Fresenius's analyses give the following as the average proportions of total acid, reduced to equivalent of malic acid:
The quantity of acids in fruits usually diminishes during ripening. The diminution is not, however, nearly so great as it appears to the taste, because the acid of ripe fruits is masked to the taste by the larger proportions of sugar and the pectous substances then present. The removal of acids is chiefly due to oxidation. It is not found that acids are neutralized, to any considerable extent, during ripening, by alkalies conveyed through the stem. The diminution of the acid in plums was shown definitely by the series of analyses before given from Mercadante. It is stated that the acids continue to oxidize away, after the sugar has reached its maximum and before it begins to diminish. Hence, perfect ripeness in fruit has been defined as that period during the maximum quantity of sugar when the quantity of acid is least. This will be, of course, just before the sugar begins to diminish.
It has been stated that both citric and malic acids are often found in unripe grapes, and are substituted by tartaric acid during the ripening. Oxalic acid is more often found in unripe than in ripe fruits. It is to be desired that closer determinations should be made as to the presence and proportion of oxalic acid in tomatoes and some other fruits. Any article of food containing oxalic acid (as the garden pie-plant) should probably be eaten with moderation, if at all.
A misapprehension sometimes occurs, from lack of reflection, as to the effect of sugar on the acidity of fruits. Sugar has no chemical effect upon acids. Its very sweet taste masks or overpowers to the sense the sour taste of free acids; but the acids remain free, all the same. Whatever effect the sugar eaten with fruits has on digestion and nutrition is due to the sugar itself; not to any change of the acids by the sugar, for there is no such change. Indeed, sugar approaches to the nature of an acid, though properly classed as a neutral body.
The varieties of tannic acids classed together as tannin are quite unlike the fruit acids above mentioned, both in sensible properties and in chemical relations. Only a few of the ripe edible fruits contain astringent acids, though these are found in many unripe fruits and in numerous ripe fruits not used for food. Most varieties of colored grapes contain a little tannin, deposited mostly in the skins and