PHYSIO-CHEMICAL CHANGES DURING REPEATED FRYING OF COOKED OIL: A REVIEW

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Today, fried foods are very famous everywhere around the world and it can be observed by the increasing number of fast food restaurants in the last few decades. Deep frying of foods at high temperature enhances the sensorial properties which include
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  PHYSIO-CHEMICAL CHANGES DURING REPEATED FRYING OFCOOKED OIL: A REVIEW PRAKASH KUMAR NAYAK 1,4,5 , UMA DASH 2 , KALPANA RAYAGURU 3 and KEASVAN RADHA KRISHNAN 1 1 Department of Food Processing Technology, Central Institute of Technology, Kokrajhar, Assam 783370, India 2 Department of Chemistry, Rajendra (Auto.) College, Balangir, Odisha, India 3 Department of Agricultural Processing and Food Engineering, CAET, OUAT, Bhubaneswar, Odisha, India 4 Center for Food Sc. & Technology, Sambalpur University, Odisha, India 5 Corresponding author.TEL:  + 91-98599-11865;FAX:  + 91-03661-277143;EMAIL: pk.nayak@cit.ac.inReceived for Publication April 19, 2015Accepted for Publication October 1, 2015doi:10.1111/jfbc.12215 ABSTRACT Today, fried foods are very famous everywhere around the world and it can beobserved by the increasing number of fast food restaurants in the last few decades.Deep frying of foods at high temperature enhances the sensorial properties whichinclude the unique fried flavor, golden brown color and crispy texture. Chemicalreactions like oxidation, polymerization, hydrolysis, etc., take place in the foodsystem, which ultimately alters the physical and chemical properties of fat. Conse-quently, so many by-products such as free fatty acids, alcohols, cyclic compounds,dimers and polymers are produced. Therefore, it is necessary to understand thephysical and chemical changes during deep fat frying to monitor the quality of fried foods. In this review, we constituted the previous studies on the changes infats during frying and methods used to analyze the quality of fried oil, in order toexplore the areas which require further research. PRACTICAL APPLICATIONS Deep frying is the most common and one of the oldest methods of food prepara-tion worldwide. To reduce the expenses, the oil tends to be used repeatedly forfrying. Repeated use of this oil has become a common practice due to low level of awareness among the public about the bad effect of this practice. Nowadays, theconsumption of deep-fried food has gained popularity which may cause increasedrisk of obesity. When heated repeatedly, changes in physical appearance of the oilwill occur such as increased viscosity and darkening in color, which may alter thefatty acid composition of the oil. In this review, we have investigated the detailmechanism of decay of oil and parameters to be measured to know the quality and safety of oil. INTRODUCTION Edible oil is a vital module of food; it provides energy,essential fatty acid and serves as a carrier of fat-soluble vita-mins. India is the world’s largest importer and the thirdlargest consumer of edible oil (Choudhary and Grover2013). In India, the annual and per capita consumption of edible oil were 11 million tons and 11.5 kg, respectively, inthe year 2007. (Data information: Ministry of Agriculture,Govt. of India). Oil is widely used for the trendiest gastro-nomic process called frying, both in industrial and domes-tic food preparations, and it increases the consumption of fatty matters (Gertz  et al  . 2000). Deep fat frying involvessimultaneous heat and mass transfers in food processingoperation by immersing the food as a whole into the hot oilaround or more than 180C temperature (Hubbard andFarkas 1999; Debnath  et al  . 2003, 2009). During theprocess, a reverse transfer of water vapor from food to theoil and finally to the atmosphere occurs (Farid 2001). Inthis process, various chemical reactions such as thermaloxidation, polymerization and hydrolysis take place. Thesereactions produce insoluble, nonvolatile matters whichincrease its viscosity, darken the color, increase the foamingand decrease the smoke point (Kalogeropoulos  et al  . 2007).As a whole, deep fat frying deteriorates the quality of oil(Table 1). Most streets vendors reuse the deep fried oil for Journal of Food Biochemistry ISSN 1745-4514 1 Journal of Food Biochemistry  ••  (2015) ••–•• © 2015 Wiley Periodicals, Inc.   Journal of Food Biochemistry  40  (2016) 371–390 V C 2015 Wiley Periodicals, Inc.  371 Journal of Food Biochemistry ISSN 1745–4514  further cooking purposes without discarding it. Azman et al  . (2012) have determined the level of knowledge, atti-tude and practice of night market food outlet operators inKuala Lumpur, Malaysia regarding the usage of repeatedly heated cooking oil. They have collected data from therespondents ( n  =  100) by face-to-face interview andobserved that majority of respondents had only moderate(53.0%) or low (18.0%) level of knowledge regarding thisissue. Most respondents (67.0%) agreed that it is not agood practice. The majority (69.0%) agreed that the usageof repeatedly heated cooking oil is detrimental to health.Despite that, most respondents (63.0%) admitted that they had used cooking oil repeatedly. Most (62.0%) of thecooking oil samples taken from the night market foodoutlets were considered fit for human consumption.Saxena 2014 conducted a study jointly with the NationalDiabetes, Obesity and Cholesterol (N-DOC) Foundation,IIT-Delhi and Diabetes Foundation of India (DFI) that hasrevealed that reuse of oil in home-cooked food forms aheavy dose of trans-fatty acids (TFA). A random samplesurvey of 402 women between the age group of 25 and 60across Delhi was conducted by a team of researchers tomonitor the pattern of usage of oil while cooking. Besideshighlighting the harmful effects of TFA, the study has alsoraised some serious concern over the cooking practice inmany households. Nearly 43.3% of the women reuse the fator oil (used for frying) at least two to three times forcooking. Also, 54% of the women left the used oil afterfrying in the same utensil in which the frying was done, andmerely 6% of women drained off the used oil beforestoring.This imparts negative effects on the health condition of consumers. The process of oxidation in the human body isassumed to be a causal factor of certain diseases such as car-diovascular, cancer, early aging, or cataract. Furthermore,discarding the deteriorated oil to the environment enhancespollution.The presence of air and water further accelerates the dete-rioration of frying oil (Chatzilazarou  et al  . 2006). It alsodepends on the nature of the fried material, the frying oil,time, temperature, intermittent continuous heating, freshoil complement, fryer model and use of filters (Lalas  et al  .2006; Mariana  et al  . 2014). Different factors such as fatty acid composition of oil, product quality, frying time, tem-perature, heating type, composition of frying oil, composi-tion of fried food, fryer type, antioxidants and oxygenavailability affect the deprivation of oil.Several studies have been carried out on degradation of oil, but a systematic study and the effect of oil deteriorationhave not been studied yet. This review is an approachtowards the investigation of the detail mechanism of decay of oil and parameters to be measured to determine thequality index of oil. Phenomenon Responsible for QualityChanges during Deep Fat Frying Phenomenal changes involve increase in the concentrationof free fatty acids, as well as polar materials, polymeric com-pounds and decrease in the unsaturated fatty acid composi-tion. (Kassama 2003). It mainly depends on three types of chemical reactions such as hydrolysis, oxidation and polym-erization. Oil becomes more stable if it contains lower levelsof linoleic, linolenic acid and higher levels of oleic acid(Aladedunye and Przybylski 2013). Decomposition of linolic acid produces 2,4-decadienal which is responsible fordeep-fried flavor. Hydrolysis It involves the breakage of triglycerides in the presenceof water and steam (Coultate 1989). It producesmonoglycerides, diglycerides, free fatty acids and glycerol,eventually (Fig. 1). The extent of hydrolysis depends on theoil temperature, interface area between oil and the aqueousphase, amounts of water and steam. Free fatty acids andlower molecular weight acidic products arising from fat oxi-dation enhance the hydrolysis process in presence of steam TABLE 1.  QUALITY PARAMETERS OF COOKING OILOil TypeFFA(AcidValue)PV(meq./kg)IV(g/100g)Viscosity(mPA s)at 40CSaponificationValueSoybean 1.15 10 139 33 195Sunflower 1.21 6.6 134 35 193Mustard 1.5 20 125 48 184Palm 1.75 3.18 45 29 202Olive 6.6 10 94 40 196 FIG. 1.  FORMATION OF FATTY ACIDS AND DIACYL GLYCEROLS(AOCS LIPID LIBRARY, OILS & FATS, FORMATION OF NEWCOMPOUNDS DURING FRYING – GENERAL OBSERVATIONS, 2009) DEEP FRYING OF OIL  P.K. NAYAK  ET AL . 2  Journal of Food Biochemistry  ••  (2015) ••–•• © 2015 Wiley Periodicals, Inc.   DEEP FRYING OF OIL  P.K. NAYAK  ET AL. 372  Journal of Food Biochemistry  40  (2016) 371–390 V C 2015 Wiley Periodicals, Inc.  during frying (Pokorny 1989). The extent of hydrolysis canbe determined by the saponification value of oil. Oxidation Three types of oxidation take place in deep-fried oil such asautoxidation, thermal oxidation and photosensitized oxida-tion. The oxidation by direct combination with oxygen atordinary temperature is called autoxidation. It results in therancidity of oil, which causes unpalatable odor and flavordue to the oxidative or hydrolytic degradation of oil.(Shukla and Bhattacharya 2004). Oxidative rancidity involves oxygen attack of glycerides and occurs in all kindsof unsaturated fats (Sangle and Daptare 2014). Thermaloxidation occurs due to heating at high temperature morethan 180C (Marinova  et al  . 2012). The rate of thermal oxi-dation is faster than the autoxidation (Choe and Min 2007).All the above said oxidation processes follow a mechanismknown as free radical mechanism (Fig. 2). It mainly involvesthree steps: chain initiation, propagation and termination.In chain initiation, the reactions goi ahead by heat, lightand metal catalyst with the formation of alkyl free radical(R) (Halliwell and Gutteridge 1984).In propagation, the radical formed in the first step reactswith oxygen to form the peroxy radical (ROO) (Halliwelland Gutteridge 1984). The levels of peroxy radicals arehigher than lipid free radicals. Peroxy radical abstractshydrogen from another lipid molecule and forms lipidradical (R   +  ROOH). The hydroperoxides are very unstableand gives rise to short chain compounds by the cleavage of O-O, C-C and C-O bonds around the peroxide group todecompose into short chain compounds. The radicals cata-lyze the reactions while the formation and decomposition of peroxide radical takes place in a reversible way.Termination involves the formation of final products likenonpolar dimer, oxidized monomer, oligomer, alcohols,aldehydes, ketones, acids, lactones, etc. Silvagni  et al  . 2010have explained formation of aldehyde directly from peroxylradical through an independent pathway during the thermaltreatment of peanut oil at 180C and it can be qualitatively analyzed by gas chromatography (Marmesat  et al  . 2008).Frying temperature, the number of frying process, the con-tents of free fatty acids, polyvalent metals and unsaturatedfatty acids of oil decreases the oxidative stability and flavorquality of oil. While antioxidants play an important role indecreasing the frying oil oxidation, the effectiveness of anti-oxidant decreases with elevated frying temperature.Oxidation is one of the most important deteriorativechanges that occur in deep-fried oil and it comprises of fivemain stages, namely induction period, peroxide formation,peroxide decomposition, polymerization and degradation,as shown in Fig. 3.Photo-oxidation occurs when normal triplet oxygen areconverted to a singlet oxygen by the exposure of ultraviolet(UV) radiation. The singlet oxygen interacts with polyun-saturated fatty acids to form hydroxyl peroxide which initi-ate the autoxidation reaction. Change in Physical Parameters during DeepFat Frying Physical changes in oil due to deep fat frying includeincrease in color, foaming and viscosity. There are variousmethods to measure these changes. The qualitative changescan be evaluated by visual inspection. FIG. 2.  SIMPLIFIED SCHEME OF THERMALOXIDATION (AOCS LIPID LIBRARY, OILS &FATS, FORMATION OF NEW COMPOUNDSDURING FRYING – GENERAL OBSERVATIONS,2009)P.K. NAYAK  ET AL .  DEEP FRYING OF OIL 3 Journal of Food Biochemistry  ••  (2015) ••–•• © 2015 Wiley Periodicals, Inc.   P.K. NAYAK  ET AL.  DEEP FRYING OF OIL Journal of Food Biochemistry  40  (2016) 371–390 V C 2015 Wiley Periodicals, Inc.  373  Color Color is a prejudiced indicator used by the food industry for rapid monitoring of frying oil quality. Darkening of oil color occurs due to the development of pigments(nonvolatile decomposition products [NVDPs] and  α -, β -unsaturated carbonyl compounds) during oxidation andthermal decomposition of fatty acids which diffuses into theoil during frying, although due to traces of carotenoidsavailable in oil, Maskan 2003 has also explained that oildarkening may be caused by caramelized scorched product,which accelerates the reduction of lightness value ( L -value)in oil when assessed by the Hunter colorimeter (Ketaona et al  . 2013). In the studies conducted by Tarmizi  et al  .(2013), the darkened frying oil samples showed higherlightness ( L *), redness ( a *) and yellowness ( b *) valuesduring frying, when assessed by the Hunter colorimeter. Itindicated that the oil were darker, more reddish and yellow-ish, when compared to the color of fresh oil. Furthermore,total color difference which combines  L *,  a * and  b *valueswas found to be increased with frying time.Aladedunye and Przybylski (2009) have observed thatcolor development in oil undergoing frying was 67% higherthan frying performed under carbon dioxide blanketingprocess, but the oil processed by vacuum frying has theleast amount of color pigments when comparing to theother process. Nor  et al  . (2008) also reported that the dark-ening of oil samples was due to the oxidation of phenolicantioxidants present in the oil while heating. Debnath  et al  .(2012) have suggested that color of rice bran oil dependupon the number of frying cycle. They observed that aftersix cycles of heating and frying, red color was increasedfrom two units to three and 4.5 units during heating andfrying, respectively. The increase in yellow color wasobserved from 15 units to 20 and 25 units during heatingand frying, respectively. The increase in color of oil wasattributed due to the Maillard reaction. In preliminary comparative studies conducted by Sulieman  et al  . (2006) onthe antiradical performance and physicochemical charac-teristics of vegetable oil upon frying of French fries, thegradual increase in darkness was observed during the fryingperiod. The color value of the vegetable oil increases withincreasing the frying time. It was due to the movement of brown pigments from the fried products into the frying oil.The combination of oxidation and polymerization of unsaturated fatty acids in the frying medium was also to beblamed for the increase in the color values (Irwandi  et al  .2000). When comparing the final results of three differentsamples of vegetable oil, the authors have concluded thatthe darkening of oil color was due to increase in linoleniccontent of the oil. It was observed that the color changes inpalm olein during frying were relatively faster than theother oil. However, studies have shown that although palmolein becomes darker with having higher color value duringfrying, it will not affect the color of the fried products(Razali  et al  . 1999). The findings therefore indicated thatdarkening is considered as a useful phenomenon as it pre-vents the continual use of edible oil which has undergoneexcessive deterioration. Density Density is an important property affecting the heat transferby natural convection and buoyant movement of gasbubbles in a liquid. The density of different types of oil isdissimilar due to differences in its composition. Kalogianni et al  . (2011) have studied that the change in density of palmoil and olive oil by varying the number of cycles of fryingfrom four to 40 (Fig. 4 – density of palm oil and olive oil asfunction of frying number of cycles). During frying and FIG. 3.  FIVE MAIN STAGES IN THEOXIDATION OF OIL DURING FRYING(PERKINS 1967) DEEP FRYING OF OIL  P.K. NAYAK  ET AL . 4  Journal of Food Biochemistry  ••  (2015) ••–•• © 2015 Wiley Periodicals, Inc.   DEEP FRYING OF OIL  P.K. NAYAK  ET AL. 374  Journal of Food Biochemistry  40  (2016) 371–390 V C 2015 Wiley Periodicals, Inc.  heating of oil, reactions like oxidation, polymerization,isomerization (in both frying and heating) and hydrolysis(in frying) occur; as a result, several polymers were gener-ated. The change in palm oil density is more than olive oil,suggesting that the generation of higher polymer content inpalm oil than olive oil and change in the density was morepronounced after the 20th cycle of frying.Zahir  et al  . (2014) have observed that the density of corn and mustard oil were decreased with the rise in tem-perature as well as with using the same oil for frying of potato for three times. The densities of oil were related tothe standard value in the range of 0.898–0.907 g/mL(SON, Standard Organization of Nigeria, 2000).They alsoobserved the density values of 0.9694 and 0.9223 g/mL formustard and corn oil, respectively, at room temperature(35C). It may be due to the pi bonds which make the morerigid bonds and the more strenuous rotation between C-Cbonds. Viscosity Viscosity is one of the indicators used to evaluate the physi-cal changes in edible oil. It depends upon density, molecularweight, melting point, degree of unsaturation and tempera-ture (Sharova and Ramadan 2012).Viscosity increasesduring hydrogenation as the increase in the chain length of tri-glyceride fatty acid and decreases during unsaturation of fatty acids (Santos  et al  . 2004). Further polymerization of oil and formation of dimmer, polymer products withmolecular weight between 690 and 1,600 Da increases theviscosity of oil (Choe and Min 2007). Oil rich in linoleicacid are more easily polymerized during deep fat fryingthan the oil rich in oleic acid (Formo  et al  . 1979; Takeoka et al  . 1997; Valdes and Garcia 2006). Santos  et al  . (2005)have observed that heating of oil (190C) for several hourswill not change the Newtonian behavior of several types of oil including olive.The different arrangement of fatty acids on glycerol back-bone of tri-glyceride molecules changes the viscosity of oil.Zahir  et al  . (2014) have studied that at 35C viscosity of mustard oil is higher than corn oil due to the degree of unsaturation of oil. Sharova and Ramadan (2012) haveinvestigated the effect of unsaturation upon the viscosity of sunflower oil (SO), cottonseed oil (CO) and palm oil (PO)and found that the major fatty acids C18:1 and C18:2appeared to make a great contribution towards the flowingbehavior. The viscosity of oil increases with the increase inthe level of C18:2 and decreases the level of C18:1 fatty acids(Fig. 5 – relation between the levels of C18:2 and viscosity in different oil and Fig. 6 – relation between the levels of C18:1 and viscosity in different oil).The presence of double bond does not allow fatty acidmolecules to stay closely together; therefore, they will inter-fere with packing in the crystalline state (Abramovic andKlofutar 1998; Aravindan  et al  . 2007). Thus, fatty acids withhigher number of double bonds will not be rigid andbehave like fluids when they are packed loosely (Kim  et al  .2010).Viscosity is strongly affected when exposed to highertemperatures, air and increase in the number of fryingcycles, and it enhances the formation of oxidative and poly-meric compounds and increases the tendency to foamduring frying (Samah and Fyka 2002). Tarmizi  et al  . (2013)have studied that the viscosity of oil increased significantly in the case of vacuum and atmospheric drainage after 2 daysof frying from 45.48 to 54.12 and 55.97 mPa·s, respectively (Fig. 7). After 80 batches of frying, polar compoundsincreased from 7.81 to 15.03% and 17.19 % in the case of vacuum drainage and atmospheric drainage, respectively.This is due to the formation of higher molecular weightpolymeric compounds upon increase in frying time.The similar effect was observed by Debnath  et al  . (2012).They observed the kinematic viscosity of rice bran oil as3.39  ×  10 − 6 m 2 /s. This value was increased by increasing the FIG. 4.  DENSITY OF PALM OIL AND OLIVEOIL AS FUNCTION OF FRYING NUMBER OFCYCLES (N). (KALOGIANNI  ET AL . 2011)P.K. NAYAK  ET AL .  DEEP FRYING OF OIL 5 Journal of Food Biochemistry  ••  (2015) ••–•• © 2015 Wiley Periodicals, Inc.   P.K. NAYAK  ET AL.  DEEP FRYING OF OIL Journal of Food Biochemistry  40  (2016) 371–390 V C 2015 Wiley Periodicals, Inc.  375
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