REACTION-INTENSITIES WITH EACH AGENT AND REAGENT. 



145 



the insoluble to the soluble non-gelatinizable form is 

 apparently not in any way related to water, inasmuch 

 as it may be brought about in anhydrous starch by anhy- 

 drous acetic acid, and is therefore an anhydrous process 

 unless water is derived in some obscure way by intra- 

 molecular disorganization. There is at all events no 

 intermolecular disorganization such as occurs antecedent 

 to and associated with obvious gelation. 



The foregoing changes in the starch molecules in 

 association with the more or less marked differences 

 exhibited by a given starch in the reactions with difEerent 

 reagents indicate clearly that beneath and overshadowed 

 by the conspicuous phenomena of gelation there lay 

 processes or reactions that vary, within even wide limits, 

 in relation to the components of the reagents. More- 

 over, raw starch presents certain very striking charac- 

 teristics in its relations to water, entirely apart from 

 the phenomena of hydration that is expressed by gelation. 

 It has been found that raw starch is not only highly 

 hygroscopic and clings tenaceously to water, but also 

 that its behavior toward water is in certain respects 

 difEerent from that of hydrated starch, the percentage of 

 water in the raw grains being influenced to a very limited 

 degree and that of hydrated starch to a maximum degree, 

 in the presence of water by changes in temperature. Air- 

 dried starches from different sources have been found 

 to contain from 9.9 to 35 per cent of water,- the figure 

 varying with the kind of starch, impurities, and per- 

 centage of moisture in the air. Freshly prepared starch 

 may contain as much as 45 per cent of water. Anhy- 

 drous starch is obtained by subjecting the starch to a 

 temperature of 120° or in vacuo at 100°. Starch that 

 has been partially or completely dehydrated and then 

 placed in water at room temperature takes up water very 

 rapidly with the evolution of heat, the amount being in 

 direct relationship to the degree of dehydration and the 

 kind and amount of starch. A preparation consisting of 

 20 grams of air-dried potato starch in 20 grams of water 

 showed an increase of temperature equal to 3° ; and a 

 similar preparation of anyhydrous starch, an increase of 

 13.8°. The formation of heat has been ascribed to an 

 actual chemical combination of the starch and water (see 

 preceding memoir, page 167), but it can satisfactorily 

 and better be accounted for upon the basis of adsorption 

 (which, however, is in fact a form of chemical union) . 



The level of aqueous saturation is maintained within 

 very narrow limits, and it is very much more influenced 

 by variations in external moisture than by changes in 

 temperature that occur below the temperature of gela- 

 tion ; and it is reached before there is the least detectable 

 change in the starch grain or starch molecule. This 

 level is, however, not only materially higher in hydrated 

 starch, but also variable within wide degrees and in direct 

 relation to moisture and temperature, and it probably 

 reaches its highest level at the baking temperature of 

 bread (Katz, Zeitsch. physiol. Chem., 1915, xcv, 104). 

 As the temperature falls, even though in the presence 

 of an atmosphere saturated with moisture, there is some 

 reversion of hydrated starch to raw or insoluble starch. 



Starch grains do not either gelatinize or pass into 

 solution in their normal state because apparently of the 

 existence of some peculiar surface condition which, like 

 10 



an osmotic membrane, serves to prevent a further inflow 

 of water after a certain level of partial saturation has 

 been reached, and which likewise prevents an outflow 

 of water as long as external conditions are unaltered — 

 in other words, maintains a state of physico-chemical 

 equilibrium as regards water within and without the 

 starch grain. That such a surface condition exists seems 

 evident in the sudden dissipation of this level at the 

 temperature of gelation and in the absence of this level 

 in comminuted and otherwise injured grains in which 

 the starch molecules of the interior of the grain are 

 freely exposed to the water. The intracapsular starch 

 thus exposed exhibits a similar but not identical surface 

 condition, which is owing to differences in the intra- 

 capsular and capsular starches, as will be noted more 

 particularly later. Therefore, in studying the phe- 

 nomena of gelatinization and absorption of water both 

 of these surface conditions must be considered, as must 

 also be both forms of starch. 



When raw starch in water is subjected to slowly ris- 

 ing temperature, at a certain temperature that varies 

 for different starches and within narrow limits for each 

 starch there occurs a loss of anisotropy (which indicates 

 an intermolecular disorganization) that is immediately 

 followed by a rapid taking up of water attended by 

 swelling and gelatinization. This disappearance of 

 anisotropy is taken to mean that immediately antecedent 

 a modification or removal of the surface condition has 

 occurred. This surface condition may likewise be 

 affected by various gelatinizing reagents such as have 

 been used in this research, and thus hydration of the 

 starch grain permitted as in the case of gelation by 

 heat ; or there may be the opposite effect, as when there 

 is present a sufficient quantity of alcohol, acetone, 

 alcohol-ether, brine or other so-called dehydrating rea- 

 gent. Analogous phenomena have been noted in the 

 study of certain other colloids, from which it seems that 

 heat and other gelatinizing agents are effective by affect- 

 ing primarily the surface condition, thus giving rise to 

 an alteration in the level of aqueous saturation. The 

 underlying cause of this peculiar surface condition is at 

 present problematical, but it seems that it is to be 

 located directly or indirectly either in a hypothetical 

 deposit on the surface of the grain by the cell-sap or in 

 the modified form of the starch that constitutes the 

 capsular part of the grain (the so-called starch cellu- 

 lose). This part of the grain is the last to be deposij;ed, 

 and it differs from the inner part (or so-called starch 

 granulose) especially in density, solubility in cold and 

 hot water, digestibility, dextrin products of digestion, 

 resistance to decomposing agents, and in both quantita- 

 tive and qualitative color reactions with iodine. The 

 degree of resistance varies in starches from different 

 sources, and it is so marked in some instances in the 

 initial stage of the reaction as to render gelatinization 

 very slow for a period varying from 1 to 10 minutes, to 

 be followed by gelatinization that varies in rapidity from 

 slow to very rapid, as will be seen by an examination of 

 Charts D 1 to D 691 that exhibit the velocities of gela- 

 tinization. Upon this assumption, any agent which 

 affects the physico-chemical condition of the capsular 

 part of the grain will modify the surface conditions or 



