Browsing by Author "Fourie, Elzanne"

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    Partial least squares calibrations and batch statistical process control to monitor phenolic extraction in red wine fermentations under different maceration conditions
    (2020-04-11) Fourie, Elzanne; Aleixandre-Tudo, Jose Luis; Mihnea, Mihaela; Du Toit, Wessel
    The extraction of phenolic compounds during maceration is of utmost importance in red winemaking. However, the monitoring of phenolic extraction is often hampered by analytical and statistical constraints. The aim of this study was to monitor phenolic extraction kinetics with the use of PLS phenolic calibrations and batch statistical process control. Eight batches of Cabernet Sauvignon and Shiraz grapes during alcoholic fermentation under different maceration conditions (pressing at 1/3rd, 2/3rd and end of fermentation) and punch down regimes (low vs. high frequency) were evaluated in the study. Cabernet Sauvignon appeared to be a more suitable cultivar for longer maceration conditions with increased tannin extraction observed. Similar trends were observed for punch down for both cultivars. The use of PLS calibrations and batch level modelling provided an enhanced interpretation and understanding of phenolic extraction during red wine fermentations.
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    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.