ACID ADJUSTMENT DECISIONS
by Jeanne L. Hintze
Wine Maker/ Goathill Winery
To malolactic or not to malolactic..., that is only (1)one question of a complex array of options for optimizing wine acidity. These decisions include: (2)when to harvest and (3)when to add (4)how much (if any) of (5)which acids, as well as (6)acid stabilization.
Monitor Brix (with refractometer or hydrometer) and TA (total acidity in grams /liter of Tartaric Acid) to determine when grapes are ripe. Simple acid test kits work well for TA determination and will provide specifics on quantities, while our club handbook provides conversion factors for the units you choose to work with. It is best to clarify musts by refrigerating the sample overnight and testing a racked portion. Be careful to only use distilled water, or at least boiled water to minimize errors due to CO2 when diluting must samples to readable levels.
Jeff Cox recommends aiming for a Brix to TA ratio in the range of 30:1 to 35:1 at harvest, but even when Nature complies, adjustments may be required. Avoid using under ripe grapes, with too high of an acidity. Dilution with water (boiled to remove chlorine), blending with lower acidity wines or chemical adjustments using calcium carbonate to lower TA levels produce wines of lesser quality. It is more convenient to harvest grapes at the desired Brix for the style of wine intended and add acid, when necessary. A good rule of thumb is to initially adjust to TA of .55 to .60 for reds and .65 to .70 for whites. Remember that as grapes ripen the acidity goes down, so your later harvest musts will tend to need more acid added.
Given a choice, it is easier to adjust for the lower acid levels of slightly over ripe grapes, than to lower the much higher acidity in very under ripe grapes.
Acid blend is a mixture of approximately 50% Tartaric acid, 30 to35% Malic acid and 15 to 20% Citric acid, which is close to the naturally occurring proportions or these acids in the grape. Using the blend is convenient for raising the acid level of musts or wine, with the following considerations. The molecular weights of these three acids are different, so for the addition of accurate amounts of H+, use 85% as much citric acid as tartaric acid or 89% as much malic as tartaric acid. Tartaric acid is recommeded for initial acid additions prior to primary fermentation, because it is not metabolized by wine bacteria and yeast as are Citric and Malic acids. However,
Tartaric acid is not recommended for final adjustments prior to bottling, because Tartaric acid solubilitydecreases with increased ethanol concentrations and colder temperatures and is therefore likely to crystalize out as bitartate (‘wine diamonds’, essentially cream of tartar crystals). Cold stabilization (2-3 weeks @ 32 F or 3-4 months at <40F) prior to bottling will precipitate any insoluable Tartaric acid, but then acid levels would need to be re-evaluated. Citric acid is the better choice for final acid adjustments prior to bottling because of its higher solubilty in the presence of ethanol. However, working with
Citric acid initially may lead to the generation of acetic acid (vinegar) during the primary fermentation. Malolactic fermentation reduces wine acidity by as much as 1/3 of the total acid concentration, and is an excellant choice for high acid musts and to add characteristics of fine Bordeaux and Burgundy wines. When to use Malic acid for increasing acidity would be dependant on your decision of whether to use this secondary fermentation, which converts Malic acid to Lactic acid (characteristic of buttery chardonnays), and carbon dioxide gas, diacetyl, and acetoin. Add Malic acid ONLY before adding the malolactic culture. Even if you opt to try to avoid malolactic fermentation, allow enough time for spontaneous malolactic (6 months? or test using paper chromatography). To avoid malolactic fermentation, keep it out of your winery! Once used it is extremely persistant and often spontaneously occurring. Keeping wine pH at less than 3.3 will help to inhibit malolactic. While wine developing a slight spritz after bottling might be charming, it is also unpredictable, and may become turbid, high in sediment, and (most insidiousl) may pop corks. Do not use Malic acid or acid blend containing Malic acid for any acid adjustments just prior to bottling. Use Citric acid for those final acid adjustments.