In this article we will discuss about:- 1. Introduction to Rancidity 2. Types of Rancidity 3. Factors Affecting Rancidity and Reversion 4. Prevention of Rancidity.
Introduction to Rancidity:
When food containing fat and oil come in contact with surrounding oxygen and these auto-oxidation leads to bad smell and change in taste, the whole process is said to be rancidity. Most any food can technically become rancid. The term particularly applies to oils. Oils can be particularly susceptible to rancidity because their chemistry which makes them susceptible to oxygen damage.
Oxidation of fats is caused by a biochemical reaction between fats and oxygen. In this process the long-chain fatty acids are degraded and short-chain compounds are formed. One of the reaction products is butyric acid, which causes the typical rancid taste. Rancidification is the decomposition of fats, oils and other lipids by hydrolysis or oxidation, or both. Hydrolysis will split fatty acid chains away from the glycerol backbone in glycerides.
These free fatty acids can then undergo further auto-oxidation. Oxidation primarily occurs with unsaturated fats by a free radical- mediated process. These chemical processes can generate highly reactive molecules in rancid foods and oils, which are responsible for producing unpleasant and noxious odors and flavors. These chemical processes may also destroy nutrients in food. Under some conditions rancidity leads to the destruction of vitamins in food.
The most important components present in oil such as soyabean oil is unsaturated fatty acid. Soyabean oil has a triglyceride composition rich in monounsaturated (23% oleic acid) and polyunsaturated fatty acids (57% linoleic acid; 7% linolenic acid). The linolenic acid contains three unsaturated bond and which can be easily undergone oxidation process. Because of the presence of high content of linolenic and linolenic acid, oil undergoes oxidative degradation and develops color change and fishy or painty smell. This process is called oil reversion.
Oxidation is the primary mechanism involved in the oil reversion. When it comes to oxidation, linolenic acid is about ten times more vulnerable than linoleic acid and about one hundred times more than oleic acid. The oxidation of double bonds is a radical-driven process.
Radical reactions typically have three steps:
1. The initiation reaction, where an energy source (heat; light) generates a radical on the fatty acid.
2. A propagation step with oxygen giving rise to peroxides, which react with more unsaturated fatty acid, creating new radicals.
3. A termination reaction, where two radicals interact forming a new single bond.
There are two types of rancidity:
The specific types of rancidity involving oxygen damage to foods are called “oxidative rancidity.” During the process, oxygen molecules interact with the structure of the oil and damage its natural structure in a way that can change its odor, its taste, and its safety for consumption, i.e. fat is oxidized and decomposes into compounds with shorter carbon chains such as fatty acids, aldehydes, and ketones all of which are volatile and contribute to the unpleasant odor of rancid fats. Oxidative rancidity leads to the formation of both unpalatable and toxic compounds.
Three distinct classes of substance occurring in oxidized fat have been shown to be toxic:
(i) Peroxidised fatty acids (peroxidised fatty acids destroy both vitamin A and E in foods).
(ii) Polymeric material (under normal food processing conditions these appear in small enough quantities to be insignificant).
(iii) Oxidized sterols (thought to be involved in the causation of atherosclerotic disease).
Fats are enzymatically hydrolyzed, the release of free fatty acids from glycerides, cause some rancid odor. This process is called hydroiytic rancidity. Hydrolysis will split fatty acid chains away from the glycerol backbone in glycerides. These free fatty acids can then undergo further auto-oxidation leads to oxidative rancidity.
Oxygen is eight times more soluble in fats than in water and it is the oxidation resulting from this exposure that is the primary cause of rancidity. Oxidation primarily occurs with unsaturated fats by a free radical-mediated process. These chemical processes can generate highly reactive molecules in rancid foods and oils, which are responsible for producing unpleasant and noxious odors and flavors. This process is called auto-oxidation or oxidative rancidity.
Triglycerides react with water under appropriate condition to form diglycerides and free fatty acid residues. Diglycerides later combine with water to form monoglycerides and fatty acids. Finally the monoglycerides completely hydrolysed to form glycerol and fatty acids. This process is called hydrolytic rancidity.
Certain microorganisms can produce the hydrolytic enzyme called lipase, which directly interferes the hydrolysis of triglcerides and produce glycerols and fatty acid. These fatty acids undergo auto- oxidation to form rancid. The microbial lipase requires suitable pH and other conditions for its activity upon fats and oil.
When a fatty substance is exposed to air, its unsaturated components are converted into hydroperoxides, which break down into volatile aldehydes, esters, alcohols, ketones, and hydrocarbons, some of which have disagreeable odours. Butter becomes rancid by the foregoing process and by hydrolysis, which liberates volatile and malodorous acids, particularly butyric acid. Saturated fats such as beef tallow are resistant to oxidation and become rancid at ordinary temperatures.
The more polyunsaturated a fat is, the faster it will go rancid. Vegetable oils have to become several times more rancid than animal fats. Presence of polyunsaturation in oils and fats makes them more susceptible to rancidity than monosaturated and other types of saturated fatty acids.
If oils and fats are chemically more complex and consists more number of double bond, more number of carboxyl or hydroxyl groups, then the chances of become rancid is high. The double bonds found in fats and oils play a role in auto-oxidation. Oils with a high degree of unsaturation are most susceptible to auto- oxidation.
The best test for auto-oxidation (oxidative rancidity) is determination of the peroxide value. Peroxides are intermediates in the auto-oxidation reaction. The peroxide value of oil or fat is used as a measurement of the extent to which rancidity reactions have occurred during storage.
These are the important factor which influences the food items rich in fat and oils become rancid. Suitable temperature and alkaline pH are required for the hydrolytic action of microbial lipase. Temperature and pH indirectly influence the auto-oxidation and hydrolysis.
Presence of heat and light accelerate the rate of reaction of fats with oxygen, i.e., heat accelerates auto-oxidation. Heat and light act as the energy source for the production of free radical in rancidity and reversion of oils and fats.
Prevention of Rancidity:
Rancidity can be prevented by several ways which are mentioned briefly:
The best method used to prevent food item from rancidity is the addition of antioxidants. Antioxidants are added to fat-containing foods in order to retard the development of rancidity due to oxidation.
There are five types of antioxidants:
(1) Natural antioxidants.
(2) Synthetic antioxidants.
(3) Semi-synthetic antioxidants – gallic acid, propylgallate.
(4) Metal chelators – citric acid, phosphoric acid.
(5) Oxygen scavengers – ascorbic acid.
Natural antioxidants include flavonoids, polyphenols, ascorbic acid (vitamin C) and tocopherols (vitamin E). Synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl-3, 4, 5-trihydroxybenzoate (also known as propyl gallate) and ethoxyquin. Natural antioxidants tend to be short-lived, but synthetic antioxidants give longer shelf life and better action.
The effectiveness of water-soluble antioxidants is limited in preventing direct oxidation within fats, but is valuable in intercepting free radicals that travel through the watery parts of foods. A combination of water-soluble and fat- soluble antioxidants is ideal, usually in the ratio of fat to water.
Sequestering agents bind metals, thus preventing them from catalyzing auto-oxidation. Examples of sequestering agents include EDTA (ethylene diamine tetra acetic acid) and citric acid.
Another method for preventing rancidity of food is the proper storage, keeping away from the action of oxygen. Rancidification can be decreased by storing fats and oils in a cool, dark place with little exposure to oxygen or free radicals, since heat and light accelerate the rate of reaction of fats with oxygen.
Do not add fresh oil to vessels containing old oil. The old oil will trigger a reaction and the new oil will become rancid far more rapidly than, if the oil was stored in a clean empty vessel. Avoid using vessels that are wet, this will also speed up the problems associated with oxidation, allow tanks to drain and dry adequately before use.