When the kids were little I made this every spring. The first gallon was gone in two days. The second in three or four. The third took a week and then I didn't make it until next spring. We always ate it over fresh strawberries with a sprinkle of brown sugar on top. We call it French yogurt.
1 gallon whole milk
2 cups sugar
1 to 1½ Tbs. vanilla extract
½ cup good quality plain yogurt with active yogurt cultures—you can use fruit yogurt but your batch will have a faint taste of the fruit. We are currently using Mountain High plain yogurt (Spring of 2007). Bring milk and sugar to a boil, stirring occasionally and constantly toward the end. Use a heat resistant spatula—you don’t want plastic in your yogurt--or use a flat bottomed wooden spoon. (Edited April 2010--this year I only got the temp up to 180 degrees because I forgot I had this recipe here and I googled making homemade yogurt and two recipes said 180 degrees. If I did it again I would probably bring it to a boil, just to make sure all the bad bacteria is killed)Cool milk to 110 degrees. THIS TAKES A LONG TIME--several hours. I cover the pot because I don't want any stray yeasts, bacteria in my yogurt. Note (edited May 13th, 2010): Lately I have been putting the pot in a sink full of cold water and it cools to the proper temperature in less than a half hour.Remove 1 cup and wisk it in a bowl with the ½ cup yogurt to make a slurry. Stir the slurry into the warm milk and add vanilla.
Put milk in something to keep it at 110 degrees. I use our dehydrator and put the milk into glass jars, Rubbermaid containers, etc. In 3-6 hours remove yogurt—it should be set—and refrigerate. You can use this to start a new batch but with each homemade batch the yogurt will not set up as firm, I don’t know why. (Edited May 13, 2010) lately I have been letting it sit longer, tonight I may even let it go all night. We'll see how it does. I'm hoping it will be more firm.
For yogurt made with powdered milk: mix according to package directions but add about a fourth—or more—extra dry milk. This will make your yogurt thicker. Bring the milk to a boil or at least to 180 degrees. This kills off any competing bacteria.
There are lots of ways to keep the yogurt at this temperature. I used to wrap a heating pan in a towel, wrap that around my gallon glass jar of warm milk and starter and then wrap the whole thing with more towels. You can also put the yogurt-milk in glass jars, in a picnic cooler and add warm water at 110 degrees. Close the cooler, check the temp of the water in an hour and remove some and add hot water if it’s too cool.
Don’t disturb the yogurt until it is set. The small six-pack coolers would be great for smaller batches. Good luck.