Experimental determination of the molecular weight of a calcium alginate gel by viscometry and 1H NMR at 25 °C

ORONÁ, J.D.; ZORRILLA, S. E.; PERALTA, J. M.

Buenos Aires

Congreso; 11th World Congress of Chemical Engineering; 2023

Asociación Argentina de Ingenieros Químicos

Calcium alginate gels are widely used in the food industry for encapsulating bioactive compounds (essential oils, vitamins, etc.) that can be degraded under food processing conditions. In general, these gels are stable at room temperature, biocompatible, non-toxic, exhibit a high encapsulation efficiency, and their products show a wide range of mechanical properties. The molecular weight (M0) of these materials is strongly related to properties (i.e., diffusive characteristics, porosity, and chemical and mechanical stability) that define their functionality. Consequently, M0 is a particularly important property for a rational design and production of alginate-based delivery systems. Usually, its determination is done using proton nuclear magnetic resonance (1H NMR) at high temperatures (> 80 °C) and requires a previous hydrolyzation of the sodium alginate to increase the spectrum resolution and move water resonance from the spectral zone of interest. Both requirements are laborious, increasing the costs and inducing to potential errors in the interpretations of the NMR spectra. The aim of this work was to simplify the determination of the M0 for a calcium alginate gel by using a combination of viscometry and nuclear magnetic resonance at low temperatures (25 °C). The value of M0 was determined from the weight-average molecular weight of a low-viscosity sodium alginate (Mw), its fraction of α-L-guluronic acid residues (G groups) (FG), and the estimated amount of Ca2+ in the gel. Firstly, the value of Mw was calculated from the intrinsic viscosity ([η]) of sodium alginate using the Mark-Houwink-Sakurada equation. Different concentrations of a 0.1 M NaCl alginate solution (0.025 to 0.1 % w:v) were used to estimate [η] by capillary viscometry at 25 °C. Secondly, the value of FG was determined from a solution of non-hydrolyzed sodium alginate in deuterium oxide (5 % w:v) by a 1H NMR technique using a longitudinal eddy current delay experiment with bipolar gradient (ledbpgp2s1d) at 25 °C. This program of pulses and the adequate setting of its parameters allowed obtaining the alginate spectrum with a suitable resolution and without overlaps. Finally, the amount of Ca2+ in the gel was estimated from its stoichiometric coefficient (Nc) based on Mw, FG and assuming a half egg-box structure of the gel. The values of Mw, FG and Nc were found to be 192 ± 2 kDa, 0.356 and 259, respectively. These results are in agreement with the literature for similar materials. It was considered that all the G groups are forming G blocks and the Ca2+ are occupying all junction zones. Consequently, the obtained M0 was 202 kDa, which is similar to values found in the literature. The proposed technique represents a novel and low-cost way to characterize alginate gels used in food systems.