What is yoghurt, really

In brief:

Yoghurt is formed by the bacterial fermentation of milk. It can be made from any milk, but cows milk is usually used. Bacteria cause the sugar in the milk to ferment, that produces lactic acid. This then reacts with the protein in the milk to form yoghurt.

In more detail:

Yoghurt is a dairy product. It is produced by the fermenting action of live bacteria.

In yoghurt (in the USA) this bacteria is either Lactobacillus bulgaricus (called L. bulgaricus) or Streptococcus thermophilus (called S. thermophilus).

The fact that the bacteria are live is critical to the formation of the yoghurt. It is the activity of the live bacteria that converts the lactose (a form of sugar) in the milk into lactic acid, which in turn, because of its acidity, reacts to cause the proteins in the milk to solidify (or at least thicken).

Some yoghurts are then pasteurised to kill the bacteria before they are sold - the container will be marked to state whether there are live bacteria present. Most people prefer the live bacteria versions for perceived or actual health reasons.

Note: In the US, the "National Yogurt Association" (NYA) has defined specific criteria for live and active culture yogurt. In order for manufacturers to mark their products with the 'live and Active' seal, refrigerated yogurt products must contain more than 100 million cultures per gram at the time of manufacture, and frozen yogurt products must contain 10 million cultures per gram at the time of manufacture.

So 100 grammes of yoghurt will contain 10 billion live bacteria!!!