Development of a quantitative visualization method to characterize the flow behavior of food particulates in a continuous aseptic sterilizer.

摘要

Presently, conventional continuous pasteurization systems which were designed primarily for liquids and semi-liquids do not satisfy either product quality specifications or safety requirements of U.S. regulatory agencies when applied to food containing particulates (Marcotte et al., 1994; Simunovic et al., 1995). One potential solution to the above problem was suggested in a recently issued patent by this author entitled 'Hydrostatic Heating Apparatus' (Shefet, 1996).;The objectives of this study were to develop a quantitative visualization tool for evaluating flow behavior of particles in a model hydrostatic heating apparatus or other flow systems and to estimate process paramenters (Reynolds numbers, Nusselt numbers, Biot numbers, heat transfer coefficients, holding tube length and process times) of a simulated aseptic process for potato spheres, based on the quantitative data collected during the study.;The three dimensional movement of polystyrene balls as influenced by ball diameter (0.95 and 1.9 cm), flow rate (10, 20 and 30 l/min) and conveyor disk design (2 configurations) were recorded in the model heating apparatus and analyzed using motion analysis software. Ball speed and net-to-gross-displacement (NGDR) ratio values were calculated for ball movement in the x;y and x;z planes. The NGDR is computed for specific particles moving on a given path. For a given point in a path, the net displacement is the distance along a straight line from the first point of the path to the given point. In contrast, the gross displacement is the distance along the actual path from the first point in the path to the given point. The ratio between these two quantities is termed the net to gross displacement ratio (Motion Analysis Corporation, 1990). As carrier liquid flow rate increased, there was an associated increase in both the mean and standard deviation speed and NGDR values. In general, larger ball sizes yielded lower speed and NGDR values (i.e., less movement). A concave (bowl-like) conveyor disk design as opposed to a 90° flat-edge disk design yielded greater speed and NGDR values when carrier velocity was 30 l/min. Speed and NGDR values having higher standard deviations were interpreted as having greater turbulent flow. Furthermore, speed and NGDR mean and standard deviations were highly correlated (r2 > 0.9) indicating that either statistic could be used to describe the flow behavior of particles. In addition, a high correlation (r2 > 0.9) was observed between measurements (speed, NGDR) taken in both planes (x;y, x;z). The average speed derived from the quantitative visualization method was subsequently used to calculate heat transfer and related properties in the model hydrostatic heating apparatus.;Based on the successful assessment of the quantitative flow visualization tool in this study, it is anticipated that this method may be useful for comparing flow characteristics of particles in other food conveying systems (i.e. continuous aseptic pasteurizers). The fact that the movement of a particle in a given system can be documented and characterized suggests that similar comparisons of particle movements can be achieved in other systems or factors influencing flow can be readily evaluated. Furthermore, this method will allow process engineers to make recommendations on specifications (i.e. conveyor disk design, liquid velocity, particulate load, etc.) of future designs of the hydrostatic heating apparatus or any other system designed for conveying particulates.