Production of high-purity neofructooligosaccharides by culture of Xanthophyllomyces dendrorhous

Purification and characterization of crude fructooligosaccharides extracted from red onion (Allium cepa var. viviparum) by yeast treatment

BACKGROUND

Yeast treatment has been used for purification of fructooligosaccharides (FOSs). However, the main drawback of this approach is that yeast can only partially remove sucrose from crude FOSs. The main objective of this research was to screen yeast strains for the capability of selectively consuming unwanted sugars, namely fructose, glucose, and sucrose, in crude FOSs extracted from red onion (Allium cepa var. viviparum) with minimal effect on FOS content.

RESULTS

Among 43 yeast species isolated from Miang, ethnic fermented tea leaves, and Assam tea flowers, Candida orthopsilosis FLA44.2 and Priceomyces melissophilus FLA44.8 exhibited the greatest potential to specifically consume these unwanted sugars. In a shake flask, direct cultivation of C. orthopsilosis FLA44.2 was achieved in the original crude FOSs containing an initial FOSs concentration of 88.3 ± 1.2 g/L and 52.9 ± 1.2 g/L of the total contents of fructose, glucose, and sucrose. This was successful with 93.7% purity and 97.8% recovery after 24 h of cultivation. On the other hand, P. melissophilus FLA48 was limited by initial carbohydrate concentration of crude FOSs in terms of growth and sugar utilization. However, it could directly purify two-fold diluted crude FOSs to 95.2% purity with 92.2% recovery after 72 h of cultivation. Purification of crude FOSs in 1-L fermenter gave similar results to the samples purified in a shake flask. Extracellular β-fructosidase was assumed to play a key role in the effective removal of sucrose. Both Candida orthopsilosis FLA44.2 and P. melissophilus FLA44.8 showed γ-hemolytic activity, while their culture broth had no cytotoxic effect on viability of small intestinal epithelial cells, preliminarily indicating their safety for food processing. The culture broth obtained from yeast treatment was passed through an activated charcoal column for decolorization and deodorization. After being freeze dried, the final purified FOSs appeared as a white granular powder similar to refined sugar and was odorless since the main sulfur-containing volatile compounds, including dimethyl disulfide and dipropyl trisulfide, were almost completely removed.

CONCLUSION

The present purification process is considered simple and straight forward, and provides new and beneficial insight into utilization of alternative yeast species for purification of FOSs.

Engineering the β-Fructofuranosidase Fru6 with Promoted Transfructosylating Capacity for Fructooligosaccharide Production

Levan-type fructooligosaccharides (FOS) exhibit enhanced health-promoting prebiotic effects on gut microbiota. The wild type (WT) of β-fructofuranosidase Fru6 could mainly yield 6-ketose. Semirational design and mutagenesis of Fru6 were exploited to promote the transfructosylating capacity for FOS. The promising variants not only improved the formation of 6-kestose but also newly produced tetrasaccharides of 6,6-nystose and 1,6-nystose (a new type of FOS), and combinatorial mutation boosted the production of 6-kestose and tetrasaccharides (39.9 g/L 6,6-nystose and 4.6 g/L 1,6-nystose). Molecular docking and molecular dynamics (MD) simulation confirmed that the mutated positions reshaped the pocket of Fru6 to accommodate bulky 6-kestose in a reactive conformation with better accessibility for tetrasaccharides formation. Using favored conditions, the variant S165A/H357A could yield 6-kestose up to 335 g/L, and tetrasaccharides (6,6-nystose and 1,6-nystose) reached a high level of 121.1 g/L (134.5 times of the mutant S423A). The β-(2,6)-linked FOS may show the potential application for the prebiotic ingredients.

A novel chicory fructanase can degrade common microbial fructan product profiles and displays positive cooperativity

Fructan metabolism in bacteria and plants relies on fructosyltransferases and fructanases. Plant fructanases (fructan exohydrolase, FEH) only hydrolyse terminal fructose residues. Levan (β-2,6 linkages) is the most abundant fructan type in bacteria. Dicot fructan accumulators, such as chicory (Cichorium intybus), accumulate inulin (β-2,1 linkages), harbouring several 1-FEH isoforms for their degradation. Here, a novel chicory fructanase with high affinity for levan was characterized, providing evidence that such enzymes widely occur in higher plants. It is adapted to common microbial fructan profiles, but has low affinity towards chicory inulin, in line with a function in trimming of microbial fructans in the extracellular environment. Docking experiments indicate the importance of an N-glycosylation site close to the active site for substrate specificity. Optimal pH and temperature for levan hydrolysis are 5.0 and 43.7 °C, respectively. Docking experiments suggested multiple substrate binding sites and levan-mediated enzyme dimerization, explaining the observed positive cooperativity. Alignments show a single amino acid shift in the position of a conserved DXX(R/K) couple, typical for sucrose binding in cell wall invertases. A possible involvement of plant fructanases in levan trimming is discussed, in line with the emerging 'fructan detour' concepts, suggesting that levan oligosaccharides act as signalling entities during plant-microbial interactions.

Kinetic and hybrid modeling for yeast astaxanthin production under uncertainty

Astaxanthin is a high-value compound commercially synthesized through Xanthophyllomyces dendrorhous fermentation. Using mixed sugars decomposed from biowastes for yeast fermentation provides a promising option to improve process sustainability. However, little effort has been made to investigate the effects of multiple sugars on X. dendrorhous biomass growth and astaxanthin production. Furthermore, the construction of a high-fidelity model is challenging due to the system's variability, also known as batch-to-batch variation. Two innovations are proposed in this study to address these challenges. First, a kinetic model was developed to compare process kinetics between the single sugar (glucose) based and the mixed sugar (glucose and sucrose) based fermentation methods. Then, the kinetic model parameters were modeled themselves as Gaussian processes, a probabilistic machine learning technique, to improve the accuracy and robustness of model predictions. We conclude that although the presence of sucrose does not affect the biomass growth kinetics, it introduces a competitive inhibitory mechanism that enhances astaxanthin accumulation by inducing adverse environmental conditions such as osmotic gradients. Moreover, the hybrid model was able to greatly reduce model simulation error and was particularly robust to uncertainty propagation. This study suggests the advantage of mixed sugar-based fermentation and provides a novel approach for bioprocess dynamic modeling.

Two-Step Production of Neofructo-Oligosaccharides Using Immobilized Heterologous Aspergillus terreus 1F-Fructosyltransferase Expressed in Kluyveromyces lactis and Native Xanthophyllomyces dendrorhous G6-Fructosyltransferase

Fructo-oligosaccharides (FOS) are prebiotic low-calorie sweeteners that are synthesized by the transfer of fructose units from sucrose by enzymes known as fructosyltransferases. If these enzymes generate β-(2,6) glycosidic bonds, the resulting oligosaccharides belong to the neoseries (neoFOS). Here, we characterized the properties of three different fructosyltransferases using a design of experiments approach based on response surface methodology with a D-optimal design. The reaction time, pH, temperature, and substrate concentration were used as parameters to predict three responses: The total enzyme activity, the concentration of neoFOS and the neoFOS yield relative to the initial concentration of sucrose. We also conducted immobilization studies to establish a cascade reaction for neoFOS production with two different fructosyltransferases, achieving a total FOS yield of 47.02 ± 3.02%. The resulting FOS mixture included 53.07 ± 1.66 mM neonystose (neo-GF3) and 20.8 ± 1.91 mM neo-GF4.