the extra-virgin olive oil handbook (peri/the extra-virgin olive oil handbook) || extra-virgin olive...

15Extra-virgin olive oil storageand handlingClaudio PeriUniversity of Milan, Milan, Italy

Abstract

Storage is critical for maintaining the quality of extra-virgin olive oil. Spoilageof oil should be minimized by carefully avoiding temperature abuse, exposure toair (oxygen), exposure to light, the presence of water and organic residues in theoil (cloudiness and deposit), lack of hygiene in the oil environment, exposure to acontaminated atmosphere, and mechanical stress during transfer, pumping or trans-portation. Time-temperature relationships and inert-gas blanketing are thoroughlydiscussed. Annex 15.1 is devoted to pumps, tanks and piping assembly for extra-virgin olive oil transfer circuits.

15.1 Introduction

Storage is critical for maintaining the quality of extra-virgin olive oil. The mainreason for the frequent spoilage of olive oil during storage, transportation anddistribution, as well as during its storage and use at home, is the underestimation ofthe perishability of extra-virgin olive oil on the part of consumers and experts alike.The fact that microbial degradation is rare in olive oil has contributed to spreadingthe erroneous belief that olive oil is a stable product and that it can be submitted tomechanical or thermal abuse without penalty. In restaurants, olive oils are normallyserved in bottles where the oil remains for days or weeks at the wrong temperatureand in contact with the air. In these conditions, finding sensory defects happensfrequently and the most common fate of an extra-virgin olive oil in a restaurant isthe loss of its most interesting sensory notes and health-promoting properties.

The Extra-Virgin Olive Oil Handbook, First Edition. Edited by Claudio Peri.© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

166 CH15 EXTRA-VIRGIN OLIVE OIL STORAGE AND HANDLING

If an excellent quality oil obtained at the producer’s factory is to reach theconsumer’s table, a revolution in the mentality of olive oil traders and users musttake place. They have to start considering that the most interesting characteristicsand properties of an excellent olive oil are highly vulnerable. Losing them results inchanging an excellent extra-virgin oil to an ordinary extra-virgin oil or, sometimes,to a virgin olive oil with detectable sensory defects. This point is thoroughlydiscussed in Annex 18.1 of Chapter 18.

The second reason for degradation is the underestimation of the role of time.While the production process from olives to oil is a matter of hours or of a veryfew days, storage of the oil goes on for months or even years. Time is one of thevariables determining the extent of spoilage, while temperature determines the rateof the spoiling reactions. A small increase in temperature has negligible effects ifit occurs over a short period of time (hours or days), but it can have catastrophiceffects on quality if it continues for weeks or months.

The third reason for oil degradation is the underestimation of some of the spoilingconditions, such as the presence of excess water and solid residues in unfiltered oils,or contamination and poor hygiene of the storage environment.

All things considered, the level of quality of an extra-virgin oil reaching the con-sumer’s table depends most of all on the storage and handling conditions during dis-tribution, transportation, and selling, storing and using at home or at the restaurant.

The consequences of improper storage conditions are summarized in Table 15.1.The main risks factors can be listed as follows:

• temperature abuse;

• exposure to air (oxygen);

• exposure to light;

• presence of water and organic residues in the oil (cloudiness and deposit);

• lack of hygiene in the oil environment and exposure to contaminated atmo-sphere;

• mechanical stress during transfer, pumping, transportation.

It must be noted that the above risk factors have synergistic effects in the sense thatspoiling reactions are greatly accelerated by the simultaneous exposure to severalrisk factors.

15.2 Prevention of temperature abuse

Temperature abuse is the single most frequent factor of spoilage and therefore it issuggested that compliance with the correct temperature conditions be documented.

Temperature should be maintained within a suitable range without interruptionfrom production to consumption. Low and high temperatures should be avoided.However, the effects of low or high temperature are very different. At temperatures

15.2 PREVENTION OF TEMPERATURE ABUSE 167

Table 15.1 Spoilage phenomena and risk factors in extra-virgin olive oil storage and handling.

Spoilage mechanisms in extra-virginolive oil storage and handling

Risk factors of oil spoilage

Oxidative degradation: loss or reduction ofsensory quality and of antioxidant potential;sensory defects

Oxygen concentration, temperature, daylight(UV rays), mechanical stress

Enzymatic degradation: sensory defects Water and solid residues, temperature,mechanical stress

Contamination by volatiles: toxic substances,sensory defects

Atmospheric contamination by volatilecompounds (solvents, lipophiliccomponents of smoke, PAHs)

Chemical contamination: toxic substances,sensory defects

Grease from pump leaks, plasticizers fromplastic materials, lipophilic components ofsmoke, pesticides from aerosols or in-housetreatments

Particulate contamination: nonconformity tohygienic standards; negative impact onconsumer perception

From the environment, people, insects androdents, fine smoke particles, dust

lower than 12 ∘C (53.6 ∘F), crystallization of triglycerides causes the oil to solidifywith consequent difficulties in handling and pouring, but other spoiling reactionsare inhibited in these conditions. On the contrary, temperatures higher than the opti-mal range of 14–16 ∘C (57–61 ∘F) accelerate degrading reactions with more seriousconsequences to the oil quality.

In general, an exponential relationship exists between the time and temperatureof any chemical transformation. Hence, a very common way to represent time/temperature relationships of technological operations is in a semilog plot, with tem-perature on the abscissa in a linear scale and time on the ordinate in logarithmic scale.

The time–temperature relationship that allows the best conditions for oil storageis shown in Figure 15.1 (Peri 2013). Temperatures on the abscissa are between 10and 25 ∘C (50 to 77 ∘F), while time is on the ordinate, in a log scale, between 1 and1000 days. The two oblique lines divide the diagram into two areas: the area belowthe oblique lines is compatible with good storage of excellent oil and the area aboveis incompatible. The space between the two oblique lines is an area of variabilitydepending on the chemical stability of the oil and its antioxidant content.

Temperatures higher than 25 ∘C (77 ∘F) are not considered in the graph becausethey are incompatible with the storage of excellent olive oil. The shaded area below12 ∘C (53.6 ∘F) should also be avoided because crystallization and solidification oftriglycerides start at this temperature.

This graph should be used as a reference for planning and controlling the stor-age temperature. For instance, an excellent oil should not be allowed to remain at24–25 ∘C (76 to 77 ∘F) for more than 2–3 days. It can also be seen that if the oil isstored for 2 months before consumption, the temperature should not be higher than18 ∘C (64 ∘F). It is also evident that at 14–15 ∘C (57 to 59 ∘F) an excellent oil canbe safely stored for 1–2 years.

The temperatures indicated in Figure 15.1 refer to the oil. Often, the temperatureof the storage room is measured and controlled by an automatic air-conditioning


1000

100

1010 15

Suitableconditions

Unsuitableconditions

Standardacceptableconditions

20 25

Tim

e, d

ays

Temperature, °C

(2 years)

(1 year)

Figure 15.1 The time-temperature relationship of oil storage (Source: Peri, C. (2013). Repro-duced with permission from Wiley).

system. The right temperature of the storage room generally results in the righttemperature of the oil, but it may not be so if the oil is transferred or decanted orfiltered or mixed. In these cases, there may be a difference between the temperatureof the oil and the temperature of the storage room, which must be taken intoconsideration for the best control and management of the storage conditions.Continuous recording of the temperature of the storage rooms and periodicalmeasurement of the temperature of the oil is the most appropriate control procedurein handling excellent extra-virgin olive oil.

Control of temperature is easier if the oil is stored in bulk than in bottles. In fact,bottles have a much higher surface-to-mass ratio than tanks and contain much lowerquantities of oil. They therefore have a much lower thermal inertia. Consequently,changes in temperature are much faster in bottles than in large containers. This pointis discussed further in Chapter 16.

15.3 Prevention of exposure to air (oxygen)

There are two systems for protecting an excellent extra-virgin olive oil against theloss of quality due to oxidation: the first is inert-gas blanketing of containers and thesecond is using floating roof containers.

15.3 PREVENTION OF EXPOSURE TO AIR (OXYGEN) 169

Inert-gas blanketing consists in substituting the air present at the top of the storagecontainers with an inert atmosphere. An ‘inert atmosphere’ refers to a nonreactivegas that contains little or no oxygen. In extra-virgin olive oil storage, nitrogen is themost common inert gas, but argon is sometimes used. In Table 15.2 the character-istics of nitrogen, oxygen and argon, the three most abundant components of air,are compared. It is worth emphasizing that nitrogen is 78% of normal air, whereasargon is less than 1%. The most important parameter to be considered is their spe-cific gravity (gravity compared to the gravity of air at 21 ∘C, conventionally takenas equal to 1). The specific gravity of nitrogen is slightly lower than that of air,whereas the specific gravity of argon is higher. This means that in the presence of asmall quantity of air in the head space of an oil container, if the inert gas is nitrogen,the small amount of residual air will probably be in contact with the oil surface,while if the inert gas is argon, the small amount of residual air will be pushed awayfrom the oil surface. In the case of nitrogen, flushing of residual air with inert gas(vent open) before seal closing of the tank can minimize this effect. Nitrogen is morewidely used due to its availability and relatively low cost.

Table 15.2 Some characteristics of oxygen and inert gases.

Gas Specific gravitya Concentration in theair (% by volume)

Nitrogen 0.972 78.08Oxygen 1.105 20.95Argon 1.377 0.93

Note: aspecific gravity relative to air at 21 ∘C (70 ∘F).

All inert-gas blanketing systems need three controlling devices:

• A pressure reducing valve. Since gas sources provide gas at a much higherpressure than desired, a pressure-reducing valve is needed to decrease the inletpressure to the tank.

• A blanketing regulating valve. The inert gas pressure inside the containershould be slightly higher than atmospheric pressure (10–20 cm of a watercolumn – which is 10–20 millibar – is enough). This is a measure to preventcontamination. If leaks should occur, some gas will leak out rather thancontaminants infiltrating the container. This condition is automatically assuredby a blanketing regulating valve. When the pressure inside the container dropsbelow a set point, the valve opens and blanketing gas enters. Once the pressurereaches the set point, the valve closes.

• A pressure vent. This consists of a vent that opens when the pressure inside thecontainer exceeds a maximum set pressure. This helps prevent the containerfrom rupturing due to high pressure.

A second system for preventing the risk of oxidation is by using containers witha floating roof that rises and falls with the level of the liquid, thereby eliminating


air in contact with the oil surface. A sliding seal between the roof and the tank shellassures a hermetic separation between the full and the empty part of the tank.

Nitrogen sources

Nitrogen for inert-gas blanketing can be supplied in traditional gas cylinders ormay be produced at the oil factory. In this case, two options are available:

• PSA (pressure swing adsorption) technology uses two towers filled withcarbon molecular sieve (CMS). Compressed air enters the bottom of the‘online’ tower and flows up through the CMS. Oxygen and other trace gases,which are small molecules, penetrate the pores of CMS, while the largernitrogen molecules by-pass the CMS and emerge from the top of the columnas nitrogen gas. After a preset time, the online tower automatically switchesto the regenerative mode, venting contaminants from the CMS. Pressureswing adsorption nitrogen generators typically produce very high puritynitrogen (higher than 99.5% purity).

• Membrane technology is based on filtration of dry compressed air throughhundreds of thousands of hollow fibre membranes, the diameter of a humanhair. Oxygen and the other trace gases permeate through the membranes,while nitrogen is retained and collected at the other end of the fibres. Mem-brane nitrogen generators are typically used in applications where the purityrequirement is below 99.5%

For oil storage, both systems have an acceptable performance.The final choice between cylinder supplied or nitrogen production with one

or the other of the two methods depends on cost, and this in turn depends on thelocation of the factory (proximity to a nitrogen supply service) and the volumeof nitrogen needed.

15.4 Prevention of exposure to light

Exposure of extra-virgin olive oil to light favours photo-oxidation of the oil. Thepresence of photosensitizing pigments like chlorophyll and carotenoids can speedup oxidative phenomena. Short-wavelength radiation, particularly the UV part of thespectrum, energizes the oxygen molecules into a more reactive form called tripletoxygen, which greatly accelerates lipid oxidation. This is usually not a problem atthe oil factory where oil is stored in stainless steel containers.

For bottles, different coloured glass is used: from brown to dark brown, fromdark green to violet. The effectiveness of the various glass types is the object offurther insight and discussion in Chapter 16. In any case, in the absence of preciseinformation, the best recommendation is to store the bottles in a dark place.

15.7 PREVENTION OF MECHANICAL STRESS 171

15.5 Prevention of water and organic residuesin the oil

Water and suspended solids may spur microbial and enzymatic spoilage of the oil.It is therefore advisable that, after the final centrifugal finish, the oil is filtered withpure cellulose pads or other suitable material. This should not be considered as anoption, but as an essential condition for good oil storage (Chapter 14).

15.6 Prevention of exposure to contaminatedatmosphere and poor hygienic standards

Extra-virgin olive oil must be stored in hermetically sealed containers in orderto prevent any particulate or volatile contamination from the environment or theatmosphere. Covers simply resting on top of containers, a common system in smallolive oil factories, do not safely protect the oil from atmospheric contaminants orintrusion of pests. At the same time, the overall hygiene of the oil factory mustbe implemented according to the requirements and practices recommended inChapter 21.

15.7 Prevention of mechanical stress

Turbulence and shearing actions on oil should be avoided. They may negativelyinfluence the perceived thickness of the oil and favour dispersion of air. In the pres-ence of water and solid residues, turbulence and shaking favour dispersion of thesolids in the oil.

Negative effects of turbulence and shearing stress are linked to the high viscosityof oil and to the use of unsuitable pumps. This problem is discussed in detail inAnnex 15.1.


Annex 15.1: Pumps, tanks and piping

Introduction

During the storage and commercial life of an oil, several operations requiringmechanical action on the oil are carried out, for example pumping, blending,pouring, filtering and bottling. The physical property that is more directly relatedto the effects of mechanical stress upon the oil quality is viscosity.

Viscosity (symbol: μ) may be defined as the resistance of a liquid to flow. It maybe considered as a sort of internal friction and is expressed in a unit called ‘poise’(actually dyne-second per cm2). The most commonly used unit is the centipoise(symbol: cP), which is one hundredth of a poise. This practical unit is used becausethe viscosity of water (something that is familiar to everybody) is 1 cP at 20 ∘C.

At 20 ∘C, extra-virgin olive oil has a viscosity of 84 cP and is therefore much moreviscous than water. This means that, under the same driving force as, for example,gravity, oil flows much more slowly than water. The high internal friction is also thereason why oil flows from a dispenser as a thread instead of breaking into drops.

When a viscous product like extra-virgin olive oil (or any other vegetable oil) istransferred through pumps, pipes and valves, great care should be taken to avoid anyturbulence and shaking. In fact, these actions not only cause large energy dissipationbut may also cause air absorption and emulsion formation, especially in the presenceof suspended solids and residual water in nonfiltered oils. These effects may impairoil stability and determine undesirable changes in sensory perception.

In Table A4 in the Appendix, the viscosity of extra-virgin olive oil is reported as afunction of temperature. It is evident that at the optimal storage temperature (15 ∘C,59 ∘F) the oil viscosity is high and handling should be particularly careful. Extra-virgin olive oil is an inherently ‘slow’ product and forcing it to flow faster can onlyresult in a stability and quality risk.

Pumps

The choice of suitable pumps is a critical detail for avoiding mechanical stress tothe oil.

Centrifugal pumps (never to be used for extra-virginolive oil transfer)

In an olive mill there are many centrifugal pumps. They are used for all water transferduties: in the plant of olive washing, in heating and cooling circuits, in wastewaterdischarge, and so on. Centrifugal pumps are the best for pumping water. Their mainadvantages are: simplicity, low cost, uniform (nonpulsating) flow, small floor space,low maintenance cost, quiet operation. Figure 15.2 presents the functional schemeof a centrifugal pump. It consists of a case containing a rotating blade impeller.Power from an outside motor is applied to the impeller. The rotating blades produce

ANNEX 15.1: PUMPS, TANKS AND PIPING 173

Dischargeline

Suctionline

Blades

Volute chamber

Shaftdrive

Rotatingimpeller

Figure 15.2 Orthogonal sections of a centrifugal pump.

A hydrostaticpressure head

A flow-regulatingvalve

The shortestpossible

feeding line

Figure 15.3 The three essential features of a centrifugal pump circuit.

reduced pressure at the entrance of the impeller, thus causing the inlet liquid to besucked from the feeding pipe. The liquid follows a spiral pattern with increasingtangential velocity. The velocity (kinetic) head is transformed into a pressure headas the liquid passes into the volute chamber and is pushed out to the discharge pipe.

Figure 15.3 is a schematic representation of the correct assembly of a centrifu-gal pump. There are three main precautions that must be followed when using acentrifugal pump: (i) the pump must be installed as closely and directly as possibleto the feed tank; (ii) the pump should be at a level lower than the feed liquid levelin order to facilitate pump priming; and (iii) a regulating valve should always bepresent at the outlet of the pump in order to create suitable pressure head losses andto adjust the discharge of the liquid at the desired flow rate.


The performance data of a centrifugal pump consist of flow rate and pressurehead: flow rate decreases with increasing pressure head losses (resistance) in thedischarge circuit.

Centrifugal pumps cannot be used with viscous and air-sensitive fluids becausethey generate very high turbulence with mechanical stress, air mixing and emulsi-fication effects. This is why centrifugal pumps should never be used for pumpingextra-virgin olive oil.

Positive displacement rotary pumps

These are the most suitable pumps for extra-virgin olive oil. They assure a constant(positive displacement) and continuous (rotary) flow, while minimizing shearingeffects.

Figures 15.4 and 15.5 represent gear and lobe pumps, respectively.Two impellers with the shape of toothed-gear wheels (Figure 15.4) or of lobed

cams (Figure 15.5) rotate in the case with very low clearance between them and thecasing. The pumped liquid flows into the spaces between the impeller teeth or lobesas these cavities pass the suction opening. The liquid is then carried to the dischargeport and forced into the discharge circuit.

Figure 15.6 represents a single rotor screw pump (‘mono’ pump) that can be usedfor viscous fluids like extra-virgin olive oil and also for semi-liquid slurries as, forexample, olive paste or pomace. In the functional scheme represented in Figure 15.6,a wormlike spiral (screw) impeller rotates in an elastomeric stationary sleeve.The liquid is transferred by means of the helical progress of concrete cavities as therotor is turned. In the representation of Figures 15.6 a and b, the screw position isrepresented 180 ∘ (half a revolution) apart. In Figure 15.6b, the liquid which was in

Liquid being transferredfrom inlet to outlet

Inlet Outlet

Figure 15.4 The functional scheme of a gear pump.


Liquid being transferredfrom inlet to outlet

OutletInlet

Figure 15.5 The functional scheme of a lobe pump.

(a)

(b)

3

OutletWorm-likeimpeller

Elastomericstationary sleeve

1 2 3

1′ 1 2

Inlet

Figure 15.6 The functional scheme of a mono pump in two subsequent positions 180 ∘ (half arevolution) apart.

position 3 in figure 15.6a has been discharged. The product that was in position 2and 1 has progressed one position toward the outlet; and a new liquid portion isentering in the new 1′ position.

This pump is extremely versatile. In the food industry it is often used for gentlehandling of large solids suspended in a liquid and in all cases of shear-sensitivematerials.

Figure 15.7 presents the functional scheme of a peristaltic pump. In this pump,the liquid is contained in a flexible tube fitted inside a circular casing. A lobe rotor


Liquid beingtransferred

from inlet to outlet

Lobe rotor

Inlet

Outlet

Figure 15.7 The functional scheme of a peristaltic pump.

compresses the flexible tube, thus trapping and transferring discrete portions of theliquid towards the outlet.

An important difference between centrifugaland positive displacement pumps

Unlike centrifugal pumps, in positive displacement pumps the flow rate is constantindependent of the head losses in the discharge circuit. Flow rates cannot be adjustedby using a valve at the pump outlet. Such a valve will have practically no effect on theflow rate and closing it completely will involve generation of very high pressures. Toprevent this risk, positive displacement pumps are often fitted with cut-off pressureswitches or a bypass pipe that allows a variable amount of fluid to return to the inlet.Flow-rate variation in pumps for extra-virgin olive oil requires that the pump has aspeed variator of turning impellers, so that the rotating speed is adjusted accordingto the desired flow rate.

Tanks and piping

Extra-virgin olive oil tanks are often vertical and cylindrical in shape, with fixed orfloating roofs. They should have hermetically closed tops to prevent contaminationof the oil from the environment. Tanks may have a flat bottom, a cone-shaped bottomor a sloped bottom. They must have rounded corners going from vertical walls to thebottom for easier cleaning and to withstand the hydraulic pressure of the liquid. Thetransfer of oil should be carried out at low flow-rate and low flow velocity, throughsmooth pipes, avoiding as much as possible sudden changes in pipe diameter or inthe direction of flow, minimizing the distances and the number of obstacles to flow,including measurement devices and valves. When discharging the oil into a tank, the


incoming flow should be directed close to the tank wall so that the oil slides downthe sides without splashing.

Figure 15.8 shows a typical piping arrangement for the transfer of extra-virginolive oil. In particular, notice the indication of a circuit-emptying valve, which mustalways be present at the lowest point of the circuit so that all the oil present in thecircuit (including pipes and pump) can be discharged by gravity at the end of theoperation. The oil should never remain in the circuit or part of it for a long timebecause the circuit conditions do not usually guarantee the proper temperature. Allpiping and valves should be part of the cleaning-in-place system.

Floating roof containers have some advantages when transporting oil. In fact,during transportation, the presence of a head space either filled with air or with aninert gas favours shaking the oil, which may accelerate spoiling effects especiallyin the presence of water and solid residues. With floating-roof arrangements, in theabsence of a head space, the shaking effect is minimized.

PG1

PG2

PG3

(o)

(n)

(k)

(h)

(j)

(g)

(a)(b)

(c)(d)

(e)

(m)

(f)

(i)

(l)

Figure 15.8 The essential features of a circuit for transferring extra-virgin olive oil from onetank to another. (a) an hermetic seal; (b) the inert gas inlet; (c) the automatic pressure regulatingvalve of inert gas; (d) vent; (e) pressure gauges; (f) rounded corners; (g) oil discharge close to thewall so that oil slides down the sides without splashing; (h) cone shaped bottom; (i) valve for totalemptying of the tank; (j) tank discharge valve; (k) opening for access, repair, maintenance; (l) apositive, rotating, low-shear, pump; (m) smooth pipes with large radius curves; (n) the valve inthe lowest position for total emptying of the oil circuit; (o) an automatic regulation of maximumpressure.


Finally, principles of hygienic design discussed in the annex of Chapter 21 mustbe accurately applied.

Reference

Peri, C. (2013) Quality excellence in extra-virgin olive oil, in Olive Oil Sensory Sci-ence (eds E. Monteleone and S. Langstaff), John Wiley & Sons, Ltd, Chichester.

Further reading

Earle, R.L. and Earle, M.D. (n.d.) Fluid-flow application, in Unit Operations in FoodProcessing, www.nzifst.org.nz/unitoperations (accessed 29 September 2013).

Peri, C. and Zanoni, B. (1994) Manuale di Tecnologie Alimentari, CUSL, Milan,Parts 1 and 3.

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