How can we detect the spoilage in food?

Following indications help us an detecting the spoilage in food :

(a) Odour : When food gives out smell, it is the indication that the bacteria has spoiled the food. 

(b) Discolouration : Growth of microbes on food results in discolouration of food. Black moulds on breads, blue green moulds.

(c) Souring : Sometimes the cooked food starts tasting sour. It is due to the production of acids by the action of certain bacteria.

(d) Sliminess : Sometimes the food becomes slimy. It is also due to action of certain bacteria.

(e) Gas formation : Due to action of bacteria gases like carbon dioxide are produced. They also spoil the food by making it swell or become spongy.