Triacylglycercols (triglycerides) are the major component of most foods, typically making up
more than 95 to 99% of the total lipids present. Triacylglycerols are esters of three fatty
acids and a glycerol molecule. Triglycerides are too large to be volatile (and hence have
aroma) but free fatty acids, particularly the small ones, can be volatile enough to have
characteristic smells.
Unsaturated (particularly polyunsaturated) fatty acids are prone to oxidation. The significant
end products of lipid oxidation are rancid-smelling flavor aldehydes derived from the fatty
acids. Lipid oxidation follows a free radical mechanism that starts with the abstraction of a
hydrogen atom from the fatty acid. The lipid radical form (R●) rapidly reacts with oxygen to
form a peroxy radical via a free radical chain reaction. The peroxy radical (ROO●) can gain
a hydrogen atom to form a lipid hydroperoxide (ROOH) which is relatively stable and exists
in significant quantities in many natural fats. The lipid hydroperoxide has no off-flavor but
rapidly break down (particularly in the presence of heat and a metal catalyst) to form rancid
flavors
-CH = CH-CH2-CH =CH- -CH = CH-CH-CH =CH-
-CH = CH-CH-CH =CH- + O2 -CH = CH-CH-CH =CH- (hydroperoxide)
The hydroperoxide breakdown starts with the loss of a hydroxy radical (●OH) to form a
lipid alkoxy radical (RO●).
The alkoxy radical rearranges and breaks the molecule into two parts including a volatile
and rancid-smelling aldehyde. The aldehydes formed are often so odor active that even a
few ppm can lead to a food being unacceptable.
●
●
-OOH
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FSN 361 Food chemistry Laboratory 1-2558
Lipid oxidation can be inhibited by eliminating oxygen, but this is practically difficult and
even if it could be done, pre-existing lipid hydroperoxides can breakdown to rancid flavors
in the absence of additional oxygen. Instead antioxidants can be added to limit the rate of
the reaction. For example, metal chelators (e.g. EDTA and citric acid) can bind the metal
ions that catalyze the breakdown of lipid hydroperoxides while BHA and BHT are chain
breaking antioxidants that react with radical intermediates and break the chain reaction
before rancid flavors are generated.
There are many intermediates and products of the lipid oxidation reaction. The selection of
a test is somewhat complex as some products (e.g. volatile aldehydes) will form relatively
late in the reaction and will only be detected after a lot of oxidation has occurred while
others (e.g. conjugated dienes) may be formed in large numbers early in the reaction but
breakdown to immeasurable levels later in the process.
● Acid Value. Acid value is defined as the number of milligrams of potassium hydroxide
required to neutralize the free fatty acids present in one gram of fat. It is a relative measure
of rancidity as free fatty acids are normally formed during decomposition of triglycerides.
The value is also expressed as percent of free fatty acids calculated as oleic acid.
● Peroxide Value. Peroxides are primary reaction products of lipid oxidation. Peroxides can
be measured based on their ability to liberate iodine from potassium iodide, or to oxidize
iron ions (from ferrous to ferric ions). The peroxide value is applicable for the early stages
of lipid oxidation. During the course of oxidation, peroxide values reach a peak and then
decline.
● Thiobarbituric Acid (TBA). TBA is the most widely used test for measuring the extent of
lipid oxidation. This test is preferred due to its simplicity and because its results correlate
highly with sensory scores. In this experiment we use a TBA test to measure the extent of
the reaction. TBA reacts with two molecules of malonaldehyde (formed as a byproduct of
lipid oxidation) to form a pink color. The color of the malonaldehyde-TBA complex can be
quanititated spectrophotometrically (530nm). Unfortunately other aldehydes (esp. reducing
sugars) can also react with TBA and give a false positive result.
Objectives
1. To evaluate lipid oxidation in fat products using different methods
2. To study effect of prooxidants and antioxidants on lipid oxidat