13-39: Robust liquid core APTES-alginate-chitosan-alginate capsules for second generation bioethanol production

Encapsulation of yeast in liquid core gel membrane capsules has been shown to improve the inhibitor tolerance and even thermo-tolerance of yeast. Encapsulation is therefore a very promising bioprocessing methodology for second generation bioethanol production using lignocellulose hydrolyzates. During pretreatment and / or hydrolysis, inhibitors are generated as degradation products from both the lignin and hemicellulose fractions. However, for industrial applications, capsules must be made robust enough to endure long periods and numerous cultivations without breaking due to chemical degradation or mechanical attrition. In this study, liquid core Alginate-Chitosan-Alginate (ACA) capsules containing Saccharomyces cerevisiae were produced by the liquid-droplet-forming method and treated with 3-aminopropyltrietoxysilane (APTES) to increase the mechanical durability of the capsules. The integrity of these membranes is not compromized by a lack of Ca²⁺ ions. Capsules were used in five anaerobic consecutive batch cultivations on dilute-acid spruce hydrolyzate after which the capsule robustness was evaluated by intense agitation in a baffled reactor. ACA capsules gave the highest initial ethanol production, but had poor mechanical robustness, since 25% of the capsules ruptured within 6 h. Capsules treated with a 1.5% APTES solution were more robust, and only 0-2% of these capsules broke in the shear test. Furthermore, the ethanol production in the fifth cultivation was nearly as high as for untreated ACA capsules. Thus, the mechanical robustness of ACA capsules can be easily improved by treating the capsules with APTES, and the obtained capsules are likely to be appropriate for industrial application.