The densities and viscosities of concentrated aqueous solutions of polyethylene glycol (10-50 mass %) have been measured. The polyethylene glycol samples had average molecular masses of 8000,3350, and 1000. The values of the density from 277 to 298 K show a linear variation with the polyethylene glycol concentration and differ, at most, by 10.07% from those of pure water at the same temperature. In order to estimate the values of dynamic viscosity, an equation is proposed which takes into account the influence of the polyethylene glycol concentration and the temperature in the range 277-313 K. The viscosity of mixtures of polyethylene glycol + magnesium sulfate + water were also studied. A linear relationship exists between the viscosities of the aqueous solutions of polyethylene glycol and the concentration of magnesium sulfate.

The liquid−liquid equilibrium for the ternary systems PEG1000 + MgSO4 + H2O, PEG3350 + MgSO4 + H2O and PEG8000 + MgSO4 + H2O has been obtained experimentally. The results are fitted to a three-parameter equation which fits the results to within ±2%. Tie lines have also been measured, along with the viscosity and density in both phases. The tie-line compositions have been fitted to Bancroft's potential equation.

The effects of temperature (25–40°C), H2SO4 concentration (31–70% (w/v)) and the acid/substrate relationship (1–5 cm3 of H2SO4 per g−1 of cellulose) on the solubilization rate of microcrystalline cellulose and on the glucose production rate have been analysed. The solubilization process was by determining reducing groups present in solution. For acid/substrate relationships of more than 1 cm3 g−1 and H2SO4 concentrations of greater than 62% (w/v), the acid promoted the total solubilization of the cellulose in the form of chains with a low degree of polymerization within 4 h. The solubilization demonstrated zero‐order kinetics in which the specific rate and time of total solubilization are a function of the variables in operation. Glucose was produced according to a mechanism of two consecutive first‐order pseudo‐homogeneous reactions. The values of the kinetic constants k1 and k2 have been correlated with temperature, the H2SO4 concentration and the acid/substrate relationship.

We have studied the enzymatic hydrolysis of whey proteins at pH 8 and50 degrees C with two proteases of bacterial origin, MKC Protease 660 L, and one of animal origin, PEM 2500 S. Our results show that a greater degree of hydrolysis is achieved under the same experimental conditions with the bacterial proteases than with the animal one. In our interpretation of the results we propose a mechanism in which the hydrolytic reaction is a zero-order one for the substrate, and the enzyme denaturalizes simultaneously via a second-order kinetic process due to free enzyme attacking enzyme bound to the substrate. Our results also indicate that there is an irreversible serine-protease inhibitor in whey proteins. (c) 1994 John Wiley & Sons, Inc.

No abstract