Browsing by Author "Malherbe, Sulette"

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    Industry-wide assessment and characterisation of problem fermentations
    (Stellenbosch : University of Stellenbosch, 2007-03) Malherbe, Sulette; Du Toit, M.; Bauer, Florian; University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    In order to remain competitive in the international wine market, wineries need to increase productivity and improve quality with the aid of available technologies. Problematic wine fermentations directly impact both productivity and the quality of wine and therefore, by reducing stuck and sluggish fermentations, less wine will be lost or downgraded. This requires early identification and classification of problem fermentations with the use of high throughput analytical tools and multivariate data analysis in order to take preventative measures. In this study, a non-directed, holistic approach was used to investigate the occurrence of problem fermentations in the South African wine industry. Data obtained with various analytical techniques was used for the purpose of multivariate data analysis. The use of high throughput analytical techniques such as Fourier transform infrared (FT-IR) spectroscopy proved extremely valuable as a fast screening method to monitor fermentation progress. Principal component analysis of the spectral fingerprints obtained with this technique indicated that all the fermentation problems occurred from the middle of fermentation onwards. In addition, successful discrimination between control and problem fermentations for different red and white cultivars was achieved. This demonstrates the feasibility of this type of methodology to investigate and monitor fermentation. Solid phase dynamic extraction (SPDE) headspace analysis was used to obtain a volatile fingerprint of the fermenting must samples. Similarly to FT-IR spectra, successful discrimination between problem and control fermentations was achieved with the use of PLS-discriminant analysis. Discrimination between red and white cultivars was achieved with headspace data. This excluded the data from tannin and colour compounds normally used for this discrimination. SPDE coupled to gas chromatography mass spectrometry (GC-MS) proved to be a novel and suitable analytical method for wine analysis, although further method development is required. A preliminary research and development strategy for evaluating FT-IR spectroscopy for quantification purposes in fermenting must was established. Although these calibration and validation results were preliminary, it was shown that the calibrations for fermenting must under South African conditions need urgent attention. In conclusion, this study illustrated the potential of using an alternative approach to investigate stuck and sluggish fermentation with definite prediction possibilities for future research endeavours.
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    Investigation of the impact of commercial malolactic fermentation starter cultures on red wine aroma compounds, sensory properties and consumer preference
    (Stellenbosch : University of Stellenbosch, 2011-03) Malherbe, Sulette; Nieuwoudt, Helene; Du Toit, M.; Tredoux, A. J. G.; Naes, T.; University of Stellenbosch. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Wine is the result of a variety of biochemical reactions and microbial interactions which contribute to the organoleptic properties of wine. Wine aroma and flavour encapsulate the sensory experience of wine and could ultimately determine wine quality and consequently influence consumer acceptance and preference of a product. A thorough understanding of potential factors influencing wine aroma and flavour is therefore needed in order to exploit such factors. The aim of this study was to evaluate the influence of commercial malolactic fermentation (MLF) starter cultures on wine composition, aroma and flavour and the potential impact on consumer preference of experimentally produced red wines. An analytical platform was established to capture the compositional changes induced by different MLF bacteria in experimentally produced red wines. A fast gas chromatography flame ionisation detection (GC-FID) method was developed to determine 39 wine volatile compounds in less than 15 minutes per sample. A 3-fold reduction in analysis time was achieved in comparison to a conventional GC-FID method (40 minutes). Analytes quantified comprise a large boiling point and polarity range which illustrates the robustness of the method. A method was also developed for the direct quantification of carbonyl compounds including diacetyl, acetoin, 2,3-pentanedione and certain aldehydes using headspace solid phase microextraction coupled to gas chromatography mass spectrometry (HS-SPME GC-MS). Both analytical methods showed satisfactory linearity, repeatability and limits of quantification. The contribution of four commercial Oenococcus oeni malolactic fermentation (MLF) starter cultures to the volatile composition, organic acid content and infrared spectral properties of experimentally produced South African red wines, showed significant strain-specific variations in the organic acid profiles, especially for the production of citric acid and lactic acid during MLF. Subsequently, concentrations of compounds related to citric acid metabolism, namely ethyl lactate, acetic acid, diacetyl and acetoin, were influenced accordingly. Bacterial metabolic activity increased the concentration of higher alcohols, fatty acids and esters, with a larger increase observed in ethyl esters compared to acetate esters. A strain-specific tendency to reduce total aldehyde concentrations was found at the completion of MLF, however, further investigation is needed to clarify this observation. Infrared spectral fingerprints were used to characterise the different bacteria and in addition, the prediction of MLF related compounds, diacetyl, acetoin and 2,3-pentanedione, from mid-infrared spectra was explored by partial least squares (PLS) models. Quantitative descriptive analysis (QDA) results depicted significant differences between wines fermented with different starter cultures, in terms of sensory attributes including buttery, fruity, nutty and yoghurt/buttermilk aroma as well as smoothness and mouth-feel attributes. Consumer preference testing results indicate that sensory differences imparted by different MLF bacteria could influence consumer-liking. Preference mapping revealed interesting relationships between sensory attributes and consumer-liking, that can be used for preliminary interpretative purposes. In conclusion, this study illustrated the potential impact of bacterial strains on wine aroma and flavour, resulting sensory properties and consumer preference through an integrative approach combining compositional, spectral, sensory and consumer data. The results presented in this study are of significance to the wine industry since they illustrate and reiterate the potential of different MLF starter cultures as an additional tool to contribute to wine aroma and flavour, and potentially influencing consumer preference and product liking.