Updated: Jan 15, 2020
1/4 cup good quality commercial plain yogurt (or previous homemade batch)
1 quart pasteurized whole milk, non-homogenized
a candy thermometer, if you want to be precise
Bring one quart of milk to the simmer stage (180 degrees) and remove from heat. Stir often to prevent scorching and sticking to the bottom of the pan.
Cover and cool to about 110 degrees. It is very important that you allow the temperature to drop so as not to kill the bacterial culture you are now ready to introduce.
Remove about one-half cup cooled milk and make a paste with one quarter cup of good quality commercial yogurt. The commercial yogurt you use should be unflavored and unsweetened. You could use a starter but why spend the extra bucks? Commercial yogurt works fine. You can use your home made yogurt as a starter for your next batch.
Mix the paste with the remainder of the cooled milk and stir thoroughly.
Pour milk into any appropriately sized shallow glass, enamel or stainless steel container, cover and let stand for at least 24 hours at 100-110 degrees up to a maximum of 29 hours. After 30 hours, it starts to kill the good bacteria. To keep the correct temperature for the culture, you can use a 60 watt bulb in the oven and leave the light on. No other heat is needed. Remember, too high a temperature will kill the bacterial culture and will prevent proper “digestion” (conversion) of the lactose. Too low of a temperature will prevent the activation of bacterial enzymes and will result in incomplete “digestion” of the lactose.
Remove from oven and refrigerate.
While this yogurt may not be as thick as commercial yogurt, it will be a true yogurt with no thickeners or extenders. For a Greek-style yogurt, strain the whey with cheese cloth or flour sack towels.