=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=.