264. Separation of Starch.—Starch being insoluble in its natural state, it is impossible to separate it from the other insoluble matters of plants by any known process. In bringing it into solution it undergoes certain changes of an unknown nature, but tending to produce a dextrinoid body. Nevertheless, in order to procure the starch in a state of purity suited to analytical processes, it becomes necessary to dissolve the starch from the other insoluble bodies that naturally accompany it. As has been shown in preceding paragraphs, there are only two methods of securing the solution of starch which fully meet the conditions of accurate analysis. These are the methods depending on the use of diastatic ferments and on the employment of heat and pressure in the presence of water. These two processes have been described in considerable detail in paragraphs [179-181]. It is important, in starch determinations, to remove from the sample the sugar and other substances soluble in water and also the oils, when present in large quantities, before subjecting it to the processes for rendering the starch soluble.
265. Desiccation of Amyliferous Bodies.—The removal of sugars and oils is best secured in amyliferous substances after they are deprived of their moisture. As has already been suggested, the desiccation should be commenced at a low temperature, not above 60°, and continued at that point until the chief part of the water has escaped. The operation may be conducted in one of the ways already described ([pp. 12-27]). There is great difference of opinion among analysts in respect of the degree of temperature to which the sample should be finally subjected, but for the purposes here in view, it will not be found necessary to go above 105°. Before beginning the operation the sample should be as finely divided as possible, and at its end the dried residue should be ground and passed through a sieve of half a millimeter mesh.
266. Indirect Method of Determining: Water in Starch.—It is claimed by Block[218] that it is necessary to dry starch at 160° in order to get complete dehydration. Wet starch as deposited with its maximum content of water has nine molecules thereof, viz., C₆H₁₀O₅ + 9H₂O. Ordinary commercial starch has about eighteen per cent of water with a formula of C₆H₁₀O₅ + 2H₂O.
The percentage of water may be determined by Block’s feculometer or Block’s dose-fécule. The first apparatus determines the percentage of anhydrous starch by volume, and the second by weight.
Block’s assumption that starch can absorb only fifty per cent of its weight of water is the basis of the determination.
A noted weight of starch is rubbed up with water until saturated, the water poured off, the starch weighed, dried on blotting paper until it gives off no more moisture and again weighed. Half of the lost weight is water, from which the original per cent of water can be calculated. This at best seems to be a rough approximation and not suited to rigorous scientific determination.
267. Removal of Oil and Sugar.—The dried, finely powdered sample, obtained as described above, is placed in any convenient extractor ([33-43]) and the oil or fat it contains removed by the usual solvents. For ordinary purposes, even with cereals, this preliminary extraction of the oil is not necessary, but it becomes so with oily seeds containing starch. The sugar is subsequently removed by extraction with eighty per cent alcohol and the residue is then ready for the extraction of the starch. In most cases the extraction with alcohol will be found sufficient. In some bodies, for instance the sweet potato (batata), the quantity of sugar present is quite large, and generally some of it is found. If not present in appreciable amount, the alcohol extraction may also be omitted. The sample having been prepared as indicated, the starch may be brought into solution by one of the methods described in paragraphs [179-181], preference being given to the aqueous digestion in an autoclave. The dissolved starch is washed out of the insoluble residue and determined by optical or chemical methods [186-194].
268. Preparation of Diastase for Starch Solution.—The methods of preparing malt extract for use in starch analysis have been described in paragraph [179]. If a purer form of diastase is desired it may be prepared by following the directions given by Long and Baker.[219] Digest 200 grams of ground malt for twenty-four hours with three parts of twenty per cent alcohol. Separate the extract by filtration and to the filtrate add about one and a half liters of ninety-three per cent alcohol and stir vigorously. After the precipitate has subsided the supernatant alcohol is removed by a syphon, the precipitate is brought onto a filter and washed with alcohol of a strength gradually increasing to anhydrous, and finally with anhydrous ether. The diastase is dried in a vacuum over sulfuric acid and finally reduced to a fine powder before using. Thus prepared, it varies in appearance from a white to a slightly brownish powder. Made at different times and from separate portions of malt, it may show great differences in hydrolytic power.
269. Estimation of Starch in Potatoes by Specific Gravity.—A roughly approximate determination of the quantity of starch in potatoes can be made by determining their specific gravity. Since the specific gravity of pure starch is 1.65, it follows that the richer a potato is in starch the higher will be its specific gravity. The specific weight of substances like potatoes is conveniently determined by suspending them in water by a fine thread attached to the upper hook of a balance pan. There may be a variation of the percentage of other constituents in potatoes as well as of starch, and therefore the data obtained from the following table can only be correct on the assumption that the starch is the only variable. In practice, errors amounting to as much as two per cent may be easily made, and therefore the method is useful only for agronomic and commercial and not for scientific purposes. The method is especially useful in the selection of potatoes of high starch content for planting. The table is constructed on the weight in grams in pure water of 10000 grams of potatoes and the corresponding per cents of dry matter and starch are given. It is not always convenient to use exactly 10000 grams of potatoes for the determination, but the calculation for any given weight is easy.[220]
Example.—Let the weight of a potato in air be 159 grams, and its weight in water 14.8 grams.