Little Miss Muffet sat on a tuffet. Eating her fermented dairy…uh, curds and whey…

The starter culture for most yogurt production in North America is a symbiotic blend of Streptococcus thermophilus (ST) and Lactobacillus bulgaricus (LB). These bacteria use lactic-acid fermentation to produce energy for growth; they lack a respiratory metabolism. Although these bacteria can grow independantly, the rate of acid production is much higher when used together than either of the two organisms grown individually. ST grows faster and produces acid. The acid stimulates LB growth. On the other hand, the proteolytic activity of LB produces stimulatory peptides and amino acids for use by ST. These microorganisms are ultimately responsible for the formation of typical yogurt flavor and texture. The yogurt mixture coagulates (curds) during fermentation due to the drop in pH. The streptococci are responsible for the initial pH drop of the yogurt mix to approximately 5.0. The lactobacilli are responsible for a further decrease to pH 4.0. The following fermentation products contibute to flavor:

• lactic acid
• acetaldehyde
• acetic acid
• diacetyl

The clear liquid is called whey, it is produced when bacteria digest the milk protein producing soluble peptides and amino acids.

Temperature: A temperature of 43° C is maintained for 4-6 h under quiescent (no agitation) conditions. This temperature is a compromise between the optimums for the two micoorganisms (ST 39° C; LB 45° C).

Milk is an emulsion. It contains droplets of fat dispersed in a mixture of water, sugars, and proteins. Homogenized milk is just milk whose emulsion has been stabilized, so that the fat-rich portion (cream) does not separate out upon standing.

Milk is pasteurized to kill disease-causing and spoilage microbes. In classic pasteurization, milk was heated to 63°C for 30 min. Most milk pasteurization today uses 72°C for 15 sec.; 140°C for <1 sec. will also achieve similar results. Pasteurization prevents transmission of tuberculosis, campylobacteriosis, brucellosis, salmonellosis and other diseases and prolongs the shelf-life of the milk. (Note, sterilization removes or kills all microbes. Milk can be sterilized by an ultra-high temperature treatment.)