THE ROLES OF LACTIC ACID BACTERIA IN COW MILK YOGHURT PRODUCTIO


INTRODUCTION
Yogurt is a widely enjoyed dairy product that is essentially an altered form of milk containing waste products from fermentation. The lactic acid that is produced from the fermentation of lactose contributes to the sour taste of yogurt by decreasing pH and allows for the characteristic texture by acting on the milk proteins (Zourari, Accolas, & Desmazeaud, 1992). Yogurt has been continually studied for its health benefits, particularly from the addition of probiotics. Current research has been investigating how to improve yogurt both in terms of its potential as a healthy food and as an appetizing product that appeals to the general population.
The role of lactic acid bacteria (BAL) are widely applied in various fields, including agriculture, especially the field of food.
In the manufacture of this paper describes the role of lactic acid bacteria (BAL) on the processing of fermented foods. The scope limitations discussed issues such as species of lactic acid bacteria and fermented food sector lactic acid bacteria, refined products fermented by lactic acid bacteria and the role and the positive impact generated by the fermentation by lactic acid bacteria such.
We could describe cow milk yoghurt production as a dairy product produced by bacterial fermentation of milk. The bacteria used to make yogurt are known as “yogurt cultures”. Fermentation of lactose by these bacteria produces lactic acid, which acts on milk protein to give yogurt its texture and its characteristic tang.
Worldwide, cow’s milk is most commonly used to make yogurt, but milk from water buffalo, goats, sheep, horses, camels, and yaks is also used in various parts of the world.
Dairy yogurt is produced using a culture of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermopiles bacteria. In addition, other lactobacilli and bifidobacteria are also sometimes added during or after culturing yogurt.
The milk is first heated to about 80 °C (176 °F) to kill any undesirable bacteria and to denature the milk proteins so that they set together rather than form curds. The milk is then cooled to about 45 °C (112 °F).The bacteria culture is added, and the temperature is maintained for 4 to 7 hours to allow fermentation.
The lactic acid bacteria (LAB) comprise a clade of Gram-positive, low-GC, acid-tolerant, generally non-sporulating, non-respiring rod or cocci that are associated by their common metabolic and physiological characteristics. These bacteria, usually found in decomposing plants and lactic products, produce lactic acid as the major metabolic end-product of carbohydrate fermentation. This trait has, throughout history, linked LAB with food fermentations, as acidification inhibits the growth of spoilage agents. Proteinaceous bacteriocins are produced by several LAB strains and provide an additional hurdle for spoilage and pathogenic microorganisms. Furthermore, lactic acid and other metabolic products contribute to the organoleptic and textural profile of a food item. The industrial importance of the LAB is further evinced by their generally recognized as safe (GRAS) status, due to their ubiquitous appearance in food and their contribution to the healthy microflora of human mucosal surfaces. The genera that comprise the LAB are at its core Lactobacillus, Leuconostoc, Pediococcus, Lactococcus, and Streptococcus as well as the more peripheral Aerococcus, Carnobacterium, Enterococcus, Oenococcus, Sporolactobacillus, Tetragenococcus, Vagococcus, and Weisella; these belong to the order Lactobacillales.

Streptococcus
Characteristics
The lactic acid bacteria (LAB) are rod-shaped bacilli or cocci characterized by an increased tolerance to a lower Ph range. This aspect partially enables LAB to outcompete other bacteria in a natural fermentation, as they can withstand the increased acidity from organic acid production (e.g., lactic acid). Laboratory media used for LAB typically include a carbohydrate source, as most species are incapable of respiration. LAB are catalase negative. LAB are amongst the most important groups of microorganisms used in the food industry.
Lactic Acid Bacteria. The term lactic acid bacteria (BAL) originally intended only for the group of bacteria that cause acidity in the milk (milk-souring organisms). BAL is generally defined as a group of gram-positive bacteria, does not produce spores, round or rod that produces lactic acid as the main metabolic end product during fermentation of carbohydrates. BAL grouped into several genera such as Streptococcus (including Lactococcus), Leuconostoc, Pediococcus Lactobacillus.
Identification of lactic acid bacteria based on morphology, physiology and biochemical properties of bacteria. Identification method according to Holzapfel and Schillinger (1992 in Widodo 2003), which states that the genus Streptococcus has characteristics that is, the final pH in MRS medium <4.6, negative catalase test, colony-shaped cocci, coccus-shaped tetrad is not, and did not grow at temperatures 100C.
Lactic acid bacteria (BAL) in the physiology of bacteria classified as Gram positive, rod shape or not kokkus berspora with lactic acid as the main product of carbohydrate fermentation. Traditionally, BAL is comprised of four genera Lactobacillus, Leuconostoc, Pediococcus and Streptococcus. For example the genus Streptococcus have been reorganized into Enterococcus, Lactococcus, Streptococcus and Vagococcus (Yang, 2000).
Among the BAL genus and species that have potential for use as probiotics can be seen in Table 2.
Lactic acid bacteria have an essential role in almost every food and beverage fermentation processes. The main role of these bacteria in the food industry is to marinade the raw material to produce the majority of lactic acid (homofermentatif bacteria) or lactic acid, acetic acid, ethanol and CO2 (bacteria heterofermentatif) (Desmazeaud, 1996). Lactic acid bacteria are widely used in dairy products like yogurt, sour cream (sour milk), cheese, butter and pickle production, and pickles (Lindquist, 1998).

III. Fermentation of Lactic Acid Bacteria
Fermentation is the process of aerobic and anaerobic, both which produce various products involving microbial activity or the extract is controlled by microbial activity (Dervish and Sukara, 1989). Fermentation is a process that has long been known to man. Fermentation is the process of converting a material into useful products for humans, such as fermented milk goat, camel in Sumaria and Babylon in Mesopotamia era. Until now, the process has undergone perbaikanperbaikan ferementasi terms of the fermentation process to produce a better product (Tamime and Robinson, 1999).

The Biochemistry Behind Yogurt

Fig. 3. Embden-Meyerhof-Parnas pathway proposed to be used by Streptococcus thermophilus with the homolactic fermentation of pyruvate into lactic acid. This diagram also outlines the conversion of galactose into glucose to be used in the EMP pathway. Courtesy of Zourari et al., 1992.
Yogurt is a product of the acidic fermentation of milk. The lactose in the milk is converted to lactic acid, which lowers the pH. When pH drops below pH 5, micelles of caseins, a hydrophobic protein, loses its tertiary structure due to the protonation of its amino acid residues. The denatured protein reassembles by interacting with other hydrophobic molecules, and this intermolecular interaction of caseins creates a structure that allows for the semisolid texture of yogurt (Zourari, Accolas, & Desmazeaud, 1992).

Yogurt production begins with the breakdown of lactose into glucose and galactose (Fig. 2), a process catalyzed by β-galactosidase. The glucose produced from this catabolic step then enters glycolysis, producing pyruvate. It has been proposed that yogurt bacteria utilize the Embden-Meyerhof-Parnas pathway of glycolysis (Fig. 3). Pyruvate then enters lactate fermentation, also known as homolactic fermentation, as it produces only lactic acid molecules. In other types of fermentation, such as ethanolic or heterolactic fermentation, the production of ethanol leads to other fermented foods and beverages such as sauerkraut, kimchi, and wine.

The production of lactic acid forms the basic structure and texture of yogurt. However, other molecules contribute to the taste of yogurt. These include acetaldehyde, an important flavor substance in yogurt, and tyrosine, a product of proteolytic activity, but can cause bitterness when the concentration is above 0.5 mg/ml (Guzel-Seydim, Sezgin, & Seydim, 2005).

Fig. 2. Lactose catabolism into glucose and galactose. Courtesy of Thomas M. Terry at the University of Hamburg

Benefits of Yogurt
The benefits of yogurt have been recognized even before microbes were discovered. The use of yogurt to treat body ailments are mentioned in the Bible, and scientists of the early ages like Hippocrates considered fermented milk to be a medicine, prescribing sour milk for curing stomach and intestinal disorders (Oberman, 1985 as cited in Lourens-Hattingh and Vilijoen, 2001). A scientific explanation for the beneficial effects of yogurt was first proposed by Eli Metchnikoff, a Russian bacteriologist at the beginning of the 20th century. Metchnikoff suggested that the lactobacilli in yogurt are responsible for the healthy and long lifespan of Bulgarian people. This led to the naming of one of the species in the starter culture as Lactobacillus bulgaricus (Lourens-Hattingh & Vilijoen, 2001).
Yogurt has also been found to protect against growth retardation in rats that are fed diets high in phytic acid, which disrupts zinc absorption, a mineral needed for normal growth (Gaetke et al., 2010). However, the level of zinc was low regardless of whether yogurt was added to the diet or not, suggesting that yogurt protects against growth retardation not by increasing zinc but by some other mechanism. Additionally, a variant of traditional yogurt, soy yogurt, has been found to help prevent hepatic lipid accumulation in rats (Kitawaki et al., 2009). Specifically, the rats fed soy yogurt had lower liver weight and hepatic triglyceride content, and their plasma cholesterol levels were also lower compared to control rats that were fed a standard rat diet without soy yogurt. Furthermore, ingestion of soy yogurt down-regulated the expression of sterol regulatory element binding protein (SREBP-1) and other lipogenic enzymes, while upregulating β-oxidation-related genes, which produce enzymes that are involved in the catabolism of fatty acids cholesterol in the rat liver.
In general, there is a consensus that yogurt has beneficial effects for gastrointestinal health, as shown in both animal and human studies. Some studies also suggest that yogurt not only helps maintain a healthy gut, but can also help with certain gastrointestinal conditions, such as lactose intolerance, constipation, diarrheal diseases, colon cancer, Helicobacter pylori infection, inflammatory bowel disease, and allergies (Adolfsson, Meydani, & Russell, 2004). The benefit of yogurt seems to extend to the reproductive system as well. In diabetic women, it has also been suggested that yogurt consumption reduces the risk of vaginal yeast infection, caused by Candida, by regulating pH and suppressing Candida overgrowth (Chauncey et al., 1999). However, the bacteria responsible for these beneficial effects are not necessarily the bacteria that produced the yogurt, as discussed below.

The health benefits of yogurt for the most part can be directly attributed to probiotics (Fig. 8). Probiotics are defined as a mono- or mixed culture of live microorganisms which benefits the host by improving the host’s microflora (Lourens-Hattingh & Vilijoen, 2001). Common probiotics added to yogurt are Lactobacillus casei (Fig. 9), Lactobacillus acidophilus, and species of Bifidobacterium (Fig. 10).

The designation of starter culture bacteria, L. bulgaricus and S. thermophilus, as probiotics is a topic of debate. The majority of the gastrointestinal benefits are attributed to the probiotics that are added after the yogurt is produced by the starter culture. However, some argue that the starter culture bacteria should also be called probiotics because they improve lactose digestion and eliminate symptoms of lactose intolerance when consumed in yogurt without any added probiotics (Guarner et al., 2005). In the rat colon, it has been found that the starter culture bacteria inactivate carcinogens, thus avoiding carcinogen-induced lesions, and also prevent DNA damage, which in effect prevents tumors (Wollowski et al., 1999). In summary, there is some research implicating that the starter cultures do confer some benefits that may deem them to be classified as probiotics, but it may take a while before this idea is accepted by the majority.

Improving Yogurt
Current Problems
Though the benefits of yogurt are recognized, what is not really understood is if the functionality of yogurt is at its maximum. The most obvious and heavily researched area is increasing the viability of the probiotics that provide the health benefits. However, increasing viability extends to the starter cultures as well, as they have also been documented to contribute some benefits and more importantly, need to be alive in order to be able to produce yogurt even in the presence of other chemicals. Viable cell count/density is the reason not all yogurt brands are equal. Those with live active cultures are indeed healthier than the pasteurized version in which the yogurt is heated, thus killing any live bacteria. However, some brands of yogurt have a higher concentration of live cultures compared to others, though for marketability reasons and overall difficulty in assessing live culture concentration, these numbers are often undisclosed. Improving yogurt by increasing or enhancing bacterial viability is a large field on its own, and is thus discussed below.

Conclusion
Yogurt has a long history and its benefits have been valued by many people, particularly those with gastrointestinal problems. The production behind yogurt is well understood, allowing for improvements and advancements in both the quality and efficient manufacturing of the product. Improving the health potential of yogurt has become a popular field, and for industrial reasons, enhancing the taste and texture, as well as storage life of yogurt is an appealing advancement for yogurt consumers. Yogurt in its basic form is a very eco-friendly product, as humans are essentially consuming the waste products of acidic fermentation. Additionally, the unique taste, texture, and potential for even better health benefits make yogurt an attractive food for people of many cultures.

REFERENCES
Adolfsson, O., S. N. Meydani, & R. M. Russell. 2004. Yogurt and gut function. Am J Clin Nutr. 80:245–256.
Akalin, A. S., G. Unal., & M. C. Dalay. 2009. Influence of Spirulina platensis biomass on microbiological viability in traditional and probiotic yogurts during refrigerated storage. Ital. J. Food Sci. 21: 356-364.
Akalin, A. S., S. Gonc, G. Unal, & S. Fenderya. 2007. Effects of fructooligosaccharide and whey protein concentrate on the viability of starter culture in reduced-fat probiotic yogurt during storage. Journal of Food Science. 72: M222-M227.
Alvaro, E., C. Andrieux, V. Rochet, L. Rigottier-Gois, P. Lepercq, M. Sutren, P. Galan, Y. Duval, C. Juste, & J. Dore. 2007. Composition and metabolism of the intestinal microbiota in consumers and non-consumers of yogurt. British Journal of Nutrition. 97: 126–13. Chauncey, K. B., L. M. Boylan, L. Thompson, R. M. Ragain, and R. Cook. 1999. Effects of yogurt with and without active cultures on vaginal Candidal infection in women with diabetes mellitus. Journal of the A

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