| Apple decay is mainly caused by spoilage moulds. In harvest, storage and transportation period, apple may inevitably be subject to mechanical damage, thus spoilage moulds are easy to latently or directly infect the fruit through the wound. Spoilage moulds attached to fruit stalks and fruit peduncles which are difficultly to remove completely during the washing process. Also, fresh-cut processing lead to the original apple texture and surface cell damage, meanwhile nutrient material that flow out of cell could also breed these remaining spoilage moulds, eventually cause decay of fresh-cut apples. This study began with separation and identification of spoilage moulds was researched in fresh-cut apple. And then conidia germination and invasion into apple wounded surface, and also colonization and extension in apple tissues, and finally the whole process of the formation of conidium and the effect of spoilage moulds on apple cells were observed by scanning electron microscopy ?SEM? and transmission electron microscopy ?TEM?. Thirdly studied spoilage mould infection impact on apple's color and texture, and analyzed their correlation of various parameters affecting the color and texture changes. The effect of spoilage infection on the gene expression of apple's own cell wall degrading enzymes ?CWDEs? also was studied. Finally the effect of aqueous ozone sterilization on fresh-cut apple was studied. This study should provide the theoretical basis for biological mechanism of spoilage mould infection, and for development of quality and safety control technology of fresh-cut fruits and vegetables. The main results were as follows:?1? Four spoilage moulds were identified, which are P. expansum, P. crustosum, A. alternata, and A. niger ?According to the spoilage ability order?.?2? It was observed that P. expansum infectd apple tissue was completely decay at 4th day, and A. alternata and A. niger infectd apple tissue was completely decay at 5th day by SEM. P. expansum and A.niger secreted a layer of mesh material on conidium surface, while A. alternata didn't. Conidium of P. expansum mainly developed germ tubes ?GTs? from the top and the base, and conidium of A. niger mainly developed GTs from the top, sides and base. These GTs could penetrate the host cell wall directly on the wounded tissues. Their conidium could also directly invade into wounded tissue through the degradation of apple's cell wall. Conidium of A. alternata mainly developed GTs from the sides, and these GTs could penetrated the host cell wall directly on the wounded tissues. It was observed by TEM that all of the three spoilage moulds colonized in the intercellular space by invading wounded tissues, and entered through the middle lamella of the cell wall, and also spread in the cell wall, then went through the cell membrane, finally spread in the cell. Wounded tissues and cells were gradually digested from the outer to the inner. The cellulose micro fibers were broken, the cell wall was broken, the cytoplasm and cell wall were separated, and the organelles were destroyed.82% of the P. expansum conidia,77% of the A. alternata conidia and 72% A. niger conidia were germinated on wounded tissues at 24 hpi. The order of GT development, infection speed and spoilage ability should be P. expansum, A. alternata and A. niger.?3? Three kinds of spoilage moulds leaded to browning on the surface of wounded tissues. The browning order should be A. niger, A. alternata and P. expansum. POD was the major browning enzyme of apple infectd by P. expansum, while PPO and POD was the major browning enzyme of apple infectd by A. alternata and A. niger. All of three spoilage moulds sped-up decrease of total phenols content in the wounded tissues, while the content of tissues around the wounds had a small change. And also, three spoilage moulds increased PAL activities of wounded tissues and tissues around the wounds, and caused apple cell membrane damage.?4? Three spoilage moulds could hydrolyze protopectin, hemicellulose and cellulose not only in the wounded tissues, but also in tissues around the wounds. All three spoilage moulds could increase the activities of 5 kinds of CWDEs both in wounded tissues and in tissues around the wounds. A. niger made the cell separation mainly by hydrolyzing pectin in the middle lamella of cell wall, which led apple to soften and decay; A. alternata broke the cell wall by hydrolyzing hemicellulose and cellulose, which led apple to soften and decay; P. expansum was through the above two ways. Physiological and biochemistry changes of wounded apple tissues infectd by spoilage moulds were happened earlier than its quality deterioration. The order of deterioration apple texture was P. expansum, A. alternata and A. niger.?5? The active infection of three kinds of spoilage moulds produced polygalacturonase ?PG???-galactosidase ??-Gal? and a-arabinfuranosidease ?a-L-Af?, which caused their activities increasing both in wounded tissues and in tissues around the wounds. The decrease in relative expression level of MdPG1, MdBGAL and MdFase1 was regulation response to spoilage moulds infection both in wounded tissues and in tissues around the wounds, which protected the integrity of apple's own cell walls.?6? It was recommended to sanitise fresh-cut apples with aqueous ozone (1.4 mg·L-1) for 5 min, which could extend the shelf life up to 10 days and maintain acceptable sensory quality indicators. In addition, aqueous ozone treatment had a reduction effect of browning enzymes activities and total phenols content of fresh-cut apple. Aqueous ozone treatment reduced the degradation of cell wall substances by inhibiting the activity of ?-Gal and ?-L-Af, which could slow down the deterioration of texture of fresh-cut apples. |
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