In order to earn more profits, cheaper substances of low quality are mixed with the foodstuffs. These substances are called adulterants and adding them to foodstuffs is called adulteration of food.
Adulterants may even be poisonous or harmful to health. Such adulterated food is impure and harmful and so unfit for consumption.
for example: (any two)
Water, chemicals like urea, starch are added to milk
Brick powder, powdered pumpkin are added to Chilli powder
Papaya seeds are added to Pepper corns
Washing soda, paper pulp are added to Ice cream
Small white stones are added to rice
Reddish pebbles/stones are added to Peanuts