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Ingredients
  • Whole Milk
  • Live Yogurt culture
  • Glass jars
Steps
  1. Clean the glass jars with hot water, fill to 80% mark with milk, then microwave until they reach 181 - 185 F (83 - 85 °C) in the microwave. In my microwave, 360 ml of refrigerated milk takes about 3 minutes at 900 watts to reach that temperature. Use coffee thermometer to check periodically.
  2. Close the jars loosely and allow to cool slowly to 43 - 45 °C (108 - 115 F), which takes about 30 minutes.
  3. Clean the rice cooker with hot water and fill it with a mixture of hot and cold tap water until the thermometer reads 45 °C / 115 F.
  4. Stir a tablespoon of yogurt starter into each jar, about 5% to at most 10% the volume of milk. Close the jars, and place them in the water-filled rice cooker, close the lid, and cover the rice cooker with a towel. Don't turn it on.
  5. If the water cools down below 35 °C (95 F), you could put it on warm mode for a while until it's back up, but be careful as it keeps going to about 55 °C which may kill the bacteria. Or just add more hot water.
  6. After 5-8 hours the yogurt should have set. Don't move or bump it in the meantime and measure temperature of the surrounding water rather than the yogurt.
  7. Once set, place the yogurt jar in the fridge for 24 hours to set properly.
  8. Either ferment from the store-bought yogurt, or save a starter culture removed at about the 4-hour mark of fermentation, before all the lactose is consumed. After one week you'd have to feed it with some fresh pasteurized milk, and use within two weeks.
Notes
  • S. thermophilus optimal growth temperature range is 35 - 42 °C (95 - 108 F), L. Casei 35 - 40 °C (95 - 104 F). 
  • Both are facultative anaerobes (don't have to use oxygen), and can ferment up to 45 °C, but not 50 °C. 
  • Do not rush the cooling down of the milk - it needs to stay hot at least 20 minutes to coagulate. 
  • Do not add the starter too early: temperature of 120 F (48.8 C) or higher damages the bacteria, shutting them down. 
  • Using less than 5% starter makes fermentation take too long, more than 10% makes it too fast for the yogurt to set. 
  • Do not let jars touch the bottom of the cooker, even on keep-warm mode that gets too hot, allow the culture to be heated by the vapours primarily.