Rancidity Of Cooking

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Cooking with oils can be considered as one of the most common cooking techniques around the world. Within the process of cooking, heating with high temperatures is often applied to break down large polymeric food molecules and for easier absorption of nutrients by our guts. However, heating oil repeatedly with the presence of air, for certain period of time, might cause undesired chemical changes within the chemical characteristics of the oils, for example lipid oxidation, which is responsible for rancidity.
The word “rancid” is used to describe the unpleasant odor and flavour produced by decomposing oils or fats. In the food industry, antioxidants normally act as food additives and are added in oils to control the deterioration of oils and
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I am interested in exploiting the chemical theories behind the rancidity of cooking oil especially for corn oil as I have encountered unpleasant odor and colour after several times of reusing it. In conclusion, I decided to propose a research question: “How do natural and synthetic antioxidants affect the oxidative rancidity of corn oil under high temperature over time?”
The aim of this investigation is not only to investigate the effects of antioxidants on the oxidative rancidity of corn oil, but also to explore the effectiveness of natural and synthetic antioxidants on preserving the corn oil.
Corn oil is extracted from a part of kernel called germ and it “contains about 85 percent of the total oil of the kernel” (Corn Refiners Association 10). Corn oil is largely composed of polyunsaturated fatty acids (PUFAs) and a low amount of saturated fatty acid (SFA). Diagram below shows a simplified version of a corn oil
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The focus on this investigation is mainly based on oxidative rancidity, which occurs when unsaturated fatty acids come into contact with oxygen under controlled high temperature. PUFAs are highly susceptible to be oxidized because of their high proportions of carbon-to-carbon double bonds.
Velíšek stated “autoxidation of fatty acids is the most common type of oxidation under conditions suitable for processing or storage of food” (154). Autoxidation of fatty acids consists of three main phases- initiation, propagation and termination. 4
Initiation
In this phase fatty acid free radical1 (R•)2 and free hydrogen radical (H•) are formed by homolytic cleavage of the covalent bond, CH, of the hydrocarbon chain, usually under high temperature, presence of light or catalysts.
RH!R• + H•
Propagation
Since fatty acid free radical is very reactive, therefore it easily reacts with an oxygen molecule (O2) to form peroxyl radical (ROO•). The resulting peroxyl radical is also highly reactive and it tends to split off a hydrogen atom (H) from another unsaturated fatty acid to form hydroperoxides (ROOH) and a fatty acid free radical (R•). These two reactions in propagation phase may be repeated numerous times because of the high numbers of PUFAs in corn oil, hence more hydroperoxides are

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