CO2 induced gelation of amidated pectin solutions: Impact of viscosity and gel formation

Abstract

CO2 induced gelation of aqueous biopolymer systems results in stable and homogeneous hydrogels with a wide field of potential applications in food and pharmaceutical industry. For the application of the process in industry, good understanding of process parameters and their impact on the overall process and the gel properties is necessary. The mechanism of the CO2 induced gelation is not fully understood; the role of viscosity has never been addressed in the literature. The first step of the overall gelation process, the dissolution of CO2 into the biopolymer systems, is limiting for the overall process. Therefore, it was investigated in this work with different non-invasive, in-situ measurement methods: magnetic suspension balance and Raman spectroscopy on the system of amidated pectin with calcium carbonate (CaCO3) as additional cross-linker. With increasing viscosity of the biopolymer system, due to increasing amidated pectin mass fraction, the CO2 dissolution is slowed down. For non-gelling, CaCO3-free biopolymer systems effective diffusion coefficients from 17.7·10-9 to 5.6·10-9 m2/s were determined for solutions with increasing complex viscosities from 0.011 to 0.379 Pa s (0.5 to 2 wt.-% amidated pectin). In this case, the effective diffusion coefficient considers mass transport due to pure diffusion and due to free convection. Inside the non-gelling biopolymer systems, free convection appears and no gradient of the CO2 concentration inside the samples occurs. Nevertheless, in gelling, CaCO3 containing, biopolymer systems the formation of the gel network (gelation) itself has a strong impact on CO2 dissolution, as it hinders free convection. Due to gelation, the effective diffusion coefficient is decreased down to about 2.5·10-9 m2/s and approaches the binary diffusion coefficient of CO2 in water; indicating pure diffusion of CO2 in pores that contain mainly pure water. At 50 bar and room temperature CO2 solubility of 0.043 ± 0.003 g of CO2 per gram of the aqueous system, nearly independent of solution composition and gelation was measured. Contrarily, increasing temperatures as well as decreasing pressure results in decrease of overall CO2 solubility, whereas increasing temperature does enhance dissolution and distribution kinetics. With the performed measurements, it was possible to distinguish between the impacts of viscosity, gelation, and the formed gel network on the CO2 dissolution kinetics inside the sample, respectively.