Acids

The acid content, titratable acidity (T.A.), of a wine is usually expressed in grammes tartaric acid per liter (g/l). Most commercial wines have an acid content of 5 to 8 g/l. White wines usualy contain a bit more acid (6-8 g/l) than red wines (5-7 g/l), because they generally contain less tannin than red wines.
Grapes contain mostly tartaric acid, other fruit contains mostly citric or malic acid.
A wine that contains too little acid doesn't keep well and tastes flat, a wine with too much acid can taste too harsh. Wines that contain more acid, need less sulphiting. Most winemakers try to keep the acidity between 5 and 8 g/l.
But in the end, it is up to your own personal preference what the acidity of your wine should be. Taking care of the acidity of your wines contributes to quality of the finished product, and more consistent results.
You can measure and adjust the acidity of a wine or must before fermentation start or after fermentation has completed.

The following topics will be covered below:

Measuring the acid content
Raising the acid content
Lowering the acid content

Measuring the acid content

I know there are some other types of wine acid test kits, but they basically work the same. Base (lye) is added to the must/wine untill an indicator changes color. The amount of added base is equivalent to the amount of acid that was present in the wine. If you wish to get a better explanation of this subject check out a highschool chemistry book.
The type of acid test kits I'll try to explain contain a test tube (a small test cylinder) with a scale printed on its side, and a bottle containing a mixture of the indicator with the base solution (testing agent).
Using another type of testing solution, it's also possible to determine the sulphite level using the same method.

First the test cylinder is filled with wine or must to the zero level indicated on the side of the test column. This can be done by using a small seringe (without needle).
Then drop by drop, and with often shaking the cylinder for good mixing, add the reagent. Once in a while, especially when dark clouds form, put your thumb on top, turn it upside down, and back again. This will help mixing.
Stop when a color change starts to appear, this can happen very rapidly.
Read the number of milliliters of reagent added. The amount of milliliters added is the same as the acid content of the wine or must expressed in grams/liter.
Dispose of the cylinder content. Do NOT add this back to your wine. This stuff is toxic!
Clean the cylinder and put it away.

Raising the acid content

There are two methods to raise the acid content of a wine:

Adding acid

Adding acid means dissolving and adding an amount of an acid salt, usually tartaric acid, citric acid, malic acid, citric acid or an acid blend of the three. Citric acid is usually not added to red wines, because its taste doesn't suit red wines well. citric acid doesn't suit those wines well.
The amount of acid to raise the must or wine 1 g/l depends on the acid you're adding. See the table below how much the TA increases when 1 gram of acid is added to 1 liter of a must or wine:

Adding acid
Kind of acid	TA increase per g/l
Tartaric	1.00
Malic	1.12
Citric	1.17

Blending

Blending with a more acid wine or must of course requires that you have such a must or wine available at the same time. You can't get a resulting acid content higher than the TA of the wine to blend with.
Blending doesn't just affect the acid content, it alters the wine overall. Try a sample blend of the mixture before you blend the whole batch to see if the two wines taste well together. We'll use the following blending ratio formulas related to the Pearson square.

Pearson square
	A = Value wine 1
	B = Value wine 2
	C = Value wanted
	D = Part of wine 1
	E = Part of wine 2
	D = C - B E = A - C proportion = D : E

The proportion in which to blend for raising or lowering the alcohol content of a can be calculated in exactly the same way.

Example

We've got 10 liters of a wine that is too low in acidity, TA = 4 g/l. The wine we are going to blend with has an acidity of 10 g/l. Using the formulas from above (see the blending cross at the right):
[blending cross]

A = 10 ('A' must be the highest number)
B = 4
We want to obtain a wine with a TA of 6 g/l, so:
C = 6
Now:
D = C - B = 2
E = A - C = 4
So we have to blend 2 parts wine 1 with 4 parts wine 2. The blending ratio (proportion) now becomes:
proportion = D : E = 2 : 4 = 1 : 2
So we have to add:
10 * 1 / 2 = 5 litres (multiplication because the amount of wine 1 is required)
of the wine to blend with.
So eventually we would end up with 15 liters of a wine that has an acidity of 7 g/l.

Lowering the acid content

There are four methods to lower the acid content of a wine:

Adding calcium carbonate

Calcium carbonate (CaCO₃) is a kind of chalk, which reacts with the acids in a wine. It neutralizes them. An amount of 0.66 grams CaCO₃ per liter lowers the TA with 1 gram per liter. Calcium carbonate can be used up to an amount of 2 grams/liter maximum to avoid its taste getting noticable in the wine. After the addition of CaCO₃, wait a day or two and rack the wine.

Diluting or blending

Blending with a less acid wine or must of course requires that you have such a must or wine available at that time. You can't get a resulting acid content lower than the TA of the wine to blend with.
Blending doesn't just affect the acid content, it alters the wine overall. Try a sample blend of the mixture before you blend the whole batch to see if the two wines taste well together. We'll use the same blending ratio formulas from above.
The blending ratio for raising or lowering the alcohol content of a wine by means of blending can be calculated on exactly the same way.

Example

We've got 5 liters of a wine that is too high in acidity, TA = 15 g/l. The wine we are going to blend with has an acidity of 3 g/l (if you dilute with water, this acidity (B) is 0). Using the formulas from the conversion tables chapter on blending wines (see the blending cross on the right):
[blending cross]

A = 15 ('A' must be the highest number)
B = 3
We want to obtain a wine with a TA of 7 g/l, so:
C = 7
Now:
D = C - B = 4
E = A - C = 8
So we have to blend 4 parts wine 1 with 8 parts wine 2. The blending ratio (proportion) now becomes:
proportion = D : E = 4 : 8 =1 : 2
So we have to add:
5 * 2 / 1 = 10 litres (division because the amount of wine 2 is required)
of the wine to blend with.
So eventually we would end up with 15 liters of a wine that has an acidity of 7 g/l.

Cold stabilizing

Cold stabilizing causes tartaric acid to precipitate as potassium bitartrate crystals when the wine is being chilled. Because the procedure works on tartaric acid, it will only work effectively on grape wines. The wine must be fermented out.
You can cold stabilize the wine by placing it in a cold spot (between -4 and 5 deg C) for a week or two. The wine will get cloudy and crystals will form on the bottom of the carboy. Wait untill the sediment has settled and rack when the wine is still cold. For best effect you can add a small amount of potassium bitartrate first, so that the crystals will form more easily.

Malolactic fermentation

This is not a fermentation conducted by yeast, but it is bacteria converting malic acid into the less harsh tasting lactic acid. Malolactic fermentation (MLF) is generally considered a good thing in red wines, but not in white wines. MLF can occur spontaneously when the SO₂ level is kept low. MLF starters also exist. To allow MLF to take place, do not add much SO₂ before MLF fermentation is complete. Specific test kits are available, but a drop in titratable acidity (TA), the acid content of a wine can be noticable.