Doctoral Degrees (Viticulture and Oenology)

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Browsing Doctoral Degrees (Viticulture and Oenology) by Author "Gao, Yu"

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    Parameters involved in the enzymatic deconstruction of the Wine Grape cell wall matrix during winemaking
    (2016-03) Gao, Yu; Moore, John P.; Vivier, Melane A.; Stellenbosch University. Faculty of AgriSciences. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Commercial enzyme preparations, consisting of pectinases and glucanases, are commonly used in the wine industry to; (1) enhance the extraction of beneficial compounds (e.g. anthocyanins, tannins, aroma etc.) from the grape berry, (2) to facilitate the clarification of the wine before the filtration, and (3) to maximize juice yield for alcoholic fermentation. All three factors are important in improving quality parameters (e.g. body, aroma, ageing potential) and market value for winemakers. The efficacy of any enzymes, including wine enzymes is dependent on their ability to target specific glycan linkages for catalysis, and thereby deconstruct polymeric architecture. However, in the case of grapes, very little information is known on grape berry cell walls, that can assist in a rational enzyme mixture design strategy for customizable applications (e.g. enhanced colour). This is partly due to the complex nature of fruit cell walls, factors such as grape maturity, inter‐vineyard variability in ripeness levels at harvest, potential cultivar differences and the crude nature of enzyme mixtures used by the wine industry. New tools are needed to understand cell wall architecture, and here the use of Carbohydrate Microarray Polymer Profiling (CoMPP) technology, which combines glycan microarrays with sets of characterised monoclonal antibodies (mAbs) and carbohydrate binding modules (CBMs) for plant cell wall polymer epitopes detection. CoMPP is extensively used with multivariate data analysis methods in this thesis to evaluate the complex nature of the grape cell walls and how enzymes are able to deconstruct them under winemaking conditions. This study consists of three parts focusing on a Cabernet Sauvignon vineyard: (1) The validation of cell wall profiling tools combined with fractionation approaches on berry pomace cell walls isolated from wine fermentations. This is to confirm the utility of the approach in the winemaking conditions, and provide a baseline reference dataset for later studies. The study revealed for that grape pomace consists of two main fractions: (i) a partially methyl‐esterified pectin‐rich outer layer and (ii) a highly methyl‐esterified pectin‐rich layer which ‘coats’ an inner layer of xyloglucan‐cellulose rich skin cells. (2) A study focusing on the inter‐vineyard variation of grape berry cell walls collected at harvest using a panel based sampling design. This was combined with how commercial enzyme preparations influenced the potential inter‐vineyard variation in harvested grape cell walls that were brought into the winemaking process. Here the study showed that the enzymes performed efficient de‐pectination, without de‐esterification, and significantly reduced cell wall variability in pomace and wines from treated panels. (3) In order to study the grape berry cell wall deconstruction process in more detail, the use of pure recombinant enzymes (singly and in combinations), were used in an extensive winemaking study for the first time that we are aware of. The use of cell wall profiling tools revealed: (i) that pectin lyase appears to be a core enzyme in probably many enzyme preparations, because it results in effective de‐pectination without de‐esterification (ii) other enzyme combinations that combine only endo‐polygalacturonase activity with de‐esterification appear to only unravel the walls and not degrade effectively. This study, plus the contexts provided by the first two, permitted us to develop a hypothetical model which illustrates the cell wall structures in different tissue layers of the grape berry. This new grape berry cell wall model apart from extending our scientific knowledge in this area, will now permit hypothesis testing in, fruit development, plant‐pathogen interactions, as well as helping designing the tailored enzymes for use in different winemaking scenarios.