| Composition of Melons. | ||||
|---|---|---|---|---|
| Total weight, grams. | Juice, per cent. | Total proteids, per cent. | Ash, per cent. | |
| Watermelons | 10330 | meat 83.99 | 6.12 | 0.37 |
| rind 81.02 | ||||
| Muskmelons | 3407 | 80.23 | 6.45 | 0.57 |
| Composition of Juice. | ||||
| Sucrose in juice, per cent. | Reducing sugars in juice, per cent. | Ash per cent. | ||
| Watermelons | meat 1.92 | meat 4.33 | meat 0.31 | |
| rind 0.34 | rind 2.47 | rind 0.38 | ||
| Muskmelons | 1.02 | 3.04 | 0.53 | |
TEA AND COFFEE.
579. Special Analysis.—Aside from the examination of teas and coffees for adulterants, the only special determinations which are required in analyses are the estimation of the alkaloid (caffein) and of the tannin contained therein. It is chiefly to the alkaloid that the stimulating effects of the beverages made from tea and coffee are due. The determination of the quantity of tannin contained in tea and coffee is accomplished by the processes described under the chapter devoted to that glucosid.
The general analysis, viz., the estimation of water, ether extract, total nitrogen, fiber, carbohydrates and ash, with the exceptions noted above, is conducted by the methods which have already been given.
For detailed instructions concerning the detection of adulterants of tea and coffee the bulletins of the Chemical Division, Department of Agriculture, may be consulted.[593]
580. Estimation of Caffein (Thein).—The method adopted by Spencer, after a thorough trial of all the usual processes for estimating this alkaloid, is as follows:[594] To three grams of the finely powdered tea or coffee, in a 300 cubic centimeter flask, add about a quarter of a liter of water, slowly heat to the boiling point, using a fragment of tallow to prevent frothing, and boil gently for half an hour. When boiling begins, the flask should be nearly filled with hot water and more added from time to time to compensate for the loss due to evaporation. After cooling, add a strong solution of basic lead acetate until no further precipitation is produced, complete the volume to the mark with water, mix and throw on a filter. Precipitate the lead from the filtrate by hydrogen sulfid and filter. Boil a measured volume of this filtrate to expel the excess of hydrogen sulfid, cool and add sufficient water to compensate for the evaporation. Transfer fifty cubic centimeters of this solution to a separatory funnel and shake seven times with chloroform. Collect the chloroform solution in a tared flask and remove the solvent by gentle distillation. A safety bulb, such as is used in the kjeldahl nitrogen method, should be employed to prevent entrainment of caffein with the chloroform vapors.
The extraction with chloroform is nearly complete after shaking out four times; a delicate test, however, will usually reveal the presence of caffein in the watery residue even after five or six extractions, hence seven extractions are recommended for precautionary reasons. The residual caffein is dried at 75° for two hours and weighed.
The principal objection which has been made to Spencer’s method is that the boiling with water is not continued for a sufficient length of time. For the water extraction, Allen prescribes at least six hours cohobation.[595] In this method six grams of the powdered substance are boiled with half a liter of water for six hours in a flask, with a condenser, the decoction filtered, the volume of the filtrate completed to 600 cubic centimeters with the wash water, heated to boiling, and four cubic centimeters of strong lead acetate solution added, the mixture boiled for ten minutes, filtered and half a liter of the filtrate evaporated to fifty cubic centimeters. The excess of lead is removed with sodium phosphate and the filtrate and washings concentrated to about forty cubic centimeters. The caffein is removed by shaking four times with chloroform. Older but less desirable processes are fully described by Allen.[596]
In France this method is known as the process of Petit and Legrip, and it has been worked out in great detail by Grandval and Lajoux and by Petit and Terbat.[597]
581. Estimation of Caffein by Precipitation with Iodin.—The caffein in this method is extracted, the extract clarified by lead acetate and the excess of lead removed as in Spencer’s process described above. The caffein is determined in the acidified aqueous solution thus prepared, according to the plan proposed by Gomberg, as follows:[598]