Various dairy products can be made from milk, such as yoghurt and cheese. Yoghurt is made by adding a mixture of lactic acid bacteria to the milk.
Yoghurt is made by adding a mixture of lactic acid bacteria to the milk known as the ‘yoghurt cultures’. The lactic acid bacteria used are Lactobacillus bulgaricus and Streptococcus thermophiles. In some countries it is allowed to add other milk acid bacteria to the yoghurt cultures. By adding the yoghurt culture the milk starts to ferment and during the fermentation process a proportion of the lactose naturally present in milk is converted to lactic acid.
The characteristics of the yoghurt depends on the composition of the milk, the yoghurt culture type and the temperature and duration of the fermentation process. Fermented milk products will keep for a longer time than non-fermented dairy products as the acid in fermented dairy products inhibits the growth of bacteria that can cause decomposition.
Fermented milk products have been made for about 4000 years. Making yoghurt and cheese from milk also had the advantage of keeping the milk for a longer period. Milk was kept in leather bags or wooden barrels and with the presence of bacteria and a high temperature, the milk started to ferment, resulting in yoghurt.