Why old butter has larger acid value than the new butter?

Answer 1

Butter when fresh is in the state of being a triacylglycerol, 3 fatty acid chains attached to a glycerol backbone. When a butter ages it undergoes a process of oxidation called lipolysis. This is where the fatty acid chains split from the glycerol backbone the result of which is the formation noticeably off flavours being present in the butter. Hence the point of using old and new butter is that the old butter should have a higher acid value than the fresh butter

Answer 2

Old butter has a larger acid value than new butter because over time, lipase enzymes in the butter can cause fats to break down and release free fatty acids. These free fatty acids contribute to the increase in acid value of the butter as it ages.