The two ways by which food industries prevent rancidity are [[1]] and [[2]].

Rancidity is the deterioration of food and food turns rancid when the fat and oils inside it undergo oxidation and the taste and aroma of food change. The oxidation of fat produces highly reactive molecules in rancid foods that cause bad smell and taste. This chemical reaction also reduces food nutrition. Also, rancid foods can have serious health consequences if eaten.

Rancidity can be prevented by using the following methods:

1. Add antioxidants to the food: Antioxidant molecules play an important role in preventing our food from spoiling very quickly by stopping the oxidation reaction. Other examples of antioxidants that we need to be aware of are ascorbic acid (vitamin C) and tocopherols (vitamin E). These are natural antioxidants. 
2. By adding nitrogen gas: Rancidity can be prevented by packing fats and oils containing Nitrogen gas because the packaged food is surrounded by non-responsive nitrogen gas and there is no oxygen or air that causes its oxidation and makes it rancid.

Hence, the two ways by which food industries prevent rancidity are by adding oxidants and by adding nitrogen gas.