HTST pasteurization

Pasteurization may be carried out by batch- or continuous-flow processes. In the batch process, each particle of milk must be heated to at least 63°C and held continuously at this temperature for at least 30 min. In the continuous process, milk is heated to at least 72°C for at least 15 s ia what is known as high temperature—short time (HTST) pasteurization, the primary method used for fluid milk. For milk products having a fat content above that of milk or that contain added sweeteners, 66°C is requited for the batch process and 75°C for the HTST process. For either method, foUowiag pasteurization the product should be cooled quickly to <7.2° C. Time—temperature relationships have been estabHshed for other products including ice cream mix, which is heated to 78°C for 15 s, and eggnog, which must be pasteurized at 69°C for 30 min or 80°C for 25 s. 

Various arrangements and configurations are available for the HTST pasteurizer. For regeneration, the milk-to-milk regenerator is most common. A heat-transfer medium, usually water, provides a milk—water—milk system. Both sides may be closed (Fig. 5) or the raw milk supply may be open. 

Fig. 6. Homogenizer used as a timing pump for HTST pasteurization. Details of bypass, relief lines, equalizer, and check valves are not included (10).

High-temperature short time (HTST) pasteurization is used in a majority of plants in the United States. HTST pasteurization is conducted at temperatures > 72 °C and holding time > 15 s in the United States (FDA, 2009). Milk may also be pasteurized using ultrahigh temperature (UHT) pasteurization. 

In a preliminary study, Tomasula et al. (2009) simulated the fluid milk process to identify energy usage and GHGs associated with HTST pasteurization and the related unit operations, such as homogenization. Physical property data for milk and cream were provided to the simulator. Packaging was not included as part of the simulation. GHGs were... 

Compatible with protein-rich beverages (e.g., milk-based soy-based) treated with high-temperature short-time (HTST) pasteurization. However, may be issues with flocculation, thickening, or sedimentation due to Ca-protein interactions when subjected to ultra-high temperature (UHT) heat treatment... 

Butter, butter oil, ghee Creams various fat content (HTST pasteurized or UHT sterilized), coffee creams, wipping creams, dessert creams Cream cheeses... 

Acid phosphomonoesterase (EC 3.1.3.2). Milk contains an acid phosphatase which has a pH optimum at 4.0 and is very heat stable (LTLT pasteurization causes only 10-20% inactivation and 30 min at 88°C is required for full inactivation). Denaturation of acid phosphatase under UHT conditions follows first-order kinetics. When heated in milk at pH 6.7, the enzyme retains significant activity following HTST pasteurization but does not survive in-bottle sterilization or UHT treatment. The enzyme is not activated by Mg2+ (as is alkaline phosphatase), but it is slightly activated by Mn2+ and is very effectively inhibited by fluoride. The level of acid phosphatase activity in milk is only about 2% that of alkaline phosphatase activity reaches a sharp maximum 5-6 days post-partum, then decreases and remains at a low level to the end of lactation. 

The acid phosphatase activity in milk increases by a factor of 4-10 during mastitic infection three isoenzymes are then present, only one of which is indigenous milk acid phosphatase, the other two being of leucocyte origin these latter isoenzymes are more thermolabile and are inactivated by HTST pasteurization. 

The occurrence of a peroxidase, lactoperoxidase (LPO), in milk was recognized as early as 1881. It is one of the most heat-stable enzymes in milk its destruction was used as an index of flash pasteurization (now very rarely used) and is now used as an index of super-HTST pasteurization. 

Significance. Apart from its exploitation as an index of flash or super-HTST pasteurization, LPO is also technologically significant for a number of other reasons ... 

When milk is heated to a moderate temperature (e.g. 70°C x 15 min), the cryoglobulins are irreversibly denatured and hence the creaming of milk is impaired or prevented HTST pasteurization (72°C x 15 s) has little or no effect on creaming potential but slightly more severe conditions have an adverse effect (Figure 9.2). 

To increase the stability of milk products. Lipoprotein lipase is probably the most important in this regard as its activity leads to hydrolytic rancidity. It is extensively inactivated by HTST pasteurization but heating at 78°C x 10 s is required to prevent lipolysis. Plasmin activity is actually increased by HTST pasteurization due to inactivation of inhibitors of plasmin and/or of plasminogen activators. 

The activity of selected enzymes is used as indices of thermal treatments, e.g. alkaline phosphatase (HTST pasteurization), y-glutamyl transpeptidase (index of heating in the range 72-80°C) or lactoperoxidase (80-90°C). 

Although batch and HTST pasteurization produces a cooked flavor in milk by activation of whey protein sulfhydryl groups, it is not severe... 

HTST pasteurization cause similar losses of these vitamins, which are less than those caused by conventional sterilization and drum drying. Deaeration before heat processing stabilizes the water-soluble vitamins against heat-induced destruction and storage loss. 

It has been suggested that some form of heat treatment, either thermi-zation (Humbert et al., 1985 Matselis and Roussis, 1998) or HTST pasteurization (Mogensen and Jansen, 1986), of milk on arrival at the factory should be performed to minimise the incidence or reduce the severity of lipolysis problems. While such treatments have been shown to be effective, they increase the cost of processing and result in double heat treatment of milk, which is not permitted in some countries. Carbonation of raw milk (with 30 mM C02) has also been reported to reduce the growth of lipolytic psychrotrophs and also to reduce the level of FFAs in cheese made from the carbonated milk (McCarney et al., 1994). 

To maintain cloud stability in fruit juices, high-temperature-short-time (HTST) pasteurization is used to deactivate pectolytic enzymes. Pectin is a protective colloid that helps to keep insoluble particles in suspension. Cloudiness is required in commercial products to provide a desirable appearance. The destruction of the high levels of pectin-esterase during the production of tomato juice and puree is of vital importance. The pectinesterase will act quite rapidly once the tomato is broken. In the so-called hot-break method, the tomatoes are broken up at high temperature so that the pectic enzymes are destroyed instantaneously. 

Figure 5.5. Diagram of the NCFST pilot HTST pasteurization plant. Reprinted from [143]. Copyright 2001 with permission from Elsevier.

The process of HTST pasteurization (Figure 5.5) is described in detail in Section 5.3. The variables used here are four temperature measurements (°C) and two PID controller outputs (mA). The hot water temperature, preheater outlet temperature of raw product, holding tube inlet temperature of pasteurized product and holding tube outlet temperature of pasteurized product are the output variables of the process (variables 1-4, respectively). The input variables of the process are the PID controller output to the steam valve (variable 5) that regulates the holding tube inlet temperature of product and the PID controller output to preheater hot wa-... 

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