powdered yogurt start (or yogurt)
1. Fill nice, clean jar(s) about 3/4 full of clean (boiled) warm water. Ideal temperature for incubating yoghurt is 43° C (110° F), just a bit warmer than a baby bottle. Water can be slightly hotter than incubating temp since some heat will be lost in warming up jars, blender, and pot.
2. Pour the warm water from the jars into a blender till it's 2/3 (or less) full.
3. Add full-cream milk powder, and jolt the blades intermittently to stir. Don't turn blender all the way on or you'll get a lot of air whipped into your milk.
4. Throw start powder (or a few tablespoons of yoghurt) in. (Start powder is pricey; you can short it by half. Save remainder of packet for next time you need powder start.) Jolt to mix.
5. Pour mixture into large pot for standardization. This makes sure that all the milk gets inoculated with yoghurt.
6. Repeat steps 2, 3, and 5 as needed till all jars are empty.
7. Check temp of mix. If too cool, put pot over low flame (do not microwave!) and stir till temp is achieved.
8. Refill jars from pot.
9. Put lids on jars, and incubate.
10. Check after 5 hrs. (3 hrs if you used yogurt to start), thenceforth hourly. When yogurt has "made" (gelled), put in refrigerator.
11. Eat at will.
Somewhere before step 7 you need to decide how much milk powder to use. You can calculate based on the amount of water you have, or you can taste and add milk powder as you go. If you use the taste-and-add method, do use a clean spoon each time you taste. I like thick yogurt, so I use more powdered milk than called for by the amount of water.
I don't use powdered start every time. I'll start once from powdered start, then carefully save out about half a cup from that batch. This half cup is my starter for maybe four or five more batches, and it lives sealed in its own little jar in the fridge, and is only visited by clean spoons on yogurt making days. The cleaner your operation, the longer you can run it before things start to go sour.* The sourness comes from the intrusion of bad guys, like wild yeasts and bacteria that drop in from the air.
Powdered yogurt start is sleeping bacteria. It takes time for them to wake up and decide that they'd better get to work. (Like some people.) Six to eight hours is not unusual for making a batch from powdered start. Ready to eat yogurt is not asleep. That's why you hear tiny screams when you put it in your mouth. Four hours is a usual time for a batch to "make" from yogurt.
I've made small batches (2 cups, say), and large batches (a big pot full). One advantage of the large pot is that it holds warmth better. Another is that you don't have to do it so often. My standard batch now is three jars; total volume is around a gallon. Seems to me that there are no limits, except practical ones, on batch size.
If I don't dally, it takes me around 10 minutes to mix and prep three yogurt jars. The good guys do all the work.
Lemme know how you do.
*This principle has been found to have applicability beyond yoghurt making.