Rancidity in Foods.                          

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1.What is Rancidity. (rancid derived from "rancidus" the Latin for stinking). 

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. When food scientists talk about rancidity, they are often talking about a specific type of rancidity involving oxygen damage to foods, and this type of rancidity is called "oxidative rancidity." During the process of oxidative rancidity, oxygen molecules interact with the structure of the oil and damage its natural structure in a way that can change its odour, its taste, and its safety for consumption.

Oxidation of fats, generally known as rancidity, 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 odours and flavours. These chemical processes may also destroy nutrients in food. Under some conditions, rancidity, and the destruction of vitamins, occurs very quickly.

Oxidative Rancidity                                                                                          Cooking oil

 

2. How can the effects be minimised.

Antioxidants are often added to fat-containing foods in order to retard the development of rancidity due to oxidation. 

Natural anti-oxidants 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. 

The natural antioxidants tend to be short-lived, so synthetic antioxidants are used when a longer shelf life is preferred. 

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.

In addition, rancidification can be decreased, but not completely eliminated, 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. (Oxidative rancidity or autooxidation is a chemical reaction with a low activation energy consequently the rate of reaction is not significantly reduced by cold storage).

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. 

 

3 Nutritional significance.

Hydrolytic rancidity caused by the release of free fatty acids from glycerides, is significantly important in terms of flavour production but is of little consequence in terms of nutrition as the fats are enzymically hydrolysed in the small bowl before they are absorbed by the body. (Hydrolytic rancidity gives strong cheeses like stilton their sharp burning taste).

Oxidative rancidity leads to the formation of both unpalatable and toxic compounds. Three distinct classes of substance occuring in oxidised fat have been shown to be toxic:

a) Peroxidised fatty acids  (peroxidised fatty acids destroy both vitamin A and E in foods).

b) Polymeric material (under normal food processing conditions these appear in small enough quantities to be insignificant).

c) Oxidised sterols (thought to be involved in the causation of artherosclerotic disease).

 

4 General information.

Human taste buds are highly sensitive to some compounds such as lactones and free fatty acids. Consequently only minute quantities are needed to spoil the taste of foods.

Research into the problem of rancidity has perused for many years however it has been given a "boost" by the recognition that it can cause damage to enzymes, vitamins, membranes and proteins. they may also play a part in the ageing process.

Ageing may be linked to oxidation of fats All oils are fats, but not all fats are oils. They are very similar to each other in their chemical makeup, but what makes one an oil and another a fat is the percentage of hydrogen saturation in the fatty acids of which they are composed. The fats and oils which are available to us for culinary purposes are actually mixtures of differing fatty acids so for practical purposes we'll say saturated fats are solid at room temperature (20C) and unsaturated fats we call oils are liquid at room temperature. For dietary and nutrition purposes fats are generally classified as saturated, monosaturated and polyunsaturated, but this is just a further refinement of the amount of saturation of the particular compositions of fatty acids in the fats.

Rancid fats have been implicated in increased rates of heart disease, atherosclerosis and are carcienogenic (cancer causing).

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. The more polyunsaturated a fat is, the faster it will go rancid. This may not, at first, be readily apparent because vegetable oils have to become several times more rancid than animal fats before our noses can detect it. An extreme example of rancidity is the linseed oil (flaxseed) that we use as a wood finish and a base for oil paints. In just a matter of hours the oil oxidizes into a solid polymer. This is very desirable for wood and paint, but very undesirable for food.

 

Smell and taste the oil before use. If it smells like oil paint or leaves a scratchy sensation in the back of your throat it is rancid and should be discarded. If you have access to a laboratory check the peroxide value on a regular basis. 

The Peroxide value of an oil or fat is used as a measurement of the extent to which rancidity reactions have occurred during storage. Other methods are available but peroxide value is the most widely used.

The double bonds found in fats and oils play a role in autoxidation. Oils with a high degree of unsaturation are most susceptible to autoxidation. The best test for autoxidation (oxidative rancidity) is determination of the peroxide value. Peroxides are intermediates in the autoxidation reaction.

The peroxide value is determined by measuring the amount of iodine which is formed by the reaction of peroxides (formed in fat or oil) with iodide ion.

2 I- + H2O + ROOH -> ROH + 20H- + I2

Note that the base produced in this reaction is taken up by the excess of acetic acid present. The iodine liberated is titrated with sodium thiosulphate.

2S2O32- + I2 -> S4O62- + 2 I-

The acidic conditions (excess acetic acid) prevents formation of hypoiodite (analogous to hypochlorite), which would interfere with the reaction.

The indicator used in this reaction is a starch solution where amylose forms a blue to black solution with iodine and is colourless where iodine is titrated.

A precaution that should be observed is to add the starch indicator solution only near the end point (the end point is near when fading of the yellowish iodine colour occurs) because at high iodine concentration starch is decomposed to products whose indicator properties are not entirely reversible.

 

 

 

 

 

 

2009

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