9 hours ago, andiesenji said:
Following is a quote from the California Milk Board:
"Yogurt is formed by the growth of two bacterial organisms in milk; Streptococcus thermophilus and Lactobacillus bulgaricus which turn the milk sugars into lactic acid. These are two separate bacteria that are active at different times during processing. Some times you will also find yogurt that contains other ""Probiotic"" cultures such as Lactobacillus acidophilus, Bifidobacterium longum, and Bifidobacterium infantis which are bacterium normally found in your intestines. Together these bacteria aid in digestion and the synthesis of vitamins. Here are the required steps. Heat milk to between 180 and 200 °F. Heating the milk is done for a few reasons. First, to sterilize/pasteurize the milk so that the yogurt bacteria/culture has a hospitable place to grow in. It is not desirable to incubate contaminating bacteria that might be present in the unsterilized milk. Heating should be done even with pasteurized milk to help make a smooth thick yogurt.
Keep in mind that killing bacteria and deactivating enzymes via pasteurization is a function of temperature and time -- and that government recommendations to consumers typically only list the temperature required to pasteurize instantly. In many cases, the same results can be achieved by longer processing at lower temperatures. The process is slower at lower temperatures, but can still be reliably effective.
For example, US FDA tests milk pasteurization by detecting "the phosphatase enzyme, a constituent that is inactivated by pasteurization at 63°C (145°F) for thirty (30) minutes or 72°C (161°F) for fifteen (15) seconds." I would imagine that instant deactivation would probably happen right around 180°F.
(As an aside -- milk labeled as "ultra pasteurized" has been shocked to 280°F for extended shelf life. Unlike regular milk pasteurized at 145°F, the enzymes are already deactivated. I've had great luck making yogurt from ultra pasteurized milk without any preheating at all, though I always open a new container directly into the pot.)
