1 cup powdered milk
1 cup sugar
8 oz plain yogurt
Remove milk and yogurt from refrigerator. Fill double boiler with water and place spoon (I use an immersion blender) and thermometer in water. Place inner (milk) pot upside-down on top of water pot. Bring water to boil and allow utensils to sterilize. Dry milk pot, spoon and thermometer with paper towel.
Pour milk and powdered milk in inner pot and mix well.
(Milk composition may be adjusted to achieve the desired fat and solids content. Often dry milk is added to increase the amount of whey protein to provide a desirable texture. Ingredients such as stabilizers are added at this time.)
Heat milk to 185°F for 30 minutes, 195°F for 20 minutes or 203°F for 10 minutes; stirring occasionally.
(A high heat treatment is used to denature the whey proteins. This allows the proteins to form a more stable gel, which prevents separation of the water during storage. The high heat treatment also further reduces the number of spoilage organisms in the milk to provide a better environment for the starter cultures to grow. )
Fill sink with about 4-6” cold water and add some ice cubes. Place inner pot into the water and stir to cool milk to 110°F.
(Cool milk to 110°F to bring the yogurt to the ideal growth temperature for the starter culture.)
Add plain yogurt to cooled milk and stir well to distribute. Place pot in oven set to 108°F and let milk sit for 3-5 hours.
(The milk is held at 108°F until a pH 4.5 is reached. This allows the fermentation to progress to form a soft gel and the characteristic flavor of yogurt. This process can take several hours.)
Add sugar* and mix well. Pour yogurt into container with well fitting lid and place in refrigerator to cool.
(The yogurt is cooled to 44°F to stop the fermentation process.)
Add desired fruit or flavors.
*For starter for next batch, remove 1 cup of yogurt. Add sugar to remaining yogurt and mix well.
Greek style yogurt or yogurt cheese:
Place a double thickness of cheesecloth in a colander and the colander over a bowl. Carefully pour yogurt onto cheesecloth. Cover with towel and let sit for about 2 hours.