Masters Degrees (Viticulture and Oenology)

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Browsing Masters Degrees (Viticulture and Oenology) by browse.metadata.advisor "Brand, Jeanne"

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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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    Role of varietal thiols in South African Colombard wines
    (Stellenbosch : Stellenbosch University, 2019-04) Van Rooyen, Rianco; Du Toit, Wessel J.; Brand, Jeanne; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Colombard is the second most planted wine grape in South Africa and research that provides insight into and additional knowledge on Colombard may prove particularly useful in the production of quality wine from this cultivar. One of the most important quality drivers is wine aroma. The aroma of a wine may be studied through the quantification of certain compounds, as well as through sensory analysis. There are limited studies that have focused on quantifying the compounds responsible for the aroma of Colombard wine. Varietal thiols are sulphur-containing compounds and include 3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA) and 4-mercapto-4-methylpentan-2-one (4MMP). These varietal thiols are normally present at extremely low levels in wine and have been known to contribute to the aromas of ‘passion fruit’, ‘guava’ and other ‘fruity aromas’ in Sauvignon Blanc and Chenin Blanc wines. The aims of this research were to determine whether certain viticultural and oenological practices may influence varietal thiol levels in Colombard wine and to explore the sensory contribution of varietal thiols to the aroma of South African Colombard wine. The effects of different vineyards, yeast strains and juice treatments on varietal thiols levels in Colombard wine were investigated. A classification of commercial wines in terms of the effect of different winemaking practices on varietal thiol levels was also undertaken. The sensory contribution of varietal thiols to the aroma of Colombard wines was also assessed. The origin of the grapes, as well as the choice of yeast strain, significantly influenced 3MH levels, while the latter also had a significant effect on 3MHA levels. Certain juice treatments were found to lead to large increases in 3MH and 3MHA levels. Varietal thiols in commercial Colombard wines were comparable to those reported in young South African Sauvignon Blanc and Chenin Blanc wines. Significant differences in varietal thiol levels were found for the origin of the grapes (3MH), vineyard yield (4MMP), method of juice clarification (3MH), and the bulk price point of the wine (3MH and 3MHA). Varietal thiols seem to play an important role in the ‘guava’ and ‘passion fruit’ aromas in South African Colombard wines. The results of this research have made a contribution to the sensorial characterisation of Colombard wines. With this new knowledge, viticultural and oenological decisions may be adapted to produce market-specific Colombard wines.
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    Sensory profiling, quality evaluation, and chemical profiling of South African de-alcoholised still wines
    (Stellenbosch : Stellenbosch University, 2023-03) Qwebani, Sinazo; Nieuwoudt, Helene; Brand, Jeanne; Pentz, Chris; Muller, Magdalena; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: A relatively new category of wines with <0.5% v/v ethanol produced through physical removal of ethanol is in demand by many health-conscious consumers. However, this physical intervention has an impact on the wines’ sensory profiles. Accordingly, there is an interest in understanding this category’s sensory aspects and chemical composition. The aim of this study was to investigate the sensory profiles and quality of South African (SA) de-alcoholised still wines, and to do chemical profiling, with focus on volatile aroma compounds, of the wines. The sensory profiles of a set of commercial de-alcoholised still wines (n=16) were generated by the free listing (FL) method. Wine industry professionals (n=52) from seven different wine cellars participated were asked to freely list the sensory descriptors they associated with the wines upon tasting. For quality evaluation, the same panel was required to score the wines’ perceived quality using a 20-point scale. The sensory modalities evaluated for quality were appearance (max=3), nose (max=7), and palate (max=10) added up to a score of 20. In addition, to obtain a chemical profile, chemical compounds of a wine set (n=14) were quantified. However, data processing was required for the text generated from the free listing sensory evaluation method. For instance, free listing generated 11627 words which were standardised to 3846 descriptions after removal of non-sensory descriptions, repetitions of descriptions and comparisons. Thereafter, lemmatisation was done where descriptors were reduced to their roots and grouped into descriptor categories. The established sensory profile consisted of positive, neutral, and negative descriptor categories. The results of the study reveal that negative sensory descriptors were cited many times compared to positive characteristics: “off-odours”, “lack of aroma”, “lacks flavour”, and “watery”. However, it is important to note that there were positive descriptors: “good aroma” and “good palate”. Even though, “fruit” a neutral descriptor was the most cited which might have contributed positively to the sensory quality of these wines. In overall these results lack cultivar specific descriptions and thus wine authenticity which indicates that the complex aroma and flavour profile of full-strength white, rosé and red wines can be lowered drastically due to the process of de-alcoholisation. These sensory descriptors were then correlated with quality scores attained using the 20-point scale scoring system in which some of the wines received very low scores. As a result, the average of the overall scores was 13 out 20 which is explained as “borderline faulty”, “unbalanced wine”. In addition, the concentrations of the wines under study had 4 folds less of esters, 17 folds less of higher alcohols, and 3 folds less of fatty acids when compared to published ideal wines values, depicting poor wine quality. This pioneering study has established the sensory, quality, and chemical profile of this wine for the improvement of this category. As a result, major concerns both in sensory quality and the chemical profile of this new category have been successfully identified and insights into the way forward have been recommended to the wine industry and future studies.
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    The use of different oak products during the fermentation and ageing of Chenin Blanc : sensory properties, perceived quality, and consumer preference
    (Stellenbosch : Stellenbosch University, 2015-12) Botha, Anri; Du Toit, Wessel J.; Brand, Jeanne; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Wooden barrels have been the preferred method for oak maturation for wines, but the use of alternative oak products have increased options of oak maturation for winemakers, since new barrels are expensive and have a limited lifetime. The main aim of this study was to investigate the effect of different oak products used during fermentation and ageing on the sensory profile, degree of liking and perceived quality of Chenin blanc wine. The different wine treatments included an unoaked tank control wine, wines matured in 5th fill barrels, wines matured in new barrels from three different cooperages, and wines matured in 5th fill barrels with stave inserts from two different cooperages. All of the treatments, except for the unoaked tank control, were performed in triplicate, rendering a total of 19 experimental wines. Sensory descriptive data were obtained with Pick-K attributes, using a trained and expert panel at three different intervals of wine maturation. These intervals included 4 months oak maturation, 9 months oak maturation, and 9 months oak maturation with an additional 6 months bottle ageing. At the third maturation interval, an untargeted consumer study was also conducted using CATA questions to obtain descriptive and hedonic data. At this interval the perceived quality of the wines were also investigated using an expert panel. The expert panel received no formal training prior to any of the testing intervals. The trained panel were trained to recognise aromas from a list of 89 descriptors used by sensory scientists and industry experts to describe Chenin blanc wines. The results obtained from the different panels at the different ageing intervals were compared. The biological repeatability of the different treatments were satisfactory from a cooperage point of view. There were clear differences between the sensory profiles of the different wine treatments. Differences between the oak products from the different cooperages used within treatments were less pronounced. There were significant differences in the degree of liking and perceived quality of the different products, but the degree of liking and perceived quality for different products were comparable. The degree of liking and perceived quality of the 5th fill barrels and new barrels were higher than the degree of liking and perceived quality of the stave treatments. The mean degree of liking for the 5th fill barrels and new barrels were higher than for the stave treatments. There were no significant difference between the degree of liking for the unoaked tank wine and the oaked treatments. The sensory descriptive data obtained from the trained, expert and consumer panel were comparable in terms of panel consensus and configurational congruence. This validates the suitability of CATA and its variant, Pick-K attributes, as rapid methods to elicit qualitative information for the sensory profiling of wine. The results from this research made significant contributions towards method validation for CATA as rapid sensory method, and the descriptive and hedonic sensory results from this research can be used by winemakers to both inform and justify the usage of specific oak products.
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    Volatiles playing an important role in South African Sauvignon blanc wines
    (Stellenbosch : Stellenbosch University, 2013-03) Van Wyngaard, Elizma; Du Toit, Wessel J.; Brand, Jeanne; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Sauvignon blanc wines have become progressively more important in the commercial market. Extensive research is being done in various countries to gain more understanding about the aroma compounds found in Sauvignon blanc wines and the interactions between them. Sauvignon blanc wines often have either have a green or tropical style. The green style is caused by the methoxypyrazines while the volatile thiols are important contributing compounds to the tropical style. Various international studies have focussed on measuring the chemical composition of Sauvignon blanc wines. However, more research is required on South African Sauvignon blanc wines. Little is known of the volatile thiols content of South African Sauvignon blanc wines, although the methoxypyrazine content has been extensively reported on. Although methoxypyrazines and volatile thiols are seen as the most important aroma compounds contributing to Sauvignon blanc character, other compounds contribute as well. Esters, monoterpenes and phenols have been found to influence Sauvignon blanc aroma and interact with the methoxypyrazines and volatile thiols. The complex interaction between the compounds responsible for the aroma of Sauvignon blanc wines are still not fully understood and further research is thus needed. The first part of the current study investigated the interaction between a specific methoxypyrazine and volatile thiol. Five different concentrations of 2-isobutyl-3- methoxypyrazine (ibMP) and 3-mercaptohexan-1-ol (3MH) were spiked in dearomatize, neutral Sauvignon blanc wine. The single compounds as well as every possible combination of the range of concentrations were evaluated using sensory descriptive analysis. It was found using various statistical approaches that ibMP suppressed the tropical attributes associated with 3MH and that 3MH suppressed the green attributes that correlated with ibMP. The concentrations at which the suppression occurred and the degree of suppression was different for each attribute. The second part of the current study focussed on commercial South African Sauvignon blanc wines. Sensory descriptive analysis and chemical analysis were used to assess the wines and measure the volatile thiol and methoxypyrazine concentrations. The concentrations of volatile thiols and methoxypyrazines were found to be in line with international Sauvignon blanc wines. It was also shown for the first time that the mutually suppressive trend between the volatile thiols and methoxypyrazines can be seen in commercial Sauvignon blanc wines as well. Future work is needed to fully understand the complex interaction between the various compounds in Sauvignon blanc wines. Further research could focus on investigating the mechanism of interaction between the volatile thiols and methoxypyrazines as well as other aroma compounds.