|
A Technical Guide for Wine Producers
|
|
RECENT ARTICLES | WYNBOER HOME
Oxygen in winemaking - Part II
W. J. du Toit, Department of Viticulture and Oenology, Stellenbosch University
O2 plays an important role in the winemaking process. The winemaker must thus be informed on how much O2 occurs in his wine at which stadium of the production process. This article will focus on the amounts of O2 occurring at different stages in the winemaking process.
The measurement of O2 is not easy in wine. One of the reasons for this is that wine is quite a complex medium. Different O2 measurement models are available on the market. The less expensive one has a probe that is inserted into the wine, which is connected to a box that gives the reading in either mg per L (parts per million) or %. More expensive models are also available which measures up to part per billion. These also have a steel rod that can be inserted into a cork, which makes it ideal for measuring the O2 concentration in bottled wine.
O2 comes into contact with must as just after crushing of the grapes, which catalyses the activation of oxidation enzymes. These enzymes oxidize phenolic molecules to their corresponding quinone, as discussed in the previous article. When juice or wine is saturated with oxygen it contains about 7-8 mg/L of O2, depending on the temperature. There are several ways available to the winemaker to avoid unwanted oxidation. The first entails removing the substrate for oxidation, the phenolic molecules. Hyper oxidation has thus been investigated as a means of removing phenolic molecules prior to fermentation by oxidation, where large amounts of O2 are added to the must. This can be done by sparging the juice in the tank for about 30 min with O2, racking the juice from one tank to another for a few times or using O2 in stead of N2 when using flotation to clarify the juice. According to literate white juice that did not receive skin contact can be saturated once with O2, which should be sufficient to remove most phenolic compounds. When skin contact has been given the juice can be left for 30 min after saturating it and then be sparged again for 30 min. It is obvious that no SO2 should be added prior to these operations. If SO2 is added before fermentation it should also be added after the juice has been racked from the precipitate after saturation. After fermentation SO2 can also be added.
Phenolic molecules can also be removed from the must and wine by fining with gelatin, PVPP or activated charcoal. The latter should however be handled with care, as it can strip a wine from its flavour compounds if added at too high dosages. Higher phenolics being extracted into the juice and wine can also be avoided by not applying too long skin contact periods, handling the grapes with care (thus not pressing at too high pressures and using small amounts of press must).
The other way of avoiding oxidation is by avoiding O2 pickup in white wine production, the so-called reductive manner of winemaking. This is done by adding dry ice to the crusher, press and in fermentation tanks and Vitamin C to the must. Inert gasses, such as N2 and CO2, can also be used to fill tanks prior to pumping the wine or to regularly replace the air space in a tank that has not been filled completely. It's important to remember that CO2 is heavier than air and should thus protect wine in a half filled tank better than N2, which is lighter than air. Argon has also been investigated, but is not really financially viable to due to it being expensive. O2 can also be removed from wine in a tank. This can be done by bubbling an inert gas from the bottom of the tank through an inlet for about 30 min, depending on the size of the tank. Here N2 is the probably the preferred choice of gas, as too high CO2 levels being dissolved into the wine is unwanted. The addition of high enough concentrations of SO2 to must (30-50 mg/L in healthy grapes) and in the wine (maintaining 35 mg/L free during the production process and 35-40 mg/L free just prior to bottling) must also be done. Certain white wines begin to start losing fruitiness after only a few saturations with O2. Procedures like centrifugation (7 mg/L), transport (2-6 mg/L) and filtration (3-7 mg/L) has all been shown to contribute to O2 pickup in wine, especially if the cellar equipment, like pump, filters etc. are faulty. When wine is also splashed into a tank or runs down the side of the tank in a wide film, O2 pickup is also higher.
The question remains whether reductive winemaking is always necessary in white wine production. It has been proven that in SA Sauvignon blanc the addition of H2O2 (which is a stronger oxidation agent than O2) to must did not decrease the methoxypirasine levels. It has also been proven however, that the precursor of the granadilla flavour in French Sauvignon blancs is sensitive to O2. It thus seems that the practice of reductive winemaking is more justified in certain white wines than others. During bottling white wine can also pickup O2. This has been true in older bottling lines, but with modern bottling lines O2 pickup should not exceed 1 mg/L during bottling. Bottling lines should also have a QC practice in place where O2 pickup is monitored during the bottling procedure. The practice of adding Vitamin C just prior to bottling should also be re-considered, as recent research has shown that Vitamin C actually enhanced ageing of white wine in the bottle.
Red wine's quality can however improve with O2 additions to certain extend. This is due to O2 catalyzing polymerization between anthocyanin and tannin molecules. This can already be done by adding O2 during fermentation with sparging or pumping overs. How much O2 actually reacts with the phenolic compounds during fermentation is however unclear, as the yeast also take up some O2. A splash pump over should in theory lead to a few mg/L O2 being dissolved in the wine, but red wine fermentations are normally conducted at around 30øC (at higher temperature lower amounts of O2 dissolves in wine) and the CO2 produced by the yeast probably also sparge some of the O2 out. O2 addition during fermentation can, however, be used to treat stuck or sluggish fermentations.
After fermentation and before malolactic fermentation O2 can be added in a process called macro-oxygenation (at a dosage of 1-4 mg/L/day for up to 4 days). This could be especially helpful for high phenolic wine, especially press wine. We even added 6 times during extended skin contact about 4 mg/L O2 to a Cabernet Sauvignon without the wine becoming oxidized. This practice is not followed normally, due to fears of VA formation, but has been done safely with the wine having less tannin and being less reactive towards proteins. Macro-oxygenation can be done with a machine, which injects tiny bubbles of O2 through a sparger into the bottom of a tank in a controlled manner. Here it's, however, important to check whether the O2 actually dissolved in the wine and did not simply escape to the top of the tank. Macro-oxygenation can also be applied in oak barrels after malolactic fermentation, but care should be taken that Brettanomyces or acetic acid bacteria growth is not stimulated.
O2 can also be added to red wine after malolactic fermentation with a process called micro-oxygenation (1-4 mg/L/month for about 1-6 months). The dosage and time of these additions dependent on the type of wine. To heavier style red wines with high anthocyanin and tannin concentrations O2 can be added 3-4 mg/L for a few months. These dosages actually imitate those, which the wine would have received in an oak barrel and micro-oxygenation can thus be used in combination with oak staves. During barrel ageing red wine normally receive about 30-40 mg O2/L/year. This is introduces into the wine by topping up the barrel, racking, fining etc. It is believed that O2 also permeates through the oak staves.
O2 can however also enhance the unwanted growth of acetic acid bacteria and Brettanomyces, both that are spoilage microorganisms in wine. Acetic acid bacteria go into a viable, but non-cultural state at low O2 concentrations in wine. They thus do not grow on selective media, which is normally used to monitor their growth in wine, but can only be enumerated with epifluoressent microscopy, with which live cells can be distinguished from dead cells. The addition of O2 to the wine however negates this state and can lead to their rapid growth. When wine tanks or barrels are filled completely the changes of AAB spoilage is however smaller, as they normally prefer to grow on the surface of the wine. It thus seems that O2 inside the wine is less of a risk than on top. Brettanomyces can also use O2 at high concentrations to grow rapidly.
When samples are taken from a tank for analysis of SO2, O2 etc. it is important to realize that wine can quickly take up O2 in the sampling procedure, which may give a wrong impression of the oxidative state of the wine. In Fig 1. one can clearly see the effect of this. A 2005 Sauvignon blanc wine at the US Welgevalen cellar was used to test this. The O2 concentration was measured in the tank and was 0.48 mg/L. Wine was then taken from the top by means of a metal container attached to a chain and added to 250 mL sampling bottles in triplicate. In three of the bottles CO2 gas was blown prior to adding the wine and in another three not. The same procedure was also done by taking samples from the sampling tap, as well as from the bottom port of the tank and then adding it in the bottles. The lids of the bottles were then sealed immediately and the dissolved O2 (DO) measured after a few seconds. It is clear from Fig 1 that where CO2 was added in the bottle little O2 came into contact with the wine, especially in the wine that was taken from the sampling tap. The wine picked up almost 1 mg/L O2 from the sampling tap where no CO2 was added prior to the bottles. Wine that was first collected from the port into an open container and then transferred into the bottles pickup up very large amounts of O2. This should be kept in mind when collecting wine samples for analyses, or when a winemaker wants to keep a control sample in a bottle when micro oxygenation will be used in a tank. It is thus possible to add more O2 to the control sample than the treated wine if done incorrectly!
It is thus clear that O2 plays a pivotal role in the winemaking process, be it negative or positive.
Literature cited
Du Toit, W.J. and Groenewald, D.P. 2003.The effect of micro-oxygenation on a South African red wine composition and quality. Proceedings of the 7th International Oenologie Symposium, Bordeaux.
Du Toit, W.J. an M.G. Lambrechts. 2002. The enumeration and identification of acetic acid bacteria from South African red wine fermentations. International Journal of Food Microbiology 74: 57-64.
Du Toit, W.J.and I.S. Pretorius. 2002. The occurrence, control and esoteric effect of acetic acid bacteria in winemaking. Annals of Microbiology, 52, 155-179.
Du Toit, W.J., Pretorius, I.S. and Lonvaud-Funel, A. (2005) The effect of sulphur dioxide and oxygen on the viability and culturability of a strain of Acetobacter pasteurianus and a strain of Brettanomyces bruxellensis isolated from wine. Journal of Applied Microbiology 98, 862-871.
Ribereau-Gyon, P. et al. Handbook of Enology: Vol 2: The Chemistry of Winemaking. 2000
Saucier, C., Little and D. Glories, Y. 1997. First evidence of acetaldehyde condensation products in red wine. . American Journal of Enology and viticulture. 48. 370-373.
Schneider,V. 1998 Must Hyperoxidation: A Review American Journal of Enology and Viticulture; 49: 65 - 73.
Singleton, V. L. 1987. Oxygen with phenols and related reactions in musts, wines and model solutions: observations and practical implications. American Journal of Enology and Viticulture. 38. 69-77.
|
|
Wynboer is incorporated in WineLand, magazine of the SA wine producers.
Visit our sister sites:
|
| UP |
COPYRIGHT (C) 2000 WineLand |