Winetech Technical Yearbook 2022

DECEMBER

FREE SULPHUR DIOXIDE MANAGEMENT IN BARRELS

SULPHUR DIOXIDE AND BARRELS ARE BOTH INTEGRAL TO WINEMAKING, ALTHOUGH THEIR PURPOSES ARE ENTIRELY DIFFERENT. SULPHUR DIOXIDE FULFILS A PRESERVATIVE ROLE, WHILE BARRELS COMPLEMENT THE WINE STYLE AND QUALITY. DUE TO THE POROSITY AND POTENTIAL ULLAGE OF BARRELS, THEY ARE PRONE TO OXIDATION AND MICROBIAL SPOILAGE. SULPHUR DIOXIDE CAN CONTROL THIS TO A CERTAIN EXTENT.

BY CHARL THERON

SULPHUR DIOXIDE (SO 2 ) has been used in winemaking for centuries. Wine in barrels is more exposed to potential oxidation and microbial spoilage than bottled wine or wine in tanks. It is consequently crucial to properly manage the sulphur dioxide concentration in barrels to ensure sound barrel maturation. BARREL DOWNGRADES Barrels are usually utilised during the winemaking of premium wines. Due to the barrel price, it is essential to ensure that these wines reach that price bracket to prevent financial losses. Due to variations in oxygen exposure, microbiome, oak character, temperature, humidity and many other factors, individual barrels will differ and may not meet the required style or quality of premium wines. Such wines will be downgraded to lower prices, which can result in a financial loss. Due to the anti-oxidative and anti-microbial characteristics of sulphur dioxide, a lack thereof can contribute to the potential decrease in wine quality. FREE SO 2 VARIATION BETWEEN BARRELS Any cellar with more than a few barrels cannot manage their free SO 2 concentrations by sampling all the individual barrels. Some barrels of a lot are usually sampled, and a composite sample is prepared by mixing these samples. This practice does, however, have a few disadvantages. It gives only an estimated concentration of the sampled barrels. If SO 2 is adjusted according to the analyses, individual barrels may be over-sulphured or lack sufficient free SO 2 . To prove this, a lot of 56 barrels containing Merlot was evaluated six months into its maturation cycle by sampling and analysing three barrels. The free SO 2 concentrations were 32, 32 and 35 mg/L individually, with an average concentration of 33 mg/L. When all 56 barrels were analysed, a significant variance of free SO 2 was found. Only six barrels had concentrations similar to the average of 33 mg/L, and the free SO 2 concentrations of the individual barrels varied between 14 and 41 mg/L. More than 2 000 barrels in 60 different barrel lots of 16 cellars were analysed to understand the barrel variance better. By analysing the molecular SO 2 concentrations of the individual barrels, it was confirmed that a broad variance is not only common, but ubiquitous. SAMPLING AND ANALYSING FREE SO 2 When the analysis of composite samples is used for free SO 2 management, the choice of the barrels sampled also confirms the variations between barrels. The experiment mentioned above with 56 barrels Merlot was repeated by sampling three different sets of three barrels each. The average free SO 2 concentration of the three sets was 20, 32 and 36 mg/L, respectively. Depending on the set that a winemaker applies, it is evident that SO 2 would have differed considerably. It is impractical to sample and analyse all barrels for practical reasons like time and labour. An appropriate analysis method is required if it is decided to draw a

higher percentage of barrels to represent a barrel lot. Speed and accuracy are decisive factors when selecting an analysis method. It is also important that analysis results are recorded carefully, because specific trends may be derived to assist winemakers in their SO 2 management. SULPHITE ADDITIONS AND STRATIFICATION After the analysis results are received, the necessary SO 2 adjustments must be made. Winemakers have different opinions regarding the minimum free SO 2 concentrations required. It will be influenced by factors like the wine type, cultivar, pH, goals, maturation period and risk tolerance. Sulphite additions can be made in different ways. It can be either powdered potassium metabisulphite (PMS) or sodium metabisulphite (SMS), or aqueous solutions of them. These salts are also available in effervescent tablets, which encourage the mixing of SO 2 in the wine. It can sometimes take weeks before SO 2 additions are reflected in an analysis. A project was launched to sample adjusted wine at different depths in a barrel to ascertain the reason for it. Effervescent tablets and an aqueous PMS solution were added to two different 228 L barrels with the same wine to obtain a free SO 2 concentration of 40 mg/L. The barrels were not stirred, topped or moved during the experiment. After the sulphite addition, samples were drawn at different depths in the barrels at different time intervals. In the case of the aqueous solution addition, most of the addition was in the lower quarter of the barrel after one hour in comparison with the tablet addition, where about half of the addition was vertically distributed due to the effervescence, and the rest was at the bottom of the barrel. After six days, significant stratification remained in both ways of addition. In the aqueous addition, 21 mg/L free SO 2 was in the middle of the barrel compared to 24 mg/L of the tablet addition. Most of the remaining added SO 2 was at the bottom of both barrels. The implications of these results are that wrong SO 2 adjustments can be made, depending on the sampling depth. Unfortunately, a “correct” depth does not exist. A conservative approach may be sampling from the middle to the top of the barrel, because oxygen and potential microbial spoilage are most likely to occur at the wine surface in a barrel. However, it is essential to fix the sampling depth for comparison reasons. Stirring barrels after sulphiting will limit stratification and ensure a homogenous SO 2 distribution, but it also has other effects. Particulate and lees are distributed, and oxygen uptake is promoted. This will all increase the rate of free SO 2 consumption. When the results of SO 2 analyses between individual barrels of a lot differ significantly, resampling is recommended to ascertain the reasons for the differences. REFERENCE https://www.wineland.co.za/free-sulphur-dioxide management-in-barrels/

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WINETECH TECHNICAL YEARBOOK 2022