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OXIDATION OF COOKING OILS DUE TO
REPEATED FRYING AND HUMAN HEALTH
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,
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 cholesterol—by oxidizing it.
II. CAUSE OF REPEATED HEATING OF THE COOKING OIL
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  and many a times it is never replaced at all, instead fresh oil is added
to already heated ,thick and highly viscous oil. 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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III. CHANGES DURING THE REHEATING OF THE OIL
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; 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
 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 170-180⁰ 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.
VI. THE PEROXIDE VALUE
Peroxide values of the heated oils were determined according to American Oil Chemists’ Society (AOCS)
Official Methods Cd 8–53 .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)
VII. RESULTS AND DISCUSSION
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. 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..
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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
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. 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.
VIII. EFFECT ON HUMAN HEALTH
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. 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 . 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.
IX. METHODS SUGGESTED TO REDUCE THE HARMFUL EFFECTS
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 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. 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  that the addition of antioxidant in
cooking oil determine the stability of oxidation during frying; and according to Tuba and , curcumin is
effective to be used as antioxidant because it can scavenge free radical by donating H atom from phenolic as its
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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INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 3, ISSUE
12, December 2014