Making homemade yogurt is a simple and inexpensive, albeit time-consuming, process. Homemade yogurt has no added sugar or preservatives, will keep for up to three weeks in the refrigerator and can be adjusted to individual preferences. Using a heating pad to incubate your homemade yogurt is an effective way to keep the temperature stable while the healthy bacteria present in yogurt do their magic. The bacterial microorganisms consume the sugars (lactose) in the milk, causing the production of lactic acid that gives yogurt it's characteristic zing and results in curdled milk, or the familiar thick texture of yogurt.
Sterilize the jars by bringing water to a rapid boil in your stockpot and dipping each piece of equipment in with your tongs.
Fill the sterilized jars with 110 degree water, 2 inches from the top.
Turn on the heating pad to a medium setting, place a towel over it and place it on a wooden surface in a draft-free corner.
Remove one cup of water from one jar and place it in the blender. Add one cup of dried skim milk and ¼-cup of yogurt. Blend only until smooth. Return this mixture to the jar from which you removed the water. Repeat for all four jars.
Place the jars on the heating pad, folding the sides up to enclose the jars. Cover with as many towels as it takes to make an incubator. Leave it alone for seven hours.
Touch the yogurt after seven hours lightly with one finger to ensure that it has set. Stir it well, turn off the heating pad and place the jars in the refrigerator overnight.
Eat and enjoy.