Briefly discuss the microorganism involved and chemical changes occurring due to spoilage in the following:

i) Fish

ii) Fruits and vegetables

iii) Bakery products

iv) Raw milk

i) Fish 

As soon as a fish dies, spoilage begins. Spoilage is the result of a whole series of complicated changes brought about in the dead fish by its own enzymes, by chemical action and by bacteria. It is necessary to understand something of the way in which these changes take place in order to make the fullest use of chilling as a means of keeping them in check.

An important series of changes is brought about by the enzymes of the living fish which remain active after its death. They are particularly involved in the flavor changes that take place during the first few days of storage, before bacterial spoilage has become marked.

Millions of bacteria, many of them potential spoilers, are present in the surface slime, on the gills and in the intestines of the living fish. They do no harm because the natural resistance of a healthy fish keeps them at bay. Soon after the fish dies, however, bacteria begin to invade the tissues through the gills, along blood vessels, and directly through the skin and the lining of the belly cavity.

In addition to bacterial and enzymatic changes, chemical changes involving oxygen from the air and the fat in the flesh of species such as tuna and mackerel can produce rancid odours and flavours.

Thus, spoilage is a natural process once the fish dies, but chilling can slow down this process and prolong the shelf life of fish as food.

Effect of temperature on spoilage

There are three important ways of preventing fish going bad too quickly - care, cleanliness and cooling. Care in handling is essential because unnecessary damage can provide access through cuts and wounds for the spoilage bacteria, thus hastening their effect on the flesh. Cleanliness is important in two ways: (i) the natural sources of bacteria can largely be removed soon after the fish is captured by taking out the guts and washing off the slime from the surface of _____ _______ ___ _________ ___.
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