1976 JOURNAL OF FOOD SCIENCE—Vol. 68, Nr. 6, 2003 Food Engineering and Physical Properties © 2003 Institute of Food Technologists Further reproduction prohibited without permission JFS: Food Engineering and Physical Properties Temperature Profiles Within Milk after Heating in a Continuous-flow Tubular Microwave System Operating at 915 MHz P. CORONEL, J. SIMUNOVIC, AND K.P. SANDEEP ABSTRACT: Milk with different fat contents (0, 1, 2, 4%, and chocolate milk) were heated in a specially designed continuous-flow microwave applicator operating at 915 MHz. The nominal power was 5 kW and the flow rates were 2.0 and 3.0 L/min to attain laminar flow. Temperature profiles at the exit of the applicator were measured. The results showed that the average increases in temperature were similar to one another, being 42°C at 2.0 L/ min and 29 °C at 3.0 L/min. Differences between the lowest and highest temperatures were 3.7 and 3.0 °C, respectively. The temperature profiles illustrated that slightly higher temperatures were achieved within seg- ments flowing close to the center of the tube. Keywords: continuous microwave heating, milk, temperature profile, dielectric properties Introduction C ONTINUOUS-FLOW MICROWAVE HEATING is a relatively new technology in the food industry, even though applications of microwave heating technology are highly evolved for household applications. Micro- waves are a part of the electromagnetic spectrum and have a frequency between 300 MHz and 3 GHz. They are primarily used for communications and radar. It has been long known that microwaves can be used for heating foods, and small batch mi- crowave oven units for household use are present in most kitchens in the U.S.A. Mi- crowaves heat foods in a rapid and direct manner, providing volumetric heating. In industrial applications, this could drastical- ly reduce the come-up time needed to achieve the required process temperature, thereby reducing the total cumulative ther- mal treatment and better preserving the thermo-labile constituents of the foods, such as aromas, vitamins, and pigments. The characteristic of instantaneous power- on and shut-off of microwave generators can help deliver energy very precisely into the food products (Metaxas and Meredith 1983; Meredith 1998). Due to the volumetric heating character- istic of microwaves, that is, heat is generated in the food as a result of the conversion of the electromagnetic energy, the cumulative heat treatment of the bulk of a product should be more uniform than in the case of conven- tional processing. In theory, volumetric heat- ing should minimize overcooking of the sur- face and undercooking of the center. This can be extremely beneficial for liquids, such as milk, in which fouling of the tubes occurs due to overheating of the product in contact with the tube wall (Kudra and others 1991). Continuous-flow microwave heating of milk could be used as an alternative to tubu- lar and plate heat exchangers in pasteuriza- tion. Milk heats faster than water when ex- posed to microwaves, with the proteins and ions present in milk being the major contrib- utors to the heating effect. The fat and sug- ars present in milk have a less significant ef- fect. Thus, milk with different fat contents can be heated with equal efficiency (Kudra and others 1991). The effects of thermal treatment of milk in a continuous microwave system was studied by Lopez-Fandiño and others (1996) by studying the denaturation of -lactoglobulin and the inactivation of al- kaline phosphatase and lactoperoxidase us- ing a modified 2450 MHz microwave oven. The results obtained were compared with those obtained by heat treatment in a plate heat exchanger, and it was found that the degree of inactivation caused by the heat treatment in both cases was similar. Villamiel and others (1996a, 1996b, 1998) showed that microwave pasteurization of milk produced lower levels of denaturation of whey proteins than conventional thermal processes and that the denaturation of -lactoglobulin was comparable between both processes. In ad- dition, it also resulted in lower microbial counts and lower lactose isomerization. Vale- ro and others (2000) studied the effect of microwave treatment of milk on the sensori- al characteristics during storage. The results showed that the sensorial characteristics of milk treated by microwaves were compara- ble to those achieved by traditional heat treatments after 15 d of storage. Research on heat treatment processes using microwaves has provided valuable insight on the dynamics of microwave heat- ing coupled with the thermal and dielectric properties of foods and variation in the en- ergy absorbed as a result of variations in properties of the processed materials (Dat- ta and Anatheswaran 2000; Zhang and Dat- ta 2000a, 2000b; Lau and others 1998). One of the limitations of the implemen- tation of microwave heating systems on an industrial scale has been the nonuniformity of temperature distribution caused by mi- crowave heating (Mudgett 1986). To ad- dress the issue of uniformity of heating of food products flowing through a cylindrical enclosure, Industrial Microwave Systems Inc. (Morrisville, N.C., U.S.A.) designed a cylindrical microwave applicator. The micro- wave field inside the applicator was mod- eled using a mathematical simulation, run by the manufacturer, and predicted a para- bolic field distribution inside the tube for a fluid with constant dielectric properties, like those of milk at 25 °C. This configuration was designed to focus the electrical field so that the material at the center would receive the highest amount of energy. This design was implemented to achieve a nearly parabolic power distribution with the maximum pow- er at the center of the tube. For a fluid with a parabolic velocity profile (fully developed laminar flow of a Newtonian fluid), the par- abolic energy field coupled with a parabol- ic velocity profile should result in every par- ticle of the fluid receiving a nearly identical