Apple Peel Multiscal Mechanical Properties Test And Microstructure Analysis

Peel characteristics and structure of fruits and vegetables have direct influence on water evaporation, gas exchange, fruit resistance capacity to microorganism and also directly affect resistance to external force. Peels of fruits and vegetables have fine functions of preserving moisture, ventilation and antisepsis because of own characteristics and structure. As the outermost tissue, apple peel has the function of "preservation packaging material". This article mainly made test researches and mechanism analysis of apple peel properties, and simulated and analyzed the apple peel microstructure to provide the theoretical support on the harvesting, processing, packaging, storing and transportation of apples, and provide fundamental support for comparing the peel microstructure changes of different apple cultivars. This study also provided knowledge basis on better apple cultivation and perfect apple texture evaluation. The main research contents and conclusions are as follows:(1) In order to understand differences in different storability different apples, fruit physical-chemical properties, for Danxia, Fuji and Starkrimson cultivars, were studied for 0,7,14,21 and 28 days of storage under room temperature. As the storage period increased, fruit has a linear increasing trend; fruit firmness and TSS content of fruit also showed a trend of gradual decline. In the storage period, apple peels tensile properties were carried on research and it was found there was a gradual decrease in tensile strength and fracture strain, but increase in elastic modulus. In the same storage period, weight loss rate, firmness and TSS content of Fuji fruits were higher than that of Danxia and Starkrimson cultivars and Starkrimson peel biomechanical characteristic was superior to Danxia and Fuji peel. This study provided theoretical foundation for apple preservation changes and peel tensile characteristic changes during the period of storage.(2) For exploring gas transmission of apple peel, water content, water vapor transmission and Oxygen and carbon dioxide transmission of apple peels were measured. The results indicated that apple peel showed a certain degree of selectivity for gas. With the three cultivars, water vapor transmission coefficient of Fuji peel was the highest and water vapor transmission coefficient of Starkrimson peel was the lowest under the same humidity; and the mean values of peel water vapor transmission coefficient decreased with the decrease of relative humidity. Under the same pressure difference, Oxygen transmission coefficient for each cultivar peel was larger than carbon dioxide transmission coefficient; and for the different cultivars, Oxygen and carbon dioxide transmission coefficient of Starkrimson peel was the lowest and that of Danxia peel was the greatest. These results can provide the references of Oxygen and carbon dioxide content on apple controlled atmosphere storage.(3) In order to acquire many evaluation indexes of apple peel biomechanics, tensile, tear, shear and puncture tests were conducted on Danxia, Fuji and Starkrimson fresh peels located at the shadow side and sunlit side and the significant difference of test data obtained was analyzed for different parts of the same apple peel and the same part of the different apple peels. For calculated mean values of mechanical properties indexes, the longitudinal sample had larger values than the transverse; the longitudinal sample had the higher values than the transverse samples, but the horizontal deformation of peel was compressible more malleable than the vertical deformation. It could be found the stiffness of the apple peel is much larger than that of the apple pulp and the apple peel is an anisotropic heterogeneous material. The results provided theoretical basis on the designing of related mechanicalequipment of the harvesting, processing, packaging and transportation of apples.(4) In order to explore crack damage of apple peel and evaluate quality of different peels varieties, Danxia, Fuji, Star-krimson were chosen to perform tests of tensile stress relaxation and creep by means of electronic universal testing machine. Stress-time curve of relaxation test and strain-time curve of creep test were obtained; and initial stress, residual stress and strain holding of peel relaxation and initial strain, stress holding and creep value of peel creep were also gain. The results indicated that under the action of the same load, discrete degree of peel initial stress was far lower than that of peel residual stress. Relaxation Five componente Maxwell model and creep Burgers models were set up by experimental data; the model parameters were obtained; and when the principle component analysis was performed for relaxation and creep model parameters of peel, contribution rate of elasticity factor and contribution rate of viscidity factor for different peel were analyzed. Those results enriched and perfected apple texture evaluation system.(5) The microstructures of three apple peels were observed and imaged using an scanning electron microscopy (SEM) and microstructure indexes were measured with MATLAB software and the combination of the mechanical properties and microstructure of apple peels was discussed. It is discovered that for the same cultivars or different cultivars and there are difference for the mechanical properties of apple peel with the different amount and width micro cracks on shadow and sunlit sides. For the different cultivars circular or elliptic and larger cell spacing of peel cells have a high-tensile than the elongated cells. Negative effect was found for width and quantity of micro crack on the peel and circular or elliptic cells of peel have a high-tensile than the elongated cells; there is non-significant effect of peel mechanical property on the thickness of the cuticle and negative effect is found for the aspect ratio of epidermal cells. This study provides some mechanical properties as a reference for selecting apple peels as a biomimetic material prototype for packaging which keeps food fresh. The results revealed the relationship between peel macromechanics and microstructure.(6) In order to achieve visual organization structure form of peel epidermis, epidermis tissue layer was acted as ideal plane; area, center of gravity, perimeter of epidermal cells could been acquired by Green formula; and cell aspect ratio and orientation direction can been obtained by inertia moment and least square ellipse fitting; and the microstructure of three kinds of peels were simulated using two-dimensional gravity Voronoi model. Results showed that distribution of epidermis cell area are similar to normal distribution; cell aspect ratio had nothing to do with the size of cell areas; there was no difference between measured area and Voronoi cell area; and the distribution of Voronoi cell direction angle are close to distribution of cell direction angle in the light micrograph. Peel microstructure simulation provided geometric model for for comparing the peel microstructure changes of different apple cultivars.