The microorganisms which are generally responsible for spoilage of preserved foods are Bacteria, Moulds, Yeasts and Enzymes.
Bacteria are the most harmful of all the microorganisms. These microscopic organisms and are found in large numbers. Slight acidic and nitrogenous foods are the best for their growth and multiplication. Some bacteria are so dangerous as they cause food poisoning.
Bacteria are unicellular in structure. The cells may spherical, cylindrical or rod like spiral. Some of the bacteria produce spores which cannot be destroyed without application of strong and continuous heat for 30-40 minutes at a temperature of 212 0F to 250 0F. Bacteria are very sensitive for acids ad can be destroyed in presence of it even at a temperature of boiling water. That is why most of the fruits are easily sterilized at 212 0F and vegetables at 240 0 F being non-acidic.
Moulds are distinguished by the formation of mycelium which is a net work of filaments or threads. These threads are termed as “Hyphae”. The mycelium is often white and cottony, Damp, Dark and close situation associated with temperature moisture and oxygene are the best for the growth of moulds. Moulds prefer sugar containing materials with acid and do not thrive in alkaline medium. It is the main reason that fruits and fruit products are attacked by moulds. Though mould spoils the flavour, appearance and food value but never cause food poisoning like bacteria. Moulds can be easily killed if heated at 165 0 F for 30 minutes.
Yeasts are single celled microscopic organisms. Yeasts need light sugar content and plenty of water for their normal growth. When the percentage of sugar is increased more than 65 % , it checks the growth of yeast completely. Yeasts cells can be killed at a temperature of 155 0F for 30 minutes. Yeats effect at an appearance and texture of fruits but they are not harmful and never cause food poisoning.
Enzyme are the chemical substances and if their activity is not checked at proper time, they spoil and cause final decay. This action can easily be controlled by regulating the temperature and exclusion of moisture and air. Some fruits when cut, change the colour of the flesh and it is due to reaction of atmosphere with enzymes.