熱物質移動連成有限要素解析によるキュウリ果実の貯蔵中における蒸散予測

概要

Water loss is one of the major postharvest quality losses in fresh horticultural produce. Significant water loss from harvested produce is occurred by transpiration. For highly perishable produce which contains a lot of moisture, such as cucumber, transpiration may bring them a severe economic loss since excessive water loss contributes to a significant reduction in saleable weight. The complicated interactions between climatic conditions and physiological responses in cucumber make the prediction of transpiration difficult. Since the finite element simulation is widely recognized as an effective tool for solving complex engineering problems, therefore, this dissertation demonstrates the prediction of transpirational water loss from harvested cucumber using the finite element simulation with various mathematical equations based on the simultaneous heat and mass transfer phenomena.

　To understand the fundamental knowledge of water movement in cucumber fruit, the changes in internal structure and moisture distribution during storage were investigated using X-ray CT technology. The CT images of cucumber successfully detected the different water loss rate between the endocarp and mesocarp layers and the difference became larger as the storage continues. To prove this observation, the different effective moisture diffusivities in endocarp and mesocarp tissues were calculated using a thin- layer drying model. The cubic-shaped endocarp and mesocarp tissues were prepared from fresh intact cucumbers and dried at various temperatures. During drying, the changes in wet weight of the cubes were measured. As a result, the initial moisture content of endocarp was statistically higher than that of mesocarp and the slope method based on the thin-layer drying model proved higher moisture diffusivity in mesocarp than that in endocarp.

　In order to predict the water loss by transpiration from fresh cucumber during storage, the transpirational water loss model of cucumber was developed using several mathematical equations based on the theory of simultaneous heat and mass transfer. The mathematical concept of transpiration in the plant was also employed for the model to determine the mass flux which refers to the transpiration rate. This model was designed for predicting the variations of temperature and moisture ratio in cucumber at certain climatic conditions during storage. The temperature and moisture data were obtained under various climatic condition storing in the environmental chambers. The finite element simulation named COMSOL carried out the prediction calculation. The water loss model showed an acceptable prediction performance for both temperature and moisture loss (MR) when a MR of less than 10% occurred in the cucumber fruits, and also the finite element simulation demonstrated the water movement similar as the real one. However, the accuracy decreased gradually as the water loss continues due to the behavior of transpiration rate reduction (TRR). To improve the accuracy of the MR prediction, various TRR models were developed and applied for the transpirational water loss model. As a result, the water loss model with TRR accurately estimated the variation of MR of larger than 10% but a considerable enhancement in temperature prediction was not resulted. Moreover, the water loss model with the effect of the TRR has successfully estimated the final state of MR in the cucumber fruits stored in the variable climatic conditions during cargo ship transportation. The statistical analysis demonstrated the MR predicted by the water loss models with TRR was not significantly different from the actual MR, however it was significantly different when using the model without TRR. Conclusively, the proposed water loss model could predict the variations of temperate and moisture ratio in cucumber fruit with acceptable performance, and this technique is positively considered as a useful and robust solution for designing the optimal storage or transportation plans to minimize the postharvest water loss in fresh horticultur al produce.