# Insights into the production of erythritol by strains of Limosilactobacillus fermentum of sourdough origin

## TL;DR

This study shows that certain sourdough bacteria can produce erythritol, a sugar alcohol, which helps balance redox reactions during fermentation.

## Contribution

The study demonstrates erythritol production by sourdough-origin L. fermentum strains and its role in redox balancing.

## Key findings

- L. fermentum IMDO 130101 and IMDO TC9L10 produced up to 0.4 mM erythritol during sourdough fermentation.
- Erythritol synthesis was linked to redox balancing and co-produced with acetate and glycerol.
- No specific genes for erythritol production were identified, suggesting shared pathways with other metabolites like mannitol.

## Abstract

Erythritol is a sugar alcohol that is occasionally produced by heterofermentative lactic acid bacteria (LAB). Although such biosynthesis has rarely been reported during sourdough production, the potential of the sourdough strains Limosilactobacillus fermentum IMDO 130101 and IMDO TC9L10 to produce erythritol was demonstrated in the context of their broader metabolic capacity both during wheat sourdough productions and in a wheat sourdough simulation medium (WSSM). Indeed, both strains produced up to 0.4 mM of erythritol during starter culture-initiated wheat sourdough productions. The strain L. fermentum IMDO 130101 was further used to carry out controlled fermentation processes in WSSM and was able to synthesize 0.53 ± 0.03 mM of erythritol. During both the wheat sourdough productions and WSSM fermentation processes, acetate was co-produced with erythritol and/or glycerol. The primary role of erythritol was likely its involvement in redox balancing. Besides, the pathways for erythritol production were probed in silico, but no genes corresponding with erythrose-producing or erythrose-converting enzymes could be identified. Therefore, it was suggested that the enzymes responsible for the production of erythritol in L. fermentum IMDO 130101, and other heterofermentative LAB species, are the same as those producing mannitol or glycerol.

Lactic acid bacteria have a limited biosynthesis capacity. Their main carbohydrate breakdown pathways enable them to produce energy and to maintain their redox balance. The latter is accomplished through the production of lactic acid in the case of homofermentative lactic acid bacteria and ethanol or acetic acid in the case of heterofermentative ones. However, under certain conditions, other branches of this pathway become active, which lead to end-metabolites that are produced seldomly. An example of such an end-metabolite is erythritol, which was already detected during sourdough production but never investigated in detail. The present study showed the production of erythritol by Limosilactobacillus fermentum inhabiting sourdough environments and its involvement in redox balancing.