Thermal decomposition of fats & oil

1. THERMAL DECOMPOSITION OF FATS & OIL AND FLAVOUR REVERSION
DEPARTMENT OF FOOD ENGINEERING & TECHNOLOGY
Sant Longowal Institute of Engineering & Technology
Longowal,Sangrur-148106 (Punjab)
ANKIT KUMAR
ankitvatsya@gmail.com
+91-8699028994

2. INTRODUCTION:-
 Fats and oils belong to group of biological substance called lipids.
 Lipids are biological chemical that do not dissolve in water.
 Fats are solid at room temperature whereas oil is liquid at Room temperature
.
 Basically oil and fats are mixture of triglycerides.
 When 3 fatty acids are combined with one glycerol molecule ,then it will
called triglyceride.
 Saturated fatty acids have all predominantly single bonds whereas
unsaturated fatty acids have double bond.
 Saturated fatty acids are generally Solid at room temperature whereas
unsaturated fatty acids are liquid at room temperature.

3. THERMAL DECOMPOSITION:-
 Thermal decomposition, or thermolysis, is a chemical decomposition caused
by heat.
 The decomposition temperature of a substance is the temperature at which
the substance chemically decomposes.
 A reaction in which a substance is broken down into 2 or more substances
by heat.
 An example is the breakdown of calcium carbonate into carbon dioxide
and calcium oxide.
 The reaction is usually endothermic as heat is required to break chemical
bonds in the compound undergoing decomposition.
 An example is when baking soda (sodium bicarbonate) is heated-
2NaHCO3(s)→CO2(g)+H2O(g)+Na2CO3(s)

4. THERMAL DECOMPOSITION OF FATS & OIL:-
 High temperature used in frying causes lipids to undergo a variety of chemical and
physical changes due to thermal decomposition.
 During deep-frying, the fat/oil is continuously exposed to elevated temperatures in
the presence of air and undergoes hydrolysis, oxidation and polymerisation, resulting
in deterioration in quality, thereby causing changes in not only the sensory but also
the nutritional properties.
 Frying is also considered as a contributory factor leading to trans fatty acid (TFA)
formation, which is believed to be a product of partial hydrogenation.
 The chemical mechanism of thermal oxidation is principally the same as the
autoxidation mechanism, but the thermal oxidation rate is faster than the
autoxidation rate.
 the temperature dependence was investigated for the development of volatile
lipid oxidation products in in vegetable oils.
 From a nutritional point of view, it should be taken into account that oils with high
amounts of saturated fatty acids and fats containing trans fatty acids are less
desirable for good health

5. Cont….
 Deep-fat frying decreases the unsaturated fatty acids and increases polar
material.
 Many methods have been used for determination of fat deterioration during
frying. They include methods for assessment of peroxide value, iodine value
dienes, fatty acids, polar components etc.
 For the quality control of frying fats or oils the determination of total polar
parts is an approved standard method.
 Heating/frying also increased the saturated fatty acids and decreased cis-
unsaturated fatty acids.
 Metabolic studies have shown that TFAs increase low-density lipoprotein
cholesterol (LDL-c) and decrease high-density lipoprotein cholesterol (HDL-c)
levels, increasing the risk of cardiovascular diseases.
 Furthermore, there is a growing concern that the risk for type 2 diabetes also
increases with TFA consumption

6. Chemical changes involved in thermal degradation of oils:-
1.HYDROLYSIS:-
))Hydrolysis is believed to be the first reaction generating detectable products
during frying, which causes major production of free fatty acids.
)) Depending on the extent of hydrolysis, products are fatty acids and,
progressively, diacylglycerols, monoacylglycerols, and glycerol .
))Chromatographic analysis of frying oils with high levels of free fatty acids
demonstrated that formation of monoacylglycerols is minimal.
))Although hydrolysis is one of the simplest reactions, previous research has
produced.
)) Some authors found hydrolysis to be the most important reaction during frying,
based on detection of high levels of free fatty acids and diacylglycerols , while
others found fatty acids to be minimal compared to other degradation products,
despite high water contents in the ood being fried.

7. Cont…
2.oxidation:-
)) Because the oxygen present in deep-frying reacts with oils, thermal oxidation has
been proposed to arise from the same reactions as in lipid autoxidation, but
accelerated by Arrhenius kinetics where the rate is doubledfor every 10 degrees
increase in temperature.
)) In the initiation stage, an alkyl radical (R ) is formed.
))In the propagation stage, oxygen adds to the alkyl radical to form peroxyl radicals
. these radicals, in turn, abstract hydrogens from other molecules to yield
hydroperoxides(ROOH) and a new radicals that react with oxygen to start a new
cycle and propagate the reaction chain. The hydroperoxide intermediates are not
stable, but are decomposed by heat to yield reactive hydroxyl radicals (HO ) plus
alkoxyl radicals that both abstract H’s to propagate and accelerate the oxidation
chain and undergo reactions to produce varied products.
)) Reactions are terminated when radicals react with each other to produce non-
radical compounds, either volatile or non-volatile, or when alkoxyl radical

8. Cont…..
3.polymerization:-
)) All commonly used fats and particularly those high in polyunsaturated fatty
acids tend to form larger molecules (known broadly as polymers) when heated
under extreme conditions of temperature and time. Under normal processing and
cooking conditions, polymers are formed. Although the polymerization process is
not completely understood, it is believed that polymers in fats and oils arise by
formation of either carbon-to-carbon bonds or oxygen bridges between
molecules. When an appreciable amount of polymer is present, there is a marked
increase in viscosity.
)) polymerizations are particularly facilitated by high temperatures. Dimerization
and polymerization occurs in three ways: 1) radical recombination,2) addition of
radicals to double bonds, and 3) Diels-Alder reactions, a reaction between a
double bond and a conjugated diene .

9. Factors affecting frying oil quality :-
1. Fatty acid (FA) composition :-
)) The common sense notion that oils composed of polyunsaturated fatty acids
should have lower frying stability than those composed of monounsaturated and
saturated fatty acids has been supported by many studies .
)) Isomers geometry also affect oxidation by altering accessibility of H atoms,
with cis fatty acids oxidize more readily than trans fatty acids.
2.FRYING CONDITION:-
)) Long time frying results in accumulation of a wide range ofoxidation products
including free fatty acids , polar compounds, dimers and polymers , most at high
levels.
)) Intermittent frying generally causesmore serious degradation than continuous
frying due to increased oxygen solubilization of oxygen in the oil during the
cooling period

10. CONT....
3.OXYGEN:-
)) Effects of oxygen on frying quality of oils have been studied for decades.
Ratesof change in viscosity and titratableacidity were found to be directly
proportional to the degree of exposure of fat surface to oxygen
4.WATER:-
)) Foods complicate the frying process in several ways. One of the most
important aspects is continuousrelease of water from food into oil. The released
moisture agitates the oil and promotes hydrolysis.
)) Moisture also catalyzed production of off-flavors in different kinds of frying
oils [60]. However, adding lower amounts of water (e.g. 0.7%) was protective.
This protection was attributed

11. TRANS FATTY ACIDS:-
 Trans fatty acids are fatty acids that contain at least one double bond in
the trans configuration.
 Dietary trans fatty acids occur naturally in meat and dairy products as a result
of anaerobic bacterial fermentation in ruminant animals.
 Trans fatty acids are also created when liquid vegetable or fish oil is
hydrogenated; this is frequently done to increase their plasticity and
chemical stability for subsequent food processing.
 During the hydrogenation process, some of the naturally occurring cis double
bonds in the vegetable oil fatty acids are changed to trans double bonds, so
that the resulting trans fatty acids resemble saturated fats more than
unsaturated fats.
 Trans fatty acids are isomers of the normal cis fatty acids, produced when
PUFAs are hydrogenated, such as in the production of margarine and
vegetable shortening.
 Trans fatty acids are manufactured fats created during a process called
hydrogenation, which is aimed at stabilizing polyunsaturated oils to prevent
them from becoming rancid and to keep them solid at room temperature.

12. Why do some companies use trans fats?
))Trans fats are easy to use, inexpensive to produce and last a long
time. Trans fats give foods a desirable taste and texture. Many restaurants and
fast-food outlets use trans fats to deep-fry foods because oils with trans fats can
be used many times in commercial fryers. Several countries (e.g., Denmark,
Switzerland, and Canada) and jurisdictions (California, New York City, Baltimore,
and Montgomery County, MD) have reduced or restricted the use of trans fats in
food service establishment.
)) Trans fats can be found in many foods – including fried foods like doughnuts,
and baked goods including cakes, pie crusts, biscuits, frozen pizza, cookies,
crackers, and stick margarines and other spreads.

13. HEALTH RISK:-
 The primary health risk identified for trans fat consumption is an elevated
risk of coronary artery disease.
 There is a growing concern that the risk of type 2 diabetes increases with
trans fat consumption.
 Research indicates that trans fat may increase weight gain and abdominal
fat, despite a similar caloric intake.(Obesity)
 Each 2% increase in the intake of energy from trans unsaturated fats, as
opposed to that from carbohydrates, was associated with a 73% greater risk of
ovulatory infertility in women.
 Intake of both trans fats and saturated fats promote the development of
Alzheimer disease.

14. FLAVOUR REVERSION IN FATS & OIL
 FlAVOR reversion in fats is probably defined most satisfactorily as the
appearance of objectionable flavor from less oxidation than is required to
produce true rancidity.
 The degree to which reversion and rancidity are separated in terms of oxygen
absorbed by the fat is widely different for different fats.
 There are some fats,however, including particularly fish oils and vegetable
oils containing linolenic acid, which revert with extremely slight oxidation.
 It is almost impossible to prepare edible products from such fats and get
them to the consumer before they have suffered some loss of palatability.