TEE WOBK OF FBUITS 315 



t'l-uits (e. g., Currant, Easpberry, etc.), a marked pro- 

 duction of acid. It is these fruits which are principally 

 used in the preparation of jams, jellies, etc. The 

 " jelling " depends on the presence of gelatinous sub- 

 stances (pectin compounds and allied substances) which 

 are present in the ripe fruit and are increased in 

 quantity by boiling the fruit; in this process, the acid 

 contained in the fruit acts on various substances and 

 transforms them into other substances which readily 

 " jell"; in the young fruit, these bodies, as well as the 

 acids, are lacking to such an extent that such fruits 

 can not be used for jelly-making. On the other hand, 

 they disappear from over- ripe fruit to such an extent 

 that it, too, is unfit for this purpose. 



Along with these changes which occur during the 

 process of ripening, go changes in the sugar -content. 

 The young fruit is either tasteless and insipid or else 

 acrid and sour; as it approaches ripeness, it gets 

 sweeter, and the sugar may accumulate to such an ex- 

 tent as to almost completely mask the taste of the acid 

 (which is, however, still present in undiminished 

 amount) . The sweetness (and the characteristic flavor) 

 of the fruit is increased by dryness and warmth: 

 mountain -grown fruit is, for this reason, sweeter; over- 

 irrigated fruit is insipid. Sweetness may also be in- 

 creased in some cases by appropriate fertilizers, and, 

 even, in the case of oranges, by spraying the fruit with 

 chemicals. 



