廣東桑（Morus atropurpurea Roxb.）果實採後生理及處 理技術之研究

Abstract

桑椹（mulberry fruit）為台灣新興果品，其採後易發生腐損，不耐貯運，櫥架壽命極短，造成農民損失與產業發展限制。目前國內外有關桑椹採後生理與相關處理技術的研究文獻甚少，因此本研究之目的為調查桑椹採後生理與探討降低桑椹採後敗壞之處理技術。 廣東桑（Morus atropurpurea Roxb.）果實最佳採收成熟度為完全轉色（紫黑色）的後熟果，此成熟度具有最高的糖酸比（7.75 ± 0.89）與花青素含量（1244.18 ± 43.64 mg•L-1 cyanidin-3-glucoside equivalent）。‘苗栗一號’與‘46C019’桑椹同屬廣東桑群，完全轉色桑椹採後呼吸率與乙烯生成量之變化，呈現典型的非更年性果實型式，無呼吸高峰與乙烯高峰的產生。其於5℃中之呼吸率為19.14∼21.62 mg CO2∙kg-1∙hr-1，20℃中之乙烯生成量為4.90~6.85 μL C2H4∙kg-1∙hr-1，所以歸類為高呼吸率與中等乙烯生成量之園產品。貯藏溫度對桑椹呼吸率與乙烯生成量有很大的影響，由0℃到20℃，呼吸率增加約18倍，乙烯生成量增加約6倍。 採後腐爛是影響桑椹劣變的主要原因，貯藏溫度對桑椹腐爛率有顯著影響，於20℃中的貯藏壽命僅有1~2天，腐爛率超過20%，在0℃中則可延長至10~12天；此外，0~5℃也可減輕果實的失重率。桑椹於低溫包裝貯藏期間，其可溶性固形物含量與可滴定酸含量變化不顯著，而花青素含量則呈現逐漸降解的趨勢，低溫貯藏8天後花青素含量僅剩貯藏前的一半。 ‘苗栗一號’桑椹採後立即進行壓差預冷可降低腐爛率與嚴重度，延長櫥架壽命，並可維持較高的可滴定酸度；但對失重率、糖度、pH值與花青素含量皆無顯著影響。高二氧化碳與低氧之氣調貯藏（22% CO2 + 6% O2或11% CO2 + 5% O2）對桑椹於5℃貯藏之腐爛率與嚴重度有顯著抑制效果，以22% CO2 + 6% O2處理組效果最佳，於貯藏一週內幾乎無腐爛發生，並可維持較高的總可溶性固形物含量，但是卻會造成桑椹花青素含量大幅下降，而對果實失重率與可滴定酸含量則無顯著影響。 貯藏前以2.40 kJ∙m-2短波紫外線（UV-C）照射‘苗栗一號’桑椹，可以降低在5℃中貯藏之腐爛率，並可提升抗菌物質trans-resveratrol含量，但是對降低病徵嚴重度並無顯著效果。桑椹經2.40 kJ∙m-2 UV-C照射，其trans-resveratrol含量於5℃貯藏4天後增加到2.83 μg∙g-1 dw，明顯高於對照組（2.21 μg∙g-1 dw），但隨後便開始下降，果實也開始腐爛；UV-C照射對另一抗菌物質trans-piceid含量則無明顯提升效果。因此，桑椹抗病力增加可能與抗菌物質含量受到UV-C誘導提升有關。UV-C照射處理效果受到照射劑量與照射後貯放溫度的影響，較低劑量（0.48 kJ∙m-2）與較高貯藏溫度（10℃）皆無法明顯抑制果腐，且UV-C處理的保鮮效果並不持久。

Mulberry fruit (Morus spp.) a newly raised fruit crop in Taiwan. It is so perishable that there is serious loss after harvest, and it is a strong limitation for the expansion of mulberry industry. Researches in the postharvest physiology and technology of mulberry are limited; therefore, the objective of this thesis is to establish the basic postharvest physiology data as well as to assess various handling technologies for extending shelf-life of fresh mulberry fruits. Mulberry fruits (Morus atropurpurea Roxb.) should be harvested at fully mature (dark-purple color) stage to achieve maximum quality; since full ripe fruits had the highest values of total soluble solids to titratable acidity ratio (7.75 ± 0.89) and anthocyanin content (1244.18 ± 43.64 mg•L-1 cyanidin-3-glucoside equivalent). The patterns of respiration rate and ethylene production of ‘Miaoli No.1’ and ‘46C019’ mulberry fruits, harvested at fully mature (dark-purple) stage exhibited typical characteristics of nonclimacteric fruits. The respiration rate at 5℃ was 19.14~21.62 mg CO2∙kg-1∙hr-1 and the ethylene production rate at 20℃ was 4.90~6.85 μL C2H4∙kg-1∙hr-1, therefore, mulberry fruit can be categorized as high respiration rate and moderate ethylene production rate. Temperature was the most important environmental factor influencing the respiration and ethylene biosynthesis in mulberry, the rates of respiration and ethylene production were enhanced by 18 and 6 folds, respectively, when temperature was raised from 0℃ to 20℃. Decay was the major cause of postharvest loss in mulberry fruits. The shelf-life of ‘Miaoli No. 1’ mulberry was only 1~2 days at 20℃; however, it was extended to 10~12 days when the fruits were stored at 0℃ due to significant suppression of pathological decay. Furthermore, lowering the storage temperature to 0~5℃ also reduced the percentage of weight loss in mulberry. There were no changes in total soluble solids and titratable acidity of mulberry fruits stored at 0℃ or 5℃. On the other hand, the anthocyanin content of mulberry gradually declined to 50% after 8 days storage at 0℃or 5℃. ‘Miaoli No. 1’ mulberry fruits cooled by forced-air cooling immediately after harvest reduced the decay incidence, decay severity index, and prolonged shelf-life. Precooled fruits maintained higher titratable acidity during storage at 5℃, but there were no significant benefits on preservation of fresh weight, total soluble solids, pH, and anthocyanin content of the product. Storing ‘Miaoli No. 1’ mulberry fruits in controlled atmosphere with 22% CO2 + 6% O2 or 11% CO2 + 5% O2 at 5℃ also effectively restrained decay percentage and decay severity index. Fruits treated with 22% CO2 + 6% O2 at 5℃ for a week show no visible decay and maintained better total soluble solids maintenance when compared with the air control fruits. However, high CO2 concentration in controlled atmosphere storage enhanced the degradation of anthocyanin of mulberry fruits during storage at 5℃, but affected neither weight loss nor titratable acidity of the fruits. Pre-storage irradiation ‘Miaoli No. 1’ mulberry fruits with 2.40 kJ∙m-2 ultraviolet-C (UV-C) induced the accumulation of the phytoalexin trans-resveratrol and reduced the percentage of decay, but not the decay severity index during storage at 5℃. The trans-resveratrol content of UV-C irradiated mulberry fruits rose to 2.83 μg.g-1dw after 4 days storage at 5℃, obviously higher than that of control (2.21μg.g-1 dw), and then declined. The content of trans-piceid, another phytoalexin in mulberry, was not induced by UV-C irradiation. Therefore, the accumulation of trans-resveratrol may be one of the factors that inhibit postharvest decay in ‘Miaoli No. 1’ mulberry fruits treated with UV-C. The efficacy of UV-C irradiation on mulberry was affected by the storage temperature and irradiation dose. Lower UV-C irradiation dosage (0.48 kJ∙m-2) and higher storage temperature (10℃) did not significant inhibit decay of mulberry fruits. In addition, the effect of UV treatment did not last long enough to protect the fruit from decay.