191. Rapid Estimation Of Starch.—For the rapid estimation of starch in cereals, cattle foods and brewery refuse, Hibbard recommends a method which is carried out as follows:
The malt extract is prepared by covering ground, dry malt with water containing from fifteen to twenty per cent of alcohol. The object of adding alcohol is to preserve the filtered extract. It exercises a slight retarding effect on the action of the diastase, but prevents the malt extract from fermenting. After standing for a few hours in contact with the malt, the liquid is separated by filtration and is then ready for use. The substance in which the starch is to be determined should be dry enough to be finely pulverized, but previous extraction with ether is omitted. Enough of the material to contain at least half a gram of starch is placed in a flask with fifty cubic centimeters of water and from one to two cubic centimeters of malt extract added. The mixture is at once heated to boiling with frequent shaking to prevent the formation of clots. The addition of the diastase before boiling is to aid in preventing the formation of lumps. After boiling a minute the mixture is cooled to 60° and from two to three cubic centimeters of the malt extract added. It is then slowly heated until it again boils, consuming about fifteen minutes, when, after cooling, it is tested with iodin for starch. If a blue color be produced the operation above described is repeated until it fails to reappear. The mixture is then made up to a standard volume, thrown on a linen filter and an aliquot part of the filtrate, representing from 200 to 300 milligrams of starch, is boiled with five cubic centimeters of hydrochloric acid, of thirty per cent strength, for half an hour. The total volume of the liquid before boiling should be completed to sixty cubic centimeters. By the method above described, it is claimed that the determination of starch in a cereal or similar substance can be completed within two hours. The chief amount of time saved is in the heating with the malt extract, which instead of being continued for two hours, as usually directed, can be accomplished in thirty minutes.[159]
192. Precipitation of Starch with Barium Hydroxid.—The tendency of carbohydrate bodies to unite with the earthy bases has been utilized by Asboth as a basis for the quantitive determination of starch.[160]
About three grams of the finely ground sample containing the starch, or one gram of pure starch, are rubbed up in a mortar with water and the detached starch remaining suspended in the wash water is poured off. This operation is repeated until all the starch is removed. In difficult cases hot water may be used. The starch thus separated is heated in a quarter liter flask to the boiling point to reduce it to the condition of paste. When the paste is cold it is treated with fifty cubic centimeters of the barium hydroxid solution, the flask closed and well shaken for two minutes. The volume is then completed to the mark with forty-five per cent alcohol, the flask well shaken and allowed to stand. In a short time the barium-starch compound separates and settles. Fifty cubic centimeters of the clear supernatant liquor are removed with a pipette, or the liquor may be passed through a filter and the quantity mentioned removed for titration of the residual barium hydroxid after the addition of a few drops of phenolphthalein solution.
The quantity of barium hydroxid remaining, deducted from the original quantity, gives the amount which has entered into composition with the starch; the composition of the molecule being BaOC₂₄H₄₀O₂₀, which contains 19.10 per cent of barium oxid and 80.90 per cent of starch.
The set solution of barium hydroxid must be preserved from contact with the carbon dioxid of the air. The burette should be directly attached to the bottle holding the set solution, by any of the usual appliances, and the air entering the bottle must be deprived of carbon dioxid. The water used in the work must be also free of air, and this is secured by boiling immediately before use.
Example.—A sample of flour selected for the analysis weighed 3.212 grams. The starch was separated and reduced to paste in the manner described above. Thirty and four-tenths cubic centimeters of tenth-normal hydrochloric acid were exactly neutralized by ten cubic centimeters of the barium hydroxid solution. After treatment as above described, fifty cubic centimeters of the clear liquor, corresponding to ten cubic centimeters of the added barium hydroxid, required 19.05 cubic centimeters of tenth-normal hydrochloric acid. Then 30.4 - 19.05 = 11.35, and 11.35 x 5 = 56.75, which number corresponds to the total titration of the residual barium hydroxid in terms of tenth-normal hydrochloric acid. This number multiplied by 0.0324, viz., starch corresponding to one equivalent of barium, gave 1.8387 grams of starch or 57.24 per cent of the weight of flour employed.
The barium hydroxid method has been given a thorough trial in this laboratory and the results have been unsatisfactory when applied to cereals. The principle of the process, however, appears to be sound, and with a proper variation of working details, it may become practical.
193. Disturbing Bodies in Starch Determinations.—Stone has made a comparison of the standard methods of starch determinations, and the results of his work show that in the case of pure starch all of the standard methods give approximately correct figures. For instance, in the case of a pure potato starch, the following data were obtained:
By inversion with hydrochloric acid, 85.75 per cent; by inversion with oxalic and nitric acids, 85.75 per cent; by solution in salicylic acid, 85.47 per cent; and by precipitation with barium hydroxid, 85.58 per cent.[161]