By Aarthi Katakam '21
Butter is seen as a staple food in many cultures, being used for cooking to being eaten on its own as a spread. How is butter made? Most people would tell you that you make butter by churning milk, but how does the churning process work, exactly? Let’s start by discussing milk.
Milk is a colloid, a special state of matter that is made by distributing small particles of one insoluble substance throughout another substance. In particular, milk is a type of colloid called an emulsion, which is a mixture of two liquids. These two liquids are milk fat and water. Milk fat is composed mostly of triglycerides, a type of lipid, in the form of globules. Each globule is covered in a membrane called the native fat globule membrane (FGM) made primarily of proteins and phospholipids. The membranes keep the globules separated in the water.
Before the churning process can begin, the cream needs to first be separated from the milk. This occurs naturally due to a phenomenon explained by Stokes’ Law, which states that the velocity a sphere will rise or fall in a liquid is directly proportional to the square of its diameter. The separation is due to the density differences between the fat globules and the surrounding water. This process is further accelerated in cold milk when immunoglobulins (antibodies) attach to the membranes of the globules and cause them to flocculate, or clump. The cream layer is then removed from the surface of the milk.
Churning is a balance of interactions between fat globules, air bubbles, and proteins. When cream is churned, or rapidly beaten, a foam is formed. This foam is stabilized by the presence of the fat globules, which partly coalesce, or join, as the FGM’s are broken. The bubbles are then linked together by the fat globules, forming a network of air bubbles and fat clumps. These clumps are called butter grains. As churning continues, the fat clumps grow larger as more globules coalesce. This results in fewer fat globules to enclose the bubbles, making them collapse and run together. They then release a liquid called buttermilk. The remaining solid is butter!
After the butter is drained of buttermilk and mixed further, it turns into another type of colloid – a water-in-fat colloid. This is because water is now dispersed in the solid fat rather than the other way around like it was in the milk. The butter is then salted and distributed — ready for you to pick up in a grocery store!
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