I used Sailor's methodology (http://cheeseforum.org/forum/index.php/topic,5165.0.html
) for making mother cultures to duplicate a wonderful Bulgarian Yogurt I found in Whole Foods.
Yesterday morning I heated, in a water bath, an unopened quart of whole milk to 190F, held it between 188 and 195F for 30 minutes. Cooled in cold water bath to 110F.
I "innoculated" the prepared milk (pouring out 1/2 cup of milk) with 1/2 cup of the bulgarian yogurt and re-closed the half-gallon container. I used some of the milk to thin out the yogurt sample, and then gently shook the container to make sure it was mixed well.
Placed the container in our microwave (just used the microwave as a closed environment) with a bowl of near boiling water (had to open the door occasionally) and maintained the heat between 102 and 110F for the rest of the afternoon and evening. Closed the door and left it overnight without adding more boiling water. Temp in the morning was 88F still, at which point I placed the container in a hot water bath at 108F for 4 more hours and then refrigerated.
Once the container had cooled sufficiently I poured the contents into a mixing bowl and mixed so that the "texture" was consistent throughout. The top of the half gallon was a bit thinner than the bottom.
Poured into jars for storage and back into the fridge. Once chilled I taste tested the "new" yogurt vs the original, and they were identical in flavor as well as thickness. Am really happy with this result, and was a bit surprised at how easy it was to totally duplicate an expensive probiotic yogurt using nothing but milk and the original yogurt as culture.