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M. UZZAN and T. P. Labuza. Dept. of Food Science & Nutrition, Univ. of Minnesota, 1334 Eckles Ave., 225 Food Science & Nutrition Bldg., Saint Paul, MN 55108-6099 Heating of liquid- food and pharmaceutical products for pasteurization or sterilization is done almost exclusively by heat exchangers using countercurrent flow, and very frequently a plate-and-frame type is applied. For product safety considerations, the thermal history of the product in the holding-tube is only used for calculations, while the come-up time and cooling stage are considered as surplus. This approach may lead to excessive thermal abuse to the product and is partially due to the inability of measuring accurate temperature profiles along the flow path. Evaluations of temperature profiles in heat exchangers have usually been achieved by numerical solutions of linear equation systems. However, this type of solution is unlikely to be used by the food industry because of the mathematical complexity and the substantial programming skills required. Our objectives were to derive and validate an analytic solution for temperature profiles within a plate-and-frame heat exchanger in countercurrent single-pass flow configuration that could be solved using a spreadsheet allowing easy evaluation of reaction rates. We applied a novel and simplified approach, based on application of the ²heat exchanger effectiveness² concept in a partial heat exchanger, to derive the analytic solution. An interactive ExcelTM workbook was developed and used in a simulated case-study to demonstrate the influence of operating conditions on the temperature profile and thermal history of a pasteurized liquid product. Validation was performed with hot versus cold water flowing in an APV Junior heat exchanger equipped with 0.25 mm diameter thermocouples connected to a data logger. The experimental data correlated well with the simulated temperature profiles, and reaction rate calculations demonstrated the importance of the temperature profiles at heating or cooling periods in a pasteurization process. The current solution can be used as a powerful food engineering tool for safe and efficient thermal processing of liquid products.
Session 111, Food Engineering: Modeling heat transfer and microbial inactivation
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