As a rule, wines which are kept for many years in cask become very dry, and the loss of alcohol by evaporation - particularly in the case of light wines - has as a result the production of acidity by oxidation.
It is found that transparent oils under the influence of light absorb oxygen, becoming deeper in colour and opalescent, while strong acidity and a penetrating odour are developed, these changes being due to the formation of various acid and phenylated compounds, which are also occasionally found in fresh oils.
A perfect soil would be such a blend of sand, clay, chalk and humus as would contain sufficient clay and humus to prevent drought, enough sand to render it pervious to fresh air and prevent waterlogging, chalk enough to correct the tendency to acidity of the humus present, and would have within it various substances which would serve as food-materials to the crops.
It has been found, for instance, that in the case of the mannitic disease the action of the micro-organism may be checked, or prevented altogether, by bringing the acidity of the must up to a certain level by the addition of a small quantity of tartaric acid.
In 1780 he proved that the acidity of sour milk is due to what was afterwards called lactic acid; and by boiling milk sugar with nitric acid he obtained mucic acid.
When the grape is ripe, the sugar has attained to a maximum and the acidity is very much reduced; the tannin has entirely disappeared.
Hence the composition and acidity of the solution, on which so much depends, must be constantly watched.
It will be seen that, compared with the dry, light red wines, the proportion of sugar, alcohol and acidity is comparatively high in champagne, and the extract (solid matter) rather low.
A slight degree of acidity seems more favourable to the crystallization of salt than alkalinity; thus it is a practice to add a certain amount of alum, 2 to 12 lb per pan of brine, especially when, as in fishery salt, fine crystals are required.
At the same time, their acidity is very low and their bouquet characteristically delicate and elegant.