|MadSci Network: Microbiology|
If you have available some plain yogurt (preferably not a fruited variety) which is labelled as having "active cultures" then it can be used as inoculum to convert fresh milk into more yogurt. Add a teaspoon of the store-bought yogurt to a quart of fresh milk mix well and allow it to stand at room temperature for about 24 hours. You will now have a quart of yogurt. I would not continue to use this new yogurt as inoculum for a new batch as foreign organisms may begin to take over and you will have spoiled milk rather than yogurt.
You should try to pasteurize the milk before adding the inoculum or "starter" in order to minimize competition from other bacteria that lead to the spoiled milk problem. There are specific recipes available online at sites like:
FiasCo Farm or
Try the links in the MadSci Library for more information on Microbiology.