Question

In yogurt, the reason the milk thickens is because:

		we add powder milk as a thickener
		the bacteria break down the proteins in milk
		proteins denature under acidic conditions
		heat causes the proteins to denature

Answer 1

Yogurt contains lactobacillus which is a pro-biotic bacteria which helps in keeping our intestine healthy, these bacteria produces lactose, an enzyme that helps in breaking down lactose the milk sugar. Fermentation of milk sugar or lactose produces lactic acid which lowers the pH of the yogurt. This lowering of pH causes the protein molecules of milk to denature as a result yogurt thickens.

So here in the question in yogurt the milk thickens because proteins denature under acidic condition, so it is the correct option.

Option-A says we add powdered milk as a thickener. Yes addition of powdered milk increases the volume of milk solids and which lead to a thicker yogurt but it is not the reason or mechanism behind thickening of milk in yogurt. So this cannot be the correct option.

Option-B says the bacteria breaks down the proteins in the milk, this option is also not correct because lactobacillus bacteria helps in breaking down lactose which is a milk sugar not a milk protein so this is not the reason behind thickening of milk in yogurt.

Option-C says proteins denature under acidic condition, this is the correct option behind thickening of milk in yogurt because lactic acid lowers the pH of yogurt as a result the protein molecules found in milk gets denatured causing yogurt to get thicken.

Option-D says heat causes the protein to denature this is also a wrong option, because heat kills the harmful bacteria present in the milk but not in denaturing the protein. Rather lactic acid helps in lowering the pH and causes denaturation of milk proteins that is casein.