|Instant non-fat dry milk||1⁄4 Cup (4 tbs)|
|Yogurt||2 Tablespoon (Your Own Or Commercially Made)|
Combine the milk and non-fat dry milk in a heavy pan; bring slowly to the boiling point, stirring.
As soon as it boils, remove from heat and cool to about 115° (you can put the pan in a bowl of cold water to speed the cooling).
If you don't have a thermometer, put a drop of milk on your wrist; it should feel warmer than lukewarm.
Blend a little of the warm milk with the yogurt until smooth, then stir the yogurt mixture into the warm milk.
At this point the object is to maintain the milk at about 115° for 6 to 8 hours (if it goes below about 95°, the action stops).
You can leave the milk in a bowl, if well covered, but it may be easier to handle if transferred to a prewarmed glass jar.
If you have a gas oven, the temperature inside with just the pilot light on may be about right€”the temperature must never go over about 120° or your culture will be killed.
Most people simply wrap the container of warm milk in some good insulating material, such as a heavy wool blanket.
Various insulated boxes and picnic bags work successfully, too; you can prewarm them by letting jars of very hot water stand in them while you are preparing the milk.
Be sure to place the insulated container of warm milk in a spot where the rate of cooling will be slow€”on a high shelf, in a cupboard above a wall oven (turned on), in the car parked in the sun with windows closed, or near the hot water heater.
During the 6 to 8 hours it usually takes to make yogurt, the milk should not be disturbed.