3. That the retardation of the assimilative process brought about by the use of coffee is owing chiefly to the empyreumatic oil, and is caused by caffeine only when taken in large quantities.
4. That increased action of the heart, trembling, headache, &c., are effects of the caffeine.
5. That the increased activity of the kidneys, relaxation of the bowels, and an increased vigour of mental faculties, passing into congestion, restlessness, and inability to sleep, are effects of the empyreumatic oil.
Professor Lehmann considers it, therefore, necessary to regard the action of coffee, and, in a less degree, that of tea, cocoa, alcohol, &c., upon the organism, as constituting an exception to the general law, that increased bodily and mental activity involves increased consumption of plastic material.
Caffeine, on careful analysis, has been found to contain in 100 parts, 49·80 of carbon, 5·08 of hydrogen, 28·83 of nitrogen, and 16·29 of oxygen. It is inodorous, but has a slightly bitter taste. The proportion in which this principle is found to be present in coffee varies between ¾ lb. and 1¾ lbs. in 100 lbs. of berries.
The peculiar essential oil which is generated in coffee in the process of roasting, by the action of heat upon some yet unascertained principle contained in the berry, is also very similar to the volatile oil in tea; but the quantity of it in coffee appears to be comparatively very small; for whilst 100 lbs. of tea-leaves contain 1 lb. of volatile oil, it takes 500 cwts. of roasted coffee to give a similar quantity; and yet it is upon the presence of this oil that the flavour and value of the several varieties of coffee mainly depend.
The tannic acid is, by some chemists, also said to be generated only in the process of roasting; others maintain that it is present in the raw bean.
The chemical properties of the coffee-berry are altered by roasting, and it loses about twenty per cent. of weight, but increases in bulk one-third or one-half. Its peculiar aroma, and some of its other properties, are due to a small quantity of essential oil, only one five-thousandth part of its weight, which would be worth about 100l. an ounce in a separate state. Coffee is less rich in theine than tea, but contains more sugar and a good deal of cheese (casein).
Schrader has analysed raw and roasted coffee, with the following result:
| raw. | roasted. | |
| Peculiar coffee principle | 17·58 | 12·50 |
| Gum and mucilage | 3·64 | 10·42 |
| Extractive | 0·62 | 4·80 |
| Resin | 0·41 | —2·08 |
| Fatty oil | 0·52 | |
| Solid residue | 66·66 | 68·75 |
| Loss | 10·57 | 1·45 |
| | 100·00 | ||