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.

Malolactic Fermentation

WHAT IS IT? Malolactic fermentation is the conversion by bacteria of malic acid into CO2 and lactic acid.

One gram of malic acid converts roughly into 0.67 grams of lactic acid and 0.33 grams of CO2.

WHY USE IT? There are several reasons:

1. The primary reason for using malolactic fermentation is to reduce acid in red wines and some selected white wines by organic rather than chemical means.
   
   Typically both red and white grapes grown in B.C. are characterized by low pH and high TA, both in combination indicating that the malic acid is probably higher than the tartaric acid.
    
2. The chances are that if it is not used under controlled conditions, it will happen spontaneously, usually after the wine has been bottled.
   
   This is the phenomenon where wines "awaken" in the Spring when temperatures begin to rise. By this time, the pH is usually higher than it was before fermentation and SO2 levels have been reduced.
    
3. A wine high in malic acid is naturally more acidic; therefore, the greater the reduction the smoother the wine.
    
4. The more aggressive and pronounced malic acid is replaced by the less aggressive lactic acid.
    
5. The young wine loses its hard and acidic edge:- its colour loses some of its vividness,
    - the grape odour becomes richer and more vinous,
    - wines become more mellow and full-bodied
    - wines tend to become buttery as a result of the formation of diacetyl during the malolactic fermentation.
   
   The latter phenomena are stylistic reasons for using malolactic fermentation.

SHOULD IT BE USED ON ALL RED AND WHITE GRAPES?No.

Typically malolactic fermentation is used only on red wines destined for aging and selected white wines such as Chardonnay, Pinot Blanc and Pinot Gris. Wines such as Gewurztraminer, Ehrenfelser, Riesling and other wines noted for their aromatic characteristics should not undergo malolactic fermentation, as they depend upon the malic acid to enhance their flavour components. They are also typically higher in acid than wines destined for table use as they also usually have residual sugar that offsets the higher acid.

Wines that have undergone malolactic fermentation require lower additions of SO2 to maintain stability than those that have not undergone malolactic fermentation. The former may be stabilized with about 100 - 120 ppm SO2 while those not undergoing malolactic fermentation will require fining and filtering as well as additions up to, perhaps, 150 ppm SO2.

WHAT ARE THE CONDITIONS CONDUCIVE TO MALOLACTIC FERMENTATION?

There are several, some critical:
 

1. pH is the single most important factor The ideal pH is about 4.0, too high for safe use, but ideal for developing a starter. Normally, the pH for reds should be above 3.3 and for whites 3.4: however, there are strains of Malolactic bacteria available that will work well below these pH's.
    
2. Temperatures should be above 20 - 30?C. So the best time to inoculate the must with the malolactic bacteria is when the wine is about one-third through the yeast fermentation, as the fermentation process generally maintains that range of temperatures.
    
3. Low alcohol is also preferred, thus the above-mentioned time of inoculation
    
4. Unclarified wine is preferred because of the contained nutrients, thus one more reason to inoculate during the latter stages of fermentation.
    
5. Low SO2 levels are also necessary, usually between 10 and 20 ppm, the amount added during crushing or settling; although, unless the grapes are less than perfect, no SO2 additions are required until either the pressing of the reds or the first racking of the whites.

WHAT MALOLACTIC CULTURES ARE AVAILABLE?

I have used three:

1. A freeze-dried culture that had to be rehydrated, was quite expensive and generally a pain in the butt.
    
2. A standard liquid culture of two strains - ER1A and EY2D - from Oregon State University available from Spagnol's. It is easy to use and is efficient at lower temperatures - down to 10?, and lower pH's - down to 2.9. It can be used for both reds and whites by making a standard starter using apple juice which has the ideal pH of 4.0. I usually build the starter to one gallon then start adding grape juice in order to build it up more as well as to sensitize it to the lower pH. I have never had a failure with this culture. A packet costs about $7.50
    
3. A dried culture - Oenos Viniflora - available from Flory Bosa. A very small amount , about one-eighth of a teaspoon in 25 gallons, is sprinkled on top of the wine. I used it for the first time last year. It works very well without making a starter. However, the larger the culture count, the more efficiently the fermentation will proceed, so this year, I will probably make a starter. It is very expensive, about $85, but it is good for about 1000 gallons and can be kept for about two years in the deepfreeze.

WHY USE A CULTURE?

As stated above, if you don't induce malolactic fermentation, it is likely to occur on its own; therefore, why use a cultured starter?

The answer is the same as for using a yeast culture: both yeast and malolactic fermentations will occur on their own and may, in fact, be quite safe. On the other hand, wild malolactic bacteria like wild yeasts are not always friendly. There are few strains of malolactic bacteria that are friendly such as leuconostoc and oenococcus oeni. Wild strains include pediococcus which produces brettanomyces and lactobacillus which produces a buttermilk character; as well, wild things include acetobacter which produces vinegar.

HELPFUL HINTS

1. If you have to add acid in order to bring the pH down, use only tartaric not an acid blend. Commercial acid blends contain tartaric, citric and malic acids. Citric acid can promote acetic acid formation. Commercial malic acid contains two forms of malic acid and malolactic fermentation converts only one of them.
    
2. Do not use any form of sorbate in a malolactic-fermented wine. In the event of renewed malolactic fermentation, sorbate will produce geraniol, an unpleasant geranium-like odour.
    
3. All my reds undergo malolactic fermentation, but I like to do partial fermentations on my whites if possible. For example and depending upon both the volume and my barrel capacity, I usually put a portion of the wine into a barrel with the malolactic culture and a portion in glass or stainless steel without the malolactic culture. Both are then blended and sterile filtered with an addition of SO2. So far, I have not had any renewed malolactic fermentation.
    
4. Check the progress using color chromatography.

Hydrogen Sulfide

Hydrogen Sulfide (H2S) has been known to plague winemakers for centuries, but it needn't.  Its causes are as simple as is its cure, if dealt with soon enough after  detection.

There are three primary causes:

1. Residual sulfur on the grapes as the result of a late spray for powdery mildew;

2. Some yeasts, such as Montrachet(UCD 522) and some strains of Steinberg, are known to produce higher levels of H2S; and more commonly,

3. Low nitrogen levels in the grapes which results in higher levels of H2S being produced by yeast cells (all yeasts produce some H2S that is dissipated during fermentation).

Early detection of the rotten egg odour and subsequent racking with deliberate splashing will usually cure the problem, as H2S is highly volatile.  However, in order to reduce the risk of H2S formation, it is wise to add yeast nutrient containing diammonium phosphate (DAP) at the rate of 100-200 ppm during the early stages of fermentation.  Do not add DAP at the beginning of fermentation, as it will overpower the yeast which has not yet had enough time to multiply to full activity.

Failure to treat H2S in its early stages will only add to your problems later, as H2S, when it interacts with alcohol, produces mono-mercaptans (sulfides) which have a range of odours - garlic, cabbage, onion, rubber, skunky - and are more difficult to remove because, unlike H2S, they are much less volatile having become bound through interaction with alcohol.  Even at this stage it is possible to treat the wine and remove the offensive odour, but it is more difficult to do so. One can successfully remove mercaptans in their early stages by a combination of aeration and passing the wine through a Molly MaidÒ copper pot scrubber stuffed into a one-inch piece of plastic (PVC) pipe.  It is absolutely essential that the wine be exposed to as much of the copper surface as possible and that the copper be free from contamination resulting from handling.

If you have not dealt with either the H2S or the mercaptans, then you are in trouble big time because now the mercaptans, if the wine has undergone any oxidation (which occurs during barrel ageing), have formed poly-mercaptans (disulfides) which will not react with copper.  Disulfide odours have been described as asparagus, corn or molasses.  Simply dropping a piece of copper into the wine and swishing it about may affect the mono-mercaptans but it will not have any effect on the poly-mercaptans; and it may work only if the copper is highly polished and clean of any contamination.  If you think the copper eliminates all the odours, you are being swayed by the power of suggestion.  The only way to deal with this problem is to reduce the disulfides back to the mercaptan stage, and there are two ways to do this:

1.     The addition of USP mineral oil will remove the disulfides (see Jackish) because they are more soluble in oil than they are in wine (And then you have to remove the oil.); and

2.     Treating the wine with ascorbic acid which will break the disulfide back down to sulfide and adding copper sulphate (CuSO4.5H2O) solution to remove the sulfide.

Testing before treatment is absolutely necessary because it is possible to confuse the off-odour for Brettanomyces, which has a barnyard odour and cannot be eliminated by treating it for mercaptans.

To test, it is necessary to first make two stock solutions:

One of copper sulphate which is done by dissolving 4.1 grams in a little water and bringing the volume up to one litre with distilled water.  (Use 10 ml of this solution with 90 ml of distilled water to make 100 ml total for the lab test.)

One of ascorbic acid which is done by dissolving 10 grams in a little water and bringing the volume up to one litre with distilled water.

Next, put 100 ml of the suspect wine into three glasses.  Use the first glass as the control.  Put 5 drops of the diluted copper sulphate solution into glass number two and stir well.  Into glass number three, put 5 drops of the ascorbic acid solution, stir well and, after a few minutes, add 5 drops of the copper sulphate solution and stir well.  The following table illustrates the possible results.

After Yair Margalit: Winery Technology & Operations.

If disulfide is not present, addition of the copper solution will help; if disulfide is present, both ascorbic acid and copper sulphate must be used.  To determine the amount of the copper solution to use, set up a series of glasses with 100 mls of wine and add 0.05 ml, 0.1 ml, 1.5 ml, etc. of the solution.  Check the smell of each glass and select the first one that no longer smells.  The addition of the copper solution used is the equivalent in parts per million of copper sulphate addition.  Thus 0.1 ml = 0.1 ppm.  To treat a 19 litre carboy of wine with 0.1 ppm requires 0.1 ppm x 19 = 1.9 ml of the stock copper solution.

Prior to adding the copper solution, add about 25 ppm of ascorbic acid, or about 0.5 grams in a 19 litre carboy.  Stir in well and wait at least one day before adding the copper solution.

Ascorbic acid in conjunction with copper sulphate works very well, but it is not instantaneous; it takes several days before the odour and taste disappear.  Do not exceed the recommended dosage of copper sulphate or you may induce a copper haze which will be difficult to remove. 

Remember,H2S (volatile) à mono-mercaptans (becoming bound) à poly-mercaptans (bound), so deal with the problem as soon as it is detected.  This process is not discrete: that is, while H2S is present, it is likely that mono-mercaptans are forming; and poly-mercaptans may be forming before the H2S in its volatile form disappears.  Research shows that mercaptan formation occurs within two days after the beginning of fermentation and is at its peak at about two months after which the poly-mercaptans become dominant.  Since most winemakers barrel-age their wines for much longer periods, if H2S has been detected and removed in the early stages, constant checking for mercaptan odours is critical since the barrel is where the mercaptans are formed, and they will continue to develop in the bottle.