称号及び氏名

博士（学術）Teeranud Romphophak

学位授与の日付

平成 17 年 2 月 20 日論

「PRACTICAL ATMOSPHERE CONTROL FOR PROLONGING文

名

SHELF LIFE AND PREVENTION OF SENESCENTSPOTTING IN BANANA FRUIT」（バナナ果実における品質保持と褐色斑点抑制のためのガス環境調整法）

論文審査委員

主査

上田 悦範

副査

堀内 昭作

副査

阿部 一博

Summary
INTRODUCTION
Banana (Musa spp.) is an economically important fruit in the tropical areawith all year round production. The fruits quality depend on their own
maturity and postharvest management. Banana ripe quickly after inductionof ripening by ethylene. Browning spots, which are called senescent spottingor sugar spotting are observed just after the fruits are fully ripened and thetaste of pulp reaches the maximum of acceptability. However, thephenomenon prevents consumers` choice when such banana decorate on theshelf of retail markets. Controlled atmosphere (CA) or modified atmosphere(MA) are beneficial for maturity control of many fruits. More practicalmodification of those effective treatments could useful for shipping andmarkets for fruits. Holding banana fruits in N2 gas for short periods
following ripening treatments can also delay ripening of banana. However,efficiacy of those treatments are not clear among former reports depending
probably on the banana ripeness and actual low O2 levels during applicationof the methods.
Our objectives were the studies of the potential gas treatment for
prolonging shelf life of banana and the prevention of senescence spotting. We

also investigated initiation of senescent spotting by microscopic observationand change of phenolic compounds during developing spotting.
CHAPTER 1. STUDY OF SENESCENT SPOTTING OF BANANA (Musasapientum L.) PEEL
There are two hypothesises on the initiation of the spots: one is the resultof infection by fungus and the other is physiological disorder during
senescence. So far the concrete mechanism of initiation of the spots is notclear.
1.1 Optical microscopic and scanning electron microscopic (SEM) observation
Less than two millimeter diameter of senescent spots of the surface peel
(Musa sapientum L.) were observed by optical microscope and SEM. We
found that the browning parts were about 0.25 mm depth, located within 7-8
layers of cells from surface. In horizontal view, the cells in browning spots
showed 2-3 big brown particles in the cells. While no such particles wereobserved in the cells at the normal part. By SEM observation, the browningpart showed sunken toward surrounding sound area like crater and wilted
cells were observed in the crater. At the center of each sunken area,prominent stoma was observed. No hypha was observed on the senescentspotting area.
1.2 Sanitizing of peel by ethanol
Ethanol (20% or 50%) was treated to the surface of banana peels everyday
for avoiding infection by microorganism during storage. There are nodifference on initiation of browning between ethanol and non-ethanol treatedbanana. The senescent spotting of the two treatments started equally at day
7 and developed same extent thereafter.
These results in 1.1 and 1.2 section, showed that senescent spotting is aphysiological disorder.
1.3 Browning and volatile chemicals
The yellow peels were separated from the pulp at day 6 after ethylene
treatment when browning did not occur yet, and they were stored forsuccessive 10 days at high humidity. The senescent spots did not occur on thepeels even after 10 days after peeling. We thought that some volatiles cameout from pulp of banana, and went through the stomata and then denatured
surrounding cells.
Volatile chemicals (5-100 µl) which reported earlier as aroma of banana
were treated on the separated surface peel. The browning on the peel
appeared in wider area, although typical senescent spots on the peel could
not appear in any cases.
CHAPTER 2. PHENOLIC COMPOUND AND POLYPHENOL OXIDASE(PPO) ACTIVITY IN THE PEEL AND PULP
2.1 Sugar and dopamine levels during ripening
The total sugar content of banana pulp increased from 0.5% on day 0 to8.7% on day 7 at 20℃. The dopamine, a catecholamine, of surface peel was
about 500 mg per 100gFW on day 0 (green color fruit) and remained at thatlevel until day 5 (yellow part). The brown part of senescent spotting on day 5
had lower content of dopamine (330 mg per 100gFW) than the yellow part.
On day 7, the dopamine content at the senescent spotting showed furtherdecrease. This indicated an inverse relationship between dopamine level andsenescent spotting.
2.2 Polyphenol and PPO activity during ripening
The tyrosine and L-DOPA of the external peel were increased, while thedopamine was decreased during storage at 20℃. Dopamine was the majority
among catecholamines. The catecholamines in pulp were low levels and not
different during storage. Activities of PPO on external and internal peel aswell as pulp were increased during ripening. These results imply that
dopamine changes to browning pigments by PPO at the senescence stage ofbanana.
CHAPTER 3. MODIFIED ATMOSPHERE STORAGE IN POLYETHYLENE(PE) BAG AND POLYVINYL CHLORIDE (PVC) WRAPPING OF
RIPENING-TREATED BANANA `SUCRIER`
3.1 MA storage in PE bags with or without ethylene absorbent (EA)
In the PE bag (0.03 mm thickness) at 20℃, the dopamine content ofsurface peel kept higher level than in air (control). The sucrose levels ofbanana in PE and PE with EA were almost identical and lower than the levelin the control. The concentrations of both carbon dioxide (CO2) and ethylene(C2H4) in a PE bag were very high (about 27-32% CO2 and 3 ppm C2H4,respectively). Slight fermented smell was observed, although PE packaging
delayed ripening of the banana. The high level of CO2 in the PE bag inhibited
ripening. Therefore, the PE packaging seems unsuitable for MA storage at20℃.
3.2 MA storage with PVC wrapping compared with PE bags
Banana in the PVC (0.01 mm thickness) wrapping at 20℃ produced about
2.6% of CO2 concentration on day 3, which increased to 4.7% on day 5 andthen decreased to 3.0% from day 7 to day 13. The concentrations did not
differ in wrappings with or without EA. The C2H4 concentration in PVC
wrappings without EA was 0.7 -1.0 ppm from day 3 to day 13, while in awrapping with EA, the concentration was about 0.2 ppm from day 3 to day 6
and then decreased to 0.02-0.1 ppm from day 7 to day 13.
At day 7, the sucrose levels of banana in PVC with EA and the control werehigher than that in PE with EA, although the glucose and fructose contents
in all treatments were almost identical. Banana in PVC with EA had
significantly higher dopamine contents than control banana and showed
lower scores of senescence spotting.
There was a slower change of ripening in bananas held in PVC or PVC
with EA at 20℃ than in the control, and senescent spotting was also delayed.
Shelf life was extended to 6-7 days, while the controls had a shelf life of only
3-4 days.
CHAPTER 4. NITROGEN (N2)-GAS TREATMENT FOR SHORT TIME IN
PE BAG FOR PROLONGING SHELF LIFE OF BANANA `SUCRIER`
N2-gas (100%) treatment of banana in dynamic system for 48 h at 20℃,
showed lower respiration rate about 40% than that in 21% oxygen (O2)- gastreatment. Then the rate increased to normal air level after banana wastransferred to the air flow condition after day 3. The treatment extended
shelf life of banana to 5-6 days, while control had shelf life of 3 -4 days. Whenthe bananas were packed in PE bags with N2 gas for 24 or 48 h at 25℃, theO2 concentrations in the PE bag were 0.98 and 3.41%, respectively, at the endof 24 and 48 h N2 -gas packaging. These bananas showed less senescent spots,
high dopamine contents and 2 days longer shelf life compared to control.
A 24 h N2-gas packing in PE bag and then PVC film wrapping extended
the shelf life of banana to 9 -11 days.
CONCLUSION
More practical controlling of surrounding N2 gas was proposed on ripeningof banana than conventional CA or MA storage. N2-gas treatment in PE bags
for short time (24 or 48 h) followed PVC wrapping made shelf life of banana
twice and suppressed senescent spotting.
It was recognized that senescent spotting was physiological phenomenon
during senescence and dopamine, a phenolic compound, decreased
accompanying with development of senescent spotting.

審査結果の要旨
バナナは熱帯亜熱帯では生食にとどまらず、食糧としても重要な果実である。バナナ
はエチレン処理等によって追熟を開始すると老化までの過程が速やかに進み、黄化の後、果皮に褐色斑点が現れる。この時期には果肉の糖濃度が最高値に達し、香りも生成されて美味になり、この斑点はシュガースポットとも呼ばれている。しかし、小売店でこのような斑点が現れたバナナは消費者に嫌われる。そこで、小売店に配送されて追熟を開始した黄緑色バナナが黄化を経て褐色斑点を示すまでの期間をバナナの棚持ち期間と呼び、なるべくこの期間を延長することが良好な商品展示を行うために重要になってくる。果実の熟度を遅延する方法としては、低温で保存することや、ガス環境を厳密にコントロールする CA 貯蔵と、プラスチック袋に果実を入れて果実の呼吸とガスの膜透過性のバランスで好適なガス環境を作るＭＡ包装貯蔵がある。小売店の棚先で熟度を遅延するにはバナナが低温障害を示すことから低温貯蔵法は使えない。唯一ＭＡ包装が考えられるが、詳細には検討されていない。また一時的に窒素（N2）ガスを処理して追熟の遅延を行うことも試みられているが、この処理の良し悪しは結論に達していない。
本研究においては、バナナ果実の棚持ち期間および褐色斑点出現を延長できる実際的な包装方法、N2 ガス短期間処理方法およびその組み合わせを検討した。また褐色斑点そのものの発現の様相を光学および走査電子顕微鏡(SEM)で観察し、その発症起源を考察している。さらに褐色色素の由来をフェノール類の消長から示唆している。
その内容は以下のように要約される。
第 1 章では、褐色斑点を光学顕微鏡および SEM で観察することから始めた。バナナの褐色斑点が現れる原因として、真菌による感染と生理的障害が以前から言われている。
現在のところ生理的障害が有力になっているが詳細は不明である。光学顕微鏡観察によって、褐色細胞は表皮の上層に限られているのが明らかにされた。ＳＥＭの観察では褐
色斑点部分はクレーター様に陥没しており、中心には気孔があってその部分から褐色斑点が始まっている様子であった。また褐色斑点を示しているどの部分からも菌糸の存在は見られなかった。果皮をアルコール(20−50％)で毎日処理し、菌の付着を防止してみると、無処理(水)との間に差は無く、同時期に褐色斑点が現れた。これらの結果から、褐色斑点はバナナ追熟末期の生理的障害であることを強く示唆している。褐色斑点が気孔から始まっているようにうかがえたので、果肉の揮発性物質が関連しているのではないかと考え、剥皮した果皮にバナナ果肉で生成される化学物質の蒸気に曝したところ、幅広い褐変は起こるものの典型的な褐色斑点は現れなかった。
第 2 章において、褐色斑点発生時期と追熟中のフェノール類の含量、ポリフェノールオキシダーゼ（OPP）活性の変化との関係を見ている。主なフェノールはドパミンでその他 L-DOPA 等が少量存在していた。ドパミンは緑熟果から黄熟果にかけて高含量であったが褐色斑点が現れる頃に減少した。同一果実においても褐色斑点を示している果皮部分は低含量であった。またドパミンは果皮表層に多く見られ、褐色斑点との関係がうかがわれた。PPO 活性は同様に果皮表層で活性が高く、追熟中増加した。これらのことはドパミンが追熟後期に PPO の働きにより褐色色素に変化したことを示している。
第 3 章では、バナナを MA 包装したときの棚持ち期間および品質について、ポリエチレン（PE）袋(厚さ 0.03ｍｍ)とポリビニールクロライド(PVC) ラッピング（0.01ｍｍ）で比較した。20℃貯蔵において、PE 袋では内部の二酸化炭素（CO2）濃度が高くなりすぎて(約 30％)発酵臭と追熟障害を起こし、包装としては適切ではなかったが、PVC でラッピングする方法は内部のガス濃度(CO2，3-4％)も適当で、無包装区(3−4 日)に比べ、棚持ち期間を延長することができた(6−７日)。
第 4 章では、100％窒素ガス(N2)短時間処理の棚持ち性および品質に与える影響を見ている。ここで述べる N2 ガス処理は実用面から考えて大型の PE 袋(厚さ 0.05mm)にバナナ果実をいれ、100％N2 ガス処理を 24 時間または 48 時間行っている。この方法
では、処理完了前の酸素（O2）濃度はそれぞれ 0.98 または 3.41％であった。この処理も前章の PVC ラッピングと同程度に棚持ち期間を延長した。上述の方法を組み合わせて、100％Ｎ 2 ガス処理を 24 時間行い、その後 PVC ラッピングすることにより棚持ち期間を 9−11 日に延長することができた。
本論文は、バナナの流通小売上の棚持ち期間の大幅な改善方法を提示するとともに、バナナの貯蔵後期に現れる褐色斑点の顕微鏡による観察やフェノール含量、PPO 活性の変化から、褐色斑点の発現機構も示唆している。この成果は実際的な流通上の問題点を改善したことにとどまらず抜本的な果実の褐変現象の解明に役立つデーターを多く含んでいる。よって、学力確認の結果とあわせて、博士(学術）の学位を授与することを適当と認める。