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NOTE ON THE EXAMINATION OF COFFEE.

By ALFRED H. ALLEN.

Read before the Society of Public Analysts, on 19th November, 1879.

IN a series of articles on "Chemistry applied to the Detection of Adulteration," published during 1874 and 1875, I described three methods as likely to be of service for the approximate determination of chicory in samples of mixed coffee.* Since the date of their publication I have acquired a large amount of additional experience in their use, and have arrived at the following conclusions.

In brief, the three methods suggested as applicable for the determination of chicory in coffee were as follows:

I. Determination of the soluble ash.

II. Comparison of the tint of an aqueous solution of the sample with that furnished. by similarly treating a standard specimen.

III. Determination of the density of a 10 per cent. infusion in hot water.

With respect to Method I., which in the paper referred to was merely suggested as of possible value, experience has shown that it is only capable of furnishing results of the roughest possible kind. This fact is due to the variations in the percentage composition of the ash of both coffee and chicory, as well as to differences in its total amount. In twenty samples of roasted genuine coffee recently examined, the total ash varied from 3.78 to 4.87 per cent. This last result was very exceptional, the next highest being only 4.39 per cent., while the average of the whole twenty samples was 4.04 per cent. The soluble ash varied in thirteen samples from 2.52 to 3.50 per cent., the average being 2.97 per cent. If the total ash be taken as 100, the highest proportion of soluble ash met with was 84 per cent., with the exception of the sample yielding 86 per cent., referred to in the above-mentioned paper. The lowest percentage of soluble matter found was 60 per cent. of the weight of the ash, while the average is 73.5 per cent.

These differences are in themselves sufficiently great, but they are exceeded by those exhibited by chicory, owing to the considerable and very variable proportion of silica present in the latter substance. The proportion of actual sand in commercial chicory varies from a trace, up to 4.5 per cent., a difference quite sufficient to invalidate deductions made from the proportion of soluble ash. By deducting the sand from the

Chemical News, XXIX., 140.

The total ash of coffee has been determined by Dragendorff, who found a minimum amount of 3.83 per cent. and a maximum of 4.87. The average ash of the twenty-five samples examined was 4.41. Why there should be so wide a difference between Dragendorff's and my own results I am unable to conjecture. I have no reason to suppose that partial volatilization has occurred.

total ash, and considering the number thus obtained to be the true ash of the sample, more concordant results are obtainable, but the variations are still too large to allow of the method being employed for any purpose beyond a check on the proportion of chicory in a mixture.

Method II., depending on the colour of the infusion, is capable of giving rapid and fairly reliable estimations of the proportions of chicory present in mixed samples, but in practice it is open to the very serious objection that a standard mixture of various coffees and chicories is apt to undergo a change which gravely affects the colour of the infusion. By comparing the infusion of the sample with a permanent coloured solution, such as can be prepared by mixing the sulphates of iron, cobalt, and copper in suitable proportions, the above-named annoyance and source of error may be wholly avoided, and the method again becomes very valuable. I am unable to perceive any advantage in the method of working suggested by Dr. Leebody* over that originally described by me.

Method III., which is based on the difference in the density of similarly prepared decoctions of coffee and chicory, is one which further experience has proved to be very valuable. The weak point in the method as originally suggested by Graham, Hofmann, and Campbell was that these chemists prepared their solution by treating a known weight of the sample with ten times the quantity of cold water, and then gradually raised the liquid to the boiling point. By operating in this manner there is no certainty that the sample will be completely exhausted, and hence accurate comparison of different samples is difficult or uncertain. As a matter of fact, I have reason to think that exhaustion of the sample is usually tolerably perfect, but it is evidently preferable to boil well, filter, and wash the residue with hot water till the filtrate measures 10 c.c. for every 1 gramme of the sample operated on.

By operating in the old manner, Graham, Hofmann, and Campbell obtained, from roasted coffee, 10 per cent. decoctions which varied in density (at 60° F.) from 1008.0 to 1009.05, the average of the eight samples being 10087, a result identical with the mean of those obtained by me in 1874.

By the exhaustion modification of the process, I have recently obtained the following results from genuine roasted coffee.

* Chemical News, XXX., p. 243.

† It will be observed that Graham, Hofmann and Campbell's method gives slightly higher density results than that by exhaustion. Either this is due to a change in the volume of the liquid, or more probably to a slight loss by evaporation when the infusion is made by raising the liquid to the boiling point, instead of making the bulk up after cooling.

These results show conclusively that the density of coffee infusions is remarkably constant, never exceeding 1009.

On the other hand, 10 per cent. decoctions of chicory are of considerably higher density, and exhibit greater variations among themselves. Thus Graham, Hofmann and Campbell obtained :

In calculating the proportion of chicory in a sample from the density of the infusion, it is of course desirable to err on the safe side, and this is done if we take the density of the chicory rather above than below the truth. From a consideration of the whole of the results, both recently and in 1874, I adopt 1023 as the normal density of chicory decoction, and by taking that of coffee at 1008.5, we cannot get far from the truth. If d be the ascertained density of the 10 per cent. decoction, and C be the (1023-d) 100 14.5

percentage of coffee in the sample, then C

=

Practically, as close an approximation as the above is obtainable by reckoning 7 per cent. of chicory for every degree of density over 1008·5.

In practice, the determination of the density of the infusion may be employed to ascertain the purity of a sample of coffee, a very small admixture of chicory causing an appreciable increase. Of course, however, all such methods are, in the case of coffee, mere adjuncts to the microscopical examination, by which the smallest admixture of chicory can be detected with the greatest facility. Personally, I prefer to examine with the microscope the residue left after boiling the sample in water, the troublesome colouring matter being thus removed without in any way interfering with the characteristic structure of the particles of chicory.

One of the many beneficial results of the appointment of Public Analysts has been to render nearly obsolete the various additions to coffee that were once far from uncommon. I have never officially examined a sample of coffee containing any admixture other than chicory, though I have invariably looked for leguminous seeds, cereals, &c.

The search for cereals, leguminous berries, and foreign matters other than chicory, is most readily effected by boiling the sample with water and testing the strained solution for starch. The liquid is allowed to become perfectly cold, and is then mixed with dilute sulphuric acid, and a strong solution of potassium permanganate added gradually till the colouring matter is nearly destroyed. The addition of solution of iodine then