The furfurol reaction is useful in detecting the presence of minute traces of carbohydrates but is of little value in discriminating between the different classes of these bodies.
It is not practical here to go into greater detail in the description of qualitive reactions. The analyst, desiring further information, should consult the standard works on sugar chemistry. [143]
175. Detection of Sugars by Bacterial Action.—Many forms of bacteria manifest a selective action towards sugars and this property may in the future become the basis of a qualitive and even quantitive test for sugars and other carbohydrates. Our present knowledge of the subject is due almost exclusively to the researches of Smith, conducted at the Department of Agriculture.[144] Dextrose is the sugar first and most vigorously attacked by bacterial action, and by proper precautions the whole of the dextrose may be removed from mixtures with sucrose and lactose.
The development of other forms of micro-organisms which will have the faculty of attacking other and special forms of carbohydrates is to be looked for with confident assurance of success.
DETERMINATION OF STARCH.
176. Constitution of Starch.—The molecule of starch is without doubt formed by the condensation of a large number of hexose bodies. On account of its great insolubility its molecular weight has not been determined with any degree of accuracy. Its formula may be expressed either as (C₆H₁₀O₅)ₙ or (C₁₂H₂₀O₁₀)ₙ. It is insoluble in cold water and other common solvents and does not pass into solution in any reagent without undergoing a change of structure. In hot water it forms a paste and when heated under pressure with water it undergoes a partial change and becomes soluble. Heated with acids or subjected to the action of certain ferments it suffers hydrolysis and is transformed into dextrin, maltose and dextrose. In analytical work an attempt is usually made to transform the starch entirely into dextrose, the quantity of which is then determined by some of the processes already given. All starches possess the property of giving an intensely blue color with iodin and this reaction serves to detect the most minute quantity of the material.
Starch grains derived from different sources are distinguished by differences in size and appearance. In most cases a careful examination of the starch particles will reveal their origin.[145] The greatest part of the cereal grains is composed of starch, the percentage ranging from sixty to eighty. Rice has the greatest percentage of starch in its composition of any substance. Certain root crops are also rich in starch, such as the potato, artichoke and cassava. Starch appears as one of the first products of vegetable metabolism, according to some authorities, preceding the formation of sugars. By reason of its greater complexity, however, it is more probable that the production of simple sugars precedes the formation of the more complex molecule. Starch granules are probably used as a food by the plant in the building of more complex structures and the excess of this food is stored in the seeds and in tubers.
177. Separation of Starch Particles.—Advantage is taken of the insolubility of the starch particles to secure their separation from the other vegetable structures with which they are associated. The substances containing starch are reduced to a pulp as fine as possible, and this pulp being placed in a fine cloth the starch particles are washed through the cloth with water. The milky filtrate carrying the starch is collected in an appropriate holder and, after some time, the particles subside. They may then be collected and dried. While this process is the one used commercially in the manufacture of starch, it can only give approximate data respecting the actual quantity of starch in a given weight of the sample. It is not quite possible by this method to get all the starch separated from the rest of the vegetable matter, and particles of foreign substances, such as cellulose and albuminoid matters, may pass through the filter cloth and be found with the deposited granules. It follows from this that the quantitive determination of the starch in a given sample by any direct method is only approximately exact.
178. Methods of Separation.—Hot acids cannot be safely employed to dissolve starch from its natural concomitants because other carbohydrate bodies become soluble under similar conditions. In such cases the natural sugars which are present should be removed by cold water and the starch dissolved from the residue by a diastatic ferment. Instead of this the sugars may be determined in a separate portion of the pulped material and the starch, together with the sugars, determined, and the quantity of sugar found deducted from the final result.
In these cases the final determinations are made on the sugars, after inverting the sucrose, and proceeding as directed for invert sugars in paragraph [141]. The starch, after separation with diastase, is converted into dextrose by one of the methods to be given and the resulting dextrose determined by one of the approved methods.