Oxalic is present in some fruit and can make some wines overly acidic in both taste and possibly chemically inhibiting the yeast form fermenting. During the many reactions as a wine ages many acids alter, malic acid turns to lactic acid for example. Oxalic acid seems more stubborn and will hang about in the final wine ruining the fun creating a initial tart taste when drunk. Screw Oxalic acid.
Apricots, figs, kiwi fruit, plums and red currants all have higher levels of oxalic acid and might need some adjustment if you want to start getting into more technical wine making. Rhubarb and apples are the poster boys for acidity and will more than likely need some tinkering.
So if you have a super villain you need a super hero in the form of calcium carbonate or its alter ego Precipitated Chalk. This should be added at the start of the wine making process before the yeast has been pitched. It is a quick easy process and doing it early allows the calcium tartrate and calcium malate crystals to settle naturally and be removed during a traditional racking.
The science! There are 2 ways to measure acidity, pH like you used to use a school with litmus papers measure the total acidity and might be good for determining technically if fermentation will start (lets face it its difficult to not ferment a wine!) Titration is the measure of taste-able acid so is good for the home wine maker like wot we are innit.
- pH Wine acidity changes during fermentation but white grape wines generally have musts of pH 3.1 to 3.3 and red wine musts generally have a pH of 3.3 to 3.5. As a home wine maker you should use the calcium carbonate to get to 3.6 and certainly no more than 4.0 as this could allow spoiling bacteria to thrive. The higher pH is becuase we lack the PHD in biochemistry and shed load of kit to allow for more complex testing and processing that commercial wine makers have.
- Titration If the test kit uses titration as a measurement get it between .55% and .75%. as a rough guide one teaspoon of Calcium Carbonate added to 1 gallon of wine, will lower it by .10%
The non-science. Rhubarb has mainly two types of acid being oxalic and malic. Oxalic is more likely to bind with the calcium carbonate leaving a higher concentration of the slightly more desirable malic acid. This is because malic acid is more likely to be respired during fermentation as opposed to the stuborn oxalic. Once the fruit has been macerated (soaked in water to extract the taste) and then removed where only the liquid is left the calcium carbonate can be added.
If you don’t want to invest in a kit (I haven’t!) you can generally use this as a decent rule of thumb. Add one teaspoon of calcium carbonate per gallon of wine – that’s 2 grams per 4.5 litres. Remember that you are not nutralising all acidity as all wines are acidic and that acidic taste changes over the maturation of the wine.
Adding it makes the must bubble like a bastard – this is normal, just carbon dioxide escaping as it reacts with the oxalic acid. The best results are obtained adding it over the course of a couple of hours. Separate some must, dissolve the calcium carbonate and the add it in 4 or so batches stirring it in thoroughly each time. The reaction is quick taking about three or so hours but the resulting crystals need time to settle as lees. This should be no problem as it occurs over the next eight weeks of fermentation and will be naturally left behind during racking.
It should be noted that you cannot cold crash during this time – not that I can think of a reason you would need to. Also total dosage should not exceed three and a half teaspoon per gallon of wine. If that was the case you were probably making battery acid wine so best just chuck it anyway.