Tuesday, April 08, 2008

La quimica del vino

Today, more grapes are grown than any other fruit in the world-and 90 percent of the grapes grown worldwide are used to make wine. From the earliest known incidences of wine production in 3500 B.C. (Assyria, Egypt Persia) to Pasteur's 1858 discovery of acetic acid forming bacteria´s role in fermentation to the present the role of chemistry in wine production has increased, although winemaking is still as much an art as it is a science. Ths three part series of articles explores the major phases of wine production: harvest, fermentation, and bottling/aging. This part begins with the agricultural factors involved in a successful harvest.Chemical analysis is used to determine when to harvest, how well fermentation is progressing, and how long to age the wine, but adhering to an exact formula is not the objective. Bouquet appearance, flavor, and mouth feel are the deciding factors; these sensory properties are evaluated using not only chemical analysis but also the winemaker's expertise in balancing the aroma, color, and flavor of wines. The goal is to produce a unique wine, not an overprocessed one.Wine contains more than 2,000 compounds that are derived from the grapes themselves and from changes the grapes undergo during fermentation and aging. Because varying microbial processes and chemical interactions affect the properties of the finished wine, wine production is more complex than simply combining individual ingredients. Over time, winemakers have learned to focus on four essential components: sugar, alcohol, acid, and tannin. Blended together, these four components create balance in wine. Monitoring and balancing these components is a unique challenge every year, because every harvest presents a different set of variables.
NURTURING THE VINEWine can be affected by soil and climate as well as by winemaking methodologies. This underscores the fact that wine production is an agricultural business, albeit a more glamorous crop than most.The US wine industry tends to diverge from its European heritage of integrated wineries. Here, grape growers manage the production cycle and winemakers turn the grapes into the finished product Grape growers nurture the vine by providing appropriate water supplies; monitoring soil drainage and irrigation; fertilizing with macro and micro mineral elements; training the vines by thinning and pruning them for efficient production and adequate penetration of sunlight pesticides, and air into the canopy; and protecting the vine and fruit from pests and toxicants. What grape growers seek to do is ensure that the flavors and tannins in the grapes mature at the same time as sugar levels, instead of afterward.Several things happen inside the vine and grape during the production cycle that affect tannin, color development, aromatic and flavor compounds, and intermediate metabolites. This cycle is dramatically affected by the terroir -the integrated growing system that includes the interactions between climate, geology, mineral nutrition, and trellising and spacing. It is vitally important, for instance, that the terroir be compatible with the root stock, because root stocks that prefer cooler climates will not thrive in hot ones.A distance of just ten miles can significantly alter the impact of the climate. The Napa Valley in Northern California (see map), for example, is five miles wide at the base, thirty-five miles long, and has three distinct microclimates. The southern end receives cooler weather, so Chardonnay and Pinot Noir grapes are planted there. The mildvalley region is warmer, and grapes that thrive in warm temperatures-such as Cabernet Sauvignon, White Riesling, and Chenin Blanc-are planted in this area. Heat-loving Zinfandel and Petit Sirah grapes are planted in the northern end of the valley, which receives very hot temperatures."The location of the vine, whether it is in a cool, foggy area or on a rocky slope or in the sun, affects the speed of ripening and the flavor," states Jac Jacobs, winemaker for Topolos at Russian River Vineyards (Forestville, CA). "The volume or tonnage of grapes on the vine itself also affects flavor. 'This is because the sugar levels increase through dehydration. And, if the vine gets too much water or has too much canopy, the flavor is affected."Ideally, the longer grapes can stay on the vine (or the more "hang time" they have), the better, because berries get their flavor from the skin. The longer the contact between the two, the more flavor is extracted, thus imparting more complexity and fruitiness to the final taste of the wine.Even with the optimum terroir, grape production and quality vary from one year to the next because of weather fluctuations such as temperature and variations in precipitation. Low temperatures prevent grapes from ripening. High temperatures cause grape sugars to rise prematurely. Rain can cause bunch rot and mold if the grapes aren't able to dry out. Indeed, when rains struck during the middle of the 1997 harvest, bunch rot set in and hang time was cut short for some grapes. The bumper crop that Sonoma and Napa grape growers had originally expected from the early ideal growing season instead turned into an estimated $70 million loss from the $470 million crop.
LABS GET BUSYMuch of the quality of the wine depends on the complete maturity of all features of the grape, so wine laboratories shift into high gear near harvest time. Winemakers start receiving grape samples from the vineyard about four to six weeks before the grapes are ready for harvest. "Me closer it comes to harvest the more frequently the grapes are sampled` Jacobs says. 'Toward the end, I can be receiving up to twenty samples a day from different Vineyards."During this time, tests for sugar, pH, and titratable acidity (TA) are performed. More sophisticated wineries may also test for potassium, nitrogen compounds, and organic acid profiles of tartaric, malic, and citric acid. A pH of 3.4 is the industry standard. Other levels are more variable.'We can sample vineyards as often as once a day as we get close to making a decision about harvesting the grapes," explains Kent Kantz, winemaking lab manager at Beringer Vineyards (St Helena, CA). 'We analyze berry samples by using a mill to extract the juice and then use a refractometer to measure soluble solids (sugars), a pH meter for pH, and an autotitrator for acidity."Soluble solids, or sugars, are measured according to the Brix scale, a hydrometer scale for sugar solutions invented by nineteenth century Austrian chemist Adolf F. Brix. One Brix roughly equals 1 g of sucrose per 100 g of juice. "The amount of sugar in a grape is one of the key measures of its maturity, so Brix is an important indicator of the proper time for harvest. Most wineries seek readings ranging from 21 to 23 Brix for whites and 23 to 25 Brix for reds. Red wines have a higher Brix and thus they normally have higher alcohol levels. In all cases, the various processing techniques used after harvest may dictate a certain Brix."Beringer is interested not only in sugar, but also acidity as grapes ripen," Kantz says.'We measure TA and pH every time we measure Brix. Malic acid starts to metabolize as the ripening process continues, so you can start to lose acidity at the same time sugars are going up. Winemakers and vineyard managers evaluate ripening by tasting and use sugar and acid levels to assist them in making decisions about harvest." Harvest seldom occurs according to a set plan. Although chemical analyses are used to determine when the grapes reach complete maturity, what the grapes taste like to the winemaker is the final blessing on whether grapes get harvested today or next week.There are other business decisions independent of the chemistries that drive harvest times. "Enhancing the flavors, aromas, and colors can depend on when you harvest," Jacobs says. "For a rich, full-bodied Zinfandel with a plum and blackberry character, we'll harvest later. Or, if we want a fruity, strawberrylike Zinfandel, well harvest sooner.""Each winery can handle only so many tons of grapes each day, depending on its crushing capacity, so it can be a real challenge to schedule picking when several grape varieties are reaching optimal harvesting conditions at about the same time," adds Kantz.Indeed, it is seldom that any vineyard is able to harvest when all criteria are perfect, because there are so many other considerations, some of which like weather-are beyond control. As a result, the vast majority of chemical analysis and monitoring is performed after the grapes are harvested. How the grapes are subsequently processed is as much a deciding factor for the quality of the wine as the agricultural variables. After the harvest, enology the science of winemaking-takes center stage. That will be the focus of Part Two of this series.HELEN GILIESPIE is an industry analyst, science writer, and editor l publisher of the LIMS I Letter. She can be reached at P. 0. Box 935, Kenwood, CA 95452. Tel: 707833-6885; Fax: 707-833-6865; e-mail: Webmaster@LIMSource.com; Web: www.LJMSource.com.
SCIENCE AND THE SENSES
Wine quality is determined by sensory properties, microbial processes, and chemical components. Although the microbial processes affect the chemical components, it is the sensory properties of the wine's bouquet, appearance, and taste that are the ultimate deciding factors in the quality of wines. The right balance for these properties is determined according to not only chemical analysis but also subjective human sensory reaction to individual wines

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The "Silver Song"

Este video es obra creativa de Armando Isaac, para el curso de Química Inorgánica I.