Large pot that holds your jars
Mason jars with lids (2 quart or 4 pint)
1/2 gallon of whole milk
4 tablespoons of plain yogurt containing live, active cultures
This recipe yields four pints or two quarts of yogurt but can easily be scaled to make more or less yogurt. Today, I made my yogurt in pint jars. I haven't had good luck making it in smaller jars than this due to the simple way I keep my yogurt warm while it ferments, sitting on the counter either wrapped in a towel or inside a cloth insulated bag.
Fill the jars with milk leaving a little room at the top because you will be adding a tablespoon of yogurt later. Loosely place the lid on each of the jars and place them into the pot. Fill the pot with water until it is about three quarters of the way up the sides of the jars. Turn the burner to medium heat and heat the milk for about 30 minutes, until it reaches 180 degrees Fahrenheit. It's okay if it gets hotter than 180 degrees.
Cool the milk to 115-120 degrees Fahrenheit. It took about 65 minutes for my jars of milk to cool, but I left them in the hot water bath while they cooled. You can cool the milk more quickly, but be cautious if you plan to place them in a cold water bath as you risk breaking the jars.
Once the milk has cooled, place one tablespoon of yogurt into each jar and stir. Put the lids on the jars and either wrap them each in a thick kitchen towel, or place them all in an insulated cooler bag. Leave the jars on the counter for anywhere from four to twenty-four hours. You can get much more complicated with this process of keeping the milk warm while it ferments, but I find anything more unnecessary. The chemical reactions taking place create some heat, and that combined with a small amount of insulation keeps the milk warm enough to ferment.
After the fermentation, it is best to place the warm yogurt into the refrigerator to cause its structure to firm and slow the production of acid, but I really enjoy eating the warm, fresh yogurt with a bit of granola and fruit.
The milk should not be warmer than 120 degrees when adding the yogurt starter. Anything hotter may kill the live bacteria in the yogurt starter.
I always use a tablespoon of yogurt from my previous batch as my starter, but you can use any yogurt containing live cultures.
From my experience, keeping the milk at 180 degrees for several minutes and leaving the yogurt to ferment for about 20 hours yields a thick yogurt. I don't notice it getting too tangy for my family's tastes unless I let it ferment for more than 24 hours. The yogurt is ready in as little as four hours if you don't want it to be very thick. Experiment to see what length of fermentation you prefer.
This is a fun activity for slightly older children to see the resulting yogurt they create from milk. Younger children can also help with this activity by measuring the temperature of the cooling milk and adding the yogurt starter to the cooled jars.