Yogurt is a thick and tasty product made from milk. It is actually made by bacteria, which thrives in milk. The flavor of yogurt actually comes from lactic acid which is produced by the bacteria fermenting the milk.

Yogurt bought in the store is usually an active, living, culture of this bacteria. As such it is very easy to make your own yogurt! Just bring some whole milk to a boil, and let it boil for 10 mins. Let it cool till you can stick your thumb in it without it being too hot (or about 97 F). Then add some plain yogurt. Cover, and let it sit out for a day or two. (actually room temperature isn't hot enough, it needs to be kept at a temperature about 95 F)

For added flavor, use Goat Milk. A nice treat can be made by adding some shredded cucumber, a little mint and a dash of black pepper. Just dip your pita bread in it for a nice traditional persian snack, called maast o'okeeyaar.

It's not really true what thecarp wrote. The main bacteria used are Lactobacillus bulgaricus and Streptococcus thermophilus, but the culture cocktail also contains more strains of lactobacilli and lactococci, as well as Bifidobacterium strains. The different types of yoghurt are produced by different mixtures, and ratios of the involved bacteria, as well as the type of milk used (the animal, low/high fat content).

During fermentation the lactose is fermented to lactic acid and polysaccharides are formed (that's the component that makes the yoghurt slimy and lumpy).
(the reasons for the high temperature during the industrial production process is, that they want to have full control over the bacteria in the milk, and has to have a longer shelf life than your home-made yoghurt.)

Making yoghurt in your kitchen can be done more easily:
just warm the milk to body temperature, add yoghurt (2-3 teaspoons per litre) and pour it into a clean jar, close it, wrap a towel around it and leave it next to the radiator for 3-8 hours. This even works to make soy yoghurt.

Thing is, you can't keep on doing this. After two or three times using you home-made yoghurt as inoculant, the ratio of bacteria has changed, because some of them grow faster than others, hence the taste will be different.

In the the movie Spaceballs (a Star Wars parody), Mel Brooks plays the character of Yogurt, the counterpart to Yoda. He is the owner of the up-side of the Schwartz, and gives his ring-sword (which looks like a light saber, of course) to Lone Star so that he can get Princess Vespa back to her home planet and out of the clutches of Dark Helmet (played by Rick Moranis, very funny guy).

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