Probiotics are living microorganisms that have beneficial effects on people's health. They are mostly species of bacteria that are either found in the human gastrointestinal tract or are similar to those that are found there. Probiotics change the environment in the human gastrointestinal system around them in ways that make it difficult for harmful bacteria to survive. One of the most common forms of probiotic preparations, and one that is relatively easy to make at home, is yogurt.
Put the jars and lids into the large pot. Cover them with water. Bring them to a boil and allow them to boil for 10 minutes. This will sterilize them, giving the yogurt probiotics that you incubate in them a chance to grow without competition.
Empty the pot and add 1 gallon of milk to it. Heat it over a medium setting on the stove. Check the milk regularly with the thermometer. When it is between 180 and 195 degrees Fahrenheit, remove it from the heat. Do not let it get hotter than this. Wait for the yogurt to cool to between 115 and 120 F.
Pour 1 cup warm milk into the measuring cup. Add enough of the yogurt that is being used as starter to the measuring cup to bring the level up to 2 cups. The yogurt used as starter contains live cultures, and should have no sweeteners, preservative, gelatin or fillers. Stir the mixture with the spoon until the ingredients are blended.
Add the starter and milk mixture to the pot of warm milk, stirring as you pour. Pour the inoculated milk into the jars. Seal the lids in place.
Warm up a gallon of water to 130 degrees Fahrenheit. Pour it into your cooler. Set the jars of milk into the cooler and close the lid. Allow the yogurt to ferment for a minimum of three hours. This is the minimum time needed to grow enough probiotics to gel the milk. Some makers will ferment it up to 24 hours, to more fully develop the product.