Studies On Rheological Properties And Texture Evaluation Of Pear

Fruit is a living viscoelastic body, which will behave their inherent rheological properties when subjected to foreign loads. Such phenomenon can provide objective and useful information for prevention from mechanical bruise, texture objective evaluation and rapid determination of fruit's quality.Therefore, the research on rheological properties of fruit body under loads is of great theoretical and practical importance. With "Dangshan crisp pear" as the main object, TA-XT2i as the instrument, this thesis mainly focuses on the aspects as follows: firstly,study the basic rheological properties of pears, and the influence of different test conditions to the results in the "slight deformation" tests , such as loading—unloading,stress relaxation,creep test; secondly, the trends of rheological properties of intact fruits during the shelf life using the optimized test conditions;thirdly, the effects of test conditions of TPA to the results;lastly, the application of three destructive tests—TPA,puncture,WBS for regularly evaluating the texture of pear flesh during the shelf life.Different test conditions such as the deformation degree, duration time, test speed etc, had been respectively applied to loading—unloading test, stress relaxation test and creep test in order to evaluate the influence of test conditions to the results. The results showed that, in the loading—unloading test, deformation degree affects the test results significantly while the effect of test speed is insignificant; in the stress relaxation test, the degree of deformation significantly affects the springiness, while the test speed and the duration time does not affect it obviously; in the creep test, the level of load affects the "Creep Deformation" more significantly than the duration time does, the more the value of load and duration time is, the more the "Creep Deformation" is, and the more susceptible to mechanical damage.The intact rheological properties of "Dangshan crisp pear" pears were regularly measured with the optimized test methods like stress relaxationtest, loading—unloading test, creep test. It was concluded that, as the preservation extends, various rheological parameters, which could be extracted from the theological models, are decreasing during the shelf life especially after 21 days on shelf (20 °C),which can be described by Unary Cubic Equation. The phenomenon reflects the changes of the internal components and their structures. The results of the tests illustrate that the changes of the rheological parameters extracted from compression—relaxation, creep curves during the shelf life have similar trends. The results of PCA (Principle Components Analysis) prove that the rheological parameters correlated with each other very well, and the prime component that synthesizes most of the parameters data can be used as the general index to reflect the trends of rheological properties of the pear during the shelf life.The study of the deformation degree, test speed on TPA test in pear flesh shows that the impact of the deformation degree on TPA parameters is significant while the test speed insignificant. Since the measurement of texture is on the basis of destructive deformation, a 75% deformation degree was selected in the TPA tests, under which the characters of the test curves are significant. The tests on "Dangshan crisp pear" during shelf life through destructive methods such as TPA, puncture, WBS show that the parameters such as hardness, brittleness, elasticity, chewiness, resilience, cohesiveness of the flesh share the same descending trend. Crispness is the trait of "Dangshan crisp pear", which can be seen from WBS shear curves: the parameters such as the number of peaks, average shear force, Ws are extracted from the WBS curves, and the test curves of WBS are smoothed by Least Squares Method using Matlab, and then the lengths of test curve and smooth curve are obtained by integral method, ratio of which is used as the measure of brittleness. The results show that these methods work well on depicting the brittleness.