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Minerals

Juice contains many minerals that affect fermentation and subsequent reactions. Diane Y. Choo 2004.

Grapes, must and wine contain dissolved non-organic salts.  These salts are minerals or metal elements, which occur naturally in grapes, which attach to berry surfaces as a result of viticultural practices, or which enter the wine during the vinification process.  The concentration of potassium, nitrogen, phosphorus, sulfur, magnesium and calcium can range from 200 to 2,000 mg/l in grape juice.  Boron, manganese, and iron are present at approximately 20 to 50 mg/l while copper, zinc and molybdenum levels rarely rise higher than 5 mg/l (3).

These minerals are distinguished by weight.  There are significant amounts of the lighter metals (such as sodium, potassium, calcium, and magnesium) in grapes, and only trace amounts of the heavier metals such as lead, mercury and cadmium and metal-like elements such as arsenic (4).

Potassium is the major cation taken up by grape vines (1).  It is an important factor in defining wine pH and tartrate stability.  Its concentration in wine ranges from 200-2000 mg/L (2).  The high level of potassium in wine is of nutritional significance.

Sodium is less common in wine, and is found in lower concentrations (10-300 mg/L).  In addition to being a natural element found in grapes, sodium can be added to wine during sulfur dioxide additions when SO2 is added in the form of sodium metabisulfite (Na2S2O5).  It can also be added when fining with bentonite (2).

Averaging about 80 mg/L, calcium can cause tartrate instability in wine.  It does not cause any problems under normal conditions.  It may enter wine during fining (bentonite, DE, filter pads) (2).

Capable of deactivating enzymes, heavy metals are toxic to organisms.  In wine, extremely small amounts of iron, copper, zinc, manganese, aluminum, lead and arsenic may be found (4).  These metals accumulate in berries via translocation through the root system or by direct contact with vineyard sprays, and then are later absorbed onto the yeast cell membrane during fermentation - about 0 to 50% of the original amount of heavy metals from the grapes in the wine (4).  In the winery, non-stainless steel equipment, fining agents and filter media are potential sources of heavy metal in wines.

The copper content of American wines range from less than 0.1 to 0.3 mg/L.  At higher levels, copper can help catalyze the oxidation of wine phenols.  Once their concentration exceeds 1 mg/L, they can be sensorially detected.  Above 9 mg/L, copper becomes a metabolic toxin inhibiting alcoholic fermentation (4).  Copper is sometimes added to reduce the levels of hydrogen sulfide and related substances in wine.

In trace amounts, iron plays an important role in metabolic activities such as an enzyme activator, stabilizer and functional component of protects (4).  Ranging from about 1-10 mg/L in wine, the instability problems begin when the iron concentration gets above 4-5 mg/L (2).  At that point, iron acts as a catalyst to wine oxidation, forms wine instabilities and alters sensory characteristics.  Once these levels exceed 20 mg/L, iron inhibits fermentation (4).

The combination of the levels of all the minor metal salts in wine is one analysis that is used to determine the origin of the grapes.  Because the vine absorbs these metals from the soil under the vine, the presence of unusual metals and their various proportions can be used to test whether a particular wine could come from a particular vineyard.

References

1.  Boulton, R.B., V.L. Singleton, L.F. Bisson, and R.E. Kunkee.  Principles and practices in winemaking.  Chapman & Hall, New York (1996).

2.  Margalit, Y.  Concepts of Wine Chemistry.  The Wine Appreciation Guild, South San Francisco (1997).

3.  The Oxford Companion to Wine.  Ed. J. Robinson.  Oxford University Press, Inc., New York (1999).

4.  Zoecklein, B.W., K.C. Fugelsang, B.H. Gump, and F.S. Nury.  Wine Analysis and Production.  Chapman & Hall, New York (1999).

Diane Y. Choo

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