Evaluation on peels texture of different apple cultivars based on rheological properties

Abstract: In order to explore crack damage of apple peel in the process of naturally growing and evaluate quality of different peels varieties, peel samples coming the shadow side and sunlit side of three apple cultivars，Danxia, Fuji, Star-krimson were chosen for longitudinal 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 from the tests; and initial stress, residual stress and strain holding of peel relaxation and initial strain, stress holding and creep value of peel creep were also gained. We then set up relaxation five component Maxwell model and creep Burgers model by use of experimental data; and established relaxation and creep characteristic parameters for correlation analysis; and used the relaxation and creep model parameters for principle component analysis. 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. The standard deviation of initial stress was within 0.4 percent of the average, but the residual stress standard deviation reached about 10% of its average. Initial stress, residual stress, strain holding, initial strain, and stress holding of Star-krimson peel had significant differences (p <0.01) from Danxia and Fuji peels. For different varieties, peel relaxation initial stress, relaxation residual stress; and creep initial strain were large, but creep value was small. For relaxation model parameters of peel, the equilibrium modulus of Star-krimson peel was much larger (p < 0.01) than that of Danxia and Fuji peel; but its relaxation time were much less (p < 0.05) than that of Danxia and Fuji peel. Viscosity factors of Star-krimson peel showed a significant difference (p < 0.05) from that of Fuji peel. For creep model parameters of peel, the mean value of instant elastic modulus of Star-krimson peel was the greatest and that of Fuji peel was the lowest; but decay time and viscosity factors of Fuji peel were the largest. There was a significant (p < 0.05) difference between instant elastic modulus, decay time, viscosity factors of Star-krimson peel and that of Fuji peel. It was found that the relaxation and creep test can objectively describe internal tissue structure differences around different peels; and organization structure of Star-krimson peel had better fluidity and stronger elasticity and that of Fuji peel had weaker fluidity and stronger viscidity. When the principle component analysis was performed for relaxation and creep model parameters of peel, eigenvalue was more than 1 for selected principle component and accumulative contribution rate of Danxia, Fuji, and Star-krimson peel reached 85.13%、83.94% and 91.68%, respectively. Contribution rate of elasticity factor which was on behalf of peel quality was relatively larger in Star-krimson peel, but contribution rate of viscidity factor was relatively higher in Fuji peel, this reflected that Fuji peel surface was easier fragmentation than Star-krimson peel surface；and Fuji peel surface was also easy to injury including harvest, packaging, transportation and storage. The results have significances to provide better apple cultivation and perfect apple texture evaluation system.