97 



With respect to the second point, investigators are by no 

 means agreed as to the best method of estimating the tannic acid in 

 hops. HERON (loc. sit.) cites several methods, one depending on the 

 absorption of tannic acid by "hide " powder, the decrease in weight 

 being put down to tannic acid absorbed. HERON condemns this 

 method, we think justly, on the score that ' hide " powder is 

 indefinite in its reaction and of greatly varying activity; it is 

 certainly not the class of reagent which a chemist cares to deal 

 with. In another process, generally known as LOWENTHAL'S 

 method, the extract containing the tannin is titrated with per- 

 manganate of potash before and after treatment with " hide " 

 powder, and this method certainly gives better results than the 

 weighing method, but still introduces the objectionable reagent. 

 The method which HERON finally used himself was a slight modi- 

 fication of that known as LOWENTHAL'S gelatin process. In this 

 last case the tannic acid is precipitated with gelatin and titration 

 made before and after with potassium permanganate. But HERON 

 prefers to calculate his potassium permanganate into terms of oxalic 

 acid, whereas by the usual method a titration of the standard 

 potassium permanganate solution is made on a solution of tannin of 

 known strength. This last method would appear to give results 

 more in accordance with facts, the only objection that can be 

 raised being in respect to the tannin employed for the solution. 

 Instead of extracting his hops by successive boilings with water 

 HERON only digests them in the boiling water bath for one hour, 

 and he gives figures to show that by this means he obtains the 

 whole of the tannic acid. The very proper objection he raises to 

 repeated boilings is that the tannic acid is liable to be partially con- 

 verted into phlobaphen. There is yet another method of estimating 

 tannin devised by KOKOSINSKI, based on the property of tannic 

 acid of absorbing iodine with great energy in the presence of alkaline 

 carbonates. It is recommended by MORITZ and MORRIS (" Text 

 Book of the Science of Brewing," p. 497), but does not appear to 

 have found favour with investigators. 



Hence it will be seen, that with varying methods of varying 

 degrees of accuracy there is great difficulty in basing any conclusive 

 argument on figures as to the relative value of tannic acid ; a 



H 



