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Food becomes tastier on deep frying, and some of food items can only be eaten, once are deep fried. Reusing this deep fried oil repeatedly for frying purposes is responsible for many health hazards in human population. Increased viscosity and darkening in colour are some of the physical changes which can alter the fatty acid composition of the cooking oil on repeated frying.Oxidation, hydrolysis and thermal polymerization are the chemical reactions occurring as a result of repeated heating of cooking oil for low and cheap food production. In this paper the degradation of the quality of cooking oil on repeated frying, its effects on human health and some ways to treat this cooking oil so as to make it more cost effective and less dangerous for human consumption are undertaken with special reference to Rajasthan.
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GarimaGoswami1, Rajni Bora2, Mahipat Singh Rathore3
1Assistant Professor (Chemistry), 2Assistant Professor (HSS), 3 ECE Department,
Department of Applied Sciences, JIET Universe, NH-62,Mogra, Pali Road, JODHPUR,
Rajasthan, (INDIA)
Food becomes tastier on deep frying, and some of food items can only be eaten, once are deep fried. Reusing
this deep fried oil repeatedly for frying purposes is responsible for many health hazards in human population.
Increased viscosity and darkening in colour are some of the physical changes which can alter the fatty acid
composition of the cooking oil on repeated frying.Oxidation, hydrolysis and thermal polymerization are the
chemical reactions occurring as a result of repeated heating of cooking oil for low and cheap food production.
In this paper the degradation of the quality of cooking oil on repeated frying, its effects on human health and
some ways to treat this cooking oil so as to make it more cost effective and less dangerous for human
consumption are undertaken with special reference to Rajasthan.
Keywords: Adsorbents, Deep Fried, Eatable Oil, Oxidation, Reheating
The biggest state of India called Rajasthan is dry and sandy area. Thardesertcovers a large portion of this region
Scarcity of water and fresh green vegetables have their effect on the Rajasthan cuisine. People are in a habit of
eating all sort of non-green food including much of fried food. Frying means cooking the stuff at app. 180°C by
dipping it fully in the heated oil. Various food products such as potato chips, pakodas, namkeens, chicken
patties,etc. are prepared at large scale by this frying process. Fried food is considered to be tastier than non-
fried.Beingunpropitious area cooking oil used for frying is heated repeatedly.Anytimeone cook food, it runs the
risk of creating heat-induced damage. The vegetable oils one choose to cook with, must be stable enough to
resist chemical changes when heated to high temperatures, otherwise one hasthe risk of damaging
one’sownhealth as well as the health of the person consuming the cooked food. One of the ways vegetable oils
can inflict damage is by converting our good cholesterol into bad cholesterolby oxidizing it.
In this fast-paced society, frying remains as one of the popular and tastier methods in food preparation. In
developing countries consumption of ready-made food is in much demand. And if it is deep-fried then its
demand becomes even higher.Frying improves the sensory quality of food by formation of compounds rich in
aroma, attractive colour, crust and texture, all highly appreciated by the consumers.Edible vegetable oil is the
major ingredient in these fried food products. Therefore, the cost of the oil becomes the most important factor to
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be considered in terms of economy. As a result, vegetable oil is often heated repeatedly to ensure cost
effectiveness. Rajasthan is not only famous for sweets but also is well known for friednamkeen items. For
cutting down the cost of fried food items and to save money, many of the household ladies and maximum of the
Namkeen makers in Rajasthan use the same cooking oil repeatedly for frying different items. The oil is reused
repeatedly and it is discarded and replaced with fresh oil, only when it becomes foamy, highly viscous, emits
bad odour and become dark coloured [1] and many a times it is never replaced at all, instead fresh oil is added
to already heated ,thick and highly viscous oil[2]. A survey was conducted in this regard clearly supports the
above statement.100 different people including household ladies and namkeen makers in the study area were
surveyed about the oil used in frying , number of times it is reheated and ultimately its fate . The results of the
survey were represented in the form of GRAPHS as given below:
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Although eating limited quantity of fried food items does not cause any kind of health problem in normal human
being. The problem begins when the same oil is used again and again by repeated heating. During deep-frying
of food at temperatures between 170° 200°C, the oil used undergo following changes: 1. Hydrolysis
Moisture from the food being fried vaporizes and hydrolyses triglycerides (TGs) in the frying oil to glycerol,
free fatty acids (FFA s ) , monoglycerides (MGs) and diglycerides (DGs); 2. Oxidation Triglyceride
molecules in the frying oil undergo primary oxidation to unstable lipid species called ―hydro peroxides‖ which
cleave to form secondary oxidation products which comprise non-volatile and volatile compounds. Some of
these secondary products polymerize ( t e r t i a r y o x i d a t i o n ), increasing the oil viscosity, cause browning
on the surface, and darken the oil[3]; and3. Thermal Polymerization High temperatures of the frying operation
produce high molecular cyclic fatty acid (FA) monomers, and TG dimers and oligomers[4,5,6]. Fried food may
absorb many oxidative products such as hydro peroxide and aldehydes, which are produced during this process
[7] thus affecting the quality of oil. The quality of oil deteriorates with increased length of frying time due to the
accelerated formation of oxidized and polymerized lipid species in the frying medium. If the physico-chemical
properties of cooking oil deteriorate, the oil must be discarded because it can prove to be harmful for human
consumption. The rate of formation of cooking oil decomposition products depends on the type of food being
fried, the type of oil used and the design of the fryer ,etc. The reactions in oil rich frying depend on factors such
as replenishment of fresh oil, frying conditions, original quality of frying oil, food materials, type of fryer,
antioxidants, and oxygen concentration. Antioxidant decreases the frying oil oxidation, but the effectiveness of
antioxidant decreases with high frying temperature.
Olive oil, Coconut Oil, Sunflower Oil, Groundnut Oil, SoybeanOil, MustardOil and Palm Oil were purchased
from the local Grocery Store. Potatoes were purchased from thehawkers in the local market.
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One- fourthkg of potato were taken , sliced, air dried for4-5 hours and then fried in app. 3 litre of each of the
above mentionedcooking oils separately, which were heated to 170-180 C. Frying was carried out in a stainless
steel pan. After the frying was completed,once heated oil was obtained. This sample was then cooled overnight
and again fresh potatoes were taken, sliced, air dried and then fried.The leftover oil was again cooled overnight.
The same process was repeated for 4 times to get5 times reheated oil.Small amount of oil each time was taken
for analysis (viscosity and peroxide determination). The sample quantity was proportionately adjusted with the
amount of cooking oil left. No fresh oil was added in between the frying processes to make up for the loss due to
uptake by the frying materials. A comparative study of the physiochemical changes of the different oils
including viscosity change and peroxide value was studied. Viscosity is a fluid's resistance to flow. Fluids resist
the motion of layers with differing velocities within them. Viscosity can be measured by using Redwood
viscometer 1 and 2.Redwood viscometer 1 is used to findout the viscosity of light oil, whereas viscometer is
used to measure the viscosity of heavy oils. In this analysis, Redwood viscometer no.1(Aditya make) is used.
An oil of different frying was taken and their viscosities were measured at 30°C. Results are listed in Table1.
Peroxide values of the heated oils were determined according to American Oil Chemists’ Society (AOCS)
Official Methods Cd 853 [8].In this 5 g of the oil samplewas taken into a 250 mL conical flask then 30 mL of
acetic acid-chloroform (3:2)was added in it. The flask was swirled and then 0.5 mL of saturated potassium
iodide was added. Then, the solution was mixed again for 1 minute and few drops of starch solution (10%) were
added. The solution was titrated against previously standardized 0.01 N sodium thiosulphate solution (Na2S2O3),
until the blue colour disappeared. The peroxide value was expressed in miliequivalents of peroxide per kg of the
sample calculated as:
Peroxide value (meq/kg) = [(Va-Vb) N × 1000]/W
Va = volume of sodium thiosulphate solution (mL)
Vb = volume of sodium thiosulphate solution (mL) used for the blank
N = normality of sodium thiosulphate
W = weight of the test portion (g)
Several chemical and physical processes took place ,when the oil was used for frying, namely: i) the fried food
being absorbs oil as well as releases some of its own lipid content (sometimes colored) into the frying medium,
ii) food particles were charred and due to lipid browning the oil darkens[9]. During frying the potentially
hazardous non-volatile polar compounds which are formed as secondary oxidation products- like epoxides,
polar dimmers, oxidized polymers, ketones and aldehydes ,and hydrolysis products of triglycerides such as free
fatty acids, monoglycerides and diglycerides.[4].
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The results are summarized in the table1given below:
Table 1 : Viscosities and peroxide values of different oils with different no. of frying
No. of fryings
Sunflower Oil
Coconut Oil
Groundnut Oil
Mustard Oil
Palm Oil
Soyabean Oil
Value (meq.
/Kg oil)
Value (meq.
/Kg oil)
Value (meq.
/Kg oil)
Value (meq.
/Kg oil)
Value (meq.
/Kg oil)
Value (meq.
/Kg oil)
1 frying
2 frying
3 frying
4 frying
5 frying
According to the compiled studies, in order to preserve bioactive components of the edible oils,heating time
should be reduced to the minimum. It was shown in the present study that the peroxide values were increased
with the increasing frequency of heating in all types of oil. Increased values indicate increased lipid peroxidation
by-products content, mainly the peroxides that were formed in the oil during heating process[10]. The extent of
oxidation in the oils was affected by the number of frying. Also the viscosity of the oil increased with the
increasing number of fryings.
Frying makes the eatables tasty. Frying of edible items includes the usage of large amount of oil. Increased oil
consumption is not considered to be good for human health[4]. Even though a certain amount of potentially
toxic products are produced during frying (such as polar compounds or polymers), fried foods are generally
considered safe [11,12,13]. It is only when frying oil is used repeatedly that it becomes toxic for human
consumption [14]. Repeated heating of the oil accelerates oxidative degradation of lipids, forming hazardous
reactive oxygen species and depleting the natural antioxidant contents of the cooking oil. Long-term ingestion of
food prepared using reheated oil could severely compromise one’s antioxidant defence network, leading to
pathologies such as hypertension, diabetes and vascular inflammation[15,16,17]. Lipid oxidation causes a high
risk for the development of coronary heart diseases. The human body is constantly subjected to a significant
oxidative stress as a result of the misbalance between ant oxidative protective systems and the formation of
strong oxidizing substances, including free radicals. This stress can damage DNA, proteins, lipids and
carbohydrates and could cause negative effect to intracellular signal transmission.
The oil can be made safe for human consumption and the harm caused by the use of repeated heated oil can be
minimised by a number of ways.Theuse of natural antioxidants [18]in cooking oil as adsorbents can make the
oil safe by retarding the formation of oildeterioration products .Antioxidants adds Tocopherols,
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butylatedhydroxyanisole (BHA), butylatedhydroxytoluene(BHT), propyl gallate(PG), and tert-
butylhydroquinone (TBHQ) which slowdown the oxidation of oil at room temperature. However, they become
less effective at frying temperature due to losses through volatilization or decomposition (Boskou 1988; Choe
and Lee 1998).Addition of different antioxidantslike sugarcane bagasse, rosemary extract,turmeric extract,etc
during frying have been found to reduce the harmful effect of the deterioration products . Also this rancidity of
the oil can be reduced by taking blended oil or an oil mixture of different types with varying concentration of
the different oils mixed.
Turmeric Extract As An Antioxidant:Addition of Curcumin, a natural antioxidant present in Turmeric olantof
zingiberaceae family can reduce the harmful effects of oil deterioration products . There was a decrease in trans
fatty acid content in the fried food with addition of turmeric extract at the concentrations of 0.03%. This was
related to curcumin that inhibit the auto-oxidation rate by modifying the lipid radical into more stable form, that
commonly inhibit the fat oxidationreaction[9]. The presence of antioxidant added into the repeatedly used
cooking oil can decelerate the oil oxidation rate during frying and contribute to sensory acceptance of fried
foods. The findings of this study are supported by the statement of [19] that the addition of antioxidant in
cooking oil determine the stability of oxidation during frying; and according to Tuba and [20], curcumin is
effective to be used as antioxidant because it can scavenge free radical by donating H atom from phenolic as its
active group.
Apart from this many adsorbents such as sugarcane bagasse ash ,magnesol XL, etc. can also be used to reduce
the formation of deterioration products produced by repeated frying.
Above studies clearly indicate that edible oil becomes highly viscous and presence of harmful products
increases when oil is heated repeatedly. This oil now becomes dangerous for human consumption. Therefore oil
should not be heated again and again and the formation of harmful products can be minimised by discarding it
(for making soap at small scale) or using it with certain antioxidants .
Authors are acknowledged to Ms. Renu Awasthi, Kapil Shivlani,Umaid Solanki, Vivek Chamoli, Kailash
Parihar, Govind Singh for their kind support during the working of this project.
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Objectives: In preparation for deep frying, the peroxide values and refractive indices of palm, sesame, and sunflower oils were measured. The peroxide value and refractive indices of the vegetable oils used to fry white Indian potato chips in three batches were determined after each stage of deep frying. According to the findings, deep-frying significantly alters the refractive index and peroxide value of vegetable oils. Material & Method: Medical and scientific indexing sites like PubMed and Google Scholar were used to find relevant medical and scientific articles. Result & Discussion: Sesame palm and sunflower oil's refractive index and peroxide value do rise when fried, but not linearly. A rise in refractive index (RI) of palm oil and a rise in the RI of sunflower oil were observed following the frying of three consecutive batches. Sesame oil's refractive index remained essentially unchanged. After three deep fryings, the peroxide values of palm oil rose from 1.9948 mEq/kg to 9.3020 mEq/kg. Sunflower oil peroxide increased from 10.6359 mEq/kg to 19.3101 mEq/kg, while sesame oil peroxide increased from 3.9914 mEq/kg to 11.9555 mEq/kg after the second batch of Indian potato chips was fried and then decreased to 11.3095 mEq/kg after the third batch of Indian potato chips was fried in the oven.
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Frying is an important cooking process due to the unique palatability and sensory characteristics of fried foods. Fried foods contain a considerable amount of fat, and have a negative perceived image due to their high caloric value and increased consumer awareness of the relationship between food, nutrition, and health. Oil consumption and especially saturated fat, is considered as one of the principal factors increasing health risks of heart disease, cancer, diabetes, and hypertension. The mechanism of oil uptake during frying is complex and affected by numerous factors, such as product and oil composition, surface-active agents, etc. Frying oil undergoes three main deleterious reactions: oxidation, hydrolysis and thermal decomposition, resulting in the formation of numerous constituents. The latter affect the organoleptic characteristics of the fried product, and could pose health risks. Frying has a significant role in the overall nutritional value of the product. Compared to other cooking methods, retention of water-soluble vitamins and vitamin E could be higher after frying. Due to the deterioration of oil after prolonged frying, regulations on the maximum levels of polar compounds and polymer concentration have been utilized. Nevertheless, an alarming number of oil samples collected from restaurants and fast food outlets in Europe failed to comply with regulations. Frying has only a marginal affect on the concentration of trans fatty acids. Yet, due to their possible connection with heart disease, the initial concentration of hydrogenated fats (that could reach 50%) should be considered. The relationships between frying and carcinogenesis and mutagenesis are inconclusive. Cyclic fatty acid monomers, which can be formed during frying, were proven harmful only in some studies. Exposure to malondialdehyde (MDA) during typical consumption of fried food constituted no actual health hazard, although MDA is known to be a very potent mutagen and carcinogen. Heterocyclic amines, formed during fish and meat frying, were related to higher cancer incidence, but only in concentrations which were higher by several orders of magnitude than those formed in typical frying. It is concluded that based on information to date, and using good manufacturing practices, fried foods pose no significant health hazard in a balanced diet.
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The antioxidative properties of Curcuma longa (turmeric) leaf extract were evaluated in refined, bleached and deodorized (RBD) palm olein using accelerated oxidation and deep frying studies at 180°C for up to 40h. The extract was capable of retarding oil oxidation and deterioration significantly (P<0.05) at 0.2% concentration, better than 0.02% BHT for the Oxidative Stability Index (OSI) in an accelerated oxidation study and also the peroxide value in deep frying studies. In sensory evaluation, the French fries were acceptable and were not significantly different (P<0.05) from one another for color, oiliness and crispiness throughout the 40-h frying study. Curcuma longa leaf extract, which had a polyphenol content of 116.3±0.2mg/g, possessed heat-stable antioxidant properties and may be a good natural alternative to existing synthetic antioxidants in the food industry.
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World consumption of soybean (Glycine max Merr.) in 2008 was over 221 million metric tonnes, with approximately 50% of this supply coming from US production, where soybean plantings on an annual basis are over 77 million hectares. Soybeans are desired on the marketplace as a valuable source of protein and oil. The former is primarily used as feed, with some food applications, while the later is more broadly incorporated into food, feed and some industrial applications (e.g., biodiesel). Protein and oil percentages in soybean, while influenced by both genotype and environmental cues, average approximately 40% and 20%, respectively. A strong indirect phenotypic correlation exists between these traits. In addition, variation in soybean germplasm for protein content is significantly greater than that observed for total oil content. Historically soybean breeders have used total protein content as a selection criterion for germplasm development. However, recently both oil content and quality has drawn much attention in soybean genetics and breeding programs due to the increased demand for vegetable oils, and increased consumer awareness of health issues around dietary fats. To this end, significant efforts have been made to increase oxidative stability of soybean oil as a means to avoid trans fats generated through hydrogenation process, and enhance omega-3 fatty acid content of the oil for use in both food and feed applications, and increase the total oil content of seeds.
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Palm olein (PO) and red palm olein (RPO) are rich in tocopherols and tocotrienols. In addition, RPO also contains a high content of carotene. This study was to determine the effect of chronic intake of diets containing palm oils, varying in their vitamin E and carotene contents, on lipid profile in rats. Weaning male Wistar rats were fed either 18% RPO, 18% PO or 18% vitamin E-stripped palm olein (SPO) for 12 weeks. Plasma total cholesterol (TC), triglyceride (TG), high density lipoprotein cholesterol (HDL) and low density lipoprotein cholesterol (LDL) were measured at weeks 4, 8 and 12. Feeding the different types of palm oil did not affect TC and HDL from week 4 through week 12, but there were reductions in TG in all dietary groups at week 12 compared to week 4 but differences between groups were not observed. The RPO group had lower LDL at week 12 (vs weeks 4 and 8) but LDL was not reduced in the PO and SPO groups. TC/HDL was reduced in the RPO group at week 12 compared to both weeks 4 and 8, but the PO group only reduced this ratio at week 12 compared to week 4. This finding suggests that chronic feeding of diets high in palm oils did not cause any detrimental effects on blood lipid profile. In addition, red palm olein which is rich in antioxidants in the forms of vitamin E and carotene, showed better effect in terms of reduction in LDL and TC/HDL.
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An optimization study on the use of oleoresin rosemary extract, sage extract and citric acid added into refined, bleached and deodorized (RBD) palm olein before deep-fat frying of potato chips was performed using a constrained mixture design. Results revealed that the use of these natural antioxidants could improve the sensory acceptability of potato chips during a 5-day repeated deep-fat frying. All three antioxidants significantly (P<0.05) improved the sensory characteristics, including appearance, taste, crispiness, odor and overall acceptability. After day 5 of frying, the most acceptable fried potato chips were those fried in oil containing a combination of 0.059% oleoresin rosemary extract, 0.063% sage extract and 0.028% citric acid. The natural antioxidants significantly (P<0.05) lowered the rate of oxidation of oil during deep-fat frying and contributed to measured sensory acceptability of fried potato chips.
This study examined how tocopherol retention is affected by the presence or absence of food coatings, and also tested the measurement of fluorescent substance levels in cooking oil to evaluate oil deterioration. Potato slices were tempura-fried (with a coating) or french-fried (without a coating). The three tocopherol isomers decreased with heating time, and better retention was found in the tempura process. The decomposition rates of tocopherol were in the order γ> δ ≥α for the three isomers for both processes over repeated fryings. The fluorescence of frying oil increased 15-and 17-fold after tempura-and french-frying, respectively, for 32 consecutive times. Changes in the amounts of tocopherol and the fluorescence correlated well with the changes found by the chemiluminescent intensity and five conventional methods of oil quality measurement. These results indicated that tocopherol retention is affected by the food coating, and that measurements of vitamin E loss and fluorescence increase in oil should be useful for assessing the progressive deterioration of frying oil with its repeated usage.
The physico-chemical changes occurring during simulated frying conditions at 180°C for 24h in trans free speciality fat, trans rich vanaspati and PUFA rich sunflower oil were evaluated. The samples became darker, the polar components and viscosity increased as the time of heating increased. The oxidative stability as determined by peroxide, anisidine values and TOTOX number, increased, whereas the total unsaturated fatty acids and iodine value decreased with time of heating in all the samples. The trans free speciality fat was as stable as vanaspati showing similar quality parameters, while sunflower oil showed a higher degree of deterioration. The layered fat used for traditional products such as Chiroti dough consisted 14% trans fatty acids (TFA), which was reduced to 4–7%, and correspondingly 18:2 was increased in the product upon frying in sunflower oil. Accordingly, trans fatty acids increased in the medium from 0 to 7.5%. Chiroti when fried in vanaspati with TFA 18%, their content in both products (16%) and in medium (17%) remained similar. Keywords Trans fat-Vanaspati-Frying-Stability-Quality changes
The biological properties of heated fats are related to their chemical properties. This chapter examines the chemical changes that occur when fats are subjected to abusive conditions and the biological properties of the abused fats. Fats used in cooking are heated in air. Therefore, oxidative changes accompany and precede the thermal changes. Even when fats are heated with exclusion of air under laboratory conditions, it is possible that the observed thermal changes depend upon either the presence of traces of oxygen in the environment or upon the presence of fat components that have suffered oxidation before the heating. For these reasons, heating and oxidation must be considered together. The chapter discusses the fats that have been oxidized at low temperatures, the extreme conditions of heating, and of heating and oxidation.