Homemade yogurt - Good to know

For better understanding of the yogurt-making process at home, it is good to know by what and in which way conversion of milk into yogurt is affected.

Homemade yogurt


The type of milk:

Yogurt and milkYogurt is commonly made from cow’s, sheep’s or goat’s milk, but it can be made from any other animal milk. Plant-based milks can also be converted into “yogurt”, but they need a special starter and a slightly different technique since plant milks do not contain lactose which is the main target of yogurt bacteria. 


Fat content in yogurt:

Fat content in milk determines fat content in yogurt. Milk fat is not affected by bacterial change during yogurt making, and since there is no waste after yogurt making, all milk fat remains in yogurt. Milk containing higher fat content will result in thicker yogurt.


Thermal processing of milk:

For yogurt making, milk needs to be boiled or pasteurized. There are important reasons for this: 

-    High temperature will denature, and thus deactivate naturally occurred enzymes in milk. This prevents natural curdling of milk, as well as the competition between milk enzymes and enzymes produced by yogurt bacteria.

-    High temperature will kill harmful bacteria, yeasts and moulds occurred in raw milk. This prevents competition between yogurt cultures with unwanted bacteria, spoilage of milk and risk of illness. 

-    Another important reason for thermal processing of milk is denaturation of milk proteins. Denatured proteins set together and retain whey inside their micelles. Thus the whey is captured and does not separate easily.

Freshly prepared yogurt


If yogurt is made from raw milk, the proteins curdle and the whey separates. Raw milk can also be used for yogurt making, but the final product will be slightly different and of thinner consistency. There are people who make yogurt exclusively from raw milk. 


Yogurt-making bacteria:

As a starter (source of yogurt bacteria), homemade yogurt from the previous batch can be used, as well as store-bought yogurt containing living bacteria or commercial powdered yogurt starter. Two main bacteria that are necessary to convert milk into yogurt are Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus salivarius ssp. thermophilus. They act in synergy, digesting milk sugars and proteins and producing lactic acid and various other compounds which contribute to taste, aroma and consistency of yogurt. Apart from these, yogurt culture may contain other bacteria like bifidobacteria and other lactobacilli to improve probiotic properties of yogurt.


Lactic acid fermentation of milk:

Thick and liquid yogurtMilk sugar (lactose) is a disaccharide derived from galactose and glucose, and it makes up approximately 5% (2-8%) of milk. The role of bacteria in yogurt making is lactic acid fermentation of milk. Yogurt bacteria convert lactose into lactic acid, causing milk to taste sour. Acidic (sour) environment causes milk proteins (caseins) to set, which results in semisolid texture of yogurt. At the same time, lactic acid acts as a preservative. 


The importance of temperature:

All yogurt bacteria are thermophyllic, which means that they work best at elevated temperature. Each of them has its own optimal temperature range and, in general, it is between 38 and 45oC (100-113oF). Temperatures near the upper limit will result in sourer yogurt. Also, if freshly made yogurt sits longer before refrigerating, it will be sourer. If milk temperature is too high, it will kill the bacteria, and if it is too low, it will slow down the bacteria activity and milk can spoil before it converts into yogurt. An optimal temperature during several hours will allow the development of acidity which protects yogurt from spoiling.

An experienced yogurt maker can determine the right milk temperature by hand, without using a thermometer. Milk should be warm, not hot to touch. For the less experienced ones, the thermometer is an excellent tool. 



Nutritional value of yogurt is similar to milk, yet improved due to partial digestion of milk compounds by bacteria and their probiotic action. Yogurt is rich in proteins, calcium, potassium and various vitamins, particularly thiamin (vitamin B1) and riboflavin (vitamin B2).