Standard Yogurt Processing

An overall scheme of yogurt processing is included in Fig. 5. Milk or standardized yogurt mixes are heat-treated to high temperatures well above minimum legal pasteurization. LTLT pasteurization at 85°C for 30 min or 88°C for 15 min or HTST pasteurization at 88-91 °C for 30 sec to 5 min are typically employed. The purpose is twofold. First, high temperatures destroy indigenous bacteria, particularly themodurics, which reduces competition and enables rapid growth of the culture bacteria. Second, high temperatures denature about 70-80% of the whey proteins. The denaturation not only improves culture nutrient utilization but also increases water-holding capacity and curd strength, minimizing syneresis.

Yogurt mix may be homogenized after preheating in an HTST system, or at the end of the LTLT or HTST pasteurization process. Although unhomogenized milk may be used for yogurt, most manufacturers homogenize yogurt mix in order to prevent creaming during the incubation period, to assure uniform distribution of the milk fat, and to stabilize the co-agulum against whey separation (26). Further, homogenization, followed by high temperatures, improves yogurt viscosity. Homogenization may be single-stage at 1800-2500 psi or two-stage at 2000/500-1500/500 psi. The air content of milk used to make yogurt should be minimized to maximize homogenization efficiency and minimize fouling during heat treatment. Air is typically added when MSNF content is modified by addition of milk powder. In such cases, the milk should be deaerated prior to yogurt making.

As the homogenized yogurt mix containing stabilizers and/or sweeteners is cooled to 38-43 °C, a 2-5% inoculum of an appropriate liquid culture is added. When concentrated

Table 4 A Standard Yogurt Formulation, Including Ingredient Functionality

Ingredient

Functionality

Skim milk

Whole milk

Fruit puree Sucrose

Nonfat dry milk

Whey protein concentrate Carrageenan

Agar

Yogurt cultures

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