The effect of oxygen on the composition and microbiology of red wine
Du Toit, Wessel Johannes
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The winemaking process involves different complex chemical and biochemical reactions, which include those of oxygen (O2). Oxygen can come into contact with the wine through various winemaking procedures and can be used by the winemaker to enhance the quality of red wine. In wine, the main substrates for oxidation are phenolic molecules, which form quinones. These can influence the sensory characteristics of the wine. O2 can be used in fresh must to remove oxidisable phenolic molecules through a process called hyper-oxidation and can also be added to fermenting must to enhance the fermentation performance of yeast. Controlled O2 additions during ageing can lead to the wine’s colour being increased and the astringency of the wine decreased. This is due to the formation of acetaldehyde from the oxidation of ethanol, which induces the polymerisation of tannin and anthocyanin molecules. The addition of too much O2 to wine can, however, lead to unwanted over-oxidation, with certain off-odours being formed. It can also enhance the growth of unwanted spoilage microorganisms, such as Brettanomyces and acetic acid bacteria. Although research on O2 in wine was started many years ago, many questions still remain. These include the general effect of O2 on the sensory and phenolic profile of red wine especially and the microbiology of wine during ageing. An effective way of measuring oxidation, especially in red wine must also be developed. In the first part of this study, the effects of O2 and sulfur dioxide (SO2) additions on a strain of Brettanomyces bruxellensis (also known as Dekkera bruxellensis) and Acetobacter pasteurianus were investigated. Epifluorescence microscopy and plating revealed that the A. pasteurianus strain went into a viable but non-culturable state in the wine after prolonged storage under relative anaerobic conditions. This state, however, could be negated with successive increases in culturability by the addition of O2, as would happen during the transfer of wine when air is introduced. The A. pasteurianus strain was also relatively resistant to SO2, but the B. bruxellensis strain was more sensitive to SO2. A short exposure time to molecular SO2 drastically decreased the culturability of the B. bruxellensis strain, but bound SO2 had no effect on the culturability or viability of either of the two types of microorganisms. Oxygen addition to the B. bruxellensis strain also led to a drastic increase in viability and culturability. It is thus clear that SO2 and O2 management in the cellar is of critical importance for the winemaker to produce wines that have not been spoiled by Brettanomyces or acetic acid bacteria. This study should contribute to the understanding of the factors responsible for the growth and survival of Brettanomyces and acetic acid bacteria in wine, but it should be kept in mind that only one strain of each microorganism was used. This should be expanded in future to include more strains that occur in wine. The second part of this study investigated the effect of micro-oxygenation on four different South African red wines. It was found that the micro-oxygenation led to an increase in the colour density and SO2 resistant pigments of the two wines in which micro-oxygenation was started just after the completion of malolactic fermentation. In one of these wines, a tasting panel preferred the micro-oxygenation treated wines to the control. In the other two red wines, in which the micro-oxygenation was started seven months after the completion of malolactic fermentation, very little colour increase was observed. One of these two wines was also matured in an oak barrel, where the change in phenolic composition was on par with the treated wines. A prolonged period of micro-oxygenation, however, led to this wine obtaining an oxidised, over-aged character. Micro-oxygenation and maturation in an oak barrel also enhanced the survival of acetic acid bacteria and Brettanomyces in this wine. Micro-oxygenation can hence be used by the wine producer on young red wines to enhance the quality of the wine, but should be applied with care in older red wines. Future research into micro-oxygenation should focus on whether it can simulate an oak barrel. More research into the effect of micro-oxygenation on the sensory profile of the wine is needed. As mentioned, the addition of O2 can lead to oxidative degradation of wine. The brown colour in wine is often used as an indication of oxidation, but oxidative aromas can be perceived before a drastic increase in the brown colour has been observed in red wine. The third part of this study was to assess the possible use of Fourier Transform Infrared Spectroscopy (FTIR) to measure the progression of oxidation in Pinotage red wines. Three wines were used in this study and clear separation between the control and aerated wines was observed by using Principle Component Analysis (PCA). Sensory analysis of these wines confirmed this observation, with a reduction especially in berry fruit and coffee characters and an increase first in potato skin and then acetaldehyde aroma characters as the oxidation progressed. PCA analysis also revealed that in certain wines the visible spectrum of light did not indicate the progression of oxidation as sensitively as with the use of FTIR. This also correlated with the inability of the panel to observe a drastic colour change. FTIR should be further investigated as a possible means of monitoring oxidation in wine and this study should be expanded to wines made from other cultivars as well.