Why butter turns rancid

If we leave the butter in the heat for a long time or in the fridge for a very long time, its surface will start to get a deep yellow color and the butter will start to smell very unpleasant. This is a process called rancidity. Yellowed butter is no longer suitable for consumption. But why does rancidity occur?

Not just butter turns rancid, but fat in general does. It is a complete or partial oxidation or hydrolysis by exposing fats to air, light, moisture or bacterial activity. In this process, shorter chains of aldehydes and ketones are formed from longer chains of fatty acids. Rancid fats also have a lower vitamin content.^[1]

Hydrolytic rancidity, which requires water for its process, causes the formation of free fatty acids during the hydrolysis of triacylglycerols. Short chain fatty acids, such as butyric acid, have a strong odor. This rancidity process is accelerated the more the formation of a short fatty acid, as these then further act as a catalyst in the reaction.^[1]

Oxidative rancidity is caused by the action of oxygen in the air. The action of free radicals on the double bonds of unsaturated fatty acids produces strong-smelling aldehydes and ketones. If the butter is exposed to sunlight in addition to air, this light acts as a catalyst, further accelerating the rancidity process.^[1]

Microbial rancidity is a process in which microorganisms such as bacteria and mold break down fats with their enzymes. This process can be prevented by the addition of antioxidants.^[1]

It is very difficult to prevent rancidity, but it is possible to significantly slow it down. Protection against rancidity includes airtight packaging, cold storage, sun protection and the addition of antioxidants.^[1]