Effects Of Controlled Atmosphere On Epidermic And Pectic Ultrastructure And Quality Of Postharvest Fruits And Vegetables

Extending the shelf-life and maintaining the quality are major goals for the storage of fruits and vegetables (F & V). Storage technology can prolong the market time for F & V, especially for fruits, since the supply of fruits mainly depends on special seasons of their harvests. Using storage technology off-season supply can be realized, which not only satisfies the market, but also greatly benefits the farmers.The principle of controlled atmosphere (CA) for the F & V storage is to artificially modify the atmosphere in the package and then to inhibit the metabolism of F & V, generally through ventilating the N₂ and utilizing the respiration of F & V to generate the CO₂. It is often companied with low temperature and a certain relative humidity when apply CA storage. The principle of modified atmosphere (MA) storage is to use films with a certain gas penetrability to pack the F & V, utilizing the respiration to change the gas composition in order to inhibit the respiration of F & V itself, and to maintain the balance between the respiration and the penetrability of the films.F & V play an important role in produce trade. It is very important to quantitatively and validly evaluate their qualities. Roughness increase of pericarp from water loss during storage would decrease the marketing value. It is meaningful to research and work out the quantitative determination method and evaluation system of pericarp roughness for impersonally evaluating the marketing value of F & V.Pectin plays a skeleton role in cell walls of F & V, together with semicellulose, cellulose and others to formulate the inter-conjunction structures and maintain the cell shapes. The quality properties, especially texture, have relation with the structures and contents of pectins, and pectinase is very important for changes of pectin structures and contents during storage. Changes of pectin structures and contents can be revealed through the changes of activity of pectinase. Through such changes we can further analyze and manipulate the texture of F & V. Then the general rule of quality changes can be interpreted and used to improve the controlled atmosphere conditions.The main research part can be divided into four parts as follows:The first part is about the visualization and quantitative roughness analysis of fruit skin by atomic force microscopy (AFM) under storage. Skin status of produce is vital in determining fruit quality. Sensory evaluation is often applied to denote this characteristic. A new method, atomic force microscopy (AFM), was proposed to denote the changes of skin status. Arithmetic roughness (Ra) and root mean square roughness (Rq) of'Jinxiu'yellow peach (Prunus persicu L. Batsch.) under CA storgage and mushroom (Agaricus bisporus (Lange) Imbach) urder MA storage were analyzed by AFM. For mushroom, the initial value of Ra was (30.035±1.839) nm, while those of 2℃, 25℃and dynamic temperature on the 2 nd day were (40.139±3.359) nm, (54.393±13.534) nm and (41.197±6.555) nm, respectively. This tendency is similar for Ra and Rq. Both values of roughness increased with increasing temperatures during duration of storage. The three-dimensional profile of the pileus epicutis could signify the process of water evaporation intuitionally. The tendency was in accordance with the roughness results, especially for the earlier stage of the storage (0～2 d). The outcome of roughness analysis could signify the differences of storage conditions. It showed that the roughness measured by atomic force microscopy effectively reflected the moisture loss degree of the mushroom pileus epicutis during post-harvest storage; For yellow peach, the Ra and Rq values increased with storage time in both CA and regular air (RA) storage, and the values of CA group were smaller than that of RA group. There is a linear correlation between Ra and Rq. The three-dimensional profiles of the skins (both mushroom and yellow peach) could also be gained by AFM. The results of yellow peaches indicate that the roughness values increase with the storage time, and the roughness of CA group increases more slowly than that of RA group.The second part is about ultrastructural changes of peach pectin by AFM during CA storage, including water-soluble pectin(WSP) and carbonate-soluble pectin(SSP). The width, length and branch of the pectin chains were studied. The structural model and the degradation of pectin were also speculated. For SSP, yellow peaches (Prunus persicu L. Batsch.) were stored under controlled atmospheres (CA) of 2% O₂ plus 5%CO₂, 5% O₂ plus 10% CO₂, 2% O₂ plus 10% CO₂, 5% O₂ plus 5% CO₂, and normal atmosphere at 2℃, to investigate the effects of different concentrations of O₂ and CO₂ on the structure of single SSP molecule; for WSP, CA of 2% O₂ plus 5%CO₂, 5% O₂ plus 10% CO₂ and normal atmosphere at 2℃were chosen. The microstructure changes including aggregates and branches were studied by atomic force microscopy (AFM) at initial,on the 15th and 45th day. The microstructures of molecules and polymers (both SSP and WSP) showed that aggregates separated gradually with the storage time. The degradation took place in the linear backbones as well as in side chains. The degradation of SSP and WSP molecules was inhibited by lower O₂ and higher CO₂. Almost all of the SSP chains were composed of four basic units with width of 11.719 nm, 15.625 nm, 19.531 nm and 17.578 nm, while it was 11.719 nm, 15.625 nm, 19.531 nm and 35.156 nm for WSP molecules. The pectin chains can be visualized and calculated exactly by AFM. It indicated that parallel linkage or intertwist between the basic units was fundamental information for both SSP and WSP molecules.The third part is about the texture properties and the pectinase activity of yellow peach under CA storage, the storage conditions were the same as the above for SSP. Kinetic model of polygalacturonase (PG) activity of peaches during CA storage were investigated. Mathematic model was proposed to describe the PG activity based on the formation from inactive predecessor and denaturation into inactive form. The experimental results show that the controlled atmosphere storage groups have lower reaction rate of PG formation and denaturation among different storage groups. The effect of CA on storage may be like that of decreased temperatures. The decrease of the firmness in later stage of storage has relation with the degradation of pectin. This might be result from the breakage of the linkage between pectin chains; the results comply well with the AFM images of pectins. The contents of WSP, chelate-soluble pectin(CSP) and SSP from the peach flesh have been assayed during the storage.There is a linear correlation between the CSP and firmness, it shows that CSP might be essential for maintaining the firmness of peaches. The relationships among the pectinase activity, the firmness and microstructure changes need to be further studied. The fourth part is to investigate the effect of CA conditions on the quality (including ascorbic acid, chlorophyll (a and b), firmness etc.) and shelf-life of broccoli. The degradations of chlorophyll and ascorbic acid of broccoli under controlled atmosphere storage had been tested and it was found that the degradations followed the first order reaction kinetics. The apparent activation energies of chlorophyll and ascorbic acid of broccoli were also determined, basing on the Arrhenius's law. The effects of storage temperatures at 2℃and 8℃, and gas compositions of 3% O₂, 2% CO₂, 95% N₂ and 6% O₂, 6% CO₂, 88% N₂,on the broccoli quality were measured. The results showed that the combination of temperature at 2℃, and the gas composition of 3% O₂, 2% CO₂, 95% N₂ was optimal. The broccoli can keep fresh over 28 days with sound ratio over 84%. The function of the storage temperature was found to outweigh the gas composition. The key to extend the shelf-life of broccoli is to inhibit the bud to effloresce and become yellow.Main experimental materials were yellow peaches, companied by mushroom and broccoli to validate some indexes. Quality and physiology properties (texture and PG activity etc.) of postharvest produces were investigated from macro scales, and the roughness of skin and structure of pectin chains from micro scales, which could be used to provide theoretical direction for physiology and quality properties of postharvest produce.