Masters Degrees (Viticulture and Oenology)

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Browsing Masters Degrees (Viticulture and Oenology) by Subject "Antioxidants"

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    Evaluating the desorption of oxygen from wine
    (Stellenbosch : Stellenbosch University, 2022-04) Sutton, Steven; Du Toit, Wessel J.; Pott, Robert William M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: The removal of dissolved oxygen through desorption is commonly done in winemaking. Winemakers have indicated that under the same conditions, this process takes place at different rates, for different wines. The mass transfer of oxygen in six wines and various model wine solutions, was examined by evaluating the oxygen desorption volumetric mass transfer coefficient (๐‘˜๐ฟ ๐‘Ž), the Sauter mean bubble diameter (D32), gas holdup (๐œ€), the interfacial area (๐‘Ž), and the oxygen mass transfer coefficient (๐‘˜๐ฟ). One of the wines used was split into two batches, with one half treated with bentonite, and the other not. A bubble column with a stone sparger was used for the experiments. The gassing out procedure and a 2nd order model was used to determine ๐‘˜๐ฟ๐‘Ž. Bubble imaging was done to determine the interfacial area, and subsequently the ๐‘˜๐ฟ. During oxygen desorption within wine, the ๐‘˜๐ฟ๐‘Ž values varied between 0.0125 s-1 and 0.0275 s-1 depending on the wine. The ๐‘˜๐ฟ ๐‘Ž value during oxygen desorption within a 10 % ethanol solution was found to be 0.0275 s1. The addition of a small amount glycerol to this system reduced the ๐‘˜๐ฟ๐‘Ž to 0.0225 s-1. Further additions of organic acids did not affect the ๐‘˜๐ฟ๐‘Ž, while the addition of protein in the form of BSA and yeast extract reduced the ๐‘˜๐ฟ๐‘Ž to approximately 0.0175 s-1. The ๐‘˜๐ฟ ๐‘Ž for during oxygen desorption within a wine that was protein unstable improved from 0.009 s1 to 0.015 s1 after being treated with bentonite. During desorption, there were no significant variations in the D32 and the interfacial area between systems containing wines or model wine solutions. Consequently, the variations between the ๐‘˜๐ฟ๐‘Ž values could all be ascribed to differences in the ๐‘˜๐ฟ. During oxygen desorption, the ๐‘˜๐ฟ values were found to be between 0.015 and 0.045 mm/s within the different wines. The ๐‘˜๐ฟ values were found to be between 0.03 and 0.04 mm/s within a 10% ethanol solution, and within the model wine solutions containing glycerol and organic acids. The ๐‘˜๐ฟ values dropped between 0.02 and 0.03 mm/s with the addition of protein to the model wine solution. Treating a protein unstable wine with bentonite increased the ๐‘˜๐ฟ value from 0.017 mm/s to 0.0225 mm/s. The combination of the reduction in the ๐‘˜๐ฟ when protein was added to a model wine solution, and the improvement of the ๐‘˜๐ฟ when wine was treated with bentonite, suggested that proteins in wine significantly affect oxygen desorption rates. It is suggested that winemakers can improve the oxygen ๐‘˜๐ฟ within their system by operating at higher gas flowrates to increase the turbulence during desorption. However, the most effective way of improving the desorption rate is by using a sparger that produces smaller bubbles, so as to increase the interfacial area. It is suggested that desorption is performed after fining, as the ๐‘˜๐ฟ will be greater.