facture (Tamime and Robinson, 1988). Texture of products also varies from liquid, such as for cultured buttermilk and liquid yogurt, to thick gel as for yogurt and sour cream. Some products such as viili from Scandinavia are characterized by their ropiness, which is intentionally induced by the use of cultures that produce exopolysaccharides to provide a thick body. Such cultures may also be used to manufacture low-fat yogurts to provide adequate body. Milk used for manufacturing fermented products is largely from the cow, but across the world milk of other species is also employed. In India, for example, the water buffalo is a common source (Aneja, 1997). Yogurt-like products in Iran are produced from milk of sheep or goats, and in some parts of Tibet milk of the yak is used (Kosikowski and Mistry, 1997). The type of milk affects endproduct characteristics partly via influence on growth of culture bacteria.
Thus, fermented milks encompass a wide range of products that possess diverse characteristics and employ a wide range of manufacturing procedures that are designed to promote optimal growth and activity of the chosen culture organisms.
The term yogurt (yoghurt) encompasses a wide range of products. Yogurt is a fermented dairy product, which is generally manufactured from pasteurized milk.
Its fat content ranges from 0 to over 4% depending on region and legislation. High-temperature pasteurization of the yogurt mix is employed to obtain a smooth and firm body. Nonfat dry milk or stabilizers may also be added to increase the water-holding capacity and therefore improve its body. The latter is particularly applicable to low-fat products.
Several different types of yogurt are commercially available. These include plain (no added flavors), flavored, liquid, carbonated, and low lactose. The flavored yogurts include the sundae-style in which fruit puree is layered at the bottom of the cup and is mixed with the yogurt before consumption. The other type is Swiss-style, in which plain yogurt is gently blended with fruit puree before packaging. Such yogurts require high levels of solids and stabilizer to obtain the desired high viscosity. Liquid yogurts are popular in Europe, Canada, and Japan, and differ from gel-type yogurt in that they are in a homogeneous, pourable state. No whey separation should occur during storage.
Manufacture of yogurt involves several key steps: standardization of mix, homogenization, heat treatment, cooling to incubation temperature, inoculation with yogurt cultures, incubation, cooling, and packaging (Rasic and Kurmann, 1978) (Fig. 2).
Many countries have their own standards of identity for yogurt with regard to composition as well as starter bacteria (Mareschi and Cueff, 1989). Most coun-
Preparation of mix: Standardization of fat and solids content via separation of fat, or addition of nonfat dry milk, or concentrated milk
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