The Encyclopedia of Fine Coffee explains the meaning of latte in detail

The chemical composition of milk is protein, lipid, carbohydrate, inorganic, phospholipid, trace vitamins and so on. The formation of milk bubbles is due to the formation of a thin film on the surface when the milk is heated to more than 40 ℃. Just like we go to the convenience store to buy a bottle of fresh milk and heat it in the microwave, there will be a thin layer of fresh milk on the surface.

That is because the skin membrane formed by heating and solidification of milk protein floats on the surface. By the way, I would like to correct the concept that has been misunderstood by partners for a long time. The main reason for the formation of milk bubbles is not milk fat. If you think about it, if the formation of milk bubbles is milk fat, the foam of low-fat milk (milk fat less than 2%) should not be played quite well, but in fact, low-fat milk still produces dense foam. The reason is simple: low-fat milk still retains the milk protein that forms bubbles. So whether you can make milk foam or not has nothing to do with milk fat.

Going back to the topic just now, when the partner is steaming milk, the steam sent by the steam rod head has an instantaneous temperature of more than 40 ℃, which is enough to heat the milk protein to form a film, and the film covers the air in the steam cyclone, forming the milk bubbles that get along with us day and night, and make the milk more fragrant!

However, if the milk is heated for more than 75 ℃, or for a long time, the protein structure that previously solidified into the foam begins to disintegrate, so the foam begins to thicken and become ugly, just like soap bubbles.

Therefore, when cooking milk into "mistress" (milk that has been steamed once and cooled and reused), because the previous protein has been solidified and combined in the first cooking, so there is no unsolidified protein in the second milk, so heating with steam is just breaking up the chain of previously solidified proteins and making the milk drink thinner. This is why "extra hot" and "mistress" (milk that has been steamed once and cooled and reused) have a smoother taste and the foam is dry and ugly.

I hope you will understand that after the chemical changes in the heating of milk, you can make a gloomy foam and use "one milk" (fresh milk that has not yet been steamed)!

Postscript: milk foam v.s milk fat?

The formation of milk foam is not milk fat, many people have this misunderstanding. If the formation of milk bubbles is milk fat, it is reasonable to say that the foam of low-fat milk (less than 0.2% of milk fat) should not play quite well, but in fact, low-fat milk still produces dense bubbles. The reason is simple: low-fat milk still retains the milk protein that forms bubbles. So whether you can make milk foam or not has nothing to do with milk fat.