This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
The estimation of the total sugar is useful in distinguishing between sweet and "dry" wines, and also in the calculation of the amount of wine extract other than sugar. Further, the separate estimation of the dextrose and lævulose is sometimes helpful in distinguishing between different kinds of sweet wine. A notable excess of lævulose over dextrose indicates a wine to which sugar has been added and then more or less fermented, since dextrose is the constituent of invert-sugar most readily destroyed during fermentation. On the other hand, if the proportions of dextrose and lævulose are substantially equal, this may indicate a fully fermented wine sweetened by maceration with raisins, or wine made from a slightly fermented must to which alcohol has been added.