Residual sugar

Terms Explained: aging, anthocyanin, aroma, astringency, blending, bouquet, brix, chaptalizing, cold stabilization, enology, filtering, fining, herbaceous, hydrometer, lees, legs, malolactic fermentation, marc, methoxypyrazines, mouthfeel, must, phenolic compounds, pomace, pressing, primary fermentation, racking, refractometer, residual sugar, secondary fermentation, sulfites, tannins, terpenes, titratable acidity, topping up, varietal character, vinify, vintage, viticulture, yeast

Enology is the study of wine and winemaking. Viticulture is the science of growing grapes, or the cultivation of grapes in general.

Before Harvest

Brix is a measure of soluble solids content in grapes, mostly as sucrose, using a refractometer and expressed in degrees. Each degree of brix equals 1 gram of sugar per 100 grams of grape juice. Brix is measured at harvest. Most table wines are harvested between 19 and 25 degrees brix. A refractometer is an instrument, usually hand-held, that measures dissolved sugar in a small juice sample in the field. Refractometers make it possible to determine ideal harvesting times of grapes so that the product arrives in an ideal state to consumers or for subsequent processing steps such as vinification.

Making the Wine

To vinify is to turn grape juice into wine. To extract the juice, grapes are pressed using a wine press. The freshly pressed grape juice that contains the skins, seeds, and stems of the fruit is called must; everything but the juice (skins, seeds, etc.) is called pomace or marc.

Primary fermentation is the initial fermentation, in which yeast convert sugars in grape juice or must to alcohol (wine) and carbon dioxide. Yeasts are single-celled microorganism that convert sugar to alcohol and carbon dioxide during the fermentation stage of wine production. During malolactic fermentation, which occurs after primary fermentation, tart malic acid, which occurs naturally in grape must, is converted to milder, softer-tasting lactic acid. Secondary fermentation is either a continuation of the primary fermentation of sugar to alcohol that takes place after the wine is moved from one type of container to another, such as from stainless steel to oak, or a supplemental fermentation triggered after the primary fermentation is complete by the addition of sugars, such as is commonly done in the production of sparkling wines. This differs from chaptalizing, or adding sugar to the must or juice before fermentation to make up for deficiencies in vine-ripened sugar levels.

At any point during the winemaking process, a hydrometer may be used to measure sugar content (brix). A hydrometer is a calibrated glass float used to measure the specific gravity (relative density) of liquids.

Several methods may be used to clarity the wine as it ferments. Lees are the solid sediments (such as dead yeast, seeds, and pulp) left after fermentation. These will be deposited at the bottom of the fermenting barrel or tank. Different winemaking methods are used to manage the lees. Racking is the process of siphoning wine off the lees into a new, clean vessel, clarifying the wine. Wine may be passed through a filter (a process known as filtering or filtration) to remove impurities. Fining involves adding a substance (fining agent) to wine to create an adsorbent, enzymatic or ionic bond with the suspended particles, making them a larger molecule that can precipitate out of the wine more easily and quickly. Unlike filtration, which removes particulates such as dead yeast cells and grape fragments, fining is effective in removing soluble substances such as polymerized tannins, coloring phenols, and proteins. In cold stabilization, wine is chilled to near freezing temperatures for several weeks to encourage the precipitation of tartrate crystals and other insoluble solids, thus clarifying the wine. This procedure is usually used only for white wines.

The sugar not converted to alcohol during fermentation is known as residual sugar and is usually measured in grams of sugar per litre of wine (g/L). Residual sugar is indicative of a wine’s relative sweetness. Titratable acidity is the acidity of a juice, must or wine determined from the amount of base needed to titrate the wine to a specific end point pH. Usually sodium hydroxide (NaOH) is used for titration to an end point pH of 8.2 in the US and 7.0 in France. Titratable acidity is reported in g/L tartaric acid equivalents and often abbreviated as TA.

Topping up involves replacing wine that has evaporated from a barrel, ensuring no air space in the container and no exposure to oxygen. Sulfites are sulfur-based compounds that occur naturally during wine fermentation, but are also often added before, during, or after fermentation.They protect wine from oxidation and the acitivity of undesirable microorganisms, particularly bacteria. Sulfites are typically added at higher levels to white and/or sweet wines to prevent browning and/or spoilage. Wines may be blended, or mixed. Often, a blending cultivar is grown specifically to be mixed with other grapes in the winery.

More about Wine

The overall sensory perception given by a wine’s physical and chemical interactions in the mouth, usually excluding taste and aroma, is referred to as mouthfeel. In the case of wine or juice, mouthfeel combines sensations related to the product’s viscosity as well as sensations related to the product’s chemical properties, such as astringency. Astringency is the lip-puckering sensation caused by excess tannins, which may disappear as the wine ages. Tannins are in a class of compounds known as phenolics and are found in grape skins and seeds. These are important in red grapes for wine production and quality. In red wine, tannins cause a dry or astringent mouthfeel. While fruit and seeds can provide tannins to finished wines, tannins are also extracted from oak barrels or oak chips during fermentation and aging. Aging is the period of time that a wine spends maturing to achieve its best flavor and aroma. Wines are aged in a variety of ways from large casks (such as oak or stainless steel) to bottles. Complex wines tend to benefit from aging, whereas simple wines should be drunk when they are young.

Phenolic compounds include several hundred chemical compounds that strongly influence taste, color, and mouthfeel. Tannins and anthocyanins (pigments that appear red, purple, violet, or blue; they give red grapes and wines their color) are examples of phenolic compounds. Some of these are naturally present in the fruit and some are created during the winemaking and aging processes. Phenolic compounds such as resveratrol have been linked to many of the health-beneficial properties of grapes and grape products.

Methoxypyrazines are a class of chemical compounds that produces herbaceous (e.g,. green bell pepper, leafy, or vegetative) odors. In white wine, the odors can be desirable. However, in red wines high levels of methoxypyrazines are very undesirable.

Terpenes are a class of organic compounds important as odorants of the highly aromatic muscat grapes and related varieties such as Riesling and Gewurztraminer.

The varietal character of a wine is the distinctive aroma and flavor characteristics of certain grapes that make them recognizable. A wine’s aroma is its smell. The term is generally applied to younger wines, while the term bouquet is used for more aged wines. A wine’s vintage is the year in which grapes were harvested to create a wine. Vintage also refers to a season’s yield of wine from a vineyard.

Wine is sometimes referred to as having legs. This is when wine adheres to the side of the glass when it is swirled, and then drips back down into the glass. This effect is associated with wines with higher alcohol content.

Recommended Resources

Enology Access, University of California