composition of Pinotage red wine
4.2 Materials and methods
4.3.7 Polymeric phenols
Polymeric phenols of all the wines increased significantly when analysed after the eight days of oxygen addition, except the control. However, the oxygenated wine’s polymeric phenol content was always significantly higher than that of the control after oxygen was applied (Fig 4.5). The
initial significant increase in polymeric phenol concentration for the oxygenated wines is in agreement with the decrease in monomeric flavanol concentration determined by HPLC. It is thus evident that the reduction in monomeric flavanols was due to the participation in polymerisation reactions. The decrease in polymeric phenol concentration witnessed over time could be due to precipitation and build-up and especially breakdown reactions of the polymers, although they were not significant (Haslam et al., 1980; Vidal et al., 2002).
4.3.8 Flavonols
All individual flavonols decreased over time, except quercetin that steadily increased (p = 0.00002) with time (Fig 4.6). The latter is not in accord with studies done by Fang et al. (2007), who found a loss in quercetin as time proceeded. The increase in quercetin could be due to hydrolysis of quercetin-glucosides in the acidic wine medium (Price et al., 1995). However, there was a significantly lower quercetin concentration in the treated wines just after the oxygen treatment compared to the control. These differences disappeared after an ageing period of two months, our finding is in agreement with Sartini et al. (2007). The greater loss of quercetin in the oxygen treated wines could possibly be due to the high reactivity of this molecule with oxygen (Park et al., 2003). Perez-Magarino et al. (2007), however, found that the oxygenated wines had the highest concentration of quercetin and not the control.
ce Before O2 After O2 After MLF 2 Months After
MLF Time
Quercitin Concentration (mg/L)
Figure 4.6: Quercetin concentrations (mg/L) as determined by RP-HPLC for the control and oxygenated wines. The control is represented as ‘TC’, oxygenated tanks receiving 16 mg/L oxygen as
‘T16’ and tanks receiving 32 mg/L oxygen as ‘T32’. The error bars denote the standard deviation of the mean between duplicate treatments. The letter above each bar represents the significant differences between tanks.
4.4 Conclusion
We showed that oxygen sparged at the bottom of the tank does not influence the phenolic composition of the wine differently at different positions in the tank. This implicates that a winemaker using this technique on commercial scale would achieve the same effect of oxygenation throughout the tank if the tank is tall enough.
The application of microoxygenation is beneficial in terms of colour as we found significant differences between the controls and oxygen treated wines in terms of colour intensity, free anthocyanins and polymeric pigments and polymeric phenols just after MLF. This technique can thus be used to increase and stabilise colour of Pinotage wine before MLF, although it seems that these effects might disappear with further ageing. The treatment did not have a significant influence on the tannin concentration and small differences in total phenol concentrations were observed. However, differences observed between other authors and ourselves indicate the need for further research on the effect of micro-oxygenation on red wine’s colour and phenolic composition.
The technique is easy to apply and can be used in most commercial wineries. Future work should include ageing the wine for longer periods of time and a tasting panel should also evaluate the wines.
4.5 Acknowledgements
The authors would like to thank Distell Winery for donating the wine, supplying the facilities to perform the experiment and the use of their equipment, Winetech, Thrip and the NRF for providing financial support and Prof Martin Kidd for analysing the data statistically.
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