How Is Yoghurt Made?

Yoghurt is made by taking milk, introducing bacteria and keeping it at a controlled temperature.  The bacteria ingest the natural sugars in milk.  This causes lactic acid to be released as a waste product.  Thsi acid then causes the milk to solidify or 'set'.  
To qualify as yoghurt as opposed to curds or whey, yoghurt has to contain the bacteria streptococcus salivarinus ssp thermophilus and lactobacillus bulgaricus.  
yoghurt cna be made at home by taking some 'live' yoghurt ie that which has the bacteria alive within it and mixing that with bolied milk and keeping it at a warm temperature.  However, the results can be mixed and it is not always successful.  mainly this is due ot the fact that the temperature has to be kept constant and this can be difficult to achieve at home.
Yogurt makers are available for home use.  They enable the bacteria to be introduced within a flavour and the temperature is thermostatically controlled to allow the bacteria to breed.  By controlling the temperature in this way, it is easier to gain consistent results.

Yoghurt is a dairy product made from milk. Technically, yoghurt can be made from any sort of milk – cow, goat or ewe – but it must be fermented before it can be called yoghurt.

Originally, yoghurt was 'invented' in Western Asia and Eastern Europe but it has become popular in the west and all over the world in the last 30 years or so. The bacteria used in the fermentation process are either Lactobacillus bulgaricus or Streptococcus thermophilus. A small starter culture of the bacteria is added to fresh, warmed skimmed or whole milk and then the mixture needs to be kept at about 37 degrees Celsius for about 12 hours.

The bacteria multiply in the warm conditions, feeding on the sugars in the milk. They ferment lactose, the disaccharide sugar in milk, producing lactic acid. This causes the milk to curdle, forming curds and whey. The curds are the yoghurt and the whey is the watery fluid that is poured off, leaving a rich, creamy thick yoghurt.