In wine, sulfur dioxide (SO2) exists in three main forms, dependent on wine pH: molecular SO2, bisulfite and sulfite. When SO2 is measured in wines it is generally reported as two results: free SO2 and total SO2. The free SO2 includes all three forms of SO2 above, however SO2 will also bind to other wine components, including aldehyde. The free and bound forms together comprise the total SO2.The presence of the molecular form is favoured by lower pH levels.
It is predominantly the molecular form of SO2 that has the anti-microbial and indirect antioxidant properties that make SO2 such an effective preservative. Thus, knowing the concentration of molecular SO2 in your wine, rather than just the free SO2, can provide greater understanding of your risk of microbial spoilage. The WineCloud will automatically calculate the molecular SO2 in your wine any time that you upload wine data that includes both free SO2 and pH.
Based on years of experience with wines affected by oxidation or microbial spoilage, the AWRI’s Winemaking Services team recommends that you add enough SO2 to achieve a molecular SO2 concentration > 0.6mg/L for red wines and > 0.8 mg/L for white wines.
Ratio of free to total SO2
An effective way of checking for microbial growth (including Brettanomyces) or oxidation problems during wine storage and ageing is to monitor the ratio of free to total SO2. If yeast or bacteria are active or if your wine is being oxidised, then you will see the proportion of bound SO2 increase, causing the free to total ratio to drop. Typical ratios of Free to Total SO2 in finished wine are in the range of 0.3 or 0.5 (equivalent to F:T ratios of 30:90 or 30:60). When a wine has a comparatively low ratio of free to total SO2 (e.g. less than 0.25) this is often an indication that oxidation and/or microbial activity have taken place at some stage of the wine’s life, because the products of such activity bind strongly to SO2. Additionally, if you are adding SO2 to a wine and finding that most of what you add is being bound up, yielding only a low percentage in the free form, this can give an immediate indication that some kind of problem is going on in the wine. The timing and magnitude of SO2 additions to a wine can also influence its final ratio of free to total SO2, with larger additions made less frequently generally yielding a higher percentage of free SO2 compared with more frequent small additions.
The WineCloud will automatically calculate the ratio of free to total SO2 any time that you upload data with both free and total SO2 values, giving you extra information to assist with early detection of wine spoilage.
Grape Total phenolics is a measure of all coloured and non-coloured phenolic molecules present in grape skin, flesh and seed. Grape total phenolics is measured in AU/g fruit.
Grape Total tannin: Tannins are a sub-class of phenolics and are characterised by their ability to precipitate proteins. Grape tannins are present in both the seeds and skins of grapes. Grape total tannin is measured in mg/ g fruit epicatechin equivalents. It’s important to note that grape tannin is chemically different from wine tannin. The relationship between grape tannin and wine tannin has not yet been fully established and can be influenced by a number of winemaking practices and techniques, including fermenter type and size, cap management, temperature etc.
Grape total anthocyanin is a measure of the red colour of a grape sample. Grape anthocyanins are measured in mg/ g fruit malvidin-3-glucoside equivalents. Most anthocyanins come from grape skin.
Wine total phenolics: This is a measure of all coloured and non-coloured phenolic molecules in wine that originate from grape skin, flesh and seed. Wine total phenolics is reported in absorbance units (a.u.).
Wine total tannin: Tannins are a sub-class of phenolics that can precipitate proteins. When grapes are crushed, the tannins present in grape skin and seeds begin to be extracted into the grape must. Those from skins tend to be more easily extracted than those from seeds. Once extracted, the grape tannins begin to chemically rearrange, turning into wine tannins, which can be significantly different in structure from the original grape tannins. Wine tannin concentration is reported in g/L epicatechin equivalents.
Wine total pigment is a measure of total red colour in the wine or ferment sample. It is reported in absorbance units (a.u.). Total pigment is predominantly made up of free anthocyanins and pigmented tannins, both of which can also be calculated using the Wine Portal. However, there are other pigmented compounds present in red wines, so total pigment is not necessarily the sum of free anthocyanins and pigmented tannin. By analysing for free anthocyanins and pigmented tannins, users can gain insight into their wine’s colour stability.
Free anthocyanins are the highly coloured compounds responsible for the colour of red grapes. They are found in grape must, ferments and young wines, but are not very stable under wine conditions so their contribution to wine colour decreases quite rapidly as wine ages. Anthocyanins are bleachable by SO2.
Pigmented tannins are stable coloured compounds formed through the reaction of anthocyanins with tannins during fermentation and wine storage. Pigmented tannins have been shown to contribute up to 90% of the colour of red wine after two years’ storage. They are not bleachable by SO2.