weight was reached; these conditions were used to prevent the cara-melization reaction. Total ash was determined by igniting the sample at 550 °C for 5 h [8] in a muffle furnace Carbolite ELF 11/14 (Carbolite Limited, Hope Valley, UK). Fat was extracted by diethyl ether using a Soxhlet apparatus [9] then the solvent was removed with a rotary evaporator IKA RVO5 Basic(IKA Laboratories, Staufen, DE). For gravimetric measurements a cause and effect diagram was established, showing how the sources of uncertainty are related to each other and indicating their influence on the combined stand-ard uncertainty (uc) of the result. Uncertainty of gravimetric

measurement was quantified with linearity (0.15 mg) of the used Mettler Toledo balance (Mettler-Toledo, Greifensee, CH) and repeatability of measurements (Standard deviation of triplicate weighting). The Expanded uncertainty (U) for each measurement was estimated using a coverage factor type (k = 2) with 95% confidence [10].

Total nitrogen was determined in triplicate by the Kjel-dahl method using Behr equipment for foodstuffs analysis (Behr Labor Technik, Düsseldorf, DE) followed by the protein calculation using the general factor 6.25 [11]. The material is digested in H2SO4 to convert the protein nitrogen to (NH4)2SO4 at a boiling point elevated by the addition of K2SO4 with a copper catalyst to enhance the reaction rate. Ammonia is liberated by alkaline steam distillation and quantified titrimetrically with standardized acid [12]. The combined standard uncertainty (Uc) was calculated from the relative uncertainty of the acid concentration, uncertainty of titrated volumes, uncertainty of weighing samples and repeatability contribution [13]. The expanded uncertainty (U) is obtained by multiplying the combined standard uncer-tainty by the coverage factor k = 2.

Sugar content was determined using a Milton, Roy 401 spectrophotometer and an enzymatic Kit Sucrose/d-Glucose/d-Fructose (R-Biopharm, Saint Didier au Mont d’Or, FR). Sucrose is hydrolyzed by the enzyme β-fructosidase to d-glucose and d-fructose. The d-glucose concentration is determined before and after the enzymatic hydrolysis of sucrose. d-Glucose and d-fructose are phos-phorylated to d-glucose-6-phosphate (G-6-P) and d-fruc-tose-6-phosphate (F-6-P) by the enzyme hexokinase (HK) and adenosine-5′-triphosphate (ATP). F-6-P is converted to G-6-P by phosphoglucose isomerase (PGI). In the pres-ence of the enzyme glucose-6-phosphate dehydrogenase

Fig. 1 Image of Tunisian Deglet Nour dates (Phoenix dactylifera L.)

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(G6PDH), G-6-P is oxidized by nicotinamide adenine dinu-cleotide phosphate (NADP) to d-gluconate-6-phosphate with the formation of reduced nicotinamide adenine dinucleotide phosphate (NADPH). The amount of NADPH formed in this reaction is stoichiometric to the amount of d-glucose. NADPH is measured by the increase of its light absorbance at 340 nm. According to the manufacturer’s technical docu-mentation, the working temperature may be between 20 and 37 °C and sensitivity is 4 mg L−1. Under the experimental conditions described above, an accuracy of 0.04 to 0.08% is expected for the d-glucose and d-fructose. For sucrose accuracy varies between 0.15 and 0.25%.

2.3 Desorption Modeling and Shelf Life Prediction

Desorption experiments were performed on fresh dates. Samples were autoclaved at 90 °C for 30 min then cut into small pieces (about 5 mm long) in order to accelerate the desorption process. Desorption isotherms were determined at nine aw between 0.11 and 0.92 each at temperatures 5, 30 and 40 °C. Water activities were obtained using air tight glass jars containing saturated solution of LiCl, FK, MgCl2, Mg(NO3)2, NaBr, KI, NaCl, KCl, and BaCl2 [14]. Each jar had the capacity for three samples, which were located over a tripod. At high aw (> 0.7), crystalline thymol was placed in the jar to prevent microbial spoilage of the samples. Experi-ments were conducted in a temperature controlled cabinet ED 53 (Binder, Tuttlingen, DE). Samples were kept at exper-imental temperatures for 21 days recommended for thermal equilibrium. The dry mass of the sample was determined at 70 °C for 48 h in a vacuum oven using an analytical balance (Mettler-Toledo, Greifensee, CH). All used chemicals were of analytical grade.

The theoretical Guggenheim–Anderson–Boer (GAB) (Eq. 1) considers that sorped water molecules lay on each other in layers.

The equation was regressed using the indirect nonlinear regression method. Three constants of the GAB equation, the monolayer moisture content (mo) on dry matter basis (db), C and K were first estimated at each temperature. The heat related to the monolayer (qc), the heat related to the multilayer (qk), Co and Ko through Eqs. (2) and (3). The experimental data were fitted using the Software Origin

(1)m =m0CKaw

(

1 - Kaw)(

1 - Kaw + CKaw)

(2)C = C0exp( qc

RT

)

(3)K = K0exp( qk

RT

)

7.0 (Origin Lab Corporation, Northampton, MA, USA). The suitability of the sorption model was based on the mean relative error P (%) and the correlation coefficient squared R2.

The expression for the steady state permeation of a gas or vapour through a thermoplastic material is presented in the following equation (Eq. 4).

where P/X is the permeance (the permeability constant P divided by the thickness of the packaging film X); A is the surface area of the package; p1 and p2 are the partial pres-sures of water vapour outside and inside the package; w/t is the rate of gas or vapour transport across the packaging film.

The prediction of moisture transfer either to or from a packaged food requires the assumptions that P/X is constant and the external environment is at constant tem-perature and humidity. A further assumption is that the moisture gradient inside the package is negligible. This is the case whenever P/X is less than about 10 gm−2 day−1.

In the multilayer sorption region, the isotherm is treated as a linear function. In this case:

The moisture content (m) can be substituted for water gain using the relationship (Eq. 6):

where w is the weight of water transported and ws is the weight of dry solids enclosed. By substitution

Then on integrating the time of packaging is expressed by

where me is the equilibrium moisture content of the food (db) if exposed to external package RH; mi is the initial moisture content of the food (db); mt is the moisture content of the food at time t (days); A is the surface area of the pack-age (m2); b is the slope of the linear desorption isotherm; po is water vapour pressure of pure water at the storage tem-perature (Pa); ws is the weight of packaged fruits (g); P/X is the permeance of polyethylene film (gm−2 day−1). The shelf life is predicted when the moisture content of the food (m) reach the critical moisture content (mc) [15].

(4)�w

�t=P

XA(

p1 − p2)

(5)m = b aw + c

(6)m =w

ws

(7)�w

�t=�mws

�t=P

XA(p0me

b−p0mt

b

)

(8)t =

Ln(

me - mi

me - mt

)

Ap0

(

P

X

)

bws

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2.4 Evaluation of Storage Stability

This section comprises two complementary stages: The first step consists in the scientific analysis of stored dates which was evaluated by colorimetric analysis, pH measure-ment and microorganisms load. The second part consists in sensory tests in order to validate the analyses carried out previously. Tasting is a socio-scientific approach based on both scientists and extended peer community [16]. The taste is an affective appreciation of the organoleptic properties during the ingestion of a substance [17]. In fact, tasting is governed by certain rules and principles but its subjective side remains predominant. The biological determinants of taste result in interindividual differences in sensitivity and perceptual flavour thresholds (sweet, salty, bitter, acidic and delicious). This difference is due to an equipment of genet-ically-determined chemoreceptors [18]. The non-biological determinants of taste are summed up in learning and sociali-zation [19].

Colour measurement, on the external surface of the sam-ples, was performed using a Minolta CR-300 Chromameter (Konica Minolta, Osaka, Japan) calibrated against standard white ceramic surface. The L*C*H*colour space was used to express dates colour before and after storage [20]. According to the manufacturer random error, due to the reliability of the chromameter, is generally equal to ± 0.05. In practice, two measures give an idea about this error which corresponds to the width of the Gaussian distribution of results.

The pH was measured using a digital pH-meter Cyber-scan 510 (Eutech Instruments, Ayer Rajah Crescent, Sin-gapore) equipped with temperature control probe and cali-brated with the help of two buffer solutions with 4.00 and 7.00 pH values. The pH values measured by commercial pH meters show an uncertainty of 0.01.

The enumeration of yeasts and moulds was performed from three dilutions using a Lab Blender 400 stomacher (Seward Medical, London, UK) which allows to achieve adequate grinding and good homogenization of the sam-ples. The culture medium used was Potato Dextrose Agar MO96 (Himedia, Paris, FR) acidified by tartaric acid to 10% leading to pH 3.5. Aerobic mesophilic bacteria were counted on Plate Count Agar (PCA Oxoid, CM 0325) dishes using the pour plate method [21]. The plates were incubated at 30 °C for 2 days. Flesh samples (10 g) were homogenized with 90 mL of sterile peptone water (Oxoid, CM0009) in the Lab Blender 400 stomacher (Seward Medical, UK) for 45 s and aliquots plated out as tenfold dilutions in peptone water. All microbial counts were reported as colony forming units per g of sample (cfu g−1). In microbiological testing the greatest sources of uncertainty arise from sampling and the non-homogeneous distribution of microorganisms in the sample. The combined standard uncertainty (uc) was based on repeatability and reproducibility data. Repeatability of

enumerations was estimated by the standard deviation of 3 experiments each time. Intra-laboratory standard devia-tion of reproducibility (SR) was 1.41 cfu g−1 [22]. Expanded Uncertainty (U) was estimated using a coverage factor type (k = 2) with 95% confidence.

The assessment of texture, taste and odor were conducted using the triangle test where three randomized unknown samples are tested [23]. In this discriminative approach, three dates was used, two of which are from the stored sam-ple and the other one is a fresh fruit conform to the require-ments of the UNECE DDP-08 codex. Twenty-four semi-trained panelists are requested to pick out the fresh date. A significant difference is considered if at least 13 panelists make a correct judgment.

3 Results and Discussion

3.1 Chemical Composition of Fresh Deglet Nour Dates

Table 1 shows the chemical composition of Fresh Deglet Nour dates on wet matter basis (wb). Deglet Nour dates contained 23.0 ± 0.1% moisture, 34.6 ± 0.3% sucrose, 10.8 ± 0.1% fructose, 11.4 ± 0.1% glucose, 4.56 ± 0.01% crude protein, 0.14 ± 0.04% crude fat and 3.82 ± 0.02% ash.

These results are similar to values reported by other researchers on date varieties [24–26]. Total sugars/mois-ture content ratio (2.38) indicated that mature Deglet Nour dates are semi-dry fruit. The chemical composition of date fruits depends on cultivar, soil conditions, and agronomic practices, as well as the ripening stage.

3.2 Regression of the GAB Equation

GAB model parameters with regression coefficients for Deg-let Nour dates at 5, 30 and 40 °C were presented in Table 2. The regression gave values for R2 very close to 1 and values for P less than 10%. So the model may be considered sat-isfactory for fitting the water vapour desorption of Deglet Nour dates.

Table 1 Chemical composition of date flesh on wet matter basis (%wb)

Component Content (%wb)

Sucrose 34.6 ± 0.3Glucose 11.4 ± 0.1Fructose 10.8 ± 0.1Moisture 23.0 ± 0.1Crude protein 4.56 ± 0.01Crude fats 0.14 ± 0.04Ash content 3.82 ± 0.02

193Chemistry Africa (2020) 3:189–197

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However, further investigation into the estimated param-eters and constants would help to reach more confident con-clusion on the applicability of the GAB equation for the water vapour desorption. Through Eqs. (2) and (3), the con-stants Co and Ko were estimated to be 2.32 and 6.24, respec-tively. Values of qc and qk were 1.38 and − 1.16 Kcal mol−1, respectively. At mo each polar group had adsorbed one water molecule, so the reaction rates in aqueous phase are mini-mal [27]. The parameter K is in keeping with sorption on multilayer above the first layer and the heat of vaporization of water. The sign of qk indicate that values of K are smaller than 1 as dictated by the GAB equation [28]. The magnitude of qc is in respect to other foodstuffs.

3.3 Desorption Isotherms

Desorption isotherms of Deglet Nour dates at 5, 30 and 40 °C were presented in Figs. 2, 3 and 4, respectively. All

isotherms were type II according to BET classification [29]. These sigmoid-shaped curves are typical for bio-logical materials with high sugar content [30]. The first bent part of a curve is related to structural bound water on polymers such as pectin, cellulose and proteins [31]. When water activity is increased beyond 0.55 the water content is dramatically raised. After salvation of available surrounding water, crystallized sugar turns into amorphous state more hygroscopic [32]. The small molecular weight of glucose and fructose increases the specific area and water affinity at molecular level with increasing tempera-ture (Fig. 5).

Table 2 GAB model parameters with regression coefficients for Deg-let Nour dates at 5, 30 and 40 °C

Temperature (°C) 5 30 40

GAB parameters mo (% db) 17.44 11.74 9.95 C 28.71 23.47 21.62 K 0.75 0.90 0.95

Regression coefficients P (%) 2.01 5.71 6.33 R2 0.996 0.976 0.971

Fig. 2 Desorption isotherms of Deglet Nour dates at 5 °C

Fig. 3 Desorption isotherms of Deglet Nour dates at 30 °C

Fig. 4 Desorption isotherms of Deglet Nour dates at 40 °C

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3.4 Heat of Desorption

The survey of the aw variation with the temperature at stationary moisture content gives access to isosteric heat of sorption (Qs). Clausius–Clapeyron equation had been widely used for calculating Qs of food and agricultural materials during water vapour sorption process [33].

This thermodynamic parameter is not in fact the meas-ure of the entirety of the energizing transition that comes with water vapour desorption. The total heat (Q) consid-ered for the energizing balances (Eq. 10) includes the latent heat of vaporization of water hV (10.5 Kcal mol−1 at 25 °C).

The variation of total heat of desorption with moisture content for Deglet Nour dates was provided in Fig. 6. At 39.8% db water content, Q was equal to the enthalpy of vaporization of pure water (10.5 Kcal mol−1). For water contents less than 20% db, Q increases abruptly indicating the presence of water strongly bonded to the constituents of the fruit. The hydrogen bonds between the water molecules of the monolayer also contribute to increased water binding energy to the adsorbent matrix [34].

Using Clausius–Clapeyron equation, the total heat of desorption needed for drying Deglet Nour dates to the monolayer moisture content, was calculated to be 16.48

(9)d(

Lnaw)

d(

1

T

) = −Qs

R

(10)Q = Qs + hV

Kcal mol−1. For most food, Q varies between 12.44 and 20.57 Kcal mol−1 when m ≤ mo [35].

3.5 Shelf Life Prediction

In the multilayer desorption region, the isotherm is treated as a linear function. At 5 °C this part of the curve is ranged between 0.20 and 0.55 aw at 5 °C (Fig. 7).

The tangent to the graph m = 0.3aw + 0.1 allowed pre-dicting 26.5% db as critical moisture content (mc) at 5 °C for Deglet Nour dates. This value is smaller than what the

Fig. 5 . Fig. 6 Total heat of desorption as function of moisture content

Fig. 7 Linear isotherm and GAB model at 5 °C

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UNECE DDP-08 standard allows for commercial whole dates (30% db).

In our study, we packaged 500 g of Deglet Nour dates with 18% db water content in 0.06 m2 polyethylene film (2  gm−2  day−1) at 5  °C and 85% RH. The drying step was conducted at 65 °C using a CT-C hot air drying oven equipped with computer control system (Jiuran Instrument Equipment, Shanghai, China). Under these conditions fruits can, theoretically, keep their organoleptic characteristics in a safe state for 154 days (Eq. 8).

3.6 Characterization of Stored Dates

The moisture content at the end of the storage period was found to be 28.3% db (22% wb). A weight loss of 1% of fresh dates (23% wb) would not be significant in the Market value, if there is a conservation of organoleptic properties.

Colour, sensorial analysis, pH and microorganisms load, were used to describe the quality loss of Deglet Nour dates after 154 days of storage under predicted conditions.

The colour difference between fresh and stored Deglet Nour dates was based on chromatic coordinates L*C*H* presented in Table 3. The total colour difference between fresh and stored dates was estimated using the overall col-our difference ΔE94 (Eq. 11) recommended by the CIE for industrial use.

were KL = KC = KH = 1, SL = 1, SC = 1 + 0.045C*fs,

SH = 1 + 0.015 C*fs and C∗

fs=

√

(

C∗

fresh dates

)(

C∗

stored dates

)

. The k coefficient accounts for the parametric effect, while S accounts for CIELAB lack of uniformity. From Eqs. (9) and (10) ΔE94 was calculated to be 2.00. A colour difference of 1 is said to be the smallest colour difference that is visibly perceptible [36]. In practice a ΔE in range of 2–3 is percep-tible only through close observation [37]. This result can be explained in part by the low protein content (0.14 ± 0.01%) which actually decelerate the Maillard reaction and increase colour stability during storage.

Fresh and stored Deglet Nour dates have pH values of 6.87 ± 0.01 and 6.70 ± 0.01, respectively so there is no significant effect of storage period on pH. The decrease

(11)ΔE94=

√

(

ΔL∗

KLSL

)2

+

(

ΔC∗

KCSC

)2

+

(

ΔH∗

KHSH

)2

in pH values in biological system is generally due to the production of acetic acid and lactic acid.

In the triangle test for texture, the fresh sample was positively identified 10 times. For taste and odor, 8 and 10 panelists correctly identify the fresh sample, respectively. A minimum of 13 correct judgments indicates a significant difference at the 95% confidence level. It is thus concluded that the sensory evaluation panel could not distinguish a difference in texture, and flavor between fresh and stored Deglet Nour dates at the 5% level of significance.

Results presented in Table  4 show microbial loads of fresh and stored date fruits from the studied cultivar. Analysis indicated that moulds and yeasts and meso-philic aerobic bacteria loads were 574.2 ± 3.6 cfu g−1 and 2018.0 ± 3.6 cfu g−1, respectively. These microbial num-bers were within the acceptable limits of dates palm fruits stability and safety [38]. Moulds and yeasts loads were reported to be 562 cfu g−1 for fresh Deglet Nour dates [39]. For Sayer date fruits, the number of mold and yeast colonies after 154 days reached 68 cfu g−1 compared to 395 cfu g−1 in the controls under a modified atmosphere packaging containing 85% CO2, 3% O2 and 12% N2 at 30 °C [40].

After 154  days of storage, the yeasts and moulds counted in Deglet Nour dates increased from 574.2 ± 3.6 to 1135.0 ± 3.4 cfu g−1 and the total mesophilic aerobic bacteria count from 2018.0 ± 3.6 to 4050.8 ± 3.6 cfu g−1. These slight increases of microbial counts were insignifi-cant in terms of organoleptic stability and safety of studied fruits. Drying Deglet Nour dates to 18% db greatly reduce molds, yeasts and mesophilic aerobic bacteria growth. It has demonstrated that an increase in microbial populations has a direct impact on shelf life. Storing Deglet Nour dates after a drying step to monolayer moisture content may pre-serve their organoleptic and sensory characteristics during a long period of storage.

Table 3 Chromatic characteristics Deglet Nour dates before and after storage at 5 °C and 85% RH during 154 days

Chromatic coordinates L* C* H*(°) ΔL* ΔC* ΔH*(°)

Fresh dates 37.20 17.38 55.4 1.39 0.64 2.0Stored dates 36.06 16.74 57.4Expanded uncertainty (U) 0.06 0.06 0.2 0.08 0.08 0.2

Table 4 Microorganisms loads of fresh and stored Deglet Nour dates (154 days)

Sample Yeast and molds (cfu g−1)

Mesophilic aerobic bacteria (cfu g−1)

Fresh Deglet Nour dates 574.2 ± 3.6 2018.0 ± 3.6Stored Deglet Nour dates 1135.0 ± 3.4 4050.8 ± 3.6

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4 Conclusion

Water vapour desorption of Deglet Nour dates was studied at 5, 30 and 40 °C. The GAB model showed a good fit with experimental data at all studied temperatures. This model can be used to predict shelf life of semi-dry fruits. The critical moisture content was predicted to be 26.5% db marking the end of the storage period. This value is less than what UNECE DDP-08 allow for whole Deglet Nour dates. To achieve maximum shelf life, a drying step to the monolayer moisture content should be applied. Packaging dried Deglet Nour Dates (18% db) with a polyethylene film at 5 °C and 85% RH may protect fruits very power-fully against microbial spoilage and avoid loss of intrinsic product quality during 154 days with only 1% weight loss compared to fresh dates.

Acknowledgements The authors would like to acknowledge the Regional Research Center in Oasis Agriculture in Degache Tuni-sia (CRRAO (for the technical supports, which made this research possible.

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