| Apple is one of the most important consumer fruits in the world, which which is attributed to its crisp texture, juiciness, sweet taste, rich aroma, a large number of carbohydrates, flavonoids, polyphenols, vitamins, minerals and phytochemicals, endurable storage and long-term supply. Therefore, apple is more and more favorable to consumers. China is the world's leading apple producer, and has an absolute advantage in both apple acreage and yield. With the continuous expansion of apple cultivation area and the development of planting technology, storage, processing and other sectors has been playing a more and more important role in the process of the sales and distribution of apple. But in the period of postharvest storage, transportation and processing of apple, the industry is demanding nondestructive instrumental techniques that are able to detect apple quality in real-time, in order to carry out a fast classification, so as to realize maximum economic benefits.Currently, the nondestructive inspection of fruit quality including mechanical properties'analysis, electrical and magnetic characteristics methods, optical reflectance and transmission, X-ray analysis method, computer vision technology and bioassay. The mechanical properties of fruit are closely related to its internal quality. In this paper, the stress-relaxation and creep properties of'Fuji'apple are analyzed by means of rheological theory. The stress-relaxation and creep property, texture profile analysis and chemical components of'Fuji'apples in different storage time were measured in order to define the proper stress-relaxation and creep models and explore the correlation between the indexes of texture profile analysis, chemical components and the variables of stress-relaxation and creep parameters. Prediction models for'Fuji'apples quality were constructed and verified based on the variables of stress-relaxation and creep parameters. studying the feasibility of predicting the apple quality by analyzing rheological property. The following conclusions were drawn:1. The optimal stress-relaxation test parameters of intact'Fuji'apple were deformation distance 1.5mm, constant time 100s, crosshead speed 1mm. The optimal stress-relaxation test parameters were load 30N, creep time 40s, crosshead speed 1.5mm.2. The three elements, five elements and seven elements Maxwell models were used to analyze the stress-relaxation data of intact sample, which were obtained under the optimal stress-relaxation test parameters. The results indicated that the three elements Maxwell model can optimally represent the stress relaxation behavior of intact apple; As the number of elements increased, the fitting effect of Maxwell model gets better; The correlation coefficient decreased with the fruit edible quality decreased during storage.3. The results of correlation analysis between texture attributes or chemical compositions and Maxwell model parameters were:for soluble solids concentration, titratable acidity, ascorbic acid, hardness, springiness, cohesiveness, chewiness and resilience there are positive significant correlations (r=0.27-0.95, P<0.05) with relaxation parameters of elastic components (Eo and Ei) and viscous components (?i); adhesiveness was negatively correlated with these relaxation parameters (r=-0.53?-0.78, P<0.01);however, the relaxation times (Ti) were correlated with chemical or texture attributes with rather lower correlations (r=-0.29?-0.34). The correlation coefficients of models for predicting the soluble solid content, titratable acidity, vitamin C content, hardness, chewiness and resilience of apple were 0.933?0.874.0.837?0.927?0.843? 0.884, respectively. The measured data were well fitted to the prediction models of these quality properties. The generated regression equation of springiness, adhesiveness and cohesiveness with R2<0.80, the accuracy of prediction models for these three attributes are not higher enough to evaluate'Fuji'apple.4. The two elements, six elements Kelvin models and Burgers model were used to analyze the creep data of intact apple, which were obtained under the optimum creep test parameters. The Burgers model can satisfactorily described the creep behavior of intact'Fuji'apples and its average correlation coefficient was 0.982; The results also indicated that the correlation coefficient increased with the number of elements. Although the maximum of the correlation coefficients for six elements Kelvin model reached 0.993, the correlation coefficients were characterized by high variability. Therefore, six elements Kelvin model was less reliable for the interpretation of creep data.5. Soluble solid content (SSC), titratable acidity (TA), vitamin C content, hardness, cohesiveness, chewiness and resilience were significantly positively correlated (r= 0.56-0.94) with the delayed elasticity coefficient Ei and the viscosity coefficient ?1, respectively. Highly negative significant correlation (r=-0.40, r=-0.45) were shown between soluble solid content, titratable acidity and viscosity coefficient ?2, and vitamin C content had a significant negative correlation(r=-0.34) with viscosity coefficient ?2. But there were no significant correlation shown between the adhesiveness, springiness and the variables of creep property. The correlation coefficients of models for predicting the soluble solid content, titratable acidity, vitamin C content, hardness, cohesiveness, chewiness and resilience of apple were 0.939,0.922,0.881,0.917,0.594, 0.857,0.584, respectively. Moreover all of the models have statistical significance (P <0.05). There was no significant difference between the predicted values of models and the measured data, which suggest that the creep property can be proposed for the reliable prediction of'Fuji'apples quality.Currently, the methods which are used to evaluate apple quality are usually destructive, time consuming, waste of material and not on-line. This work studied the correlation between chemical compositions or texture attributes and rheological properties of intact apple, aiming to predict and evaluate apple fruit quality by rheological method, and also improve the fruit quality evaluation system. Therefore, this study has the positive sense regarding the storage, transportation, processing and selling. |
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