Doctoral Degrees (Horticulture)
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Browsing Doctoral Degrees (Horticulture) by browse.metadata.advisor "Crouch, Elke"
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- ItemRole of cell membrane integrity and oxidation stress in development of external browning of white seedless table grapes (Vitis vinifera L.)(Stellenbosch : Stellenbosch University, 2023-03) Botes, Wilhelmina Jacoba; Jooste, Mariana; Crouch, Elke; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: Berry skin browning is an important quality problem in white seedless table grapes - it negatively affects the cosmetic appearance, causes reduced market value, financial losses and restricted market access. In a high browning incidence season the accumulation of saturated fatty acids (SFAs) and high levels of total sterols were the driving factors for high browning incidence at harvest in ‘Regal Seedless’ (highly susceptible to external browning). The higher unsaturated:saturated phospholipid fatty acid (FA) ratio, monounsaturated:polyunsaturated fatty acid (MUFA:PUFA) ratio and higher ascorbic acid levels explained the lower browning levels in ‘Regal Seedless’ in the low browning incidence season. ‘Thompson Seedless’ is less susceptible to external browning than ‘Regal Seedless’ due to its high glutathione levels. The higher levels of MUFAs, total sterols and antioxidants resulted in lower browning incidence for ‘Thompson Seedless’ between seasons. Variables determined in grapes, harvested at two maturities and stored at an intermittent warming (IW) regime, at -0.5 °C (control) and 4 °C could explain the postharvest variance in external browning between a high and low browning incidence season in ‘Regal Seedless’. The IW regime effected the lowest browning incidence in both harvest maturities after 6 weeks of cold storage, because it caused higher levels of antioxidants and MUFAs than PUFAs in the berry skins. Storing grapes at 4 °C for 6 weeks resulted in high external browning incidence, especially in more mature grapes in the high browning incidence season, because it could not maintain the antioxidant levels and caused a low MUFA:PUFA ratio in the membranes. The -0.5 °C regime (control) caused high browning levels in especially the less mature grapes in the low incidence season (due to the ratios of the SFAs, MUFAs, PUFAs being less optimal in the less mature compared to the more mature grapes), but in both harvest maturities in the high incidence season (due to the lower MUFA levels and MUFA:PUFA ratio, and higher SFA concentrations in the cell membranes). The variables determined in this study could not discriminate between the different browning types, but could successfully separate the green and brown berry skins of the two harvest maturities. Therefore, although the external browning types look visually different on the berry skin, there is not a difference in the mechanism of how each develops on a cellular level. The variables that discriminated between green and brown berry skins differed between the low and high browning season. In a low browning season, berries of both harvest maturities developed the same level of external browning after storage. The brown berry skins had higher levels of total sterols and total PUFAs, and lower ascorbic acid than the green berry skins. In the high browning season both maturities had the same percentage of external browning after 6 weeks of storage at -0.5 °C. The brown berries had higher oxidized glutathione and malondialdehyde levels, higher total sterol:total phospholipid ratio and %SFAs than the green berries. This result indicates that berries on the same bunch differ in maturity and that this might be the reason for their differing susceptibilities to develop browning pre- and postharvest. It is also confirmed that external browning is initiated pre- and not postharvest. Future work should, therefore, focus on preharvest factors and vineyard manipulations that will ensure grape bunches of more uniform maturity. Postharvest, different IW regimes and/or preconditioning at higher temperatures before cold storage could be tested in combination with lower dose SO2 sheets, to prevent the development of external browning during cold storage.