ANALYSIS OF TRANSIENT CONDUCTION HEAT TRANSFER IN THE THERMAL PROCESSING OF FOODS USING THE FINITE ELEMENT METHOD

Transient heat transfer in conduction heating products packed in hermetically sealed containers undergoing heat sterilization was analyzed theoretically using a variational finite element approach, and experimentally using thermocouples. The problem areas analyzed and major findings were: (1) Overshooting of temperatures after steamoff: the continued temperature rise after steamoff at the slowest heating zone (SHZ) of conduction heating products was calculated numerically and measured experimentally for a range of container sizes, product thermal diffusivities and cooling medium conditions. The effect on process Fo of the overshooting is significant and results in gross process overdesign when the Formula method is used. Rho (the ratio of Fo(heat) to Fo(heat and cool) was expressed as a function of g only for given values of m+g, z and Biot number. Numerically generated values of Rho agreed with laboratory measurements for conduction heating products in large cans and jars. A simple mathematical method for process design and evaluation that includes the contribution of overshooting to process lethality was developed. (2) Heating rates in glass containers: heating rates obtained from laboratory measurements and from results of computer simulations of a finite element formulated jar, were in good agreement. A semiempirical method to convert heating parameters (fh) for different size jars when product thermal properties are unknown was developed. (3) Fluctuating medium temperatures during sterilization: The effect on sterilization value and nutritional losses of normal retort cycling was found to be unimportant. To adjust process design for severe cycling conditions, a method employing an effective temperature approach and dimensionless response charts that relate cycling medium temperatures to the harmonious response at the SHZ was developed. (4) Heating rates in squat containers processed with an end flat against the retort bottom: under these conditions gross underprocessing may occur. Heating curves are broken and the slowest heating zone moves towards the container bottom during heating. A numerical model was developed that enables safe process design for this condition. (5) Effect of a change in fh on process Fo: Numerically calculated charts relating (DELTA)u/u/(DELTA)fh/fh to g enable rapid evaluation of the effect of an error in fh on the change in process Fo.