Can. Ins!. Food Sei. Teehnol. 1. Vol. 16, No. 4, pp. 246-253, 1983

Lipid Changes in French Fries andHeated Oils During Commercial Deep Frying

and Their Nutritional and Toxicological Implications

Lilian U. Thompson and Rae Aust

Department of Nutritional SciencesUniversity of Toronto

Toronto, Ontario M5S IA8

Abstract

Lipid changes in the heated oil and french fries (FF) in a shortorder restaurant were determined over a 100 h frying period duringwhich the oil was replenished three times. The percent cis polyun­saturated fatty acids and Irans fatty acids remaining after 100 h fryingwere 48 and 87010, respectively, in the frying oil and 46% and 67%,respectively, in the FF. The polymer content rose to levels highenough to be of toxicological importance. Due to lipid exchange,the composition of FF differed from that of the frying oiL Lipidchanges were influenced more by the quantity of potatoes fried thanby the frying time. Intermittent heating was no more destructiveto lipids than continued heating. Replenishment of 1/3 of the oilin the fryer every 16.5 h or after frying 25 kg potatoes can preventlarge changes in the nutritional and toxicological properties of theoil and FF.

ResumeLes changements lipidiques furent determines dans I'huile chauffee

et dans les pommes de terre frites d'un casse-croGte pendant uneperiode de 100h durant laquelle l'huile a ete remplacee trois fois.Les pourcentages d'acides gras polyinsatures cis et Irans qui resterentapres 100h de friture furent respectivement 48 et 87% dans J'huileet 46 et 67% dans les frites. La teneur en polymeres s'est elevee ildes niveaux d'importance toxicoligique. Acause des transferts lipidi­ques, la composition des frites fut differente de celle de I'huile defriture. La quantite de pommes de terre frites eut plus d'influencesur les changements lipidiques que le temps de friture. Les lipidesne furent pas affectes plus par le chauffage intermittent que par lechauffage continu. Le reapprovisionnement du 1/3 de I'huile dansla friteuse il chaque 16.5h ou apres la friture de 25 kg de pommesde terre peut prevenir de grands changements dans les proprietesnutritionnelles et toxicologiques de \'huile et des frites.

IntroductionOil oxidation results in the loss of essential fatty

acids such as cis polyunsaturated fatty acids (PUFA)and the accumulation of various oxidation products(Dugan, 1976; Chang et al., 1978). A number of theoxidation products when fed in substantial quantitiesappeared to have detrimental effects in test animals(Crampton et al., 1951; Kaunitz et al., 1956; Artman,1969; Kantorowitz and Yannai, 1974; Alexander, 1978;Changetal., 1978; Meltzeretal., 1981). Thus it is im­portant to determine the extent of oxidation during

food processing and also the processing conditionswhich influence the oxidative process.

In this study the commercial deep frying of foodswas examined because oxidation during this type ofprocessing is not well documented. Most investigationshave studied oils heated in the laboratory simulatingcommercial deep frying conditions. In some studies,oils were severely abused by heat treatment at hightemperatures for long periods of time. In many, nofood was fried at all. Some have tried to adjust forfrying by frequent aeration of the oil or frying water­logged cotton balls. Those studies in which food wasfried usually involved frying small quantities of foodover a short period of time or frying fresh foods in­stead of pre-fried foods which are now commonly us­ed by fast food restaurants. In many instances thecommon commercial operating practices, such asperiodic replenishments of the oil in the fryer and in­termittent heating (ov.ernight cooling followed by all­day heating) have been overlooked (Firestone et al.,1961a; Fleischman et al., 1963; Krishnamurthy et al.,1965; Perkins, 1967; Rao and Rao, 1968; Kilgore andBailey, 1970; Waltking and Zmachinski, 1970; Kilgoreand Windham, 1973; Morrison et al., 1973; Kan­torowitz and Yannai, 1974; Chang et al., 1978; Meltzeret al., 1981).

The objectives of this study were to determine thelipid changes in french fries and heated oils during ac­tual commercial deep frying and to assess the influenceof some common commercial frying practices on thelipid changes.

Materials and MethodsThe short order restaurant selected for the study us­

ed potatoes obtained from Carnation Co. Ltd. Theywere 0.95 cm wide, straight cut, partially fried andfrozen. The lightly hydrogenated soybean oil fromMonarch Fine Foods Co. Ltd., contained 0.1070butylated hydroxy anisole and 0.45% methyl silicone.The fryer had a thermostat set at 177°C and contain­ed 11.6 kg of oil. The used oil was filtered once a day

Copyright Cl 1983 Canadian lnstilute of Food Science and Technology

246

TOTAL SAT FA

100

138.1

100

138.1

80

80

_OIL

._. FF

107.3

60

_OIL__ • Ff

• TOTAL PUFA

• CIS PUFA

82.7

40

63.6

KILOGRAMS OF POTATOES FRIED

20

20

35.9

KILOGRAMS OF POTATOES FRIED

35.9 63.6 82.7 107.3

.........---'?"---...~,..--~ ....

o

o

o

o

10

40 60

HOURS OF FRYING

Fig. I. Polyunsaturated fatty acid (PUFA) content of frying oiland french fries (FF) over a 100 h frying period.

10

40

...........---

HOURS OF fRYING

Fig. 2. Oleic acid and total saturated fatty acid contents of fryingoil and french fries (FF) over a 100 h frying period.

remaining after 100 h of frying were 48,52 and 34%,respectively in the frying oil and 46, 47 and 33%,respectively, in the FF. The greater losses of linolenicthan linoleic acid was expected since linolenic is moresusceptible to autooxidation (Dugan, 1976). The lossesof PUFA in the frying oil used in this study were atthe upper limit of losses found by others in com­mercially used frying oils (Thompson et al., 1967;Waltking and Zmachinski, 1970). When the slope wascalculated as change in FA over hours of frying andalso over kilograms of potatoes fried for the times

20 --

40 o:::.-_~

30oii::::;...oif.

20

~ 30:::;...o;!.

and usually discarded after a week. The restaurantoperated 7:00 to 23:00 Sunday to Thursday and 7:00to 1:00 on Friday and Saturday. The fryer was turnedoff overnight and the oil allowed to cool. The oil wasusually replenished three times a week.

The study was started when the fryer was filled withfresh oil (0 h) and ended when this was discarded,which was 100 h of frying time (6 d). During thisperiod, the oil in the fryer was replenished at 20.5,45and 60 h of frying time with 2.84, 0.91 and 3.29 kgof oil, respectively. At 7:00, 12:00, 14:00, 17:00 and23:00 on each day, and after each oil replenishment,samples of the oil in the fryer, of the partially friedfrench fries (PF) and of the finished fried potatoes (FF)prepared at that time were taken. In all there were 32sampling periods. The PF and FF in plastic bags andthe oil in plastic bottles were flushed with nitrogen andstored at - 20°C until lipid extraction and analyseswere conducted.

The oil used during the study was from the samebatch. Composition data suggest that five separate bat­ches of PF were used during the period. Finish fryingtime was not controlled but left to the discretion ofthe fryer operator. Variation in the total lipid contentof the FF (12.4 ± 0.3070) suggested that cooking timevaried from sample to sample.

Four months after the start of the study, samplesof fresh oil and oil about to be discarded were obtain­ed from the same restaurant for lipid analysis. The oilhad also been used for 100 h.

Lipids were extracted by a modified Werner-Schmidtype extraction method (Pearson, 1962) as recom­mended by Sheppard et al. (1974). Percentage lipid wasdetermined by evaporation of the extracts. The cisPUFA was assessed according to the enzymaticmethod outlined by MacGee (1959). Fatty acid (FA)profiles were determined by gas liquid chromatography(GLe) according to AOCS (1971) methods; an inter­nal standard (triheptadecanoin) was used to obtainquantitative estimates. The percentage of non-elutedmaterials from the GLC was determined according toWaitking (1975). To assess the trans FA, infra-redspectroscopy (AOCS, 1971) with modification byHuang and Firestone (1971) was employed. Free FAdetermination was according to AOCS (1971) methodsand non-urea-adduct forming fatty acids (NUAF) wasby the method of Firestone (1961 b). All samples wereanalysed at least in duplicate. Statistical analysis whereindicated was according to Steele and Torrie (1960).

Results and DiscussionChanges in composition offrying oil and french fries

After 100 h of frying, the cis PUFA dropped from31.7 to 15.1% of lipid in the oil and from 29.8 to14.5% of lipid in the FF (Figure 1). Total PUFA(linoleic and linolenic) as measured by GLC showedsimilar type of changes (35.3 to 17.8% in the oil and35.1 to 16.0% in the FF). However, there was a greaterquantity of total PUFA as measured by GLC than cisPUFA, the difference probably being trans PUFA.

The percent cis PUFA, linoleic and linolenic acid

Can. Inst. Food Sci. Technol. J. Vol. 16, No. 4. 1983 Thompson and Aust / 247

HOURS OF FRYING

Fig. 4. Free fatty acid content of frying oil and french fries (FF)over a 100 h frying period.

before (0-20.5 h), during (20.5-60 h) and after(60.5-100 h) replenishment, both the frying oil and FFshowed losses of PUFA which were greater in the in­itial stages of frying than towards the end. As ex­pected, the smallest losses occurred during thereplenishment period .

The oleic acid in the oil and in the FF slightly in­creased initially and then decreased with a negligiblenet effect (Figure 2), as found by others (Firestone etal., 1961a; Thompson et al., 1967; Kilgore and Bailey,1970; Waltking and Zmachinski, 1970). The saturatedFA, on the other hand, decreased initially but in­creased in the later stage of frying (Figure 2), i.e. after60 h frying. Since the apparent increase which OCcur­red after 60 h of frying was primarily due to palmiticacid, it could likely be due to oxidation products whichhave the same retention time as palmitic acid, e.g. anine carbon aldehyde ester reported by others (Keeney,1962).

The total trans FA (Figure 3) increased initially butthen decreased to show a net loss by the end of thefrying period. This was true for both the frying oil andFF. The losses were not as extensive as those observ­ed with cis PUFA. The percent of total trans FA andcis PUFA which remained after 100 h of frying were87 and 48 respectively in the frying oil and 67 and 46respectively in the FF. Firestone et aI., (1961a) foundan increase in isolated trans FA in cottonseed oilheated for 300 h in the laboratory. Trans FA have notbeen measured in other commercially heated fryingoils.

Free FA accumulated according to the patternshown in Figure 4. The replenishments account for thelower free FA values observed at 20.5 and 60.0 h offrying time. A small volume of fresh oil was addedat 45.0 h, resulting in a smaller change in free FA. Themaximum free FA level (5.6070 in frying oil and 5.0%in FF) was attained at 83.5 h after which the free FAdecreased. The increase in free FA at the early stagesmay be due to both hydrolysis of the triglycerides andoxidation of the FA. The latter decrease may be dueto further oxidation of the FA and/or involvement ofthe FAin the polymerization. The maximum valueobserved in the frying oil in this study was in line withfree FA values (0.26 - 8.54%) found by Thompsonet al., (1967) in several commercially used frying oils .

Polymers are formed during oxidation and heattreatment of the oil (Dugan, 1976). They are commor,­ly measured by determining the NUAI: componentsof the oil since straight chain FA readily complex Wilhurea whereas branched and cyclic FA found inpolymers do not complex (Firestone et al., 1961 b).Waltking and Zmachinski (1970) showed that the noneluted materials from the GLC is also a good indicatorof polymer concentration and that it correlates wellwith NUAF. However both NUAF and non elutedmaterials from GLC have been shown by others (Art­man and Alexander, 1968; Waltking et al., 1975) tocontain not only polymer but also other oxidationproducts.

The NUAF content of the frying oil at various in­tervals in the frying period is illustrated in Figure 5.

100

lOO

138.1

lOO

138.1

138.1

80

80

107.3

80

107.3

GO

_ OIL

•••• FF

60

60

82.7

82.7

40

40

40

63.6

63.6

............................................................

KILOGRAMS OF POTATOES FRIEO

20

20

20

35.9

35.9

KILOGRAMS OF POTATOES FRIED

35.9 63.6 82.7 107.3

KILOGRAMS OF POTATOES FRIED

... NON ELUTED MATERIAL

o NON UREA ADDUCT FORMING (NUAFI FATTY ACIDS

• CIS PUFA LOSS F ROM HOUR 0

- OIL

._- FF ..""",;

;'"......... IT......

_-...-----111''''',.--

o

HOURS OF FRYING

Fig. 3. Total Irans fatty acid content of frying oil and french fries(FF) over a 100 h frying period.

HOURS OF FRYING

Fig. 5. Oxidation products and loss in cis polyunsaturated fattyacids (cis PUFA) in frying oil and french fries (FF) overa 100 h frying period.

o

o

4

6

10

20

10

o

30

o

o0::::i...of. 20

248 / Thompson and Aust J. Inst. Can. Sci. Technol. Aliment. Vo!. 16. No. 4, 1983

Table I. FallY acid composition of fresh oil and oil after 100 h of frying (% lipid).

Collection I Collection 2

FallY AcidQsPUFALinolenic acidLinoleic acidOleic acidTotal sat FATotal trans FAFree FA

Oh31.683.02

32.3239.4418.8413.710.39

100 h

15.101.03

16.7837.8625.5511.983.94

Change

-16.58-1.99

-15.54-1.58

+6.71-1.73

+ 3.55

Oh36.903.75

36.9642.2117.1313.120.35

100 h

19.151.13

18.8337.6426.077.723.42

Change-17.75-2.62

-18.13-4.57

+8.94-5.40

+3.07

There was only trace amount of NUAF in the oil upto 27 h, after which there was a gradual increase to76.5 h when the value rose more sharply. The noneluted material from GLe followed a similar pattern,although there was a greater quantity present especiallyin the earlier stages of frying. This is probably due tosome polar oxidation products of linoleic acid whichare included in the non eluted material but not in theNUAF values (Waltking et al., 1975). The loss of cisPUFA (Figure 5) followed the same pattern as NUAFand non eluted material.

The maximum amount of NUAF (14.2010), (Figure5) attained after 100 h of frying time was higher thanthe 0.5 to 5.36010 reported by Thompson et al., (1967)in commercially used frying oils, but lower than the23 to 45010 levels found in laboratory heated oils andshortenings (Perkins and Van Akkeren, 1965; Waltk­ing and Zmachinski, 1970; Kantorowitz and Yannai,1974).

Four months after the first samples in the longitu­dinal study were collected, another sample of fresh oiland oil about to be discarded were collected from thesame restaurant. In the second collection the oil aboutto be discarded was the same brand as that used in thefirst study, had been used also for 100 h and had beenreplenished three times. Analysis of the two sets offresh oil and oils used for 100 h (Table I) showed nolarge differences in their FA composition except fora greater loss of trans FA in the oil at the second col­lection period. This suggests good reproducibility ofthe lipid changes observed in the longitudinal study.

Frying oil vs. french fries compositionThere were significantly less cis and total PUFA

(Figure 1) and trans FA (Figure 3) and more oxida­tion products (non eluted material) (Figure 5) in theFF than in the frying oil. Kilgore and Bailey (1970)also reported less PUFA in the FF than frying oil whileHussain and Morton (1974) found more oxidation pro­ducts in the fried food product than the frying oil.

The differences observed in this study could not beattributed to the lipid extraction process since 100010recovery of the lipid was obtained. Since the potatoesused were partially fried, it is likely that their lipid com­position will have influence on the final compositionof the finished fried FF. This issue was discussed indetail in a previous publication (Aust and Thompson,1981). The data suggested that all of the lipid in thePF did not remain during frying and that an exchangeof lipid in the PF and frying oil took place. The com­position of the frying oil, however, has been shownto exert the major influence on the composition of theFF.

Effect ofquantity ofpotatoes fried vs hours offryingIn numerous studies, a fixed amount of food was

fried every given hour (Kilgore and Luker, 1964; Beareet al., 1968; Rao and Rao, 1968; Kilgore and Bailey,1970; Morrison et al., 1973). As a result, the two in­dependent variables, frying time and quantity of foodfried, were constantly in the same proportion and itwas impossible to determine which factor had a greaterinfluence on the changing composition of the frying

Table 2. Relationshipl between fatty acid concentration in french fries and frying oil and hours of frying, kilograms of potatoes fried or in­teraction of hours times kilograms for 26 samples

Hours Kilograms Hours x kilograms

-0.183-0.224-0.260-0.267

+0.2110.201

Fatty Acid

Frying Oilcis PUFAlinolenic acidlinoleic acidoleic acidtotal sat FAtotal trans FA

French Friescis PUFA -0.143linolenic acid -0.147linoleic acid -0.082oleic acid -0.124total sat FA + 0.235totaltrans FA -0.412*

-0.068-0.157-0.129-0.427*

+0.170-0.350

-0.076-0.184-0.011-0.127

+0.162-0.495*

-0.461*-0.293-0.584***

+0.401*+0.183+0.396*

-0.715***+0.110-0.269-0.009

+0.325+0.102

r13.2

-0.434*-0.247-0.552**

+0.511*+0.150+0.582**

-0.706***+0.158-0.257

+0.026+0.279+0.244

-0.277-0.463*-0.525**

+0.151+0.242+0.220

-0.298+0.008-0.043-0.154

+0.329-0.224

Ir = simple correlation coefficientr12.3 = partial correlation coefficient between fatty acid concentration and hours with kilograms being constant.r132 = partial correlation coefficient between fatty acid concentration and kilograms with hours being constant.*p < .05 **p < .01

Can. Insl. Food Sei. Teehnol. J. Vo!. 16. No. 4, 1983 Thompson and Aust / 249

oil. Some workers (Krishnamurthy et al., 1965; Perkinsand van Akkeren, 1965; Rao and Rao, 1968) have gonea step further by comparing changes in one fryer inwhich the oil was heated alone to a second in whichfood was fried. However no one has examined the ef­fect of quantity of food fried and frying time in onesystem.

In this study, the changes in lipid composition inthe FF and in the oil were correlated with hours of fry­ing between sample collection intervals and also thequantity of potatoes fried in the same interval. Thecorrelation coefficients are presented in Table 2.

The changes in cis PUFA and linoleic acid in thefrying oil and cis PUFA in the FF significantly negativecorrelated with kilograms of potatoes fried. Changesin oleic acids and total trans FA in the frying oilpositively correlated with kilograms of potatoes fried.There were no significant correlations between hoursof frying and changes in any of the FA except totaltrans FA in the FF.

Since the two variables (hours of frying andkilograms of potatoes fried) could be interrelated, theirinterrelationships was investigated in two ways. First,the changes in FA was correlated with an interactionterm, expressed as hours of frying times kilograms ofpotatoes fried. Correlation coefficients between thechanges in FA and the interaction term which arelarger than the correlation coefficients for either of thevariables alone, would be evidence for interaction.Table 2 shows that in all cases except for linoleic acidin the frying oil, correlation coefficients with the in­teraction term were lower than those with eitherkilograms of potatoes fried or hours of frying alone.

Second, a partial correlation coefficient was assessedfor each independent variable. An appreciably lowervalue than the simple correlation coefficient would beevidence for a strong association between the two in­dependent variales. Table 2 shows that the partial cor­relation coefficients between kilograms of potatoesfried and cis PUFA or linolenic acid are still signifi­cant and not appreciably lower than the simple cor­relation coefficients. The simple correlation coefficientbetween kilograms of potatoes fried and hours of fry­ing between each collection time is 0.272 (p < .05).Thus, there is no evidence for a strong association bet­ween kilograms of potatoes fried and hours of frying.

Results therefore suggest that the amount of foodfried has a greater influence on changing the lipid com­position of the frying oil and fried food than hoursof frying. The amount of food fried may be a bettercriteria for deciding when to discard the frying oil thanhours of frying.

It is known that the frying of food introduces waterinto the fat and results in increased hydrolysis whichis then reflected by high values of free FA(Krishnamurthy et al., 1965; Perkins and Van Ak­keren, 1965). From results in this study it would ap­pear also that the frying of food enhances oxidationas indicated by the increased loss of PUFA associatedwith kilograms of potatoes fried. Factors related tofrying food which could increase oxidation include in­creased agitation, and thus aeration of the oil as the

250 / Thompson and Aust

food is placed into and removed from oil and, the dripback into the oils of fat and moisture which cool andcondense in the hood of the fryer.

Effect of intermittent heatingThe fryer heater was turned off in the evening and

the oil allowed to cool until morning when the oil wasreheated. The heating and cooling cycle is known asintermittent heating. To determine the effects of in­termittent heating, paired t-test of the FA contents ofthe frying oil and of the FF before and after the over­night cooling was done. Samples were collected justbefore the heater was turned off in the evening andagain when the fryer oil reached 177°C in the morn­ing. Five sets of samples were compared. The samplesat 60 h were omitted because the oil was replenishedbefore being heated in the morning. Results showedno significant effect on the compositiOli of the fryingoil or the FF except for a slight change in linoleic(+0.21010) and in oleic acid (-1.15%) in the fryingoil and in non eluted material (- 0.63%) in the FF.

It has been suggested that cooling the oil results inan increased accumulation of peroxides which decom­pose rapidly when the oil is heated again (Perkins andVan Akkeren, 1965). If this is the case, a decrease inPUFA might be expected as a result of overnight cool­ing. Based on our data, this is obviously not true.

It may be argued that if peroxides were decompos­ing rapidly upon heating, changes in FA may not benoticeable until the early morning period (7:00 to12:00). Thus FA changes in the intervals from 7:00 to12:00, 12:00 to 14:00, 14:00 to 19:00 and 19:00 to 23:00were compared in this study using Duncan's multiplerange test. Changes were expressed as changes per hourper kilograms of potatoes to eliminate effects due tohours of frying and amount of potatoes fried. Resultsshowed no intervals in which there was significantlymore or less activity than another. Therefore, the over­night cooling of the oil or intermittent heating was nomore destructive than continual heating.

Effect of oil replenishmentThe fryer held 11.6 kg oil and a total of 7.04 kg oil

was used for replenishment. Therefore the replenish­ment averaged 0.0704 kg of oil per h over the 100 hfrying period and the turnover time was 11.6/.0704or 164.8 h.

The extent to which the replenishment oil regener­ated the frying oil was measured as the ~ime taken forthe FA to return to pre-replenishment levels. This wasalso measured as the quantity of potatoes fried untilthe FA return to pre-replenishment levels. As shownin Table 3, the maximum effect was equal to one dayof frying as seen in the case when about 30% of thevolume of the fryer was added in the replenishmentat 60 h. This would suggest that an amount of oil equalto one third of the fryer or about 3.6 kg should be add­ed at least once a day (16 h frying time), in order tomaintain the quality of the oil. This would reduce theturnover time to approximately 50 h of frying. Thisvolume of replenishment is close to that used byKrishnamurthy et al. (1965) in a laboratory study.

J. InSI. Can. Sci. Techno/. Aliment. Vol. 16. No. 4, 1983

They found that the addition of one third the volumeof the fryer every 12 h resulted in only small changesin free FA, peroxide value and iodine value in the fry­ing oil after 90 h of frying.

If this frequency of replenishment was assessed interms of quantity of potatoes fried (Table 3), it wouldappear that one third the volume of the fryer shouldbe added after approximately 25 kg of potatoes werefried. This would provide an oil turnover of 74.8 kgof potatoes.

Nutritional and toxicological implications0/ the lipid changes

There were considerable changes in the compositionof the frying oil and FF over the frying period.Therefore, in determining the nutritional and tox­icological properties of FF from commercial outletsit is necessary to collect samples at the beginning andend of the frying cycle. One randomly collected sam­ple would only be representative of the FF at that pointin the frying cycle. If only one sample can be collected,then the length of time the oil had been used for fry­ing and the quantity of food fried should be known.Since the composition of the oil does nor necessarilyrepresent the composition of the FF, samples of bothshould be analyzed.

To reduce the risk of heart disease, several healthagencies (Inter-Society for Heart Disease Resources,1970; Food and Nutrition Board, National Academyof Science - National Research Council, 1972;American Heart Association Committee on Nutrition,1974; Health and Welfare Canada, 1977) recom­mended reduction in total fat intake to 30-35070 of totalcalories. Of that amount, less than 10% of totalcalories should come from saturated FA and up to10% from PUFA with the remainder of the fat sup­plied by monounsaturated FA. Thus at least a 1: 1 ratioof PUFA to saturated FA should be present in a nutri­tionally acceptable fat. In this study, the fresh fat us­ed may be considered as a good source of PUFA witha cis PUFA:saturated FA ratio of 1.8:1 (Table 4), but,the quality decreased with frying time with the ratioboth in the oil and in the FF falling below 1: 1 after76.5 h. Nevertheless, with the current recommenda­tion of 3% of the total calories coming from cisPUFA, the 20% cis PUFA in the oil after 76.5 h fry­ing time (Table 3) could still meet the requirement ifthis oil was the sole source of dietary fat.

Since the metabolism and other properties of frans

Table 3. Kilograms of potatoes fried and hours of frying requiredfor fatty acids in the frying oil to return to pre·replenishment levels after each addition of oil.

Replenishment Time I

20.5 h 45 h 60 hFatty Acid h kg h kg h kg

Cis PUFA 6.5 13.6 3.5 1.8 10.0 16.1Linolenic Acid 6.5 13.6 3.5 1.8 16.5 24.7Linoleic Acid 6.5 13.6 15.0 17.0 16.5 24.7Free fatty acids 6.5 13.6 3.5 1.8 16.5 28.6

J2.84, 0.91 and 3.29 kg oil were added at 20.5, 45 and 60 h of fryingrespectively.

FA generally differ from those of cis unsaturatedisomers and tend to more closely resemble thesaturated FA, a low dietary intake of trans FAespecially the frans dienes, has also been suggested(Health and Welfare, Canada, 1980). The fresh fat inthis study cntained frans FA (13.7%) wich was higherthan the 5% recommended for margarines andshortenings (Health and Walfare Canada, 1980). Therewas a decrease from 13.7 to 12.0% in the frying oiland from 16.0 to 10.7% in the FF after 100 h frying(Figure 3), however, the small loss is probably not ofgreat nutritional significance. The total saturated plusfrans FA even after 100 h frying (Table 1) did not ex­ceed the recommended 40% maximum level fordesirable fat source.

A relationship between non-volatile NUAF contentof the heated oils and toxic responses have been sug­gested by early work of Crampton et al. (1953, 1956).Subsequent researchers not only confirmed these fin­dings but also suggested that the non adductablemonomer and oxidative dimers were the main sourceof the toxicity (Michael ef al., 1966; Artman, 1969;Artman and Smith, 1972). Since the monomers anddimers were not analyzed, the toxicity of the oil usedin this study could only be speculated from themonomer and dimer levels of other oxidized oils ofknown NUAF or non eluted material concentration.

Trilinolein heated at 185°C for 74 h reportedly con­tains 26.3% NUAF and 2.8% non cyclic dimers(Chang ef al., 1978). A rat diet containing 15% of thisfat should therefore contain 0.42% dimers. Since ratsfed diets containing 0.75% non-cyclic dimers did notresult in enlarged or fatty liver and other toxic symp­toms (Perkin and Taubold, 1978), the dimer level inheated trilinolein should give no danger to health. Inthis study, the NUAF in the oil after 100 h was 14.2%

Table 4. Cis polyunsaturated to saturated fatty acid ratio of the frying oil and french fries during 100 h of frying.

Potatoes Frying Oil French FriesFried time(kg) (h) cis·PUFA sat FA Ratio cis·PUFA sat FA Ratio

0 0 31.7 17.9 1.8:115.0 9.0 28.3 16.6 1.6:1 27.7 16.5 1.7: I15.4 10.5 29.2 16.8 1.7: I 26.5 16.6 1.6:150.6 27.0 25.4 16.9 1.5:1 20.7 16.2 1.3:165.5 43.5 21.2 18.8 1.1: I 19.0 19.8 1.0:182.6 60.0 21.5 18.5 1.2:1 19.7 18.0 1.1: I

104.3 76.5 20.1 19.6 1.0:1 20.3 21.4 0.8:1128.1 93.0 16.9 23.2 0.7:1 18.5 23.0 0.8:1138.1 100.0 15.1 25.6 0.6:1 14.5 21.3 0.7:1

Can. Inst. Food Sci. Technol. J. Vol. 16, No. 4, 1983 Thompson and Aust / 251

(Figure 5) and much lower than that of heatedtrilinolein. Therefore the dimeric component of the oilis probably also lower than that of trilinolein andshould also pose no toxicological problems.

Regarding the monomer, a hydrogenated soybeanoil heated continuously for 104 h contained 20.7"70noneluted material and 0.573"70 cyclic monomer(Meltzer et al., 1981). In a diet containing 15"70 of thisoil, 0.086"70 cyclic manomer would have been fed tothe rats. This could pose a health threat since in a studyof Hsieh and Perkins (1976), animals fed diets con­taining 0.02 to 0.15"70 cyclic monomers developed fattylivers, the severity increasing with the monomers con­centration. Similarly in this study, the oil and FF after100 h frying which contain 21 and 26"70 non-elutedmaterials, respectively, probably contain high levelsof cyclic monomers which may be of toxicologicalimportance.

The polymer content of the oil in the early stage offrying was low (Figure 5). If the oil was discarded on­ly after 27 to 30 h of frying or if replenishment of 1/3of the oil in the fryer is done every 16-17 h of fryingor after frying 25 kg of potatoes as recommendedearlier, polymer formation would probably be less ofa problem.

In conclusion, extensive lipid changes occurred inthe frying oil and FF after 100 h frying or 138.1 kgpotatoes. The cis PUFA fell below the desirable nutri­tionallevel and the polymer content rose to levels suf­ficiently high to be of toxicological importance. Largechanges may be prevented by replenishment of 1/3 ofthe oil in the fryer every 16.5 h of frying or after fry­ing 25 kg of potatoes. The quantity of potatoes friedhave greater influence on the lipid changes than thefrying time. Intermittent heating was no more destruc­tive than continued heating. The composition of theFF may differ from the composition of the oil fromwhich it was fried due to lipid exchange.

Acknowledgement

The authors thank the Atkinson Charitable Foun­dation for financial support.

References

Alexander. J .C. 1978. Biological effects due to changes in fats dur­ing heating. J. Amer. Oil Chem. Soc. 55:711.

American Heart Association, Committee on Nutrition, 1974. Dietand coronary heart disease. Nutr. Today 9:26.

AOCS. 1971. Official and Tentative Methods of Analysis of theAmerican Oil Chemists Society. AOCS. Champaign, IL.

Artman, N.R. 1969. Chemical and biological properties of heatedand oxidized fats. Adv. in Lipid Res. 7:245.

Artman, N.R. and Alexander, j.c. 1968. Characterization of someheated fat components. J. Amer. Oil Chem. Soc. 45:643.

Artman, N.R. and Smith D.E. 1972. Systematic isolation and iden­tification of minor components in heated and unheatedfats. J. Amer. Oil Chem. Soc. 49:318.

Aust, R. and Thompson, L.V. 1981. Lipid composition of finish­ed fried potatoes in relation to partially fried potatoesand frying oils. Nut. Rep. Internat. 24:957.

Beare, J.L., Kennedy, B.P.e. and Heroux, e.M.A. 1968. The eis,cis linoleic acid content of heated oils. Can. Inst, FoodTechnol. J., 1:48.

252 / Thompson and Aust

Chang, S.S., Peterson, R.J. and Ho Chi-Tang. 1978. Chemical reac.tions involved in deep fat frying of foods. J. Amer. OilChem. Soc. 55:718.

Crampton, E.W., Farmer, F.A. and Berryhill, F.M. 1951. The ef­fect of heat treatment on the nutritional value of somevegetable oils. J. NUlr., 43:431.

Crampton, E.W., Common, R.H., Farmer, F.A., Wells, A.F. andCrawford, D.R. 1953. Studies to determine the natureof the damage to the nutritive value of some vegetableoils from heat treatment. II. Segregation of toxic and non­toxic material from the esters of heat-polymerized linseedoil by distillation and by urea adduct formation. J. Nutr.49:333.

Crampton, E.W., Common, R.H., Pritchard, E.T. and Farmer,F.A. 1956. Studies to determine the nature of the damageto the nutritive value of some vegetable oils from heattreatment. IV. Ethyl esters of heat polymerized linseed,soybean, sunflower seed oils. J. Nutr. 60: 13.

Dugan, L. 1976. Lipids. In: Food Chemistry. O.R. Fennema (Ed.),Academic Press Inc., New York, NY.

Firestone, D., Horowitz, W., Friedman, L. and Shue, G.M. 1961a.Heated fats. I. Studies of the effects of heating on thechemical nature of cottonseed oil. J. Amer. Oil. Chem.Soc. 38:253.

Firestone, D., Nesheim, S. and Horowitz, W. 1961b. Heated fats,Ill. Determination of urea filtrates, J. Assoc. OfficialAnal. Chem. 44:465.

Fleischman, A.I., Florin, A., Fitzgerald, J., Caldwell, A.B. andEastwood, G. 1963. Studies on cooking fats and oils. J.Amer. Diet. Assoc. 42:394.

Food and Nutrition Board, Nat. Acad. Sci - Nat. Res. Council andCouncil on Foods and Nutr., Amer. Med. Assoc. 1972.Diet and coronary heart diseases. J. Amer. Diet Assoc.61:379.

Health and Welfare Canada, Committee on Diet and CardiovascularDisease. 1977. Recommendations for prevention pro­gramme in relation to nutrition and cardiovasculardisease. Health and Welfare Canada, Ottawa.

Health and Welfare Canada. 1980. Report of the Ad Hoc Com­mittee on the composition of special margarines. Healthand Welfare Canada, Ottawa.

Hsieh, A. and Perkins, E.G. 1976. Nutrition and metabolic studiesof methyl esters of dimeric fatty acids in the rat. LipidsII :763.

Huang, A. and Firestone, D. 1971. Determination of low levelisolated Irans isomers in vegtable oils and derived methylesters by differential infrared spectrophotometry. J.Assoc. Official Anal. Chem. 54:47.

Hussain, S.S. and Morton, O.D. 1974. Characteristics of oil ab­sorbed by fried products. J. Sci. Food Agric. 25: 1042.

Inter-Society Commission for Heart Disease Resources. 1970. Reporton primary prevention of the atherosclerotic disease. Cir­culation 42:55.

Kantorowitz, B. and Yannai, S. 1974. Comparison of the tenden­cies of liquid and hardened soybean oils to formphysiologically undesirable materials under simulated fry­ing condition. Nutr. Rep. Inter. 9:331.

Kaunitz, H., Slanetz, e.A., Johnson, R.E., Knight, H.B., Saunders,D.H. and Swern, D. 1956. Biological effects of thepolymeric residues isolated from autoxidized fats. J.Amer. Oil. Chem. Soc. 33:630.

Keeney, M., 1962. Secondary degradation products. In: Lipids andtheir oxidation. H.W. Schultz, E.A. Day and R.O. Sinn­huber (Ed.). The Avi Publishing Company, Inc.,Westport, CN.

Kilgore, L. and Luker, W.D. 1964. Fatty acid components of friedfoods and fats used for frying. J. Amer. Oil Chem. Soc.41:496.

Kilgore, L. and Bailey, M. 1970. Degradation of linoleic acid dur­ing potato frying. J. Amer. Diet. Assoc. 56: 130.

Kilgore, L. and Windham, F. 1973. Degradation of linoleic acidin deep-fried potatoes. J. Amer. Diet. Assoc. 63:525.

Krishnamurthy, R.G., Kawada, T. and Chang., S.S. 1965. Chemicalreactions involved in the deep fat frying of foods. I. A

J. Inst. Can. Sel. Technol. Aliment. Vo!. 16. No. 4, 1983

laboratory apparatus for frying under simulated res­taurant conditions. 1. Amer. Oil Chem. Soc. 42:878.

MacGee, 1. 1959. Enzymatic determination of polyunsaturated fattyacids. Anal. Chem. 31 :298.

Michael, W.R., Alexander, 1.C. and Artman, N.R. 1966. Thermalreactions of methyllinoleate. I. Heating conditions, isola­tion techniques, biological studies and chemical changes.Lipids 1:353.

Meltzer, 1.B., Frankel, E.N., Oessler, T.Rand Perkns, E.G. 1981.Analysis of thermally abused soybean oils for cyclicmonomers. 1. Amer. Oil Chem. Soc. 58:779.

Morrison, W.H., Robertson, 1.A. and Burdick, 0.1973. Effect ofdeep-fat frying on sunflower oils. 1. Amer. Oil Chem.Soc. 50:440.

Pearson, O. 1962. Chemical Analysis of Food. 1. and A. ChurchillLtd., London. p. 297.

Perkins, E.G. 1967. Formation of non-volatile decomposition pro­ducts in heated fats and oils. Food Tech. 21:125.

Perkins, E.G. and Taubold, R. 1978. Nutritional and metabolicstudies of noncyclic dimeric fatty acid methyl esters inthe rat. 1. Amer. Oil Chem. Soc. 55:632.

Perkins, E.G. and Van Akkeren, L.A. 1965. Heated fats, IV.Chemical changes in fats subjected to deep fat frying pro­cesses: cottonseed oil. 1. Amer. Oil. Chem. Soc. 42:782.

Can. Insr. Food Sci. Technol. J. Vol. 16. No. 4, 1983

Rao, C.N. and Rao, B.S.N. 1968. Effect of heating on fats and oilsdue to cooking. 1nd. 1. Med. Res. 56: 1732.

Sheppard, A.l., Hubbard, W.O. and Prosser, A.R. 1974. Evalua­tion of eight extraction methods and their effects upontotal fat and gas liquid chromatographic fatty acid com­position of food products. 1. Amer. Oil. Chem. Soc.51:416.

Steele, R.G.O. and Torrie, 1.H. 1960. Principles and Proceduresof Statistics. McGraw Hill Book Company, New York,NY

Thompson, 1.A., Paulrose, M.M., Reddy, B.R., Krishnamurthy,R.G. and Chang, S.S. 1967. A limited survey of fats andoils commercially used for deep fat frying. Food Tech.1:87A.

Waltking, A.E. and Zmachinski, H. 1970. Fatty acid methodologyfor heated oils. 1. Amer. Oil. Chem, Soc. 47:530.

Waltking, A.E., Seery, W.E. and Bleffert, G.W. 1975. Chemicalanalysis of polymerization products in abused fats andoils. 1. Amer. Oil. Chem. Soc. 52:96.

Accepted luly 27, 1983

Thompson and Aust / 253