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Potassium metabisulphate What is it? and why is it used in wine?
Potassium metabisulphate (a.k.a. KMS) is a white crystalline powder
Antiseptic action Many microbes are killed by sulphur dioxide and when added early during winemaking has the effect of selectively promoting the growth of “wine yeast” that are relatively sulphur dioxide tolerant. Although sulphur dioxide "loses" its antiseptic properties during fermentation, its initial influence in culling spoilage microbes ensures that the relatively alcohol tolerant “wine yeasts” are present later in the fermentation. Besides having sulphur dioxide tolerance, wine yeasts are also able to tolerate high levels of alcohol thus ensuring the completion of fermentation. While sulphur dioxide is not a “magic bullet” for unwanted microbes - as a number of other spoilage microbes have similar tolerances as “wine yeasts” - its use, combined with an understanding of the populations present and the environments they exist in , provides an excellent winemaking tool, that more often than not results in superior quality wines. Antioxidising action Grape juice and wine are not stable when exposed to air and deteriorate to produce oxidised characters over time. The "bruised apple/nutty characters" that occur when wine oxidises result in either, the loss, and/or the masking of the primary fruit aromas. Rather than acting directly on dissolved oxygen sulphur dioxide acts to bind oxidised byproducts, that if left unchecked will further oxidise a wine. In addition to losing aroma, oxidised wine (both red and white) takes on a distinctive brown colouration and loss of texture (mouthfeel).
Organic wines The fundamental premise of the organic food movement is that much of the ill health in society today in caused by the use of chemicals in our food. These chemicals include herbicides, pesticides, antibiotics and hormones on our farms and an ever increasing list of additives in our factories, and by removing the chemicals we will all lead healthier, happier lives. Sulphur dioxide, is one such additive that causes a small proportion of the population (estimated 5% of asthmatics) an allergic reaction, while for the rest of us it poses no danger at all. While it is acknowledged that the estimated 1,000,000 people in the States who have this reaction should be warned of its presence, perhaps it would be better if everyone just accepted that wine contains sulphur dioxide and thereby negating the need to declare it on the label. Let some enterprising winemaker cater for the demands of this section of the public. Unfortunately, this is not the case and sulphur dioxide is now a known allergen that is added to wine, and as such has been “tarred with the same brush” as all the synthetic chemicals used in “conventional” as opposed to organic production, albeit naturally occurring in the product in the first place. The addition of sulphur dioxide is officially forbidden in organic wines, and American legislation stipulates that all wines containing over 10 mg/l (total) sulphur dioxide must carry an “added sulphites” declaration irrespective if it is added or not. There is however, a further category of “wine made from organic grapes” that allows up to 100 mg/l of sulphur dioxide and this is where many health conscious wine lovers will find their favorite wines. While I can see an argument for additives like isinglass, casein and albumen which come from other species, the addition of bentonite clay and sulphur dioxide, in my opinion should be deemed “normal” and intrinsic in the production of wine.
How much KMS should be added? Unfortunately, the addition of sulpur dioxide requires a little chemistry, a little maths, a little experience and some basic laboratory skills. Let's start with the chemistry, the molar mass of KMS is 222 and the two molecules of sulphur dioxide it produces when added to an acidic medium (i.e. wine) have a molar mass of 128 or approximately 57% of the weight. Next, when added to wine only 50% (assuming the total sulphur dioxide concentration is less than 60 mg/l) remains as free sulphur dioxide (the remainder binding to wine constituents). As it is the free sulphur dioxide that has the antiseptic and antioxidising properties the degree to which it binds is important. Besides the total amount of sulphur dioxide already in the wine (both free and bound), the sugar content, general condition of the grapes and stage of fermentation all affect this and there is no substitute for experince in this matter. Next, the potency of the free sulphur dioxide as an antiseptic is determined by the wine's pH - the lower the pH the more potent. Charts are available to help with this and provide the required free sulphur dioxide levels to be effective at a given pH. Given that the juice/wine has relatively low pH and previous additions have been kept to a minimum, then a molecular sulphur dioxide level of 0.8 mg/l should be achievable. Sulphur dioxide concentrations at this level provide effective antiseptic action to inhibit most microbial activity.
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that, when added to wine, forms sulphur dioxide (gas). This colourless [dissolved] gas has antiseptic and antioxidising properties that act to preserve a wine from deterioration. While sulphur dioxide (gas) can be added directly to wine, its containment (under pressure) and pungent aroma (rhino stopping) make it's stable white salt forms (either potassium or sodium) the preferred option for many winemakers. As it is manditory in most countries to declare its addition, labels often say "contains sulphites (sulfites)" or give an "E number" (sulphites E220). Depending on the form added, sulphur dioxide (E221) uses the coding from E222 to E228 with Potassium metabisulphate (KMS) and sodium metabisulphite having the codes E224 E223, respectively.
