This study aimed to analyze and improve the postharvest conditions for a leafy vegetable concerning with minimizing the respiration of spinach. The study was divided into 2 parts: (1) Examination of models for spinach respiration (2) The effect of nano-mist on the respiration of spinach.

1. Examination of models for spinach respiration

Anaerobic respiration is a major problem of fresh produces packaged under low O2 atmosphere. The problem becomes more severe and causes high losses in the packages handling at ambient conditions without any cooling systems. In designing the modified atmosphere packaging, the risk of anaerobic development greatly depends upon the accuracy of respiration rate prediction, therefore, the respiration rate model for a particular produce has to be identified. The objective of this study was to examine the respiration rate models in order to indentify the simple and suitable model for spinach storage under low O2 atmosphere at ambient temperature (25oC).

1.1 Material and methods

The spinach samples were stored in closed system using acrylic chambers, controlled at 25oC for 77 hours. Seven different initial atmospheric conditions were given to examine the dynamic change of gas compositions, respiration rate and respiratory quotient (RQ) at different initial O2 concentrations and different void volumes in relation to the amount of spinach stored.

Six respiration rate models were applied to the experimental data: Michaelis-Menten without inhibition, Michaelis-Menten with competitive inhibition of CO2, Michaelis-Menten with uncompetitive inhibition of CO2, Michaelis-Menten with noncompetitive inhibition of CO2, Michaelis-Menten with mixed inhibition of CO2 and Langmuir adsorption [1]. One of the seven storage conditions was selected for estimating the models' parameters using non-linear regression. From the parameters obtained the model(s) was validated by simulation (MATLAB 2007b, The Mathworks Inc, USA) to predict the gas composition of the other six storage conditions. The agreement between the simulated and the experimental results was evaluated by the mean relative percentage deviation of modulus (%E).

1.2 Results

The very low void volume of storage atmosphere (21% initial O2 with amount of spinach per unit mass of spinach 236.8mmol/kg) caused the respiratory metabolism shifted from aerobic respiration to anaerobic respiration or fermentation in the latter period of storage. In this condition, the RQ value sharply increased and became higher than 1.3. The lower limit O2 to maintain aerobic respiration was found to be around 1.0-0.8mmol/l. For the rest of conditions, the RQ value was constant throughout the storage period of 77 hours.

From the six respiration rate models proposed, only three models that the experimental data could be fitted and the parameters could be estimated, viz. Michaelis-Menten without inhibition, Michaelis-Menten with uncompetitive inhibition of CO2 and Langmuir adsorption. These models gave good fitting results with correlation coefficient (R2) higher than 0.95. The adaptability of models was dependent on the CO2 level. Therefore, the Michaelis-Menten model with uncompetitive inhibition was selected as it can be applied in extensive range of CO2. Moreover, it gave the best fitting result and widely accepted.

The estimated parameters with the average RQ of 0.8 were used to simulate the gases concentration change during storage at different initial conditions. The simulated results showed fairly good agreements to the experimental data both in O2 and CO2 concentrations. The E values for O2 concentration lower than 20% suggesting adequacy of the model and parameters to describe the respiration rate and predict the O2 concentration.

1.3 Conclusions

For aerobic conditions, the prediction of spinach respiration rate was described with a constant RQ by Michaelis-Menten model without inhibition, Michaelis-Menten model with un-competitive inhibition and Langmuir adsorption model. Among these models, the Michaelis-Menten with un-competitive inhibition was found to be more generic and suitable for practical applications as it can be applied in extensive range of CO2 and O2 concentrations.

2. The effect of nanomist on the respiration of spinach

Water loss is an important issue in relation to the deterioration fruits and vegetables, especially in leafy vegetables. Water mist humidification is a technique commonly used to diminish water loss by increasing relative humidity of the storage atmosphere. The previous studies on water mist humidification discussed the advantages of this technique on maintaining fresh produce quality. However, the size of water mist particle and its influence on the respiratory metabolism have not been investigated. This study aimed to (1) characterize the storage atmosphere as influence by the nano-mist humidification, analyzing through particle size distribution and (2) investigate the influence of nano-mist on the respiration and quality of spinach.

2.1 Materials and methods

The nano-mist was produced by the nano-mist generating system using ultrasonic principle. The water used to produce the nano-mist was control at 20oC. The nano-mist was introduced into the storage chamber that was installed in an incubator controlling at 20oC, the rate of introduction was controlled by 95% relative humidity (RH) of the storage atmosphere. The particle size distribution of the water mist particle in the storage chamber before, during and after the nano-mist introduction was examined.

For investigation of particle size distribution of the water mist particle during storage of spinach, spinach sample was stored in a storage chamber at 20oC for four days. The nano-mist was introduced into the storage chamber in daytime and stopped in nighttime. The particle size distribution of water mist particle inside the storage chamber during and after stop the introduction was examined and compared to the control condition without misting.

For the influence on the spinach respiration and quality, spinach sample was stored in a storage chamber at 20oC for six days. The nano-mist was introduced and stopped 6 hours daily for respiration rate determination. The respiration rate was determined by O2 consumption rate. The quality attributes: moisture content, leaves color and electrolyte leakage were measured on the first and the final day of the storage. The result was compared to the control condition without the nano-mist introduction.

2.2 Results and Discussions

The particle size distribution of water mist particle was analyzed through the total concentration and the geometric mean diameter. The nano-mist generating system can supply the water mist particle in the larger geometric mean diameter than water mist particle of an ambient air, while the total concentration was lower.

In comparison to control condition without misting during storage of spinach, it was found that, during the nano-mist introduction, the total concentration was lower while the difference was not observed after stop the nano-mist introduction. The geometric mean diameter was larger in the misting condition for both during and after stop the nano-mist introduction.

For the six days of storage, the O2 consumption rate decreased in the first four days and then increased in the last two days of storage due to the increasing of microbial population for both the misting and control condition [2], but the different between misting and control conditions was not observed. This was explicated as a result of similar total concentration and geometric mean diameter of water mist particle during stop the nano-mist introduction. The moisture content and leaves colors were not influenced by misting, but the tissue electrolyte leakage was higher in the misting sample after the six days of storage.

2.4 Conclusions

Nanomist application was found to alter the storage atmosphere by reducing the total concentration and increasing the geometric mean diameter of water mist particle. However, its influence on respiration rate and quality of spinach was still unclear.

3. General conclusions

The un-competitive inhibition Michaelis-Menten model with a constant RQ was found to be the most suitable model for the prediction of spinach respiration under modified atmosphere. The influence of nano-mist on the respiration was still unclear. Besides the misting treatment, the experimental set up could be the factors influencing on the results. To assure the influence of misting, the experimental set-up should be improved.

呼吸と蒸散は、収穫後の果実や野菜の劣化をもたらす主要因であるため、品質を保持するためには呼吸を抑制し、環境湿度を維持する技術が必要になる。本研究は、ホウレンソウを対象に、MA包装設計に不可欠な呼吸速度モデルおよびナノミスト(微細水滴)による加湿について検討することにより、特にコールドチェーンなどの設備が利用できない途上国において、常温付近で呼吸を最小限に抑制するための呼吸速度モデルおよびナノミストの品質保持効果について検討することを目的とした。

本論文は4章からなり、第1章では、呼吸代謝および呼吸と蒸散、果実や野菜の収穫後流通技術に関する既往の研究、さらに、ナノミストに関する研究事例を概観し、劣化が早いとされるホウレンソウの鮮度保持に関する既往の研究と合わせて、低酸素環境におかれた果実や野菜に既往の呼吸速度モデルを適用する場合の問題点を指摘した。

第2章では、ホウレンソウのMA包装設計に適した呼吸速度モデルについて検討した。まず、MA包装設計においては、呼吸速度の推定精度が低いと包装された農産物が嫌気呼吸に陥る危険性が高くなるため、所要の推定精度を得るためには個々の農産物に対して適切な呼吸速度モデルを選定する必要があることを指摘した。その上で、密閉した容器内に異なる貯蔵ガス環境を実現し、温度25℃で低酸素濃度下に置かれたホウレンソウに対して、好気条件下で呼吸商を一定であると仮定した場合、(a)阻害のない場合のミカエリス・メンテンモデル、(b)CO2による不拮抗阻害のある場合のミカエリス・メンテンモデル、(c)ラングミュア吸着モデルについては、それぞれ誤差の程度が異なるが、モデルによる環境ガス濃度の推定が可能であることを示した。その上で、誤差解析により、CO2による不拮抗阻害のある場合のミカエリス・メンテンモデルが、MA包装設計において、嫌気条件に陥ることなく呼吸速度の抑制に効果的な環境ガス濃度を予測する上で、最適なモデルであることを示した。

第3章では、ナノミスト(微細水滴)を導入して、貯蔵環境における水滴の数密度および水滴径を測定すると共に、ナノミストがホウレンソウの呼吸速度に及ぼす影響について検討した。従来の研究では水滴径や数密度に関する知見は報告されておらず、蒸散抑制に関する利点を指摘したに留まっているが、本研究では水滴の数密度と水滴径の定量的な評価を行った。

その結果、ホウレンソウを貯蔵した環境にナノミストを導入した場合には、試料からの蒸散による微細水滴のみの対照区に対して数密度が小さく、水滴径は大であった。すなわち、対照区における微細水滴の数密度が1850±157 個/cm3、幾何平均径が74±3nmであったのに対し、ナノミストを導入した直後は、それぞれ、934±49 個/cm3、92±6nmであり、両者の差は小さくなるものの、この傾向は2日程度維持されることが認められた。また、ナノミスト区、対照区とも高湿度(95%RH以上)に保たれたため目減りや葉色に顕著な差異は生じなかったが、貯蔵初期の2日間はナノミスト区において呼吸速度が抑制される傾向が認められ、微細水滴の数密度および水滴径が呼吸速度に影響を及ぼす可能性を指摘した。

以上、本論文は、常温におけるMA包装設計に適した呼吸速度モデルについて検討し、最も予測精度の高い呼吸速度モデルを特定すると共に、ナノミストの微細水滴径および数密度を定量化し、呼吸速度への影響の可能性を指摘したものであり、学術上・応用上貢献することが少なくないと考えられる。よって審査委員一同は、本論文が博士(農学)の学位論文として価値あるものと認めた。