Prediction and quantification of apple bruising : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Agricultural Engineering at Massey University, Palmerston North, New Zealand
Mechanical handling subjects fruit to impacts which often cause bruising. Such bruising is a major source of quality loss in the fruit industry. In this study, a range of experiments was carried out to investigate the quantification of bruises and the prediction of bruising in relation to mechanical handling systems. In order to understand apple bruising, a study of free normal impact between pairs of apples was conducted. There was a 2/5 power relationship between contact area and impact energy. The coefficient of restitution varied in a non-linear manner with impact energy, decreasing as impact energy increased. Bruise damage produced by a typical New Zealand-made fruit grader was critically analysed. A large percentage of individual bruises was under 1 cm2 in area and it was rare to have any bruises above 3 cm2. The total number of bruises was found to be the best indicator of bruise susceptibility. A new method of predicting such bruises has been developed involving a new term, the Bruise Factor, which was related to bruising sustained during handling operations, allowing for the variation in fruit size, shape and mass. An Instrumented Sphere (IS) was used to characterize impacts on commercial packing lines. It was found that the IS could be used to identify apple-to-apple impacts likely to cause bruising in commercial packing operations, providing care is taken with interpretation of the data. Typical impacts on packing lines were represented by impacts onto a flat steel surface, a rubber pad, a plastic tube, a solid plastic bar, and onto another fruit. Impact tests were conducted on freshly picked Gala, Splendour, Fuji, Braeburn, and Granny Smith apples, all grown in Hawkes Bay, New Zealand. Bruise areas produced by impact onto flat steel, rubber, plastic tubing, and a solid plastic bar were found to be linearly correlated with the peak acceleration recorded by an Instrumented Sphere dropped from the same heights. Following fruit-to-fruit impacts, bruising was generally more severe on one of the two apples. When the results of apple-to-apple and IS-to-apple impacts were compared, it was found that the area of the larger of the two bruises produced in fruit-to-fruit impacts was directly related to the peak acceleration recorded by the IS when it was dropped onto a fruit from the same height. For each variety and each surface the drop height required to produce a critical bruise with a surface area of 1 cm2 (as measured with the skin removed) was determined. By joining the threshold points on each surface response line, a threshold potential bruise boundary was formed on a velocity change against peak acceleration graph. The boundary curve, which included apple-to-apple impact, was hyperbolic in shape, rather than the linear boundary described in other studies. The implications of the results to the fruit industry are discussed in this study.