Industrial Frying Process

284(C18:2) and linolenic (C18:3) acids, giving the oil a lowiodine value (IV) of about 80, which renders it resistant tooxidation. Its long shelf-life and high resistance tooxidation are also, in part, related to the advantageouscombination of various phenolic antioxidants present inolive oil. Olive oil is available in several grades, extravirgin olive oil being the best and usually the mostexpensive. This high-grade olive oil is seldom used forindustrial frying, although some high-class restaurateursmay justify the expense. For industrial frying, the cheaperrefined olive oil grades are usually satisfactory.Soya-bean oil and rapeseed oil are similar to oneanother in that they have high iodine values and lowlevels of saturated fatty acids, and are fully fluid evenat low ambient temperatures. The main difficulty withsoya-bean and rapeseed oils is the 8-10% or so oflinolenic (C18:3) acid present, as this makes thesetwo oils prone to oxidation and off-flavourdevelopment. In this respect, it must be rememberedthat, with industrial frying, it is not only oxidationduring the frying process that must be considered butin many cases also oxidation of the oil duringsubsequent storage of the fried food. Crisps are aparticular problem in this respect as they contain ahigh level of absorbed oil, are often stored intransparent bags and expose the oil on a largesurface area to the surrounding atmosphere.However, rapeseed oil and soya-bean oil are cheapand are widely used in many frying operations,especially in the fast-food area, where storage of thefried product is not necessary.Animal tallows are also used for frying, beef tallowbeing popular in Yorkshire and eastern England,while lard is popular in Lancashire. Although animalfats have high levels of saturated fatty acid and aresolid at room temperature, fresh fats have pleasantdistinctive flavours, which are imparted to the friedfood. There is a low tendency to oxidation of thefresh fat as the level of polyunsaturated fatty acid isquite low. Unlike vegetable oils, however, animalfats contain no natural tocopherol antioxidants.Some fast-food chains have used beef tallow in theirfrying operation, usually blended with a small amountof liquid vegetable oil.Fish oils are seldom used for frying, as they havea tendency to cause excessive foaming. Fish oilshave extremely high iodine values, reflecting the highlevels of polyunsaturated fatty acids with 20 or morecarbon atoms. These long-chain polyunsaturatedacids render fish oils extremely prone to oxidationand in general fish oil is unsuited for fryingoperations. Producers of fried fatty fish thereforeensure that their normal frying oil does not becomeaccidentally contaminated with fish oil.Table III includes the compositions of some oilsmanufactured for industrial frying, as well as those ofoils used for batch frying.(c) Consejo Superior de Investigaciones CientíficasLicencia Creative Commons 3.0 España (by-nc)Grasas y AceitesGroundnut oil was at one time a premium fryingoil. It is sufficiently fluid at ambient temperature tocause few problems of pipe blockage due tosolidification, although it might solidify in pipes ordrums exposed to very cold winter weather. Its mainattraction is its low iodine value, and low level ofpolyunsaturated fatty acids. Its most unsaturatedcomponent is linoleic acid (C18:2), which rangesfrom 15-40% depending on the origin of the oil. Theoil should contain less than 0.1% linolenic acid.However, some manufactured products may containtrace contamination with other oils, and slightlyhigher levels of linolenic acid, up to 0.3%, might attimes be experienced. In terms of fryingperformances, the low iodine value and the nearzero level of linolenic acid make it an admirable oil.Unfortunately, groundnut oil has declined inpopularity, owing in part to its cost but also toproduction problems related to the aflatoxin problemwith groundnuts, and to public worries about thepeanut allergy to which some people are sensitive.Table IIIIndustrial frying oilsMost unsaturatedcomponent(% level)IodineValue% Unsaturatedfatty acidsGNO85-105851.3C18:2 (15-40)P. Oléine56-60530.3018:2(10-12)116-119884.6C18:3 (8-10)88-96840.9C18:3 (1-2)SBC130-135843.5C18:3 (7-9)HSBO100-115803.5C18:3 3018:2(37-43)RSOHRSOPolyunsats:sats ratioGNO groundnut oilP. Oieine palm oléineRSO rapeseed oilHRSO hydrogenated rapeseed oilSBO soya-bean oilHSBO hydrogenated soya-bean oilHWSBO hydrogenated winterised soya-bean oilHSun hydrogenated sunflowerseed oilPalm oléine is similarly of low iodine value (55-65units), and has an even lower content of polyunsaturatedfatty acids. Its most unsaturated component islinoleic acid, which occurs at concentrations ofaround 10%. Palm oléine has a relatively high levelof saturated fatty acids, and may thus be frowned onfrom a dietary point of view. However, it is more fluidthan palm oil and should seldom cause problems ofpipe blockage or lack of pourability due tosolidification unless, as with groundnut oil, oil pipesor drums are exposed to very cold wintry weather. Inbatch installations, palm oléine may partly solidify indrum storage, and it is usually recommended thatunopened drums should be warmed before use, e.g.by being placed alongside the frying kettle. Onehttp://grasasyaceites.revistas.csic.es

Vol.49. Fase. 3-4(1998)disadvantage with palm oléine is that the oil tends todarken during frying, causing kitchen staff to presume thatthe oil is deteriorating in quality. In fact, this has been shownby the Palm Oil Research Institute of Malaysia (PORIM) to bedue to the presence of low levels of polar compounds suchas parahydroxybenzoic acid and vanillin The dari ening ofpalm products during frying is therefore in no way areflection of any inferior performance. The uses of palm oiland oléine in frying have been reviewed by Berger ' .Rapeseed oil and soya-bean oil are illustrated inTables II and III, and it has already been mentioned thata problem with these two oils is the high level of linolenicacid. Some of the newer varieties of rapeseed oilrecently introduced into the UK do in fact have evenhigher linolenic acid contents " . Rapeseed oil andsoya-bean oil can, therefore, be slightly hydrogenatedduring manufacture of oils for industrial frying, in order toreduce the linolenic acid content. This procedure canalso be applied to sunflowerseed oil, as illustrated inTable III. These hydrogenated oils, therefore, havereduced levels of polyunsaturated fatty acids. In the caseof hydrogenated rapeseed and hydrogenated soya-beanoils, the level of linolenic acid is reduced to below 3%, thecorresponding iodine values being around 100 units,similar to that of some varieties of unhydrogenatedgroundnut oil. Sunflowerseed oil has no linolenic acid,and the linoleic acid content can be reduced to about40%. Hydrogenated sunflowerseed oil may be attractivewhere a high polyunsaturated to saturated (P/S) ratio isneeded for dietary purposes, together with a goodshelf-life. However, hydrogenated sunflowerseed oil willbe more expensive than alternatives based on othervegetable oils, and attention should be paid to the transacid content of the hydrogenated oil, an aspect currentlycausing concern in nutritional debates . In fact, the use ofhydrogenated sunflowerseed oil may not be seen as anattractive option, since the main dietary attraction ofsunflowerseed oil is its high linoleic acid content, buthydrogénation destroys this.These hydrogenated oils do, of course, have highermelting points, and in some cases a somewhat harderfrying fat is an advantage. In doughnut production forinstance, such a fat will solidify quickly, giving betteradhesion to the sugar dusting. Palm oil is thereforeattractive here. In cases where cold weather fluidity isimportant, the hydrogenated oils can be fractionallycrystallised and filtered to remove high-meltingcomponents, as in the production of palm oléine. Thisis illustrated in the case of soya-bean oil in Tables IVand V . Soya-bean oil with an initial iodine value (IV) of131 can be selectively hydrogenated at either of twotemperatures, of 170 C and 130 C, to IVs of 106.6and 104.0, respectively. In both cases, the residuallinolenic acid content will be close to the target level of2%. However, there is an appreciable trans isomercontent in both oils, and a clanger of solid-phaseformation during storage. The oils are therefore slowlycooled to approximately 5 C and then held at this(c) Consejo Superior de Investigaciones CientíficasLicencia Creative Commons 3.0 España (by-nc)285temperature overnight to allow complete crystallisation.Several methods may be employed to separate solidcrystals and liquid oil, a rotating drum filter being thecheapest to install, but other types of flat-band filter andmembrane/diaphragm units are also used. A moresophisticated procedure is to mix the oil with aqueousdetergent solution, which preferentially wets the solidphase, enabling centrifugal separation, this is the so-calledLipofrac system. Winterised oils of the type illustrated inTable V may be obtained. These have good fluidity andresistance to oxidation and off-flavour development.The production of frying oils from soya and rape isreviewed in Leatherhead Food RA SymposiumProceedings No 31 , and by Hastert .TABLE IVProduction of partially hydrogenatedsoya-bean oll Feedstock oilIVHydrogénation Hydrogénationat 170 Cat 130 X103.9106.6104.0Fatty :00.40.50.5Others0.50.30.4Trans isomers {%)0.521.919.7TABLE VProperties of hydrogenated winterisedsoya-bean oil Fraction170 C Hydrogénation130 "0 3.176.9IV96.3108.993.0105.310.3Fatty 92.31.71.9Others1.10.71.20.724.421.622.019.5Trans isomers (%)http://grasasyaceites.revistas.csic.es

286Another aspect of quality influenced by thecomposition of the oil is the nature of the flavours thateventually develop during storage of the fried food.Although these may all be described as off-flavours,some are more acceptable than others. Thus coconut oildevelops an initial coconut flavour, but, in the presence ofmoisture, a strong soapy off-flavour can be produced ,these soapy off-flavours also being a problem with palmkernel oil. Soya-bean and rapeseed oils develop painty,fishy, or beany off-flavours, while those in corn (maize)are slightly burnt or corny, and those in cottonseed orpeanut are often nutty. The off-flavours that may developin hydrogenated oils are more varied, as there is a widervariety of unsaturated fatty acid isomers in the oil. Insome cases, they are not detected when the food iseaten, but lingering cardboardy notes can develop on thepalate some time after the food has been consumed.Fortunately, however, this is a rare occurrence, and isprobably encountered only with oils hydrogenated underpoorly controlled conditions. Blumenthal, Trout & Changefound that the flavours developing in hydrogenatedsoya-bean oil were less attractive than those developingin hydrogenated corn, cottonseed or peanut oils.The criteria that should be taken into account inselecting a frying oil are therefore price, fluidity, a lowtendency to foam or smoke formation, oxidativestability of the oil with products that are eaten after aperiod of storage, and good flavour stability of thefried product. Where slight off-flavours do develop,they should be acceptable in terms of the food fried.When dietary requirements play a role, the ratio ofpolyunsaturated to saturated fatty acids (P/S) mustbe taken into consideration.Although COMA discounted dietary cholesterol asa major factor in heart disease, some consumersnevertheless prefer frying oils with low cholesterolcontents. The concentrations of cholesterol in somefoods are therefore listed in Table VI, olive oil having thelowest level of all. It is ironic that some consumers mayask for fish portions battered with a compositioncontaining egg yolk, or a chicken piece, to be fried inoil substantially free of cholesterol, when the basefood contains much more cholesterol than the cookingoil.In many cases the above requirements areconflicting, and a balance or compromise must bemade in selecting an oil for industrial frying.The frying oil must also, of course, be fresh andsubject to normal quality control criteria, as illustratedin Table VII. The peroxide value should be less than0.4 mEq/kg and the colour should be less than 2Lovibond red units in a 5.25-inch cell. In many cases,the linolenic acid level should be below 3%. Forfreshly refined oils, the free fatty acid level should bebelow 0.05%, but with some natural oils, like olive oil,or unrefined animal fats (e.g. dripping), higher levelsof free fatty acid may be tolerated. In fact, Matz claimsthat a free fatty acid concentration of 0.4% is optimum(c) Consejo Superior de Investigaciones CientíficasLicencia Creative Commons 3.0 España (by-nc)Grasas y Aceitesfor heat transfer to the fried food, as it reduces theinterfacial tension between the hot oil and the film ofsurface moisture on most uncooked foods. The lowconcentrations of free fatty acid in most grades ofolive oil are therefore a benefit in the frying process.TABLE VICholesterol contents(range (mean) - mg/kg)Safflower oilSunflower oilMaize oilGroundnut oilCotton oilRapeseed oilOlive 5- 80(50)1-24(7)Soya oilFish oilDairy butterfatBeef tallowEgg yolkFishChicken, lamb chop20-35(28)2,000-6,0002,200-2,8001,000-1,200ca 12,600500-7,000ca 1,000Blumenthal & Stier developed this concept further,and claim that one of the most important aspects offrying performance is the influence of surface-activeagents in the oil breaking down interfacial tensionbetween hot oil and wet food. Thus partly oxidised fattyacids, which form in the hot oil, act as surface-activeagents and improve frying performance as an oil«matures». This is termed «breaking in» the oil.Another possible advantage in allowing an oil tomature is that some of the fried food flavours can arise asa result of interactions between aldehydes in usedcooking oils and proteins in the food. Such flavours maybe absent in foods fried in very fresh refined oils'' ''" .However, assessment of an oil intake at a bulk fryinginstallation must require the flavour to be bland, as anydevelopment of cooked, fried or partially oxidised flavourbefore the oil is used must denote some deterioration orpremature breakdown of the oil, and indicates poorquality. In addition, the flash and smoke points should beabove 315 C and 200 C, respectively, although oliveoil, because it is unrefined, will have flash and smokepoints at lower temperatures - of about 285 C and 169 C, respectively. This will also be the case with unrefinedanimal fats. Oils with very low flash points can give rise topossible fire or explosion dangers.TABLE VIIQuality limits for a frying oilPeroxide value (mEq/kg)0.4 max.Linolenic acid (%)3.0 max.Colour (Lovibond Red units,5.25 inch cell)2.0 max.Free fatty acid (%)0.1% max. (but 0.4% is optimumaccording to Matz to providebetter heat transfer)FlavourBlandFlash point315 CSmoke poir t200 CMoisture0.05% max.http://grasasyaceites.revistas.csic.es

Vol. 49. Fase. 3-4(1998)Minor ComponentsAll oils contain minor components, some of whichare beneficial to performance during frying, and othersdetrimental. Table VIII lists the minor components thatmay be considered to be beneficial. Tocopherols arenatural, phenolic antioxidants, and are present in allvegetable oils . Processing of the oil, and itssubsequent storage and handling, should be arrangedso that the natural tocopherols are preserved for aslong as possible. However, high levels of tocopherols,e.g. over 1,000 mg/kg, can promote oxidation, soadditional quantities should not normally be added inlarge amounts. Any tocopherol additions shouldtherefore be adjusted to replace tocopherolsunavoidably lost during processing, orto achieve someagreed optimum level bearing in mind the intendeduses. Animal fats do not contain natural tocopherols,and, with these frying media, addition of moderateamounts of tocopherol can give significantimprovement. Some sterols - for example, A -avenasterol,also act as antioxidants ' '"' . Rice-bran oil containsmore A -avenasterol than most other liquid oils, andthis may confer good frying performance on this oil.Low levels of phospholipids (lecithin) act synergisticallywith tocopherols in alleviating risk of oxidation .Concentrations of lecithin should, however, be limitedto a maximum of about 100 mg per kg as higher levelscause foaming. Carotenoids can also inhibit oxidationduring frying and it is sometimes claimed that an oillasts longer if small pieces of carrot are added to thefrying bath during frying. Various additives can also beused to enhance or retain oil quality. Syntheticantioxidants such as BHT or BHA are permitted atconcentrations of up to 200 parts per million.Although BHT and, to a-lesser extent, BHA, aresteam-volatile and are lost from the oil during thefrying operation, some benefits are passed on tothe fried food. Citric acid is also useful because itcomplexes transition metals such as iron and copper,thus limiting their catalytic pro-oxidant influence.Most suppliers of refined oil will add up to 100 mg/kgcitric acid to the oil immediately after the deodorisationif requested.TABLE VIIIBeneficial minor componentsNatural:Additives:Tocopherols - up to 1,000 ppmSterols, e.g. A avenasterolPospholipids (lecithin) up to 100 ppmCarotenoidsBHA (BHT) up to 200 ppmCitric acid up to 100 ppmSilicon oil up to 2 ppmMirOil Life Powder(c) Consejo Superior de Investigaciones CientíficasLicencia Creative Commons 3.0 España (by-nc)287Silicon oil (polydimethyl siloxane) is useful in thatit is claimed to form a film on the surface of the fryingoil, limiting uptake of atmospheric oxygen, as shownin Table IX .MirOil Life Powder is claimed to enhance theproperties of the frying oil. It is a mixture of citric acidand moisture absorbed onto the surface of afood-grade volcanic ash. It is claimed that, when thepowder is added to a used frying oil, the citric acidand moisture react with alkaline materials in the oil,such as soaps, precipitating these in a form in whichthey can then be removed along with the earthdunng filtration. Any residual iron or coppercompounds are doubtless removed at the sametime, and any suspended particulate matter isabsorbed and removed by the earth during filtration.The filtration must be very thorough, however, toprevent carry-over of any powder into the frying bath,as this would contaminate the food with the powder,which, although it may be used as a processing aid,is not an approved food ingredient.Schulz claims, however, to have tested MirOilLife Powder in a large-scale industrial fryingoperation with no beneficial effect.TABLE IXInfluence of polydimethyl siloxane(per cent non-oxidised triglyceride insunflowerseed oil)Silcon anti-foam concentration (ppm)Heating timeat180 6(h)505456728(h)40445164From Freeman, Padley & SheppardTable X illustrates minor harmful components thatmay be present in an oil. Lecithin and partial glyceridescause excessive foaming if they are' present inmoderate or high concentrations. Free fatty acid willcause smoke when present at levels of over 2%. Thisinfluence depends on the volatility of the free fatty acid,being more of a problem with the shorter-chain acidspresent in palm kernel or coconut oils, as discussedearlier. The most detrimental minor components aretrace metals such as iron and copper. These promoteoxidation if present at levels of over 0.1, or 0.01 mg/kg,respectively and all efforts should be taken to minimisecontamination of the oil with these transition metals,especially copper. Residual solvents will not normallybe present; if they are, they will reduce the flash pointhttp://grasasyaceites.revistas.csic.es

288Grasas y Aceitesand give potential explosion danger. Proteinaceousresidues may be present in the oil when purchased.This is a more serious problem in unrefinedcold-pressed oils such as groundnut oil or beefdripping. Residues of the food fried will also fall intothis category, and should be removed by effectivefiltration or skimming. The hot oil should not continueto flow through a filter bed of hot debris, however, asthis will negate the benefits of filtering it off. Filtersshould therefore be discharged on a regular basis.When animal fats are used as frying media, thesewill of course contain much higher levels ofcholesterol than vegetable oils, as shown in Table VI,and fried vegetable products will then acquire anincreased cholesterol level. However, this is notregarded as a problem by nutritional experts .Alkaline-reacting materials (ARM) such as sodiumoléate are also reputed to cause deterioration of thefrying oiP, as illustrated in Table XI, where it can beseen that the concentration of alkaline-reactingmaterial (as sodium oléate) increased during the use offrying oil in a restaurant. When the level of ARM hadrisen to 43 mg/kg (as sodium oléate) it becamenecessary to discard the oiP.Foaming can be a major problem in fryingoperations, and can cause a fire. Figure 1 shows thedifference between foaming and bubbling oil.Foaming is when the oil surface becomes coveredwith a mass of thick bubbles that do not disperseduring frying. This is dangerous because the foamcan rise over the top of the fryer and cause a fi

KEY-WORDS: Oil evaluation during use - Oil properties - Oil storage - Oil transport. The quality of oil, fat or shortening used for frying is of paramount innportance with regard to the quality of the fried food. Table I illustrates this by listing the amount of