丙烯誘導‘珍珠拔’番石榴果實香氣產生及對關鍵酵素活性之影響

Abstract

非更年型‘珍珠拔’(Psidium guajava L)採後乙烯生成量極低，果實香氣不若更年型品種濃郁，乙烯似乎扮演調控的關鍵角色。由於ʻ珍珠拔ʼ番石榴果實自身不會產生系統二乙烯之特性，可排除內生乙烯影響，適合作為探討乙烯調控番石榴果實香氣生合成的材料。本研究探討丙烯(乙烯類似物)處理對非更年型‘珍珠拔’誘導香氣圖譜、各香氣成分對乙烯的敏感性、乙烯對主要香氣生合成關鍵酵素之影響，並與更年型‘香拔’、‘中山月拔’後熟香氣圖譜比較。‘珍珠拔’番石榴經丙烯處理後共鑑定出13種揮發性化合物，包括酯類9種、醇類1種、醛類1種及萜烯類2種，其中3-hexenyl acetate、hexyl acetate、ethyl hexanoate和ethyl butanoate含量隨著丙烯處理濃度和處理時間增加而顯著增加，可能是‘珍珠拔’主要香氣成分。這4種主要酯類成分生成對丙烯具有高度依賴性，接近100%，顯示乙烯可能為番石榴果實香氣生合成主要調控因子。與更年型‘香拔’、‘中山月拔’後熟果實比較，經丙烯處理之‘珍珠拔’果實揮發性化合物雖然種類較少，但主要香氣成分含量及總揮發性化合物含量差異不顯著，另在總可溶性固形物及可滴定酸含量也無明顯差異，加工後風味不受影響，表明‘珍珠拔’催熟後具有作為加工原料之潛力，可作為調節鮮果產銷的方法。酯類是‘珍珠拔’番石榴果實經人工催熟後主要香氣組成成分，進一步測定酯類生合成關鍵酵素脂氧合酶(LOX)和醇醯基轉移酶(AAT)活性後，發現經丙烯處理之ʻ珍珠拔’與‘香拔’果實酯類化合物生成的時間早於AAT酵素活性上升，表明影響番石榴果實香氣生成的重要因素是AAT催化反應前驅物的供給，即由LOX催化產生的醛類，而不是AAT活性。同時番石榴果皮組織中有較高的LOX活性，且其對丙烯有高敏感性(1.219 μL·L-1)及依賴性(91.0%)，顯示果皮LOX活性可能是乙烯調控香氣生合成的關鍵因素。

The fruit of ‘Jen-Ju Bar’ (‘JJB’) guava is a non-climacteric produce, which emits less ethylene and aroma volatiles than those of climacteric cultivars. Ethylene appears to play a key regulatory role in the aroma production of the fruit. Since the System 2 ethylene production is almost abolished, ‘JJB’ provided a good material for exploring the aroma synthesis controlled by ethylene during guava ripening. This research investigated the effects of various treatments of propylene, an ethylene analogue, on compositions of the aroma volatiles as well as the enzymatic activities of the aroma-related enzymes in peel and pulp tissues of ‘JJB’, compared with ‘Shiang Bar’ (‘SB’) and ‘Chung Shan Yue Bar’ (‘CSYB’) which are climacteric guava fruits. Thirteen main aroma compounds were identified in ‘JJB’ fruit after propylene treatment, including nine esters, one alcohol, one aldehyde and two terpenoids. Among the volatiles found, 3-hexenyl acetate, hexyl acetate, ethyl hexanoate and ethyl butanoate showed significant increase in production with the increase of propylene dosage and duration, which may be the main aroma component of ‘JJB’. These four main esters production showed a high dependency for propylene (nearly 100%), suggesting that ethylene might be the major regulator of aroma synthesis in guava fruit. Compared with ‘SB’ and ‘CSYB’ ripening fruits, the volatiles type of ‘JJB’ fruit treated with propylene were less, but there was no significant difference in the content of the main aroma components and total volatile compounds as well as total soluble solids and titratable acidity. So the flavor will not be affected, indicating that ‘JJB’ has the potential to be a processing material. The main aroma components of ‘JJB’ guava fruit after artificial ripening were esters, then further study were about the key enzymes, lipoxygenase (LOX) and alcohol acyltransferase (AAT), for ester biosynthesis. The volatiles production in ‘JJB’ fruit treated with propylene and ‘SB’ fruit was earlier than the enhanced AAT activity, indicating that an important factor affecting the aroma production in guava fruit is the supply of precursor, that is, aldehydes produced by LOX catalysis, rather than AAT activity. At the same time, there was a higher LOX activity in peel tissue than pulp of guava, and it had a high sensitivity (1.219 μL·L-1) and dependency (91.0%) for propylene, suggesting that LOX activity in peel tissue may be important point for ethylene to regulate aroma production in guava fruits.