Die Hydratation der Stärke: II. Tensimetrische Untersuchungen der Ad‐ und Desorption von Wasser durch Stärke

For the purpose of comparative research in the water sorption of potato, wheat, corn and rice starch, the equilibrium values in water saturation and the adsorption and desorption isothermal lines were established by two different processes. Furthermore, the influence of the various drying processes on the extent of the sorption were examined. The influence of the starch structure on the water sorption was investigated on gelatinized potato, wheat, corn and rice starches precipitated with methanol. The measurements had the following results: The values for water sorption increase in rice, corn, wheat, potato starch in that order; the differences between the starches are highest in water saturation, reducing with decreasing rel. humidity and disappear in the cereal starches below 68% rel. humidity and between potato and cereal starches below 35%. In the non‐gelatinized starches, the extent of the water sorption is practically independent of the applied methods — measurement at atmospheric pressure within 20–120 days or measurement under reduced air pressure in 10–50 hours — in all humidity ranges at same temperature. The ability to take up water at water saturation of the atmosphere is reduced by thermic drying processes in all four starches. The same starches show an increased ability to water adsorption, however, after methanol dehydration and vacuum drying. Rolled‐dried starches obtained by gelatinization and following methanol precipitation showed a sorption ability increased to more than 40 % H₂O at water saturation: The differences between the starches almost completely disappeared. Below 79–68% rel. humidity, the sorption isothermal lines are practically identical with those of the non‐gelatinized starches. The isothermal lines, when evaluated according to the Freundlich' isothermal equation log a = f (lg x) in a double logarithmic diagram, between 0 and 68 % rel. humidity with different inclination coefficients, show approximately linear course and change to a steeply rising branch without noticeable transition on increasing water saturation. From the deviation from validity of the Freundlich isothermal lines, water binding by starch in the range of higher rel. humidity through capillary condensation is deduced, the extent of which is dependent on the surface characteristic of the sorbing substance and the sorption conditions. Below this critical point of the isothermal lines, the water binding by the starch is mainly a material property specific to starch carbohydrates and fulfills the Freundlich equation valid for a pure adsorption process.