Buttermilk

True buttermilk is composed of the low-fat portion of milk that remains after the cream has been churned into butter. Traditionally, its thickness and flavor develop by mild fermentation.  It has remnants of fat globules that make it an excellent emulsifier like lecithin; a characteristic that make it valuable in the preparation of finely-textured foods. True buttermilk is slightly acidic and has a subtle, complex flavor. Regrettably, it is prone to spoilage and off-flavors.

A shortage of true buttermilk after World War II led to the development of imitation “cultured buttermilk.” It is made from ordinary skim milk that is fermented.  The process follows that of yogurt, but the fermentation is stopped abruptly by rapid cooling. The gelled milk is then agitated to produce a thick, smooth liquid. Most buttermilk sold in the US is not true buttermilk but rather imitation cultured buttermilk.   

Bulgarian buttermilk has yogurt cultures that have replaced the cream cultures. It has increased acidity by fermentation also at higher temperatures; thus it resembles yogurt.

Sour cream

Sour cream is a leaner, firmer version of crème fraîche. It contains 20% milk fat and enough protein to curdle when cooked. It appears to have originated from Central and Eastern Europe and brought to the US in the 19^th century.  Since then, Americans have added a small amount of rennet, which contains enzymes that cause protein coagulation. The result is a heavier, firmer sour cream than its European counterpart.

Acidified sour cream is made by coagulating milk with acid instead of through fermentation. It is therefore also known as non-fermented sour cream. Manufacturer versions of low fat and non-fat sour cream replace butterfat with starch, plant gum, and dried milk protein.

Crème fraîche

Crème fraîche is 30% milk fat pasteurized at moderate temperatures. It is not made from UHT (Ultra High Temperature) pasteurized or sterilized milk. Two versions are available: liquid and thick. The liquid crème fraîche is unfermented. It has a shelf life of 15 days. The thick version is fermented with a typical cream culture. It has a shelf life of 30 days.

A home-made version of crème fraîche is made by adding cultured buttermilk or sour cream to heavy cream (1 tbsp per cup) and letting it stand at cool room temperature for 12 to 18 hrs or until thick.

Cream cultures

Cream cultures such as sour cream, crème fraiche, and buttermilk are indigenous to Western and Northern Europe. These products result from the slow fermentation produced by the mesophilic bacteria Lactococci and leuconostoc species. These bacteria have three important characteristics that make them ideal for production of creams and buttermilk: 1. They grow best at moderate temperatures. The process of fermentation can be kept at lower temperatures than those that produce yogurt. 2. They are moderate acid producers. Again, it prevents the formation of yogurt from too acidic a condition. 3. They complement flavor by turning citrate into diacetyl; a compound which gives the fermented milk product a characteristic buttery flavor.

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.

Health benefits of lactic acid bacteria in fermented milk

Lactic acid bacteria do more than just pre-digest lactose and produce yogurt. Back in the early 1900s, the Russian immunologist Ilya Metchnikov proposed that lactic acid bacteria in fermented milks help eliminate toxic microbes in our digestive system. To support Mr. Metchnikov’s prescient claim, research in recent decades suggests that Bifidobacteria, fostered in breast milk, colonizes the infant intestine and keep it healthy through acidification and production of antibacterial substances. Once the infant is weaned, the Bifidobacteria recede in favor of a mixed population of Streptococcus, Staphylococcus, E. coli, and yeasts.

Bacteria such as L. fermentum, L. casei, L. brevis, and L. acidophilus adhere to the human intestinal wall. Their presence shields it from other microbes by the secretion of antibacterial compounds, and by boosting the immune response to infection. Furthermore, research suggests that these bacteria also dismantle cholesterol and reduce the production of carcinogens.

The consumption of fermented milks for health purposes, however, is debatable. At the time of Mr. Metchnikov’s claim, milk fermentation involved a dozen or more microbes. Industrial versions nowadays usually limit it to two or three. This biological narrowing does not only affect flavor and consistency, but its health value as well. In addition, live cultures in industrial buttermilk and yogurt grow well in milk but cannot survive in the human body. Some manufacturers are adding "probiotic" Lactobacilli and Bifidobacteria to their cultured milk products in an attempt to mimic the original fermented milks. Such products are advertised for the health benefits afore noted.

Lactic acid bacteria and fermented milk

Lactose is almost uniquely found in milk. Few bacteria can break down lactose into usable forms of energy. Lactic acid bacteria, also known as Probiotic bacteria, use the enzyme lactase to break down the disaccharide lactose into the more usable monosaccharides glucose and galactose. The bacteria produce lactic acid as a by-product of lactose digestion. The low pH preserves the milk as other microbes cannot survive in acidic conditions. In addition, casein proteins come together in semi-solid curds. The flavor and texture favorably change in a process referred to as milk fermentation.

The history of milk fermentation dates back at least 2,000 years. In the early years, people believed that the fermentation was a spontaneous process. Later, the process was managed by the inoculation of fresh milk with fermented milk. By the late 19^th century, bacteria had been identified as a causative agent, though the process was not entirely understood. By the 1900s, starter cultures of unknown mixed bacteria became commercially available. By the 1930s, pure single-strain cultures had evolved for the specific production of sour creams, yogurts, and cheese.

Fermenting lactic acid bacteria include species in the genera Lactobacillus, Lactococcus, Streptococcus, Leuconostoc, and Pediococcus. Some of these species also colonize the mouth, intestine, and vagina of mammals as normal flora.