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Fermented milks and yogurt are unique dairy products that have been a part of human diet for centuries. They are special milk preparations obtained by coagulating milk without removing the whey. This coagulation is achieved through the action of specific microorganisms, which must remain alive and vital until the product is consumed. These microorganisms ferment lactose into lactic acid, causing a drop in pH, coagulation of caseins, and formation of a soft, not very compact clot.
There are two main categories of fermented milks: sour milks and acid-alcoholic milks. Each category is defined by the type of microbial strains used, and the fermentation process they undergo.
Sour milks include products like buttermilk, yogurt, sour cream, filmjölk. These are produced through a process known as "homolactic" acid fermentation, where bacteria ferment lactose into lactic acid. This simple fermentation process results in products with a distinctively tangy flavor.
Yogurt is perhaps the most well-known and widely consumed sour milk product. It is produced using two specific microbial strains - Lactobacillus bulgaricus and Streptococcus thermophilus. The ratio of these strains in the milk can affect the final product's acidity and texture. For instance, a 1/1 ratio of these strains results in a more acidic yogurt, while a 1/2 ratio results in a creamier product. The raw material for yogurt production can be skimmed, partially skimmed, or whole milk.
Acid-alcoholic milks, such as kefir and kumis, undergo a more complex fermentation process known as "heterolactic" fermentation. This process results in the production of both lactic acid and ethyl alcohol, giving these products a unique flavor profile.
Kefir and kumis are typically fermented, foamy milk drinks. Kefir is produced using a combination of yeasts and bacteria, including torula yeast, Lactobacillus brevis, L. casei, Leuconostac mesenteroides, and Saccharomyces cerevisiae. On the other hand, kumis is produced from milk fermented with a pure culture of Kumiss.
The production of fermented milks involves several steps, starting with the standardization of the milk. This process adjusts the milk's lipid content and dry residue to ensure a consistent product. The milk is then homogenized and subjected to heat treatment, which denatures the proteins and kills any present microorganisms. After this, the functional microbial strains are inoculated into the milk, initiating the fermentation process.
The specific steps and conditions of the production process can vary depending on the type of fermented milk being produced. However, the fundamental principles remain the same: the transformation of milk into a unique, flavorful product through the action of specific microorganisms.
In conclusion, fermented milks are a fascinating category of dairy products with a rich history and a wide variety of flavors and textures. Whether it's the tangy acidity of yogurt or the complex flavor of kefir, there's a fermented milk product to suit every palate.