On the Cooling Rates of Agricultural Products

Thermal Characteristics of Citrus Unshu as Related to Hydrocooling

Abstract

1. This report presents data on the fundamental and applied heat transfer characteristics of citrus Unshu being cooled with natural covected cooling water and air.
2. Solutions of cooling problems presented in terms of half-cooling times are based on the assumptions that the logarithm of the temperature reduction is linear and temperature reduction at every point in the substance occurs simultaneously at an equal rate.
But actually, there is a large time lag in temperature response at the fruit center. The presence of a time lag means that cooling rate data reported in terms of half-cooling times are not precise.
This report suggests that a slope of the cooling curve m or cooling coefficient Cc obtained by substituting (Z-τyes) for Z in equation 6 afford a describable criterion for evaluating cooling rate of substance.
3. The term effective thermal diffusivity α is a more precise characterization for whole citrus Unshu, consisting of components having dissimilar physical properties are not conforming to an expressible geometric configuration.
Differences in values of α were found to be statistically significant.
However, when considered in the sense of variation in actual cooling time, it is likely that the differences have a negligible significance in practice. Therefore, the average mean effective thermal diffusivity 0.00034 [m²/hr] within 0-15°C sample temperature, can be used in practice to predict expected cooling rate.
4. A chart (Fig. 14, 15) can be used to predict the time required to achieve a specified final temperature of fruit center having a given size and given initial temperature.
Computations for the chart were based on average effective thermal diffusivity of 0.00034 [m²/hr] and a water temperature of 0°C.
5. Also, Fig. 11, 12 (or equation 10-13) can be used to predict the time required to achieve a specified final temperature.