RANCIDITY: LIPID OXIDATION - TYPES, MECHANISM, FACTORS

Rancidity is the chemical decomposition of lipids.

Oxidation of lipids is a major cause of deterioration in the quality of lipids, dairy, meat and meat products and affects many characteristics such as flavor, color, texture and nutritive value.

Rapid development of rancid flavors during storage is a major problem facing the oil, dairy and meat industries

Many of the off-flavours develop during the processes known as hydrolytic rancidity and oxidative rancidity. Rancidity is the term associated with the development of off-flavours in foods and is defined as the subjective organoleptic appraisal of the off-flavour quality of food. It is subjective because the ability to perceive an off-flavour varies from person to person.

Hydrolytic rancidity is associated with the presence of water, while oxidative rancidity requires the interaction of oxygen.

Hydrolytic rancidity

Water and fat can exist in contact with one another for months at a time. The rate of reaction between water and fat can become significant if there is a suitable catalyst present and the temperature is raised. The typical catalysts are lipase enzymes and acidic catalysts. Most usually the lipase comes from bacterial contamination. An alternative enzyme system which can cause rancidity, the lipoxygenases, can be introduced from vegetables.

Lipolysis, the enzymic hydrolysis of milk lipids to free fatty acids and partial glycerides, is a constant concern to the dairy industry because of the detrimental effects it can have on the flavor and other properties of milk and milk products. However, free fatty acids also contribute to the desirable flavor of milk and milk products when present at low concentrations and, in some cheeses, when present at high concentrations.

The enzymes responsible for the detrimental effects of lipolysis are of two main types: those indigenous to milk, and those of microbial origin. The major indigenous milk enzyme is lipoprotein lipase. It is active on the fat in natural milk fat globules only after their disruption by physical treatments or if certain blood serum lipoproteins are present. The major microbial lipases are produced by psychrotrophic bacteria. Many of these enzymes are heat stable and are particularly significant in stored products.

When a food is contaminated by bacteria and is subsequently heated, we have the conditions for hydrolytic rancidity. In these situations, the triglycerides in the food are hydrolysed, first to diglycerides, then monoglycerides and finally to fatty acids. Lipolysis may be a problem in milk powder, creams. butter, ice cream and cheese.

Hydrolytic rancidity refers to the hydrolysis of triglyceride into its component fatty acids and glycerol. The reason it causes an odor and flavor deterioration is because we taste individual fatty acids more than the total triglyeride. Since lipase naturally occurs in dairy products, it happens that short chain fatty acids are a major component. These short chain fatty acids like butyric acid are particularly able to be perceived by the tongue sensory buds. Since hydrolytic rancidity occurs naturally, the best defense is to keep butter in the refrigerator.

  

Oxidative rancidity

This is caused by auto-oxidation of lipid components in food.

Auto-oxidation is defined as spontaneous oxidation of a substance in contact with molecular Oxygen. It results into formation of off-flavours and off-odours. Colour deterioration, nutritional deterioration and impairment on texture are other attributes affected by lipid oxidation.

Oxygen attacks the double bond in fatty acids to form peroxide linkages -therefore, phospholipids which contain a high content of unsaturated fatty acids, mainly linoleic and arachidonic acids are more susceptable to oxidation.

Rancidity is a spoilage factor affecting all aspects of food acceptability. The lipid component most susceptible to auto-oxidation are the unsaturated fatty acids, especially with double bonds.

Mechanism:

1. Initiation:

RH              R^.          +          H^.

At this stage few molecules of the lipid RH are sufficiently activated by heat, light or metal catalyst to decompose into unstable free radical R^. +  H^.

2. Propagation:

      R^.          +          O₂                    ROO^.

      ROO^.    +          RH                    ROOH   +  R^.

In the presence of O₂ the free radical reacts with R^. to form peroxide; Or the radical can react with fresh molecule of lipid RH producing Hydroperoxide (ROOH) and a free radical R^.. The free radical is formed without the help of the initiator or activator.

3.Termination:

The free radical combines with other free radical or free radical inactivator to yield stable compounds which accumulate in the system.

R° + R                                                      RR

ROO° + ROO°                               ROOR + O₂

RO° + R°                                                   ROR

ROO° + R°                                                ROOR

2RO° + 2ROO°                              2ROOR O₂

Free radicals can damage living tissues unless antioxidants, ie. tocopherols

or vitamin E, are present to react with free radicals

When no radicals are available for further reaction with oxygen, necessary for a new initiation reaction to occur if oxidation is to continue

The hydroperoxides (are very unstable) and  enter a series of reaction leading to more free radicals and stable final products. These final products include short chain Carboxylic compounds (aldehydes and ketones) responsible for the rancid flavour and the side reactions leading to the overall deterioration.

RO^.             +          R^.                                           Aldehydes and ketones

Antioxidants are often added to fat-containing foods in order to delay the onset or slow the development of rancidity due to oxidation.

Antioxidants Used To Decrease Oxidative Rancidity of Fat or Oil

butylated hydroxyanisole (BHA)	improves oxidative stability, antioxidants
butylated hydroxytoluene (BHT)	improves oxidative stability, antioxidants
carotene (Pro-Vitamin A)	enhances color of finished foods; color additive
citric acid	inhibit metal-catalized oxidation and production of dark colors; metal chelating agents.
Diacetyl	provides buttery odor and flavor
Lecithin	water scavenger to prevent lipolytic rancidity; emulsifier
methyl silicone	inhibits oxidation; antifoam agent
tertiary butylhydroquinone (TBHQ)	improves oxidative stability, antioxidants
tocopherols	natural antioxidant, improves oxidative stability

The natural antioxidants tend to be short-lived, so synthetic antioxidants are used when a longer shelf-life is preferred.  In addition, rancidity 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. The addition of antimicrobial agents can also delay or prevent rancidity due to the growth of bacteria or other micro-organisms.

Factors which affect the rate of rancidity:

1.     Temperature:

The rate of autooxidation increases with temperature. High temperature accelerates the generation of free radicals. Temperature increases the rate of autooxidation as well as the reaction mechanism.

2.     Light

Ultraviolet light accelerates the initiation of the chain reaction especially the acceleration of peroxide decomposition.

3.     Oxygen:

The rate of autooxidation increases with increasing Oxygen pressure, until a constant oxidation rate is reached beyond a given pressure eg. Exclusion of air (vacuum or Nitrogen packs, use of packaging material with low Oxygen permeability) is used for retardation of oxidative deterioration of fatty foods.