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A Technical Guide for Wine Producers
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Micro-Oxygenation In South African Wine: Part 1
WJ du Toit
Micro-oxygenation is receiving more attention today. The following article will deal with some practical aspects regarding micro-oxygenation, as well as the effect it has on the phenolic composition of wine. The second article will focus on the effect it has on the microbial population, sulphur compounds, SO2 levels, micro-oxygenation in combination with oak wood and its effect on the taste of wine. Some general recommendations will also be given.
During micro-oxygenation small, controlled amounts of oxygen (O2) are bubbled into wine to bring about positive changes in the wine. This is achieved by filling a known volume with gas at a high pressure. The volume is then transferred via a low-pressure circuit to the diffuser and into the wine. The latter normally consists of a ceramic or stainless steel sparger that produces small bubbles, which can dissolve in the wine. The aim of micro-oxygenation is to introduce O2 into the wine at a rate equal to or slightly less than the wine’s ability to consume that O2 to avoid too much O2 build up in the wine. It has to be managed in such a way that, after addition, all O2 has been used up, while sufficient SO2 is still left to protect the wine against excessive oxidation and microbial spoilage. These changes include the following (some of them claimed by the producers):
How is micro-oxygenation brought about? The equipment entails using a gas canister which contains oxygen, linked to a special micro-oxygenation machine which can dose the O2 accurately. Different machines exist on the market. Some can dose in mL/L while others dose in mg/L. From the dosing machine a small pipe bubbles the O2 in a fine bubble form into the wine from the bottom of a tank. The sparger used for this can either be made from stainless steel or ceramic. The latter is reputed to lead to smaller bubbles, which can dissolve better in the wine, but stainless steel is obviously more robust. The tank also needs to be at least 2.5 m tall to facilitate dissolving of the bubbles in the wine. When micro-oxygenation is applied care should be taken that the bubbles dissolve in the wine and does not simply move to the top of the tank. The pressure from the gas canister to the dosing machine should also be monitored regularly. The sparger should be cleaned after a wine has been treated, by leaving it in a caustic solution for a few hours and then rinsing it thoroughly with clean water. Also make ensure that the pipe leading from the dosage machine to the sparger is free of liquid.
Oxygen can be supplied during different stages of the winemaking process. It can be supplied at 1-5 mg/L/day for a few days just after malolactic fermentation, especially to press wine fractions that are rich in polyphenols. The stage when micro-oxygenation is normally applied is during the ageing period after malolactic fermentation, when between 1-6 mg/L/month is introduced into the wine, although certain researchers recommended adding even up 10 mg/L/month. The wine’s temperature must be around 15°C because temperatures that are too high will lead to poor solubility of O2 and temperatures that are too low to chemical reactions taking place too slowly.
One of the main aims of micro-oxygenation is to bring about changes in the colour and phenolic composition of wine. It is well known that the addition of O2 to wine leads to polymerization of certain phenolic compounds, such anthocyanin and flavanol moieties (catechin and condensed tannins). This polymerization occurs due to the formation of H2O2 from the oxidation of a phenolic molecule. H2O2, being a strong oxidant, oxidizes ethanol, which yields acetaldehyde. The latter forms a bridge between phenolic molecules, leading to the polymerization. Just after fermentation a large percentage of anthocyanins are still in the colourless form. Slight oxidation, as would happen with a racking of wine, leads to these colourless anthocyanin molecules binding to tannins and changing into red or brown red compounds. When red wine is matured in oak barrels O2 also comes in contact with the wine, but this does not happen when it’s matured in stainless steel tanks, without O2 addition such as micro-oxygenation.
Experimental observations
We decided to investigate the effect of micro-oxygenation on different South African red wines.
Materials and methods
Four different SA red wines were monitored in terms of their phenolic and colour development during micro-oxygenation (Table 1). Note that in wines A and B the treatment started just after malolactic fermentation and wines C and D seven months after malolactic fermentation. Different spectrophotometric analyses were conducted during the course of the experiments. These included wine colour density, modified wine colour density (the modified version of the analysis negates the effect of pH and SO2 on the analysis), wine colour hue, total red pigments, total phenolics and estimate of SO2 resistant pigments. The fractions of co-pigmented anthocyanin, free anthocyanins and polymeric colour pigment content in the red wines were also determined. The estimate of SO2 resistant pigments and modified colour density were used to analyse these fractions. Total anthocyanin concentrations, total tannin concentrations, HCl index value (index of polymerisation of procyanidins) and gelatin index values (index of reactivity of phenolic molecules in wine towards gelatin) were also conducted. Additional phenolic analyse were performed with HPLC.
Table 2 - Concentrations of different phenolic compounds in wine C initially and after 24 weeks treatment.
Compound Concentration (mg/L) for each
treatment Initially Control 1.5 mg O2/L/month 3 mg O2/L/month Barrel Gallic acid 46.4 56.1 50.3 57.2 47.3 Gentisic acid 1.5 2.6 nd 1.5 nd Caftaric acid 17.3 16.3 17.5 16.8 17.3 Vanillic acid 2.6 4.1 5.1 3.3 43.7 Catechin 790.2 784.0 704.4 698.6 659.5 Caffeic acid 64.2 59.0 56.8 57.0 52.6 Procyanidin B1 60.0 91.0 93.5 55.6 78.5 p-Coumaric acid 3.6 3.8 4.2 6.4 12.2 Procyanidin B2 49.1 40.8 40.4 40.0 39.8 Epicatechin 90.1 73.6 76.2 68.8 80.5 Delphinidin-3-glucoside 11.2 7.7 7.4 9.6 7.2 Petunidin-3- glucoside 13.8 7.6 9.3 8.5 8.7 Peonidin-3- glucoside 5.5 2.9 4.1 3.4 4.1 Malvidin-3- glucoside 117.9 63.4 83.8 72.0 79.6 Ellagic acid 3.7 4.8 4.5 5.4 3.3 Quercetin-3- glucoside 15.8 12.3 13.6 11.1 14.7 Myricetin 4.3 3.2 5.1 5.1 4.3 Quercetin-3-rhamnoside 5.0 4.2 3.4 4.7 4.0 Malvidin-3-Acetate 40.1 18.4 23.8 24.2 22.4 Quercitin 7.8 4.8 6.4 5.8 5.2 Malvidin-3-p-coumaric acid 19.6 6.8 11.1 9.6 10.0 Polymeric pigment 28.8 29.0 33.9 34.8 35.3 Polymeric phenols 791.6 782.9 947.1 1021.5 1132.0
Results
It is clear from Fig 1 that the colour density of wine A increased with increasing concentrations of O2 added. In wine A, micro-oxygenation led to a decrease in total phenolic concentrations (which was high in this wine) after seven to nine weeks and were lower in the treated wines after 15 weeks (results not shown). The colour pigment in this wine also increased with O2 addition. The free anthocyanin fraction was also smaller in the treated wines (results not shown), as well as the difference between the colour density and modified colour densities. The latter shows that the colour became more resistant towards the bleaching effect of SO2. This indicated that more free anthocyanins were incorporated into polymeric pigment form, leading to the increase in colour.
The same trend happened in wine B (Fig 2) where the micro-oxygenation led to a drastic increase in the colour density (which could be observed with the naked eye), but not the modified colour densities (where the bleaching effect of SO2 is negated). The fraction of colour in the polymeric pigments were much higher than that of the control wine (Fig 3).
However, the addition of O2 with micro-oxygenation does not always lead to drastic increases in colour density. This was true for wines C and D (Fig. 4). Micro-oxygenation, if used to increase the colour of red wine, would rather be used just after malolactic fermentation, where a large part of the anthocyanins are still in the colourless form. New research also showed that the addition of O2 between alcoholic fermentation and malolactic fermentation also lead to less colour being lost during malolactic fermentation. Total red pigments decreased, as would happen when red wine is aged in a barrel in all the treatments, but were slightly higher in the treated wines after 15-18 weeks than in the control (Fig. 5).
The HPLC results for wine C can be seen in Table 4.3. Vanillic acid was much higher in the barrel treated wine than in the other treatments, probably due to the higher oak contact. Catechin and procyanidin B1 concentrations decreased with the increasing O2 addition over time, with the malvidin-3-glucoside concentration being lower in the control wine. The procyanidin B1 concentration can also increase over time due to catechin associations, explaining the higher concentrations in the control wine after 24 weeks. The O2 treated wines were also higher in polymeric pigment and polymeric phenols, due to acetaldehyde formation and polymerisation. The polymerisation of procyanidins in wine A was also reflected in an increase in the HCl index of the treated wines (Results not shown).
A small decrease in the total tannin concentration and a small increase in colour hue in the treated wines were observed (results not shown). The gelatin index of wine C also varied over time, but it must be kept in mind that this index only gives an indication of astringency and is not always directly correlated with it. During micro-oxygenation the wine is supposed to first go through a structuring phase with an increase in astringency. This is followed by a phase where softening of the wine occurred. If too much micro-oxygenation is applied for too long a period of time the wine can become hard or "dried" out. These phases has however not been scientifically proven, but it has been found that if micro-oxygenation is applied for too long a period of time the mean degree of polymerization of catechins increases to a large extend and the wine can become too hard. Research has also showed that the decrease in anthocyanin level was also higher where larger percentages of oak were added to the wine. This is probably due to oak tannins being oxidized easier that grape tannins, leading to enhanced polymerization.
It thus seems that micro-oxygenation is most effective in terms of colour development when applied before or after malolactic fermentation. It is doubtful whether a wine, which is older than 6 months, will still benefit from micro-oxygenation for its colour. We have also seen that the colour of wines with a high colour density (<15) after malolactic fermentation does not increase to such a large degree if micro-oxygenation is applied that it can be observed with the naked eye. Micro-oxygenation can however be a useful tool if applied to young red wines to increase the colour.
It is clear that the sensory monitoring of wine during micro-oxygenation is still very important and will be discussed in more detail in the next article.
Acknowledgements
The authors would like to thank Reines Trading (agents for Parsec in South Africa) for the use of the micro-oxygenation equipment; Winetech for financial support; D.P. Groenewald and L. Kotzé for technical support.
References
Boulton, R., 2001. The copigmentation of anthocyanins and its role in the color of red wine: a critical review. Am. J. Enol. Vitic. 52, 67-87.
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