MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.1. MICROBIOLOGY OF MILK AND DAIRY PRODUCTS

15.1.3. Fermented Dairy Products

The manufacturing technology of fermented dairy products relies on specific microflora whose metabolic activity determines the flavor, aroma, and texture of the final product. These specific microbial cultures typically comprise combinations of lactic acid Bacteria (LAB), occasionally supplemented with bifidobacteria, Yeasts, acetic acid bacteria, and propionic acid bacteria.

The primary metabolic product of LAB is lactic acid. The presence of lactic acid in fermented dairy products helps to some extent inhibit the growth of putrefactive and pathogenic microorganisms. Nevertheless, bacteria of the genera Salmonella and Shigella, as well as staphylococci, can survive in these products for extended periods and act as agents of infectious diseases or food poisoning. Therefore, the most critical prerequisites in The production of fermented dairy products are high raw milk quality, proper pasteurization, high starter culture activity, and the Prevention of post-pasteurization contamination of both the milk and the final product by pathogenic bacteria.

Yogurt is a fermented dairy product with an increased solids content (16%), which is achieved by evaporating a specific amount of moisture or by adding skim milk powder. The starter culture for yogurt, as well as for southern, Mechnikov's, and Bulgarian sour milk, includes two species of LAB: Bulgarian bacillus (Lactobacillus delbruekii subsp. bulgaricus) and thermophilic streptococcus (Streptococcus salivarius subsp. thermophilus). The ratio between these species (cocci to rods) in yogurt, Mechnikov's, and Bulgarian sour milk should be 1:1, whereas in southern sour milk it should be 4:1. Milk Fermentation is carried out at 40—45 °C for 3—4 hours. The acidity of yogurt, Mechnikov's, and Bulgarian sour milk must not exceed 110 °T, while that of southern sour milk should not exceed 140 °T. The concentration of LAB in 1 cm³ of the finished product ranges from 10⁷ to 10⁸ Cells.

To prevent an excessively sour taste in the final product caused by the overgrowth of Bulgarian bacillus, The amount of added starter culture should be reduced to 1.0—1.5%, the fermentation Temperature should be maintained within 39—42 °C, and the product should be rapidly cooled after fermentation down to 5—6 °C.

Plain sour milk (Prostokvasha). Pasteurized milk cooled to 30 °C is inoculated with 3—5% of a starter culture based on mesophilic lactococci: Lac. lactis ssp. lactis, Lac. lactis ssp. cremoris, Lac. lactis ssp. diacetylactis. Plain sour milk is produced primarily by the tank (thermostat) method. The inoculated milk is dispensed into retail packaging and placed into a thermostatically controlled incubator at (30 ± 2) °C. Milk fermentation under these conditions lasts for 6—8 hours, after which the product is transferred to cold storage. The finished product should form a firm, uniform curd without whey Separation, with an acidity ranging from 75 to 80 °T.

To improve the texture of the sour milk, an additional 0.5% starter culture based on Bulgarian bacillus is sometimes added to the milk at a ratio of 1:4 to 1:10 relative to lactococci. When selecting this ratio, it is essential to consider the fermentation duration, the CHARACTERISTICS OF THE resulting curd, and the acidity of the final product.

Ryazhenka is a fermented dairy product made from milk subjected to high-temperature Treatment with prolonged holding, which gives the final product a creamy color and a characteristic caramelized flavor. Ryazhenka is produced from high-fat raw milk ranging from 3.2% to 6.0% fat. The product blend is heated to 92—95 °C and held for 2—3 hours. During this "simmering" process, the mixture is stirred 2—3 times to prevent Skin (film) formation. Following heat treatment, the milk is cooled to 43—45 °C and inoculated with 3—5% of a Streptococcus thermophilus starter culture. Ryazhenka is manufactured using both thermostat and tank Methods. Practical experience shows that using a starter based solely on thermophilic streptococcus often results in a bland product with an underdeveloped flavor. Consequently, Bulgarian bacillus is additionally introduced into the ryazhenka starter culture at a ratio of 1:4 to 1:5 relative to the streptococci.

The LAB Cell count in the finished product is 10⁷—10⁸ CFU/cm³. The acidity of the final product is 80—90 °T.

Varenets is prepared from sterilized milk or milk heat-treated at 95 ± 2 °C with a 1-hour holding time. Subsequent production technology for varenets is analogous to that of ryazhenka.

Acidophilus milk. This product is obtained by fermenting pasteurized milk with a starter culture based on acidophilus bacillus (Lactobacillus acidophilus). A 1—5% starter culture is added to milk at 37—39 °C and fermented for 3—4 hours until an acidity of 70—80 °T is reached. The acidity of the finished product is 90—120 °T. The number of acidophilus bacillus cells in the final product reaches 10⁸—10⁹ per 1 cm³.

The most frequent difficulties encountered in the production of acidophilus milk are the proliferation of contaminating mesophilic lactic acid streptococci (if the fermentation temperature drops below 37 °C) or thermophilic streptococci (if the fermentation temperature exceeds 40 °C). The multiplication of these microorganisms leads to a plain sour-milk taste and a reduction in the therapeutic Properties of the final product.

Acidophilus Yeast-milk (Acidofilin). The product is manufactured from pasteurized milk by fermenting it with equal Proportions of the following starter types: acidophilus bacillus starter, mesophilic lactococcus starter, and kefir grain starter. Fermentation is conducted at 30—35 °C for 6—8 hours until the curd acidity reaches 80—85 °T.

Kefir is the only fermented milk beverage produced using a natural symbiotic starter: kefir grains. They have an irregular shape, a folded or bumpy surface, and a resilient consistency. The diameter of kefir grains ranges from 1—2 to 5—6 cm. The primary microflora of kefir grains includes mesophilic lactococci, lactobacilli, yeasts, and acetic acid bacteria. Lactobacilli isolated from kefir grains have been identified as L. plantarum, L. casei, L. kefir, L. brevis, L. buchneri. Among the yeasts isolated from kefir grains, the most common are Saccharomyces lactis, S. cartilaginosus, S. unisporus, Candida kefir, Kluyveromyces fragilis. Lactic acid streptococci belonging to the species Lac. lactis subsp. lactis, Lac. lactis subsp. cremoris, Lac. lactis subsp. diacetylactis, Leuconostoc cremoris reside in the surface layers of the kefir grains. Acetic acid bacteria (Acetobacter aceti) are also part of the permanent microflora of kefir grains; they apparently play a significant role in the Symbiosis of the grain organisms. Thus, both homo- and Heterofermentative lactic acid fermentation as well as Alcoholic Fermentation take place in kefir.

Defects in kefir include gas-pocket (eye) formation and a yeasty flavor caused by violations of Sanitary and hygienic production standards. Gas bubbles in kefir can be produced by yeasts, aroma-forming lactococci, heterofermentative lactic acid rods, and coliform bacteria.

Research by Japanese scientists has demonstrated that the polysaccharide kefiran, found in kefir grains, exhibits anti-carcinogenic activity.

Kumis (Koumiss) is a traditional fermented dairy beverage of the steppe peoples of Asia, produced from mare's or cow's milk through mixed lactic acid and alcoholic fermentation. Kumis is prepared by fermenting mare's milk with cultures of Bulgarian and acidophilus bacilli and lactose-fermenting yeasts. Strains of these lactic acid bacteria and yeasts exhibit high antagonistic activity against Mycobacterium tuberculosis and undesirable intestinal microorganisms.

Mare's milk contains less casein and fat, but more lactose than cow's milk, with casein and albumin present in equal amounts. Consequently, when mare's milk is fermented, a solid curd does not form; instead, it precipitates as loose, fine flakes. Mare's milk is richer in Vitamins C, B₁, B₂, and Trace Elements than cow's milk.

A technology for producing kumis from a specialized milk blend consisting of whole and skim cow's milk, cheese whey, with added lactose and Vitamin C, has been developed. A key feature of this technology is the Introduction of a large volume of starter culture (up to 20%) into the prepared blend, which stimulates the intensive proliferation of yeasts and lactic acid bacteria.

Tvorog (cottage cheese / curd) is a protein-rich fermented dairy product obtained by fermenting milk with mesophilic lactococci, followed by the separation of whey to achieve a specific moisture content in the final product.

The production of tvorog utilizes starter cultures of the following composition: Lactococcus lactis ssp. lactis, Lactococcus lactis ssp. diacetylactis (with or without The addition of Lactococcus lactis ssp. cremoris).

The main challenges most commonly encountered during the production of tvorog are:

✵ multiplication of heat-resistant lactic acid rods, leading to the "excessively sour taste" defect in curd;

✵ slowed milk fermentation resulting from seasonal changes in milk composition, presence of inhibitory substances in the milk, or bacteriophage infection of starter cultures;

✵ contamination of the finished product with coliform bacteria;

✵ stringiness of the clot and poor whey separation due to acetic acid bacteria entering the milk.

During the production of curd products (curd bars, cakes, creams), strict SANITARY AND BACTERIOLOGICAL control must be maintained over auxiliary Materials (sugar, salt, raisins, dried apricots, nuts, candied fruits, flavoring and aromatic additives) as well as all Stages of the technological process (grinding, mixing with ingredients, cooling, packaging).

The most common defect in curd products, especially during the summer season, is Swelling. This defect is caused by the accumulation of metabolic products from yeasts that enter the products via sugar, raisins, and other ingredients.

Curd and curd products can be contaminated with pathogenic or opportunistic microorganisms at various stages of their preparation. Specifically, Staphylococcus aureus was detected in 7–10% of curd product samples; yersiniosis pathogens (Yersinia enterocolitica) were isolated from 9% of curd samples. Pathogenic microorganisms can persist in curd for quite a long time at a storage temperature of 2–4 °C. Outbreaks of disease have been described following the consumption of curd and curd products contaminated with salmonellae, shigellae, yersiniae, and enteropathogenic Escherichia coli.

Sour cream (smetana) is a fermented milk product obtained by fermenting cream of varying fat content with a starter culture based on mesophilic lactococci: Lac. lactis ssp. lactis, Lac. lactis ssp. cremoris, Lac. lactis ssp. diacetylactis. During smetana production, cream is pasteurized at high temperatures (95 ± 2 °C with a holding time of 2–10 min), which is why the residual microflora is primarily represented by spore-forming bacteria.

After pasteurization and homogenization, the cream is cooled to the inoculation temperature—18–22 °C in summer and 22–23 °C in winter—after which 0.5 to 5.0% of the bacterial starter is added. Due to its elevated fat content, lower protein content, and fewer available minerals, cream serves as a less favorable nutrient medium for LAB development compared to milk. Consequently, the fermentation time for cream is significantly longer than for milk, averaging 12–16 hours. The end of fermentation is determined by titratable acidity (60–75 °T) and the density of the formed clot. During cooling and ripening, the multiplication of lactococci slows down, but the acidity continues to rise to an optimal value of 85–100 °T. The number of lactococcus cells in the finished product is approximately 10⁷ CFU/g.

The main defects of sour cream are:

✵ excessively sour taste caused by the proliferation of heat-resistant lactic acid rods (up to 10⁷);

✵ swelling and yeast flavor resulting from product contamination with lactose-fermenting yeasts (up to 10⁶);

✵ bitter, rancid taste caused by protein and fat breakdown by Enzymes from psychrotrophic bacteria of the genera Pseudomonas, Achromobacter, Flavobacterium;

✵ unclean taste due to contamination by coliform bacteria (over 10⁴);

✵ molding caused by the growth of dairy mold Endomyces lactis and other species of mycelial Fungi On the surface.

Fermented milk beverages (excluding thermized ones), curd, and sour cream are permitted to be stored in the retail network for no more than 72 hours (from the time of production) at a temperature of (4 ± 2) °C.

Probiotic products. The Development of the "Probiotics and functional Nutrition" concept is one of the most important achievements of the late 20th century. Today, probiotics are defined as living microorganisms and substances of microbial origin that exert a positive effect on the host's physiological Functions, biochemical reactions, and behavioral responses by optimizing its microecological status. Probiotics include bifidobacteria, LAB, propionic acid bacteria, E. coli strain M17, and others. Among lactic acid bacteria, the following lactobacilli are commonly used: L. acidophilus, L. casei, L. rhamnosus, L. plantarum, L. helveticum, L. brevis, L. fermentum, L. buchneri.

Probiotics help maintain the balance of the intestinal normal flora, inhibit the colonization of pathogenic and opportunistic microbes, exert anti-infectious and immunostimulatory effects, improve protein and mineral METABOLISM in the Body, synthesize certain vitamins and Essential Amino Acids, and lower Blood Cholesterol levels. Products containing probiotics are classified as functional foods.

In recent years, lactobacilli and bifidobacteria have found widespread application in the production of fermented dairy products with therapeutic and prophylactic properties (Table 10).

Class="center">Table 10. Fermented dairy products containing lacto- and bifidobacteria

Substances that stimulate the growth of bifidobacteria in The Human Body are called prebiotics or bifidogenic factors. These include Oligosaccharides (lactulose, lactitol), Polysaccharides (inulin, Pectins, pullulan), and plant or microbial extracts. Functional products containing both probiotics and prebiotics are called synbiotics.

In recent years, bacterial preparations or so-called direct vat set cultures (DVS), which consist of dried lacto- and bifidobacterial biomass, have become widely used in the production of fermented dairy products. The main suppliers of direct vat set cultures to Russia are primarily foreign companies: Chr. Hansen (Denmark), the Texel concern with plants in France and Biolacta-Texel in Poland, and Danisco (Germany). Direct vat set cultures offer the following advantages over traditional liquid starters:

✵ ease of use;

✵ guaranteed stable quality and starter activity, along with an improved microbiological profile of the finished product;

✵ reliability and reproducibility of the technological process, ensuring uninterrupted enterprise operation;

✵ reduction of the risk of bacteriophage infection and degeneration of starter cultures;

✵ ability to rapidly adapt the range of manufactured products.