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    Influence of handling at harvest on the softening behaviour of kiwifruit : a thesis presented in partial fulfilment of the requirements for the degree of Master in Horticultural Science at Massey University, New Zealand
    (Massey University, 1992) Davie, Ivan John
    The New Zealand Kiwifruit industry in 1991 lost an estimated $54 million due to premature softening of kiwifruit during postharvest storage. The present study sought to determine if premature softening might be associated with the physical damage resulting from handling at harvest. The influence of physical damage on fruit was investigated on fruit from eleven kiwifruit properties from the Bay of Plenty region in New Zealand. Fruit were harvested and stored at 0°C and 20°C; firmness was destructively assessed. Softening behaviour of fruit sampled immediately after harvest from the vine (no physical damage) were compared with fruit from the same orchard block handled through the normal postharvest handling chain and packed in a packhouse. Analysis of variance and nonlinear regression using two, three and four parameter models were used to help in describing any differences in firmness values for fruit held in cool storage (0°C). The advantages and disadvantages of using analysis of variance and nonlinear regression to describe differences in firmness values between treatments are discussed. Analysis of variance determined that the packhouse and vine fruit on average were of a similar firmness. Nonlinear three parameter model: Firmness = a exp-bt + c (starting values: a = 6, b = 0.01 and c = 0.5) where: a = difference between initial and final asymptotic firmness b = exponent describing rate of decline in firmness c = final asymptotic value for fitted firmness was found to best characterise changing fruit firmness values over time. An analysis of variance was then performed on the resulting parameter values a, b and c which found that vine fruit on average had a slightly faster rate of softening than packhouse fruit. Packhouse fruit were not expected on average to have a similar firmness to vine fruit, as packhouse fruit were thought to have been exposed to potentially damaging impacts during handling. This may have been due to vine fruit being of a smaller size, position of fruit trays in cool storage or the rewarming of fruit during transportation. A non-destructive measure of firmness would help to identify the factors leading to premature softening and help to quantify fruit to fruit variability. A second part of this study therefore involved development and evaluation of a non-destructive instrument for measuring kiwifruit firmness (softness meter) compared with a penetrometer and its ability to repeatedly measure an individual fruit's firmness over time. The non-destructive softness meter characterised fruit firmness by measuring changes in deformation over time. Plots of deformation versus the natural log of time were linear and the gradient of the line was used as the measure of firmness (softness coefficient). Fruit with a range of firmness values were assessed using the softness meter, then penetrometer readings were obtained on the same location of each fruit and the relationship between the two instruments established. Within-fruit variation for both softness coefficients and penetrometer data was strongly related to fruit firmness, with coefficients of variation remaining approximately constant at about 10% for each variable. The softness meter will help to identify how localised treatments applied to fruit affect firmness and help to identify premature softening causes in individual fruit.
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    Evaluation of Massey twist tester for textural assessment of fruits and vegetables : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Agricultural Engineering at Massey University
    (Massey University, 2000) Tautakitaki, Tevita Pasinamu
    The Massey Twist Tester is an instrument developed to assess the texture of fresh fruit and vegetables rapidly. Since its original development in 1990, the Twist Tester has been modified extensively and numerous prototypes have been developed. In principle a small rectangular flat blade is rotated inside the fruit, and the torque required is measured. The current version incorporates a motor driven unit rotating inside a set of needles which hold the fruit firmly. Although measurements of fruit properties have been reported in previous studies, these have all been based on earlier designs, and no data on the new version of the Twist Tester have yet been published. The main aim of this study is to evaluate the performance of the new version of the Twist Tester by comparing it to the standard penetrometer, which has been widely used in many parts of the world for several years. Samples of fruit and vegetables were stored in different conditions to vary the level of firmness in order to expose how well each instrument performed in detecting the changes of textural properties. Generally, both Twist Tester and Penetrometer readings declined with storage time. In the testing of Braeburn apples, the Twist Tester has highly correlated with storage time as compared to penetrometer, The Twist Tester and Texture Analyser produced results for the Royal Gala apples which were highly correlated with those obtained from the penetrometer, suggesting that this test could be used, as it is more reliable for determining the maturity of apples. For plums, the correlation of the Twister with storage time at three storage conditions were high (r =0.92, 0.95 and 0.92), compared to the correlated of penetrometer with storage time which was (r = 0.83, 0.44 and 0.77). The penetrometer has a slightly higher degree of correlation with storage time for pears, compared to the Twist Tester. Pears declined in crushing strength and penetrometer readings with storage time, but over the last 7 days the value of firmness increased. The literature review showed that when water loss from the fruit is extreme, it forms a rubbery texture, produces a higher degree of firmness. Further research work would need to be done to obtain a more reliable result. The Twist Tester performed well in predicting the changes of textural properties of nashi, which showed a stronger correlation with storage time than the penetrometer relationship with storage time. During storage of kiwifruit, the penetrometer could not detect any changes after 14 days, while Twist Tester obtained a reliable result. This showed that penetrometer could not test the firmness of texture of any soft fruits. The relationship between the crushing strength and storage time produced a high coefficient in all three storage conditions (r = 0.91, 0.89, 0.80) while the penetrometer readings showed the following correlations with storage time (r =0.77, 0.76, 0.44). Thus the Twist Tester can determine the maturity of kiwifruit as well as any soft tissue products. Changes in the textural properties of potatoes also were well detected by the Twist Tester, which showed a stronger correlation with storage time than did the penetrometer. Firmness and crispness as measured by both the Twist Tester and penetrometer readings were highly correlated, while other variables showed only a poor relationship with instrumental measurement. Further research is needed to improve these results by using a well-trained taste panel. Changing the speed of Twist Test has no significant effects on the crushing strength of fruit and vegetables within the range of 5-10 rpm. The Twist Tester is more accurate, easy to operate and may be used to determine the quality and maturity of a wider range of products than penetrometer.