Have you ever taken a sip of a steely crisp white wine such as a New Zealand Sauvignon Blanc or a Chablis and been delighted with its clean and refreshing character?  That’s the treat of acidity at work.

Wine is inherently an acidic drink.  Even wines that taste sweet to the consumer are very high in acid; the residual sugar simply masks the acidic character.  One of the key characteristics of a sweet German Ries-ling is the “racing acidity” underlying the sweetness, providing an ideal balance.

The level of acidity in the grapes (measured by the pH of the juice) is one of the primary indicators of when to harvest the fruit.  Young grapes are usually characterized by high acidity and very low sugar producing a very tart taste.  Most wine makers try to pick their grapes at the point where sugars and acids are at their maximum.  Beyond that “ideal” point, sugars continue to rise as acid levels drop.  Without the acids, the wine becomes what is known in wine language as “flabby” or dull, loos-ing much of its personality.

Acids are also a key component for wine preservation along with alcohol and sugar.  A German Riesling with high residual sugar and high acid can easily age for 20 years.  Acid in wine (along with alcohol) also re-tards bacterial growth.  With the exception of a few harmless bacteria, nothing harmful can live in wine.  That may explain some (but clearly not all) of the preference for alcohol to the risks of tainted water in colo-nial days.

The unfermented grape contains three primary acids:  Tartaric, Malic, and Citric, although a few additional acids play a role as well such as Lactic, Succinic, and Acetic.  Tartaric and Malic acids are known as non-volatile acids; that is, they do not evaporate when boiled.  This is in distinction to Acetic Acid which is a volatile acid and an undesirable quality in a wine.  When you hear someone refer to the distasteful scent of volatile acid in a wine, they are usually referring to a vinegary scent.

All three primary acids are produced by the grapes as they develop.  In warmer climates, Tartaric and Malic acid levels are reduced due to the process of respiration.  This helps to explain why grapes in warmer cli-mates (Zinfandel grapes from the Napa Valley) possess less acidity whereas those from cooler climes (i.e., Alsace) retain more.  Just for ref-erence, acid and sugar production move in opposite directions.  While a warmer climate will reduce acid production, it will enhance sugar levels expressed in percent or Brix and thus increase alcohol production as with the Napa Valley Zinfandels.  On the other hand, a cooler climate while supporting the retention of acids will result in a lower Brix in re-gions such as Chablis.

The acid balance of bottled wine can be traced back to several factors, the most important of which is the varietal of grape.  If the vineyard manager is attentive and the weather cooperates, the grapes will be ma-ture (ripe) when harvested while retaining a nicely balanced acidity.  Think of a dry Australian Riesling, a Sauvignon Blanc or a cold climate Pinot Grigio that is harvested at the perfect time, expressing its best in white fruit flavors while retaining maximum acidity.  However, there are some grapes that upon reaching full maturity will have lost some of their acidity allowing other components such as fruit or tannins (reds) to take a front seat.  Semillon, for example, tends to loose some of their acidity by the time they are ready for harvest.

The vineyard manager and winemaker also play a pivotal role in achiev-ing the desired acid balance of the bottled wine.  The vineyard manager is responsible for vine health, grape selection and harvesting decisions while the winemaker has the ability to influence the acidity of the final product during and after fermentation.  If the must (grape juice, skins, stems and pulp prepared for or going through fermentation) requires acidification, Tartaric Acid may be added or alternatively, a must of dif-ferent acidity can be incorporated.  If there is excessive acidity, the must can be diluted with water or de-acidified through the addition of a base chemical.  Additional actions taken by the winemaker during fermenta-tion such as altering the temperature and oxygen levels (oxidation) may alter the must acidity levels as well.

Malo-lactic fermentation (MLF) is a term that you will hear used often to describe the process of “softening” the wine’s acidity.  MLF, which actually is NOT a chemical fermentation, converts some of the tart Ma-lic acid (think green apples) into Lactic acid (softer acid) and Carbon Dioxide.  This process is accomplished through Lactic Acid Bacteria ei-ther naturally present in the winery or through the addition of cultured stains.  While the process is sometimes called a secondary fermentation, it is not uncommon for it to occur during the primary alcoholic fermen-tation and be completed by the end.  Additional maneuvers or decisions that can be considered post fermentation but prior to bottling include cold/heat stabilization (processes that affect the appearance of the wine through the removal of acidic crystals), draining off what is known as “free run” (the juice from grape pressing), racking (draining the wine off of the remaining sediments), choice of oak barrel or stainless steel for aging, blending and even the addition of water.

Even after bottling, the winemaker can manage acidic balance through bottle aging and determining when to release the wine.  Generally, al-lowing the wines to age allows some of the acids to break down and lower the total acidity of the wine.

So next time you take a sip of wine, whether red or white, think about the complex decisions that affect the wine’s acidity from the grapes themselves to the finesse and attentiveness of the vineyard manager and winemaker.  If they have made the right decisions, your wine will taste fresh and balanced.

David Muhleman, PhD, CSW

Tags: acetic acid, acidity, brix vineyard manager, Citric acid, cold stabilization, fermentation, free run, heat stabliization, lactic acid, Malic acid, malolactic fermentation, pH, succinic acid, tartaric acid, volatile acid, winemaker