tonyhibbett
Landlord.
I've had a lot of problems this year with acetification of wine in casks. I think the problem is to do with the long hot summer. I don't have the luxury of a cellar, where conditions are generally cool, damp and stable. Under such conditions, water slowly evaporates through the pores of the wood and air is sucked in by the vacuum created. As a result, the wine becomes more concentrated and the slow oxidation makes it taste smoother. However, in sustained hot and dry conditions the rate of evaporation is significantly increased and the level of liquid needs to be topped up to prevent the top of the cask from drying out, which allows bacteria to enter through the wood joints. The worst offender is acetobacter which, given warm conditions and an oxygen supply, turns alcohol into acetic acid and acetone, i.e. vinegar. Left undetected and unchecked, you get sour, undrinkable wine with an unpleasant smell. Typically vinegar contains 5% acetic acid, which is 10 times the amount normally present in wine. Although my wine had not got to that stage, it was far too acidic and smelled 'off'. One cask contained 10 gallons of 1 year old pinot noir and I was not about to tip it down the drain!
I kept adding potassium bicarbonate, which caused it to foam violently. In all it took about 400 g plus 200 g of calcium carbonate to get rid of the smell and reduce the acidity to an acceptable, drinkable level. The dosage was far in excess of that recommended and supposedly this creates an off-taste of it's own, caused by the by-product salts created. Fortunately this did not happen.
Normally, the main acids in wine are tartaric and, to a lesser extent, malic. I couldn't find any information about the effect of potassium bicarbonate on acetic acid but it is well established that it starts to work on tartaric acid initially, whereas calcium carbonate works on both tartaric and malic acid at the same time, thus preserving the balance of the two. I have no way of measuring the levels of the individual acids now present other than by taste. All I can say is that it is now smoother and more pleasant to drink but lacks the really good flavour an aroma it once had and is somewhat deficient in tannin, which is also an acid. So I added 5 level teaspoons, dissolved in enough sulphited warm water to top up the cask.
Using a vinometer, I measured the abv as 12%. Considering that the acetobacter must have used up some of the alcohol, this suggests that only water evaporated from the cask, thus concentrating the alcohol (and the acidity).
Anyway, lessons learned. As I can't control the temperature and humidity, I should keep a supply of topping up wine and regularly maintain the level during hot dry weather. Of course, each time you remove the bung you run the risk of exposing the wine to infection so a little sulphite added to the topping up wine for protection is a good idea.
Cask size is an important factor. The smaller the cask, the greater the rate of evaporation. I have a 14 gallon cask which was subjected to the same weather (stored outside). The wine was also 1 year old and did not suffer the same fate. I have smaller (5 gallon) casks which were affected but to a lesser extent, possibly because they are stored indoors where the temperatures 915-20) and humidity (no central heating) are less extreme, due to an air conditioning unit.
I kept adding potassium bicarbonate, which caused it to foam violently. In all it took about 400 g plus 200 g of calcium carbonate to get rid of the smell and reduce the acidity to an acceptable, drinkable level. The dosage was far in excess of that recommended and supposedly this creates an off-taste of it's own, caused by the by-product salts created. Fortunately this did not happen.
Normally, the main acids in wine are tartaric and, to a lesser extent, malic. I couldn't find any information about the effect of potassium bicarbonate on acetic acid but it is well established that it starts to work on tartaric acid initially, whereas calcium carbonate works on both tartaric and malic acid at the same time, thus preserving the balance of the two. I have no way of measuring the levels of the individual acids now present other than by taste. All I can say is that it is now smoother and more pleasant to drink but lacks the really good flavour an aroma it once had and is somewhat deficient in tannin, which is also an acid. So I added 5 level teaspoons, dissolved in enough sulphited warm water to top up the cask.
Using a vinometer, I measured the abv as 12%. Considering that the acetobacter must have used up some of the alcohol, this suggests that only water evaporated from the cask, thus concentrating the alcohol (and the acidity).
Anyway, lessons learned. As I can't control the temperature and humidity, I should keep a supply of topping up wine and regularly maintain the level during hot dry weather. Of course, each time you remove the bung you run the risk of exposing the wine to infection so a little sulphite added to the topping up wine for protection is a good idea.
Cask size is an important factor. The smaller the cask, the greater the rate of evaporation. I have a 14 gallon cask which was subjected to the same weather (stored outside). The wine was also 1 year old and did not suffer the same fate. I have smaller (5 gallon) casks which were affected but to a lesser extent, possibly because they are stored indoors where the temperatures 915-20) and humidity (no central heating) are less extreme, due to an air conditioning unit.
