SUNScholar - Browsing by Author "Du Toit, W. J."

Browsing by Author "Du Toit, W. J."

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Additions of glutathione or specific glutathione-rich dry inactivated yeast preparation (DYP) to sauvignon blanc must : effect on wine chemical and sensory composition
(South African Society for Enology and Viticulture, 2017) Gabrielli, M.; Aleixandre-Tudo, J. L.; Kilmartin, P. A.; Sieczkowski, N.; Du Toit, W. J.
Although the addition of pure glutathione (GSH) is not allowed under current regulations, the concentration of this compound can be increased in wine through the addition of glutathione-enriched dry yeast preparations (DYP). These preparations have been observed to have antioxidant properties and could thus influence wine aroma and sensory characteristics. The main aim of this study was to investigate the effect of DYP and GSH juice additions on the sensory and chemical composition of Sauvignon blanc wine. Four juice additions were performed and compared against a control treatment: 5.5 mg/L of GSH; 0.4 g/L of DYP; 80 mg/L of GSH; 0.4 g/L of DYP plus 80 mg/L of GSH. After three months of bottling, the volatile and sensorial composition was investigated. The addition of DYP preparations to must increased the concentration of certain wine volatile compounds, with increased attributes of riper tropical fruit aromas, which was not always observed with the GSH additions. The addition of DYP influenced the concentrations of some volatile compounds, which modified the white wine aroma. The release of compounds other than GSH by the yeast products is proposed as the reason for these changes. The results observed in this study can assist winemakers to modify the aroma profile of Sauvignon blanc wines.
Biogenic amines in wine : understanding the headache
(SASEV, 2008-09) Smit, A. Y.; Du Toit, W. J.; Du Toit, M.
The presence of biogenic amines in wine is becoming increasingly important to consumers and producers alike, due to the potential threats of toxicity to humans and consequent trade implications. In the scientific field, biogenic amines have the potential to be applied as indicators of food spoilage and/or authenticity. Biogenic amines can be formed from their respective amino acid precursors by various microorganisms present in the wine, at any stage of production, ageing or storage. To understand the large number of factors that could influence the formation of biogenic amines, the chemical, biochemical, enzymatic and genetic properties relating to these compounds have to be considered. Analytical and molecular methods to detect biogenic amines in wine, as well as possibilities that could enable better control over their production levels in wine will also be explored in this review.
Comprehensive survey of the distribution of colour and phenolics of different red grape wine vineyard blocks from the Robertson area in South Africa
(South African Society for Enology and Viticulture, 2012) Van der Merwe, H.; Nieuwoudt, Helene; De Beer, D.; Du Toit, W. J.
Colour and phenolic content of red grapes are two of the most important constituents required to produce a quality red wine. In the Robertson grape growing area, difficulty is sometimes experienced with colour development of grapes. This is especially linked to location and most probably greatly influenced by season. Forty four vineyard blocks of the cultivars Pinotage, Merlot, Cabernet Sauvignon and Shiraz, were studied over 3 seasons primarily to focus on colour and phenolic content, but secondly on total soluble solids, titratable acidity and pH of the grapes. High performance liquid chromatography and spectrophotometric methods were used to determine various colour and phenolic parameters present at harvest. This data was used to indicate how colour and phenolic constitution of a part of the Robertson grape growing area was distributed in relation to various factors, such as cultivar and season. GPS points were used to map data for the blocks visually. Results showed variable colour and phenolic content for these grapes based on blocks and phenolic compounds investigated. Shiraz displayed a wider distribution of certain phenolic compounds over the three seasons than the other 3 cultivars. Seasonality had a great influence on these results, with outlying blocks being identified.
Correlations between South African red grape and wine colour and phenolic composition : comparing the Glories, Iland and Bovine Serum Albumin tannin precipitation methods
(South African Society for Enology and Viticulture, 2012) Du Toit, W. J.; Visagie, M.
Phenolic compounds in red grapes might give an indication of phenolic and colour compositions of the resulting wine. This work compared the Glories, Iland and Bovine Serum Albumin (BSA) tannin precipitation methods for phenolic characterization of South African Pinotage, Merlot, Shiraz and Cabernet Sauvignon red grape samples (n=31). Significant positive correlations were found for certain phenolic characteristics in the grapes measured by these methods. Levels of phenolic compounds in the grapes and correlating wines were in line with literature. Merlot samples often associated more with higher concentrations of seed tannins, which were also reflected in the wines. Significant correlations were also found with the colour characteristics of the resulting wines and some anthocyanin related measurements in the grapes with the Glories and Iland methods, with the latter correlating slightly better. Significant positive correlations were also found between grape and wine tannins as measured with the BSA method. However, malolactic fermentation changed some of these correlations and this needs to be investigated further. This work might give wine producers as well as wine analyses laboratories valuable information regarding the suitability of these methods to characterize the phenolic composition of South African red grapes and their resulting wines.
Do differences in the colour and phenolic composition of young shiraz wines reflect during ageing?
(South African Society for Enology and Viticulture, 2017) De Beer, P.; Strever, A.; Du Toit, W. J.
This study investigated the evolution of colour and phenolic compounds of Shiraz wines during wine ageing. Wines made from the same vineyard, but with a different phenolic composition, were used in this study. Analyses included colour density, anthocyanins, tannin, hydroyxycinnamic acids, flavan-3-ols, polymeric pigments and polymeric phenols. Differences in phenolic compositions observed in the young wines were generally still observed after 6 and 12 months of ageing. These results indicate that significant differences in terms of phenolics and colour in young Shiraz red wines might thus be expected to be reflected in the wines after ageing as well.
The effect of grape temperature at pressing on phenolic extraction and evolution in Méthode Cap Classique wines throughout winemaking
(South African Society for Enology and Viticulture, 2018) Mafata, M.; Buica, A. S.; Du Toit, W. J.; Van Jaarsveld, F. P.
Maintaining the chemical composition of a wine is essential for the wine industry. Although the sugar-acid balance of a wine is of primary sensory importance, individual acids and oenological parameters are equally important. The main focus of this study was to investigate the impact of grape temperature at harvest, on the oenological volatile acidity (VA), titratable acidity (TA), pH and alcohol levels and organic acid (citric, malic, pyruvic and succinic) characteristics of Méthode Cap Classique (MCC) wines during winemaking, produced from grape cultivars obtained from two regions. Chardonnay and Pinot noir grapes were obtained from Robertson (warmer) and Elgin (cooler) regions and were subjected to different temperature treatments, i.e. 0, 10, 25 and 30oC before further processing, including pressing, primary fermentation, blending, tirage, secondary fermentation, riddling and disgorging. Grape temperature was mostly responsible for a significantly higher pH of Robertson (0 and 10ºC) and lower pH (0ºC) of Elgin post-tirage wines. Chardonnay base wines from both regions that were vinified from grapes at lower temperatures (0 and 10oC) were richer in malic- and succinic acid, while Pinot noir wines from both regions were characterised by higher malic-, citric- and pyruvic acid. Pyruvic acid was only detected after the secondary fermentations in wines from both regions. To our knowledge, this study is the first to investigate the influence of grape temperature on the oenological and organic acid characteristics of MCC wines in different regions, and throughout different production stages.
The effect of micro-oxygenation on the phenolic composition, quality and aerobic wine-spoilage microorganisms of different South African red wines
(South African Society for Enology and Viticulture, 2006) Du Toit, W. J.; Lisjak, K.; Marais, J.; Du Toit, M.
The effect of micro-oxygenation treatments on the composition of different South African red wines was evaluated. In some wines, micro-oxygenation led to an increase in colour density, with a corresponding change in colour, due to the formation of polymeric pigments. This increase also led to the red colour becoming more resistant to the bleaching effect of SO2. Micro-oxygenation also seemed to be more effective in increasing the colour densities of younger red wines than the older ones, although total red pigments were found to be highest in an older red wine that had received micro-oxygenation. Differences in the gelatine index were also observed over time. The micro-oxygenated and barrel-matured wines also had lower concentrations of catechin and the procyanidin B1, and showed a corresponding increase in polymeric pigment and polymeric phenols. Micro-oxygenation led to higher acetic acid bacteria counts, although no increase in volatile acidity was observed in these wines. A tasting panel preferred younger red wines that had undergone micro-oxygenation. In an older red wine prolonged micro-oxygenation led to the wine becoming over-aged, with an increase in the barnyard/medicinal character, which corresponded with an increase in Brettanomyces counts. Micro-oxygenation can be used to increase the quality of young red wines, but further research is needed in this area.
Effect of skin contact before and during alcoholic fermentation on the chemical and sensory profile of South African Chenin Blanc white wines
(South African Society for Enology and Viticulture, 2015) Aleixandre-Tudo, Jose Luis; Weightman, C.; Panzeri, V.; Nieuwoudt, Helene; Du Toit, W. J.
The volatile and phenolic composition of Chenin blanc wines made with different skin contact treatments was studied. One batch of grapes was used to make a dry white wine according to two different treatments, namely pre-fermentative skin contact and complete fermentation on the skins. A white wine fermented without any skin contact was used as control. Fermentation on the skins and skin contact before fermentation led to significantly lower levels of terpenes, esters, acids and thiols, and the highest significant levels of alcohols and phenolic compounds. However, this effect was less pronounced in wines with skin contact before fermentation. Sensory analysis of all the experimental wines was also performed. The results showed a significant shift from the sensory attributes of fresh and tropical fruits of the control Chenin blanc wines towards riper fruit notes in the skin contact treatments. This observation was correlated with the length of the skin contact period. Possible reasons to explain the results observed in this study are discussed.
Evolution of phenolic composition during barrel and bottle aging
(South African Society for Enology and Viticulture, 2020) Aleixandre-Tudo, J. L.; Du Toit, W. J.
During red wine ageing, phenolic compounds undergo several reactions that have an impact on wine colour and mouthfeel properties. The evolution of phenolic content is affected during wine ageing. The aim of this study was to investigate the phenolic content and evolution of 82 commercial red wines subjected to barrel and bottle ageing. The phenolic content evolution of wines that underwent an ageing period of 12 months in commercial 225 litre barrels, followed by 12 months in the bottle, was monitored. While the total phenolic content remained stable, the anthocyanin fraction was affected the most, which led to substantial changes in the colour properties of the wines. Differences were found during both ageing regimes, indicating certain phenolic reactions being favoured or compromised under different ageing conditions, with an impact on wine colour properties. This paper provides the first large-scale study on the phenolic evolution of commercial red wine during ageing.
The impact of different tannin to anthocyanin ratios and of oxygen on the phenolic polymerisation over time in a wine-like solution
(South African Society for Enology and Viticulture, 2019) Garrido-Banuelos, G.; Buica, A. S.; Sharp, E.; De Villiers, A.; Du Toit, W. J.
Colour and phenolic stability during ageing are influenced by the levels of distinct classes of phenolics in young red wines. The ratios between different classes of phenolic compounds also determine the colour and phenolic development of red wines. The present study evaluated the impact of forced oxidation on different anthocyanin/tannin (A/T) extracts and its consequent effect on the colour and phenolic evolution over time. The results showed that higher contents of seed tannins could enhance phenolic polymer formation, especially in the presence of oxygen. The addition of oxygen seemed to favour certain polymerisation reactions between tannins, leading to higher concentrations of monomeric anthocyanins in solution. A slower oxygen consumption was also observed as the phenolic composition of the wine-like extract evolved over time.
Impact of time, oxygen and different anthocyanin to tannin ratios on the precipitate and extract composition using liquid chromatography-high resolution mass spectrometry
(South African Society for Enology and Viticulture, 2019) Garrido-Banuelos, G.; Buica, A.; De Villiers, A.; Du Toit, W. J.
Wine colour and phenolic stability over time are influenced by the amount and nature of phenolics in young wines. The ratio between different phenolic compounds can also be determinant in the colour and phenolic development of red wines. Three different anthocyanin to tannin ratios extracted in a wine-like system were saturated with oxygen several times during sample storage. A LC-HRMS method was used to evaluate the impact of a forced oxidation and of the different extracts on the wine-like composition and on the precipitate formed over time. The extract composition was found to be the most determinant factor for the precipitate formed. Time was also found to be a relevant factor according to the precipitate composition.
Influence of yeast strain, extended lees contact and juice composition on glutathione levels in wine.
(2011) Kritzinger, E. C.; Bauer, Florian; Du Toit, W. J.
Influence of yeast strain, extended lees contact and nitrogen supplementation on glutathione concentrations in wine.
(AUSTRALIAN SOC VITICULTURE OENOLOGY, PO BOX 197, GLEN OSMOND, AUSTRALIA, SA5064, 2013) Kritzinger, E. C.; Bauer, Florian; Du Toit, W. J.
Investigation of thiol levels in young commercial South African Sauvignon Blanc and Chenin Blanc wines using propiolate derivatization and GC-MS/MS
(South African Society for Enology and Viticulture, 2018) Coetzee, C.; Schulze, A.; Mokwena, L.; Du Toit, W. J.; Buica, A. S.
In this work, the ethyl propiolate method for analysing thiols in white wine by GC-MS, originally proposed by Herbst-Johnstone et al.) (2013), has been adapted to GC-MS/MS and has been validated. The method performance has shown improvement in terms of sensitivity (limit of detection, LOD) and of the number of compounds measured. In addition to 3-mercaptohexanol (3MH), 3-mercaptohexyl acetate (3MHA), and 4-mercapto-4-methylpentan-2-one (4MMP), the adapted method can also measure 2-furanmethanethiol (FMT) and makes use of a commercially-available internal standard (IS), 4-methoxy-2-methyl-2-butanethiol (4M2M2B, IS). The proposed method was applied to determine thiol levels in young commercial South African Sauvignon and Chenin Blanc wines. The samples (n=20 for each cultivar) were chosen according to a high frequency of the typical descriptors associated with this class of impact compounds. 3MH was found at 178-904 ng/L and 99-1124 ng/L, and 3MHA at 23-151 ng/L and 5-253 ng/L in Sauvignon and Chenin Blanc respectively. 4MMP was present in Sauvignon Blanc in concentrations up to 21.9 ng/L, but in none of the Chenin Blanc samples.
Monitoring the effect of micro-oxygenation before malolactic fermantation on South African Pinotage red wine with different colour and phenolic analysis
(South African Society for Enology and Viticulture, 2012) Geldenhuys, L.; Oberholster, A.; Du Toit, W. J.
The use of micro-oxygenation and its effect on the quality of the red wine varietal, Pinotage, is largely unknown. The influence of adding different oxygen dosages before malolactic fermentation on the phenolic composition and colour stabilisation of wine made from Pinotage was studied, and the suitability of certain spectrophotometric and RP-HPLC analyses to determine these changes were assessed in tanks of commercial lengths. Total oxygen dosages of 0 mg/L, 16 mg/L and 32 mg/L were applied, after which the wines underwent malolactic fermentation and maturation in the same tanks for two additional months. Decreases in anthocyanin concentration showed a strong inversely proportional correlation with increasing polymeric pigment concentration. This suggests that the addition of oxygen contributed to the early stabilisation of wine colour in Pinotage red wine. Overall, tannin concentrations were not significantly influenced by the oxygen treatments, although lower levels were observed in wines treated with oxygen. An important finding of the study was that there appeared to be little difference in the colour and phenolic composition of the wines between the 16 mg/L and 32 mg/L oxygen treatments. Good correlations were found between certain spectrophotometric techniques and the RP-HPLC analysis used to study changes induced by micro-oxygenation (total anthocyanins, total phenols). The colour and phenolic composition of Pinotage wine can be influenced before malolactic fermentation by micro-oxygenation. Some spectrophotometric phenolic analyses showed the same tendencies as observed with RP-HPLC (anthocyanins, monomeric flavan-3-ols, tannins), indicating their suitability to follow phenolic and colour changes induced by micro-oxygenation in Pinotage red wine.
Oxygen in must and wine : a review
(South African Society for Enology and Viticulture, 2006) Du Toit, W. J.; Marais, J.; Pretorius, I. S.; Du Toit, M.
Oxygen can play an important role during the winemaking process. It can influence the composition and quality of the must and wine. Phenolic compounds are the main substrates for oxidation in must and wine. Oxygen addition leads to colour changes and the polymerisation of phenolic molecules in wine. Oxygen can, however, also influence the flavour and microbial composition of wine drastically, with certain off-flavours being formed and spoilage micro-organisms able to grow at too high oxygen additions to wine. A state-of-the-art, up-to-date review on the effects of oxygen in must and wine has, however, not been published recently. This review focuses on the effects of oxygen in must, during alcoholic fermentation, extended lees contact and during ageing of white and red wines. The effects it has on acetic acid bacteria and Brettanomyces are also discussed, as well as micro-oxygenation, a relative new technique used in wine production.
Pinking in white wines - a review
(South African Society for Enology and Viticulture, 2020) Nel, A. P.; Du Toit, W. J.; Van Jaarsveld, F. P.
In the late 1960s, a phenomenon was discovered in white wines. It was noted that certain white wines turned pink in the bottle. This phenomenon was dubbed as pinking. Research was done on the pinking to establish its cause and effect. Analysis of SO2, pH and polyvinyl polypyrrolidone (PVPP) showed that a minimum of 45 mg/L of SO2 were needed for the wine not to be susceptible to pinking. Tests on the decrease in pH showed that there was no increase in pink colour with a decrease in pH, which meant that monomeric anthocyanins were not the cause of pinking. Recent research claims that malvidin-3-Oglucoside is the most abundant monomeric anthocyanin found in pinked wines and could be the cause of pinking. This led to the theory that phenols contribute to pinking susceptibility, and this was accepted as fact in recent years. The establishment of a pinking assay in 1977 made the testing for pinking easier and cheaper for winemakers. The sales of PVPP increased as winemakers worked preventatively with their wine to decrease susceptibility to pinking. This review attempts to describe the history of pinking, the establishment of the assay, as well as to describe factors that could lead to pinking susceptibility in white wines.
The production of reduced-alcohol wines using Gluzyme Mono® 10.000 BG-treated grape juice
(SASEV, 2009) Biyela, B. N. E.; Du Toit, W. J.; Divol, Benoit; Malherbe, D. F.; Van Rensburg, P.
High alcohol wines have become a major challenge in the international wine trade. Several physical processes are used to produce wines with reduced-alcohol content, all of which involve the selective extraction of ethanol based on volatility or diffusion. In this study, the possibility of Gluzyme Mono® 10.000 BG (Gluzyme) (Novozymes, South Africa) to reduce the glucose content of synthetic grape juice before fermentation was investigated in order to produce wine with reduced-alcohol content. Gluzyme is a glucose oxidase preparation from Aspergillus oryzae, currently used in the baking industry. Glucose oxidase catalyses the oxidation of glucose to gluconic acid and hydrogen peroxide(H2O2) in the presence of molecular oxygen. Gluzyme was initially used in synthetic grape juice, where different enzyme concentrations and factors influencing its efficiency were investigated under winemaking conditions. The results showed up to 0.5% v/v less alcohol at an enzyme concentration of 20 kU compared to the control samples. This reduction in alcohol was increased to 1 and 1.3% v/v alcohol at pH 3.5 and pH 5.5 respectively in aerated (8 mg/L O2) synthetic grape juice using 30 kU enzyme. Secondly, Gluzyme was used to treat Pinotage grape must before fermentation. Gluzyme-treated wines at 30 kU enzyme concentration after fermentation contained 0.68% v/v less alcohol than the control wines. A decrease in acetic acid concentration of the treated compared to control wines was also observed.
Quantification of glutathione, catechin and caffeic acid in grape juice and wine by a novel ultra-performance liquid chromatography method
(2011-05) Fracassetti, D.; Lawrence, N.; Tredoux, A. G. J.; Tirelli, A.; Nieuwoudt, Helene; Du Toit, W. J.
This research aimed at the development and validation of an ultra-performance liquid chromatography (UPLC) method for the quantification of glutathione (GSH) in grape juice and in white wine after derivatisation with para-benzoquinone. The phenolic compounds catechin and caffeic acid that occur in white wine and have antioxidant effects, are also quantified in the same analysis. Catechin is the basic monomeric unit of grape and wine tannins and caffeic acid, when esterified with tartaric acid, plays a relevant role in Grape Reaction Product (GRP) formation. The analytical method proposed showed good linearity, repeatability and intermediate repeatability, as well as high recovery (>85%). It was applied for the quantification of GSH, catechin and caffeic acid in South African juices (12) and white wines (43). This novel method will have a large impact on the time and costs of the analyses for the wine industry through enabling rapid routine quantification of GSH, catechin and caffeic acid. © 2011 Elsevier Ltd. All rights reserved.
Research note : effect of simulated shipping temperatures on the sensory composition of South African chenin blanc and sauvignon blanc wines
(South African Society for Enology and Viticulture, 2014) Du Toit, W. J.; Piquet, C.
This work investigated the effect of constant and simulated shipping temperatures on the sensory composition of Chenin blanc and Sauvignon blanc wines. Wines exposed to elevated temperatures during the trial developed unwanted aromas, such as over-aged, sulphur-like and a yellow colour, while those left at a cooler temperature retained tropical aromas. However, varying temperatures did not lead to large aroma differences in wines compared to those left at a constant lower temperature. It thus seems that average temperatures play a larger role in the development of unwanted over-aged aromas in certain South African white wines than temperature variations.

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