ABV

Alcohol by volume. The % of alcohol in a wine.

Acetaldehyde

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!

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.

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.

Balance

A wine is balanced when it has all the ingredients present in the correct proportion: acid, fruit, tannin, sugar, alcohol.

Base

the original ingredienst form which a wine is made.

Bentonite 

A fining agent made of clay particles which swell in water. 

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. 

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. 

Carbon Dioxide 

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. 

Demijohn

Glass jar for fermentation. It has a capacity of Just over a gallon (4.5 litres) and fills six standard wine bottles.

Diastase 

The enzyme which converts starch to sugar and thus prevents starch haze in a finished wine. Also known as fungal amylase. 

Dry 

The opposite of sweet. A dry wine has no taste of sweetness, although it may still contain a tiny amount of sugar.

Enzyme 

A chemical compound which can effectively convert complex molecules (such as starch) to simpler ones (such as sugar). Enzymes produced by yeast cells convert sugar into alcohol during fermentation.

Esters 

The chemical compounds responsible for the bouquet of wine. 

Fermentation 

The conversion of sugar into alcohol and carbon dioxide under certain conditions by yeast cells. (See also malo-lactic fermentation.) 

Fermentation lock or trap See air-lock.

Filtration

The removal of minute particles suspended in a wine by passing it through a filter medium. 

Fining 

The clearing of a wine by adding any substance which will coagulate suspended matter and form a sediment.

Finished wine

A wine in which the processes of fermentation and clearing are complete. 

Flor 

A growth of yeast cells formed on the surface of a wine during the making of sherry. 

Fortification 

The addition of alcohol to a finished wine to increase its alcoholic strength. 

Fructose 

A simple sugar, one of the constituents of sucrose. 

Glucose 

Another simple sugar; the other constituent of sucrose (see fructose). 

Grape sugar 

Glucose and fructose. 

Hydrometer 

An instrument for estimating the sugar content of a must or finished wine. 

Invert sugar 

A preparation of glucose and fructose obtained by boiling sucrose solution with some added citric acid. Yeast can ferment invert sugar, but has to convert sucrose enzymatically to glucose and fructose before fermentation.

Invertase

The enzymes produced by yeast which convert sucrose into glucose and fructose.

Isinglass 

A proteinaceous compound used to fine wine. 

Lactic acid 

An acid produced during the malo-lactic fermentation. Can also be added to a wine must in place of citric, malic or tartaric acid; it helps develop a good bouquet. 

Lees See sediment. 

Malo-lactic fermentation 

A reaction caused by the lactic acid bacterium converting malic acid into lactic acid in a wine where the normal fermentation is complete. Can be avoided by careful hygiene and sterilization. 

Maturation 

The change in wine which takes place during storage. It involves subtle chemical changes that produce a good bouquet and the precipitation of excess tannin (in red wines) to form a sediment. The flavor of a wine mellows during its maturation. 

Metabisulphite 

Sodium metabisulphite powder. It dissolves in water to form a potent sterilizing agent, sulphur dioxide gas.

Nutrients 

Chemical compounds (ammonium sulphate, phosphate and vitamin B) which encourage the growth of yeast.

Oxidation 

A process whereby alcohol is converted to aldehyde compounds, thus spoiling a wine's flavor, colour and bouquet. However, carefully controlled oxidation can be used to produce sherry.

Pectic enzyme 

An enzyme needed to break down pectin, a gummy carbohydrate substance found in fruit which may cause haziness in a finished wine. 

Primary fermentation 

See aerobic fermentation. 

Pulp fermentation 

Any fermentation conducted in the presence of the pulped fruit or other ingredients. 

Racking 

A process of siphoning a wine off the sediment formed after fermentation. 

Secondary fermentation See anaerobic fermentation. 

Sediment 

The deposit formed at the bottom of the fermentation vessel when fermentation is complete. It is composed of dead or dormant yeast cells, fruit sediment, and so on. 

Sucrose 

The chemical name for ordinary household sugar. It is composed of glucose and fructose molecules combined to form a complex sugar. 

Sulphur dioxide 

The gas produced by dissolving sodium metabisulphite powder in water. It acts as a sterilizing agent. 

Tannin 

An astringent substance which is an important ingredient of many wines, providing a vital element of the overall flavor. 

Vinegar 

Acetic acid. This may be formed in a wine if it becomes infected by types of bacteria often carried by the "vinegar fly" or fruit fly  Drosophila. 

Yeast 

A small, single-celled organism, Saccharomyces, which carries out fermentation. Only the true wine yeast can work adequately for the home-winemaker. 

Zymase 

The enzymes produced by yeast which convert glucose and fructose into alcohol.