Package and cool to 10°C
Figure 6 Steps for the manufacture ofreduced fat sour cream. (From Singer et al., 1992.)
acid content prevents growth of contaminants. During extended storage, however, enzymes of bacteria that survived pasteurization may cause development of bitter and unclean flavors via proteolysis. Good manufacturing and sanitation practices should be employed to prevent such defects.
Acidophilus milk is a fermented milk produced mainly by the use of lactobacilli and is believed to have therapeutic properties (Gilliland, 1989). It can have an acid content of up to 2%, which is unpleasant to some, so consumption is limited. Manufacture of this product first involves sterilization of nonfat or low-fat milk followed by inoculation (5%) with an active Lb. acidophilus culture. Incubation is for 24 h at 36°C, and this generally results in a titratable acidity of 1%. After incubation, the product is cooled and packaged. In addition to tartness, the product also has a strong cooked flavor from sterilization of milk before fermentation. Because of these qualities, the product is not popular. These drawbacks have been overcome in a product from Finland, which is manufactured by fermenting demineralized, lactose-hydrolyzed whey with Lb. casei GG and then adding fruit flavors.
An alternative for ingestion of Lb. acidophilus is sweet acidophilus milk. Initially, this product contained only Lb. acidophilus (Speck, 1975) but now also includes bifidobacteria. Pasteurized, low-fat, skim, or whole fluid milk is packaged with added viable Lb. acidophilus and bifidobacteria. As the inoculated fluid milk is held refrigerated, growth of these bacteria does not occur during storage but occurs in the intestinal tract after consumption. Such growth depends on strain of Lb. acidophilus used (Collins and Hartlein, 1982). Because these organisms are present, the milk must always be refrigerated. Shelf life under such conditions is 2 weeks. Extended storage and/or storage at high temperatures will lead to curdling of milk from acid produced by the added bacteria. A similar Swedish fluid milk product contains Lb. reuteri in addition to Lb. acidophilus and bifido-bacteria.
Kefir is originally a Russian liquid fermented milk product (Tamime et al., 1999). Approximately equal amounts of lactic acid and alcohol are produced during fermentation. Typical flavor results from a balance between lactic acid, diacetyl, aldehyde, ethanol, and acetone. Fizz is provided by the carbon dioxide that is also produced during fermentation. In the manufacture of kefir, milk is heated to 85°C for 30 min and cooled to an inoculation temperature of 22°C. It is then inoculated with kefir grains and fermentation occurs over 12-16 h. The kefir grains are then filtered out and reused.
Kefir grains consisting of yeasts, bacteria, and polysaccharides are used for kefir production (Tamime and Marshall, 1997). The yeasts include Saccharomyces kefir and Torula spp. or Candida kefir and bacteria include Lb. kefir, leuconostocs, lactococci, and various others. Takizawa et al. (1998) isolated 120 strains of lactobacilli from kefir grains; the most prominent was Lb. kefirogranum. The grains require proper care and should be held using routine sanitary practices. Contaminants such as coliforms, micrococci, and bacilli, if present, will lead to a variety of flavor defects.
Kefir-like products with only small amounts of alcohol and with flavors such as strawberry are also manufactured in the United States. Yeasts and various Lactobacillus spp. and Lactococcus spp. are used.
Koumiss also is a product of Russian origin and is largely used in that country for therapeutic purposes (Kosikowski and Mistry, 1997; Moreau, 1992). It is made with a combined acid and alcohol fermentation traditionally from mare's milk but cow's milk also can be used. Even though the acid content of koumiss is high, no curd is visible because of the relatively low protein content of mare's milk (2%) (Kosikowski and Mistry, 1997). Fermentation is accomplished with a combination of Lb. delbrueckii subsp. bulgaricus and a lactose-fermenting yeast, Torula spp. The finished product contains 1.0-1.8% lactic acid, 1.0-2.5% etha-nol, and carbon dioxide. The latter makes for a frothy product.
Scandinavians are among the highest consumers of fermented milk products. It is not surprising, therefore, that some unique fermented products have originated in Scandinavian countries. Examples include viili, langfil, keldermilk, skyr, ymer, and several others. Some of these products possess unique characteristics such as a heavy, ropy body obtained by the use of specially selected cultures, which, in some instances, includes mold (Tamime and Marshall, 1997; Tamime and Robinson, 1988).
Viili is a fermented product of Finland that may be either plain or flavored with fruit. The fat content may vary from 2 to almost 12%, depending on classification (such as low fat, full fat). Milk is heated to a high temperature (83°C for 20-25 min), tempered to the incubation temperature of 20°C, and inoculated with 4% starter culture consisting of Lc. lactis subsp. lactis, a diacetylactis culture, Leuc. mesenteroides subsp. cremoris, and Geotrichum candidum, a mold. Incubation occurs in consumer cups at 20°C for 24 h (final acidity of 0.9%). The purpose of incubation in consumer cups is to allow fat to rise to the surface during incubation where the geotrichum mold will grow and contribute to the typical musty aroma. Furthermore, complex carbohydrates formed by the organisms used give the product a heavy, ropy characteristic.
Ymer is a fermented product of Denmark that has a high protein content of 5-6%. Current commercial procedures use ultrafiltration technology to concentrate the milk protein before fermentation (Tamime and Marshall, 1997). Concentration by some of the more traditional procedures involves either allowing curd to drain or applying heat to curd to induce syneresis. Before fermentation, milk receives a high-heat treatment (90-95°C for 3 min). Incubation is at 20-22°C with an inoculum consisting of Lc. lactis subsp. lactis (biovar. diacetilactis) and Leuc. mesenteroides subsp. cremoris. Consequently, the product has a pleasant acidic flavor balanced with hints of diacetyl.
Another concentrated fermented product of Scandinavia is skyr. This product is from Iceland and has almost 13% protein. Such a high concentration is achieved commercially with the help of a centrifugal separator similar to one used in the manufacture of quarg. Skim milk is fermented with thermophilic lactic acid bacteria similar to those used for yogurt along with lactose-fermenting yeast. Small amounts of rennet may also be added to obtain proper body. With active cultures, a pH of 4.6 is obtained within 4-6 h at 40°C. After an additional 18 h at 18-20°C, the pH drops to 4, the product is pasteurized, and is then centri-fuged at 35-40°C for concentration. Because of the presence of yeast, ethanol occurs in the final product along with lactic acid, diacetyl, acetaldehyde, and acetic acid.
India, the largest milk-producing country in the world today, has a long history of dairying (Aneja, 1997). Production and consumption of milk and milk products date back many thousands of years. Today, numerous indigenous products are available locally. Of these, fermented milk products such as dahi, lassi, srikhand, and misti doi are important parts of the diet.
Dahi is a product made by fermenting milk of the cow or water buffalo milk with lactic acid bacteria. It has a clean, acidic flavor with slight hints of diacetyl. The texture is similar to that of yogurt. Much of the dahi consumed in India is either made at home or by small dairies. In both instances, the culture usually consists of the previous day's product, but pure cultures are also available. Hence, composition of culture and consequently flavor can vary from batch to batch. The legal standards of identity for dahi that is produced commercially and sold in the market are the same as for milk from which dahi is made (Aneja, 1997). The manufacturing procedure for dahi is simple. Milk of the cow, water buffalo, or a mixture is briefly boiled and cooled to room temperature. It is then inoculated with 0.5-1.0% culture and incubated at room temperature for 12-16 h. With an active culture, the final pH is 4.5-4.7. Because room temperature in tropical countries varies according to the season, it is not uncommon to find thermophilic cultures in dahi. Dahi typically contains a mixture of S. thermophilus, Lb. delbrueckii subsp. bulgar-icus, Lc. lactis subsp. lactis, Lc. lactis subsp. cremoris, Lc. lactis subsp. lactis (biovar. diacetylactis), Lb. helveticus, Lb. casei, and Lb. acidophilus (Masud et al., 1991). The initial boiling step eliminates undesirable organisms from the milk, but it is important to have an active culture. After repeated transfers, the culture may lack activity and, in the absence of adequate acid production, undesirable flavors from growth of yeasts and mold may occur. Because yeasts tolerate acid, it is important to prevent postheating contamination of the milk with these microbes.
Lactic acid bacteria of dahi have antimicrobial effects against pathogenic and spoilage bacteria (Balasubramanyam and Varadaraj, 1994; Dave et al., 1992; Srinivasan et al., 1995). Some of these effects come from cell-free extracts and are believed to be associated with production of H2O2 by lactobacilli and bacterio-cin-like compounds by some lactococci (de Vuyst and Vandamme, 1994).
Dahi is typically stored at room temperature; hence lactic acid continues to develop rapidly after its manufacture. Researchers have attempted to eliminate this by introducing nisin (25 IU/mL) in dahi after fermentation is completed (Kumar et al., 1998).
Dahi is consumed as such and is also used as a base for producing other products. Examples include lassi, srikhand, and ghee. Lassi is a liquid product that is manufactured by blending water and dahi and mixing to a uniform consistency. The ratio of dahi to water depends on the consistency desired. The product is lightly salted or sweetened.
Srikhand is a popular product that is manufactured at home and also commercially (Patel and Chakraborty, 1988). Fresh dahi is drained either with a cheesecloth overnight or with the help of a centrifuge. The drained curd is mixed with an equal proportion of sugar and enough cream to adjust the fat content to 5-6%. Additional flavorings such as fruits, nuts, and spices may be added. The final product has 40-45% moisture, 5-6% fat, 40-45% sugar, and a shelf life of at least 30-35 days at 10°C (Patel and Chakraborty, 1987). Postproduction acidification is restricted by the presence of a large amount of sugar, but spoilage occurs through growth of yeasts and mold and the presence of heat-stable proteo-lytic and lipolytic enzymes that cause undesirable flavors. The shelf life can be improved to almost 2 months by pasteurizing the product before packaging (Pra-japati et al., 1991). The use of nisin as a preservative has also been suggested (Sarkar et al., 1996b). It is also important to ensure that good-quality sugar is used, such as that which is hot-air treated to improve the microbial quality of srikhand (Patel and Chakraborty, 1987). Antibacterial effects of dahi described above also apply to srikhand (Sarkar et al., 1996a).
Ghee is clarified milk fat and has been used for cooking in India for thousands of years. Although it is not a fermented product, some procedures to manufacture ghee use dahi as a base. Dahi, when churned, is separated into a fat-rich product (butter) and buttermilk. Butter is then heated to 110-120°C, cooled, and filtered. When cooled, it has a granular texture. Much of the flavor of this product results from metabolites of the lactic fermentation during dahi manufacture.
A fermented product similar to dahi called misti doi is popular in eastern India. The manufacturing procedure is similar to that of dahi except that before boiling 6-6.5% sugar is added to milk. The intense heating concentrates milk and gives it a slight brownish color. Approximately 1% culture (previous day's product) is added and incubation occurs at approximately 40°C for 12-15 h. Thermophilic lactic organisms predominate. For example, in one study, 45% of total isolates were S. thermophilus, 35% were S. lactis, and 20% were Enterococ-cus faecalis (Sarkar et al., 1992). Although this product is commonly produced at home and in small-scale dairies, standardized commercial procedures for large-scale production have been developed.
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