Enzymatic oxidation as a potential new route to produce polysaccharide aerogels

Abstract

Specific enzymatic oxidation of terminal galactosyl-containing polysaccharides (guar galactomannan (GM), and tamarind seed galactoxyloglucan (XG)) was used to prepare hydrogels. The hydrogels were lyophilized to form novel types of polysaccharide aerogels: biobased and biodegradable, lightweight, and stiff materials. The compressive moduli of the aerogels were greatly dependent on the oxidation, polysaccharide type, freezing method, and ambient moisture. Ice crystal templated, oriented aerogels from oxidized XG (XG-OX) showed the highest compressive modulus, 359 kPa, when determined parallel to the freezing and drying direction (i.e., the vertical direction). The water vapor sorption of freeze-dried GM and XG was not significantly affected by oxidation, even though the oxidized GM (GM-OX) and XG-OX aerogels were no longer water-soluble. GM-OX and XG-OX aerogels absorbed liquid water 40 and 20 times their initial weight, respectively. Focused ion beam scanning electron microscopy showed that the inner structure of oriented aerogels from GM-OX consisted of a honeycomb architecture with a pore diameter of some tens of micrometers. On the other hand, corresponding aerogels from XG-OX seemed to contain longer capillaries oriented in the freezing direction. This observation was supported by imaging the XG-OX aerogels using high-resolution synchrotron X-ray microtomography. The enzymatic hydro- and aerogel preparation method is considered a green way to obtain novel, functional products from polysaccharides.