米糕類蒸製過程的熱傳研究

Abstract

蒸煮是食品工程中常見的加工手法，文獻中的蒸煮的對流熱傳係數值都相當高，但在實際觀察中並非如此。因此本研究目的在於利用蘿蔔糕當作媒介量測電鍋內蒸煮環境中的對流熱傳係數。蘿蔔糕質地細緻，對水分傳遞的阻力大，蒸煮過後蘿蔔糕的重量變化僅2.3 %，除此之外水分吸收對於熱傳影響也不顯著。這個現象能降低在於量測蒸煮環境中對流熱傳係數的複雜度。藉由量測蘿蔔糕的各食品物理參數，並藉著整體模式法以及熱通量法量測蒸煮環境中的對流熱傳係數，最後由商業軟體COMSOL Multiphysics模擬驗證對流熱傳係數的正確性。 在整體模式法中，利用銅片Bi數小於0.1的限制，推估出的對流熱傳係數約800~5800 W/m2 K；而熱通量法中，則利用PID控制致冷晶片維持恆溫並將熱通量片貼於其上進行量測，推估的對流熱傳係數值為50~480 W/m2 K。由模擬與實際蒸煮蘿蔔糕的結果顯示，整體模式法中所量測的值較符合實際對流熱傳係數值，熱通量法則是低估了實際對流熱傳係數值。 在澱粉糊化熱的驗證中，以70 %含水率的秈米漿來進行實驗。在DSC的數據中顯示秈米漿的糊化溫度範圍為65 ~ 90 ℃。藉由糊化溫度範圍內秈米漿比熱的上升模擬糊化焓值，並將其代入COMSOL Multiphysic軟體進行模擬。由蒸煮秈米漿實驗與模擬結果比較，其相對誤差僅為7.4 %，顯示糊化溫度範圍內比熱的上升能有效模擬蒸煮秈米漿時糊化熱的存在，同時也驗證了量測的物理參數。

Steaming is a common process in food processing, and many studies pointed out that the high convection heat transfer coefficient in steaming. However, in real processing, the steam quality had a significant effect on reducing the heating efficiency. Therefore, the purpose of this study was concentrated on estimating the convection heat transfer coefficient by steaming of rice cake. Mass transfer was less significant in steaming of rice cake for the mass difference was only 2.3 % after steaming process. In addition, the mass transfer of water barely affects the convection heat transfer. The phenomena reduced the complexity in estimating the convection heat transfer coefficient. In the study, COMSOL Multiphysics was applied to simulate the steaming of rice cake with the measured food properties as well as the convection heat transfer coefficient which was estimated by the lumped mass method and heat flux method. The estimated convection heat transfer coefficient was 800~5800 W/m2 K in lumped mass method; and was 50~480 W/m2 K in heat flux method. The results showed that the lumped mass method had the greater estimation in heat transfer coefficient than heat flux method by comparing the results between the simulation and experiment data. In the experiment of rice flours solution, the DSC result indicated that the gelatinization enthalpy appears at the range of 65 ~ 90 ℃. To simulate this phenomenon to predict the gelatinization enthalpy, different specific heat referred to DSC result was modified in COMSOL Multiphysics calculation. The model was successfully constructed with relative error 7.6 % between the experiment and simulation, so the increment of specific heat on predicting gelatinization enthalpy is valid.