Department of Viticulture and Oenology

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Development of novel methods for tannin quantification in grapes and wine
(Stellenbosch : Stellenbosch University, 2017-03) Terblanche, Elsa; De Villiers, Andre J.; Du Toit, Wessel J.; Aleixandre-Tudo, Jose Luis; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: Phenolic compounds, and condensed tannins in particular, are of utmost importance in red grapes and wine due to their contribution to the sensory properties and potential health benefits. However, the detailed analysis of these compounds is hampered by their complexity and the lack of reliable quantitative analytical methods. In this study, the analysis of wine tannins using different chromatographic methods was evaluated in order to develop an improved methodology for their accurate characterisation and quantification. Standard compounds for use in calibration were isolated from cocoa using semi-preparative high performance liquid chromatography or purchased commercially. Calibration curves were constructed and relative response factors based on degree of polymerisation (DP), class of compound and mobile phase composition were determined. Response factors were found to vary as a function of DP and class, indicating the errors associated with quantification as (epi)-catechin equivalents as is often done due to the lack of standards. Both hydrophilic interaction chromatography (HILIC) and reversed-phase liquid chromatography (RP-LC) methods for tannin analysis were developed. For HILIC, an amide column was used, which provided separation according to DP as well as a separation of isomers within specific elution windows. In RP-LC compounds were separated based on hydrophobicity, resulting in separation of isomers, with compounds of various DPs overlapping. In both separation modes, three detectors were connected in series: a photodiode array ultraviolet (UV) detector, a fluorescence detector (FLD) and a quadrupole-time-of-flight mass spectrometer (Q-TOF-MS). FLD was found to be the most sensitive for procyanidins (PCs), while UV demonstrated the best sensitivity toward gallated PCs. Negative electrospray ionisation (ESI)-Q-TOF-MS proved essential in identifying 161 tannin species based on accurate mass data, and was the most selective of the detectors when using extracted ion chromatograms. Quantification of tannins in 9 red wine samples and a grape seed extracts indicated that each of the detectors was useful for particular compounds. Co-elution caused overestimation of some compounds by UV and occasionally by FLD as well. Nevertheless, there was good agreement between the HILIC and RPLC methods, as well as between the various detectors in each mode. Quantitative data for the red wine and seed samples were in agreement with those obtained in previous studies. The total number of compounds identified (161) and quantified (74 and 41 in HILIC and RP-LC, respectively), was greater than could previously be obtained. Both methods were shown to be viable options for the analysis of condensed tannins in grape and wine samples. HILIC was found to be more sensitive, and therefore HILIC-UV-FLD-Q-TOF-MS is recommended as the method of choice for detailed quantitative condensed tannin analysis.
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.
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.
Phenolic extraction of South African red wines
(Stellenbosch : Stellenbosch University, 2019-12) Fourie, Elzanne; Du Toit, Wessel J.; Aleixandre-Tudo, Jose Luis; Mihnea, Mihaela; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: Phenolic compounds are well known for their crucial role in red wine quality parameters such as colour, flavour and mouthfeel attributes. Knowledge about phenolic extraction during fermentation and possible wine practices or techniques to possibly modify phenolic content during red wine production is becoming a necessity in the wine industry to improve overall quality. In addition, the industry requires suitable, rapid, accurate and affordable monitoring tools to be able to improve and modify phenolic content during the process. In this study phenolic levels of fermenting samples were quantified through UV-Visible spectroscopy in combination with PLS calibration models. Furthermore, phenolic extraction was evaluated with batch statistical process control as a statistical monitoring tool. Eight grape batches of the commercial cultivars Cabernet Sauvignon and Shiraz were randomly selected from the Western Cape wine region. Wines were pressed at 1/3rd, 2/3rds and near the end of alcoholic fermentation and two punch down regimes were evaluated for each pressing time. Standard punch down (Cb) consisted of 3 punch downs per day during the maceration stage of skin contact, whereas increased punch down (Ta) consisted of 12 punch downs applied during different stages of fermentation. Firstly, significant variance was observed between grape batches for the four measured phenolic parameters (anthocyanins, tannins, colour density and total phenolics), with less prominent differences observed for colour density between batches. Furthermore, soft independent modelling of class analogy (SIMCA) showed wines classified according to the grape batch. Moreover, wines produced under different maceration conditions separated according to the vineyard the grapes were sourced from in the batch level model (BLM). All of the analysis indicated significant phenolic variance between grape batches. In the BLM OPLS-DA model the fermentation samples collected during maceration separated according to the different pressing times for both cultivars, regardless of punch down level. Fermenting samples obtained from pressing time 1 were associated with low phenolic content, whereas wines pressed at 2/3rds and near the end of fermentation showed trends of higher phenolic content. The BLM OPLS-DA between pressing time 2 and 3 revealed clearer trends for Cabernet Sauvignon, whereas fewer phenolic differences were observed for Shiraz samples. Fermenting samples obtained from pressing time 2 showed higher levels of anthocyanins in comparison with wines pressed near the end of fermentation were associated with higher levels of tannin, polymeric phenol and gallic acid. Shiraz fermenting samples pressed at time 3 showed only the latter. Results indicated Cabernet Sauvignon may possibly be a better suited cultivar for longer maceration, since fermentations pressed near the end of fermentation i.e. longer skin maceration, were associated with higher tannin, polymeric phenol and pigment content. In addition, results also reflected phenolic extraction as a diffusion process driven by maceration length. Furthermore, evaluating the BLM OPLS-DA a poorer separation was observed between standard and increased punch down frequency. Increased punch down frequency showed trends of higher phenolic content, whereas fermenting samples produced with three punch downs a day was strongly associated with high phenolic acid content only. Phenolic differences were observed between Cabernet Sauvignon and Shiraz indicating results may be batch or variety dependent. Additionally, taking a closer look at both punch down levels for each pressing time, similar results were observed in the OPLS-DA models. Overall, BSPC allowed for the monitoring of phenolic extraction and identification of possible deviations during maceration. However, this study data was only evaluated after process completion. This methodology could be potentially used to monitor phenolic extraction in real time for future red wine fermentations. Additionally, the approach of PLS calibrations proved to be a suitable, rapid, accurate and cost-effective method to measure phenolic levels of fermenting samples.
The quantification of red wine phenolics using fluorescence spectroscopy with chemometrics
(Stellenbosch : Stellenbosch University, 2021-03) Dos Santos, Isabel Anne; Du Toit, Wessel J.; Aleixandre-Tudo, Jose Luis; Bosman, Gurthwin W.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: The organoleptic and perceived quality characteristics of red wine are largely influenced by important phenolic compounds extracted throughout fermentation from the grape berry to the final wine matrix. These complex secondary metabolites have resulted in numerous equally complex analysis methods, the implementation of which are yet to form part of routine phenolic analysis during winemaking. In this study, front-face fluorescence spectroscopy was investigated for its suitability in quantifying phenolic parameters of unaltered samples and the subsequent implications for non-invasive analysis throughout fermentation. A front-face accessory and fluorescence spectrophotometer were successfully optimised in order to analyse samples directly, eliminating the need for sample dilution as with conventional fluorescence spectroscopy. A diverse dataset comprising 289 fermenting musts and wine were analysed using the optimised fluorescence protocol and the most commonly used UV-Vis spectrophotometric methods for the following phenolic parameters; total phenolics, total condensed tannins, total anthocyanins, colour density and polymeric pigments. Different statistical analysis methods were explored for their suitability in model development, specifically Parallel Factor Analysis (PARAFAC) and a gradient boosting machine learning algorithm (XGBoost). Subsequent to the investigation of the most optimal chemometric method, a machine learning pipeline was generated to develop accurate regression models per phenolic parameter. Successful models were obtained for total phenolics, total condensed tannins and total anthocyanins while polymeric pigments and colour density require further investigation and refinement. Following model development and optimisation, an external validation experiment monitoring a Cabernet Sauvignon fermentation was used to examine prediction accuracy under fermentation conditions, specifically investigating the effect of carbon dioxide and must turbidity. No effect of sample preparation treatment was found and the potential for analysing unaltered samples directly during fermentation was possible. Fluorescent properties of fermenting musts and wines were explored and the responsible spectral regions of interest tentatively identified. Differences in fluorescence between musts and wines were found and upon closer inspection, unique changes were monitored and identified throughout fermentation using the Cabernet Sauvignon experiment. The unique fluorescent profiles of wines is widely accepted, and the classification of South African red wine cultivars was successfully conducted using Neighbourhood Component Analysis (NCA). These results may have beneficial implications for authentication and quality control by industry bodies. Overall, front-face fluorescence spectroscopy holds several advantages including it being non- invasive, user-friendly, relatively economical, rapid and accurate, and thus presents itself as a promising alternative to the current phenolic analysis methods with the added benefit of direct phenolic analysis throughout red wine fermentation. The potential for implementation within on- line automated systems or portable optical devices may be of interest to producers and allow for monitoring of phenolic content and extraction directly from the fermentation vessel throughout red wine production.
The quantification of white grape juice phenolics using various spectroscopic methods and chemometrics
(Stellenbosch : Stellenbosch University, 2021-12) Clarke, Sarah; Aleixandre-Tudo, Jose Luis; Du Toit, Wessel J.; Bosman, Gurthwin W. ; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: Phenolic compounds are aromatic, secondary metabolites found in plant tissues. They have a number of bioactive properties as well as positive effects on health. Phenolic compounds, although found at lower levels in Méthode Cap Classique (MCC) and white wines, contribute to the mouthfeel and flavour as well as having antimicrobial and antioxidant properties. The levels of the phenolic compounds present in a wine depend on the variety, ripeness of grapes at harvest, soil type and the vinification processes applied. During the pressing stages, juice is obtained from the grapes and skins in contact with the juice. This maceration time, although limited, allows for phenolic compounds to be extracted and dissolved in the juice. The ability to monitor the phenolic concentration during the pressing stages of MCC and white wine could potentially increase the yield recovery of quality juice, furthermore, allowing increased control in the vinification process and can lead to benefits such as improved and consistent wine quality. Current phenolic compound analysis methods can be outdated and unreliable due to interferences. In order for phenolic monitoring techniques to be useful in the wine industry they must be compatible with process control methods. Spectroscopy techniques, alongside chemometrics, for the quantification of phenolics have the potential to be implemented into wineries as in-line and on-line systems. These techniques provide increased accuracy and reliability. This research explores a range of analytical techniques which may be applied to the quantification of phenolic compounds with the use of calibration models. Infrared (IR), Raman and fluorescence spectroscopy were the analytical methods explored and the reference total phenolic index (TPI) data was collected using Ultra-Violet/Visible (UV/Vis) spectrophotometry. These spectroscopic techniques were chosen as they are suited for the implementation into portable devices and hence could be of use to the wine industry for process control analysis. The spectroscopic analyses performed are: - Attenuated Total Reflectance Mid-Infrared Spectrometer (ATR-MIR). - Multi-Purpose Analyser (MPA) Transmission Fourier Transform Near-Infrared Spectrometer (T-FT-NIR). - Matrix F Diffuse Reflectance Fourier Transform Near-Infrared Spectrometer (DF-FT-NIR). - Raman spectroscopy with a central wavelength of 532nm. - Fluorescence spectroscopy with emission spectra between 300nm and 575nm and excitation wavelengths between 300nm and 575nm. Partial Least Squares (PLS) regression models were built for all analytical methods explored and the robustness of these models were examined using a range of statistical parameters. Further techniques, such as machine learning, were explored for the data obtained in the fluorescence spectroscopy. T-FT-NIR provided the best model for TPI with 0.547 and 2.12 RMSEP and RPDval, respectively. Moreover, high prediction accuracy was observed with DF-FT-NIR for the MCC dataset with 0.457 RMSEP and 2.01 RPD. The models obtain form the Raman and fluorescence spectra underperformed those of the IR instruments. However, improvements in fluorescence model performance were achieved when the use of a machine learning analysis pipeline was explored. The statistical parameters used to determine model robustness did not indicate that all of the predication models constructed are of immediate use to the wine industry. Despite these results, it is believed that the aim of this research is worth further investigation. The observed models do indicate results which could be of potential use for screening purposes. Further research could be the key to unlocking the potential of these spectroscopic methods for phenolic quantification as this would reduce the number of variables which are believed to have caused the results observed.
A study of spatial and temporal natural intra-block variability of plant water status in a commercial vineyard
(Stellenbosch : Stellenbosch University, 2021-03) Jasse, Aladino; Poblete-Echeverria, Carlos; Aleixandre-Tudo, Jose Luis; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: Water is one of the main elements responsible for plant growing and functioning. The most reliable method to measure the availability of water in plants is through the analysis of plant water status. Since plant water conditions have a significant role in grape and wine composition it is of utmost importance to control and manage the level of water deficit during the whole phenological cycle, more specifically during the ripening period. Plant water status variations during the ripening period have several consequences for the evolution of grape berry parameters and the biosynthesis of phenolic compounds in grape berries. In this period, water-stressed plants are prone to increase the concentration of certain parameters, such as sugar, and modify the biosynthesis of phenolic compounds. Plant water status is variable between plants within the vineyard block because several surrounding aspects influence the availability of water, and therefore, the absorption of water from the soil and the loss of water from the plant. Therefore, plant water status variability can be temporal according to the variability in the ripening period or spatial depending on plant conditions, soil characteristics and topography. Temporal and spatial variability can be monitored and evaluated by Precision Viticulture techniques. Precision Viticulture is an agricultural management approach that evaluates variability in vineyards with a focus on applying differentiated management practices to control intrinsic variability of vineyard blocks. The aim of this study was to evaluate natural temporal and spatial variability of plant water status during the growing season and to further investigate the effect that plant water status has on grape composition parameters (pH, TA, sugar) and wine phenolic compounds (Anthocyanins and Tannins). The existing literature concerning spatial variability of plant water status in vineyards was reviewed. Most studies in the field devote attention to the effect of Regulated Deficit Irrigation in the spatial variability of plant water status between plants. However, further work is needed to fully understand the implications of the natural spatial variability between plants in a vineyard block where also the surrounding characteristics (soil, diseases, microclimate) of each plant affect grape and wine composition. In Chapter 3, the natural spatial and temporal variability of plant water status in a commercial vineyard of cv. Cabernet Sauvignon was evaluated and mapped. Remote sensing measures of soil electrical conductivity, trunk circumference and satellite NDVI (Normalized Difference Vegetation Index) were used to select target vines in the block in addition to an unsupervised k- means function in order to create a grid of the spatial variability. Soil analysis and topography were also included in the analysis. As a result of the evaluation, 43 target plants were selected to be monitored from September to February. At the end of the season, three classes of plant water status were defined, and each target vine was assessed for yield, ripeness as well as standard juice parameters. Micro-vinification was done for each target vine and the concentration of anthocyanins and tannins was analysed. The results showed that the spatial and temporal variability of plant water status was evident along the season, affecting the grape parameters and wine concentration of phenolic compounds. An evident relationship between plant water status and soil texture and topography characteristics was noticed. Plants with a lower water status were located in steeper regions of the block with a higher water run-off and less water penetration in the soil. Compared with plants with a higher plant water status distributed in more flat regions of the block, plants with a low water status presented a significant increase in sugar content, anthocyanins and tannins and a strong decrease in yield. The findings of this thesis are a contribution to understand and quantify how spatial variability is naturally distributed and its effect on grape and wine parameters.
A survey of the YAN status of South African grape juices and exploration of multivariate data analysis techniques for spectrometric calibration and cultivar discrimination purposes.
(Stellenbosch : Stellenbosch University, 2018-12) Petrovic, Gabriella; Buica, Astrid; Aleixandre-Tudo, Jose Luis
ENGLISH ABSTRACT: Yeast assimilable nitrogen (YAN) has been identified as one of the main drivers of wine quality, influencing the production of various aromas and ensuring a successful fermentation to dryness. Due to the number of factors affecting YAN concentration and composition, paired with the complexities of yeast metabolism, more data is required to enable a comprehensive understanding of this important component of the grape juice matrix. Thus, there is a need for simple, rapid, and cost-effective methods to measure YAN status. The main aims of this research were to gain insight into the nitrogen status of grape juices used for commercial winemaking in the South African wine industry, and subsequently, to assist in a more comprehensive understanding of grape juice nitrogen status. Therefore, in Chapter 3, an unsupervised survey of the YAN, FAN, and ammonia concentrations of 805 grape juice samples of various (industrially relevant) cultivars and geographical origins are reported. Subsequently, an overall average of 191 ± 64 mg N/L, 138 ± 46 mg N/L and 53 ± 24 mg N/L was observed for YAN, FAN, and ammonia, respectively. Trends of nitrogen deficiency and excess could be found for various cultivars and geographical origins. Analysis of variance tests and exploratory data analysis techniques such as hierarchical agglomerative clustering and CART analysis established ‘cultivar’ as the most important factor in determining the YAN concentration and composition of the resulting grape juice. In Chapter 4, using the data collected in Chapter 3, plus an additional vintage (2018), the viability of infrared (IR) spectroscopy for the accurate quantification of YAN, FAN, and ammonia was tested. IR spectroscopies compared included: Fourier-transform infrared (FT-IR), Fourier-transform near infrared (FT-NIR) and attenuated total reflection mid-infrared (ATR-MIR) spectroscopy. FT-IR and FT-NIR were found to outperform ATR-MIR in a variety of tasks assigned to each instrument and were deemed robust and capable of accurate quantification as RPDVAL > 2.5 were repeatedly obtained for both spectroscopies. The achievement of accurate calibration models is owed to the large amount of variability included in both the calibration and validation sets and the application of proper external validation strategies. Thus, both industry and research are presented with a simple, rapid and cost-effective method to measure this important component of the grape juice matrix. In Chapter 5, a deeper look into the FAN component of YAN was conducted by quantifying individual amino acids. Overall, proline, arginine, glutamine, alanine, tryptophan and GABA were found to be the most abundant while glycine, lysine, methionine and, ornithine were found to be the least abundant. Subsequently, the discriminatory power of the amino acid profile of the various cultivars were tested. This was done to identify key differences in amino acid profiles which could possibly serve as the basis for further research investigating yeast metabolism and aroma production during fermentation. The results of this research have contributed a wealth of information regarding the nitrogen status of various cultivars of Vitis vinifera, together with a rapid and easy-to-use method for the quantification of the nitrogen status of the grape juice matrix. This was done in hope of furthering the research efforts in this field to aid the production of quality wines, capable of meeting consumer demands.

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