1. SHELF LIFE
Hande Türkçapar 12-C

2. All foodstuffs gradually degrade because of changes in flavor, smell, texture and appearance, or because of the growth of organisms, they become undesirable and unfit for consumption.
Shelf life is the recommendation of time that products can be stored, during which the defined quality of a specified proportion of the goods remains acceptable under expected (or specified) conditions of distribution, storage and display.

3. The degradation can occur from a number of processes:
1. Change in water content as a result of contact with the air
When moist foods lose water, they become dry and change in texture. When solid foods are exposed to air, the rate of oxidation increases, leading into a decrease in nutrient value, discoloration of the surface and rancidity.
Conversely, when dry foods absorb water vapor from the air they become moist and more vulnerable to microbial degradation.

4. The degradation can occur from a number of processes:
2. Chemical Reaction
When chemical reactions occur within the food it results in pH change( becoming sour) or the development of other undesirable flavors. They can also result in color changes and a decrease in nutrient value.
The presence of oxygen often leads to oxidative degradation of foodstuffs.

5. The degradation can occur from a number of processes:
Light
Light provides energy for photochemical reactions to occur, and that leads to rancidity, fading of color and the oxidation of nutrients, especially vitamins.

6. The degradation can occur from a number of processes:
Temperature
Just like all chemical changes, increase in temperature results in increase in degradation of foods.
Water is an important component of food that determines its texture, softness etc. these water molecules are either chemically bonded to carbohydrate and protein polar groups or be free but linked to protein and polysaccharides via hydrogen bonding.

7. Aldohexose as a carbohydrate
Effect of temperature
A change in the forces of attraction that hold carbohydrates or protein chains in place determines how much water can be held.
So if there is a change in pH or if temperature increases, these forces will be disrupted and thus change the amount of water these foods can hold-in the same time affecting its texture, softness and how juicy it remains.
Aldehyde group
Aldohexose as a carbohydrate
Two amino-acids

8. Rancidity One common form of degradation is rancidity in lipids.
It is the development of unpleasant smells in fats and oils- along with changes in their texture and appearance.
Rancidity can develop in two ways:
1. Hydrolytic Rancidity
2. Oxidative Rancidity

9. Hydrolytic Rancidity C-O-CO-R + H2O C-O-H + HO-CO-R
In Hydrolytic Rancidity, the ester bond is broken down with the presence of lipase, heat and moisture to yield free fatty acids.
C-O-CO-R + H2O C-O-H + HO-CO-R
Oil/Fat water glycerol free fatty acid
Free fatty acids such as butanoic acid, hexanoic acid and octanoic acid can give an unpleasant rancid smell and taste to butter and milk that have been stored too long.
Longer chain acids are less volatile, so the smell is less noticeable, however the presence of free fatty acids such as palmitic, stearic and oleic acids or lauric acids makes the texture soapy.
Micro organisms and deep frying induces the presence of lipase, which increases the rate of hydrolytic rancidity.

10. Oxidative Rancidity (auto-oxidation)
It involves the reaction of carbon-carbon double bonds in unsaturated lipids with oxygen from the air.
This results in complex free radical reactions that can produce a variety of products and thus create unpleasant odor and taste.
For example, the presence of light (can produce free radicals leading to photoxidation), and enzymes accelerate its rate.
This is less of a problem with saturated lipids, but in highly unsaturated lipids such as fish oils, it is a big problem.

11. Oxidative Rancidity has three stages
1. Initiation
Form free radicals requires a high activation energy, so it usually occurs by exposure of unsaturated lipids to light. (photo-oxidation) It causes homolytic fission of a carbon hydrogen bond
R-HR+H
2. Propagation
Once hydrocarbon free radicals are formed, they react rapidly with oxygen molecules to form peroxide radicals. These abstract hydrogen from other substrate molecules and reform hydrocarbon radicals.
R + O2 R-O-O
R-O-O+ H-RR-O-O-H+ R
3. Termination
During termination free radicals are removed from the system by reactions in between them. For example:
R + R R-R
R-O-O +R R-O-O-R
R-O-O+ R-O-OR-O-O-R + O2

12. Rancidity can be prevented in a number of ways:
FURTHER…
The hydroperoxides (R-O-O-H) in the propagation step are very reactive molecules so they are gradually converted into aldehydes and ketones.
These have unpleasant smells and tastes that spoil the food.
Rancidity can be prevented in a number of ways:

13. Packaging
Opaque packaging and colored glass bottles reduce light induced oxidative rancidity.
Gas impermeable wrapping film will reduce the exposure to oxygen and water vapor.
The free space in the container should be kept to minimum. More advanced solutions would be vacuum packaging or filling it with inert gas.

14. Storage
Refrigeration can 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.
Dairy products are almost always should be stored at low temperatures.
Drying and smoking techniques reduces hydrolytic rancidity and the growth of micro-organisms.

15. Additives
Substances can be added to foodstuffs to reduce rancidity. Some procedures like salting (of bacon) and high sugar contents (preserves) reduces he amount of water by osmosis. This reduces hydrolytic rancidity and growth of microorganisms.
Fermentation and pickling are also ways to prevent formation of microorganisms; the vinegar added when pickling and ethanol during fermentation reduces the pH, creating an unfavorable environment unsuitable for microorganism growth.

16. Antioxidant There are many natural antioxidants such as:
Vitamin C (ascorbic acid): citrus fruits and green vegetables
Vitamin E (a tocopherol): nuts, seeds, soya beans, whole grains…
β-carotene: carrots and broccoli, tomatoes, peach
Selenium: fish, meat, eggs, grains
There are also synthetic antioxidants. Most common are: BHA, BHT, THBP, TBHQ
Most have a hydroxyl group attached to a benzene ring.
They work by reacting with oxygen containing free radicals, neutralizing them, so preventing these from degrading food by e.g. oxidative rancidity.

17. How do they prevent auto-oxidation?
1. Free Radical Quenchers
These species (represented as H-A) react with free radicals to produce less reactive free radicals.
R-O-O+ H-AR-O-O-H + A
2. Chelating Agents
Free radicals can also formed by the reaction of transition metals and hydroperoxides initially produced by auto-oxidation
R-O-O-H + Fe2+R-O+ OH-+ Fe2+
These form very stable complex ions with transition metals and reduce the concurrency of auto-oxidation.
3. Reducing Agents
RA’s (Electron Donors) can react with both oxygen in the food and hydroperoxides initially formed by auto-oxidation. Examples of naturally occurring reducing agents are: vitamin C and carotenoids.