The effect of oxygen on the composition and microbiology of red wine

[摘要] The winemaking process involves different complex chemical and biochemicalreactions, which include those of oxygen (O2). Oxygen can come into contact with thewine through various winemaking procedures and can be used by the winemaker toenhance the quality of red wine. In wine, the main substrates for oxidation arephenolic molecules, which form quinones. These can influence the sensorycharacteristics of the wine. O2 can be used in fresh must to remove oxidisablephenolic molecules through a process called hyper-oxidation and can also be addedto fermenting must to enhance the fermentation performance of yeast. Controlled O2additions during ageing can lead to the wine's colour being increased and theastringency of the wine decreased. This is due to the formation of acetaldehyde fromthe oxidation of ethanol, which induces the polymerisation of tannin and anthocyaninmolecules. The addition of too much O2 to wine can, however, lead to unwantedover-oxidation, with certain off-odours being formed. It can also enhance the growthof unwanted spoilage microorganisms, such as Brettanomyces and acetic acidbacteria. Although research on O2 in wine was started many years ago, manyquestions still remain. These include the general effect of O2 on the sensory andphenolic profile of red wine especially and the microbiology of wine during ageing. Aneffective 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) additionson a strain of Brettanomyces bruxellensis (also known as Dekkera bruxellensis) andAcetobacter pasteurianus were investigated. Epifluorescence microscopy and platingrevealed that the A. pasteurianus strain went into a viable but non-culturable state inthe wine after prolonged storage under relative anaerobic conditions. This state,however, could be negated with successive increases in culturability by the additionof 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 strainwas more sensitive to SO2. A short exposure time to molecular SO2 drasticallydecreased the culturability of the B. bruxellensis strain, but bound SO2 had no effecton the culturability or viability of either of the two types of microorganisms. Oxygenaddition to the B. bruxellensis strain also led to a drastic increase in viability andculturability. It is thus clear that SO2 and O2 management in the cellar is of criticalimportance for the winemaker to produce wines that have not been spoiled byBrettanomyces or acetic acid bacteria. This study should contribute to theunderstanding of the factors responsible for the growth and survival ofBrettanomyces and acetic acid bacteria in wine, but it should be kept in mind thatonly one strain of each microorganism was used. This should be expanded in futureto include more strains that occur in wine.The second part of this study investigated the effect of micro-oxygenation on fourdifferent South African red wines. It was found that the micro-oxygenation led to anincrease 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. Inone of these wines, a tasting panel preferred the micro-oxygenation treated wines tothe control. In the other two red wines, in which the micro-oxygenation was startedseven months after the completion of malolactic fermentation, very little colourincrease 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. Aprolonged period of micro-oxygenation, however, led to this wine obtaining anoxidised, over-aged character. Micro-oxygenation and maturation in an oak barrelalso 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 toenhance 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 anoak barrel. More research into the effect of micro-oxygenation on the sensory profileof the wine is needed.As mentioned, the addition of O2 can lead to oxidative degradation of wine. Thebrown colour in wine is often used as an indication of oxidation, but oxidative aromascan be perceived before a drastic increase in the brown colour has been observed inred wine.The third part of this study was to assess the possible use of Fourier TransformInfrared Spectroscopy (FTIR) to measure the progression of oxidation in Pinotage redwines. Three wines were used in this study and clear separation between the controland aerated wines was observed by using Principle Component Analysis (PCA).Sensory analysis of these wines confirmed this observation, with a reductionespecially in berry fruit and coffee characters and an increase first in potato skin andthen acetaldehyde aroma characters as the oxidation progressed. PCA analysis alsorevealed that in certain wines the visible spectrum of light did not indicate theprogression of oxidation as sensitively as with the use of FTIR. This also correlatedwith the inability of the panel to observe a drastic colour change. FTIR should befurther investigated as a possible means of monitoring oxidation in wine and thisstudy should be expanded to wines made from other cultivars as well.

[发布机构] Stellenbosch University