Updated: Jul 4, 2020
What is acid blend? Acid blend contains a mixture of Tartaric, Malic, and Citric acid. The ratios of the acids can vary wildly between acid blend manufactures with some in the 40-40-20 Tartaric-Malic-Citric range, while others are in the 10-50-40 (LD Carlson). In a grape, around 50-60% of the acid is tartaric, 30-40% is malic, and less than 10% is citric. Rather than add acid blend, normally you would make grape wine adjustments with Tartaric acid. On a very rare occasion you can add malic acid or citric acid to achieve certain goals that we will cover in this article.
Tartaric acid plays nice with a fermentation and does create any off flavors or smells. Generally, this is the acid that you will want to add when making adjustments to the must. If potassium is present, some tartaric acid will become insoluble and fall out as potassium bitartrate (Cream of Tartar). This is not a bad thing, but requires you to add a bit more than you think in certain musts. I generally like to adjust my pH down to 3.6 or lower before fermentation with a red wine (3.0-3.2 with a white wine). If potassium bitartate falls out above pH 3.6, the pH will rise dramatically. If it falls out at 3.6, very little change should occur. Below 3.6, the pH will drop slightly. I can't fully wrap my head around the chemistry of this, but I have seen it in practice many times. Clark Smith mentions this in his book Postmodern Winemaking, and that guy knows his stuff...
This acid is responsible for the crisp, apple like character of a white wine, and is the dominant acid in an apple. In a red wine or a chardonnay, it is most common to allow the wine to complete malolactic fermentation. In malolactic fermentation, bacteria convert the harsher malic acid to a silkier lactic acid. This process creates Diacetyl, which is responsible for the buttery smell that a wine can have. A little malic acid is a good thing in most wine musts, but too much of a good thing can be bad. Too much malic acid can make a white wine taste a little too sour. In a wine that will undergo malolactic fermentation, too much malic can lead to too much diacetyl which can put off a little bit of an old milk vibe. I generally will not add any malic acid to a red wine but will consider it in a white wine (though very rarely).
This acid is strong and very effective at adjusting a pH, but also can easily ruin a wine. Citric acid can create excess diacetyl and acetic acid (vinegar) when metabolized by malolactic bacteria. Citric acid also has a distinct citrus taste which is generally not welcome in a red wine. The little bit of citric acid that is present in a grape generally will not cause issue with your wine, but if added in high doses to adjust the pH, trouble can occur. In a white wine that will not undergo malolactic fermentation, some citric acid can be used to enhance the citrus character, but use it cautiously.
All of these acids are readily available from internet retailers, or at your local wine shop. I use tartaric acid about 10 to 1 over malic acid, and always keep some on hand before the wine harvest. I very rarely use malic acid and almost never use citric acid, though I do keep some on hand for the rare instance that I want to liven up a white wine. In fruit wine making (non-grape), the dominant acid of the fruit is generally the first acid to reach for. In some instances an "Acid Blend" can be used, but I would rather add my own blend for each specific fruit. Be careful when adding too much of any one acid to a fruit wine, as it can sometimes dominate the flavor. Also, take steps to prevent malolactic fermentation in fruit wines, like a 50-75ppm SO2 addition after primary fermentation and otherwise managing SO2 like a boss.
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