Whether the wine is aging in tanks or barrels, tests are run periodically in a laboratory to check the status of the wine. Common tests include °Brix, pH, titratable acidity, residual sugar, free or available sulfur, total sulfur, volatile acidity and percent alcohol. These tests are often performed throughout the making of the wine as well as prior to bottling. In response to the results, a winemaker can then decide if more sulfur needs to be added or other slight adjustments before it is bottled.
°Brix is a measure of the soluble solids in the grape juice and represents not only the sugars but also includes many other soluble substances such as salts, acids and tannins, sometimes called Total Soluble Solids (TSS) However, sugar is by far the compound in greatest quantity and so for all practical purposes Brix is a measure of sugar level. The level of sugar in the grapes is important not only because it will determine the final alcohol content of the wine, but also because it is an indirect index of grape maturity. Brix (Bx for short) is measured in grams per hundred milliliters, so 20Bx means that 100ml of juice contains 20gm of dissolved compounds. There are other common measures of sugar content of grapes, Specific gravity, Oechsle (Germany) and Beaume (France). The French Beaume (Be for short) has the benefit that one Be gives approximately one percent alcohol. Also one Beaume is equal to 1.8 Brix, that is 1.8 grams of sugar per one hundred milliliters. This helps with deciding how much sugar to add if the juice is low in sugar; to achieve one percent alcohol add 1.8 grams per 100 ml or 18 grams per liter. This is the process of chaptalization, legal in some countries illegal in others. However, perfectly acceptable for the home winemaker. Generally, for the making of dry table wines a Bx of between 20 and 25 is desirable, this is equivalent to Be of 11 to 14.
A Brix test can be ran either in the lab or out in field for a quick reference number to see what the sugar content is at. Brix is usually measured with a refractometer whilst the other methods use a hydrometer. Generally, hydrometers are a cheaper alternative. For more accurate use of sugar measurement it should be remembered that all measurements are affected by the temperature at which the reading is made, suppliers of equipment generally will supply correction charts.
Volatile acidity test verifies if there is any steam distillable acids in the wine. Mainly present is acetic acid but lactic, butyric, propionic and formic acids can also be found. Usually the test checks for these acids in a cash still, but there are new methods available such as HPLC, gas chromatography and even enzymatic methods. The amount of volatile acidity found in sound grapes is negligible. It is a by-product of microbial metabolism. It's important to remember that acetic acid bacteria require oxygen to grow. Eliminating any air in wine containers as well as a sulfur dioxide addition will limit their growth. Rejecting moldy grapes will also prevent possible problems associated with acetic acid bacteria. Use of sulfur dioxide and inoculation with a low-V.A. producing strain of Saccharomyces may deter acetic acid producing yeast. A relatively new method for removal of volatile acidity from a wine is reverse osmosis. Blending may also help—a wine with high V.A. can be filtered (to remove the microbe responsible) and blended with a low V.A. wine, so that the acetic acid level is below the sensory threshold.