Yogurt is the product of culturing certain strains of live, active bacteria that eat up milk sugars (lactose) and excrete lactic acid, which ferments milk. During the first phase of the yogurt-making process you heat the milk for two reasons:
(1) you want to kill off any bacteria that may be in the milk (we only want specific bacteria to be culturing here!)
(2) heating denatures the proteins in milk- this is the difference between curdled yogurt and creamy yogurt
Then you let the milk cool a bit until the it's warm but not too hot- the way bacteria like it. Once the milk has cooled you stir in yogurt containing live, active cultures (or you can purchase active freeze-dried cultures ). This acts as your starter or seed, introducing the bacteria you want into the milk. You cover this and insulate it to keep it warm and prevent contamination. Then you you give the bacteria plenty of time to do their thing. The longer you let this process go on then less milk sugar you'll have and the more lactic acid you'll have. The more lactic acid, the more tangy. The less lactic acid, the more sweet. When you strain out the yogurt, you reduce the whey content and increase the content of the "curds", which are essentially milk proteins, hence why Greek yogurt has a much higher protein content than un-strained yogurt.
Are you feeling better about this now? Ready to give it a go?
Heat your milk to 185 F, then let it cool to about 110. Whisk in about 1/2 cup of yogurt. Cover and insulate to keep warm.
Depending on the size of your slow cooker (you can also do this on the stove top- but be sure to heat your milk gently) and how much milk you use, the time it takes for the milk to come up to and down to temperature will vary. So you first few times you do it, you'll need to pay attention. Once you've got a routine down, it's easy peasy.
I prepare my yogurt in a 6 quart slow cooker. If I'm using 1 gallon of milk, I heat it on low for 2 1/2 hours, let it cool for 3 hours, and then let is culture for about 12-14 hours. Keep in mind I like mine real tangy. While it's culturing I keep my slow cooker off but insulate it with a heavy towel or winter coat. If I use 1/2 gallon of milk, I heat the milk for 1 1/2 hours, then let it cool for about 2 hours. I let it culture for the same amount of time.
To strain your yogurt , you'll probably need to divide it up into a couple batches. Use a sieve lined with a few layers of cheesecloth or a single layer of paper towels (unless they're the super cheap-o kind, then you might want to shoot for two layers). Set the sieve on top of a small bowl or pot. It's important to use a bowl or pot that has a small circumference. If the body of the sieve drops more than a couple of inches in to your container of choice, your "straining" yogurt will end up sitting in the liquid it's trying to get rid of. Not ideal. Cover the top of the sieve with tinfoil or plastic wrap and place it in the fridge while it strains.
How long you strain it is entirely up to your preference. If you like Greek yogurt, you'll want to strain for at least 8 hours. I typically strain mine for 12-14 hours.This also makes it extra easy to remove the yogurt from the sieve. I usually just fold sections of the cheesecloth or paper towel towards the center, forming a compact ball of yogurt (see the picture above for reference).
Oh no, I've over-strained it! Not to worry- you can just whisk a bit of the whey back in to thin it out more.
Making yogurt (especially in a slow cooker) is a very low-input process. Really all you have to do is press a few buttons and whisk in your starter cultures. It is, however, a time-consuming and time-sensitive process. I like to start my yogurt at around 3-4 pm. That way I stir in the yogurt and insulate shortly before bed. By the time I wake up and get through with my morning routine, the yogurt is ready to be strained. I'll strain it before work and by the time we're done with dinner for the evening, it's nice and thick and ready to be jarred. Figure out what timing works best for you. The important thing is to think ahead!
I'm perfectly happy to eat this yogurt plain, but sometimes I like to jazz it up or make it more satisfying. I also know plenty of people that are just really not into plain yogurt and that's totally fine- you don't have to be! Below are a few of my favorite ways to add flavor and texture to my yogurt. I would highly recommend adding flavor on a per serving basis. Adding sugar to the whole batch could decrease it's shelf life and alter the consistency.
-Want a fruit-flavored yogurt? Add a spoonful or two of your favorite jam, preserves, or curds.
-Want something a little more on the sweet side? Add a bit of honey.
-Want a french vanilla-flavored yogurt? Add a touch of vanilla extract.
-Want to pack in a little more flavor and fiber? Add some fresh berries.
-Want to amp up the protein and add some texture? Add some toasted nuts or granola
My personal favorite: fresh peaches and nutty coconut granola
Store in a jar or air-tight container for up to 7-10 days. If you can stand not to eat it all, keep 1/2 cup reserved to seed your next batch of yogurt!