What is shelf life? Length of time a product can be stored until degradation occurs. >Foodstuffs will gradually degrade, (growth of organisms)

>undesirable and unfit for consumption >changes in flavour, smell, texture, and appearance

Factors that affects the food shelf:

• 1) Changes in water content (result of contact with the air)example : -moist foods becoming dry

-dry food absorb water vapour from air, become moist, vulnerable to microbial

degradation>Increase exposure to air , increase the rate of oxidation-decrease in nutrient value, discolouration of the surface, rancidity.

• 2) Chemical reaction

>result in pH changes, development of other undesirable flavours, colour changes in food, decrease in nutritional value.

• 3) Light

>provides energy for photochemical reaction to occurs.>rancidity, fading of the colour, oxidation of the nutrients (vitamins).

• 4) Temperature>increase in temperature, increase in the rate of degradation of food.>water molecules will either chemically bonded to carbohydrate or protein polar group.>changes in temperature, change in the force of attraction between carbohydrates or protein chain with water molecules.>influence how much water molecules it can hold. Thus affecting texture, softness of the food.

RANCIDITY

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 Oxidative rancidity (auto-oxidation)

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 due to the oxidation of fatty acid chains, typically by the addition of oxygen across the C=C bond in unsaturated fatty acids.

Occurs more rapidly in the presence of enzymes such as lipase, and with heat and moisture.

The process proceeds by a free radical mechanism catalysed by light in the presence of enzymes or metal ion.

The water present in the food and the high temperature will increase the rate of hydrolysis to fatty acids.

The complex free radical reactions will produce a wide variety of products, many of which have unpleasant odours or tastes.

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.

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