Foods are sometimes spoiled by the action of various microorganisms. Microorganisms include moulds, yeasts and bacteria. Some microorganisms exist in both vegetative and spore forms. Spores are more resistant to destruction by heat or other agents than the vegetative form.

1. Food spoiled by Moulds: Mould growth on food with its cottony appearance makes food unfit for consumption. Moulds are most likely to develop in warm, damp and dark places between 25 to 30°C. Sunlight is an inhibitor for mould growth. Certain Moulds in mild quantity are not harmful but a few can produce mycotoxins i.e, aflatoxin specially on groundnut, wheat, etc. If these are not dried as soon as they are harvested.

2. Food spoilage by Yeast: Yeasts grow usually on food such as fruits which contain water and sugar. The typical musty smell of spoiled grapes is due to the growth of yeast. Yeast is reproduced by budding of cells. Yeasts generally grows better in acid medium in the presence of sufficient oxygen between 25°C to 30°C. Yeast converts sugar of the fruit into alcohol and carbondioxide. Honey, jams, jellies are contaminated by Yeast.

3. Food spoilage by Bacteria: Bacteria are unicellular and much smaller in size. Aerobic bacteria grows in the presence of oxygen only while anaerobic bacteria grows in the absence of oxygen. Bacteria usually spoils food which are neutral in reaction such as milk, eggs, meats and vegetables.

4.Action of enzymes: Enzymes are organic catalyst produced by living cells. They are protein in nature and can act from 0°C to 60°C. Beyond a certain stage, enzymes themselves can render the fruit unfit for human consumption. For example, ripening of banana us caused by enzymes but beyond stage of ripening they spoil the banana.
Certain worms, bugs and fruit flies may also spoil food.
Three conditions for spoilage of food by Microorganisms

Microbes works best when these conditions are met
1. Presence if moisture
2. Presence of warmth
3. Presence of oxygen(air)

Ways to prevent decomposition of foods by Microorganisms
There are four ways to keep foods persevered for a longer period of time
1. Air tight/Vacuum parking (eliminates air or oxygen
2. Drying and dehydration (Removes moisture)
3. Refrigerating or freezing (eliminates warmth)
4. Sterlizarion by blanching or boiling (kills bacteria, moulds and enzymes).
Any one if these methods will break the chain of action that is causing food delay. Combining more than one methods will further improve matters. The above technique of preserving the food belongs to the subject of "food preservation" which is a vast subject in itself.