Alternative tests for the mechanical properties of fruit : a thesis presented in partial fulfillment of the requirements for Master Degree in Agricultural Science (Agricultural Engineering) at Massey University
A newly patented twist test for studying the mechanical properties of fruit is described. This test measured the force required to rotate a small rectangular blade inserted into a fruit and was expressed as the twist strength of the fruit being tested. In testing Granny Smith apples during cool storage, the test was highly correlated (R2 of 0.965) with storage time and comparable with the penetrometer test (R2 of 0.968). In testing Royal Gala and Gala apples, the twist test was more sensitive in distinguishing the apples of different degrees of maturity than the penetrometer test and the Brix test. The twist test was more highly correlated with harvesting dates based on colour for Royal Gala and Gravenstein apples, compared with the penetrometer test and the Brix tests, suggesting that this test could be used and was more reliable as a means of assessing apple maturity. Utilised during kiwifruit maturation, the twist test was more highly correlated with soluble solid contents (SSC) measured with the Brix tester (R2 of 0.979), compared with the penetrometer test (R2 of 0.858), suggesting that this test could also be used as a means of predicting kiwifruit maturity. During cool storage of kiwifruit, the relationship between the twist (crushing) strength and storage time produced a higher R2 value than those between the penetrometer reading and storage time, or between SSC and storage time. The twist test is accurate, easy, fast and flexible and may be used on a wide variety of fruit to assist in the determination of quality and maturity.