Autoxidation (factors and prevention measures): Fat & Oils

                                                   Autoxidation

Autoxidation is any oxidation that occurs in open air or in presence of oxygen (and sometimes UV radiation) and forms peroxides and hydroperoxides.

HYDROGENATION

• Liquid oils can be converted to solid fats by the process known as hydrogenation.
• In this process there is an addition of hydrogen to unsaturated fat thus converting oils into solid fats.
• Hydrogenation takes place in a reactor where hydrogen gas is bubbled through the liquid in the presence of nickel as a catalyst.
• In this process some of the double bonds between the carbon atoms of the fatty acids portion of the triglyceride molecule are broken and hydrogen is added. This chemical change makes the fatty acid more saturated. The melting points of the fats are thereby increased.
• Hydrogenation increases the stability of oils and prevents it from spoilage due to oxidation which results in rancidity.
• Hydrogenation is utilized in the manufacture of a wide variety of fats such as vanaspati and margarine.
• Sometimes additives such as antioxidants, Vit. A, D are added to the fat.
• Air maybe whipped in, to impart a snow white color.
• Palm oil, palmolein, rice bran, cotton seed, sunflower, maize, soyabean, groundnut, and sesame oils are generally hydrogenated.

Winterization of oil

is a process of removing the higher melting point parts from oillike waxes or triglycerides by slowly cooling vegetable oils and felling saturated glycerides from the solvent. Used primarily for oils in salad dressings. Mainly applied in bioindustry.

It is a process in which the temp of the oil is lowered to a degree at which the high melting point triglycerides crystallize which are then removed by filteration. Thus oils donot become cloudy when stored in refrigerator.

RANCIDITY

• The development of any disagreeable odor and flavor in fats and oils causing spoilage is known as rancidity. Under-processing, repeated exposures to high temp, air, water and improper packaging and storage can cause the spoilage.
• Different types of fats and oils show varying degrees of resistance to spoilage, thus most vegetable oils deteriorates only slowly whereas animal fats deteriorate on more rapidly, and marine oils which contain a relatively high proportion of highly unsaturated fatty acids deteriorate most rapidly.
• Vegetable oils resist oxidation because of the presence of antioxidants. The antioxidants get readily oxidized themselves and protect the oil from oxidation.

Types of rancidity:

• Hydrolytic rancidity-is also known as lipolytic rancidity. It involves the breakdown of fat into free fatty acids and glycerol. The rate of hydrolysis is hastened by: presence of enzymes eg. Lipase and micro organisms like moulds and yeasts.

The presence of FFA lowers the smoking point of fats. The products fried in such fat tend to absorb greater amount of fat and develop cracks on their outer surface.

• Oxidative rancidity– It is also called auto-oxidation. Fats when in contact with oxygen produce products with undesirable characteristics. It is promoted by heat, light, ionizing radiations, catalyst and enzymes like lipoxygenase.

MECHANISM OF  AUTO-OXIDATION: Takes place in 3 steps. The rate of oxidation increases as the reaction proceeds.

1. the first stage is INITIATION or formation of free radicals
2. PROPOGATION or free radical chain reaction
3. TERMINATION or terminal point i.e. formation of non radical products. Termination occurs when two free radicals interact to form a non- radical.

FACTORS LEADING TO RANCIDITY

1. temperature
2. moisture content
3. air
4. light
5. metals
6. degree of unsaturation
7. absence of anti-oxidants

PREVENTION

1. Store fat at low temp in cool dark place.
2. Use air tight containers
3. Use opaque containers to cut off the exposure o light and air.
4. Do not keep strong smelling foods in the vicinity of fats and oils as they absorb foreign odor.
5. Copper container and rusted iron accelerate rancidity. Only steel or aluminium containers should be used.
6. Natural antioxidants like Vit E can be used.
7. Artificial antioxidants like BHA, BHT  can be used.
8. If fats and oils have to be stored for some time, they should be hydrogenated and stored. Hydrogenation increases the shelf life of fats and prevents  rancidity