香菇中甲醛含量之研究

Abstract

香菇為東方常見含有豐富營養成分的食用菌之一，乾燥過之香菇具有濃厚的特殊香氣。香菇中特有香氣成分香菇精 (lenthionine) 為香菇酸經酵素催化生合成的環狀含硫化合物，而甲醛為此生合成過程所產生的副產物，文獻研究顯示香菇中天然甲醛含量可高達 600 mg/kg。本研究擬探討鮮香菇採後之預冷是否改變其香氣生成途徑所需 γ-麩胺基轉移酶 (GGT) 之活性，進而影響香菇中甲醛含量；另外亦觀察不同儲存及乾燥溫度對乾香菇中甲醛含量之影響。分別將香菇儲存於 5˚C 及 25˚C 下 7 天，其中香菇以 5˚C 儲存 7 天內甲醛含量沒有顯著差異；而以 25˚C 儲存，在第 5 天香菇中甲醛含量急遽下降，此現象可能與其腐壞有所關聯。第二部份熱風乾燥溫度試驗中，40˚C 乾燥處理後，殘餘甲醛含量百分比為最高，可能與此條件下香菇未完全脫水及酵素尚具有活性相關。第三部份預冷條件試驗分析探討預冷溫度對甲醛含量及 GGT 酵素活性之關聯，結果顯示，5˚C 預冷條件甲醛含量為最高，而其 GGT 酵素活性為最低，此現象可能與低溫預冷香菇水分散失較少及低溫存放能保有較佳品質相關。綜合以上結果顯示，低溫儲存 7 天內不影響香菇中甲醛含量，而以 40˚C 乾燥處理殘量甲醛為最高。另外，低溫預冷後甲醛含量與 GGT 酵素活性不完全相關。

Lentinula edodes, a common nutrient-rich edible mushroom in Asia, has special flavor after dehydration. Lenthionine is a cyclic organosulfur compound produced from lentinic acid through an enzymatic biosynthesis in L. edodes. Formaldehyde is a by-product produced from this pathway. Previous studies from the other authors have showed that the formaldehyde content in L. edodes could be as high as 600 mg/kg. This study investigated the activity of γ-glutamyl transferase (GGT) which is the important enzyme of the flavor synthesis pathway and the formaldehyde content of L. edodes after postharvest precooling. The formaldehyde contents in L. edodes at different storage and drying temperature were also examined. First, L. edodes was stored at 5˚C and 25˚C separately for 7 days. The formaldehyde contents of L. edodes stored at 5˚C did not have significant differences within 7 days. However, the formaldehyde content of the sample stored at 25˚C decreased rapidly at day 5. This was probably due to the decay of the L. edodes sample. In the second part, the residual formaldehyde content of L. edodes was found to be the highest after drying at 40˚C. This could be attributed to an incomplete dehydration and enzyme activity after drying at 40˚C procedure. In the third part, the formaldehyde content and GGT enzyme activity in L. edodes were investigated at different precooling temperatures. Our results showed that the formaldehyde content was the highest and the GGT activity was the lowest after precooling at 5˚C. This could be due to the lesser water loss and better quality conserve of L. edodes precooled at low temperature. In conclusion, storing under low temperature within 7 days would not affect the formaldehyde content of L. edodes. The highest formaldehyde content of L. edodes was found after dried at 40 ˚C. After precooling at low temperature, the formaldehyde content was not directly proportional to the GGT activity.