From Grape to Table: The Chemistries of Wine I I
The multiple
facets of
fermentation
Helen Gillespie
After the harvest, the variables in winemaking that can be manipulated are few. Sugar cannot be added, although acids, grape concentrate, and limited amounts of water can be added to achieve a balance. Fining agents can be used to remove proteins or tannins. "You can give the same grapes to ten different winemakers and you'll get ten different wines," says Jac Jacobs, winemaker for Topolos at Russian River Vineyards (Forestville, CA). Everything that happens from this point on, from the type of barrel fermentation to the nature of the indigenous yeast and bacteria to the length of maceration or steeping, affects the final wine. Every winemaker makes slightly different choices when managing the volatile chemicals in grapes that are mainly responsible for differences between wines. There are three distinct phases when flavor, aroma, body, and color can be manipulated: fermentation, aging, and bottling.
PROCESSING GRAPES
Although the winery can choose to use grapes from a single outstanding vineyard to produce a premium wine, most wineries selects their grapes from a variety of vineyards in the general area. This enhances the ability of the winemaker to produce a complex wine, protects the winemaker from the variability that a single vineyard faces between dissimilar growing seasons, grapes from a single and helps ensure consistency. When the grapes are picked and send to the winery they are destemmed, crushed, and placed into containers. For white wines, the skins and seeds are also removed. 'Me crushed grapes are called must.
Depending on the winery, the must for a red wine may be inoculated with an active yeast culture, sulfur could be added to help preserve freshness and prevent browning and microbial spoilage, and the must could be flushed with carbon dioxide to displace oxygen before being loosely covered in preparation for the primary fermentation, which begins almost immediately. The process for white wine grapes is more complex In addition to sulfur, pectic enzymes could also be added prior to pressing, as these can aid in improving the extraction rate and clarity of juice. The skins and seeds are removed and the resultant juice is transferred to refrigerated stainless steel tanks for settling away from gross lees or sediment. The juice is then ready for yeast inoculation.
Wine yeasts are yeasts that carry out a complete fermentation of grape juice and tolerate high concentrations of ethanol and sugar. Not all wineries choose to inoculate the must, however, because there are naturally occurring yeasts on the grape surfaces that will carry out fermentation, often with the formation of flavorful esters. 'Topolos doesn't use any yeast other than the natural yeasts in the grapes for our red wines," declares Jacobs. "This means that our fermentation processes take longer to get the desired result. Well ferment a red anywhere from 21 to 45 days. 'Me longer fermentation makes the tannins more complex, and a more complex tannin doesn't get picked up by the tastebuds."
"On the other hand, we use two techniques for our Chardonnay: a 45 to 50 degree cold fermentation in stainless steel that is inoculated, and an oak fermentation," Jacobs adds. These are later blended to get a crisp, fruity flavor from the cold fermentation and complexity from the oak"
YEAST IS POWERFUL
"Yeast makes powerful contributions to wine characters," explains John Kelly, technical director at Vinquiry (Healdsburg, CA), a specialty contract lab that offers winemaker services and research laboratory facilities to the industry. "And it aids the wine in reaching maximum character and complexity. Some yeast strains provide lower acidity while enhancing fresh fruit and floral aromas; others are optimized for rapid fermentation. Depending on the yeast strain used, the base aromas can be shifted toward the softer, riper, and balsamic; maintain varietal fruit characters; or lean toward a younger fruit and mildly herbal varietal background."
'We use several different yeast strains for fermentation, because different yeast strains can affect the flavor and aroma of the wine," says Kent Kantz, winemaking lab manager at Beringer Vineyards (St Helena, CA). "We also add nutrients, such as diammonium phosphate, if needed, to ensure a viable fermentation, and then monitor Brix and temperature as fermentations proceed. If necessary, we also adjust acidity to inhibit unwanted microbial activity." (Me Brix scale is a hydrometer scale for sugar solutions; one Brix roughly equals I g of sucrose per 100 g of juice. See Part One of this series for details.)
More than 90 percent of-the carbohydrates or soluble solids in musts are naturally occurring grape sugars. The primary sugars in grapes, glucose and fructose, are normally found in equal amounts in a mature grape. Small amounts of sucrose, a disaccharide of glucose linked to fructose, and nonfermentable sugars are also present. During fermentation, yeast converts the glucose and fructose from the grape to ethyl alcohol, carbon dioxide, heat, and they byproducts. The amount of alcohol a wine depends on the sugar content of the grapes and the desired style of the finished wine. Most wine contains 10 to 14 percent alcohol. In dry wines, all the sugars have been converted by the action of yeast. It is possible to stop a fermentation before this point by mechanical means such as chilling (used often for such wines as Rieslings, Gewurztraminers, and white Zinfandels) or by adding brandy (used for fortified wines such as port).
The fermentation process takes six to thirty days or more depending on the type of wine to be produced. White wine is refrigerated during fermentation; red wines are not. Because the fermentation process raises the temperature of the must, heat is closely monitored during fermentation. The temperature and the density of the sugar contained in the juice are measured regularly. Temperature can be maintained or lowered by thermoregulation so that the yeast can work in the best conditions for extraction.
During fermentation, the must naturally makes its way up the container through the cap formed by the floating grape skins. The cap is turned several times a day to improve extraction while keeping the cap moist and releasing some of the heat of the fermentation. Because desirable color, aroma, and flavor extraction are complete relatively early in the primary fermentation, the wine can be pressed off the skin after bout six days. If malic acid levels are high, an inoculation of malolactic bacteria can be made at this stage to encourage a secondary malolactic fermentation (MLF) that will transform the malic acids lactic acids, making the wine and more harmonious.
"We process lots of samples malolactic fermentations," Kelly states. "We confirm that the primary yeast fermentation is complete, that there is little residual sugar, and that the alcohol is at a certain level. 'Ths is followed by the MLF process. Of the white wines,until the lactic acids reach ~ certain level, then sulfur dioxide is adeed to aid in settling the wine."
Not all wines go through the secon( ary MLF process. Of the white wine, typically is Chardonnay that undergoes MLF. Topolon uses MLF only for red wines.
On the other hand, this smooth, buttery aspect is precisely why some wineries use MLF, in particular to develop diacetyl, which is the flavor compound with the distinct buttery character. This buttery malolactic flavor is readily distinguishable from nonmalolactic flavors, which are floral and fruity. Aroma is also affected by MLF. MLF generates a buttery, "oaky" odor, but the intensity of most fruity and floral aromas is not affected, and vegetative odors such as 1-octanol and 1-octene-3-ol are eliminated.
Because fermentation depends on yeasts and bacteria to carry out the process, sometimes wine-spoilage microbes and microorganisms affect fermentation. Problem fermentations can involve those with hydrogen sulfide production, sticking or sluggishness from insufficient nitrogen, and high levels of volatile acidity (acetic acid/ethylacetate).These are A indicators of musts that lack the sufficient nutrient levels, are microbially contaminated, contain pesticide residues, or are not properly monitored.
According to Kelly, "Adding nutrients when needed can do a lot to ward off some of the problems during fermentation. Many wineries evaluate juice nitrogen status by measuring ammonia using an ion-specific electrode. Amino acids from the grapes also provide significant amounts of nitrogen to the yeast during fermentation."
Fermentations are monitored by daily temperature checks and various measurements. Each fermentation tank has a temperature gauge. "The ideal temperature is high 80s to low 90s," Jacobs says. "If it gets hotter, it shuts down the fermentation, so well pump the must over to cool it down." Brix is measured by hydrometer or refractometer. "If the sample shows over 2 percent residual sugar," he adds, "we use a standard test for reducing grape sugar in the tank to get the test result closer to +/-0.01 percent residual sugar."
'We use a robotic pipettor in the lab for analyzing residual sugar and organic acids using enzymatic chemistry," states Kantz. "For measuring alcohol, we use a near-infrared analyzer (NIRA), an instrument that has high sample throughput and is much simpler to use than a gas chromatograph. But the NIRA is only accurate for wines that contain less than about 3 percent sugar, so we do have to use the gas chromatograph for measuring alcohol in sweet dessert wines and in port."
Beringer monitors residual sugar and alcohol frequently as the fermentation nears completion. "We also monitor malic acid frequently on wines that are undergoing MLF," Kantz explains.' "For such wines, malic acid is converted into lactic acid, and additional flavor' and aroma compounds are also produced. While we want this to happen certain wines, we don't want it to happen in others such as Rieslings 0 Gewurztraminer that are distinguished by their fruity flavor and aroma."
When testing for sulfur dioxide in white wines, Beringer uses the classic "Ripper" method, which is an iodine titration. "For red wines, it is difficult to see the endpoint using the Ripper method so we use an aeration/oxidation method for measuring sulfur dioxide," he continues. 'We measure volatile acidity by using a Cash still to do a step distillation, and we measure ammonia levels by using a specific ion electrode.
Typically, Beringer's lab might perform up to ten different measurements on each sample. a lot of effort to monitoring fermentations, and we do analyses juice or wine receives a treatment or is moved from tank to tank or from tank to barrel," Kantz states. "This means constant attention to detail throughout the making and aging of wine."
As the fermentation process nears completion and all the sugar is converted to alcohol, the yeast pulp, seeds, skins settle to the bottom of the tank. The wine is racked and the sediment moved so that only the juice remain ready for aging and bottling. The latter .will be the subject of the third and final part of this series.
HELEN GUIESPIE is an industry and editorlpublisher of the LIMS/Letter, a webmaster of the LIMSource. She can reached at F0. Box 935, Kenwood, 95452. Tel.~ 707-833-6885,- Fax: 70 6865; E-mail: Webmaster@LIMSou com; Web: http://www.LIMSource.com.
SULFUR IN WINE
Sulfate occurs naturally in grape juice, and yeast produces sulfur dioxide during fermentation from inorganic sulfate. Additionally, winemakers add small amounts of sulfites to wine to prevent oxidation and spoilage, The federal government regulates the use of sulfites in wine, but the most important factor I limiting the use of S02 in winemaking is its undesirable effect on flavor.be Iegal upper limit for sulfites is 350 ppm. Premium wines contain much lower levels, rarely exceeding 150 ppm for dry wines. Regulations require a label statement if the wine contains more than 10 ppm. Since the tolerance of error of the standard te
st is 5-20 ppm, nearly all wines carry the label statement.
CONTAINERS: BARRELS OR TANKS
The container used for the fermentation process affects the end product. Winemakers may choose stainless steel tanks or oak barrels. Stainless steel does not affect the flavor, but "barrel fermentation can add some interesting flavors and aromas, particularly to Chardormay, as components of the yeast and oak barrels get introduced into the wine," explains Kantz. There is a very notable difference in barrel-fermented wines, and you often get thicker, creamier texture this way."
Beringer ferments its Chardormay in barrels and leaves the wine to age 'in barrels for approximately nine months to achieve desired results. "Such barrel fermentation is along the fines of traditional French winemaking methodology," Kantz continues, "and we spend considerable effort stirring, topping, and monitoring these barrels to ensure that wine quality is maintained while in barrels. Unfortunately, barrels don't last forever. After two to three years their attributes start to decline. New barrels can impart a toasty or vanilla aroma to the wine, so the age of the barrel and the type of barrel will influence the final style of the wine.
