Gather and sterilize your 4 quart jars and lids (and a pint jar, if you want to have a starter for your next batch).
Pour a gallon of whole milk into a stainless steel pot and heat it to 185-195 degrees Fahrenheit. (You can tell it’s getting close to this temperature by the small bubbles that form on the edge of the pot.) Stirring mostly keeps milk from sticking to the bottom of the pot (although I have never succeeded in not having some milk stick to the bottom).
Heat some water to around 120 degrees Fahrenheit. To sterilize your jars and lids, put this hot water and the jars and lids in the cooler that you will later put the jars of milk into. (Or you can use a separate pot and sterilize your jars in a pot on the stove.)
Once the milk reaches 185 degrees, remove the pot from the burner and place the pot in a sink filled with 3-4 inches of cool water. Cool the milk mixture to 120/125 degrees Fahrenheit. Stirring the milk helps release heat as well…or just let the pot sit in the sink if you are busy doing something else. (I have found that the milk cools much quicker in the wintertime and much slower in summertime because of the indoor temperature of the house.)
Once the milk has reached 120/125 degrees, add the 1 cup of yogurt starter. This can be a cup of yogurt from a previous batch or a cup of plain yogurt purchased from the store. (I have found that Dannon works well. Yogurt with whole milk will produce a thicker yogurt.) It is very helpful to use a whisk to stir in the starter so it evenly distributes the yogurt cultures throughout the milk…very important for the consistency of the final outcome.
Then you can fill your sterilized jars with the milk mixture. (I remove the bubbles at the top using a spoon…just to remove excess air.) Put the lids on and place the jars in the cooler with the warm water (120 degrees)…making sure the water level comes about ¾ of the way up the jar. Place the lid over the cooler and put the cooler in a draft-free place so the temperature stays stable inside the cooler; set a timer for 3 hours.
After 3 hours, you can transfer the jars from the cooler to the fridge. At this point, the yogurt may still be a little runny. But once it’s been in the fridge for a few hours, it will solidify more.
If you like how the yogurt turns out, you should be able to constantly use this yogurt to make your next round of yogurt. This means, you may never need to buy yogurt again.
