1. 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
2. Factors that affects the food shelf:
• 1) Changes in water content (result of contact with
example : -moist foods becoming dry
-dry food absorb water vapour from air,
become moist, vulnerable to microbial
>Increase exposure to air , increase the rate of
-decrease in nutrient value, discolouration of the
3. • 2) Chemical reaction
>result in pH changes, development of other
undesirable flavours, colour changes in
food, decrease in nutritional value.
4. • 3) Light
>provides energy for photochemical reaction
>rancidity, fading of the colour, oxidation of
the nutrients (vitamins).
5. • 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.
What is rancidity?
Rancidity is the development of unpleasant
smells in fats and oils, which are often
accompanied by changes in their texture and
Two types of rancidity:
Oxidative rancidity (auto-oxidation)
7. Hydrolytic rancidity Oxidative rancidity
Caused by the breaking down of a lipid into Occurs due to the oxidation of fatty acid
its component fatty acids and glycerol. chains, typically by the addition of oxygen
across the C=C bond in unsaturated fatty
C-O-CO-R + H2O → C-O-H + HO-CO-R acids.
Occurs more rapidly in the presence of The process proceeds by a free radical
enzymes such as lipase, and with heat and mechanism catalysed by light in the
moisture. presence of enzymes or metal ion.
The water present in the food and the high The complex free radical reactions will
temperature will increase the rate of produce a wide variety of products, many of
hydrolysis to fatty acids. which have unpleasant odours or tastes.
The free fatty acids have an unpleasant In highly unsaturated lipids, such as fish oils,
smell giving a rancid smell and taste to milk oxidative rancidity can be a major problem.
and butter that have been stored for too
long. Longer chain acids are less volatile, so
the smell is less noticeable.
8. Ways to minimize rate of rancidity
- 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
-Example: Potato crisps which are usually packed in thick foil
packets filled with nitrogen.
9. • Storage
- Refrigeration will reduce the rate of most reactions that
- 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
10. • Additives
- The process of salting and having a high sugar content (as in
preserves) will reduce the amount of water in the foodstuff
- The rate of hydrolytic rancidity will also be reduced as well as
making the environment less suitable for the growth of micro-
- 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.
11. • 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
12. Traditional methods to extend shelf
life of foods
- The use of vinegar creates an environment that is too acidic
13. • Fermentation
- Ethanol is produced which limits the growth of bacteria
- Wine keeps much better than fresh fruit juice and distilled
- Higher alcohol concentration will last longer
14. • Salting – reduces amount of water
• Preserves – high sugar content will also reduce the
amount of water