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ABSTRACT
During the past few decades, food industry has explored various novel thermal and non-thermal processing technologies to minimize the associated high-quality loss involved in conventional thermal processing. Among these are the novel agitation systems that permit forced convention in canned particulate fluids to improve heat transfer, reduce process time, and minimize heat damage to processed products. These include traditional rotary agitation systems involving end-over-end, axial, or biaxial rotation of cans and the more recent reciprocating (lateral) agitation. The invention of thermal processing systems with induced container agitation has made heat transfer studies more difficult due to problems in tracking the particle temperatures due to their dynamic motion during processing and complexities resulting from the effects of forced convection currents within the container. This has prompted active research on modeling and characterization of heat transfer phenomena in such systems. This review brings to perspective, the current status on thermal processing of particulate foods, within the constraints of lethality requirements from safety view point, and discusses available techniques of data collection, heat transfer coefficient evaluation, and the critical processing parameters that affect these heat transfer coefficients, especially under agitation processing conditions.
Funding
This research was partially supported by funds from the Natural Sciences and Engineering Research Council (NSERC) of Canada.
Appendix A: List of symbols
A.1. Nomenclature
| α | = | thermal diffusivity, m2/s |
| a | = | radius of sphere, m |
| A | = | total external surface area, m2 |
| Bi | = | Biot number |
| cp | = | specific heat capacity, J/kg °C |
| cpf | = | specific heat capacity of liquid, J/kg °C |
| cpp | = | specific heat capacity of particle, J/kg °C |
| D | = | negative reciprocal of slope of the thermal destruction curve (D-value), minutes |
| fh | = | heating rate index, minutes |
| F | = | lethality, minutes |
| hfp | = | fluid to particle heat transfer coefficient, W/m2 °C |
| k | = | thermal conductivity, W/m °C |
| m | = | mass, kg |
| N | = | population density |
| ρ | = | density, kg m−3 |
| r | = | radius, m |
| t | = | time, seconds |
| τ | = | Fourier number |
| T | = | temperature, °C |
| U | = | overall heat transfer coefficient, W/m2 °C |
| z | = | temperature sensitivity (z-value), °C |
Subscripts
| c | = | can |
| o | = | reference |
| i | = | initial condition |
| f | = | fluid |
| p | = | particle |
| ps | = | particle surface |
| R | = | retort |
| s | = | surface |
| m | = | model |
| TTI | = | time-temperature integrator |