Biological conversion of industrial by-products/wastes into value-added bacterial exopolysaccharides

Abstract

This thesis is focused on the production of value-added bacterial exopolysaccharides (EPS) by the bacterium Enterobacter A47 (DSM 23139), using industrial by-products/wastes as sub-strates. Enterobacter A47 has demonstrated the ability to synthesize a high molecular weight (Mw) fucose-rich EPS, namely, the heteropolysacharide named FucoPol. This new EPS is com-posed of fucose, glucose, galactose and glucuronic acid, which present interesting functional properties. Cheese whey was the first industrial by-product studied. Under standard controlled param-eters for FucoPol’s production, Enterobacter A47 successfully grew using lactose as carbon source and an EPS concentration of 6.40 g L-1 was reached within 3.2 days of cultivation, corre-sponding to a volumetric productivity of 2.00 g L-1 d-1. The produced EPS was mainly composed of glucuronic acid and fucose, which confers it a great potential for use in high-value applications, such as cosmetics and pharmaceuticals. The use of out-of-specification tomato paste as substrate resulted in the highest production (8.77 g L-1) and overall volumetric productivity (2.92 g L-1 d-1), which were obtained with con-tinuous substrate feeding at a constant flow rate of 11 g h-1. The polymer produced had the typical FucoPol composition: 30-36 mol% of fucose; 22-29 mol% of galactose; 25-34 mol% of glucose; 9-10 mol% of glucuronic acid and 12-22 wt.% of acyl groups. The ability of Enterobacter A47 to grow and produce EPS using as carbon source a mixture of glucose and xylose (75:25%) was also tested using pure sugars and a brewer’s spent grain (BSG) hydrolysate. The use of BSG as substrate resulted in an EPS concentration of 2.30 g L-1 with a low volumetric productivity (0.57 g L-1 d-1). The produced EPS was mainly composed of glucose. On the other hand, 5.71 g L-1 of EPS was achieved after 4 days using commercial glu-cose/xylose mixture, giving an overall volumetric productivity of 1.43 g L-1 d-1. The EPS pro-duced revealed to be similar to FucoPol composition. Although that the ability of Enterobacter A47 to use BSG hydrolysate as sole substrate to grow and produce EPS was demonstrated, further studies need to be developed to increase the polymer’s productivity. In order to evaluate the potential of the different EPS produced by Enterobacter A47 in this study, their properties in aqueous solutions, emulsion forming and stabilizing capacity, and film-forming capacity were studied. EPSCW presented the lowest apparent viscosity (ηa), but it was able to stabilize emulsions for over 4 weeks with olive oil, cedarwood oil and paraffin oil and its films had higher elongation capacity. EPSTP showed the lowest intrinsic viscosity ([η]) and angular frequency at dependence of storage (G’) and loss moduli (G’’) cross-over, good emulsion capacity and stabilization with peanut oil, almond oil and olive oil, and the produced films are slightly more rigid. In contrast, EPSGX reached the highest [η] and ηa, the emulsions formed were very strong with most of the tested oils at low O/W ratios, although having low stability. Moreover, EPSGX films were slight stiffer.