- A cooler to keep the yogurt warm while it ferments. I’m sure there are many ways to keep yogurt warm, but I find the cooler straightforward, and that’s what I’m going to describe here. We make two quarts at a time in a little six pack cooler.
- Very clean canning-type jars
- Hot water bottle (optional)
- Towel(s) for insulation
- Your last store bought container of yogurt. You need live yogurt to start the culture, only a few spoonfuls. The label should say something about containing live, active cultures. You’ll need 1 Tablespoon of live yogurt for every quart of milk you’re transforming.
- Milk, of course. Make sure your milk doesn’t say “Ultra Pasturized” or UP on the label. That stuff is just nasty. Otherwise, you can use whole, 2%, 1% — and even skim, I presume, though I’ve never tried it. How much milk? As much as you want. But it seems to me that for the trouble, a quart would be the minimum it would be worthwhile making. After all, it keeps a long time.
- Heat milk gently to 180 degrees Fahrenheit. If you’ve got a thermometer, great. If you don’t, 180F is where the milk starts to simmer. Just watch for those first tiny bubbles to start rising. When they do, turn off the heat. (Heating the milk makes for thicker yogurt. You could skip this step if you like. I would skip it if I got my hands on some nice raw milk.)
- Let the milk cool down to about 110F. This is the only hard part–waiting for it to cool. 110F is about as hot as a hot bath. You can put your finger in it and keep it there.
- While you’re waiting, boil water to heat your jars. I like to fill my jars with boiling water, cap them, and let them sit until it’s time to use them, at which point I pour the water out. I know it’s not really sterilization, but it’s something, and it pre-heats the jars, which is important. You could also pull the jars straight from a hot dishwasher, or actually boil them. Also, you’ll want to pre-heat your cooler. Pour hot water in it as well and let it sit until the last moment. And fill up your hot water bottle, if you have one.
- Stir in 1 Tablespoon of yogurt for every quart of milk in your pot. Use no more than that. Stir until dissolved.
- Transfer the inoculated milk into warm jars, cap them, and stuff them into the warm cooler (which you’ve emptied of water). Do all this fast so you don’t lose much heat. Your mission is to fill the cooler up, so there’s no empty space, with some combination of jars of yogurt, towels and heating devices like hot water bottles or lacking one of those, just more jars filled with hot water. My routine is to put 2 quart jars in a six pack cooler, slide a hot water bottle between them, and pack the top of the cooler with an old towel, so that I can just barely manage to lock the lid in place.
- The goal is to keep the yogurt very warm for about 8-12 hours. You might not be able to keep it at 110F the whole time, but it should be in that neighborhood. Certainly above body temperature. My set up described above seems to do that well enough. I’ve never checked the temp. inside, fearing to lose the heat. It just works.
- After 8-12 hours the milk in the jars should look yogurty and taste yogurty. It might not appear thick enough, but remember that it is quite warm. It will thicken some after it goes in the fridge.
- If it doesn’t look yogurty at all, add a smidge more starter, rewarm the cooler and everything, and try it again for another 8 hours. Consider that your starter–your store bought yogurt–may not be alive. Either that or the cooler wasn’t warm enough.