Yogurt

Yogurt originated in the warm climates of Southwest Asia and the Middle East. Though it has been produced for thousands of years, it only gained popularity in Europe in the early 20^th century. By the 1920s, yogurt attained factory-scaled production. Broader popularity for yogurt came after the French developed a means to give it a creamy texture and added fruit flavors.

Yogurt production follows the common path of fermented milks: heat and fermentation.  First, the milk is prepared by heating it to concentrate proteins and denature the whey protein lactoglobulin. This treatment improves the consistency of the yogurt. A denatured lactoglobulin allows casein proteins to bond and form a fine matrix that retains liquid in its small interstices instead of coagulating into semi-solid curds.  The milk is then cooled to a warm temperature optimal for bacterial production of lactic acid.

The bacteria used for the fermentation of yogurt are very thermophilic. Industrially, these bacteria include Lactobacillus delbrueckii, subspecies bulgaricus, and Streptococcus salivarius, subspecies thermophilus. These bacteria stimulate each other and in combination acidify milk rapidly. They are also notable for their production of flavor compounds dominated by acetaldehyde, which gives yogurt its fresh, tart flavor.

Fermentation of yogurt at high temperatures of 40 to 45 degrees Celsius (104-113 degrees F) grow bacteria that multiply quickly and produce large amounts of lactic acid. Milk proteins set in 2-3 hrs, but produce a coarse protein network. The proteins assemble in thick strands which give it firmness but leak whey protein readily. In comparison, fermentation of yogurt carried out at 30 degrees Celsius (86 degrees F) grows bacteria more slowly and produces a finer, more intricate network that better retains whey. Yogurt prepared at this lower temperature takes 18 hours to set.

Reduced fat milk yogurt is firmer than the regular kind due to the addition of milk proteins used to mask the lack of fat. The extra milk proteins add density to the coagulated protein network. Manufacturers also add gelatin, starch and other stabilizers.  

Frozen yogurt is somewhat of a misnomer. Commercial frozen yogurt is made from iced milk with small dose of yogurt in it, usually in a ratio of 4:1.