Understanding of swelling behaviors of polymer gels is essential for the development of superior gel materials for foods and other applications. This article describes equilibria and kinetics of their swelling, especially of polyelectrolyte complex gels, which can show pH-responsive swelling behaviors. Polyelectrolyte complex gels can be prepared from natural polymers. Examples of their pH-responsive swelling behaviors were described. The swelling equilibria were successfully explained by pH-dependent change in the degree of dissociation (deprotonation) of the chargeable functional groups fixed on the polymer chains. Theoretical analysis of swelling equilibria for dextran sulfate/chitosan complex gels showed that the complex formation lowered the acid dissociation constant of amino group. Thus the analysis of swelling equilibrium should include not only Donnan effect of fixed charge but also lowering of the dissociation constant due to complex formation. Cooperative diffusion of polymer chain has been known to control swelling rate of non-ionic gels, whereas diffusion of such mobile ions as Na+ was found to be the rate limiting factor for swelling of ionic gels.
We developed novel method for producing glucoamylase (GA) from gelatinized rice flour by co-culturing Bacillus amyloliquefaciens NBRC 14141 and Rhizopus cohnii P5. In liquid culture, the acidification of the growth medium due to the growth of R. cohnii prevented the growth of B. amyloliquefaciens; however, the B. amyloliquefaciens protease lysed R. cohnii cells, which decreased the production of GA. This antagonism was repressed by high concentrations of ammonium acetate in the growth medium. However, since low pH or high ammonium acetate concentrations inhibited the initial growth of B. amyloliquefaciens, it needed to be precultured without ammonium acetate. However, preculturing B. amyloliquefaciens for 48 h led to overproduction of protease, which inhibited the growth of R. cohnii. As a result, the maximum GA activity (740 U/ml) was obtained when B. amyloliquefaciens was precultured for 24 h followed by inoculation with R. cohnii in the presence of 3.84% (w/v) of ammonium acetate. These results indicated that GA can be produced at high levels from gelatinized rice flour by using a submerged co-culture system.
Hydration behavior of a rare sugar, d-psicose, was investigated based on measurements of the dielectric relaxation and the water activity in its aqueous solutions. Dielectric relaxation spectra in the frequency domain were obtained from time domain reflectometry. The dielectric relaxation time was then determined by fitting the spectra to a Cole-Cole type equation and used to evaluate the average hydration number representing the average number of water molecules forming a complex with a sugar molecule at infinite dilution. The concentration dependence of the dielectric relaxation time in aqueous solutions of all examined sugars generated a single master curve. The average hydration number of psicose was found to be identical with that of fructose. Only slight differences were noticeable among the concentration dependences of the water activity of monosaccharides. The equilibrium constant of hydration of sugar was calculated by considering material balances in an aqueous solution of sugar and using experimentally obtained values of the average hydration number and the water activity. The equilibrium constant values increased in the order of glucose < fructose ≈ psicose < trehalose < sucrose < maltose. The present results suggest that both fructose and psicose interact with water in a similar manner.