Making Yogurt

Making yogurt is not a matter of assembling, preparing and combining the correct ingredients as it is a matter of propagating an existing organism: to make yogurt, you need to have yogurt. By and large, the organism manages itself, you just have to take away all excuses for the organism to do otherwise.

Tools:

1. Thermometer.
   A candy or meat thermometer will do, so long as it will read quickly and accurately.
2. Heat source.
   Some people do this in a crock pot. I've never tried, so satisfied am I with the stove. You can even use a microwave if you want to. I have. It turned out fine, I just have a vendetta against my microwave.
3. Pot or pan or bowl in which the milk will be heated.
4. Jars with lids or sealed containers of the appropriate size to contain and incubate the treated milk.
5. A method of incubation.
   I personally use quart jars in a small cooler and periodically pour warm water into it. Some combine the sealed container with the incubator and use a thermos. Some will heat their oven, turn it off before loading the yogurt and use that as their incubator. You want something that can maintain a steady, warm-but-not-hot temperature for hours at a time. Yogurt can even be incubated by putting it in a sunny window or a warm place for a few hours, I haven't tried that. I have too many control issues to try that.
6. Yogurt starter.
   Again, to make yogurt, you need yogurt. You can get just the bacterial cultures from online sources, or you can start with a little leftover yogurt from a previous batch. A few times, I have discovered to my deep shame that my yogurt had gone awry before I could use it. On those occasions, I just ran to the store and bought a little cup of single-serve yogurt.
7. Strainers. (Optional)
   Commercial yogurts will often use pectin to thicken their yogurt. This isn't Greek yogurt, it's just a cosmetic decision, and a good one, too. Thick yogurt is more pleasant than runny yogurt. The thickness of your yogurt is affected by how long you hold the milk at the 180 degree temperature and how long you incubate it. But if you want Greek yogurt, put down the bag of tricks, you all out need a strainer to do the job. Greek yogurt strainers exist and I love mine, but if you haven't got that by some terrible twist of fate, you can accomplish the task by laying down layers of cheese cloth over a strainer, setting that in a bowl and then pouring in your yogurt.

The basic method:

1. In a pot, heat the milk to 180 degrees Fahrenheit.
   This is not a question of sterilization, though I'm sure that's an added benefit, it's preparing the proteins in the milk for the cultures they need. If you maintain the milk at this heat for a longer period of time, say, half an hour, the result will be naturally thicker yogurt. I'm lazy and impatient. I don't do that, I just use a strainer.
   The milk will of course develop a skin which can be stirred right back into the pot.
2. Allow the milk to cool down to 115 degrees Fahrenheit.
   Walk away for a while. Put the dish in an ice bath, whatever. If you add the cultures before it cools down, you'll kill the cultures.
3. Mix in the cultures.
   Rule of thumbs are nice. I haven't yet found a specific one for how much culture to how much milk and tend to go with “However much culture I set aside last time I made yogurt” and “however much milk I've decided to use this time” and haven't disappointed myself yet. But that isn't very encouraging for someone who's never done this and isn't sure they know what they are doing. So let's say this: You should use probably 2 tablespoons of yogurt to 1 quart of milk.
   Here's the best way I've found to mix it: dump the yogurt into a resealable jar, ladle out about a cup of the warm milk onto the yogurt, seal the jar and shake it vigorously. Pour the mixture back into the rest of the milk and stir well.
4. Incubate the cultures.
   Pour the mixture into whatever method you've chosen to incubate them. When I do it, I pour them into quart sized glass jars, stick them into my cooler, pour some warm water into the bottom of the cooler and walk away. If you're using the oven, heat the oven to 115 BEFORE you get to this stage and turn it off.
5. Let set.
   Leave the mixture alone, utterly alone, for 2 hours. Once you've set the cooler down/closed the oven door, poured it into the thermos, don't jostle it.
   It is still not done, not even after 2 hours, but it's okay to examine after that. It's okay to pop the oven open, or pull a jar out of the cooler and stare at it speculatively. If it's still liquid at this time, the cultures failed and nothing more is going to happen, but you can reheat it and add new cultures. I've only had this happen to me once, and it was because I had frozen the cultures beforehand, misled into thinking that would be okay. Apparently it wasn't.
   You should keep letting it incubate for at least 6 hours. It'll be fine to leave alone overnight. The longer it sets, the thicker it gets, and the stronger the flavor as well.
6. Cool.
   Stick the yogurt, for yogurt it now is, in the fridge. You can eat some of it now, but to stop the incubation process, put it in the fridge. If you prefer to strain your yogurt to make Greek yogurt, put it in the strainer, THEN put it in the fridge
   If you plan to gobble it up as is, set aside a portion of your work into a separate container. This is your starter for next time. Make it clear that this is not to be devoured, put it in a reserved part of the fridge, mark it, repeatedly lecture the other members of the household on the subject, whatever it takes. 'Course, if they eat it, you can just go out and buy a single serving for a buck.
7. Strain (optional)
   Your Greek yogurt strainer has the mosey on this. Strain out a third of the milk volume's worth of whey, and then you've got Greek yogurt. This usually takes 6-8 hours. If you strain TOO long (18 hours, for example) you're headed toward yogurt cheese, which is fantastic, but not necessarily what you want. After you strain, don't forget to set aside your starter.