Here’s what you will need: a candy or meat thermometer, glass jars with lids to hold your yogurt mixture (2 quarts + 1/2 cup), wooden spoon, pot big enough to heat the milk, 1/4 cup of starter yogurt per quart of milk, heating pad that does not have an auto-shut-off, and a box or cooler (I used a soft sided picnic cooler).
I started with a half gallon of whole milk (I’ll experiment with 2% and 1% later now I’ve got the method right!) and ½ cup of whole milk plain yogurt. It is important to make sure that your starter yogurt contains active cultures.
First, measure out your yogurt starter and sit it in a bowl on the counter to bring it to room temperature. You will need 1/4 cup of starter for every quart of milk. Then, heat the milk on medium (or lower) until the top is frothy but not boiling. Make sure to stir constantly so that it does not burn. The milk should be about 185 degrees. Turn the heat off and go do 20-30 minutes of other chores while the milk cools down to 90-120 degrees. Milk that is warmer than 120 degrees will kill the cultures in your yogurt starter, so err on the side of cooler.
Once the milk has cooled, stir in the starter. I’ve heard and read that it’s important to NOT use a metal spoon from this step forward in the process and that if you use a metal spoon to serve your final product it will break down the culture and your yogurt will get watery. I cannot speak to this nor have I found a good reason to follow this wisdom, but yet I do. So, stir in your starter with a wooden or plastic spoon.
Once you’ve stirred your mixture, pour it into your jars. Any glass jar will do as long as you’ve thoroughly cleaned it. I have a pickle jar and salsa jar that work well for me – you may use whatever you like.
Place the heating pad in the bottom of your box or cooler,
then gently set your jars on top of the heating pad. Set the lids on the jars, but do not tighten them so that the mixture has room to breath and you won’t end up with as much condensation inside the jar. Place towels around the jars as necessary for stability. Turn the heating pad on and set a timer for 2 hours. Come back and check the temperature of your yogurt to make sure that it is staying below 120 degrees. Close it up and come back and check again later.
Within 5 hours my yogurt had thickened to the point that it stuck to the thermometer when I put it in and by 8 hours it would pull away from the jar as a blob –nice thick yogurt! It is normal to have a bit of water-like fluid on the top of your yogurt. I like to pour off as much as I can. You can also just stir it in. Many commercial yogurts do not have as much fluid on top because they have pectin or gelatin added.
So, my starter yogurt was $3.49/quart, my milk was $1.50/half gallon. For 3+ quarts of yogurt this first time around (1 quart starter + 2 quarts I made) it comes out to about $1.66 per quart, next time, since I can use my homemade yogurt as a starter, the cost will be down to about $0.75/quart for fresh, homemade, I-know-what’s-in-it yogurt! I’ll post details of how future yogurt adventures turn out if I can master a low-er fat version and as I track down my mom's granola recipe.
3/5/2010 - Edited to Add: Check out my more recent post for tips on how to achieve a sweeter, milder yogurt.