Heating Characteristics of Milk Constituents in a Microwave Pasteurization System T. KUDRA, F.R. VAN DE VOORT, G.S.V. RAGHAVAN and H.S. RAMASWAMY ABSTRACT A continuous pasteurization system was designed using a domestic microwave oven and heating characteristics were studied on water, cream, and milk constituents. Parameterswere determined as func- tions of flow rate and power using water zz 75g/min (minimum flow at full power without boiling). Operation at less than 100% caused unacceptable temperatureoscillations. Cream, milks and solutions of the individual milk constituents (fat, protein and lactose), were also assessed in attempts to determine relative contributions of each. Milk heatedmore rapidly than water and protein was the major contributor to heating, with fat and lactose-effects being negligible. The micro- wave system could heat solutions to temperaturesneeded for HTST pasteurization and was controlled by adjusting flow rate. INTRODUCTION PASTEURIZATION is a conventional process applied to milk for destruction of pathogenic (vegetative) bacteria, yeastand fungi. As a time-temperature dependent process, equivalent microbialdestruction may be achieved by subjecting milk to 72°Cfor 15sor 127°C for 4 set (Hall andTroyt, 1968;Sale et al., 1970). Temperature sensitivity of mostvegetative mi- croorganisms is greater than enzyme denaturation, nutrient de- struction or loss of qualityfactors (Ramaswamy et al., 1990). Thus hightemperature-short timeprocesses favorbetter quality retention. A common problem encountered with pasteurization is foulingof theheat exchanger overtime due to high surface temperatures. Fouling reduces heat transfer andcauses flavor changes as fouled materialdecomposes. Microwave heating could provide a means to avoid foulingby eliminating thesteep temperature gradient of conventional pasteurization and heat a fluid directlyandefficiently. Other workers haveconsidered microwave energy for pas- teurization, although relatively little work hasbeen reported. Jeppson (1964) was first to reportapplication of microwaves to pasteurization, while Hamid et al. (1969) pasteurized milk in batchand continuous modes. The continuous system was devised using a glass tube placed obliquely across a waveguide and the pasteurization efficiency of batch andcontinuous sys- tems wererated at 99.99and92.7%, respectively. Stenstrtim (1972), patented an annular microwave heat exchanger and Jaynes (1975) developed anexperimental continuous flow mi- crowave pasteurization system using a 12cm/O.635 cm Teflon tube placed across a 2450 MHz waveguide. The lattersystem hada 15 set hold time, with a two stage regeneration system whichprewarmed and cooled theincoming and outgoing milk. Adequate pasteurization was indicated based on phosphotase test, standard plate and coliform counts.Unilever Research Laboratories developed a pressurized microwave chamber to pasteurize free fallingmilk, however, thedetails of thatprocess are proprietary (Sale, 1976). Microwave energy has also been usedfor nominalheat treatments (45-6O’C) of milk in the Author Kudra is Visiting Professor, Dept. of Chemical & Process Engineering, Lode Technical Univ., Lodz, Poland. Authors van de Voort end Remaswamy are with the Dept. of Food Science and Author Raghavan is with the Dept. of Agricultural Engi- neering, Macdonald College of McGill Univ., Ste. Anne de Belle- vue, PQ, Canada H9X ICO. Address inquiries to Dr. Ramaswamy. household to extend shelf life and dramatically lowerthepsy- chotroph load and standard plate count (Chiu et al., 1984; Chin et al., 1982). Merlin and Rosenthal (1984) indicated thatbatch microwave heating (i.e., 65PC for 30 min) might be more effective thanconventional heating, resulting in better micro- bial count reduction than equivalent conventional temperature- time treatments. To a large extent, microwave heating characteristics of milk and its components have not been considered and maywell be a variable in heating characteristics of theproduct, should the raw material vary in composition. Our objective was to de- scribe a simple laboratory scale microwave system for thecon- tinuous heating of biological fluids anddetermine the heating characteristics for milk andits constituents in solution. MATERIALS & METHODS A CONTINUOUS FLOW microwave heating system was designed using a 700 W, 2450 MHz Eaton-Imperial microwave oven for the cavity (Fig. 1). The sample was contained within a centrally located helical coil, 100 mm x 100 mm, of tygon tubing (12.5 mm o.d., 10 mm i.d. and 2.1 m long) within the cavity portion (165 mL capacity). A polyethylene frame was used to support the tubing with cavity inlet and outlet ports of Swagelock fittings screenedwith wire mesh to prevent microwave leakage. Inlet and outlet temperatures were mon- itored continuously using 30 AWG chromel-alumel thermocouples (Omega Engineering Co., Stanford, CT) positioned just outside the swagelock fittings. The original freely rotating mode stirrer was re- placed with a DC motor driven stirrer to obtain a more uniform power distribution within the microwave capacity. The distribution was op- timized using thermal paper stacked at various levels throughout the cavity. The normal air flow used to cool the magnetronwas redirected outside to avoid forced convection inside the cavity and reduce its influence on heat loss. The magentron power was verified and cali- brated using the water load technique. The solutions to be run through the microwave were tempered to 4°C in a refrigerated bulk tank to simulate cold milk. The solution was fed into the cavity by a calibrated variable speed metering pump (Smith and Nephew Watson Marlow, COLD A STORAGE rl Fig. 1 -Schematic diagram of continuous microwave fluid heat- ing system. Volume 56, No. 4, 1991-JOURNAL OF FOOD SCIENCE-937