About a year ago I got fed up with spending $0.50-1.00 on a container of yogurt. Especially ones with ingredients on the label that I can't pronounce. So I started on a new adventure--making my own yogurt. I was amazed at how simple the process actually is. They key is making sure you watch the temperature carefully.
All you need to get started is a couple quarts of milk and a small amount of unsweetened, plain yogurt with active cultures -- make sure the container has the NYA's seal on it that guarantees a certain percentage of live and active cultures. After you make the first batch you can use use some of your homemade for future batches. (I've used both whole milk and 2% with great success; I've read that you don't want to use less than 2% or it will get "grainy".)
2 quarts of milk = 1 half a gallon. Pour it into a heavy saucepan. Let the yogurt sit out, allowing it to come to room temperature.
Over medium-high to high heat, warm the milk to 175-180 degree Fahrenheit, stirring frequently. A candy thermometer works well because it clips onto the side of the pan.
Remove from heat and allow milk to cool to a temperature between 110 and 120 Fahrenheit.
I usually cheat at this point and drop the saucepan into the kitchen sink, filling the basin with cold tap water. With constant stirring it drops the temperature of the milk to it's desired range within a couple of minutes.
Take about a cup of the warmed milk and whisk it into about 1/4 cup of your room-temperature, unsweetened yogurt. This makes it easier to incorporate the yogurt evenly into the larger quantity of milk.
After whisking, pour mixture into the saucepan and mix everything well.
(This is where the directions can vary, depending on your home and your oven. The goal is to allow the yogurt to incubate at about 100 degrees Fahrenheit until thickened; taking any where from 6-14 hours depending on the environment. Some people have luck putting the pot in a clean ice chest on top of a heating pad. My heating pad shuts off after a given amount of time so that doesn't work for me. I've also found that doing this in the crockpot works wonderfully; especially if you have an digital thermometer that will beep at set temperature.)
Wrap the saucepan in a heavy towel and put into a conventional oven that has been preheated for about one minute. Turn the oven light on. Let it sit and incubate for 10-14 hours. The time varies depending on the environment and how thick you want your yogurt. Mine typically takes 12-14 hours as it sits between 102 and 104 degrees with the light on. I start it after dinner and let it go overnight, pulling it mid-morning the next day.
When it's done incubating, pull the yogurt from the oven and stick it in the refrigerator to chill.
If you want regular yogurt go ahead and stop here. If you're like me and want to try your hand at Greek yogurt, keep going.
After chilling the yogurt, completely, I strain it to thicken the consistency. My system is weird but it works well -- I line a strainer with muslin, put the yogurt into cheesecloth and then the cheesecloth goes into the muslin-lined strainer. (It's cheaper than buying a $75 bullion strainer and a standard kitchen strainer doesn't have a fine enough mesh.) To speed the process I place a plate on top of it all and add some weight.
Then I let it sit for another hour or so.
I let it go until it quits dripping whey into the collection pan. Ideally you want to reduce it by half. I've found that getting about 3 cups of liquid out makes for a perfect consistency.
Then put your thickened, creamy yogurt into a tightly sealed container and refrigerator. It will thicken more as it cools.
Apparently I got so excited about the finished product that I forgot to take shots of the yogurt before it went into the fridge. Oops!
So far this has been a huge hit. I bought some vanilla Torani syrup to sweeten it and it works really well -- toss in some fresh fruit or granola and it's hands down better than anything you could buy at the store.