1G6 



DIFFERENTIATION AND SPECIFICITY OF STARCHES. 



(7) The refractive index. 



(8) The reaction in polarized Ught. 



(9) Characters of the starch-paste and starch-solution. 

 (10) The phenomena of digestibility. 



THE HISTOLOGICAL METHOD. 

 The findings by the histological method ha^'e been referred to in the preceding chapters 

 and will be (jnitc fully reviewed in subsequent chapters, especially in Chapter V, so that 

 nothing need be stated at this place. 



PROXIMATE CONSTITUENTS AND OTHER FEATURES OF GENERAL CHEMICAL COMPOSITION. 



THE PROXIMATE PRINCIPLES. 



Starch-grains consist approximately of from 80 to 85 per cent of starch-substance, 15 to 

 20 per cent of water, and small and variable amounts of organic and inorganic substances, 

 including fat, cell residues, cholesterin, dextrin, sugars, protein matters, phosphates, tannin, 

 copper, and other substances. The figures in table 14 are by Konig (Die manschl. Nahrungs- 

 u. Geniissmittel ; Die Rohstoffe des Pflanzenreiches, Wiesner, Leipzig, 1903, Bd. I, 580). 

 (See Jessen, i^age 29.) 



Table 14. 



Commercial starches, and starches as ordinarily prepared in the laboratory, contain 

 large percentages of water and more or less impurities that can be removed by succes- 

 sive treatment with dilute hydrochloric acid, alcohol, and ether, and subsequent drying. 

 Salomon (Reportorium f. Analyt. Chem., 1881, 274; Jour. f. prakt. Chemie, 1882, xxv, 348, 

 and XXVI, 324) recommends that desiccation be carried on at 120, because at lower tem- 

 peratures water is retained, while at higher temperatures the starch becomes discolored and 

 decomposition processes set in. 



THE STARCH-SUBSTANCE. 



The starch-substance may be regarded, in the light of the more recent investigations, 

 as consisting of a number of modified forms of a single substance, and it is probable that 

 not only are certain of these modifications peculiar to the species, etc., but also that vari- 

 ations exist in the kinds and proportions of these modifications in different grains, and even 

 in a given grain from the starch of a given plant. Meyer {loc. cit.) records that the per- 

 centages of a-amylose differ in different starches, as follows: potato 0.6, rice 0.9, corn 1, 

 wheat 0.5, and arrowroot 2.5. Day (U. S. Dept. Agriculture, Office Expt. Stat. Bull. 202, 

 1908, 40) states that bhie-amylose represents the entire inside of potato, arrowroot, tapioca, 

 and sago starches, and 90 jier cent or more of wheat, corn, rice, and barley starches ; that 

 red-amylose constitutes the outer layer of starch-grains ; and that rose-amylose forms about 

 10 per cent of the inner part of wheat, corn, rice, and barley starches, but is absent from 

 potato, arrowroot, tapioca, and sago starches (see page 58). See also references to the 

 investigations of Kraemer, Salter, Denniston, and others in Chapter II. 



