Abstract
d-lactic acid is a building block for heat resistant polylactic acid, a biobased polymer with a high potential. Nevertheless, an economically efficient industrial process for d-lactic acid production still needs to be implemented. Yeast extract is an expensive nutrient source, which is used to fulfill the complex nutritional requirements in lactic acid fermentations. The substitution of yeast extract by cheap alternative nutrient sources is a challenge in many fermentation processes. In this study, chemical and enzymatic hydrolysis techniques for protein rich agricultural residues and their effectiveness are compared, as well as their impact on the d-lactic acid production of Sporolactobacillus inulinus. An efficient substitution of yeast extract could be achieved by a variety of agricultural residues, hydrolysed with 3M H2SO4, demonstrating the much higher versatility and effectiveness of this method compared to enzymatic methods. In a fed-batch experiment with chemically hydrolyzed rapeseed meal and minimal supplementation, a lactic acid titer of 221 g L−1 and an overall productivity of 1.55 g (L h)−1 (96% yield) were obtained.
Highlights
The utilization of alternative nutrient sources to yeast extract is a challenge in many fermentation processes
An efficient substitution of yeast extract could be achieved by a variety of agricultural residues, hydrolysed with 3M H2 SO4, demonstrating the much higher versatility and effectiveness of this method compared to enzymatic methods
This mild chemical hydrolysis method was tested for various various residues lactic acid titers andand lowlow productivities of S.of inulinus, compared to yeast residues but butresulted resultedininlower lower lactic acid titers productivities
Summary
The utilization of alternative nutrient sources to yeast extract is a challenge in many fermentation processes. Yeast extract is still the nutrient source of choice, leading to positive results due to its low molecular mass distribution and high B-vitamin content [3,4]. Cheap protein-rich agricultural residues can be used as alternative nutrient sources, but a prior disintegration of high molecular cell components is necessary. The predominant method is enzymatic hydrolysis using proteases in a separate or simultaneous hydrolysis and fermentation processes [6,7]. This method benefits from a low salt content in the hydrolysate since a neutralization step is not required [8]. In a simultaneous process the enzyme incubation conditions often do not correspond to the cultivation conditions
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