Rancidity

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 odours. 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.

How does a Substance Turn Rancid?

Rancidity, in general terms, is always used for food products that have oil and fatty acids in them. Fatty acids are generally composed of fats, cholesterol and steroids. These are mostly carboxylic acids that consist of a long aliphatic chain. They are either saturated which means there is only a single linkage between the carbon atoms or unsaturated that comprises multiple linkages between the carbon atoms. 

The reaction of rancidity typically occurs in three steps, and they are:-

1. Initiation reaction: This reaction leads to the formation of the radicals on the food substances because of the external factors like heat and air that stimulates the reaction. A radical can be defined as an atom molecule or ion that has an unpaired electron. These unpaired electrons make the radicals very reactive chemical substances. 

RH———> R*+H*

2. Propagation reaction: In this stage, the oxygen present in the atmosphere gives rise to the peroxides. These peroxides then react more and more with the unsaturated fatty acids and then produce new radicals.   

R*+ O2———> ROO**

(peroxide)

3. Termination reaction: in this stage, the two radicals combine together to form a new single bond.

ROO*+ROO*———> end products

The fats, lipids and the other compounds are decomposed at the end of the rancidification, thus forming high reactive molecules. This is the reason that the food starts smelling unpleasant and tastes bad after rancidity. In some cases, there is a loss of vitamins due to the process of rancidity.  

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Types of Rancidity

There are three types of rancidity. They can be classified as follows:

1. Hydrolytic Rancidity

In some fats, there exists the predominance of short-chain fatty acids. The LIPASE enzyme can break down the linkage between glycerol and fatty acid. A few fatty acids (when not bound to the glycerol) contain flavour that, when present 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.

2. Oxidative Rancidity

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 temperatures (exposure to air, cooking, and some chemical contaminants can accelerate oxidation). Antioxidants are added to the oils to reduce oxidative rancidity.

3. Microbial Rancidity

This type of rancidity takes place when the microorganisms such as bacteria use their enzymes to break down the 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.  They 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). Also, 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, some people are sensitive to walnuts that have gone even 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.’ 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 off free radicals, and since rancidification is based on the formation of free radicals, BHT preserves foodstuffs from this particular reaction.

Factors Responsible for Rancidity

Some of the important factors that can result in rancidity of the food items are discussed below:

1. Oxygen: The major cause of the rancidity of food is the exposure of the food items in the open atmosphere where there is an abundance of oxygen. The oxygen results in the oxidation reaction that results in the formation of the free radicals that leads to food damage. 

2. Microorganisms: An enzyme known as lipase is released by many microorganisms which leads to hydrolysis of lipids. They use their enzymes on the food material that leads to the destruction of the ideal composition of the food and they lose their desired property. 

3. Physical factors: Heat, light and air play an important role in rancidification. As the process of oxidation is also stimulated by heat, light and air primarily, thus it is the main source of the production of free radicals. Light promotes the decomposition of unsaturated fatty acids.

4. Trace elements: Trace elements like Fe and Zn also increase the rate of rancidity. 

What Causes 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 still 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. They should and also be consumed very quickly, or not at all if they smell 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, which can be indicated as the product or food has been deteriorated reducing 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 are the common causes of rancid taste or causes of rancidity.

Prevention of Rancidity

It is very important to preserve the food properly so as to avoid rancidity in the food items, especially in the food that contains fatty acids and oils, to maintain their desired quality. 

One of the prime ways to prevent the food from getting rancid is to preserve it in a dark place away from light and air. To maintain the same, the food items can be stored in airtight containers. To prevent the auto-oxidation action in foods containing fats and oils, it is also an effective way to add antioxidants. The antioxidants can either be natural or synthetic. All vitamin C, vitamin E, flavonoids and polyphenols are examples of natural antioxidants. Sequestering agents like EDTA also prevent or slow down oxidation and can effectively prevent rancidity.