Calculated on the proportion that dextrose is to starch, as ten is to nine, this is equivalent to 142 milligrams of starch. The percentage of starch in the original substance, therefore, was equivalent to 142 multiplied by 100, divided by 240, viz., 59.17.
182. Principles of the Methods of Determination.—In the approximately pure state in which starch exists in the trade, it may be determined by conversion into dextrose and estimating the latter by one of the methods given. It is probable that there is no known method by which starch can be entirely converted into dextrose, and all the methods of hydrolysis, when used for quantitive purposes, must be standardized, not by the theoretical quantity of dextrose which a given weight of pure starch should yield, but by the actual quantity obtained. Starch is not largely converted into dextrose by any of the diastatic ferments which produce principally maltose and dextrins. Recourse must therefore be had to strong acids. In practice, hydrochloric is the one usually employed. By the action of a hot mineral acid, not only is starch converted into dextrose, but also the dextrose found is subjected to changes. In such cases an opposing action seems to be exerted by the hydrolytic agent, a part of the dextrose formed suffering a partial condensation, and thus assuming a state of higher molecular weight, approaching the constitution of the dextrins. Another part of the dextrose may also suffer oxidation and thus disappear entirely in respect of the further steps in starch analysis.
In such cases, the best the analyst can do is to conduct the hydrolysis in as nearly as possible constant conditions, and to assume that the percentage of dextrose present at a given time bears a constant ratio to the quantity of starch hydrolyzed. In reality almost all the starch appears finally as dextrose, and by proceeding on the assumption noted above a fairly satisfactory accounting may be made of the remainder.
Starch being insoluble, it cannot be determined directly by its rotatory power. When heated for a few hours in contact with water at a high pressure, starch becomes soluble, and in this state has a fairly constant gyrodynat, viz., [a]D = 197°.
Starch is also rendered soluble by rubbing it in a mortar for about ten minutes with an excess of strong hydrochloric acid, and in this way a quick approximate idea may be obtained of the percentage present. Starch prepared in this manner, however, has a strong reducing power on metallic salts, showing that a part of it has already, even in so short a time, assumed the state of maltose or dextrose. The gyrodynat of pure anhydrous starch in such conditions varies from [a]D = 197° to [a]D = 194°. Starch is also rendered soluble by boiling with salicylic acid, whereby a solution is obtained having a gyrodynat of [a]D = 200°(circa). The methods of procedure for the analysis of starch will be set forth in detail in the following paragraphs.
183. Estimation of Water.—In prepared or commercial starches the water may be determined by heating in a partial vacuum. The temperature at first should be low, not exceeding 60°. After drying for an hour at that heat the temperature may be gradually increased. The last traces of water come off from starch with difficulty, and the final temperature may be carried a little beyond 100° without danger of decomposition.
Ost recommends the use of an atmosphere of hydrogen or illuminating gas.[148] One and a half grams of the finely powdered sample are placed in the drying tube described in paragraph [23], and heated in a stream of dry hydrogen. The temperature at first is kept at about 60° for several hours and is then gradually increased to 120°. Ost states that even at 150° the sample preserves its pure white color, but so high a temperature is not necessary. Maercker, at the Halle station, makes use of the same process, but employs illuminating gas instead of hydrogen. The importance of beginning the desiccation at a low temperature arises from the fact that at a higher temperature, before the greater part of the water is driven off, the starch will suffer a partial fusion and form a paste which is very difficult to dry. The dried sample must be kept in a stoppered vessel to prevent the absorption of hygroscopic moisture.
184. Estimation of Ash.—When the drying is accomplished in a flat platinum dish, the same sample may serve for incineration. Otherwise the incineration may be accomplished in another portion of the sample by following directions already given.[149]
185. Nitrogen.—Even very pure samples of starch may contain a little nitrogen which is most conveniently determined by moist combustion.[150]
As a rule, in commercial starches of good quality, the quantity of pure starch may be considered to be the remainder after subtracting the sum of the weights of water, ash and nitrogen multiplied by 6.25, from the original weight of the sample taken.