Consolidated bioprocessing of starch based on a bilayer cell factory without genetic modification of yeast

Abstract

The exploitation of starchy wastes for the production of value-added products and the consolidated bioprocessing (CBP) have a positive environmental and cost-effective impact. For this reason, a cell factory (CF) was employed to perform three bioprocesses in one step (CBP) for alcoholic fermentation of starch without genetic modification. CF is a bilayer biocatalyst consisting of an inner layer of immobilized Saccharomyces cerevisiae on tubular cellulose (TC) and an external layer of immobilized Aspergillus awamori on alginates (ALG). In first, the suggested CF was proved to be more effective compared with co-immobilized S. cerevisiae and A. awamori on ALG beads for starch fermentation. Subsequently, the effect of the (i) S. cerevisiae concentration (g S. cerevisiae/g TC) during its immobilization, (ii) CaCl 2 concentration (% w/v), (iii) form of bilayer CF (wet or freeze-dried) and (iv) freeze-dried CF concentration on 5% (w/v) starch fermentation was examined. It was found that the higher ethanol production (32.17 mL/L), productivity (4.60 mL/L/d) and yield (0.51 g ethanol/g starch) reaching the theoretical one were obtained when the CF in freeze dried form fermented 100 mL of 5% (w/v) starch solution. Finally, two repeated fermentation batches were performed using the best CF. The ethanol yield was decreased during the repeated fermentation batches, but remained in acceptable levels. The successful preparation of CF was verified with FTIR spectroscopy, SEM and TEM analysis. The results indicated that the objective of designing CF was successfully achieved, and the results are promising to be the base for the exploitation of starchy wastes and the development of an innovation in brewing industry by eliminating the malting stage. This CF can be applied as model for different bioprocesses of White Biotechnology, e.g. substituting S. cerevisiae with the appropriate microorganism to produce other metabolites of added value.