Assessment of the relationship between kiwifruit skin topography and its quality and storability using fringe projection : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand. EMBARGOED until further notice.

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Massey University
Embargoed until further notice
Figure 3-1 is reproduced with permission.
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Kiwifruit harvested from different growing locations tends to have variable fruit quality and storage performance due to many preharvest factors that contribute to fruit variation at harvest. This variability of fruit between batches makes the prediction of postharvest storage quality difficult, causing postharvest fruit losses. One of the preharvest factors that introduces fruit variability is the growing environment in which the fruit are exposed to. Fruit skin, a protective layer that covers the entire fruit, plays an important role in fruit development and is the first point of interaction with the surrounding environment. The objective of this research was to investigate a novel non-destructive technique that utilised at-harvest skin topography to link with fruit quality and storage performance of kiwifruit. The ‘G3’ (marketed as SunGoldᵀᴹ) kiwifruit cultivar was chosen for consideration in this thesis because it has distinctive skin properties with protruding lenticels and is a high-value cultivar that is of considerable importance to the New Zealand kiwifruit industry. The potential for fringe projection to extract skin physical properties in kiwifruit was demonstrated through surface roughness quantification and image analysis technique. Characterisation of lenticels on the surface of kiwifruit was achieved by developing an automated image processing algorithm. The knowledge of the skin properties of kiwifruit was revealed through a comparison of skin topography and cuticle compositions of different kiwifruit cultivars. Skin topography differences revealed genotype related diversity as well as the effect of environmental factors that fruit were exposed to. The most abundant cutin monomer composed mainly of C₁₈. Predominant cuticular waxes such as fatty acids and phenolics were identified. The knowledge of lenticel development was confirmed through monitoring the skin topography during fruit development and fruit bagging. Lenticel formation becomes visible from 45 DAFB and is dictated in the early stage of fruit growth before 77 DAFB. Lenticel properties are set and established before harvest. An orchard bagging experiment revealed that the difference in the growing environment modified the development of lenticels in kiwifruit. The lenticel coverage was positively correlated with the humidity condition that the fruit is exposed to during fruit development. Lenticel density and size at harvest had little influence on the water loss and storage performance of fruit. Lenticels were found to become a low resistance pathway for water loss if there is evidence of microcraking and splitting. The hypothesis of using at-harvest skin topography to predict the post-storage quality of kiwifruit was explored by developing a blackbox machine learning model. Unfortunately, both quantitative and qualitative predictions of soluble solids and flesh firmness in storage were not successful due to a low level of accuracy across models. The storability of fruit is affected by many factors, and improvements can be made to include additional information such as other non-destructive techniques to help in prediction. While skin topography using fringe projection may not be a good indicator of kiwifruit storability, the application is useful to characterise skin properties that are related to fruit quality. The work found that skin roughness generally increases after storage which is likely to be caused by shrivel development or skin scuffing. There is an opportunity to rapidly and reliably quantify skin defects. Another potential application for fringe projection is to use in a kiwifruit breeding program as a high-throughput phenotyping tool to capture the surface properties of different genotypes, enabling the identification of desirable traits.
Kiwifruit, Color, Quality, Storage, New Zealand, Lenticels