Cheese, yoghurt and fermented milk drinks are made using cultures that can produce acid from the milk sugar lactose, break down milk proteins and add flavour compounds and sometimes gas.
Yoghurt is a thickened, fermented milk made using a mixture of bacteria which work together to produce the desired acid and flavours. The microbes are only killed if the product is pasteurised to extend its shelf life. Many people believe that eating live yoghurt bacteria is good for their health.
Cheese manufacture depends on microbial activities at several stages to produce the required flavour, texture and appearance. There are many different types of cheeses. Blue cheese is made by inoculating the whole cheeses with a fungus which gives the typical taste and appearance.