LD2668 .T4 1954 H85 / Master of Science

The deterioration of freshly harvested horticultural crops prior to storage can be minimized by rapid pre-cooling. This deterioration occurs for two reasons: (i) the respiration rate is typically high due to high ambient temperatures at harvest, and (ii) physiological activity of the produce is at peak levels. Thus, precooling is essential. / Many precooling techniques are used in the horticulture industry today. Their main purpose is to rapidly extract heat from the produce by using a suitable fluid for the heat extraction/rejection process. / The efficiency of a cooling method depends on many factors: the cooling fluid used, the morphological characteristics of the product, the type and shape of packing, the system design, etc.. It is therefore relatively difficult to compare the efficiencies of cooling systems or to evaluate the effect of each parameter on system efficiency. A new method for assessing cooling system efficiency was therefore developed. The technique was based on the capacity of the system to maintain the product temperature at the surface as low as possible without affecting the morphological characteristics of the product. It was shown that this method of assessing efficiency was effective under field conditions. / A low cost liquid-ice system was designed and tested for broccoli precooling. It is based on ice particle injection into a water stream, the mixture then being pumped into the box of produce to be precooled. The effects of different ice particle sizes and ice-water ratios on the medium temperature of broccoli, the mass of ice remaining in the boxes of produce and the icing efficiency were analyzed. The results led to the establishment of optimum conditions for the parameters of the proposed system.

This thesis focuses on diffusion channel design for broccoli storage, an important cash crop and consumer commodity in Canada. Experiments were first conducted to determine the respiration rate of broccoli in modified atmosphere conditions created by the produce metabolic activity inside closed chambers, at four different temperatures in the cold room of 3, 7, 13, and 24$ sp circ$C. The respiration rate decreased with a reduction of O$ sb2$ concentration inside the chamber. A model to predict the respiration rate of broccoli as a function of O$ sb2$ concentration at different temperatures was developed. Experiments were then carried out to determine the cross sectional area and length of diffusion channel leading to various final O$ sb2$ (steady-state) concentrations during the storage of broccoli. Combinations of cross sectional area of 0.04, 0.18, and 1.15 cm$ sp2$ and length of 0.6, 3, 7, 12, 18, and 25 cm were tested. The carbon dioxide produced through respiratory process was absorbed by placing hydrated lime inside the chambers such that the two-component molecular diffusion model by Ratti et al. (1993) could be evaluated. The length and cross sectional area of diffusion channel have a significant effect on the final level of O$ sb2$ (steady-state) concentration. The Ratti model then served as a basis for a modified model for predicting the length of diffusion channel required to obtain a given O$ sb2$ concentration as a function of the mass of stored broccoli and the cross sectional area of the diffusion channel. (Abstract shortened by UMI.)

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