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Chemical Analysis of Wine

Dateline: 07/13/98

By Alan Bruzel

There are dozens of terms describing wine's diverse flavors and aromas. For quality control and classification purposes, many of these chemical components are under scrutiny using the latest instrumentation and techniques. The following is a brief survey of some organic and inorganic constituents that populate a typical bottle of wine, and the methodologies employed for their quantitation.

Trained to use the sensory inputs of taste and odor, experienced wine tasters are adept at determining a wine's geographical locale. The complexities of this regional terroir (French for soil) are slowly being worked out by means of analytical chromatography. Gas chromatography using a mass spectrometer as a detector identifies and quantitates, at the part per billion level, terpene alcohols such as linalool, believed responsible for the citrus-like quality of certain wines. High-performance liquid chromatography with refractive index and ultraviolet light detectors can similarly identify and quantitate organic acids and carbohydrates generated during the fermentation process. In this manner, the production of acetic acid (which gives wine a vinegary taste) and glycerol (which gives wine a smooth savor) can be closely monitored.

Of course, concentrations of other alcohols can be measured, such as ethanol and the adulterants diethylene glycol and methanol. The latter two compounds are added for sweetening and alcohol enhancement, respectively, and add more terreur than terroir to some economically priced products. Alcoholic beverages usually contain some traces of methanol, but a higher methanol level than 0.1% by volume constitutes adulteration in the eyes of the US Food and Drug Administration (FDA).

Sulfur appears in wine in a variety of forms. Grape vines are dusted with elemental sulfur to kill powdery mildew. (Grapes are beset with a variety of pests including mites, insects, and nematodes. The Oregon State University Extension Service describes grape pests and the pesticides used to control them.) Gravimetric procedures used to measure sulfur residues involve tedious weighings of barium sulfate precipitates. This time-consuming technique may be replaced by atomic absorption spectroscopy, which easily measures the part per million levels of sulfur remaining on harvested grapes.

The preservatives sulfur dioxide and sulfites are added to the finished product to prevent microbial growth. Whether the sulfites are intentionally added or not – even preservative-free wines contain 6 to 40 parts per million sulfite due to natural fermentation processes – US law mandates that wines with more than 10 parts per million sulfite carry on their labels the statement "Contains Sulfites."

A search is underway for the source of hydrogen sulfide (H₂S) generated during fermentation. So far, it has been found that it is the yeast strain itself that is responsible for H₂S production, not amino acid levels in the grape juice or sulfur dusting in the vineyards. In the future, a genetically engineered yeast may produce less of this odoriferous gas. The H₂S problem is treated today either by aeration of the wine or by addition of copper sulfate to precipitate the H₂S as copper sulfide before bottling.

DNA technology may also come to the rescue in ascertaining parentage of wine grape rootstocks. Viticulturists, like horse and dog breeders, have the same reasons for finding and establishing sturdy lineages: good stock produces good offspring. The standard rootstock classification methods, leaf identification and isoenzyme patterns, may soon be replaced by DNA microsatellite analysis. This technique is currently used in identification of crime scene samples and in determining paternity in humans. (See this site's DNA Fingerprinting for an explanation of this methodology.)

The observation that undiluted wines contain high percentages of the heavier oxygen isotopes led to a new technique that distinguishes undiluted from water-diluted wines. Because each geographical region gives its lot of wine a characteristic oxygen isotope ratio, a possible outcome of this research is an isotopic wine signature as distinctive as a rootstock DNA signature.

Another means of wine identification has been described, again using naturally occurring constituents. At a 90% level of accuracy, products from neighboring vineyards grown with the same grape varieties show similar abundance patterns of certain anthocyanins and amino acids. This can be used to authenticate a wine's region of origin. What are anthocyanins? They are glucosides of anthocyanidins (polyhydroxy derivatives of flavanols). Both are natural constituents of plants. They have an astringent flavor, and are responsible for the color of many flowers. The blue pigment in the cornflower is composed of two identical anthocyanin molecules complexed with either iron or aluminum. In the rose, this anthocyanin molecule is not coordinated with a metal ion, and appears red.

Analytical chemists are also paying attention to corks (which are preserved with sulfur dioxide and kept at a moisture content of 5% to 8%) and their foil coverings. The FDA has banned these foil capsules if they are composed of lead.

Recommended Web resources for additional information:

Breath Alcohol Analysis
An article from this Web site describing new technologies used in enforcement.

Understanding Wine
Online syllabus and slides from Cornell University.

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