WHAT IS WINE? 
39 
of other homologous acids, such as propionic acid, butyric 
acid, &c. &c. It is to these latter that the production of the 
bouquet is chiefly due. 
In good sound wines the total amount of free acid, that is, acid 
not combined with or neutralised by an alkali, varies from three 
to six parts per thousand, calculated as if all the acid were tar- 
taric acid. This, as before stated, is not strictly correct, but it is 
the conventional method of expressing the acidity of a wine. In 
the case of white wines, not fortified, not more than about one- 
fourth of the total acidity should be due to volatile acids ; in 
the case of red wines, or fortified wines, the proportion of volatile 
acid is generally higher, but should not, even in these, amount 
to more than about one-third of the total free acid. 
Sugar in Wine . — Grape juice contains two kinds of sugar in 
equal proportion — grape sugar and fruit sugar. Both these 
sugars are capable of fermentation, both are destroyed by being 
heated with a solution of an alkali, and both reduce copper 
salts from their warm alkaline solutions. The first, however, is 
crystallisable, and turns the plane of polarised light to the 
right ; the second forms an uncrystallisable syrup, and turns 
the plane of the polarised ray to the left about twice as much 
as the other turns it to the right. A mixture of both in equal 
proportion, such as is found in the grape, will therefore turn 
the ray about half as much to the left as pure grape sugar 
turns it to the right. This same mixture of sugars is produced 
by the action of yeast, or acids, on cane sugar, and is then called 
invert sugar, since, as above explained, it turns the plane of 
polarised light to the left, cane sugar turning it to the right. 
Cane sugar then, when added to must , or wine, very soon 
becomes changed into invert sugar — a mixture of sugars identical 
with that found in the grape — and a few weeks after its addition 
is no longer found as cane sugar. During the fermentation of 
the must grape sugar chiefly is destroyed, so that the sugar 
remaining in the wine is in greater part fruit sugar. 
By the action of sulphuric acid on starch, a sugar is produced 
chemically identical with grape sugar (not with the mixture of 
sugars as found in the grape), and this is frequently employed 
in the adulteration of must and wine. 
Natural wines, when more than a few years old, contain, as 
a rule, little or no sugar. Young wines of this class not unfre- 
quently, however, retain an appreciable quantity of sugar, 
amounting in some rare instances even to 6%, though more 
usually to 05 or 2% only. As long as the wine contains this 
amount of sugar, it is very apt to enter again into fermentation, 
and must therefore be kept for some years in cask, and in a 
cool cellar, before it can be safely bottled. In time this sugar 
gradually diminishes, even without any decided fermentation, 
