Every winemaker, at one time or another, has had
to deal with grapes having less than ideal pH and acid balances.
Whether the grapes come from California, a warm climate, or Oregon,
Washington and the Okanagan, cool climates, the winemaker is faced
with the problem: how to adjust the acid and pH.
It may be necessary to adjust the grape acid
level for different reasons:
1.
The pH is too high and the acid too low;
2.
The pH is too low and the acid is too high; or
3. Both
the pH and the acid are too high, usually the result of excessive
amounts of malic acid and potassium.
Both 2 and 3 are phenomena common in cool climate
grapes; although 3 occurs in warm climate grapes also.
Before continuing, it must be pointed out that
when Total Acidity (TA) is referred to it will be
in grams per litre. Some books refer to acid as a
percentage: ie - 0.7 percent. To make the transition to grams
per litre, move the decimal one space to the right; thus 0.7 becomes
7 grams per litre.
All white and rosé musts should be adjusted to a
maximum of pH 3.3 and all reds should be adjusted to a maximum of pH
3.4 in order to achieve optimum flavour extraction
and to minimize bacterial infection. These adjustments should
be made with
tartaric acid. Even if this procedure increases
the TA above desired levels, the tartrates can be precipitated with
cold stabilization. The rule-of-thumb for tartaric acid
addition is 1 g/l to reduce the pH by 0.1.
However, there are some cautions:
1.
Different grapes have different buffering capacities;
2.
High pH drops, say from 3.6 to 3.3 may require about 4 g/l of
tartaric acid instead of the rule-of-thumb 3g/l as the ratio is on a
curve rather than being linear;
3. Adding
acid can result in some precipitation of potassium hydrogen
tartrate (KHT) which may affect both pH
and TA values. Therefore, lab tests should be performed.
The most convenient way of testing for the
appropriate amount of tartaric acid to add is to prepare a 10%
solution (10 grams in 100 ml of water) of tartaric acid in
distilled water. Dissolve the acid in a little water
and add water to exactly 100ml. Set up several glasses
containing 100ml of juice and, using one as a control, to the other
glasses add 1, 2, 3, etc. ml of the 10% solution and measure the pH
changes. The volumes of the 10% solution used are equivalent
to the grams per litre of acid necessary to make the required
adjustment. That is, one ml of the solution equals one gram of
acid. The same process can be used if post-fermentation acid
adjustments need to be made. By using the 10% solution,
results are instantaneous and less bothersome than dissolving the
acid volumes one at a time.
Be aware that all the acids - tartaric, malic and
citric - will affect the TA values differently. While a
one-gram addition of tartaric acid will increase the TA by one gram
per litre, malic acid will increase the TA by about 1.12 and citric
acid by about 1.17. They also affect the flavour differently.
So lab tests are essential.
Whether the grapes are pressed immediately after
crushing or let stand on the skins for flavour extraction before
pressing, once the juice sample has settled and cleared, the acid
and pH readings should be accurate. The same cannot be said
for red grapes, however. Most winemakers take their samples
immediately after crushing, but the readings are not accurate.
Doing a test twenty-four hours later will see an increase in pH of
between 0.1 and 0.2 as the direct result of potassium
extraction. The TA change will be minimal. A further
increase in pH will be observed after pressing due to maceration
during fermentation and greater extraction of skin constituents.
Many grapes, particularly in climatically
unfavourable years, may require the acid to be reduced prior to
fermentation. This can be achieved in several ways.
Water Addition:
Adding ten to fifteen percent water, particularly with the more
floral grape varieties, will achieve the desired TA without making
significant changes in the pH; however, sugar will have to be added
as the result of dilution.
Occasionally grapes from California, even the
Okanagan, have high Brix (or SG),
high pH and high TA; and it may be desirable to add some water in
order to decrease the potential alcohol of the high sugar.
While adding water will also reduce the acid, it will still be
necessary to add acid in order to reduce the pH.
Cold Stabilization:
This procedure is usually performed after fermentation, when the
weather is slightly below freezing. Putting the wine into a
refrigerator is an alternative. In either case, it also helps
to "seed" the wine with cream of tartar crystals in
the amount of 2 - 6 g/l. Do bench trials to determine the
optimum amount to be used. Cold stabilization will not work if
the pH is too low, less than 3.2, because the malic
acid content will be higher than the tartaric acid content; and
malic acid does not precipitate its salts as does tartaric acid. As
well as reducing acidity, cold stabilization reduces the probability
of tartrate precipitation when the wine is chilled before serving.
Prior to cold stabilization, the pH should be below 3.65, otherwise
any precipitation of potassium bitartrate will lead to a decrease in
both the pH and the TA, possibly necessitating the addition of acid
prior to bottling. Conversely, if the pH is much above 3.65,
the pH will increase.
ACIDEXâ:
This so-called double salt of calcium carbonate, in
theory, reduces both tartaric and malic acids equally. Before
fermentation a portion of the juice is treated
wherein all the acid is removed and then added back to the rest of
the juice. Do not use this procedure on wine, as the portion
treated will have a pH close to 8.0 and the wine will oxidize
irrespective of its SO2
content.
Do not be alarmed at the dark brown colour of the deacidified
juice; fermentation will clear it up. It is advisable not to
use any SO2
in the treated juice, otherwise the colour may become "fixed" or
bound. In order to use AcidexÒ
effectively, it is necessary to consult the
Desired Acidity table
below.
Unlike the following carbonates, the juice
must be stirred into the AcidexÒ
in order to reduce the malic acid as well as the tartaric acid.
In order to reduce the malic acid, the pH must be
higher than
4.5, preferably above 5.0, during the
entire process, so stir the juice in slowly. (See
Explanations for Acid Reduction
p. 4) Some winemakers have added AcidexÒ
to the juice or the wine and observed an acid reduction, but only
the tartaric acid has been reduced. The same result could have
been obtained by using one of the carbonates described below at a
fraction of the cost.
Calcium Carbonate (CaCO3)
or Chalk:
Use at the rate of between 0.67 and 1.53 g/l to reduce TA by 1.0g/l.
Seeding with cream of tartar crystals and chilling hastens the
process. If the pH is too low, calcium carbonate will not work
for the same reason that cold stabilization will not work. If
this procedure is used, do it well before bottling, at least three
months, or a chalk haze or crystalline deposit could occur in the
bottle. I prefer to use calcium carbonate before filtering.
Calcium carbonate is not the preferred method of acid reduction by
wineries because of the length of time it takes to complete the
process, as well as the possibility of tartrate precipitation in the
bottle. They do, however, have metatartaric acid
at their disposal. This acid prevents tartrate precipitation
for up to a year. Metatartaric acid is temperature sensitive,
and wines should be held below 20
°C
in order to retain the activity of the acid. According to
Peynaud (Knowing and Making Wine), this acid should be used
only in wines that are not going to be kept very long. Since I have
not used it, I cannot comment on its effectiveness.
Potassium Carbonate (KCO3):
Use at the rate of about 1.0
g/l to reduce the TA by 1.0 g/l. The wine should be chilled,
although it will work at cellar temperature, and unlike calcium
carbonate, potassium carbonate reacts immediately and does not leave
a deposit.
As with tartaric acid, for the purpose of testing
for the proper additions of potassium carbonate, make a 5% solution.
Put one litre of wine into a refrigerator and chill to about -3 or
-4°C.
Set up a few glasses with 100 ml of the chilled wine. Using
one as a control, add 1, 2, 3, etc, mls. of the solution which will
be the equivalent of 0.5, 1.0, 1.5, etc, g/l. Refrigerate for
two hours or so stirring regularly - 7 or 8 times. Let the
samples warm up to cellar temperature and taste to determine the
amount to add to the batch. It is necessary to taste the wine after
the potassium carbonate has been added to the glasses in order to
determine whether there is a resulting flabby taste. I have
found that some wines, particularly aromatic wines lose their
crispness when potassium carbonate is used even in very small
amounts.
NOTE:
-
The last two procedures are generally carried out on wine.
It is always best (safest) to do lab trials before treating the
entire volume of wine.
-
Potassium will increase the pH very quickly compared to
calcium, so do not use it if the wine pH is already high, say
above 3.5 or for large reductions. Potassium carbonate is
best used to "fine tune" the acid balance.
-
When using either of the carbonates, put it into a small
quantity of wine, mix well, add back to the larger volume and
stir it well. Always leave plenty of space in the larger
container, as foaming can be violent unless the wine is very
cold.
-
Because both carbonates reduce only the tartrates, it is quite
possible to notice that there may be little drop in TA if the
malic acid is predominant despite the increase in pH.
-
The rule of thumb is to use 1 g/l to reduce acidity by 1 g;
however, this equation is not linear. If it is necessary
to reduce acidity by, say, 5g, it will require more than 1g/l.
Malolactic Fermentation
(MLF): Unlike the four
procedures described previously which remove tartrates, malolactic
fermentation removes malic acid. It does so by converting
malic acid into
lactic acid. One gram of malic acid is converted into
0.67 grams of lactic acid and the rest is given off as
carbon dioxide. This procedure is often used for two
reasons: it is a natural means of reducing acidity; and as a
stylistic tool: it changes the character of the wine by making it
softer and slightly buttery. It also influences microbial
stability: if used in marginal to high pH's - between 3.5 to 4.0 -
it can encourage growth of spoilage forms of lactic acid bacteria.
Leuconostoc Oenos
is normally the only lactic acid bacteria inducing MLF in lower
pH wines (<3.5). Higher pH wines support the growth of
Lactobacillus and Pediococcus, both spoilage bacteria.
One of the most readily available MLF cultures
was developed at Oregon State University. It consists of two
strains, Erla and Ey2d, now referred to as OSU1 and
OSU2, which were developed to tolerate both low temperature
fermentation (about 15C°)
and low pH (about 2.9), similar conditions for winemaking in B.C.
If you use this culture, make a starter. It
is in liquid form and the package states that it is good for 5
gallons, and at about $7.00 per package, that's expensive. Buy
some apple juice, which has the desired pH of about 4.0 (it also has
lots of malic acid), and start the culture in this medium.
Gradually add white grape juice to sensitize the bacteria to the
lower pH of the must and add directly to the ferment. This
starter can be used for both red and white grapes.
Freeze-dried cultures have recently become
available. They are very easy to use and very effective - just
sprinkle them into the wine after alcoholic fermentation has been
completed. The drawback is that they are very expensive.
MLF is routinely carried out on red wines and a
few white wines (Chardonnay, Pinot Blanc, Pinot Gris, Sauvignon
Blanc). Some winemakers like the taste; others don't. It
also has some important catches to it:
l. Sulfite
levels must be kept dangerously low - 30 ppm - during malolactic
fermentation. Malolactic bacteria are sensitive to sulfite;
they may be stunned, but they are not killed. If the MLF is
not completed before bottling, when the free sulfite degrades, the
bacteria can become active again; and the wine will undergo
malolactic fermentation in the bottle.
2. Since
home winemakers cannot test for either tartaric or malic acid
levels, they don't really know how much TA drop to expect; and the
only way to find out if the MLF is completed is to use color
chromatography. See
Charles Plant's explanation of this procedure.
Obviously getting involved with MLF is another
dimension of winemaking that many people may not want to step up to,
but it bears serious consideration. If MLF is not practiced and the
sulfite levels are not high enough to inhibit the ML bacteria, there
is the real danger that the wine, once it has been corked, will
undergo MLF in the bottle. So it is best to use the process
for no other reason than to avoid the potential problem.
Blending:
Blending high acid wines with low acid wines is a method of
balancing acid that many winemakers prefer. It is safe, uses
no chemicals and yields immediate results. Many winemakers do
an MLF on a portion of the wine and blend it with the non-MLF
portion in order to balance the softer lactic characteristics with
the more aromatic and fruitier characteristics of the grape
resulting in a more complex wine. It can be a lot of fun to
experiment in this way, and the rewards can be considerable.
All of the above-mentioned procedures are useful
and safe; indeed, it is unlikely that many commercial wineries make
their wines without using one or another - or a combination - of
these methods of dealing with acid imbalances.
Explanations for Acid Reduction
All
acids have an isoelectric point (also
isoionic or dissociation point), the pH at
which 50% of the acid is in its free
form and 50% is still in its bound form.
However, the isoelectric point pH's differ for each acid. The
two main acids,
tartaric and malic, have isoelectric pH's
of 2.96 and 3.41 respectively. It is only after the
isoelectric pH's have been attained that optimum acid reduction can
occur.
Once the isoelectric pH of tartaric acid is
reached and continues to increase, the bound tartaric acid becomes
free tartaric acid (H2T-)
which changes into potassium bitartrate (HT-)
which, in turn, changes into dipotassium tartrate (T2-)
both of which are salts. While the free tartaric acid can be
reduced with carbonates, the potassium bitartrate and the
dipotassium tartrate can be reduced with a combination of cold
stabilization and carbonates.
Malic acid reacts in the same way as tartaric
acid; however, malic acid (H2M-)
does not respond the way tartaric acid does to carbonate treatment,
and the potassium malate (HM-) and
dipotassium malate (M2-)
salts will not precipitate during cold stabilization. Thus the
use of AcidexÒ
to reduce the high acidity often associated with cool climate
grapes. Because the isoelectric point of Malic acid is so much
higher than that of tartaric acid, it is necessary to increase the
malic acid pH even higher in order to complete the dissociation of
the acid into its salts. In order to achieve this, it is
necessary to add the juice slowly into the Acidex©
in order to maintain a pH higher than 4.5,
preferably close to 5.0. Stirring the Acidex©
into the juice will result in the tartrates being reduced without
having any reducing effect on the malates; and, while the desired
acid reduction may be achieved, malic acid will be predominant
giving the resulting wine a sharper rather than a softer edge, which
was the purpose of the exercise in the first place. Acidex©
Cool Climate Grapes
The most frequently encountered problem, if it is
a problem, is low pH and high TA. It is not unusual, however,
to encounter both high pH and high TA due to a higher concentration
of malic acid and high potassium which results in high pH. Under
normal growing conditions, some of the malic acid is metabolized
into sugars and some disappears through transpiration. And the
decrease in TA is directly parallel with the decrease in malic acid.
(See
graph) Tartaric acid and its
salts, on the other hand, remain fairly constant during the ripening
process; and ultimately the level of malic acid is metabolized to
the point where the dominant acid is tartaric.
One method of dealing with the high pH/high acid
phenomenon is the following:
1.
Add tartaric acid to adjust the tartaric/malic acid balance;
2.
Use Acidex© to reduce the potassium; and
3.
Add tartaric acid to achieve the desired pH and TA if
necessary.
Since it is unlikely that few, if any, any home
winemakers can determine both the tartaric and malic acid levels, it
is best to consider the above procedure only in terms of the
standard practice of reducing the pH to the desired level prior to
fermentation.
A second problem arises when red grapes exhibit
both high pH and high TA. Whereas with white grapes it is
necessary to press off the juice, make adjustments with Acidex© and
add the juice back to the must, this is not practical with red
grapes. The best way to deal with red grapes is to add
tartaric acid to reduce the pH; use malolactic fermentation to
reduce the malic acid; employ cold stabilization; and, if necessary,
make further deacidification adjustments with potassium carbonate.
The problem can be dealt with in the same way
with those white grape varieties that can be enhanced by malolactic
fermentation. However, white grape varieties that depend upon
malic acid for their aromas and flavours do not respond well to
malolactic fermentation and require other treatments, including the
use of higher levels of SO2
to prevent spontaneous malolactic fermentation after bottling.
On the other hand, grapes from warm
climates
can also exhibit both high pH and high TA. When this
situation occurs, the high pH is due to high potassium, and the high
TA is almost entirely tartaric. Tartaric acid must be added to
reduce the pH, and the necessary acid reduction must be accomplished
by using procedures discussed previously. It is unlikely,
however, that malolactic fermentation will result in any significant
reduction in TA due to very low levels of malic acid.
Index of Acidity (IA) or Acid Taste Index
Ultimately, acid balance is a matter of taste and
there is no "rule of thumb" that will determine what the correct
acid balance is. However, research has been done that provides
some general guidelines that can be helpful in determining whether
the acid balance is within the "desired" range for the type and
style of wine. It is simply a matter of subtracting the pH
from the TA. For example: dry red wines should have an
IA range of about 2 to 3, dry white wines
about 2.7 to 3.7 and off-dry white wines about 3.8 to 4.8. Too
far below these levels and the wine will be flabby or soapy; too far
above them and the wine will be sharp and acidic.
Specialty wines such as dry Sherries, sparkling
wines, dessert and after dinner wines are not as easy to assess
using this method: Sherries because of their general low pH and low
TA; sparkling wines because their low pH and high acid are mitigated
somewhat by carbonation; dessert and after dinner wines in
particular require a much higher pH-to-TA ratio because they usually
have a lower pH with higher acids in order to balance the sweetness.
Icewines, for example, may have an IA as high as 12
or more. Of significance in red wines is the level of
astringency: high astringency will tend to make wines on the high
side of the IA seem more acidic than they really
are.
While numbers may be useful tools and can be used
as aids in striving for good pH/acid balance, the final arbiter of
proper balance is the taste buds.
Prepared by Bill
Collings
January 27, 2002 (amended June 17, 2007)
Acidex®
Calculations
Desired Acidity
|
Initial
Acidity |
10 g/l |
9 g/l |
8 g/l |
7 g/l |
|
Acidex®
grams |
Juice
litres |
Acidex®
grams |
Juice
litres |
Acidex®
grams |
Juice
litres |
Acidex®
grams |
Juice
litres |
|
9.5 g/l |
* |
* |
8 |
2.8 |
15 |
4.4 |
46 |
8.7 |
|
10.0 g/l |
* |
* |
15 |
3.6 |
23 |
5.2 |
47 |
9.7 |
|
10.5 g/l |
8 |
2.0 |
23 |
4.4 |
31 |
6.0 |
62 |
10.0 |
|
11.0 g/l |
15 |
2.8 |
31 |
5.2 |
39 |
6.7 |
69 |
10.7 |
|
11.5 g/l |
23 |
3.6 |
39 |
6.0 |
46 |
7.4 |
77 |
11.4 |
|
12.0 g/l |
31 |
4.4 |
46 |
6.7 |
47 |
8.1 |
85 |
12.0 |
|
12.5 g/l |
39 |
5.2 |
47 |
7.4 |
62 |
8.7 |
92 |
12.7 |
|
13.0 g/l |
46 |
6.0 |
62 |
8.1 |
69 |
9.7 |
101 |
13.3 |
|
13.5 g/l |
47 |
6.7 |
69 |
8.7 |
77 |
10.0 |
108 |
14.0 |
|
14.0 g/l |
62 |
7.4 |
77 |
9.7 |
85 |
10.7 |
117 |
14.5 |
|
14.5 g/l |
69 |
8.1 |
85 |
10.0 |
92 |
11.4 |
124 |
14.7 |
|
15.0 g/l |
77 |
8.7 |
92 |
10.7 |
101 |
11.7 |
* |
* |
Note: The table is set up for 23
litres (5 Imperial gallons) and is calculated for unfermented grape
juice. If you are adjusting more than 23 litres of juice
divide the Acidex© and juice amounts by 23 and multiply the result
by the number of litres you have. Acidex© won’t work if it’s
simply dumped into the entire amount of wine. Instead you must add
the indicated amount of juice to the Acidex. The procedure is
as follows:
-
Determine the initial acidity
of your juice. Decide the level to which you wish to reduce it
and find the correct figures in the table above.
-
Carefully measure the juice
sample indicated. Do not use more - it won’t work.
-
Weigh the indicated amount of
Acidex© and place it in a container at least 20% larger in
volume than the juice sample. This will allow for foaming.
-
Slowly stir the juice into
the Acidex©. Stir for at least 10 minutes to thoroughly
distribute the acid salts. You should see some active foaming.
-
Allow the mixture to settle
for several hours, preferably overnight Put it into a
refrigerator if possible or, alternately, put it in the coldest
place in your wine making area.
-
Filter the juice through a
lint-free cloth, cheesecloth, or a wine filter. This will remove
the chalky precipitate.
-
Stir the de-acidified and
filtered sample back into the main portion of the juice.
-
Test and record your acidity
again to ensure your reduction has had the desired effect
BIBLIOGRAPHY
 |
Iland, W; Ewart A, Sitters, J; Markides, A;
Bruer, N: (2000), Techniques for chemical analysis and
quality monitoring during winemaking, Australia, Patrick
Lland Wine Promotions.
| 