Masters Degrees (Institute for Wine Biotechnology)

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Browsing Masters Degrees (Institute for Wine Biotechnology) by browse.metadata.advisor "Du Toit, Wessel J."

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    The effect of grape temperature on the phenolic extraction and sensory perception of Méthode Cap Classique wines
    (Stellenbosch : Stellenbosch University, 2017-03) Mafata, Mpho; Van Jaarsveld, Francois; Du Toit, Wessel J.; Buica, Astrid; Stellenbosch University. Faculty of AgriScience. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: The first sparkling wine in South Africa was released in 1971. The South African Cap Classique Producers Association (CCPA), formed for the appreciation of Méthode Cap Classique (MCC) traditional style sparkling wines (TSW), was established in 1992 and has since contributed to the growth of these wines on a competitive footing with the international market. Generally, studies on TSW have focused primarily on the foam capability, volatile composition and autolytic character of the wines and very little on phenolic content of the wines. Phenolic compounds are important quality indicators of wine. Their composition in wine is determined by various factors including grape variety, terroir, viticultural practice, and oenological practices. In this project, MCC wines were made by the traditional method using Chardonnay and Pinot noir grapes harvested from two regions (Robertson and Darling) and stored at 0, 10, 25 and 30ºC, over two vintages (2014 and 2015). The phenolic concentration of the wine samples throughout the winemaking process was analysed by spectrophotometer and the aroma and taste of the final 9 month old sparkling wines performed. The study was aimed at investigating the effect of the grape storage temperature on the phenolic content and the sensory properties of MCCs through a quantitative phenolic analysis. The study found that MCCs made from grapes stored at lower temperatures (0 and 10ºC) had lower total phenolic content, colour intensity and total hydroxycinnamates than wines made from grapes stored at higher temperatures (25 and 30ºC) showing that there was greater phenolic extraction from grapes stored at 25 and 30ºC. The total phenolics, as measured by spectrophotometer, was below the range cited in literature for Champagne made from the same cultivars. The sensory evaluation of the MCCs comprised a sorting analysis similar to that used for beers. Separating the aroma and taste sorting of the MCCs, the study showed a grouping of the MCCs according to temperature treatments for both vintages. There were, however, clear vintage differences in terms of the attributes cited and the frequency of citations. Based on frequency of citation, 2014 MCCs made from grapes stored at 0 and 10°C were described by judges as being fruity, fresh and crisp whilst those made from grapes stored at 25 and 30°C were described as having oxidised fruit, volatile acidity and solvent-like aromas. The judges perceived less oxidation and VA (in terms of the frequency of citation) in the aroma of 2015 MCCs, although higher temperature treatments were still associated with less desirable attributes compared to lower temperature treatments. Judges were better able to separate the Darling wines according to treatments compared to the Robertson wines. This study has shown that the grape storage temperature has an effect on the phenolic extraction and the sensory perception of MCCs aged 9-months with no changes in the phenolic content observed throughout winemaking.
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    The evaluation of malolactic fermentation starter cultures under South African winemaking conditions
    (Stellenbosch : Stellenbosch University, 2008-03) Van der Merwe, Hanneli; Du Toit, M.; Du Toit, Wessel J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: With ever increasing pressure on wine producers to lower the financial costs involved in winemaking to be able to compete in the market, all while maintaining a high level of wine quality, the focus on maintaining control over all aspects of the winemaking process are greatly emphasized. Malolactic fermentation (MLF) is one of the important processes in red wine production. The advantages of this process, when performed successfully, is widely known and accepted. One way to gain control over MLF is the use of MLF starter cultures. Starter cultures usually consist of Oenococcus oeni that has been isolated from grapes or wines and is in most cases available in a freeze-dried form ready for direct inoculation into the wine when MLF is desired. Starter cultures are induced into wine and usually ensure the immediate onset as well as a fast and clean execution of the process. Starter cultures used in South Africa are in most cases isolated from cooler viticultural regions in the Northern hemisphere. The constitution of wines from cooler viticultural regions, differ from those in South Africa, which has a warm climate. The most important difference is the acid content of the wines which is lower in South African must/wines and results into a higher pH. The three most important changes that develop in wine during MLF are a decrease in acidity due to the conversion of malic acid to the less harsh lactic acid, enhanced flavour and aroma of wine and an increase in the microbiological stability of wine. The decrease in acidity is very important for wines produced for grapes grown in cool viticulture regions. In South Africa though, the climate is warm and higher pH’s are present in the musts and wines and the de-acidification due to MLF is not the main aim but rather the microbiological stabilisation. One of the compounds that could be produced by lactic acid bacteria (LAB) is biogenic amines (BA’s). These compounds can be hazardous to human health. This thesis focussed on the performance of MLF starter cultures in high pH South African red wines. The first objective of the study was to stretch MLF starter cultures in high pH red wines of South Africa. Stretching means to use less than the prescribed dosage or the re-use of starter cultures. The difference in MLF rate, the influence of the natural occurring LAB and the levels of biogenic amines formed during MLF were determined for the different stretching treatments. The results showed that different rates in malic acid degradation were experienced between the treatments, but in all cases MLF fermentation was completed. Biogenic amines were formed at various levels and the influence of the natural occurring LAB also played a role. The second objective of the study was the evaluation of the effect of a wine isolated LAB (Lactobacillus) and an acetic acid bacteria (AAB), inoculated with a MLF starter culture had on MLF at different wine pH’s. It was found that especially in the case where the Lactobacillus was inoculated in combination with the MLF starter culture a possible stimulatory effect was experienced with regards to malic acid degradation rate. Biogenic amine concentration was measured at the end of MLF and it was found that no histamine and tyramine were formed in any of the treatments, while the putrescine and cadaverine levels were found to be at approximately similar levels for the different treatments. The third objective was to evaluate the possible influence of commercial tannin additions and a pectolytic enzyme on rate of MLF and phenolic composition of high pH red wine. The commercial tannins had possible inhibitory as well as stimulatory effects on the rate of malic acid degradation especially during the initial stages of MLF, with the highest dosage having the significant effect. The BA results showed difference in the levels produced due to tannin additions as well as strain differences could exist. The phenolic content showed a decrease in colour density, total red pigments, total phenolics and anthocyanins between AF and MLF. The fourth objective was to evaluate inoculation time of MLF starter cultures. The results showed that the fastest AF/MLF time was with simultaneous inoculation of the yeast and MLF starter cultures. It was also for this treatment where no histamine or tyramine was detected at the end of MLF compared to the other inoculation strategies (before the end of AF and after AF). This study generated a large amount of novel data which made a valuable contribution with regards to MLF in high pH red wines of South Africa.
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    In-Line Monitoring of Red Wine Fermentation
    (Stellenbosch : Stellenbosch University, 2021-03) Lambrecht, Kiera Nareece; Aleixandre-Tudo, Jose Luis; Du Toit, Wessel J.; Nieuwoudt, Helene; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Phenolic compounds may only account for a small percentage of the final composition of a finished red wine but are vital to its sensory attributes. During red wine making, extraction of these phenolic compounds takes place, whereby there is mass transfer from the solids of the grape into the liquid phase. The rate and the extent to which this extraction occurs is dependent on many factors. There are many different methods employed in the wine industry which can influence the composition of the wine. These techniques are varied and can involve manipulating process conditions such as temperature or the addition of certain oenological products. As the final composition of the wine is a major contributing factor to the quality of the wine, it is vital to be able to monitor and control this process. It has been demonstrated through a variety of studies that the use of infrared spectroscopy along with chemometrics provides an avenue for implementation of monitoring and control systems in wineries. However, the limiting factors in these studies are the extensive sample pre-treatment to remove solids before scanning as well as their discrete and off-sight sampling. In the contents of the first research chapter (Chapter 3), a system was designed for the purpose of automatic sampling directly from vessels containing fermenting wines. This was an extensive design process which required separate sampling pumps and sampling lines which delivered samples to a single instrument. Another requirement was automation of different components and synchronisation of these in an individual system. The resulting design was put through a series of stress tests to ensure functionality and reliability. The results showed that the automated system was capable of full-time operation without experiencing component failures and, therefore, it was applied to actual fermentations. For this, 24 hours of real time monitoring was achieved. The turbidity remained a challenge as a perfectly clarified sample was not achievable. This led to the development of partial least squares (PLS) calibrations for three different spectroscopy techniques where the samples used incorporated differing degrees of sample pre-treatment to reduce turbidity. The results of this endeavour compiled in Chapters 4 and 5 showed favourable results for samples with different levels of turbidity as well as for contactless methods of conducting analysis. With further optimisation of the models using spectral pre-processing and wavenumber selection, it was possible to develop models suitable for application in an industrial setting. Finally, in Chapter 6, these models were deployed for use with a series of fermentations, where the ability to monitor phenolic extraction of fermentations receiving different treatments was explored. The results show that the system can be used to monitor trends in phenolic extraction in an industrial set-up. In addition to this, the system has the capacity for updated models and different methods of process control, thereby allowing it to be tailored to each unique scenario.