Pour the milk into a clean pot, and gently heat up to 180° f. I use a 10 quart stock pot placed inside a 20 quart stock pot. Nestle the smaller stock pot into the larger, and then fill the rest of the outside stock pot with water almost up to the top. (Note: This is a bain marie, or water bath cooking method, and raises the temperature of the milk slowly and gently. This is different than a double-boiler, where the level of the water in the outer pot would never touch the bottom of the inner pot, so that the water quickly would come to a boil.)
When the milk reaches 180°, then cool it back down to about 110°. This initial heating kills any other bacteria that might be present, and also starts to pre-process the protein in the milk. You can take the inner pot out of the hot water, and place into a sink of cold water to speed the process. You can also add ice cubes to the water in the sink to cool it even faster. Don’t let the cold water in the sink come up over the top of the pot of milk.
Add non-fat dried milk powder to the milk, either while heating or while cooling. Just add the powder, not the extra water. This adds more protein and milk solids, without adding more liquid or fat. I use about 1 cup non-fat dried milk powder per gallon of milk. The non-fat dried milk powder is optional.
When the milk has cooled to about 110°, mix in the starter yogurt. Use a couple of teaspoons of yogurt per gallon of milk. More starter yogurt will speed up the process slightly, but don’t user more than 3 tablespoons of starter per gallon of milk. To help the thicker yogurt mix well into the milk, I put the yogurt starter into a small bowl, add a half-cup of milk, and stir that well before adding to the larger pot. Stir the starter well into the pot.
Cover the top of the yogurt pot with a lid or aluminum foil, and keep somewhere for at least five or six hours at 110°. You might want to use a fancy electronic thermometer that can be programmed to alarm you if the temperature goes over, say 120°, or under, say 105°. If the yogurt mixture gets too cool, it will only slow down the process. (Solution: gently rewarm to 110°). If the yogurt mixture gets too warm, say over 120°, then it may kill the starter yogurt. (Solution: Cool to 110° and add fresh starter). The time is variable, because it all depends on how well your starter culture thrives and how consistent the temperature is kept.
After four or five hours, you should start to see the mixture thickening. When you see pools of clear greenish whey form on top of the yogurt, you can do one more thing to speed the process: Gently run a wire whisk or long knife through the thickened yogurt. This will break up the thickened yogurt and help the whey separate out. DO NOT STIR the yogurt, just run the whisk slowly through the thickened yogurt to break it up a little. After this process, let the yogurt continue to brew for another hour or so.
The total time to finish this part of the process depends on the factors mentioned above, but also how thick you like your yogurt. The longer you let the yogurt sit at 110°, it will become thicker, and will also become tangier. If you don’t like tangy yogurt, then stop the process a little earlier.
Strain the yogurt to separate the yogurt from the whey. Pour the yogurt mixture into the cheesecloth-lined colander. You should see the thin greenish whey fall through the strainer into a bowl below. If the whey looks very milky, then either the cheesecloth is too coarse, or the yogurt has not cooked long enough. If the entire batch of yogurt will not fit in the strainer at one time, wait a little until more whey has strained out, and that makes more room in the strainer for the rest of your yogurt.
The longer you leave the yogurt in the strainer, the thicker it will become as more whey drains off. You can drain the whey into another bowl if you plan to use the whey for something, or just let it run into the sink. If you drain whey into a bowl, remember to keep a few inches between the bottom of the strainer and the level of whey that is draining. We often set the strainer in a larger bowl, and put the whole assembly in the refrigerator overnight. If the yogurt is too thick for your tastes, thin with some of the whey or some milk. We like a very thick yogurt, so we drain off a little more than one gallon of whey from two initial gallons of milk, leaving a little less than one gallon of finished yogurt. One last optional step is to put the strained yogurt into a large bowl and beat with a hand-mixer to smooth out the texture. Put into plastic containers with tight-fitting lids, and store in the fridge.
Adapt the recipe and process to your own tastes. Longer cooking time makes thicker and tangier yogurt. Longer straining time also lets the finished product thicken even more.
Serve with honey, preserves, fresh or frozen fruit, to taste. We always add the flavorings to each bowl as you are eating it, not into the larger plastic tubs of yogurt.
You can use your home-made yogurt as your starter base for your next batch, but we found it to be a little tangier. We prefer to go no more than one batch from store-bought and then one batch from that homemade batch, then back to store-bought. We usually divide an 8 oz or 6 oz package of store-bought yogurt into starter for three or more batches, but only if those later batches are still within the expiration date of the original container.
You are not done yet. Thoroughly clean all pots and pans. Dried-on Yogurt is very difficult to clean, so be prompt. Thoroughly hand-wash your cheesecloth with soap and water, then rinse thoroughly in cold water until all suds are gone. When the suds are rinsed out, refill a container with hot water and a capful of bleach. Soak your cheesecloth in the bleach solution for at least a minute, then wring out and hang up to dry. When I use my cheesecloth the next time, I give it the sniff-test. It should smell slightly of bleach, with no sour or moldy smell to it.
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