Dairy products are consumed daily and are one of the most desirable foods by a large part of the world’s population.
To obtain them, fermentation technologies are used with the participation of various lactic acid bacteria (LAB). Lactic acid fermentation improves the taste, increases the digestibility of milk and provides producers with a wide range of valuable products. Fermented milk is very useful for many reasons. The related products have a long shelf life. They are safe because LAB act as preservatives and inhibit the growth of pathogenic microflora, are very good for absorbing nutrients from milk, and are beneficial due to the effect of lactic acid bacteria on various body functions.
The first yogurt-like dairy products were invented in Mesopotamia around 5000-6000 BC. Fermented mare’s milk was also used for food by early Bulgarians, and was stored in leather bags made from the stomach. The resulting product was called koumiss and was a staple food during military campaigns.
In Europe, the therapeutic effects of Bulgarian yogurt have been known since at least 1542, when the French king, François I, was cured of chronic diarrhea with a simple yogurt diet. However, the discovery of the yogurt microbiota (as the cause of yogurt fermentation) occurred only in the 20th century. In 1905, Stamen Grigorov, a Bulgarian medical student, was the first to describe a rod-shaped lactic acid bacterium (named Bacillus bulgaricus Grigorov) along with a spherical streptococcus in Bulgarian yogurt. Based on Grigorov’s findings, in 1909, Russian biologist and Nobel laureate Elie Metchnikoff, developing his theory of longevity, was the first to suggest that daily consumption of yogurt extended the lifespan of the Bulgarian peasant population, especially in mountainous areas, suggesting that there was a correlation between yogurt consumption and the number of Bulgarian centenarians. Compared to other EU countries, plain yogurt (156,610 MT) and brine white cheese (899 MT) in Bulgaria were in limited quantities, but they are known for their very high quality.
Overview of traditional Bulgarian dairy products: appearance, nutritional value, production technologies, and shelf life
Yogurt
Bulgarian yogurt is a traditional Bulgarian dairy food, considered a distinctive feature of the country, produced by microbial lactic acid fermentation of pasteurized milk, inoculated with starter cultures of only two species of LAB (L. delbrueckii subsp. bulgaricus and S. thermophilus).
Bulgarian yogurt is usually made from cow’s milk, but can also be made from buffalo, goat, or sheep’s milk, and, according to the FAO/WHO definition, must contain at least 107 viable colony-forming units (CFU) of symbiotic starters per gram of product. Another important requirement for raw milk is the absence of inhibitors (antibiotics or natural plant compounds) that can inhibit the growth of the LAB starter culture. The total acidity of raw milk should not exceed 18-23°T.
Bulgarian yogurt is white, sometimes slightly yellow. Its surface may have a visible layer of fat. The product’s curd is dense, slick, and smooth. When tilted, the coagulum could rupture and exude some clear, slightly yellowish, and opalescent milk serum. When raw buffalo or sheep milk is used, the consistency of yogurt can be homogeneous and creamy. Its taste and aroma are specific, sour, and pleasant, and depend on the type of milk.
According to the A 99 (1987) standard of the International Dairy Federation (IDF) in Brussels, yogurt is evaluated by five indicators: appearance, color, taste, odor, and consistency.
Both manual and industrial technologies for the production of yogurt are as follows.
They start with filtration of raw milk to remove solid particles, homogenization, and pasteurization by heating to 93-95°C for 3-5 seconds and cooling to 45-50°C. Then, the raw milk is inoculated with a starter culture, consisting of L. bulgaricus and S. thermophilus, in a ratio of 1:2 and 1:5. The starter culture is 0.5 to 2% of the raw milk. The inoculated, liquid, warm milk is poured into sterile containers or packages, usually from 150 g to 1 kg. The packages are immediately sealed after filling and incubated in a warm place or heat chamber at 42-45°C for 2.5 to 4 h. Lactic acid bacteria multiply rapidly, reaching 107-108 CFU/g (L. bulgaricus) and 108-109 (S. thermophilus) CFU/g. Lactic acid bacteria ferment lactose to lactic acid, which binds calcium ions (Ca2+) from casein in the milk, thereby causing it to coagulate. To produce high-quality yogurt, the temperature during fermentation must be stable, and there must be no shaking. The finished product is cooled to 6-12°C and can be stored for up to 21 days.
Yogurt is considered a nutrient-dense food that significantly contributes to the intake of macronutrients and micronutrients, most importantly calcium and protein. It is a source of iodine, potassium, phosphorus, and vitamins: water-soluble riboflavin (B2) and cobalamin (B12), and fat-soluble secosteroids such as vitamin D.
White brined cheese
White brined cheese is the second most common and widely consumed traditional dairy product after yogurt. This cheese originated in Bulgaria and was widely distributed in the Balkan Peninsula, former Soviet republics, Turkey, the Middle East, and Mexico in the second half of the 20th century. This cheese is made from cow’s, sheep’s, goat’s, buffalo’s milk or a mixture of them, which is fermented and subsequently preserved in brine. White brined cheese is obtained by coagulating milk using a special technology. Cheese made from two different types of milk usually has improved nutritional properties.
White brined cheese belongs to the group of hard cheeses with low water content and high solids, which requires a long ripening process of intense acidification, proteolysis, and lipolysis. Its quality is determined by chemical, physical, microbiological, and organoleptic characteristics that ensure safety, long shelf life, and high nutritional value of the product. One of them, for assessing the appearance of the cheese, is the “cut surface”. According to the Bulgarian National Standard, the cheese mass should appear in the form of a cube with rectangular walls and a homogeneous porcelain cut surface, without the presence of bacterial pores. In general, the process first involves pre-heated milk, which is cooled and fermented using rennet, an enzyme mixture that curdles the casein in milk. Rennet contains the enzymes aspartic endopeptidase chymosin, pepsin, and lipase and is usually isolated from the abomasum of newborn small ruminants, but is alternatively produced by fermentation using recombinant E. coli, Aspergillus niger var. awamori, or Kluyveromyces lactis. The next technological steps are: curd smoothing, cheese ripening period for at least 45 days at 15°C, salting and packaging in boxes containing brine (6-10% sodium chloride), and the shelf life of the product is 12-18 months.
White brine cheese has a high energy and nutritional value, which is due to proteins and fats, as well as the presence of easily digestible peptides, essential free amino acids, vitamins (A, B, E and D), minerals (calcium, zinc, magnesium, and phosphorus), and lactic acid. Its unique taste and aroma are due to a rich range of volatile components such as aldehydes, alcohols, carboxylic acids, methyl ketones, ethyl esters, sulfur compounds, and aromatic hydrocarbons.
Kefir
Kefir originated in the Caucasian Mountains, Russia, and Tibet, and later became popular in Central and Eastern Europe. Its production is due to a combination of lactic acid and alcoholic fermentation. Since the final product is a concentrated drink containing carbon dioxide, kefir belongs to the carbonated dairy drinks, which, in addition to a specific sour taste and a slightly alcoholic aroma, also give a “fizzy” sensation. In Bulgaria, kefir can be natural (plain) or with added fruits (flavored).
Fermentation begins by adding kefir grains to raw milk. The original kefir grains are white to slightly yellow, 0.3 cm in diameter, cauliflower-shaped, and semi-hard granules, containing polysaccharides, fats, proteins, and a symbiotic association of yeast and bacterial strains. These grains are insoluble in water, but when suspended in milk, they swell and form a gelatinous product, the polysaccharide kefiran, consisting of repeating hexasaccharide units. Kefir is a heteropolysaccharide with glucose and galactose units. LAB are the main microorganisms that produce exopolysaccharides and are responsible for the texture and rheological properties of kefir. The technology of kefir preparation allows the use of sheep, cow, or goat milk. For large-scale production, kefir grains are not used, but sterilized milk is directly incubated with selected microorganisms. On an industrial scale, the milk is sterilized by pasteurization after homogenization, kept for cooling to 20°C, and incubated with specific strains for 24 hours. Total acidity in the range of 95-100°T (pH 4 to 4.5) indicates the completion of the fermentation process and the formation of a stable coagulate.
Conclusion
Traditional Bulgarian dairy products contain a unique bacterial microflora that has evolved over centuries under specific climatic conditions. In addition to excellent technological properties resulting in excellent taste, aroma, organoleptic properties, and shelf life, LAB strains, both starter and indigenous, contribute to the performance of dairy foods.
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