146 



REACTION-INTENSITIES OF STARCHES. 



surface tension so that hydration may be augmented or 

 inhibited. 



As stated elsewhere (see preceding memoir, pages 95 

 and 96), while there can be no doubt of the essential part 

 played by water in the swelling, gelatinization, pseudo- 

 solution, and true solution of starch, it seems that none 

 of these phenomena is due to either hydrolysis (de- 

 composition in which molecules of water are taken up and 

 become an integral part of the molecules) or hydration 

 in the strictly chemical sense (the formation of deriva- 

 tives in which basic matter is substituted by hydrogen 

 atoms of water, or the actual combination of water so 

 that the molecules of water constitute intramolecular 

 components of the derivatives). The terms hydrolysis 

 and hydration are often used synonymously, but at times 

 incorrectly, because while hydration may mean hydro- 

 lysis, it may on the other hand signify a union or im- 

 pregnation with water which is an extramolecular and 

 not an intramolecular phenomenon. According to the 

 recent developments of physical chemistry, none of the 

 processes concerned in the conversion of raw starch into 

 the so-called soluble starch, of which starch-paste and 

 pseudo-solution and true solution are simple modifica- 

 tions, is one of hydrolysis or hydration in the strictly 

 chemical sense, but one of adsorption, that is, an extra- 

 molecular union with water that is of a physico-chemical 

 character, such, for instance, as is observed in the depo- 

 sition of moisture on glass and the taking up of water by 

 hygroscopic substances in which there may be no true 

 chemical union in the conventional meaning, but a mere 

 surface combination or surface condensation. The com- 

 bination is, of course, actually chemical, but it is not 

 chemical in the customary sense any more than is the 

 solution of sugar in water chemical, and thus in the form 

 technically of a hydrate. Starch in common with other 

 organic colloids is hygroscopic, and the so-called process 

 of hydration or hydrolysis that is associated with swelling 

 and gelatinization is explicable upon the basis of adsorp- 

 tion that is, a physico-chemical affinity that is specific 

 and selective, and supplemental to satisfied affinities ac- 

 cording to the laws of stoichiometry. This, however, 

 does not preclude the possibility or probability of the 

 occasional occurrence, of reagent reactions that are 

 strictly speaking those of hydration. 



It seems clear from the foregoing that in the gela- 

 tinization of normal starch grains the first and essential 

 step is the modification or dissipation of the surface 

 condition that prevents an inflow of water after the nor- 

 mal point of partial saturation, or state of physico- 

 chemical equilibrium as regards water, has been reached. 

 This barrier it seems is not mechanical but physico- 

 chemical, as is suggested by the fact that corresponding 

 or analogous phenomena have been observed in the be- 

 havior of other colloids in vitro and in the living cells, 

 where it seems to have been clearly demonstrated that 

 they are manifestations of surface tension. Heat, when 

 a certain temperature is reached, is assumed to give rise 

 to a surface alteration or change in surface tension that 

 causes a mass action of the molecules of water with a 

 consequent inflow of water and attendant gelatinization, 

 and it has been found that the addition of various sub- 

 stances to the water may lower or raise the temperature 

 of gelatinization in other words, aid or oppose the 



action of heat in altering the surface tension. The 

 various gelatinizing reagents which are active at room 

 temperature are undoubtedly effective by causing similar 

 or identical alterations in surface tension, for evidence 

 has been found that the ions do not form an adsorption 

 union with the starch molecules but give rise to the 

 surface alteration that leads to an adsorption union of 

 molecules of water and starch ; and it would seem to 

 follow, in accordance with our knowledge of the be- 

 havior of other colloids with ions and molecules of dif- 

 ferent kinds, that this surface change, as well as subse- 

 quent phenomena, are modifiable in relation to the kinds 

 and concentrations of ions and molecules taking part in 

 the reactions. Hence, the phenomena of gelatinization 

 brought about in distilled water by heat would likely 

 be different in certain respects from those due to some 

 chemical reagent, such as chromic acid ; and those of any 

 given reagent will differ from those of every other reagent. 

 Such is in fact what has been found in this research 



Samac (Studien iiber Pflanzenkolloide I. Die L6- 

 sungsquellung der Stiirke bei Gegenwart von Kristal- 

 loiden. Dresden, 1912, S. 42) made studies with potato 

 starch in which he used equimolecular solutions of 

 various electrolytes and non-electrolytes in concentra- 

 tions varying from 0.25 to 10 gram-molecules to the 

 liter. Both cations and anions were found to be effec- 

 tive. Lithium, sodium, potassium, ammonium, mag- 

 nesium, calcium, strontium, and barium chloride in weak 

 solution raised the temperature of gelatinizntion ; and 

 with increasing increments of concentration there 

 occurred with some a further elevation followed by a 

 fall, but with others a fall, the effects being different 

 according to the kind of cation present. Sulphate, oxa- 

 late, tartrate, acetate, chloride, bromide, nitrate, iodide, 

 sulphocyanate, and carbonate of potassium, and also 

 calcium nitrate, sodium sulphate, and ammonium sul- 

 phate, behaved differently in accordance with the kind 

 of anion. With some, in any concentration, the tem- 

 perature of gelatinization was raised ; with others, with 

 increasing increments of concentration a rise was fol- 

 lowed by a fall; and with others there was a fall with 

 any concentration. Sulphuric acid, hydrochloric acid, 

 and acetic acid likewise caused varying effects. With 

 sulphuric acid and hydrochloric acid increasing incre- 

 ments of concentration caused a rise followed by a fall, 

 while under the same conditions acetic acid caused a fall. 

 Both potassium hydroxide and ammonia in all concen- 

 trations caused a fall. Dextrose and glycerin, which 

 are in any concentration without detectable gelatinizing 

 action at room temperatures, caused with increasing in- 

 crements of concentration a steady elevation of the tem- 

 perature of gelatinization; and urea and chloral hydrate, 

 under the same conditions, caused a steady lowering. 

 Both acetic acid and potassium hydroxide in any con- 

 centration caused a fall ; but acetate of potassium in in- 

 creasing increments of concentration caused a rise 

 followed by a fall. These results are in harmony with 

 those obtained by various investigators in swelling and 

 precipitation experiments with proteins. 



The starch molecule like the protein molecule has the 

 property of acting as an acid or base to form salts, this 

 being explicable upon the assumption that both starch 

 and protein molecules are produced by a condensation 



