Foods are spoilt by the action of: (1) Micro-organisms (2) Enzymes and (3) Insects.
1. Micro-organisms:
The micro-organisms responsible for food spoilage are moulds, yeast and bacteria.
Moulds:
Moulds are in the form of threads developed on perishable foods and are easily visible to the eye. They contain spores which can spread through the air and start new mould plants. When these moulds find a favorable environment, they germinate and produce a fluffy growth, often white or grey but sometimes bluish-green, red, orange or some other colour, depending upon the variety of the mould. Most moulds are not harmful. A relatively small proportion of the moulds, found on foods are capable of producing toxic materials known as mycotoxins of which aflatoxins is an example.
Yeasts:
Yeasts are tiny organisms which are not visible to the naked eye, but which can be seen through the microscope. They multiply very fast and cause fermentation by acting on certain components of the perishable foods like fruit juices, syrups etc. During yeast fermentation, the sugars present in the food are broken up to form alcohol and carbon dioxide. Foods liable to be spoiled by yeasts are fruit juices, syrups, molasses, honey, jams and jellies.
Bacteria:
Bacteria are unicellular organism and are much smaller in size than either yeasts or moulds. They occur in different sizes and shapes and are classified as coccus (spheroidal), bacilli (cylindrical) or spirillae (spirillar) on the basis of their shape as seen under the microscope. They also vary in their requirement for food, moisture, acidity, temperature and oxygen. Bacteria can grow and develop rapidly between 20°C and 53°C. Bacteria are classified according to the temperature ranges that they need for growth:
1. A higher temperature than 45°C are known as thermopile, (e.g. in canning industry and milk processing plants).
2. Temperatures between 20-25°C are called Mesophiles.
3. Temperature less than 20°C are called psychrophites (e.g. in Refrigerator and in cold storages).
2. Spoilage by Enzymes:
Enzymes are organic catalyst present in living cells. The life of every living cell depends upon the chemical reactions activated by these enzymes. Hence, they cause food spoilage due to the chemical reactions as in cutting apples; it becomes brown while tomato cause develops a black scum. Enzymes are sensitive to heat and are easily destroyed by heat. They can act from 0°C to 60°C; their optimum temperature of reaction is usually 37°C. All enzymes are inactivated by temperatures above 80°C. Therefore, enzyme activity can be prevented by heating foods to temperature which inactivate the enzymes. It can also be prevented by cooling (as in freezing and refrigeration) by elimination of air, by protection from light and by addition of anti-oxidants.
3. Spoilage by insects:
Worms, bugs, weevils, fruit flies, moths cause extensive damage to food and reduce its nutritional value and make it unfit for human consumption.
