Kinetics of acid hydrolysis of k-Carrageenan by in situ rheological follow-up

Water based solutions of κ-Carrageenan (1.5% weight concentration), a high molecular weight polysaccharide, were subjected to acid-catalyzed hydrolysis under controlled pH (≈3) and temperature (from 40 to 70 °C). The hydrolysis process involves selective attack of the Carrageenan glycosidic bonds α(1–3) and β(1–4), determining changes in the weight average molecular weight of the macromolecule and, as a consequence, on the gelling behavior. In this study we used transient rheological tests to monitor the hydrolysis reaction and its consequences on both the sol-gel transition and the final gel properties. Hydrolysis reaction has been modelled using a bi-exponential kinetics. To this end, a rotational rheometer was equipped with a batch reactor configuration, consisting of a cylindrical vessel and a double helix mixing tool. Both viscous and viscoelastic properties of the samples were measured quantitatively and related to the biopolymer molecular weight, replacing more complex molecular weight determinations by an easier methodology. By using the above experimental approach, kinetic constants and activation energies for the κ-Carrageenan hydrolysis were obtained as a function of the hydrolysis temperature. The latter, along with the hydrolysis reaction time, was also shown to be a relevant parameter in determining the possibility of gel formation and of the gel mechanical strength (in the range 10–600 Pa).