I tried to make homemade yogurt a few weeks ago and failed miserably. Yogurt cultures are more temperamental than yeast and the margin for error is much slimmer. Temperatures under 105F will make the cultures dormant and temperatures over 125F will kill them. The first time I tried I left the yogurt in a warm oven overnight (not turned on). I think I got overly paranoid about killing off the cultures and ended up cooling the milk too much before I added them. This time I was careful to keep the milk hot, but not too hot, before I stirred in the starter yogurt.
The other issue was the incubation. While other people swear by the oven method, it obviously didn't work for me. This is where the engineer comes in.
C hates yogurt but took it upon himself to come up with a solution to my problem, because that's what he does. He solves problems. We have a basement laboratory full of gadgets and tools, so he built me an incubator out of a cooler, a temperature probe, an electric blanket, and some contraption that monitors the temperature recorded by the probe that would turn on the blanket when it got too cool and turn off the blanket when it got too hot.
This morning I came downstairs to three beautiful jars of yogurt.
1/2 gallon skim milk (you can use 1%, 2%, or whole milk for creamier and probably tastier results, but in the interest of the P90X diet I went with skim)
1/4 cup nonfat powdered milk (supposedly if you're using full-fat milk you can leave this out)
1/4 to 1/2 cup starter cultures - you can use store bought yogurt as long as it contains "live, active cultures" (for future batches I'll just reserve some of my own)
Heat the milk and milk powder in a large stock pot until it reaches 180-185F. It's best not to let it boil because then you run the risk of scalding the milk, especially on the bottom of the pot.
Once you reach the temperature, remove from heat and cool down to 120F. This could take a while - you can move things along by stirring it or by setting the pan in a sink of cold water. Just make sure you don't cool it down too far or the cultures won't wake up.
Ladle out a few scoops of the warm milk into a bowl. Stir in the starter culture, then add back to the larger pot.
Pour the mixture into airtight jars (or one big jar).
Now comes the incubation period. There are a variety of recommended methods - heating pad, warm oven, water bath, sunny window - I used my engineer husband. Either way, the goal is to maintain a milk temperature as close to 115F as possible (of course, a yogurt maker will do this for you, but who wants that unitasker taking up space in the kitchen?).
Either way you choose, let it sit from 5-8 hours. The longer it sits the firmer and tangier it will be. I prefer Greek-style yogurt, so I let mine go all night long and then strained the whey out. Next time I'm going to make it during the day and probably pull it out of the heat a little sooner. It's just a tad too sour to eat plain.
You can eat it plain or stir in flavorings. Make sure to put aside 1/2 cup of plain, unflavored yogurt for your next batch! My favorite yogurt add-ins are strawberries, granola, and honey. Yum!
Here is the whole setup:
The finished product: