Milk treatment

Yogurt can be made from skim (non-fat), reduced fat, or whole milk.As is true for all dairy products, but especially so for yogurt and other cultured milks, it is important to use good quality milk, free of antibiotics and other inhibitory substances. The first step (Figure 42) involves adding nonfat dry milk to the milk to increase the total solids to 12% to 13%, sometimes to as high as 15%.Alternatively, the total milk solids can be increased by concentrating the milk via evaporation. Other permitted ingredients (see below) may be added, and the mix (only if it contains fat) is then usually homogenized, although there are some specialty manufacturers that produce unhomoge-nized, cream-on-the-top whole milk versions.

Yogurt is considered a fluid milk product by the U.S. Food and Drug Administration (FDA) and must be made using pasteurized milk. However, most yogurt mixes receive a heat treatment well above that required for pasteurization. Thus, instead of pasteurizing milk for 71.7°C for 15 seconds (the minimum required), mixes are heated to between 85°C and 88°C for up to 30 minutes. Other time-temperature conditions can also be used, but kinetically they are usually equivalent. Heating can be done in batch mode (i.e., in vats), but continuous heating in plate or tube type heat exchangers is far more common.

The high temperature treatment not only satisfies all of the normal reasons for pasteurization (i.e., killing pathogens and spoilage organisms and inactivating enzymes), but these severe heating conditions also perform two additional functions. First, even heat-resistant bacteria and their spores are killed, making the mixture essentially free of competing microorganisms. Second, the major whey proteins, a-lactalbumin and p-lactoglobulin, are nearly 100% denatured at the high pasteurization temperatures.These proteins exist in globular form in their native state but once denatured, amino acid residues are exposed and their ability to bind water, via hydrogen bonding, is significantly enhanced. Denatured whey proteins also reduce the Eh and stabilize the milk gel.

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