A
ABV
Alcohol by volume. The % of alcohol in a wine. Acetification The formation of acetic acid from alcohol - what we'd call the conversion of wine to vinegar by bacterial infection. To avoid acetification, fermentation vessels should be sterilized and kept full. You can prevent infection by Acetobacter, responsible for the conversion, by using good hygiene and sterilization procedures, but once a wine is infected you won't be able to save it. Don't use any equipment or vessels to make your wine which may have held vinegar!
Acetaldehyde
Organic chemical molecule produced as a by-product of fermentation, and as a result of oxidation of the alcohol to acetaldehyde. It's needed for the special aroma of oxidized wines like sherry and Madeira, but too much spoils the flavor of all wines.
Acid
The acids in a wine must provide sharpness and flavor; they also ensure the yeast has an environment suitable for fermentation. There are three main types of acid: citric, the acid of citrus fruits; tartaric, the acid of grapes and other fruits; and malic acid, the acid chiefly of apples. A good blend for use in home winemaking is 50% tartaric, 30% malic and 20% citric.
Acidity
Too much acid makes a wine sharp or acidic; too little makes it flabby or bland. You can test for acid levels using a pH meter or titration. Otherwise you can just follow a recipe. It's important to have the right levels of acidity in a must so fermentation can proceed correctly and the finished wine will be balanced and enjoyable.
Activated Yeast
A dried yeast which has been cultivated using a Yeast Starter. Once activated, the yeast is ready to ferment the must. Activation is essentially a process of developing a thriving yeast colony with enough yeast cells to ferment a must.
Aerobic fermentation
The first part of the fermentation, conducted in the presence of air, in a tank, fermentation vat, pail or other such vessel, during which yeast builds up a strong colony of cells.
Aftertaste
The lingering residue of flavor after you have swallowed a wine, which should impart some memory of the wine's essential character - fruity, dry, sweet, etc.
Aging
The process of maturing a wine, in anaerobic conditions, so that the chemical reactions essential for the development of full flavor and aroma can proceed to completion. The chief reaction is the formation of volatile esters by reaction of wine acids and alcohol to produce esters, aromatic molecules which give a wine its bouquet. In additions, tannin will react with wine acids and precipitate out. The reactions proceed more slowly in bottle than cask.
Air-lock (fermentation lock, air trap, bubbler)
A glass or plastic device which excludes external air from the fermentation vessel, but allows carbon dioxide to escape.
Alcohol
The compound responsible for the formation of character and flavor, not to mention the intoxication of the drinker, in a wine, is a 5 carbon atom chain alcohol also known as ethanol or ethyl alcohol. Other alcohols such as propanol or butanol with more carbon atoms form during fermentation, and contribute to the flavor and body of wine. Methanol, or methyl alcohol, is extremely poisonous.
Ameliorate
To add a substance to a wine must or finished wine to modify its flavor and quality. For example: adding water to a must, or concentrated grape juice to sweeten a finished wine.
Amino acids
Constituents in grapes and wines that contain nitrogen and are metabolised to form protein and other compounds
Amylase
An enzyme that converts starch to fermentable compounds.
Anaerobic fermentation
The second part of the fermentation, conducted under air-lock, during which most of the alcohol is formed.
Antioxidant
Anything designed to stop a wine oxidizing and losing quality of flavor at any stage of its preparation. Sulfur dioxide is the best antioxidant.
Aroma
The fragrance of a wine due to its original ingredients. Compare with bouquet.
Astringency
A taste and a mouthfeel that seems constricting or puckering - hard to describe, but easy to recognize. Due mostly to tannins in a wine, it tends to mellow with age as the tannins precipitate out. It is not the same as bitterness.
Autolysis
Breakdown of dead yeast cells in a wine, giving a rich flavor, structure and body to wines like champagne, and those made from chardonnay or sauvignon blanc.
B
Balance
A wine is balanced when it has all the ingredients present in the correct proportion: acid, fruit, tannin, sugar, alcohol.
Base
The original ingredients from which a wine is made.
Baumé
Pertaining to, noting, or callibrated according to a Baumé scale. The Baume scale provides a measure of specific gravity, which is the ratio of the density of substance to the density of a reference substance (typically water). More commonly expressed as ° Brix in winemaking (the weight in grams of sucrose dissolved in 100 grams of a solute, such as grape juice.)
Bentonite
A fining agent made of clay particles which swell in water.
Bisulfite
A source of sulphur dioxide.
Bite
The astringency of a wine, produced by tannin. Without sufficient tannin, a wine may taste flat and insipid.
Bitterness
A sense of taste in a wine which is not pleasant. Not the same as astringency, which is felt in the mouth rather than tasted. Bitterness is most often associated with polyphenolic compounds, especially tannin, but high sulfate (not sulfite) content can also produce bitterness. Bitterness can be partially alleviated by fining, partially masked by sweetness and partially eliminated by aging.
Bottles
These come in many sizes: A Magnum is equivalent to 2 standard bottles (1.5 litres); a Double Magnum is 3 litres; Jerobaom is 3 litres of sparkling wine; Methusalah, 6 litres; and a Nebuchadnezzar is 15 litres of sparkling wine!
Bottle sickness
Immediately after bottling a wine may seem unpalatable, bland or flat. This blandness or flatness will last for only a short period of time.
Bottle stink
An unpleasant aroma which may be apparent after opening an aged bottle of wine. It soon disperses, leaving the true aroma and bouquet of the wine.
Blending
The process of mixing wines with differing qualities so that their faults or deficiencies cancel each other out.
Body
The body of wine refers to the sense of fullness one gets when drinking it. The opposite is "thin", which means a wine tastes thin and watery. A wine of full body will contain more glycerol and higher alcohols than a thin wine.
Bouquet
The aroma or "nose" of a wine which develops during storage in bulk containers or bottles. Produced by slow chemical reactions between acids and alcohol in the wine.
Brix, 0Brix, Degrees Brix
Value used to express the weight in grams of sucrose dissolved in 100 grams of a solute, such as grape juice.
Browning
Undesirable brownish (amber, tawny, dark yellow) colour change in table wine resulting from oxidation.
C
Campden Tablets
Small tablets of compressed sodium metabisulphite powder which are used to make up sterilizing solution or to protect a wine against oxidation or infection during storage.
Cap
Grape solids ( skins and pulp) that separate from the juice of crushed grapes (must) and tend to float and compact on the juice surface.
Carbon Dioxide (Co₂)
Odourless, harmless gas produced during fermentation by the action of yeast on sugar dissolved in the must. Clearing The natural process by which sediment drops out of a wine after fermentation, to form a deposit of lees and leave the wine clear.
Chaptalization
Adding sugar to must or juice before fermentation to make up the deficiencies.
Under Excise laws in Australia it is illegal to add cane sugar to grape wine. Only grape juice concentrate may be used legally to sweeten wines.
Cold stabilisation
Removal of excess potassium bitartrate to prevent its crystallisation and precipitation in wine stored under cold conditions.
Concentrate grape
Refers to concentrated grape juice in which the water content has been reduced so that the soluble (dissolved) solids are increased threefold: usually is 680 to 700 Brix.
Conversion
Conversion tables here