Part 53

Example.—Let the weight of a potato in air be 159 grams, and its weight in water 14.8 grams.

Then the weight of 10000 grams of potatoes of like nature in water would be found from the equation 159: 10000 = 14.8: x.

Whence x = 931 nearly.

In the table the nearest figure to 931 is 930, corresponding to 24.6 per cent of dry matter and 18.8 per cent of starch. When the number found is half way between the numbers given in the table the mean of the data above and below can be taken. In other positions a proper interpolation can be made if desired but for practical purposes the data corresponding to the nearest number can be used.

TABLE FOR CALCULATING STARCH IN POTATOES FROM SPECIFIC GRAVITY.

10000 grams of potatoes weigh in Per cent Per cent water. dry matter. starch. Grams.

750 19.9 14.1 760 20.1 14.3 770 20.3 14.5 780 20.7 14.9 790 20.9 15.1 800 21.2 15.4 810 21.4 15.6 820 21.6 15.8 830 22.0 16.2 840 22.2 16.4 850 22.4 16.6 860 22.7 16.9 870 22.9 17.1 880 23.1 17.3 890 23.5 17.7 900 23.7 17.9 910 24.0 18.2 920 24.2 18.4 930 24.6 18.8 940 24.8 19.0 950 25.0 19.2 960 25.2 19.4 970 25.5 19.7 980 25.9 20.1 990 26.1 20.3 1000 26.3 20.5 1010 26.5 20.7 1020 26.9 21.1 1030 27.2 21.4 1040 27.4 21.6 1050 27.6 21.8 1060 28.0 22.2 1070 28.3 22.5 1080 28.5 22.7 1090 28.7 22.9 1100 29.1 23.3 1110 29.3 23.5 1120 29.5 23.7 1130 29.8 24.0 1140 30.2 24.4 1150 30.4 24.6 1160 30.6 24.8 1170 31.0 25.0 1180 31.3 25.5 1190 31.5 25.7 1200 31.7 25.9 1210 32.1 26.3 1220 32.3 26.5 1230 32.5 26.7 1240 33.0 27.2 1250 33.2 27.4 1260 33.4 27.6 1270 33.6 27.8 1280 34.1 28.3 1290 34.3 28.5 1300 34.5 28.7 1310 34.9 29.1 1320 35.1 29.3 1330 35.4 29.6 1340 35.8 30.0 1350 36.0 30.2 1360 36.2 30.4 1370 36.6 30.8

=270. Constitution of Cellulose.=—The group of bodies known as cellulose comprises many members of essentially the same chemical constitution but of varying properties. The centesimal composition of pure cellulose is shown by the following numbers:

Carbon, 44.2 per cent Hydrogen, 6.3 ” ” Oxygen, 49.5 ” ”

corresponding to the formula C₆H₁₀H₅.

According to the view of Cross and Bevan, cellulose conforms in respect of its ultimate constitutional groups to the general features of the simple carbohydrates, but differs from them by reason of a special molecular configuration resulting in a suppression of the activity of constituent groups in certain respects, and an increase in activity of others.

=271. Fiber and Cellulose.=—The carbohydrates of a plant insoluble in water are not composed exclusively of starch. There are, in addition to starch, pentosan fibers yielding pentose sugars on hydrolysis and furfuraldehyd on distillation with a strong acid. The quantitive methods for estimating the pentosan bodies are given in paragraphs =150-157=. The method to be preferred is that of Krug (=155=).

In the estimation of cattlefoods and of plant substances in general the residue insoluble in dilute boiling acid and alkali is called crude or indigestible fiber.

The principle on which the determination depends rests on the assumption that all the protein, starch and other digestible carbohydrates will be removed from the sample by successive digestion at a boiling temperature with acid and alkali solutions of a given strength. It is evident that the complex body obtained by the treatment outlined above is not in any sense a definite chemical compound, but it may be considered as being composed partly of cellulose.

=272. Official Method of Determining Crude Fiber.=—The method of estimating crude fiber, adopted by the Association of Official Agricultural Chemists, is as follows:

Extract two grams of the substance with ordinary ether, at least almost completely, or use the residue from the determination of the ether extract. To this residue, in a half liter flask, add 200 cubic centimeters of boiling 1.25 per cent sulfuric acid; connect the flask with an inverted condenser, the tube of which passes only a short distance beyond the rubber stopper into the flask. Boil at once, and continue the boiling for thirty minutes. A blast of air conducted into the flask may serve to reduce the frothing of the liquid. Filter, wash thoroughly with boiling water until the washings are no longer acid, rinse the substance back into the same flask with 200 cubic centimeters of a boiling 1.25 per cent solution of sodium hydroxid, free or nearly free of sodium carbonate, boil at once and continue the boiling for thirty minutes in the same manner as directed above for the treatment with acid. Filter into a gooch, and wash with boiling water until the washings are neutral, dry at 110°, weigh and incinerate completely. The loss of weight is crude fiber.

The filter used for the first filtration may be linen, one of the forms of glass wool or asbestos filters, or any other form that secures clear and reasonably rapid filtration. The solutions of sulfuric acid and sodium hydroxid are to be made up of the specified strength, determined accurately by titration and not merely from specific gravity.

The experience of this laboratory has shown that results practically identical with those got as above, are obtained by conducting the digestions in hard glass beakers covered with watch glasses. The ease of manipulation in the modification of the process just mentioned is a sufficient justification for its use.

=273. Separation of Cellulose.=—Hoppe-Seyler observed that cellulose, when melted with the alkalies at a temperature as high as 200°, was not sensibly attacked.

Lange has based a process for determining cellulose on this observation.

The process, as improved by him, is carried out as follows:

From five to ten grams of the substance are moistened with water and placed in a porcelain dish with about three times their weight of caustic alkali free of nitrates and about twenty cubic centimeters of water. The porcelain dish should be deep and crucible shaped and should be placed in an oil-bath, the temperature of which is easily controlled. The contents of the dish are stirred with the thermometer bulb until all foaming ceases and the temperature of the mixture is then kept at from 175° to 180° for an hour. After the melt has cooled to 80° about seventy-five cubic centimeters of hot water are added to bring it into solution and it is then allowed to cool. The solution is acidified with sulfuric and placed in large centrifugal tubes. After being made slightly alkaline with soda lye, the tubes are subjected to continued energetic centrifugal action until the cellulose is separated. The supernatant liquid can be nearly all poured off and the separated cellulose is broken up, treated with hot water and again separated by centrifugal action. The cellulose is finally collected upon the asbestos felt, washed with hot water, alcohol and ether, dried and weighed. With a little practice it is possible to complete the separation of cellulose in two and one-half hours.

=274. Solubility of Cellulose.=—Cellulose resembles starch in its general insolubility, but, unlike starch, it may be dissolved in some reagents and afterwards precipitated practically unchanged or in a state of hydration. One of the simplest solvents of cellulose is zinc chlorid in concentrated aqueous solution.

The solution is accomplished with the aid of heat, adding one part by weight of cotton to six parts of zinc chlorid dissolved in ten parts of water.

A homogeneous sirup is obtained by this process, which is used in the arts for making the carbon filaments of incandescent electric lamps.

In preparing the thread of cellulose, the solution, obtained as described above, is allowed to flow, in a fine stream, into alcohol, whereby a cellulose hydrate is precipitated, which is freed from zinc hydroxid by digesting in hydrochloric acid.

Hydrochloric acid may be substituted for water in preparing the reagent above noted, whereby a solvent is secured which acts upon cellulose readily in the cold.

A solution of ammoniacal cupric oxid is one of the best solvents for cellulose. The solution should contain from ten to fifteen per cent of ammonia and from two to two and a half of cupric oxid.