9 Heat- induced changes in milk 9.1 Introduction In modern dairy technology, milk is almost always subjected to a heat treatment; typical examples are Thermization eg65C×15s Pasteurization LTLT (low temperature, long time) 63C X 30 min HTST(high temperature, short time) 72C X 15s Forewarning(for sterilization) eg90C×2-10min 120°C×2min Sterilization UHT (ultra-high temperatur 130-140°C×3-5s 110-115°C×1020min The objective of the heat treatment varies with the product being produced. Thermization is generally used to kill temperature-sensitive micro-organisms, e.g. psychrotrophs, and thereby reduce the microflora of milk for low-temperature storage. The primary objective of pasteurization is to kill pathogens but it also reduces the number of non-pathogenic micro-organisms which may cause spoilage, thereby standardizing the milk as a raw material for various products. Many indigenous enzymes, e.g lipase, are also inactivated, thus contributing to milk stability. Forewarming (preheating) increases the heat stability of milk for subsequent sterilization (as discussed in section 9.7. 1). Sterilization renders milk shelf-stable for very long periods, although gelation and flavour changes occur during storage, especially of UHT-sterilized milk Although milk is a very complex biological fluid containing comple protein, lipid, carbohydrate, salt, vitamins and enzyme systems in soluble, colloidal or emulsified states, it is a very heat-stable system, which allows it to be subjected to severe heat treatments with relatively minor changes in comparison to other foods if subjected to similar treatments. However, numerous biological, chemical and physico-chemical changes occur in milk during thermal processing which affect its nutritional, organoleptic and or technological properties. The temperature dependence of these changes9 Heat-induced changes in milk 9.1 Introduction In modern dairy technology, milk is almost always subjected to a heat treatment; typical examples are: Thermization Pasteurization e.g. 65°C x 15 s LTLT (low temperature, long time) 63°C x 30 min HTST (high temperature, short time) 72°C x 15 s Forewarming (for sterilization) e.g. 90°C x 2-10 min, Sterilization 120°C x 2 min UHT (ultra-high temperature) 130-140°C x 3-5s In-container 110-115°C x 10-20min The objective of the heat treatment varies with the product being produced. Thermization is generally used to kill temperature-sensitive micro-organisms, e.g. psychrotrophs, and thereby reduce the microflora of milk for low-temperature storage. The primary objective of pasteurization is to kill pathogens but it also reduces the number of non-pathogenic micro-organisms which may cause spoilage, thereby standardizing the milk as a raw material for various products. Many indigenous enzymes, e.g. lipase, are also inactivated, thus contributing to milk stability. Forewarming (preheating) increases the heat stability of milk for subsequent sterilization (as discussed in section 9.7.1). Sterilization renders milk shelf-stable for very long periods, although gelation and flavour changes occur during storage, especially of UHT-sterilized milks. Although milk is a very complex biological fluid containing complex protein, lipid, carbohydrate, salt, vitamins and enzyme systems in soluble, colloidal or emulsified states, it is a very heat-stable system, which allows it to be subjected to severe heat treatments with relatively minor changes in comparison to other foods if subjected to similar treatments. However, numerous biological, chemical and physico-chemical changes occur in milk during thermal processing which affect its nutritional, organoleptic and/or technological properties. The temperature dependence of these changes