Acetobacter:

A group of bacteria that oxidatively convert wine to vinegar (ethanol into acetic acid) through an aerobic (oxygen present) fermentation.

Acid blend:

A generic name for any commercially available blend of acids (usually citric, tartaric, and possibly malic) sold for the acidification of homemade wines.

Aeration:

The process of incorporating air into a wine, must, or juice. Usually through splashing while racking, sparging with air, or simply by stirring a container very vigorously. This is sometimes done to "blow off" undesirable aromas such as hydrogen sulfide or to give an initial dose of oxygen to a fermentation just getting under way.

Antioxidant:

Compound that retards oxidation and slows its effects in wine (browning, sherry-like aromas). Sulfur dioxide, SO2, is the most widely used winemaking antioxidant. It also serves as an antimicrobial agent.

Astringency:

The dry, puckery sensation caused by tannin in wine. The tannins actually denature the salivary proteins, causing a rough "sandpapery" feel in the mouth.

Bulk aging:

As opposed to aging wine in its final bottles, the term "bulk aging" is used to describe aging that might be done after fermentation but before bottling. Typically for home winemakers, this maturation occurs in five-gallon carboys or small oak casks.

Campden tablets:

A convenient way of delivering sulfites to wine. One tablet contains one-half gram of potassium or sodium metabisulfite.

Cap:

Fruit skins, stems, and pulp that float to the surface during a fermentation. It is essential to "punch down" the cap into the wine during a red wine fermentation to extract valuable tannins and colored compounds as well as to discourage the proliferation of spoilage organisms in the cap.

Carboy:

A glass container that looks like an office water-cooler bottle or a large jug. Carboys usually come in 2.8, 3.0, 5.0 & 6.5 gallon volumes and are used for fermenting juice, carrying out secondary fermentations, and for long-term storage.

Chillproofing:

Deliberately exposing wines to very cold temperatures prior to bottling to, primarily, precipitate any tartrate crystals that might come out of solution later. It is seen as more of a quality-control step than a necessity for home winemakers.

Clearing:

The natural settling-out of small particulates and suspended matter in finished wine over time. The material that settles out to the bottom of the container is called the "lees."

Cuvée:

French winemaking term relating to the specially blended base white wine that will be made to undergo a secondary fermentation in the production of sparkling wines. It also refers to a blend of different wines in general.

Degrees Brix:

The amount of sugar in a wine, usually measured by a hydrometer, which is a floating instrument that determines the density of solution. Based on a system calibrated to the density of water, the pre-fermentation degrees Brix of most table wines are between 22 and 24, meaning 22 to 24 percent sugar (really the percentage of total soluble solids, including unfermentable sugars). Knowing the Brix helps predict the final alcohol percentage, which should be high enough to retard growth of microbial contaminants as well as to provide sensory characteristics. For example a dessert wine (high alcohol and residual sugar) typically starts with must or juice that has 30° to 40° Brix and results in 15 to 20 percent alcohol.

Demijohn:

Somewhat of an archaic winemaking term. Demijohns (or carboys) are bulbous, long-neck bottles that can hold three to 10 gallons of liquid. Traditionally they were covered with wicker weaving to protect them from breaking.

Dry:

No sugar left, i.e. from a chemical standpoint (the degrees Brix is approaching 0) and/or a sensory standpoint (the wine is no longer perceptibly sweet).

Enology:

The science of winemaking.

Fermentation Lock:

Also called airlocks, these plastic devices fit over the tops of carboys or into bung holes of barrels to allow any gas produced by wine to escape while keeping out air.

Fining:

The process of adding an agent (such as bentonite or gelatin) to help clarify and stabilize the finished wine. This operation is done before bottling to help ensure the product will not be cloudy or flocculant in the bottle.

Glycerin:

Also known as glycerol, glycerin is a carbohydrate (sugar) that is not a substantial food source for most wine yeast strains, though it can be consumed by some lactic acid and acetic acid bacteria. It is sometimes added to wine to increase a wine's body and, in higher amounts, sweetness.

Hydrometer:

An inexpensive and widely available analytical device that measures the specific gravity (relative density) of a solution. Very useful to measure the amount of sugar (in Balling or degrees Brix) in a juice or wine. Because density depends on temperature, a thermometer reading of the solution being tested is critical for accurate results. Hydrometers are calibrated to be used at 60° F.

Invert sugar:

Common sugar (sucrose) that has been broken down into fructose and glucose. It does not contain dextrins. One pound of invert sugar is only two-thirds as sweet as cane sugar, so you have to use 50 percent more to achieve the same sweetness.

Lactic acid:

An acid present in wines that have undergone a malolactic fermentation, in which the malic acid (see below) has been transformed into lactic acid by malolactic bacteria. Lactic acid is less acidic than malic acid.

Lees:

The spent yeast cells that accumulate on the bottom of winemaking vessels after the population has completed the fermentation and has died out. Wine is usually racked (siphoned) off the lees to make it more presentable and to exclude any undesirable sensory effects that extended lees contact might impart.

Maceration:

The processes through which red wine grape (or other fruit) skins, seeds, and pulp are mixed and mashed in with the fermenting juice to extract tannins, colored compounds, and aroma from the grapes. Different maceration programs have different effects. For example if you stir red wine while it ferments (often called "punching down") twice a day as opposed to once a week, you should extract more color and tannin from the skins and seeds of the grapes into the finished wine than if your strategy was less aggressive.

Malic acid:

A naturally occurring grape acid that decreases with ripening. It is one of the principal components of a wine's total acidity. If a wine is too acidic (the grapes hadn't ripened fully), it can be de-acidified by a malolactic fermentation, in which the malic acid will get metabolized by malolactic bacteria and excreted as lactic acid.

Malo-lactic Fermentation:

Very different than alcoholic fermentation, malo-lactic fermentation occurs when a strain of lactic acid bacteria is introduced by chance or on purpose into a finished grape wine. These bacteria convert the malic acid (a natural grape acid) in the wine into lactic acid, a less potent acid, as well as contributing some flavor and aroma to the wine. Usually described as "buttery" or "caramel," this malo-lactic aromatic profile is especially desirable in quality red wine production as well as some whites, such as Chardonnay.

Must:

A red-wine making term that refers to the soupy mass of squished skins, seeds, and pulp that are fermented together. "Must" can also be applied to fruit winemaking; it refers to the gloppy pulp/skin mixture to which the yeast are added, essentially the winemaker's raw material. In contrast if the pulp and other solids are pressed off before fermentation, the raw material is simply "juice."

Oxidation:

Chemical term relating to the reaction of juice, must, or wine with oxygen. Typical negative side effects of such reactions are browning of wine and juice and "cooked" flavors and aromas. Limited amounts of oxidation are actually healthy for wine, because yeast need oxygen to grow during the initial stages of fermentation. Protracted, slow oxidation is a key physiological change that takes place when a wine ages.

Pectins:

Complex carbohydrate chains naturally occurring in fruits that can contribute to the viscosity and haziness of a wine. They can be shortened and solubilized (dissolved) by pectic enzymes, which are sometimes used in winemaking when dealing with non-grape fruit.

pH:

The pH of a wine or juice is the measure of the number of hydrogen ions that can be detected in the solution and is a measure of a wine's relative acidity. The pH can really only be accurately determined by titration (see Titratable Acidity).

Primary Fermentation:

The first vigorous "rolling" fermentation, in which yeast convert sugar in the wine to alcohol and carbon dioxide. At this stage it is all right for the fermentation vat to be exposed to the air because the yeast are producing so much carbon dioxide that it forms a "blanket" of this inert gas over the fermenting juice or must. Once the yeast start to die down and the fermentation is less active, it's advisable to move your wine into a carboy or barrel to exclude air and possible contaminants (see Secondary Fermenation).

Residual Sugar:

Any sugar left in the wine after the fermentation is complete and the yeast have completed their life cycles and have died out. Sometimes residual sugar is desired, as in sweeter white wines or dessert wines. Residual sugar that is perceptible on the palate is seen as a defect in most red table wines.

Reverse Osmosis:

An expensive and inconvenient commercial process through which alcohol and acetic acid can be removed from the wine so that it meets aesthetic or, more commonly, regulatory levels.

Ropiness:

A condition in which wine resembles slime, raw egg whites, or mucous. It is caused by an extreme microbiological contamination that produces long-chain carbohydrates (polysaccharides), hence the "ropiness."

Secondary Fermentation:

A bit of a misnomer, secondary fermentation can refer to two things: 1) A true second fermentation that follows completion of the first. Usually purposely started by adding yeast and extra sugar to the finished wine to make CO2 for a sparkling wine effect. 2) The second stage of the primary fermentation. After vigorous primary, wine is transferred to a carboy or barrel (secondary fermenter) to finish the last, protracted "secondary fermentation" when the yeast are slowing down and the wine needs to be protected from oxygen and any air-borne microbial contaminants.

Sulfites:

A class of sulfur-containing compounds used in winemaking as an antimicrobial agent, as an antioxidant, and as a preservative. A respiratory hazard in its undiluted state, sulfites need to be handled carefully but are entirely safe at the levels in which they are used for winemaking. If you are asthmatic and lack the enzyme sulfite oxidase, you should not consume foods or beverages that contain sulfites.

Sulfur dioxide:

In the form of potassium metabisulfite crystals, liquid sulfur dioxide, or sulfur dioxide gas, sulfur dioxide is an effective and safe preservative, antioxidant, and antimicrobial agent that has been used for millennia to facilitate the winemaking process. It is a respiratory irritant in high concentrations, so it should always be handled with care. Usual levels of free sulfur dioxide in table wines is about 20 to 40 parts per million.

Sweet wine:

Any wine in which there is perceptible residual sugar. Sugar is perceptible, depending on the individual taster and the composition of the individual wine, at about 1.5 percent.

Tannin:

The astringent phenolic anthocyanins found in grape skins, seeds, and stems that make your mouth pucker and feel dry when you drink red wine. Tannins are extracted from the grapes during the maceration process.

TCA:

2,4,6-Trichloroanisole, a chemical compound that is one of the major sources of the "cork taint" off-odor.

Titratable acidity:

Also known as "total acidity," titratable acidity is the total amount of all hydrogen ions (what makes acids "acidic") in a solution of juice, must, or wine. It is the measure of all aggregate acids and a sum of all volatile and fixed acids.

Titration:

A scientific analytical method used to determine total (or titratable) acidity. A strong base (such as sodium hydroxide), the opposite of acid, is added to a must, juice, or wine in measured amounts. If an indicator chemical (such as phenolphthalein) has been added to a sample of the liquid being tested, then a color change will occur at the point when all of the available hydrogen ions in the acids have been neutralized by the base. The total (or titratable) acidity of the must, juice, or wine can then be determined in relation to how much base it took to neutralize all of the acids in the wine.

Viticulture:

The science of grape growing.

Volatile Acidity:

Acid created during fermentation by spoilage organisms that are introduced by contact with fruitflies or other air-borne insects and contaminants. Refers usually to acetic acid (vinegar) produced by contamination by acetobacter bacteria. Keep fermenting wine covered to avoid problems.

Yeast Food (or Yeast Nutrient):

A pre-calculated commercial mix of vitamins, minerals, and amino acids added to juice or must to ensure a clean, complete fermentation. Adding it to non-grape wines is essential because many fruits lack high enough nitrogen levels to support healthy yeast growth.

Wild yeast:

Sometimes referred to as "natural yeast," wild yeast are the yeast fungi that are present naturally on grapes, on winery equipment, and just in the air itself. Many wineries rely on these itinerant microorganisms to start their wine fermenting, but since these yeast strains are far from uniform in population and "good" fermenting ability, using "natural fermentation" (and not inoculating with a proven pure culture) can be a serious risk. Wild yeast have been known to cause stuck fermentations, high hydrogen sulfide concentrations, and visual defects in finished wine, as well as a host of other spoilage reactions.