Masters Degrees (Viticulture and Oenology)

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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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    Bunch quality and fertility of Vitis vinifera L. cv. Prime as affected by gibberellic acid (GA3) and s-abscisic acid (s-ABA).
    (Stellenbosch : Stellenbosch University, 2022-12) Masikane, Nonkululeko Nosipho; Avenant, Eunice; Avenant, J. H. ; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Table grapes are one of the most cultivated, economically important fruit crops in the world and is one of the major fruit industries in South Africa. There is an increase competition in the market that leads to an increase production costs for South African table grape industry, to produce quality grapes that meet market requirements nationally and internationally. Market requirements for grapes are consumer driven, and are based on bunch structure, berry size, colour uniformity, flavour, texture, seedlessness and firmness. Plant growth regulators (PGRs) have become an important tool in producing grapes of high export quality, and to meet these market requirements. Gibberellic acid (GA3) is one of the registered plant growth regulators that is used as a chemical thinner in the grapevine, it is has been found to play a role in berry set, thinning and development. The effectiveness of PGRs dependent on cultivar, application time and concentration. This study aimed at establishing the effect of gibberellic acid (GA3) and abscisic acid (s-ABA) for thinning, combined with GA3 for berry sizing (applied at the recommended dosages, using different application volumes obtained with commercial spray equipment), on fertility of a 5-year-old Vitis vinifera cv. Prime block, grafted onto Ramsey rootstock on the farm Newgro at Kanoneiland in the Orange River Region. Four thinning treatments were combined with four berry sizing treatments, resulting in 16 treatment combinations. The thinning treatments comprised of a Control (No thinning); 1 ppm GA3 at 10% set and 3 days later; 1 ppm of GA3 x 5, with the first application at 10% set, followed by four more applications at 3-day intervals; and 1 ppm GA3 plus 400 ppm s-ABA at 10% set, followed by 1 ppm GA3 3 days later. All thinning treatments were applied at a volume of 1000 L.ha-1 with a Nobili 2000 T mist blower spray pump. The berry sizing treatments, applied at 7 to 8 mm and 8 to 10 mm berry diameter, respectively, comprised of a Control (Dipping) and three treatments applied by spraying. The same active ingredient dosage (2 x 20g GA3 ha⁻¹) was used for all treatments, however different application volumes were used for the spraying treatments (90 L ha-1 with an electrostatic spray pump (ESS); 250 L.ha-1 and 500 L.ha- 1 respectively with a Cima A T50S2.15.L11 mist blower spray pump. Bunches of all treatments obtained commercially acceptable bunch lengths (>20 cm) and Extra-Large berry size (> 18 mm in diameter). The control had the most compact bunches, but there was no significant difference between the three chemical thinning treatments, regarding bunch length, bunch compactness, berry size and berry size distribution. Therefore, producers could use any of these three chemical thinning treatments for Prime. There was a tendency that the largest berry size was obtained with the Dipping and ESS berry sizing treatments. Although there were significant differences, the percentage of extra small berries of all sizing treatments was very low (< 1) in both seasons. Potential fertility was assessed through forced budding in a glasshouse and bud dissections. Actual fertility was assessed in the vineyard. None of the thinning or sizing treatments negatively affected fertility. All treatments obtained commercially acceptable levels (> 80%) of bud break (ranging from 92 % to 97%) and fertility (more than one bunch per sprouted bud). Fertility was not affected by the GA3 treatments applied over three consecutive seasons, probably due to the dosage range used (1 to 5 g.ha-1 for thinning and 40 g.ha-1 for berry sizing). When making the final decision of which chemical thinning treatment and which berry sizing application technique to use, the effect on berry size and bunch quality should also be considered. None of the treatments evaluated negatively affected berry size and bunch quality. Any of the application techniques evaluated could be used, if GA3 within the recommended dosage range is used and applied at the recommended phenological stages for Prime. This study also provided data on the efficacy of GA3+ABA as thinning agent for table grapes, to be used to extend the registration of s-ABA on table grapes to include its use as a thinning agent.
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    Effect of post-harvest summer pruning on carbohydrate reserve status, bud break and fertility of Sultanina H5 in the Lower Orange River region.
    (Stellenbosch : Stellenbosch University, 2022-12) Toolo, Keboneilwe Boitumelo; Avenant, Eunice; Avenant, J. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Carbohydrate reserves produced from photosynthesis are stored in perennial tissues of the grapevine in the form of starch and free-sugar fractions or soluble sugars, mostly sucrose, glucose, and fructose. These reserves are highly affected by viticultural practices altering the source-sink relationship in the grapevine. Post-harvest pruning is a practice widely applied by several table grape producers in South Africa aiming to channel carbohydrate reserve accumulation to the remaining shoots. Due to the high input costs of table grape production, any manipulation, including post-harvest summer pruning, should be applied only if it is scientifically proven to have practical and economic benefits. This study, comprising of two trials, aimed to determine whether post-harvest pruning results in increased carbohydrate reserve status, improved bud break and fertility, as well as to establish a base for quantifying and practically assessing the carbohydrate reserve status of grapevines. The first trial focused on establishing the seasonal dynamics of non-structural carbohydrate (NSC) reserves of Vitis vinifera L. cv. Sultanina H5 in the semi-arid Lower Orange River (LOR) and the Mediterranean Hex River Valley (HRV). Root, trunk, cane and/or shoot tissues were sampled monthly and analysed for NSC. The Anthrone method was used to analyse soluble sugars and starch, while enzymatic analysis was used to quantify specific sugars (sucrose, d-fructose and d- glucose). Starch and sucrose were the most abundant forms of NSC in all tissues in both regions. In both regions, soluble sugars in permanent tissues (roots, trunks, canes) reached their highest concentration during dormancy (June-July). The starch concentration was low in all tissues in winter (July), during grapevine dormancy, whereafter it increased to a peak occurring in August (before bud break). A steep decrease in starch concentration was recorded from dormancy to flowering in both regions, indicating a dependency of the vine on carbohydrate reserves during that period. Accumulation of NSC reserves began after flowering to the post-harvest period, reaching their second peaks in autumn. The overall higher soluble sugars and starch (roots and canes) concentrations in the tissues of the Mediterranean region is ascribed to the earlier accumulation of reserves, lower crop load and a shorter post-harvest period characteristic of this region. A basis was established for sampling grapevine tissues for qualitative assessment of grapevine NSC reserve status, linking sampling time to occurrence of peaks in soluble sugars and starch concentrations. It is recommended that sampling for qualitative assessment of soluble sugars should be done after leaf fall, during dormancy (June-July under the conditions of this study). Starch concentrations should be assessed before bud break (August under the conditions of this study). Based on significant positive correlations between NSC concentrations of different tissue types, tissue types that could be sampled for indication of the overall NSC status of the grapevine were identified. Canes and shoots could be sampled and analysed to indicate the overall NSC reserve status of the vine. These tissues are considered the most practical tissue types to sample for determination of grapevine NSC reserve status. The second trial investigated the effect of post-harvest summer pruning applied in the semi-arid Lower Orange River region on NSC reserve status, bud break and fertility of Sultanina H5 grapevines. Five post-harvest pruning treatments were applied, namely an early 33% and a 66% shoot removal pruning treatment one day after harvest (33_1dAH and 66_1dAH respectively), a late 33% and a 66% shoot removal pruning treatment 45 days after harvest (33_45dAH and 66_45dAH respectively) and a control (Ctr), in which no post-harvest summer pruning was applied. To quantify pruning severity, the number and length of removed shoots, as well as the number of leaves and leaf area removed were determined at the time of the post-harvest summer pruning treatment application. After winter pruning was applied, the removed canes and shoots were measured to calculate the overall shoot length and leaf area removed per vine. The day after pruning treatments were applied, photosynthetically active radiation (PAR), as well as photosynthetic activity and related physiological parameters were measured. Cane and/or shoot, stem and root tissue, were sampled on 4 dates for assessing the effect of pruning treatments on NSC reserve status. Bud break and fertility were assessed through forced bud break and bud dissection for potential bud break and fertility, while actual bud break and fertility were assessed in the vineyard. Post-harvest pruning proved to be beneficial for light penetration, but it did not improve the photosynthetic rate of the leaves. A few significant differences were recorded on the impact of the treatments on TNC. These however, do not show a clear trend. Post-harvest summer pruning did not have a significant effect on final bud break and potential fertility of grapevines in the season following the treatment. Based on this one season’s results, post-harvest pruning did not have overall practical benefits. Repeating the treatments for two more seasons on the same data vines, would indicate whether there is a carry-over effect of the practice on NSC, bud break and fertility. It is recommended that in a further phase of this project, available rapid and accurate methods to quantify carbohydrate reserves should be used and/or evaluated for use in grapevine studies, including Near-Infrared spectroscopy, as well as the starch iodine test (already commercially used in the apple and forestry industries).
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    The evaluation of tannin activity in South African red wines
    (Stellenbosch : Stellenbosch University, 2022-04) Du Preez, Brannigan Jovern; Aleixandre-Tudo, Jose Luis; Du Toit, Wessel J.; De Villiers, Andre; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Proanthocyanidins are complex and important phenolic compounds, which play a significant role in defining overall red wine quality. In fact, some of the beneficial bioactive components associated with moderate red wine consumption can be attributed to these non-volatile, polar compounds. Additionally, they are involved in multiple chemical reactions with other compounds from the onset of fermentation throughout periods of long-term ageing, which ultimately influence the organoleptic properties of red wine. Therefore, it is beneficial to quantify proanthocyanidin content and composition in red wine. Several chemical methods exist for the analytical quantification of tannin content and composition in red wines and/or their grape skin-, seed- or tannin extracts. These methods are based primarily on spectrophotometry and chromatography. Many methods have been used in chemosensory studies to understand factors affecting red wine mouthfeel. In recent years, the use of tannin activity has gained traction as a useful chemical measurement to understand red wine mouthfeel, with astringency in particular focus. However, from a South African context, such studies are lacking. To address this, an explorative study was performed on experimental red wine extracts, separated into various mass fractions by Sephadex LH-20 by flash column-chromatography. Following chemical characterisation of these fractions based on tannin concentration and composition, tannin activity values were shown to be strongly influenced with proanthocyanidin molecular mass. Moreover, tannin activity-concentration relationships were identified using HPLC-tannin estimation methods, while MCP tannin concentrations were poorly correlated with activity values. Although tannin estimation methods were well correlated with one another, the study showed that other factors were also involved, setting the basis for the second part of the study. The effect of wine vintage on tannin composition and activity values was investigated across multiple vintages. Sixteen Pinotage wine samples (2003-2018) isolated by an optimised FCC method rendered four fractions (F1-F4) differing in degree of polymerisation and subunit composition, increasing in size from F1 to F3, with F4 pooled from these three samples. Similar findings were reported as in the explorative study. The extent to which MCP and RPLC- based tannin estimation methods were correlated with one another, was strongly dependent on wine age and tannin size. Activity values increased with tannin size but decreased in older wines, as a function of DP. This was indicative of reactions related to age, such as oxidation and precipitation. Finally, chemosensory analysis illustrated that low positive correlations were observed between fractions of high molecular weight, and bitterness and astringency, suggesting that oligomeric and polymeric proanthocyanidins play a greater role in modifying tannin activity values. More research is needed to understand how purified tannin fractions of various sizes affect tannin activity and mouthfeel attributes, by a chemosensory approach. Additionally, this may also be integrated with other experiments to evaluate how activity values are modified by tannin origin or maturity, and other processes such as tannin oxidation or precipitation.
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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 (𝑘𝐿 𝑎), the Sauter mean bubble diameter (D32), gas holdup (𝜀), the interfacial area (𝑎), and the oxygen mass transfer coefficient (𝑘𝐿). 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 𝑘𝐿𝑎. Bubble imaging was done to determine the interfacial area, and subsequently the 𝑘𝐿. During oxygen desorption within wine, the 𝑘𝐿𝑎 values varied between 0.0125 s-1 and 0.0275 s-1 depending on the wine. The 𝑘𝐿 𝑎 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 𝑘𝐿𝑎 to 0.0225 s-1. Further additions of organic acids did not affect the 𝑘𝐿𝑎, while the addition of protein in the form of BSA and yeast extract reduced the 𝑘𝐿𝑎 to approximately 0.0175 s-1. The 𝑘𝐿 𝑎 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 𝑘𝐿𝑎 values could all be ascribed to differences in the 𝑘𝐿. During oxygen desorption, the 𝑘𝐿 values were found to be between 0.015 and 0.045 mm/s within the different wines. The 𝑘𝐿 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 𝑘𝐿 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 𝑘𝐿 value from 0.017 mm/s to 0.0225 mm/s. The combination of the reduction in the 𝑘𝐿 when protein was added to a model wine solution, and the improvement of the 𝑘𝐿 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 𝑘𝐿 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 𝑘𝐿 will be greater.