Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) by Subject "Apple industry -- South Africa"

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    Browning and watercore disorders in 'Fuji' apples explored by means of X-ray computed tomography (CT)
    (Stellenbosch : Stellenbosch University, 2020-12) Chigwaya, Kenias; Crouch, E. M.; Crouch, Ian; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: The Fuji apple cultivar occupies 9% of land under apple production in South Africa. To ensure all year-round fruit availability, ‘Fuji’ apples are stored for extended durations. However, ‘Fuji’ apples are prone to internal browning (IB) during storage. IB is characterized by patches of brown flesh in the apple tissue, which makes fruit unmarketable and causes financial losses. Browning symptoms that have been identified in apple cultivars include radial browning, diffuse browning, combination browning, CO2 damage and core-flush. Techniques such as X-ray computed tomography (CT) that can evaluate IB disorders non-destructively are important. This study aimed to explore IB types and watercore in ‘Fuji’ apples quantitatively and qualitatively using X-ray CT. Exposure of fruit to high CO2 conditions for 3 days after harvest at 21 °C induced IB in the core region of fruit. The construction of porosity maps for intact fruit enabled characterization of tissue structure before and after disorder development. Porosity distribution of ‘Fuji’ was higher in the cortex region compared to the core region. High-resolution X-ray CT scans performed on IB affected and unaffected fruit tissue showed differences in microstructural properties such as porosity, pore size distribution and pore connectivity. Fruit size had a significant effect on the susceptibility of ‘Fuji’ apples to CO2 stress-induced IB. Radial porosity profiles did not differ significantly between fruit that developed IB and fruit that did not develop IB. However, porosity along the axial profile was generally higher for fruit that did not develop IB, particularly in the region between the calyx end and the core region. This was the first X-ray CT study carried out on South African ‘Fuji’ apples to evaluate how fruit microstructural properties relate to the IB types identified under different storage conditions. A further study was done to determine microstructural properties of watercore affected fruit tissue and the effects on storability of ‘Fuji’ apples. X-ray CT scans showed that fruit tissue with watercore had a significantly low porosity and connectivity of pores. This may have a negative impact on respiratory gas diffusion in the fruit and could increase susceptibility to IB during storage. Furthermore, watercore affected tissue had significantly smaller cells due to plasmolysis. For long-term storage experiments, it was found that regular atmosphere (RA) stored fruit had a significantly higher incidence of core-flush compared to fruit from controlled atmosphere (CA) and delayed controlled atmosphere (delayed CA) storage. Although CA and delayed CA were effective in reducing core-flush incidence, they both resulted in a significantly higher incidence of radial browning. Fruit with CO2 damage and cavities were also evaluated in this study. CO2 damage was associated with cell damage and increased pore sphericity. All IB types evaluated resulted in an increased tissue porosity and altering of pore sphericity, anisotropy and pore size distribution. IB after short-term exposure to CO2 stress occurred only in the core region while IB types observed after long-term storage occurred in all fruit tissue regions. This study provided unique insights into the microstructural properties of different IB types occurring in ‘Fuji’ apples.
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    The potential of dynamic controlled atmospheres and possible mechanisms in mitigating superficial scald in Apples cv. ‘Granny Smith’
    (Stellenbosch : Stellenbosch University, 2015-12) Mditshwa, Asanda; Opara, Umezuruike Linus; Crouch, E. M.; Vries, Filicity Ann; Van der Merwe, Jacobus; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: The development of a postharvest method for controlling superficial scald, maintaining quality and reducing postharvest losses of ‘Granny Smith’ apples is essential in maintaining the competitiveness of the South African apple industry. Previously, the South African apple industry relied on diphenylamine (DPA) for controlling scald disorder; however, increasing consumer concerns and reductions in maximum residue levels (MRLs) have highlighted the urgent need for alternative control strategies. Currently, there is no effective non-chemical method for controlling superficial scald for South African apple producers. The overall aims of this study were (a) to examine the potential of dynamic controlled atmospheres (DCA) in controlling superficial scald in apples, and (b) to investigate the mechanism of action of DCA in controlling scald, should it be effective. To get a deeper understanding of superficial scald etiology and physiological dynamics of apples, studies in paper 2 and 3 were conducted. In paper 2, studies on antioxidants contents and phytochemical properties of apples harvested at pre-optimal and optimal maturity were conducted. Significant increases in fruit antioxidant capacity and ascorbic acid concentration occurred with increasing maturity. Fruit harvested at optimal maturity had lower total phenolic contents compared to pre-optimal maturity. Phenolic compounds including catechin and quercetin were also higher in pre-optimal compared to optimal maturity. In paper 3, an attempt was made to classify apples with different levels of scald severity based on metabolomics analysis. The results showed that ethylene, α- farnesene, 6-methyl-5-hepten-2-one (MHO) and reactive oxygen species (ROS) increased with scald severity but declined in severely scalded fruit. Discriminant analysis successfully classified fruit based on scald severity. Ethylene, ROS and lipid peroxidation were identified as the major contributors in separating the five scald severity levels studied. Studies in paper 4 focused on whether DCA is effective in controlling superficial scald. The minimum period for the exposure of fruit to DCA before an extended shipment period of 10 weeks was also investigated. The results showed that DCA was highly effective in controlling scald for both pre-optimal and optimal harvested fruit. The results further demonstrated that DCA stored fruit can be shipped for 6 weeks; however, extending the shipping period up to 10 weeks might lead scald development and undesired fruit quality. Fruit stored in DCA before shipment generally had higher flesh firmness and ground colour. It was also shown that DCA inhibit scald by retarding the accumulation of scald-associated metabolites such as α-farnesene and MHO. Paper 5 focused on the impact of DCA on ROS, antioxidant capacity and phytochemical properties of stored apples. Using principal component analysis, two clusters which could be identified as DCA and RA stored fruit were noticed. Compared to RA stored fruit associated with higher ROS and lipid peroxidation, fruit stored at DCA was characterized by higher contents of ascorbic acid, total phenolics and antioxidant pool. The research reported in paper 6 investigated the efficacy of repeated application of DCA on apples with high scald potential. During the marketing season, an unexpected demand of fruit often leads to the opening and resealing of storage chambers. Thus, the efficacy of a repeated DCA treatment after an interruption period at RA was investigated. Fruit were stored for up to 16 weeks in DCA with a 14 d interruption in RA at -0.5 °C, 95% RH. The results showed that efficacy of DCA was not significantly affected by the interruption. However, the development of 1% scald after 4 months of storage could be an economic setback for fruit producers. In paper 7, the influence of DCA on aroma volatiles was assessed. DCA stored fruit had significantly lower total amount of volatiles detected compared to fruit stored in RA. Notably, the production of 1-butanol, 1-hexanol and 1-hexenol by fruit stored in DCA were only 42%, 38% and 39%, respectively, of the amounts detected in the RA. The known characteristic flavour of ‘Granny Smith’ apples was attributed to the production of ethyl-2-methylbutyrate, ethyl hexanoate and hexyl acetate. The contribution of these three aroma volatiles was higher with increasing storage duration. In paper 8, the research identified effective variables that could be used to develop prediction models for superficial scald incidence in harvested ‘Granny Smith’ apples. Stepwise multiple regression found MHO, antioxidant capacity (FRAP), ascorbic acid and lipid peroxidation to be the best combination of predictive variables for scald. After validation, this combination gave a good prediction of scald incidence (R2 = 0.94). The identified variables proved to be effective regardless of fruit maturity status. The results from this thesis provide an alternative non-chemical postharvest technology for the South African apple industry. The study further provides insights on the mechanism of action of DCA in controlling scald and maintaining fruit postharvest quality of ‘Granny Smith’ apples. Overall, the results contained in this thesis will be very instrumental in future optimisation of DCA technology in the apple industry, and provides a valuable guide for improved the storage of apples susceptible to superficial scald.