Specific pollutants or classes of pollutants are well known to have harmful effects on the environment, such as ozone depletion (triatomic oxygen) and global warming, and on our health when the pollutants are beyond “acceptable” levels. Vineyards are usually located in areas with low levels of pollution and are therefore not of great concern. This does not mean that contamination should be ignored; however, there has been limited research on the impact of contamination on vineyards. It is a very complex area of research. Let’s examine the chemistry of contaminants and their impact on viticulture specifically.
The Earth’s atmosphere comprises a number of gases, primarily diatomic nitrogen and oxygen, as well as carbon dioxide and water vapor. And every second of every day, pollutants from automobile emissions, landfills, and industrial processes are released into the atmosphere and react with their constituents. The main pollutants are sulfur oxides, carbon dioxide, nitrogen oxides, and chlorofluorocarbons (CFCs), and when they reach the atmosphere, they react with sunlight and atmospheric (diatomic) oxygen to form harmful substances.
Sulfur dioxide is known to have damaging effects on plants when in high concentrations; it is the result of oxidation of elemental sulfur, as in coal burning processes. It also reacts with atmospheric oxygen to form sulfur trioxide, which then reacts with water vapor to form sulfuric acid in the form of acid rain. A second component of acid rain is carbonic acid, formed in a similar reaction from the emitted carbon dioxide. And the most damaging component is hydrofluoric acid, formed from hydrogen fluoride, released by smelting operations and the production of phosphorus-containing fertilizers, and water vapor in the atmosphere. When acid rain hits the soil in vineyards, it lowers the pH, throwing soil chemistry out of balance and making it harder to grow premium grapes. Some of these acids are particularly corrosive and toxic, and detrimental to the vine crop.
Nitrogen oxide is found in automobile exhaust gases and is the result of high-temperature combustion of (diatomic) nitrogen. In the atmosphere, nitrogen oxide reacts with molecular oxygen to form nitrogen dioxide, the reddish-brown gas responsible for smog. The nitrogen dioxide is then photochemically converted back to nitrogen oxide along with an oxygen atom. The oxygen radical then reacts with molecular oxygen to form ozone in the lower atmosphere. Ozone is a strong irritant and damages vines and crops.
Automobile exhaust gases also release hydrocarbons that react with nitrogen oxide to form peroxyacetyl nitrate, a compound that belongs to the class of peroxyacetyl nitrates or PAN, powerful toxiirritants present in photochemical smog that cause the accumulation of ozone. PANs are very damaging to the physiology of the vine, which reduces yields. And until leaded gasoline (tetraethyl lead) was phased out, wines produced in vineyards located in close proximity to high-traffic highways demonstrated higher levels of lead, a potent neurotoxin responsible for lead poisoning. If lead enters the bloodstream, it can interfere with and deactivate the delta-aminolevulinic dehydratase (ALAD) enzymes responsible for producing hemoglobin. Hemoglobin is the iron-containing protein pigment found in the red blood cells of vertebrates and functions primarily in transporting oxygen from the lungs to body tissues. Lead poisoning can cause irreversible neurological damage, as well as abdominal pain, gastrointestinal problems, headaches, anemia, reproductive problems, and a host of other effects.
Chlorofluorocarbons (CFCs) belong to the class of haloalkanes, that is, they include alkanes, such as methane or ethane, with halogens, such as chlorine or fluorine, and are known to have harmful effects associated with ozone depletion. The halogen in CFCs reacts with ozone to form an oxide of the halogen plus molecular oxygen.
Regarding pesticides, it has been clearly shown that, despite the environmental impacts, there are negligible residues in the wine. Of course, this assumes that the pesticides are applied correctly and within the recommended pre-harvest treatment period. And the different processes of vinification, crushing, pressing, fermentation, clarification, filtration and aging, make pesticide residues disappear.
And what about the thick, dense smoke from the devastating summer 2008 wildfires that blanketed Northern California’s wine country? Some red wines from hard-hit areas like Mendocino County have a decidedly smoky, charred aroma and an ashy burnt-wood flavor. The compounds responsible for the smell of smoke are guaiacol and 4-methylguaiacol, volatile phenols that are absorbed into the skin of the grape, especially in thin-skinned varieties such as Pinot Noir, and then extracted during maceration and exacerbated by fermentation. The egg whites are mainly respected since there is no maceration of the skin with the juice. Much research has been done on smoke-tainted wines in Australia, where, in 2003, smoke from bushfires also heavily affected vineyards.
Using reverse osmosis and nanofiltration technologies, the Australians have been able to reduce the culprit compounds to undetectable levels. VA Filtration (VAF), a company specializing in services such as volatile acidity (VA), Brett (yeast infection) and TCA (corked wine) removal, claims they can now remove up to 99 percent of specific sensory characteristics by treating affected wine. with a food-grade resin developed in Germany. Interestingly, if not puzzling, the VAF website states that the “offensive compounds being removed are not yet [known].”
But the astute reader will also know that guaiacol and 4-methylguaiacol are compounds found in toasted oak-aged wines, where they are highly desirable. It’s quite a dichotomy!