I just finished eating my first bowlful of fresh homemade yogurt. It is very tasty, and was ridiculously easy to make. Yogurt making has been a form of food preservation for over a millennium, and it is easy to see why.
All you need to do to make yogurt is mix milk with a couple tablespoons of already prepared yogurt, and let it sit for 6 to 12 hours at 90-110 degrees F until it sets. The fermentation process makes the yogurt more acidic than milk, which is why it lasts longer. The acid makes it inhospitable to the growth of bacteria.
I used an unopened bottle of milk so that I was sure that there were no contaminates, but you can also bring the milk to 185 degrees for 5 minutes to kill off germs. Just be very careful not to burn the milk if you do that. Also make sure to let the milk cool to 120 before you mix in the yogurt, or the yogurt culture will be killed off.
You don’t need a fancy yogurt machine to make yogurt. All you need is an insulated waterproof container to use as a water bath. I used an insulated plastic bag that I have for carrying frozen food home from the grocery store, but a cooler would work as well. Fill the container with enough warm (approx. 120 degree F) water to come up to the bottom of the jar’s screw top. You probably want to avoid soaking the top in water because they do rust.
Let the yogurt sit for at least 6 hours, refilling the water as needed to keep the temperature over 90 degrees. Be careful not to disturb the yogurt too much, or the curds and way will separate out, although you can stir them back together.
The best part of making yogurt yourself is the sense of accomplishment that comes from being a bit more self-sufficient, without having to do a ton of work.