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Lactic acid fermentation is the process of degradation of the feast to lactic acid. As a result glycolizis (in anaerobic conditions), a molecule of glucose receive two molecules piruvat , which is reduced to lactic acid molecule. Place in muscles, where oxygen, which supplied the blood is not sufficient to meet the energy needs of the body. The process is done in some bacteria and fungi.





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lactobacillus bulgaricus
Lactobacillus bulgaricus

(Lactobacillus delbrueckii subspecies bulgaricus)


Lactobacillus Bulgaricus is one of the bacteria involved in lactic acid fermentation. (Another is Streptococcus termophilus - the only harmful to human streptococcus bacterium).        Lactobacillus bulgaricus refers to homofermentative lactobacillus. Bacterium cells have stick forms with width below 2 mkm. Grow at 45o C, even at 50-53o C. Optimum temperature for growth is 40-43o C. Any change in the composition of the environment and temperature cultivation causes changes in morphological and physiological characteristics of the species.


Lactobacillus Bulgaricus was called after Bulgaria - which was first discovered and used. Bacterium occurs naturally free of Bulgaria. In falling in milk and under certain conditions Lactobacillus Bulgaricus (and Streptococcus termophilus) causing natural lactic acid fermentation, resulting in obtaining what we call yogurt.


Streptococcus Thermophilus( Streptococcus Salivarius subsp. Thermophilus) is other bacteria involved in lactic acid fermentation. It is the only      Safer for human alpha-haemolytic streptococcus bacterium, which does not form spores.They are classified as lactic acid bacteria (MKB).The bacteria are used to produce yogurt, when it is not probiotic, and don`t survive in the stomach.


Bacterium cells are spherical or oval shaped with a diameter 0,7-0,9 mkm. It is located in pairs in the short and long chains. Its optimal temperature for growth is between 40 and 45o C. Then it increases at 50o C, but stops increasing at 53o C. It doesn`t grow below 20o C.


Lactobacillus bulgaricus has been reported for the first time in 1905. This is done by the Bulgarian Stamen Grigorov , student , in a report presented to the Institute “Pasteur” in Paris. It explores microflora of the Bulgarian yogurt and discovers that yogurt is produced due to a stick and a globe bacteria. Later the stick bacteria is called Lactobacillus bulgaricus, a globe one - Streptococcus thermophilus.The origin of the bacteria is plants. It is believed that due to animal contact with plants during grazing, bacteria clog the animal's udder when milking and they fall in milk. Thus, this makes possible the fermentation of milk.



Yogurt is milk product which is produced by fermentation of milk. There are two main organisms that are involved in fermentation of yogurt-Lactobacillus bulgaricus and Streptococcus thermophilus, which are in simbiotic relationship and this connection between the two bacteria is beneficial for their survival and efficiency only when they are together. The fermentation is not possible in the presence of a single bacterium. Each of the two bacteria have a role in fermentation .


Fermentation ends naturally, by cooling. During the stay in a warm place for a few days (outside the refrigerator) fermentation continues, milk spoils and the taste becomes unpleasantly sour.


History of yogurt

Accurate data about the origin of the Bulgarian yogurt lacks. One of the theories about its occurrence is associated with the Thracians. Ancient Thrace own fertile soil and rich vegetation succulent pastures. All this contributed to development ruddy sheep. And because of that the main domestic animal was sheep. Thracians noticed that sour milk can be kept fresh for a long time. By adding the sour milk to the fresh boiled milk , they received leavened product known as milk or “prokish” .


Another theory is associated with proto-Bulgarians . It is that Bulgarian yogurt goes back to lactic acid drink "kumiss” , which proto-Bulgarians prepared from mare`s milk. Once settled in our land and domesticated sheep began to make "kumiss" from sheep's milk. Proto-Bulgarians produced yogurt as well under the name "katak”, as fresh sheep milk curdle with dilution of cheese. This product is usually ready by the end of the summer, when milk has a high content of dry matter.


Yogurt has a higher nutritional value than fresh milk. Reason for this are the changes under the action of Lactobacillus bulgaricus and Streptococcus thermophilus. The content of the milk sugar in yogurt is reduced by 20-30%. The remainder sugar milk serves as a source of energy worth 4.1 kcal. Yogurt is a rich source of calcium to meet the needs of man. Calcium maintains volatility of the heart muscle and has great importance for the proper functioning of the nervous system , stimulating effects of endocrine and accelerating blood clots.



Петър Атанасов – Мишо

Никола Куванджиев