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 browning 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 biochemical reactions.28 First, rate constants for microbial destruction are usually much larger than those for the chemical reactions responsible for nutrient degradation, and second, rate constants for microbial destruction are usually more sensitive to temperature increases (z(thiamin) = 48, z(peroxidase) = 36.1, and z(C. botulinum) = 10°C).29 Methods for rapidly reaching the target temperatures therefore 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 associated with conventional retorting can activate protease to degrade myofibrillar proteins before the protease is eventually heat-inactivated.31 Tests for conventional 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 15mm 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 temperatures 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 unstable 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 autooxidation with dissolved oxygen in the food or cooking water. This reaction is catalysed 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 tunately, studies on food nutrients affected by ohmic heating are sparse in the literature. Ohmic heating is an effective method to pasteurise milk (220 V, 15kW AC, C electrodes-70 C for 15 seconds) and has been used successfully to produce quality viscous products and to foods containing various combinations of particulates such as meat, vegetables, pastas, or fruits in a viscous medium,33 including a wide variety of high acid (ratatouille, pasta sauce and vegetables, vegetables Provençale, fruit compote, strawberries, apple sauce, sliced kiwi fruit) and low acid (tortelline in tomato sauce, cappaletti in basil sauce, tagliatelle a la crème, beef bourguignonne, Beijing lamb, beef and vegetable stew, lamb Wala Gosht, vegetable curry, minestrone soup concentrate) food products. Sensory evaluations of ohmically heated food dishes such as carbonara sauce, California Beijing beef, winter soup, mushroom à la Greque, ratatouille, and cappaletti in tomato sauce produced good to very good ratings.34

Recent published data35 compared the application of conventional and ohmic heating on the kinetics of ascorbic acid degradation in pasteurised orange juice exposed to identical temperature-time profiles in each case. The reaction followed pseudo-first-order kinetics and the kinetics parameters obtained from the Arrhenius plot in each case are similar. The data also indicate that ascorbic acid degrades as a result of thermal treatment and that the electric field contributed no additional influence on the degradation of the vitamin. Yongsawatdigul and co-workers36,37 in their studies on gel functionality of Pacific whiting surimi found that ohmic heating can rapidly inactivate protease to avoid the enzymatic degradation of myofibrillar proteins, and hence increases the gel functionality of Pacific whiting surimi without the addition of enzyme inhibitors. Ohmic heating has been found to inactivate other enzymes.38,39 Enzyme inactivation should help prevent or reduce enzymatic degradation of nutrients. However, more studies are warranted.

There are several reports on relationships between ohmic heating and changes in properties of carbohydrates and fats. These studies did not directly address the nutrition issues of ohmically heated foods, although the physical changes that occur during ohmic heating affect the heating characteristics of the solids and liquids, which may have impact on thermal destruction of nutrients. Halden et al40 suggested that changes in starch transition, melting of fats and cell structure changes of the food material were responsible for changes in electrical conductivity that influenced the heating rate in foods such as potato during ohmic heating. In conventional thermal processes, starch gelatinisation was found to cause rheological and structural changes, and similar changes were observed for ohmic heating. Wang and Sastry41 indicated that ohmic heating caused significant changes in physical properties including viscosity, heat capacity, thermal and electrical conductivity. They found that conductivity decreased with degree of gelatinisation. When we design ohmic heating processes, we must take changes in electrical conductivity caused by physical property changes of major compounds such as starch, fats and proteins into account so that no significant under-cooking of solids or over-cooking occurs.

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