Masters Degrees (Viticulture and Oenology)

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Browsing Masters Degrees (Viticulture and Oenology) by browse.metadata.advisor "Buica, Astrid"

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    Chemical evaluation and sensory relevance of thiols in South African Chenin Blanc wines
    (Stellenbosch : Stellenbosch University, 2017-03) Wilson, Christine Leigh; Buica, Astrid; Brand, Jeanne; Du Toit, Wessel J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: South African Chenin Blanc is gaining recognition for its high quality both domestically and abroad. As the most widely-planted cultivar in the country, there is interest in research which can provide additional knowledge to producers and further increase Chenin Blanc wine quality. One of the sensory modalities contributing to wine quality is wine aroma, which is studied through sensory analysis and the chemical quantification of volatile compounds. Commercially-available South African Chenin Blanc wines had been characterized previously for a variety of chemical compounds, but not for thiols. Thiols, including 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA), are volatile sulphur compounds which are important to the ‘tropical’ and ‘green’ aromas of many wines, especially Sauvignon Blanc. The main aims of this research were to chemically characterize 3MH and 3MHA levels in a variety of commercially-available dry South African Chenin Blanc wines and explore the sensory contribution of these compounds to Chenin Blanc wine aroma. Chapter 3 reported the chemical analysis results of 3MH and 3MHA in South African Chenin Blanc Wines and explored trends within the chemical results. Chapters 4, 5, and 6 addressed the sensory relevance of thiols to South African Chenin Blanc wines. In Chapter 3, both 3MH and 3MHA were quantified in South African Chenin Blanc wines at levels above their odour thresholds. The average levels found were 893 ng/L for 3MH and 23 ng/L for 3MHA, with ranges of 380-2929 ng/L for 3MH and 0-305 ng/L for 3MHA. Significant differences were found for 3MHA levels by wine age, vine age, wood contact, price, and lees contact were found, while 3MH only differed significantly for wine origin. In Chapters 4 and 5, the sensory contribution of thiols was analysed through interaction studies. In Chapter 4, interactions of a thiol (3MH), an ester (ethyl hexanoate), and a terpene (linalool) in partially-dearomatized Chenin Blanc wine were analysed by descriptive analysis. Interaction effects were identified, such as the antagonism between the ‘tropical’ attributes of 3MH and the ‘floral’ character of linalool. The second interaction experiment, reported in Chapter 5, analysed combinations of 3MH and 3MHA in different matrices by projective mapping (PM) with intensity. This study showed that the perception of thiols was affected by the volatile and non-volatile wine matrix. The addition of an intensity measure to the ultra flash profiling step of the method provided more detailed data, which made the rapid sensory method better suited to interaction studies. In all sensory studies, wines with high thiols, especially high 3MHA, were described with ‘tropical’ and ‘green’ terms In Chapter 6, polarized projective mapping (PPM) was used to characterize commercial South African Chenin Blanc wine aroma, and sensory results were compared with extensive volatile chemical analyses. Results showed a sensorial and chemical opposition between wooded and unwooded wines. The levels of 3MHA in the wines correlated with the unwooded wines and thiol-related descriptors. PPM was applied for the first time to wine, validating a method which increases the maximum sample size of wines in rapid sensory analysis. The results of this research made contributions to the sensorial and chemical characterization of South African Chenin Blanc wines, as well as the validation of PPM and PM with intensity in wine. The knowledge that thiols are present in Chenin Blanc wines, together with existing research on practices affecting thiols can help inform viticultural and oenological decisions in the future of Chenin Blanc winemaking.
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    Development and change that occurs in table grape berry composition during growth
    (Stellenbosch : Stellenbosch University, 2015-03) Sonnekus, Nastassja; Raath, Pieter J.; Buica, Astrid; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Grape quality is important for the producer, exporter and the consumer. Consumers judge table grapes according to their size, colour, taste and shelf life. The consumer’s prerequisites will influence the producer. Therefore, it is essential to know how the table grape berry develops so that it can be manipulated, favouring the postharvest quality and shelf life. This study was performed on Prime and Crimson Seedless, both grafted onto Ramsey, in the Paarl district of South Africa. The aim of this study was to describe and quantify table grape berry development and compositional changes taking place throughout growth and ripening. The effect of sugar:acid ratio on postharvest shelf life was also evaluated. To evaluate whether berry size influenced the measured development parameters, three berry sizes were induced for both cultivars by using plant bioregulators such as gibberellic acid (GA3) and forchlorfenuron – synthetic cytokinin (CPPU) or girdling. The following sizes were obtained for Prime: (i) small berries (<20 mm) with no treatment, which acted as the control; (ii) medium berries (20-24 mm) obtained by 15 ppm GA3 application at 8 mm berry size; (iii) large berries (>24 mm) obtained by combination of 15 ppm GA3 and 1 ppm CPPU application at 8 mm berry size. Crimson Seedless berry sizes were as follows: (i) small berries (<18 mm) with no treatment, which acted as the control; (ii) medium berries (18-22 mm) treated with 10 ppm GA3 at 7 mm berry size; (iii) large berries (>22 mm) treated with 10 ppm GA3 and vines were girdled at 7 mm berry size. To evaluate the effect of sugar:acid ratio on postharvest shelf life, grapes were stored for five weeks at -0.5 ˚C and another week at 7.5˚C. The bunches were evaluated for loose berries, browning, soft tissue breakdown, decay and berry split. The following components were analysed for both cultivars to determine changes in berry composition throughout the season: berry fresh weight, total soluble solids (TSS), glucose, fructose, titratable acidity (TA), tartaric acid, malic acid, abscisic acid (ABA) and total phenols. Total and individual anthocyanins were analysed for Crimson Seedless. Differences were obtained for the three berry sizes for both cultivars. Véraison, representing the start of ripening, started at the same time in successive seasons: 21 days after pea size berry (5 mm berry diameter) for Prime and 28 days after pea size berry (5 mm berry diameter) for Crimson Seedless. A lag stage was not observed, at seven day sampling intervals, for either of the cultivars. Components such as TSS, glucose, fructose and TA content per berry were influenced by berry size in either one or in both seasons for both cultivars. Significant changes in component concentration were detected at the start of, or around véraison. Sugar concentrations (TSS) already started to increase for both cultivars before the start of véraison. At véraison, concentrations of glucose, fructose and ABA increased while concentrations of TA, tartaric acid, malic acid and total phenols decreased. Total anthocyanins in Crimson Seedless started to increase one week after véraison commenced. The main anthocyanin found in Crimson Seedless was peonidin-3-glucoside. During ripening a 1:1 glucose:fructose ratio was detected in both cultivars. Prime tartaric:malic acid ratio was lower than Crimson Seedless tartaric:malic acid ratio in both seasons. Tartaric acid was the main organic acid found in Prime, while malic acid was the main organic acid found in Crimson Seedless. No significant differences were found in the postharvest defects between the different berry sizes. However, tendencies for differences were observed which led to the assumption that medium size berries were more prone to loose berries in both cultivars. Large berries showed a higher percentage berry split for both cultivars. Crimson Seedless second harvest date took place 24 hours after rainfall which could have very likely led to the higher percentages berry defects compared to the first season. Greater berry decay was found with later harvest dates for both cultivars. No significant differences were found for the TSS:TA ratio between the three berry sizes for both cultivars. Postharvest defects were therefore found not only to be influenced by TSS:TA ratio but rather by harvest date and packing procedures. Environmental conditions prior to harvest also had an impact on postharvest shelf life.
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    Oenological evaluation of Chenin Blanc wines elaborated from different trellising systems
    (Stellenbosch : Stellenbosch University, 2019-12) Iipinge, Hilaria; Buica, Astrid; Panzeri, Valeria; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Grape composition plays a significant role in defining wine style and typicality; and there are ways in which grape composition can be altered or modified. Among these ways are viticultural practices which include trellising systems. Trellising systems are driving mechanisms that alter canopy microclimates, consequently affecting yield and the quality of grapes. Due to the fact that most grape primary metabolites contribute to the production of secondary compounds in final wines, it is important to assess if changes in canopy microclimates induced by trellising systems can reflect in final wines. Therefore, the aim of this study was to characterise (chemically and sensorially) grape must and wines of Chenin Blanc grapes made from grapes of different trellising systems. Basic oenological parameters (including Brix, pH, TA and alcohol) were measured for grape juice from harvest and then throughout the winemaking process. Sugar level varied in the first season with a significant difference between systems, but no significant differences were observed in other parameters. The other important factor evaluated was yield, which had differences between systems; mostly, open canopies by horizontal division produced higher yield than vertical dividing canopies and closed non-dividing canopy systems. However, those differences were not statistically significant between systems. YAN, ammonia, total (FAN) and individual amino acids in musts and major volatiles and thiols were subsequently measured. The concentrations were above critical levels for YAN. No significant differences were found across all the treatments, when the data were subjected to analysis of variance, and even by multivariate analysis no distinctive groupings were formed. However, the additional fingerprint of wine samples by high-resolution mass spectrometry produced groupings of samples according to trellising systems. In Chapter 3, the wines of the six different trellising systems were profiled using a rapid descriptive method (CATA), using both analytical and expert panellists; further, wine overall quality was evaluated by experts. Wine samples could not be differentiated by aroma according to trellising systems. On the other hand, taste and mouthfeel profiles implied that there were differences between wine samples according to trellising systems. Additionally, the overall wine quality rating showed significant differences between wines of according to trellising systems, mostly contributed by significant differences in taste and aroma ratings. In Chapter 4, a detailed discussion of results from Chapters 2 and 3 is presented by comparing sensory profiles with chemical composition by correspondence analysis and principal component analysis respectively. Aroma compounds showed no significant differences between trellising systems (Chapter 2). Aroma description profiles from the correspondence analysis (CA) biplot (Chapter 3), illustrated that aroma profiles of the wines were similar since no clear groupings observed., The CA score plot from taste and mouthfeel results produced a similar configuration pattern with the PCA score plot from the high resolution mass spectrometry (HRMS) data, which indicated that the trellis system may have an influence on chemical aspects also related to the wines’ taste perception. The results of this research contribute to information that winemakers or growers may require regarding decisions they can make in relation to choosing an appropriate trellising systems. However, other factors such as cultivar, climate, vintage, and economic advantage should not be disregarded.
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    Study of Durif (Vitis vinifera L.) berry ripening phases in relationship to wine styles
    (Stellenbosch : Stellenbosch University, 2016-03) Wiid, Stephanie Cezanne; Deloire, Alain; Buica, Astrid; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: The wines of Durif (Petite Sirah) are known for their exceptionally dark colour and astringency (mouth feel). The goal of this research project was to assess Durif wine styles, profiles and diversity using sequential harvest. Grapes were harvested from a Durif vineyard in Paarl, South Africa, at different stages of ripeness on different dates (sequential harvesting) influencing the grape composition, which, in turn, influences wine composition and sensory attributes. There is not only one optimal harvest date for a vineyard or specific site and several harvest dates could be adequate. This research project spanned the 2012 and 2013 vintages. The 2012 was used as a diagnostic year to characterize the vineyard plot and the options for harvest timing. Sequential harvesting was used to determine if Durif follows a berry aromatic evolution (Deloire, 2011). The berry aromatic evolution describes wine styles, which can be achieved by determining harvest time/period using sugar loading as a berry physiological indicator. These wine styles include a fresh fruit (FF) style, a neutral, spicy or pre‐mature fruit style, a mature fruit (MF) style and later an over ripe style (OR). One of the major findings of this research project was calibrating the model of berry sugar accumulation for Durif to determine picking windows/dates for specific wine styles. The wines made from sequentially harvested grapes were sensorially assessed using the citation frequency method (Campo et al., 2010).The sensory analysis was used to find the sensory attributes of wines from two harvest time points corresponding to fresh fruit and mature fruit style wines. The main aromatic attributes associated with Durif wines in South Africa were identified. Descriptive analysis was used to rate the intensity of astringency, bitterness, sweetness, sourness and alcohol perception of the wines and how it changes with harvest timing Berry composition was analysed (fructose, glucose, tartaric acid, malic acid, total anthocyanins and total phenolics) and it was shown that there were significant changes in berry composition during ripening. The evolution of berry composition during the ripening period highlights the importance of optimization of the picking date. The wine composition, however, is not directly related to grape composition as it is compounded by the impact of extraction during fermentation (with increasing levels of ethanol with increasing sugar levels when using sequential harvesting) on tannin and anthocyanin extractability and yeast metabolism (Gil et al., 2015, Fournand et al., 2006). Must composition (pH, TA, Brix) was monitored and showed significant differences between early and later harvest dates. Wine composition of each sequentially harvested wine was analysed. From the trends observed in the frequency of citation method it is suggested that the berry aromatic ripening sequence for Durif exists and fresh or mature fruit style wines can be made. Harvest timing could be defined as picking ten days after the keypoint to produce fresh fruit style wines and twentythree days to produce mature fruit style wines according to the results of the 2013 vintage.