Add two gallons of milk to a large stockpot. You may use whole, 2% or skimmed milk. Heat the milk slowly over a medium heat (not so hot that it burns on the bottom). It may need stirred a couple times depending on the quality of the stockpot. Heat milk until the temperature is 180*F. It is not necessary to boil the milk.
Note: Use a sturdy, heavy stockpot to heat the milk. It will avoid scorching the milk. We are so thankful for the high-quality stainless steel cookware Daddy got for our kitchen many years ago! Thank you Daddy!
A double-boiler can also be used to heat the milk, if a stockpot is thin.
When the milk reaches 180*F, turn the heat off and let the milk cool to 110*F. Putting the stockpot into a sink of cold water will quicken this step. Otherwise, this is a good opportunity to move on to other projects for a bit!
When milk has cooled to 110*F, add 1 six ounce cup of Yoplait yogurt. Any flavor can be used, because it doesn't flavor the entire two gallons very much.
With a wire whisk, beat the milk & yogurt starter for a minute. It is important for the starter to get completely dissolved into the milk. The milk mixed with yogurt starter is now called inoculated milk.
Once thoroughly mixed, pour the inoculated milk into the desired jars. Fill to the neck of the jar. We use gallon jars - however, for the smaller family - half gallon or quart jars are much easier to manage and work just as well!
Put lids on tightly and place jars into a cooler. It is helpful if the jars fit snugly into the cooler - it all stays warmer and turns to yogurt faster. Warm water [110*f] can also be added to help stabilize the temperature.
Place the lid on cooler and set in a warm place for 3-4 hours. The yogurt is done when it has jelled. This can be easily checked by gently tipping the jar to see if it is milky or firm. If the milk has not become firm in 4 hours, the cooler may not be quite warm enough and it will just need a few more hours. When it is jelled, place in the fridge overnight to completely cool down and set.