Complete Utilization of the Major Carbon Sources Present in Sugar Beet Pulp Hydrolysates by the Oleaginous Red Yeasts Rhodotorula toruloides and R. mucilaginosa.

Luís C Martins,Isabel Sá-Correia,Elke Nevoigt,Margarida Palma,Angel Angelov,Wolfgang Liebl

doi:10.3390/jof7030215

Abstract

Agro-industrial residues are low-cost carbon sources (C-sources) for microbial growth and production of value-added bioproducts. Among the agro-industrial residues available, those rich in pectin are generated in high amounts worldwide from the sugar industry or the industrial processing of fruits and vegetables. Sugar beet pulp (SBP) hydrolysates contain predominantly the neutral sugars d-glucose, l-arabinose and d-galactose, and the acidic sugar d-galacturonic acid. Acetic acid is also present at significant concentrations since the d-galacturonic acid residues are acetylated. In this study, we have examined and optimized the performance of a Rhodotorula mucilaginosa strain, isolated from SBP and identified at the molecular level during this work. This study was extended to another oleaginous red yeast species, R. toruloides, envisaging the full utilization of the C-sources from SBP hydrolysate (at pH 5.0). The dual role of acetic acid as a carbon and energy source and as a growth and metabolism inhibitor was examined. Acetic acid prevented the catabolism of d-galacturonic acid and l-arabinose after the complete use of the other C-sources. However, d-glucose and acetic acid were simultaneously and efficiently metabolized, followed by d-galactose. SBP hydrolysate supplementation with amino acids was crucial to allow d-galacturonic acid and l-arabinose catabolism. SBP valorization through the production of lipids and carotenoids by Rhodotorula strains, supported by complete catabolism of the major C-sources present, looks promising for industrial implementation.

Highlights

This study provides strong evidence that the concept of sugar beet pulp (SBP) valorization through the production of lipids and carotenoids by Rhodotorula spp. from the complete catabolism of all major C-sources present in SBP hydrolysates is promising for implementation in an economical biotechnological process
Sugar beet pulp (SBP) is likely an interesting material for the isolation of different microorganisms since at room temperature, SBP can be degraded by the exo- and endopolygalacturonases as well as pectin and pectate lyases produced by filamentous fungi, such as Aspergillus niger or Trichoderma reesei, that hydrolyze the pectin backbone [45]
A SBP sample suspended in water with chloramphenicol was used as growth medium for the isolation of yeast strains envisaging the full catabolism of the sugars present in pectin, in particular the more challenging sugars D-galacturonic acid and L -arabinose

Summary

Introduction

Among the agro-industrial residues available, those rich in pectin (e.g., sugar beet pulp (SBP), citrus peels, apple pomace) have potential as feedstocks for the production of biofuels and other industrial bioproducts [3,6]. They are generated in high amounts worldwide from the sugar industry or the industrial processing of fruits and 4.0/). They are partially pre-treated and have a low lignin content, facilitating biomass processing for saccharification and fermentation [8]

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Results

Conclusion