Jet fuel wines with soft acidity In the Winery


The hot summer was a tough one for those who didn’t have irrigation under tight control between fruit set and véraison.


By Gary Strachan W


hat a weird year it’s been! In the wines I dealt with, the norm has been high pH; sometimes over 4.0, especially in reds. Just to add that touch of whimsy that the moment lacked, I’ve also encountered an occasional pH in the range of 3.0. Since pH is a logarithmic scale, this means that the actual acidity difference between pH 3.0 and pH 4.0 is tenfold. So much for treating all wines with the same recipe. A lot (but not all) of the high pH can be attributed to the warm summer we had with related accelerated ripening. If grape varieties are matched to their growing site, we expect about three weeks of cool weather before a cool climate harvest. This accelerates the production of secondary metabolites: colour compounds, tannins, and flavour compounds. In an extended warm season, sugars continue to accumulate and malic acid respires, producing jet fuel wines with soft acidity but less of the varietal character and refreshing sourness we’ve come to know and love. The hot summer was a tough one for those who didn’t have irrigation under tight control between fruit set and véraison. Growth in a warm year can be explosive. The shading enhances potassium deposition in fruit, and contributes to high pH.


Always keep in mind that the rate of chemical reactions approximately doubles with each increase of temperature by 10 degrees Celsius. Although tartaric acid is a constant in grapes throughout the season, malic acid is respired by the Krebs Cycle (which describes the biochemical breakdown of many organic acids within living cells) The warmer the summer and autumn, the lower will be malic acid at harvest. There goes our refreshing sourness. All of these factors are described mathematically by the Henderson Hasselbalch equation. The weak organic acids of grapes each have a different dissociation constant and molecular weight, so calculating the net effect of a


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variable mixture of organic acids is seldom performed. It’s easier to try a sample in the lab, get an empirical answer and treat the batch accordingly. Just keep in mind that the equation indicates that the dominant base, potassium, raises


the pH and any of the various acids lower the pH. The complication is that tartaric acid is less soluble than malic acid or citric acid and will eventually precipitate, removing potassium along with it. The empirical relationship that I use is that one gram per litre of tartaric acid added to wine will lower the pH by approximately 0.2 pH units. I understand that sales of tartaric acid were particularly good this year in BC. I like the effect of tartaric acid because it lowers the pH with little effect on the titratable acidity (related to sourness). More soluble acids such as malic or citric also lower the pH but concurrently raise the titratable acidity.


Whether or not to adjust pH during winemaking is a matter of choice of wine style. At higher pH, flavours evolve toward a less fruity, more oxidative style. Berry flavours are suppressed in red wines, and tree fruit flavours are suppressed in white wines. If you want to retain fruity flavours, bring down the pH as early in the process as possible. Lower pH also minimizes the need for sulfite. This is another exponential relationship which can require up to 100 times as much sulfite at pH 4.0 as at pH 3.0 in order to maintain microbiological and oxidative stability.


If I am dealing with a high pH must I


prefer to add a moderate amount of tartaric acid at crush and do final


adjustments after fermentation. Keep in mind not to overdo the acidification if you intend to do a malolactic fermentation. Maintain the pH above 3.4. For white wines, you may wish


to go as low as 3.2. The flip side of the coin is low pH must, often with accompanying high titratable acidity. I encountered a low pH must from a block that grew from secondary buds after the primaries had freezing injury. From the H-H equation it’s obvious that potassium addition will bring the pH up but we also know that adding potassium will result in the precipitation of tartaric acid, which brings the pH down. Potassium bicarbonate is often the addition of choice. Try it on the lab bench before you treat a tank. Another option is double salt precipitation, also known as acidex. Acidex substitutes calcium for potassium because calcium compounds are less soluble than potassium. On the other hand calcium tartrate doesn’t crystallize as readily as potassium bicarbonate so it’s harder to achieve cold stability. The advantage is that both malic and tartaric acid precipitate, maintaining the acid balance. Acidex also contains potassium bicarbonate to seed crystallization. If you dislike doing must adjustments and have tight control of harvest composition, you may be able to avoid acid adjustments, but if your clients expect consistent wine character, acid adjustment is hard to avoid.


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