Courtney Ngai And Selina Wang, PhD - UC Davis Olive Center
ReportFilter or not?A Review of the Influence of Filtration on Extra Virgin Olive OilCourtney Ngai and Selina Wang, PhDOctober 2015Copyright 2015 UC Regents, Davis campus. All rights reserved.Photo: iStockphoto/afinocchiaro
Filter or Not? UC Davis Olive Center, October 2015Filter or Not?A Review of the Influence of Filtration on Extra Virgin Olive OilCourtney Ngai and Selina W ang, PhDExecutive SummaryFiltration removes suspended solids and moisture in olive oil before storage and can produce positive, negative orneutral effects on the parameters of stability, phenolics, volatiles, sensory, appearance, pigments and shelf life. Theimpact of filtration on the oil depends on the initial chemical and sensory profile, varietal of the oil, filtration systemand storage conditions. Regardless of method, filtration will require additional expenses for equipment, labor, andprocessing time. After reviewing the current literature on filtration research, we found that there is no simple answerto the question of whether to filter or not. While the research lacks consensus regarding many of the effects offiltration as summarized in Figure 1, there are some general areas of agreement as summarized in Figure 2.Figure 1. Filtration can have positive and negative effects on key parameters of extra virgin olive oil quality1
Filter or Not? UC Davis Olive Center, October 2015Figure 2. Areas of general agreement on extra virgin olive oil filtrationBackgroundExtra virgin olive oil (EVOO) is produced solely through mechanical means, which includes washing of the olives,malaxing (or kneading) of the olive paste, centrifuging to separate the oil from the olive paste, and decanting of theoil before storage. While EVOO can be consumed without further treatment, some processors filter the oil to removesuspended solids or moisture that can promote oil deterioration. However, quantitative and qualitative changesoccur during and after filtration, especially on the minor components that are very important for the oil quality andits health benefits.More than 98 percent of olive oil (by weight) is triacylglycerols while the remainder, known as minor components, ismade of phenols, pigments, volatiles and other non-glyceride chemical compounds. There are many agronomic andtechnological factors that affect the composition and concentration of these minor components, including olive fruitripeness, fruit quality, milling conditions, malaxation (time, temperature, and oxygen exposure), centrifugation, andstorage conditions.2
Filter or Not? UC Davis Olive Center, October 2015There are several types of filter aids for removing suspended solids and moisture, and within the same filter aids,there are often additional options (e.g., paper filtering with or without Na2SO4). The different filtering systems arebriefly described below:Filters to remove moisture. In olive oil with a high moisture content, traditional filter presses such ascotton, paper or anhydrous Na2SO4 are used. The process is carried out after the filtration process to removesuspended solids, prior to storage. The system is pressurized by a hydraulic closure, and the oil is passed througheach chamber of the filter as water is removed. Cotton filters removed water more consistently than paper filters,and filters with Na2SO4 dehydrated more efficiently than filters without it.15,20Filters to remove suspended solids. In olive oil with a high amount of suspended solids, cellulosefibers can be used. Cellulose fibers have a rough surface, large porosity and elastic behavior providing a stablestructure with higher flow rates and longer cycle times in comparison with mineral filter aids.13,20,21Membrane filtration. Membrane filtration is often appropriate for filtering large volumes of olive oil,or oils with high levels of solids. The continuous flow over the membrane reduces impurities. Filtration is done witha perpendicular-flow through the filter, and the process is mild with little change to oil composition. The fluid isdirected at high speed to the membrane surface allowing for high-filtrate fluxes.20Inert gas filtration. The flow of inert gas (nitrogen or argon) generates a circular movement of the oilmass that facilitates the separation of the suspended solids. The process prevents organic materials from coming intocontact with the oil and is considered by Italian and Spanish researchers to be an effective filtration system for waterremoval.20 Moreover, at the end of the filtration step, the EVOO is already under inert gas providing suitableconditions to prolong the shelf life of the oil.18,20Filter-Bag System. This filtration system contains two compartments, a cylindrical tube and a filter bagmade of polypropylene. The filter bag is introduced into the cylindrical tube, and the system is pressurized by ahydraulic closure. Olive oil is taken directly from storage tanks to filtration equipment. The fluid passes across thefilter bag, and suspended solids are removed. On occasion, similar to the other filtration systems described, variousmaterials selected by the manufacturer may be used as filter aids to improve the filtration process. The mainadvantages of this system are its wide versatility and easy maintenance, which permits an optimal level of clear andtransparent oil.20Effects of FiltrationResearch has shown that olive oil filtration can produce both positive and negative outcomes on olive oil’s stability,phenolics, volatiles, sensory, appearance, pigments and shelf life. The published literature has mixed results andrecommendations largely due to the use of the different types of filtering systems and the variable initial quality of theoils. Summarized below are the overall effects of olive oil filtration described in the current literature that focusspecifically on stability, phenolics, volatiles, sensory, appearance, pigments and shelf life. Our summary alsoprovides explanations and reasons relating to whether or not to filter the oil. More detailed descriptions of eachstudy are shown in Tables 1 - 7.3
Filter or Not? UC Davis Olive Center, October 2015Stability. By removing the suspended solids, the oil is at reduced risk of enzymatic spoilage anddevelopment of anaerobic microorganisms that can cause sensory defects and, subsequently, cause the oil to nolonger meet extra virgin grade standards.22 By removing the moisture, the rate of hydrolysis is reduced, which canhelp lower the free fatty acidity level.26 However, some research suggested that the suspended solids and particlesare important for stabilizing the oil and protecting it from both oxidative and hydrolytic degenerations.11 Thesemixed results are mostly due to the difference in initial quality and characteristics of the oil (e.g., varietal, ripeness,robustness) and the filtration system used. For most oils, filtration does not cause significant changes to basicchemical parameters such as free fatty acidity, peroxide value and UV absorbance,25 although, some studies haveshown that a slight increase (with cotton, ultrafiltration) or decrease (with inert gas, filter bag) may take place.6,18Rancimat, an accelerated aging test, is commonly used to measure the oxidative stability of an oil. It provides quickinformation on the relative stability of oils, though it does not always resonate with real-time studies. Most researchfound that filtered oils have lower oxidative stability than unfiltered oils do. Studies tend to evaluate the commonlyused chemical parameters of free fatty acidity, peroxide value, and ultraviolet absorbance and have not addressedhow other useful parameters such as diacylglycerols (DAGs) and pyropheophytins (PPP) change after filtration.Phenolics. After filtration, the concentration of total phenols drops due to the amphiphilic nature ofphenolic compounds near the water droplets in olive oil. The percentage of loss is dependent on the type of filtrationsystem and the oil. Filtration can not only reduce the total phenolic content but also change the phenolic profile.21Some research shows that there is sometimes an increase in total phenolics in filtered oils, however, this is most likelydue to an effect of the analytical method used. Before filtration, the hydrophilic phenols are in a more polar matrix,and their affinity to the extraction solvent is low. After filtration, these compounds are more available to theextraction solvent. This results in an apparent increase in their concentration in filtered oils.2,3,15In addition, the concentration of individual phenols changes during storage; some increase (i.e., hydroxytyrosol,tyrosol, from the hydrolysis of secoiridoid aglycones) and others decrease. A number of research papers haveshown that in filtered oils, the levels of hydroxytyrosol and tyrosol had decreased and these oils showed a morerapid loss in total phenolic compounds compared to unfiltered oils.7,12,15,25Volatiles. In EVOO, the most important volatiles, C5 and C6, consist of aldehydes, alcohols, ketones andesters. Knowledge about influence of filtration on volatiles is very limited and the pathways are not well understood.The changes in volatile profile are highly dependent on the characteristics of the oil and the filtration system used.Sensory. Filtration can remove unwanted particles that cause hydrolysis, lipid oxidation and microbialfermentation, which would end up producing sensory defects during storage.1,4 However, positive attributes such aspungency, fruitiness and bitterness may also be affected depending on the filtration system. Research results arevery mixed on the sensory impacts of filtration, which are highly dependent on the sensorial attributes of theunfiltered oil, the type of filtration used, and the time in storage.Appearance. Filtration increases the transparency of the oil and reduces the intensity of green color.204
Filter or Not? UC Davis Olive Center, October 2015Pigments. The reduction of suspended solids decreases pigment concentration leading to lowerchlorophyll and carotenoid concentration in filtered oils. Chlorophyll is a pro-oxidant in the light and an antioxidantin the dark; therefore, its effect on the oxidative stability of the oil depends on the storage conditions.14Shelf life. Filtration helps to reduce the rate of hydrolysis of the triacylglycerol matrix, especially in oilswith higher initial free fatty acidity. A study showed that the effect of filtration on shelf life is dependent on the oilvarietals and storage temperature (25 C vs. 40 C).12 Some research showed that individual phenols such as thehydroxytyrosol and tyrosol25 decreased while some showed that they increased7,12 in both filtered and unfiltered oilsduring storage, using paper filtration. The rancid defect appears in filtered and unfiltered oils of Arbequina after aten-month period.12 Unfortunately, research on shelf life using newer filtration systems is lacking. (Note: Table 7omits a study that examined the effect of cellulose fiber on shelf life after a nine-month storage period – the studyfound no significant differences in free fatty acidity, peroxide value and UV absorbance after nine months.23 )ConclusionThe literature shows mixed results on the effects of filtration on parameters of stability, phenolics, volatiles, sensory,appearance, pigments and shelf life that can affect the oil positively, negatively, or not at all. Some studies showthat filtration can achieve specific goals such as prolonging shelf life, reducing moisture and removing suspendedsolids. The suspended solids contain water and enzymes that impair oil stability, increase fermentation anddegrade the oil’s sensory quality; by removing these solids, filtered oil has less water activity, clearer appearance,less green color, and no deposits in the storage container. On the other hand, the literature also shows thatfiltration can have negative impacts on a variety of parameters. Ultimately, the effect of filtration depends on thechemical and sensory profiles, quality of the initial oil, the type of filter aid and system, and storage conditions.Research does not show consistent results using the same filtration system due to variance in factors such as olivevariety, ripeness, water and solid contents, and the initial chemical and sensory parameters of the oils after milling.Thus, currently, the literature does not provide simple answers on whether to filter or not. While there have beensome carefully designed research on olive oil filtration, many studies did not characterize the complete chemicaland sensory profiles of the initial oil before filtration and during storage after filtration. In addition, a large numberof studies relied on lab-scale equipment that may or may not correlate with results from industrial-sized systems.To provide additional clarity on the question of whether to filter or not, future studies should seek to include all ofthe following features: collection of chemical and sensory data of the oils, both prior to and after filtration;inclusion of oils made from a variety of cultivars and maturity levels;evaluation of oil shelf life, particularly using newer filtration systems;examination of how filtration affects olive oil volatiles and sensory attributes over time; anduse of industrial-scale filtration systems to complement the existing body of data on lab-scale systems.Regardless of method, filtration will require additional expenses for equipment, labor, and processing time.5
Filter or Not? UC Davis Olive Center, October 2015Table 1.Positive,negative, and neutral effects of different oil filtration systems on olive oil stability.STABILITYPaper Showedno effect onperoxidevalues andUVabsorbancein mostoils.25Paper withNa 2 SO 4Decreasedthe oxidativestability index(OSI) using theRancimatmethod andwere lower inOSI than usingcotton filters.15CottonDecreasedthe oxidativestability index(OSI) usingthe Rancimatmethod butwere higher inOSI thanusing paperwith Na2SO 4filters.15 Increasedslightly in freefatty acidity,peroxide valueand UVabsorbance inCoratina andSalentina oilsmade fromripe fruits, butno significantdifference inboth oils fromless ripefruits.10Cellulosefiber Produced nosignificantdifferencebetween filteredand unfilteredoils in free fattyacidity,peroxide valueand UVabsorbanceexcept for theoil filtered withthe Tami T50membraneshowed ahigher peroxidevalue.6 Showedno significantdifference infree fattyacidity,peroxide valueand UVabsorbance forthe filtered oils.5Membrane Produced nosignificantdifferencebetween filteredand unfiltered oilsin free fattyacidity, peroxidevalue and UVabsorbanceexcept for the oilfiltered with theTami T50membraneshowed a higherperoxide value.6 Showed nosignificantdifference in freefatty acidity,peroxide valueand UVabsorbance forthe filtered oils.56Inert gasFilter bagDecreased the peroxidevalue to half of the initialvalue in unfiltered oils.18Decreased the peroxide valueto half of the initial value inunfiltered oils.18Decreased slightly theoxidative stability indexusing the Rancimat methoddue to the reduction in watercontent and were morepronounced in effect usingargon gas than nitrogengas filters.18Decreased slightly the oxidativestability index using the Rancimatmethod due to the reduction inwater content and were morepronounced in effect than usingnitrogen gas filters.18Obtained highconcentration ofortho diphenols (e.g.hydroxytyrosol andoleuropein) using theRancimat method but hadthe lowest oxidative stabilityindex compared to unfilteredoils due to reduction in watercontent where thesecompounds are moreprotective against oxidationin water in oil emulsion.18Obtained high concentrationof ortho diphenols (e.g.hydroxytyrosol and oleuropein)using the Rancimat method buthad the lowest oxidative stabilityindex compared to unfiltered oilsdue to reduction in water contentwhere these compounds are moreprotective against oxidation inwater in oil emulsion.18 Produced no effect on the fattyacid composition.18
Filter or Not? UC Davis Olive Center, October 2015Table 2.Positive,negative, and neutral effects of different oil filtration systems on olive oil phenolics.PHENOLICSPaperDecreasedthe degradationrate ofsecoiridoidphenolics.7Reduced thetotal phenoliccontent.17 Showed noeffect on alphatocopherolcontent.7,12Paper withNa 2 SO 4Increased thehydroxytyrosolcontent.15Decreasedthe totalphenolic contentfor most of theoils.25 Decreasedthe tyrosolcontent in somefiltered oilswhile showedan increase inothers.15CottonCellulose fiberMembraneIncreased thehydroxytyrosoland tyrosolcontent in oilsmade from riperolives.10Decreased the phenolic alcoholsand flavones compounds using a100% cellulose and 70%cellulose/30% ligand filters andwere less affected when using the70% cellulose/30% ligandfilter.3Decreasedsignificantly thehydroxytyrosolcontent.5,6,7Decrease thetotal phenolicand tocopherolcontent asripeningproceeded.10Decreasedsignificantly thehydroxytyrosolcontent.15Removed humidity and reducedwater content along with a portionof phenolic compounds.15Lowered significantly thehydroxytyrosol and 3,4 DHPEA EAcontent.23Inert gasDecreasedthe totalphenoliccontent aftercross ncreased thequinic acidcontent(responsible forastringency) usingargon andnitrogen gas.18 Showed noeffect on lipophilicphenoliccompounds (e.g.tocopherols).18 Showed no significant change intotal phenolic compounds.237Increasedsignificantly thetotal phenoliccontent usingargon gas.18Filter bagIncreasedsignificantly thetotal phenoliccontent.18Showed adecrease in phenylalcohols, lignansand flavonescompounds.18Removed quinicacid (responsiblefor astringency).18 Showed no effecton lipophilicphenolics (e.g.tocopherols).18
Filter or Not? UC Davis Olive Center, October 2015Table 3.Positive,negative, and neutral effects of different oil filtration systems on olive oil volatiles.VOLATILESPaper Changed the initialflavor of the oil throughthe formation of volatileproducts caused byoxidation.4,24Paper withNa 2 SO 4CottonCellulose fiberMembraneIncreased volatile2 methylbutanaldescribed as sweet andmalty.23Decreasedvolatiles trans 2 hexen 1 ol and cis 2 penten 1 ol describedrespectively as greengrass, leaves and sweetand banana.23 Showed no significantchange in a majority ofthe 38 volatilecompounds analysed.238Eliminated some undesired volatilecompounds.6 Reduced significantly the short andmedium chain carbonyl compounds (C5–13)which contributes to its flavor and fruity tasteusing cross flow filtration.5Reduced the concentration of short andmedium chain carbonyl compounds (C5–13)which contribute to oil flavor and fruitinesswhen using cross flow micro and ultrafiltrationand produced less effect on the loss of volatileswhen using microfiltration.6Inert gasFilterbag
Filter or Not? UC Davis Olive Center, October 2015Table 4.Positive,negative, and neutral effects of different oil filtration systems on olive oil sensory attributes.SENSORYPaperIncreased mildly thefruity attribute ofCornicabra andArbequina oils.12Decreasedsignificantly the fruity,pungent andbitter attributes afterremoval of suspendedsolids and moisture.12Paper withNa 2 SO 4Increasedmildly the fruityattribute ofCornicabra andArbequina oils.12Decreased mildly theintensity of pungency.9Decreasedsignificantly thefruity, pungentandbitter attributesafter removal ofsuspended solidsand moisture.12 Showed no changein the fruity, bitter andsweet attributes.9Decreasedmildly the intensityof pungency.9CottonCellulosefiberIncreased theapple and grassattributes whilefruity, bitter andsweet attributesshowed nochange.9Decreasedmildly thepungent attributein Picual oils dueto the removal ofphenoliccompounds.21Decreasedsignificantly therancid defect.12Increased thepungent and fattyattributes.23MembraneRetained thepleasant aromainfiltered oils.5 Obtainedsimilar aromavalues forfiltered andunfilteredoils.5,6Inert gasFilter bagEnhanced bitterand pleasantflavors.18Enhanced bitterand pleasantf
Inert gas filtration. The flow of inert gas (nitrogen or argon) generates a circular movement of the oil mass that facilitates the separation of the suspended solids. The process prevents organic materials from coming into contact with the oil and is considered by Italian and Spanish researchers to be
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