308 DIFFERENTIATION AND SPECIFICITY OF STARCHES. 



THE ANILINE METHOD. 



The use of anilines lias demonstrated some differences, as in the finer histological 

 characters of different starches, and also in different grains of the same starch and in 

 different parts of the same grain. The experiments of Denniston (Chapter II, page 56), 

 in which it is shown that the outer or capsular part of the grain will permit the passage 

 of the aniline to the intracapsular part without itself becoming colored, unless an excess 

 of stain be present, is most significant of an important difference in the composition of 

 the two parts. Moreover, the differences shown by starches from different sources in the 

 rapidity with which they combine with a given aniline and different anilines suggest 

 distinct differences in the nature of the starch-substance. Whether or not we regard the 

 union of anilines with starch as being a phenomenon of adsorption or of chemical union 

 in the coiiA^entional sense (both are chemical), it is certainly a manifestation of variations 

 in starch properties which can not be accounted for on the basis of varying physical or 

 physico-mechanical conditions, because, for instance, in one form of starch the reaction 

 will be greater with one aniline than with another, while with another starch they will 

 be of equal degree, and with another the reverse of what was found in the first place. Were 

 it a matter, for instance, merely of varying porosity, such differences would not be observed. 

 On the whole, much may be expected of the use of anilines in differentiating different starch- 

 substances, especially in the individual grain. 



THE TEMPERATURE METHOD. 



The temperature of gelatinization of starch-grains and the melting-point of crystal- 

 line substances generally are terms that have the same significance, that is, they express 

 the temperature at which the intermolecular structure of the substance is broken down. 

 Differences in melting-point have been an important means of differentiating isomers; simi- 

 larly, differences in the temperature of gelatinization have served to differentiate starches 

 from different sources, but the value of this method has been very low; partly because of 

 the heterogeneity of starch and partly and chiefly because of improper methods of research. 

 Hence, very variable and contradictory records have resulted. In fact, one of the latest 

 experimenters states that the results obtained by the ordinary method of determining 

 gelatinization temperatures were so conflicting as not to be worthy of publication. 



The absence of homogeneity of the grains constituting any given form of starch can 

 not be overcome, but a proper method will eliminate very important sources of fallacy. 

 In the methods heretofore used, the determinations were based upon observations with 

 the microscope, dependence being placed upon gross changes in the size and appearance 

 of the grain, and in the reactions with iodine, the starch with water being placed in a test- 

 tube or beaker, and this in turn in a water-bath which is slowly heated, etc. In the present 

 research the reactions in the polarizing microscope were depended upon. 



As already stated, when raw starch is heated in water, a certain temperature is reached 

 at which the intermolecular arrangement of the starch molecules begins to undergo dis- 

 organization, with an attendant loss of optical activity; and the progress of gelatinization 

 can be followed clearlj^ until the last vestige of the grain has been disorganized. Gelatin- 

 ization begins in some of the grains and in parts of a given grain before it appears in others, 

 and it is likewise earlier complete in some grains and parts of grains than in others. In 

 these determinations, the temperature of gelatinization means the lowest temperature at 

 which disorganization in practically all of the grains was complete. With care the range 

 of error is, with rare exceptions, less than a degree centigrade. Occasionally a starch is found 

 in which rare grains resist gelatinization at temperatures even much higher than the others. 

 In such cases these exceptional grains are disregarded. In many starches the outer part is 

 distinctly more resistant than the inner. When the degree of polarization is low, especially 

 in the very small grains, it may be found impossible to get accurate temperatures. 



