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Unformatted text preview: J. Dairy Sci. 88:41724182 American Dairy Science Association, 2005. Comparison of Sensory, Microbiological, and Biochemical Parameters of Microwave Versus Indirect UHT Fluid Skim Milk During Storage D. A. Clare, W. S. Bang, G. Cartwright, M. A. Drake, P. Coronel, and J. Simunovic Southeast Dairy Foods Research Center, Department of Food Science, North Carolina State University, Raleigh 27695-7624 ABSTRACT Shelf-stable milk could benefit from sensory quality improvement. Current methods of heating cause flavor and nutrient degradation through exposure to over- heated thermal exchange surfaces. Rapid heating with microwaves followed by sudden cooling could reduce or eliminate this problem. The objectives for this study were focused on designing and implementing continu- ous microwave thermal processing of skim fluid milks (white and chocolate) to compare sensory, microbiologi- cal, and biochemical parameters with conventionally prepared, indirect UHT milks. All test products were aseptically packaged and stored at ambient tempera- ture for 12 mo. Every 3 mo, samples were taken for microbiological testing, reactive sulfhydryl determina- tions, active enzyme analysis, instrumental viscosity readings, color measurements, and descriptive sensory evaluation. Microbiological plate counts were negative on all milks at each time point. Enzymatic assays showed that plasmin was inactivated by both heat treatments. 5,5 -Dithio-bis(2-nitrobenzoic acid) analy- sis, a measure of reactive sulfhydryl (-SH-) groups, showed that the initial thiol content was not signifi- cantly different between the microwave-processed and UHT-treated milks. However, both heating methods resulted in an increased thiol level compared with con- ventionally pasteurized milk samples due to the higher temperatures attained. Sulfhydryl oxidase, a milk en- zyme that catalyzes disulfide bond formation using a variety of protein substrates, retained activity following microwave processing, and decreased during storage. Viscosity values were essentially equivalent in micro- wave- and UHT-heated white skim milks. Sensory anal- yses established that UHT-treated milks were visibly darker, and exhibited higher caramelized and stale/ fatty flavors with increased astringency compared with the microwave samples. Sweet aromatic flavor and sweet taste decreased during storage in both UHT and Received May 13, 2005. Accepted August 2, 2005. Corresponding author: M. A. Drake; e-mail: mdrake@unity. ncsu.edu. 4172 microwave milk products, whereas stale/fatty flavors increased over time. Sensory effects were more appar- ent in white milks than in chocolate varieties. These studies suggest that microwave technology may provide a useful alternative processing method for delivery of aseptic milk products that retain a long shelf life....
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