413 19.4 The effect of ohmic heating on nutrient loss thermal destruction Since systematic research on ohmic heating has a much shorter history than has conventional heating, food scientists and technologists might look to microwave heating for information on nutrient changes. In general, many improvements in nutritional quality were found using microwaves(cooking in a minimum of water retained more K, vitamin B, and vitamin C, and the absence of surface brown- ing retained more amino acid availability, especially lysine), and microwave heating induces no significant effects different to those induced by conventional The benefit of attaining food safety with less nutrient degradation using HTST rocesses such as ohmic heating or microwave heating is based on differences in the kinetics parameters(k, Z, Ea) for bacterial spores compared to those for bio- chemical reactions. First, rate constants for microbial destruction are usually much larger than those for the chemical reactions responsible for nutrient degra- dation, and second, rate constants for microbial destruction are usually more sen- sitive to temperature increases(z(thiamin)=48, z(peroxidase)=36. 1, and z(c. botulinum)=10C).Methods for rapidly reaching the target temperatures there- fore tend to destroy microorganisms while giving less time to compromise the nutrient content and other quality attributes. .In fact, the slow heating rate asso- iated with conventional retorting can activate protease to degrade myofibrillar proteins before the protease is eventually heat-inactivated. Tests for conven- tional heating showed that heating large(25 mm) particulates in a liquid medium at 135 C to achieve Fo= 5 at the particulate center required extensive overpro- cessing of the liquid phase(Fo=150 for the liquid). For this reason, the common process conditions for scraped surface heat exchangers are maximum particulate sizes of 15 mm and sterilisation temperatures of 125-130oC(producing liquid Fo= 25) while limiting particulates to 30-40%o so that there is enough hot liquid available to heat particulates. For ohmic heating, direct heating sterilisation tem- peratures can reach 140C(the temperature limit of plastics in the machinery) without grossly overheating the liquid phase and can support greater particulate loading suspended in highly viscous carrier liquids. For comparative purposes, conventional heating at 130C to produce a lethality of Fo=8 produced a cook alue Co(based on thiamin degradation) of Co =8, whereas ohmic heating at 140C produced Fo= 24 and Co= 4 Vitamin losses in foods are determined by the temperature and the moisture and destroyed at relatively low temperature, articularly temperature sensitive of the applied heating method. Vitamin C is par o heating foods must be for as short a time as possible to retain the vitamin C. Thiamin and riboflavin are un stable at higher temperatures such as those used in rapid grilling. Vitamin C is also water soluble and can be lost when cooking with moist heat or by autooxi dation with dissolved oxygen in the food or cooking water. This reaction is catal- ysed by adventitious iron and copper ions. By comparison, thiamin is the most water soluble vitamin and vitamin a and vitamin d are water insoluble Unfor19.4 The effect of ohmic heating on nutrient loss: thermal destruction Since systematic research on ohmic heating has a much shorter history than has conventional heating, food scientists and technologists might look to microwave heating for information on nutrient changes. In general, many improvements in nutritional quality were found using microwaves (cooking in a minimum of water retained more K, vitamin B12, and vitamin C, and the absence of surface brown￾ing retained more amino acid availability, especially lysine), and microwave heating induces no significant effects different to those induced by conventional heating.11 The benefit of attaining food safety with less nutrient degradation using HTST processes such as ohmic heating or microwave heating is based on differences in the kinetics parameters (k, z, Ea) for bacterial spores compared to those for bio￾chemical reactions.28 First, rate constants for microbial destruction are usually much larger than those for the chemical reactions responsible for nutrient degra￾dation, and second, rate constants for microbial destruction are usually more sen￾sitive to temperature increases (z(thiamin) = 48, z(peroxidase) = 36.1, and z(C. botulinum) = 10°C).29 Methods for rapidly reaching the target temperatures there￾fore tend to destroy microorganisms while giving less time to compromise the nutrient content and other quality attributes.26,30 In fact, the slow heating rate asso￾ciated with conventional retorting can activate protease to degrade myofibrillar proteins before the protease is eventually heat-inactivated.31 Tests for conven￾tional heating showed9 that heating large (25 mm) particulates in a liquid medium at 135°C to achieve Fo = 5 at the particulate center required extensive overpro￾cessing of the liquid phase (Fo = 150 for the liquid). For this reason, the common process conditions for scraped surface heat exchangers are maximum particulate sizes of 15 mm and sterilisation temperatures of 125–130°C (producing liquid Fo = 25) while limiting particulates to 30–40% so that there is enough hot liquid available to heat particulates. For ohmic heating, direct heating sterilisation tem￾peratures can reach 140°C (the temperature limit of plastics in the machinery) without grossly overheating the liquid phase and can support greater particulate loading suspended in highly viscous carrier liquids. For comparative purposes, conventional heating at 130°C to produce a lethality of Fo = 8 produced a cook value Co (based on thiamin degradation) of Co = 8, whereas ohmic heating at 140°C produced Fo = 24 and Co = 4. Vitamin losses in foods are determined by the temperature and the moisture of the applied heating method. Vitamin C is particularly temperature sensitive and destroyed at relatively low temperatures,32 so heating foods must be for as short a time as possible to retain the vitamin C. Thiamin and riboflavin are un￾stable at higher temperatures such as those used in rapid grilling.3 Vitamin C is also water soluble and can be lost when cooking with moist heat or by autooxi￾dation with dissolved oxygen in the food or cooking water. This reaction is catal￾ysed by adventitious iron and copper ions. By comparison, thiamin is the most water soluble vitamin, and vitamin A and vitamin D are water insoluble. Unfor￾Ohmic heating 413