氏 名 | アトゥングル グリフィス グレゴリーATUNGULU, Griffiths Gregory | 本籍(国籍) | ケニア |
学位の種類 | 博士 (農学) | 学位記番号 | 連研 第274号 |
学位授与年月日 | 平成16年3月23日 | 学位授与の要件 | 学位規則第4条第1項該当 課程博士 |
研究科及び専攻 | 連合農学研究科 生物資源科学専攻 | ||
学位論文題目 | Direct Current High Electric Field Application in Post Harvest Preservation of Fruits (高圧直流電場処理の収穫後果実貯蔵への適用) |
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論文の内容の要旨 | |||
This study focused primarily on evaluating freshness of apples and plums fruit stored under high electric fields. Because fruits contain a very high percentage of their fresh weight as water, they exhibit relatively high metabolic activity when compared with other plant-derived foods such as seeds. This metabolic activity continues after harvest, and thus making most fruits highly perishable commodities. It becomes imperative that better storage methods be applied to keep the fruit freshness at the consumer acceptability level. The typical cold storage methods for keeping freshness generally require much energy for refrigeration. In this regard, a novel technique with low energy consumption that uses direct current high electric fields was considered with an aim of assessing its efficacy in freshness extension of apple (Malus domestica Borkh, cv. Fuji, Starking Delicious, Golden Delicious, Sansa, Rome Beauty) and plum(Prunus salicina Lindl, cv. Ooishi and Sodom) fruit. This research was conducted using laboratory-assembled equipment that consists of a high electric field generator. The generator had output 0 to 100 kV and 200μA maximum. Ionisation in negative corona discharge was feasible using needles as cathode and aluminum plate as grounded anode. Color Spectrophotometer, NIR type soluble sugar measuring equipment, CO2 sensor and dynamic loading equipments were utilized in assessing physicochemical properties of the fruit. In Fuji apple storage physicochemical parameters varied with water core content. Therefore the Fuji apples were matchid for same water core content using a water core content grader. This research primarily falls into four categories. Fistly, corona discharge(pin-plate electrode) effects on storability i.e. evaluating the effect of electric field treatment (corona discharge) on apple soluble sugar, respiration, color, surface reflectance and tissue mechanical properties. It was noted that corona discharge treated samples had a smaller total color difference compared to untreated samples. The displacement of the reflectance percentage from the initial target in the considered waveband 400 to 700 nm was lower for the continuously corona discharge treated sample than in the controls after seven days of storage. Continuous electric field treatment influenced apple fruit mechanical properties. The treated apples tended to show tougher and stiffer characteristics than the untreated ones. Continuous and intermittent corona discharge treatment suppressed CO2 evolution in apples stored at room conditions. Intermittent direct current electric field treatments of apples stored at 0℃ and 10℃ suppressed climacteric peaks in terms of CO2 evolution. Treatments with corona discharge suppressed soluble sugar concentration change. Secondly, effects due to corona discharge electrode configuration with parallel plate electrode configuration were compared. It was observed that positive and negative ion flux and positive and negative corona current depended on electrode gap and applied voltage. Negative ion flux and negative ion current values were higher for a constant electrode gap, applied voltage and electrode geometry. The effents on the physicochemical properties depended on the type of electrode configuration used. Apples stored between parallel plate electric field had suppressed respiration and climacteric peak. However, those stored in the corona discharge field had suppression in the initial seven days followed by increased respiration in the treated sample over the untreated. Weight loss was supperssed by treatment in the parallel plate electric field. Thirdly, the effects of field polarity reversal in both pin-plate electrode and parallel plate electrode configurations were examined. Investigations revealed that the electric field influence on apple physicochemical properties depended on the magnitude as well as the direction of the applied electric field. The 'reversed'(apples on cathode plate) electric field treatment gave higher weight loss than the corresponding 'foward'(apples on anode plate) electric field treatment for the parallel plate electrode configuration. Generally the differences in untreated and treatment values were smaller in parallel plate configuration than in corona discharge configuration. The input electric energy required to evaporate water was higher in negative corona discharge electrode configuration than in positive corona discharge. Further the energy efficiency ratio was lowest in negative corona discharge. In corona discharge, the multiple of evaporation rate (ratio of water loss in treated sample to that of untreated sample) was higher for the 'forward' electric field direction (negative needle polarity) than the 'reversed' electric field direction(positive needle polarity). Fourthly, investigetion of the effects of field configuration and treatment timing on putrefaction and moulding were conducted. It was realised that the a values in plums representing reddening tendency of the samples treated pre-ripening increased by 21% more in the electric field treated sample than the untreated sample. The deviations of the chromatic aberration depended on the stage of treatment. Samples treated pri-ripening had higher chromatic aberration values than the untreated ones. Non continuous pre and post ripening treatment in corona discharge as well as parallel plate electrode configurations did not control mold on plums(Ooishi and Sodom variety). However, continuous treatment with corona discharge electrode configuration, unlike parallel plate delayed mold development. Electric field treatment had promising effects in postharvest storage life of apple and plums fruit. It should, however, be further proved that there exists no lethal effect on the chemical constituents of the fruit if the treatment are to be used to elongate shelf life of fruit. Further, because of variability that is predominant in the characteristics of agricultural fruit, performing optimization of various parameters(field strength, air temperature, air humidity, product variety, etc) is important if more candid conclusions are to be drawn. 和文要旨 本研究は市販の高直流電圧発生機などを使用して、研究室で組み立てた実験装置で行った。 高電圧発生機の出力は0~100kV、最大電流200μAである。鋸歯極を陰極にアルミニウム板を接地陰極として、 陰コロナ放電によってイオン化を起こさせた。カラー分光光度計、近赤外線(NIR)式可溶性糖含量測定器、 二酸化炭素メータおよび動的荷重計測機を使って、果実の物理化学的特性を調べた。ふじ(りんご)の貯蔵では、 蜜の含量によって物理化学特性が異なるので、蜜含量計測機(FANTEC)を使って蜜含量を計測し、試料の蜜含量を揃えた。 本研究は基本的に次の4部に分けられる。 Ⅰ.鋸歯-平板電極で発生させたコロナ放電による貯蔵性への効果、すなわち、コロナ放電による電場処理による、
りんごの可溶性糖含量、呼吸速度、色、表面反射率、組織の力学特性への影響を調べ、次の結果が得られた。 Ⅱ.コロナ放電電極型と平行平板電極型の電極型の違いによる影響を比較した。陽イオン流束と陰イオン流束および
陽コロナ電流と陰コロナ電流の大きさは、電極間隔と印可電圧によることが認められた。
また、印可電圧、電極間隔と電極形状が同じであれば、陰イオン流束および陰イオン電流は、陽イオン流束および
陽イオン電流より、それぞれ高い値を示した。 Ⅲ.鋸歯電極と平行平板電極において、極性を逆にしたときの影響を検証した。貯蔵りんごの物理化学的特性は、
印加する電場の電界強度と電場の方向によって変わることが明らかになった。上部陰極・下部平板陽極(接地)を
「順極」、上部陽極(接地)・下部平板陰極を「逆極」と呼ぶ。電場処理は試料を下部平板極上に載せて行う。 Ⅳ.プラムの電場処理型と処理時期の腐敗やかび発生への影響を検証した。 電場処理はりんごやプラムの収穫後貯蔵に顕著な効果があることが認められた。しかし、果物の化学的な 成分に有害な影響が無いことを証明して始めて、この高静電圧場処理が果実の寿命を延ばす方法として使用することが できる。また、これは農産製品の特徴ではあるが、果実の種類によって特性が異なるから、より信頼性の高い結論を 得るためには、更に、電界強度、空気温度及び湿度といった操作パラメタの最適化を行うことが重要である。 |