882 



FOODS AND FOOD ADULTERANTS. 



SPENT OR EXHAUSTED LEAVES. 



The detection of spent or partially exhausted leaves in teas is not a 

 simple problem. The chemical methods all depend upon the proportion 

 of certain of the soluble > constituents which remains in the tea leaves. 

 A large number of analyses of genuine teas made by different chemists 

 show a very wide range in the percentage composition of teas 5 hence 

 the difficulty in obtaining a basis for comparison. A genuine tea may 

 contain as little as 10 per cent tannin or even less, and as high as 25 

 per cent. It is evident from this that the percentage of tannin can not 

 be depended upon for the detection of the admixture of spent leaves. 

 A large quantity of such exhausted leaves would be required to reduce 

 a tea containing a high percentage of tannin to the average percentage. 

 The same may be said, in a measure, of each of the soluble constituents 

 of the leaves. Hence, we see the difficulty in securing data of value on 

 this point from a determination of the soluble matters. The only 

 estimation of this kind that is of value, and its value it must be said is 

 but relative, is the percentage of extract matter. A large number of 

 analyses by different authorities shows a range in the extract matter 

 of from 26 to 50 per cent of the air-dried tea. This extract is not the 

 total soluble matter of the prepared leaf, bat is the soluble matter 

 extracted under certain conditions which are fully described on page 892. 



One of the most valuable series of analyses of genuine teas that have 

 been made is that of Mr. Joseph F. Geisler. 1 This chemist obtained 

 the following averages of extract matter: 



Formula' have been given, notably by Allen, lor the calculation !' 

 the proportion of spent leaves from t lie a mount of extract matter. Little 



'.Josuph V. (icislcr in AIM. (Jror.-r, O-t>l>rr L'li. 1SSI; :i|s,, in 1'ivsrot f O 

 Analysis, |ip. r>()f (<> fill.'. 



