Some detail on dahi making:

Dahi (homemade yoghurt) is fermented milk in which some of the lactose has been converted to lactic acid by the action of bacteria. Dahi is a highly valued foodstuff because it has the same nutritional value as milk, but 90% of yoghurt is digested with one hour, whereas only 30% of milk digests in the same period. This is the case because the bacteria in Dahi produce enzymes that continue to work favorably in the digestive tract. Dahi that has been pasteurized or processed at high temperatures causes the bacteria to be inactive, and these benefits are then lost. to make Dahi, begin with 1 quart of regular Milk, to which you may add 2 Tablespoons of starting culture. To begin, you can use a natural regular yoghurt as starter, then just reserve some of the produced yoghurt to use as a starter in future. Details follow:

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Bring milk to a boil, being careful not to burn or boil over. The milk must at least reach 185° F for two minutes. Desirable temperature is 200° F. Exposure to high heat changes the protein structure in the milk, needed to make Dahi. If the protein structure is not changed, the milk proteins do not coagulate to a firm yogurt, making it stringy. The pasteurization process does not change the protein structure enough to make Dahi. The commercial pasteurization process heats milk to only 162° F for 15 seconds, killing unwanted harmful bacteria only. Ultra-Pasteurization process on the other hand heats milk to 280° F for 2 seconds killing unwanted harmful bacteria and changing the protein structures needed to make Dahi. You can also use evaporated milk thinned with 50% water to make Dahi. It has the suitable protein structures. If you elect to use evaporated milk or the Ultra-Pasteurized milk, you will have to warm it to about 120° F before introducing the starting culture.

Turn off heat. Let it cool to tepid warm, cooling to 122° F but not below 105° F. Add starting culture. Incubate in a warm place for 7 hours. You have to keep the milk between 98° F to 105° F for proper incubation. Wrap it in a blanket and put it on a heating pad at low for about 7 hours. When 7 hours have passed, you should have a perfect batch of Dahi. Store it in the refrigerator.

Be sure to save culture from the new batch of Dahi each time. All the commercial Yogurts have stabilizers and texture enhancing compounds added. When you make Dahi, the cultures grow and the stabilizers don't. So, with each new batch you get a purer bacteria than the previous batch. By the time you get to third batch, it is almost same as if you were having homemade Dahi in India.

Why use an electric heating pad? The human body temperature is 98.6° F. The heating pads are designed so that at "low" setting they will not cause burns. They may go up to 115° F, still in a safe range for the culture.

Temperature & Culture Relationship of Dahi:
Culture becomes dormant below 90° F.
Culture is most active at 115° F.
Culture starts to die over 125° F.

To make thicker Dahi, just add 4 Tablespoons or more of non-fat dry milk. Stir it in when milk has cooled down to 120° F. To make a creamier Dahi, add 1 pint (2 cups) of heavy whipping cream as the milk warms up. Heavy whipping cream available in the U.S. is Ultra-pasteurized, meaning it was heated to 280° F for two seconds. So, the protein structure is suitable to make Dahi. You can add the cream at any time to milk. Continue heating till the milk has reached 200° F. Turn off heat.

Dahi can be stored for up to 15 days at 40° F in the refrigerator. You can also freeze it for about a month without affecting Dahi’s cultures. To do so, whip Yogurt and put in a freezer bag and freeze. If frozen, thaw it in the refrigerator overnight.