What is rancidity?
Rancidity is the development of unpleasant smells in fats and oils, which are often accompanied by changes in their texture and appearance.Two types of rancidity: Hydrolytic rancidity Oxidative rancidity (auto-oxidation)
Hydrolytic rancidity
Caused by the breaking down of a lipid into its component fatty acids and glycerol.
C-O-CO-R + H2O → C-O-H + HO-CO-R
Occurs more rapidly in the presence of enzymes such as lipase, and with heat moisture
The water present in the food and the high temperature will increase the rate of hydrolysis to fatty acids.
The free fatty acids have an unpleasant smell giving a rancid smell and taste to milk and butter that have been stored for too long. Longer chain acids are less volatile, so the smell is less noticeable.
Oxidative rancidity (auto-oxidation)
Occurs due to the oxidation of fatty acid chains typically by the addition of oxygen across the C=C bond in unsaturated fatty
The process proceeds by a free radical mechanism catalysed by light in the presence of enzymes or metal ion
The complex free radical reactions will produce a wide variety of products, many of which have unpleasant odours or tastes
In highly unsaturated lipids, such as fish oils oxidative rancidity can be a major problem
Ways to minimize rate of rancidity
Packaging
- Opaque packaging and coloured glass bottles will reduce light induced oxidative rancidity.
- Gas impermeable wrapping film will reduce the exposure of the product to oxygen and water vapour in the air.
- Free space in the container should be kept to minimum to reduce the amount of oxygen and water vapour present. The best way is to use vacuum packaging or fill the package with inert gases.
-Example: Potato crisps which are usually packed in thick foil packets filled with nitrogen.
Storage
- Refrigeration will reduce the rate of most reactions that produce rancidity.
- Storing fat and oil rich foods in the dark will reduce the rate of photo-oxidation, which is less affected by temperature.
- Reducing the water level content of the foods (such as drying or smoking) and then storing them away from moisture helps to reduce hydrolytic rancidity and discourage the growth of micro-organisms.
Additives
- The process of salting and having a high sugar content (as in preserves) will reduce the amount of water in the foodstuff via osmosis.
- The rate of hydrolytic rancidity will also be reduced as well as making the environment less suitable for the growth of micro- organisms.
- Sulfur dioxide and sodium sulfites (used with fruit products), along with sodium and potassium nitrites (used in curing meats) are examples of reducing agents that function to prevent the oxidative reactions that will lead to browning of many substances when stored too long.
Anti-microbial agents
- Many organic acids and their salts are added to discourage the growth of moulds and bacteria in foods.
- Examples : → addition of benzoic acid and sodium benzoate to fruit juices
→ addition of propanoic acid along with its sodium and calcium salts to bread and cheese
Traditional methods to extend shelf life of foods
• Pickling
- The use of vinegar creates an environment that is too acidic for micro-organisms
Fermentation
- Ethanol is produced which limits the growth of bacteria
- Wine keeps much better than fresh fruit juice and distilled spirits - Higher alcohol concentration will last longer
Salting
– reduces amount of water• Preserves
– high sugar content will also reduce the amount of water.
Rancidity is the development of unpleasant smells in fats and oils, which are often accompanied by changes in their texture and appearance.Two types of rancidity: Hydrolytic rancidity Oxidative rancidity (auto-oxidation)
Hydrolytic rancidity
Caused by the breaking down of a lipid into its component fatty acids and glycerol.
C-O-CO-R + H2O → C-O-H + HO-CO-R
Occurs more rapidly in the presence of enzymes such as lipase, and with heat moisture
The water present in the food and the high temperature will increase the rate of hydrolysis to fatty acids.
The free fatty acids have an unpleasant smell giving a rancid smell and taste to milk and butter that have been stored for too long. Longer chain acids are less volatile, so the smell is less noticeable.
Oxidative rancidity (auto-oxidation)
Occurs due to the oxidation of fatty acid chains typically by the addition of oxygen across the C=C bond in unsaturated fatty
The process proceeds by a free radical mechanism catalysed by light in the presence of enzymes or metal ion
The complex free radical reactions will produce a wide variety of products, many of which have unpleasant odours or tastes
In highly unsaturated lipids, such as fish oils oxidative rancidity can be a major problem
Ways to minimize rate of rancidity
Packaging
- Opaque packaging and coloured glass bottles will reduce light induced oxidative rancidity.
- Gas impermeable wrapping film will reduce the exposure of the product to oxygen and water vapour in the air.
- Free space in the container should be kept to minimum to reduce the amount of oxygen and water vapour present. The best way is to use vacuum packaging or fill the package with inert gases.
-Example: Potato crisps which are usually packed in thick foil packets filled with nitrogen.
Storage
- Refrigeration will reduce the rate of most reactions that produce rancidity.
- Storing fat and oil rich foods in the dark will reduce the rate of photo-oxidation, which is less affected by temperature.
- Reducing the water level content of the foods (such as drying or smoking) and then storing them away from moisture helps to reduce hydrolytic rancidity and discourage the growth of micro-organisms.
Additives
- The process of salting and having a high sugar content (as in preserves) will reduce the amount of water in the foodstuff via osmosis.
- The rate of hydrolytic rancidity will also be reduced as well as making the environment less suitable for the growth of micro- organisms.
- Sulfur dioxide and sodium sulfites (used with fruit products), along with sodium and potassium nitrites (used in curing meats) are examples of reducing agents that function to prevent the oxidative reactions that will lead to browning of many substances when stored too long.
Anti-microbial agents
- Many organic acids and their salts are added to discourage the growth of moulds and bacteria in foods.
- Examples : → addition of benzoic acid and sodium benzoate to fruit juices
→ addition of propanoic acid along with its sodium and calcium salts to bread and cheese
Traditional methods to extend shelf life of foods
• Pickling
- The use of vinegar creates an environment that is too acidic for micro-organisms
Fermentation
- Ethanol is produced which limits the growth of bacteria
- Wine keeps much better than fresh fruit juice and distilled spirits - Higher alcohol concentration will last longer
Salting
– reduces amount of water• Preserves
– high sugar content will also reduce the amount of water.
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