Fermented milk products are very popular throughout the world. These are prepared by controlled fermentation of milk to produce acidity and flavour to the desired level. Pawas Goswami, Smita Singh and Kanti Prakash Sharma explain briefly here the method of production and associated important properties of fermented milk products.
Milk contains sugars, fats, proteins and calcium along with a number of vitamins and minerals, which provide nourishment to the body. It is a rich source of nutrients and can be classified as a complete food. The concentration of various milk components differs from the animal e.g. fat content of cow’s milk is lower as compared to that obtained from buffalo (Zicarelli, 2004). It also depends upon a number of factors like the breed of cow/buffalo, stage of lactation, age, feed, the season of the year, the interval between milking, disease etc. (Nickerson, 1999), but all kinds of milks have one thing in common i.e. all of them support prolific growth of microorganisms which include microflora of raw milk as well as both the spoilage and pathogenic types such as Salmonella, Mycobacterium, Campylobacter, etc. that pose a health hazard as observed in the milk-borne outbreak of several diseases (Parry, 1966).
This problem of microbial load can be prevented by several treatments given to raw milk such as Bactofugation, Pasteurization, Sterilization, Microfiltration etc (Rysstad and Kolstad, 2006). Besides the presence of unwanted microorganisms other problem associated with the consumption of milk is that of indigestion, which occur in the people suffering from lactose intolerance (Ferguson, 1981) i.e. the inability to digest significant amounts of milk sugar lactose. It happens due to a shortage of enzyme lactase, which breaks down milk sugar. This leads to water accumulation in the small intestine and lactose fermentation in the large intestine, resulting in mucosal irritation and cramps, flatulence and diarrhea. Lactose intolerance and other problems associated with consumption of milk can however be taken care of by fermentation of milk to which this article is devoted.
Fermentation of milk
Fermentation is the chemical transformation of organic substances into simpler compounds by the action of enzymes, complex organic catalysts, which are produced by microorganisms such as molds, yeasts, or bacteria and are easier to be assimilated by the body. Enzymes act by hydrolysis, a process of breaking down or predigesting complex organic molecules to form smaller (and in the case of foods, more easily digestible) compounds and nutrients. The important enzymes associated with fermentation of milk nutrients are protease, amylase and lipase.
Fermentations can be of various types depending on the end product such as acidic fermentations in which the end product is lactic acid or mixture of other acids including acetic acid, propionic acid or alcoholic fermentations that include those fermentations in which the end product is alcohol.
During fermentation of milk by microorganisms, lactose is transported via permease into the cell, where beta-galactosidase hydrolyses the disaccharide into glucose and galactose. Galactose is not fermented and is discharged out of the cell. Glucose, on the other hand, is rapidly phosphorylated and, by the intervention of aldolase, is converted to pyruvic acid in accordance with the glycolytic pathway. Pyruvic acid is then converted to lactic acid by the lactate dehydrogenase enzyme.
Benefits of Fermentation Milk Products
Fermented milk products are very popular throughout the world. These are prepared by controlled fermentation of milk to produce acidity and flavour to the desired level. Consumption of fermented milk products has numerous advantages such as:
- Fermented milk products are more stable than fresh milk because they are more acidic and/or contain less moisture hence does not putrefy easily.
- Fermented milk products are wholesome and readily digestible.
- Fermentation leads to improved flavour and texture, appearance and aroma, as in the case of ripened cheeses, yogurt.
- It leads to the synthesis of certain vitamins (including B-12 (Karlin, 1961), which is difficult to get in vegetarian diets).
- Fermentation is helpful in reducing or masking undesirable or beany flavours.
- It reduces or eliminates lactose by converting into another product like lactic acid eliminating problems associated with lactose intolerance.
- Enzyme inhibitors and other anti-nutrients are eliminated in fermented milk products.
- Minerals can be more easily absorbed; the calcium and phosphorus of curd are more easily assimilated.
- Fermented milk products are ideal vehicle for delivery of probiotic bacteria in gut, by which intestinal microflora is replenished and other beneficial effects as desired from probiotics achieved.
Delivery of probiotics through the fermented milk products is an important point and needs more description, as it is an emerging trend in functional foods.
Probiotics in Fermented Milk Products
Probiotics, as defined by Fuller (1989) are ‘live microbial feed supplements which beneficially affects the host animal by improving its intestinal microbial balance’. An organism to be used as a probiotic in dairy foods should have certain prerequisite properties–
- acid tolerance and bile tolerance for transit in the intestine.
- ability to adhere to the intestinal mucosa and to colonize gut.
- technological compatibility to be used in fermented milk products and maintain viability under storage conditions.
Tannock (1998) reported use of several bacteria including Lactobacillus acidophilus, Lactobacillus rhamnosus, Lactobacillus casei Shirota strain, Lactobacillus delbrueckii, Bifidobacterium adolescentis, Bifidobacterium bifidum, Bifidobacterium breve, Bifidobacterium longum, Bifidobacterium infantis and Streptococcus thermophilus as probiotics in dairy foods to produce desired effects of –
- Enhancement of both specific (Haghighi et al., 2005) and nonspecific immune response.
- Inhibition of pathogen growth and translocation thus reducing the chance of infection from common pathogens.
- Inhibition of superficial bladder cancer (Aso et al., 1992).
- Prevention of food allergies (Majamaa et al., 1997).
- Prevention of traveler’s diarrhea (Oksanen et al., 1990), antibiotic-associated diarrhea (Siitonen et al., 1990) and treatment of rotaviral diarrhea (Isolauri et al., 1991).
- Lowering of blood cholesterol (Klaver and van der Meer,1993).
Hence the combined health-promoting properties of probiotic microbes along with the beneficial properties of carrier-fermented milks create ideal health-enhancing dietary supplements. The addition of correct probiotic strain is however highly essential for this purpose, which can be achieved by testing of these strains by a number of molecular techniques, namely, PCR, ARDRA, Ribotyping, specific probes, PFGE, sequencing of 16S rRNA gene or other conserved house keeping genes etc. The most commonly and easily employed method however is of PCR, which can be used for genus, species and even strain identification either directly or with variations of PCR like RAPD (Coeuret et al., 2003; Kaufmann, et al., 1997).
These benefits associated with the fermentation process has led to the development of a number of fermented milk products like, acidophilus milk, yogurt, sour cream, kefir, etc. They are popular among people and provide nourishment as well as promote health. The method of production and associated important properties of such products are briefly given in this article.
Acidophilus milk is traditional fermented milk product with Lactobacillus acidophilus (LA), which has been thought to have therapeutic benefits in the gastrointestinal tract. Skim or whole milk is heated to high temperature, e.g., 95°C for 1 hour, to reduce the microbial load and favour the slow-growing LA culture. Milk is inoculated at a level of 2-5% and incubated at 37°C until coagulated. Some acidophilus milk has acidity as high as 1% lactic acid, but for therapeutic purposes, 0.6-0.7% is more common.
Production of yogurt starts by conditioning the milk. The water content of milk is first lowered 25% by vacuum evaporation and 5% of milk solids are added. As a final conditioning step, the milk is heated to 86-93°C for 30-60 minutes. This higher temperature heat treatment is necessary to produce a relatively sterile and conducive environment for the starter culture and to denature and coagulate whey proteins to enhance the viscosity and texture of the final product and also to kill contaminating microbes that may compete with the starter culture. After cooling to 45°C a 1:1 mixture of Streptococcus thermophilus and Lactobacillus bulgaricus is added. Fermentation takes place at 45°C until the desired degree of acidity is reached. This usually occurs in 3-5 hours.
The nonfat milk-solids content of the finished product must be more than 8.25%, and the final titratable acidity must be at least 0.9%, expressed as lactic acid. The synergistic interaction between Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, is favourable to acidification, production of polysaccharides, and formation of the typical yogurt aroma, which is primarily due to the presence of lactic acid and acetaldehyde. Small quantities of acetaldehyde (2-4 mg/100g) are enough to confer the typical aroma to yogurt. Other carbonylic compounds of fermentation such as acetone and acetoin also contribute to create aroma and taste.
Fruits and other edible ingredients can also be included in the finished product. Yogurt is stored at 0-4°C until consumed.
Like yogurt or other fermented milk products, kefir contains much less lactose than milk. Its live organisms also harbour beta-galactosidase. Kefir’s titratable acidity is about 1%, and it has an alcohol content of about 0.01% to 0.10%. Production starts with whole, low-fat or skim milk, adjusted for body with nonfat milk solids. The milk is pasteurized, and then heat-treated at 95°C for 10 to 15 minutes to totally denature the whey proteins. This product is then cooled to 18° to 22°C, and 2% to 5% kefir grains are added. This mixture is incubated at 18° to 22°C for 24 hours. The kefir grains are then sieved out, and the product is chilled and packaged. Kefir grains can be rinsed and re-used, or dried and stored for later use. The final kefir product can be flavoured in a manner similar to yogurt.
It is usually defined as the liquid product remaining when fat is removed from milk or cream by churning. It contains not less than 8.25% nonfat milk solids. Milk is usually heated to 95°C and cooled to 20-25°C before the addition of the starter culture. Starter is added at 1-2% and the fermentation is allowed to proceed for 16-20 hours, with a targeted titratable acidity of 0.75% to 0.90%, which correlates to a diacetyl content of about 2.0 ppm. This gives the product an optimum buttery flavour. Viscosity can be increased by adding stabilizers, nonfat dry milk, or polysaccharide-producing cultures. Both lactic-acid-producing and flavour-producing strains are used for culturing. The culture most frequently used is Lactococcus. Lactis ssp. lactis and. perhaps also L. lactis ssp. cremoris.
This product is frequently used as an ingredient in the baking industry, in addition to being packaged for sale in the retail trade.
Cultured cream usually has a fat content between 12-30%, depending on the required properties. The starter is similar to that used for cultured buttermilk. The cream after standardization is usually heated to 75-80°C .It is then homogenized at >13 MPa to improve the texture. Inoculation and fermentation conditions are similar to those for cultured buttermilk, but the fermentation is stopped at an acidity of 0.6%.
Cheese is also a product of milk fermentation, but its production is more complex. Different bacteria play a role and production periods are much longer than yogurt. There are 20 classes and hundreds of varieties of cheeses. The initial manufacturing process is however surprisingly similar.Cheese making was started as a way of preserving milk, to a long life product. Principally, cheese making involves concentration, preservation and ripening. Milk is pasteurized and cooled to inoculation and setting temperature, additives like saltpetre or salt or annatto colour are added, starter culture is inoculated, the milk is coagulated using rennet, the curd is cooked, pressed, and salted, and the cheese is ripened. These three processes are common for all cheese varieties. By controlling these processes in different ways – more or less whey drainage, stronger or weaker acidification, different mouldings, different surface treatments, the addition of different micro-organisms, storage at different temperatures, etc. make the different varieties It is possible to manufacture a large number of very different cheese varieties from the same raw material i.e. milk. Broadly, cheeses are classified as soft, semi-hard, hard, and very hard types. The ingredients used in making cheese include whole milk, skim milk, cream or a combination of two of them, starter
cultures, rennet or organic acids like citric acid. Spices like pepper, cumin, black peppers and other may optionally be added in processed and special types of cheeses.
Other fermented milk products
There are a number of other fermented milk products like koumiss, taette and tarhana etc. which are consumed in specific areas and countries.
The advent of fermented milk products has not only provided a highly efficient method of conserving surplus milk nutrients for a longer period, it has also contributed largely to the well being of the ancient forefathers of human species as well. They are also equally important in today’s context as they provide a natural and reliable way to keep healthy without any side effects. With India emerging as the biggest milk producer in the world their potential to reach more and more population has opened new dimensions for their popularity. However the development of more such products by identifying new starters with beneficial properties are needed. Probiotic yeast Saccharomyces boulardii and other organisms must also be studied in detail to incorporate them in fermented milk products. Their consumption should be encouraged over other milk products for their daily inclusion in the diet due to the range of health-promoting properties associated with these products.
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