Rancidity in Chemistry, which is also called Rancidification, is a condition that is produced by the aerial oxidation of unsaturated fat present in foods and also other products that are marked by unpleasant flavours or odour. When unsaturated components of a fatty material are exposed to sunlight, they can be converted into hydroperoxides, which break down into esters, volatile aldehydes, ketones, alcohols, and hydrocarbons, some of which have unpleasant odours.
It is also possible in preventing rancidity using some preventive measures.
Examples of Rancidity
Oil becomes rancid (rancid oil) because of the decomposition of fats it has, or sometimes milk becomes rancid because of not heating it in the humid atmosphere, etc.
Types of Rancidity
There are three types of Rancidity. They can be classified as follows:
Let us Discuss Hydrolytic Rancidity here.
In some fats, there exist the predominance of short-chain fatty acids. The LIPASE enzyme can breakdown the linkage between glycerol and fatty acid. A few fatty acids (when not bound to the glycerol) contain flavour that in high concentration is undesirable (same as in Butter). Long time storage at unrefrigerated temperatures, butter contamination with microbes producing lipases, and more. can accelerate hydrolytic Rancidity.
Let Us Discuss Oxidative Rancidity here.
The large unsaturated fatty acids are more susceptible to oxidation. The oxidation products are given as aldehydes, ketones, and related ones that can give off-flavour at high temperature (exposure to air, cooking, and some chemical contaminants can accelerate oxidation).To reduce the oxidation, antioxidants are added to the oils.
Let us discuss microbial rancidity here.
This type of Rancidity takes place when the microorganisms such as bacteria use their enzymes to break down chemical structures of fat.
Causes and Effects of Rancidity
Unsaturated fatty acids can be found in all solid foodstuffs and oils. These contain carbon-carbon double bonds that can be broken in the air by different reactions between those bonds and oxygen. Besides the shortening of hydrocarbon chains in the lipids, which contain them, the oxygenated products taste and smell different. The polyunsaturated fatty acids, which are supposed to be so good for us, contain a minimum of 2 C=C double bonds, and they are specifically prone to oxidation reactions and quickly turn rancid compared to the fatty acids containing only a single carbon-carbon bond.
A good example is Linseed oil (turned into rancid oil) because it can be used to harden oil-based paints (for example, artist’s ‘oil’ paints). And, the hardening reaction results from the polymer formation of these oxygenated fatty acids. It is also believed that walnut oil turns rancid rapidly. For this particular reason, polyunsaturated fatty acid oils should be stored in the refrigerator to slow down the reaction.
Also, a few human beings are sensitive to walnuts that have even gone slightly rancid. The idea that some sort of polymers forming in the stomach is particularly repulsive, and one will not eat any walnuts that smell the slightest bit ‘off.’ And, if we notice the labels on the packaged foods that we eat, we can find something called ‘BHT’ in the ingredients list. The butylated hydroxytoluene scavenges of free radicals, and since rancidification is based on the formation of free radicals, BHT preserves foodstuffs from this particular reaction.
Responsibility for the Rancidity of Fats
All fats oxidize more readily, and the oxidized fat molecules probably smell bad to us. We can say this bad smell is “rancidity.”
A few fats go rancid faster than the others, and exposure to heat and/or light accelerates the process. At the same time, saturated fat is the most stable one and can last for some months or even a year or two. The monounsaturated fat quickly goes rancid, but it is still fairly stable. For example, we can store lard (around 50–50 monounsaturated or saturated) in a non-airtight and unrefrigerated container for several months, and still, it will be fine.
In fact, this was very common prior to the advent of refrigeration. In the same way, we can also keep olive oil (mostly monounsaturated) unrefrigerated either in an opaque or dark airtight container for many months, and it will still be fine. Polyunsaturated fats very rapidly go rancid and should be kept refrigerated and also consumed very quickly, or not at all if they smell like rancid when we buy them.
Even if we keep fat in an airtight container still, it will eventually go rancid. If there is something mixed with the fat that has even a slight oxidation potential, then it will oxidize the fat molecules. Also, fat molecules can even self-oxidize since one end of each fat molecule contains oxygen. Eventually, they will do so if something else does not get to them first. Rancidity can also be postponed by mixing the antioxidants with fat (some commonly-used ones are given as BHT BHA and vitamin E), but still, the oxidation will happen eventually.
Mostly, the rancid taste is associated with fat products and food that have fats. Majorly this rancid taste will exist after a very long time, and this can be indicated as the product or food has been deteriorated and will reduce the product quality. Generally, rancidity is increased if fat is exposed to some conditions that help the process such as exposure to high heat, humidity, oxygen, and sometimes, the type of containers (iron). The taste is due to the volatile chemicals, which are produced by the reactions with exposure to the conditions above. Volatile chemicals such as aldehydes and ketones formed are the common causes of rancid taste or causes of rancidity.