THE CHEMISTRY OF FRUIT-RIPENING. 469 



Fresenius's analyses give the following as the average proportions 

 of total acid, reduced to equivalent of malic acid : 



Currant 2.04 per cent. 



Raspberry 1.48 " " 



Strawberry 1.31 " " 



Sour cherry 1.28 " " 



Apple 0.75 " " 



Grape 0.74 " " 



Peach 0.67 " " 



Red pear 0.07 " " 



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 w T ill 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 ap- 

 proaches to the nature of an acid, though properly classed as a neu- 

 tral 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 



