Coupling two sizes of CSTR-type bioreactors for sequential lactic acid and xylitol production from hemicellulosic hydrolysates of vineshoot trimmings

This study develops a system for the efficient valorisation of hemicellulosic hydrolysates of vineshoot trimmings. By connecting two reactors of 2 L and 10 L, operational conditions were set up for the sequential production of lactic acid and xylitol in continuous fermentation, considering the dependence of the main metabolites and fermentation parameters on the dilution rate. In the first bioreactor, Lactobacillus rhamnosus consumed all the glucose to produce lactic acid at 31.5°C, with 150 rpm and 1 L of working volume as the optimal conditions. The residual sugars were employed for the xylose to xylitol bioconversion by Debaryomyces hansenii in the second bioreactor at 30°C, 250 rpm and an air-flow rate of 2 L min−1. Several steady states were reached at flow rates (F) in the range of 0.54–5.33 mL min−1, leading to dilution rates (D) ranging from 0.032 to 0.320 h−1 in Bioreactor 1 and from 0.006 to 0.064 h−1 in Bioreactor 2. The maximum volumetric lactic acid productivity (QP LA = 2.908 g L−1 h−1) was achieved under D = 0.266 h−1 (F = 4.44 mL min−1); meanwhile, the maximum production of xylitol (5.1 g L−1), volumetric xylitol productivity (QP xylitol = 0.218 g L−1 h−1), volumetric rate of xylose consumption (QS xylose = 0.398 g L−1 h−1) and product yield (0.55 g g−1) were achieved at an intermediate dilution rate of 0.043 h−1 (F = 3.55 mL min−1). Under these conditions, ethanol, which was the main by-product of the fermentation, was produced in higher amounts (1.9 g L−1). Finally, lactic acid and xylitol were effectively recovered by conventional procedures.