Question 69 of the day: Why does hot milk have a layer of skin on the surface after a period of time?

This layer of skin is produced by protein denaturation and fat aggregation during milk heating. We call this skin "milk skin" in our lives. The milk skin is white or yellowish, the outside is protein, the inside is fat, the outer protein can protect the fat on the inside from oxidation or is slowly oxidized, and its main components are generally: protein 10%-20%, fat 40%-50%, water 10% or less, the rest are carbohydrates and minerals. It is more common when milk is heated and cooled.

During milk heating, milk proteins, especially whey proteins (mainly β-lactoglobulin and α-lactalbumin), are heat denatured and enriched on the surface. During the heating process, the fat balls will also be heated up, and the fat ball membrane will rupture, resulting in a decrease in fat viscosity, and eventually adsorb proteins and accumulate on the surface.

Milk skin is commonly found in fresh milk, the finished milk on the market may not have milk skin, modern milk processing, there is a "homogeneous" process, this process is under high pressure to break the large fat balls in milk into small fat particles, so that it is evenly dispersed in milk, because the fat particles are small, so it is not easy to appear milk skin.

In addition, whether there is milk skin, the thickness of the milk skin, etc. are not the criteria for judging the quality of milk, and the milk skin does not contain all the fat in the milk, and skimming off the milk skin is not equivalent to defatting the milk. Although there is a lot of fat in the milk skin, there are also certain nutrients in it, so if you do not need to control the fat intake, you can eat the milk skin. The absolute fat content of a single milk skin is not high, and it will not gain weight when eaten.

Resources:

Scientific understanding of milk "milk skin" - China Rural Network

Answer: Frost white

Edit: yr Lewis