Milk is composed of almost 90% water. In addition to water, the three main components in milk are: Fat, sugar (lactose) and protein. When making yogurt, proteins are the main players involved in the process of solidifying the milk. Milk contains two main groups of proteins, curds and whey. The two groups of proteins can be distinguished by their behavior under acidic conditions. Curds, or casein proteins coagulate under acidic conditions forming insoluble protein aggregates, while whey proteins stay suspended under acidic conditions. It is the curd proteins, which enabling yogurt and cheese making processes.
In milk, the four proteins within the casein family are suspended as micelles, or clusters of a few thousand proteins intermixed with calcium in the form of calcium phosphate. The calcium phosphate helps to keep the proteins from scattering. In addition, the surface of the micelle is covered by a specific casein protein known as kappa casein. Kappa casein is negatively charged, which keeps the individual micelles from aggregating, and helps keep the micelles suspended in solution.
Milk is slightly acidic, with a pH around 6.7. Further acidifying the milk, either through natural or controlled fermentation results in molecular changes to the casein micelle. At a pH around 5.5, two important things happen, first, the negative charges of kappa caseins are neutralized, and second, calcium phosphate begins to dissolve. Both of these changes result in micelles that begin to scatter. Even further acidification, to a pH around 4.7, results in casein proteins that begin to re-aggregate. However, instead of forming micelles, a fine network of casein strands are produced, which give yogurt its structure.
The first step in making yogurt is heating the milk up to 180F. The casein micelle is unaffected by heat, even boiling temperatures do not alter its molecular structure. However the whey protein lactoglobulin denatures around 178F, and once denatured binds more water molecules and freed (due to the acidic conditions discussed above) casein proteins to influence the texture of the casein curd.
The temperature of the milk is then reduced to between 80-113F before adding yogurt culture. The reason for lowering the temperature of the milk prior to adding the bacterial culture is that the bacteria used in the yogurt making process cannot survive at temperatures above 113F. The two most common strains of bacteria added to yogurt cultures are Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus salivarius subspecies thermophilus. The bacteria grow and reproduce by consuming the lactose (sugar) present in the milk and in the process release lactic acid, thereby acidifying the milk. As noted above, this acidification results in the solidification of the milk. The two bacterial strains work in tandem, and one aides the others growth.
By controlling the temperature at which the milk and bacteria are incubated one can control the time it takes for the milk to solidify as well as the texture, taste and whey retention potential of the final yogurt product. Milk and yogurt culture that is incubated at temperatures close the ideal temperatures for the growth of the culture, around 104-113F, will set within a couple of hours and the final product will be mildly acidic, firm and whey will leak out readily. Milk and yogurt culture incubated closer to 86F may take up to 12 hours to solidify, will have a more acidic taste, be less firm, and leak whey less readily.
McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004.
“Milk Facts.” Home. N.p., n.d. Web. 11 Apr. 2016.