I have made several batches of yogurt in the last two weeks – for giving away, for showing friends how to make their own, and for eating and baking with here at home. Yogurt making is not an exact science, which is apparent if you do a Google search on the topic. Still, once you make a successful batch you're likely to do it the same way ever after. Here's my technique; try it, and then tweak it to your own liking.
Makes one litre
In the top of a double boiler, whisk together
1 litre whole milk
1/2 cup full cream powdered milk
and bring to 180ºF, or just below boiling.
Set pan of milk aside and let cool to 110ºF. (You can accelerate this by setting the pan in cool water.) Place a lid over the hot water in the bottom of double boiler, and set into a towel-lined camping cooler; drape with another towel & close cooler. This will be your incubator.
As milk cools, skim off any film that forms. In a small bowl, dissolve
1 packet dried yogurt starter or 2 tablespoons fresh yogurt
and return mixture to the pan of milk; stir thoroughly to combine.
Pour mixture into very clean container, cover, and set in the cooler next to the pan of hot water (drape towel over both to make a tent). Close cooler. Check yogurt after 4 hours; if texture is to your liking, place in refrigerator to stop process. If thicker or tarter yogurt is desired, leave in cooler for up to 7 or 8 hours.
Use as a sour cream substitute, a buttermilk substitute in baking, or enjoy on its own (it's especially good with a sprinkling of brown sugar or a bit of honey).
I like the "High Quality Milk" brand here in Taiwan, but use whatever milk tastes good to you. I have used both low-fat and full-fat powdered milk with similar results, so again, choose what you like. Yogurt starter is available at health food stores in the States as well as by mail-order (Yogourmet is a popular brand). Here in Kaohsiung a friend found dried starter at a health food store in the Zuoying district (on Fumin, just south of Mingcheng Rd., very near the Finnish restaurant). Look for the little green box in the cooler at the back left corner of the store.
This yogurt will keep well for at least a couple weeks. After you dig in, you may notice some separation of the whey; pour this off or mix it back in, depending on how thick you want to keep the yogurt (or use the back of the spoon to level off the yogurt each time, and you won't have the problem in the first place). I like my yogurt quite thick, but for a thinner product reduce the powdered milk by half or more.
For future batches, freeze several 2-tablespoon portions in an ice-cube tray and label them "first generation." When you're ready to make more yogurt, remove a frozen starter cube and allow it to come to room temperature while the fresh milk is heating. Use as for fresh yogurt, above. This process can be repeated once more (this time labeling the frozen yogurt "second generation"). Do not freeze the third generation, however; start over, or use another first of second generation frozen cube. Be aware, the yogurt will be tarter with each succeeding generation.