Acidity in a must has been described as the cornerstone upon which flavours are built. Without sufficient acid a wine tastes medicinal, will not ferment well, is prone to infection, will not keep and lacks balance.

The common acids found in fruit, other than grapes, are citric or malic. Most winemakers add citric acid to their musts, partly because of its general availability and partly because it stimulates a quick fermentation. Tartaric acid, the major acid in grapes, is used by some winemakers, however, because any excess is precipitated during maturation as potassium tartrate - little glass-like crystals. Malic acid also has advantages in that it is frequently subject to a malo-lactic fermentation by members of the lacto-bacillus group. The sharp-tasting malic acid is then converted to the mild-tasting lactic acid. A combination of the three acids has all the advantages, but is less widely used than it deserves.

As a general rule a dry wine needs an acidity of between 3.5 and 4.5 parts per 1,000. A sweet wine needs a little more, between 4.5 and 5.5 parts per 1,000 to balance the sugar. Strong wines which it is hoped to mature for 10 years or so, need more still, and 6 or 7 parts per 1,000 would not be too much.

As a general guide a mixture of 2 parts tartaric acid to 1 part malic acid should be added to fruits containing citric acid, and a mixture of equal parts citric and tartaric acids to fruits containing malic acid. To vegetable, flower, cereal and herb musts a mixture of the three acids in the proportion 2 parts citric, 2 parts tartaric and 1 part malic acid, make a suitable blend.

For must containing no acids at least 20 grams (J oz.) and preferably 28 grams (1 oz.) of acid should be added per demijohn of must, ie for 6 bottles of wine. This quantity may be increased up to 40 grams (1^ oz.) for a strong dessert wine, that is to be matured for a number of years.

Acid, in the form of crystals, keeps for years in an airtight jar stored in a cool dry place. It is best to buy the crystals in bulk so that you always have plenty when needed.

Tannin

This is the substance which gives wine 'zest' or 'bite' and contributes substantially to a wine's character. Wines lacking tannin have an insipid taste, do not keep well and seem rather 'spineless'. Grapes, of course, have an abundance of tannin, to be found in skin, stalk and pips. Many fruits have some, including apples, apricots, bananas, blackcurrants, bilberries, damsons, elderberries, peaches, pears and sloes. Most fruits have insufficient, however, especially after dilution. Flowers, vegetables, cereals and herbs have no tannin. It is readily available to the home wine-maker in the form of grape tannin powder and about half a level 5 ml spoonful should be added to all musts. Some winemakers add a quantity of strongly brewed cold tea left in the pot, varying from 1 table-spoonful to cup. The process of fermenting a must on the pulp, extracts the maximum amount of tannin from fruit.

Nutrients

It is essential that they must should contain sufficient nourishment for the yeast cells, if a satisfactory fermentation is to be effected. This can be added in the form of a mixture of different salts of ammonium phosphate and sulphate and vitamin B. Experience has shown, however, that some fruit mucilage is also needed to produce a good wine. Some grape juice concentrate, fresh grapes or dried grapes should therefore be added to every must and especially to flower, herb, grain and vegetable musts.

Pectic Enzyme

Fruit juice is contained in a cell surrounded by a wall composed of cellulose and a mixture of substances called pectin. Pectin acts as a protective colloid or haze-forming particles and unless it is destroyed prevents a wine from clearing and becoming star-bright. It does not respond to fining agents or to filtering and has to be treated with an enzyme called pectinase. This enzyme is present in many fruits, especially the grape, and will clear many musts during fermentation. Pectinase is inactivated by heat, however, and the pouring of boiling water over fruit damages it.