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冷凍香蕉電阻之測定及其應用於冷凍幹燥
The Determination of Frozen Banana Resistance and Its Application in Freeze-Drying
摘要
設計一套可量測冷凍香蕉電阻與溫度變化關係之試驗裝置。電阻與溫度關係為電阻（對數座標）=（a+b×溫度+c×溫度^2）。a，b和c為常數。a介於1.586到1.780，b介於－0.039到-0.0190，c介於0.0100到0.0846；這些常數與香蕉品種，成熟度和樣本大小有關。 試驗結果顯示，香蕉共晶點約為-52℃，平均最初凍結點為-0.9℃，與ASAE Standards資料所載-0.8℃，略有差異，可能與成熟度，品種和測定方法有關。 以香蕉共晶點-52℃為參考點在真空壓力40 Pa，加熱板溫度20℃，冷凝溫度-50℃下，分別以預冷凍溫度-60℃、-52℃、-40℃、-30℃、-20℃，初含水率約75％（濕基），成熟度6級之香蕉進行冷凍乾燥試驗，乾燥到含水率為8到10％（濕基）。試驗結果顯示，預冷凍溫度-40℃（約98.6％水凍結）所得產品色澤最佳。預冷凍溫度-30℃（約97.9％水凍結）所得產品不甚穩定：預冷凍溫度-20℃（約96.6％水凍結）以上則不推薦使用。預冷凍溫度-60℃及-50℃（分別約99.3％、99.1％水凍結）所得產品太硬且中間部分會發生三角星形裂縫，故也不推薦使用。 在真空壓力40 Pa，冷凝溫度-50℃下，改變加熱板溫度為0℃，預冷凍溫度為-60℃；加熱板溫度30℃，預冷凍溫度-40℃；加熱板溫度40℃，預冷凍溫度-40℃；加熱板溫度50℃，預冷凍溫度-30℃，各作一次試驗，共四次實驗以供對照。加熱板溫度0℃的乾燥成品因供熱不足，冰凍無法完全昇華而發生溶化，產品有收縮現象，顏色泛黃；加熱板溫度30℃及40℃的乾燥成品顏色較加熱板溫度20℃的乾燥成品的顏色為黃，且加熱板溫度愈高成品顏色愈黃；加熱板溫度50℃的乾燥成品表面出現泡狀物，顏色泛黃，故不推薦使用。 香蕉冷凍乾燥重量與乾燥時間的關係為重量=EXP（a+b×時間），a和b為常數，與加熱板溫度、真空度及冷凝溫度有關。
並列摘要
An experimental appartus was designed to determine the relation between the frozen banana resistance and its temperature during freezing process. According to the experiment result, the relation between frozen banana resistance and the temperature could be expressed as resistance (log) = (a + b×temperature + c×temperature^2). a, b and c are constants which are somewhat varied from varieties、maturity and sample sizes of bananas. The experimental results showed that the eutectic point and the average initial freezing point of bananas was -52℃ and -0.9℃ respectively. Because of the difference between the average initial freezing point and -0.8℃ stated by ASAE Standards, the reason may be related to the maturity and species of bananas, and the method of measurement. By using the eutectic point -52℃ as a reference point, vacuum pressure 40 Pa, heat plate temperature 20℃, condensor temperature -50℃,different prefreezing temperatures were -60℃, -52℃, -40℃, -30℃ and -20℃ respectively, maturity at grade six and initial moisture content 75% (wet basis), the final moisture contents of freeze drying products were between 8 and 10% (wet basis). Under the prefreezing temperature -40℃ (about 98.6% frozen water), the color of the products was the best. Under the prefreezing temperature -30℃ (about 97.9% frozenwater), the product was not stable. Above the prefreezing temperature -20℃ (about 96.6% frozen water) was not recommended. The prefreezing temperatures -60℃ and -50℃(about 99.3% and 99.1% frozen water respectively), the products were too hard and with triangular stress cracks in the center, therefore, the above two temperatures were also not recommended. Under vacuum pressure 40 Pa, condensor temperature -50℃ and prefreezing temperatures -60℃, -30℃ and -40℃, four experiments were carried out by changing four heating plate temperatures 0℃, 30℃, 40℃ and 50℃. Obviously heating plate temperature 0℃ was too low to sublimate the ice which resulted in melting, to shrink the product and to make the color of the product yellow. Under heatingplate temperatures 30℃ and 40℃, colors of products had deeper yellow thanthat of heating temperature 20℃. The higher heating temperature would result in deeper yellow color. With heating temperature 50℃, the surface of product showed foam and deeper yellow, there fore, it was also not recommended. The sample weight during freeze drying could be expressed in terms of Weight =EXP (a + b×time). a and b are constants which may be effected by heating plate temperature, vacuum pressure and condensor temperature.