Microbial xylitol production

Utilization of agricultural wastes for co-production of xylitol, ethanol, and phenylacetylcarbinol: A review

Corn, rice, wheat, and sugar are major sources of food calories consumption thus the massive agricultural waste (AW) is generated through agricultural and agro-industrial processing of these raw materials. Biological conversion is one of the most sustainable AW management technologies. The abundant supply and special structural composition of cellulose, hemicellulose, and lignin could provide great potential for waste biological conversion. Conversion of hemicellulose to xylitol, cellulose to ethanol, and utilization of remnant whole cells biomass to synthesize phenylacetylcarbinol (PAC) are strategies that are both eco-friendly and economically feasible. This co-production strategy includes essential steps: saccharification, detoxification, cultivation, and biotransformation. In this review, the implemented technologies on each unit step are described, the effectiveness, economic feasibility, technical procedures, and environmental impact are summarized, compared, and evaluated from an industrial scale viewpoint.

Detoxification Approaches of Bagasse Pith Hydrolysate Affecting Xylitol Production by Rhodotorula mucilaginosa

In this study, the potential of bagasse pith (the waste of sugar and paper industry) was investigated for bio-xylitol production for the first time. Xylose-rich hydrolysate was prepared using 8% dilute sulfuric acid, at 120 °C for 90 min. Then, the acid-hydrolyzed solution was detoxified by individual overliming (OL), active carbon (AC), and their combination (OL+AC). The amounts of reducing sugars and inhibitors (furfural and hydroxyl methyl furfural) were measured after acid pre-treatment and detoxification process. Thereafter, xylitol was produced from detoxified hydrolysate by Rhodotorula mucilaginosa yeast. Results showed that after acid hydrolysis, the sugar yield was 20%. Detoxification by overliming and active carbon methods increased the reducing sugar content up to 65% and 36% and decreased the concentration of inhibitors to >90% and 16%, respectively. Also, combined detoxification caused an increase in the reducing sugar content (>73%) and a complete removal of inhibitors. The highest productivity of xylitol (0.366 g/g) by yeast was attained after the addition of 100 g/l non-detoxified xylose-rich hydrolysate into fermentation broth after 96 h, while the xylitol productivity enhanced to 0.496 g/g after adding the similar amount of xylose-rich hydrolysate detoxified by combined method (OL+AC2.5%).

Exergetic analysis and optimization of process variables in xylitol production: Maximizing efficiency and sustainability in biotechnological processes

This study presents an exergetic analysis of xylitol fermentative production from hemicellulose hydrolysate, aiming to optimize operational conditions in a fluidized bed bioreactor. The aerobic fermentation conditions evaluated in this study (gas flow rate - x1, hydrolysate concentration factor - x2, and recirculation flow rate - x3) were optimized using various exergetic parameters and xylitol yield as objective functions. Four objective functions were defined for the mono-objective optimization process: rational exergetic efficiency, normalized destroyed exergy, thermodynamic sustainability index, and xylitol yield factor. The results reveal that the optimization problem involves conflicting objectives when considering both yield-based and exergy-based approaches. Thus, the bioreactor's performance was formulated as a multi-objective problem, where the yield factor and thermodynamic sustainability index were simultaneously maximized. For the multi-objective optimization, the ideal operational variable ranges were found to be: 594 ≤x1≤ 619 mL/min, x2= 7 e 37 ≤x3≤ 57 L/h.

Effects of electrospun nanofiber fabrications on immobilization of recombinant Escherichia coli for production of xylitol from glucose

A suitable nanofiber sheet was formulated and developed based on its efficacy in the immobilization of recombinant Escherichia coli (E. coli) to enhance xylitol production. The effects of different types of nanofibers and solvents on cell immobilization and xylitol production were studied. The most applicable nanofiber membrane was selected via preliminary screening of four types of nanofiber membrane, followed by the selection of six different solvents. Polyvinylidene fluoride (PVDF) nanofiber sheet synthesized using dimethylformamide (DMF) solvent was found to be the most suitable carrier for immobilization and xylitol production. The thin, beaded PVDF (DMF) nanofibers were more favourable for microbial adhesion, with the number of immobilized cells as high as 96 × 106 ± 3.0 cfu/ml. The attraction force between positively charged PVDF nanofibers and the negatively charged E. coli indicates that the electrostatic interaction plays a significant role in cell adsorption. The use of DMF has also produced PVDF nanofibers biocatalyst capable of synthesizing the highest xylitol concentration (2.168 g/l) and productivity (0.090 g/l/h) and 55-69% reduction in cell lysis compared with DMSO solvent and free cells. This finding suggests that recombinant E. coli immobilized on nanofibers shows great potential as a whole-cell biocatalyst for xylitol production.

Bioremediation of an industrial soil contaminated by hydrocarbons in microcosm system, involving bioprocesses utilizing co-products and agro-industrial wastes

The present study describes practical implication of bioaugmentation and biostimulation processes for bioremediation of an industrial soil chronically contaminated by hydrocarbons. For this purpose, biomass production of six autochthonous hydrocarbon-degrading bacteria were evaluated as inoculum of bioaugmentation strategy, by testing carbon and nitrogen sources included co-products and agro-industrial waste as sustainable and low-cost components of the growth medium. Otherwise, biostimulation was approached by the addition of optimized concentration of nitrogen and phosphorus. Microcosm assays showed that total hydrocarbons (TH) were significantly removed from chronically contaminated soil undergoing bioremediation treatment. Systems Mix (bioaugmentation); N,P (biostimulation) and Mix + N,P (bioaugmentation and biostimulation) reached higher TH removal, being 89.85%, 91.00%, 93.04%, respectively, comparing to 77.83% of system C (natural attenuation) at 90 days. The increased heterotrophic aerobic bacteria and hydrocarbon degrading bacteria counts were according to TH biodegrading process during the experiments. Our results showed that biostimulation with nutrients represent a valuable alternative tool to treat a chronically hydrocarbon-contaminated industrial soil, while bioaugmentation with a consortium of hydrocarbon degrading bacteria would be justified when the soil has a low amount of endogenous degrading microorganisms. Furthermore, the production of inoculum for application in bioaugmentation using low-cost substrates, such as industrial waste, would lead to the development of an environmentally friendly and attractive process in terms of cost-benefit.

Food-grade xylitol production from corncob biomass with acute oral toxicity studies

Xylitol, a sugar substitute, is widely used in various food formulations and finds a steady global market. In this study, xylitol crystals were produced from corncob by fermentation (as an alternative to the chemical catalytic process) by a GRAS yeast Pichia caribbica MTCC 5703 and characterized in detail for their purity and presence of any possible contaminant that may adversely affect mammalian cell growth and proliferation. The acute and chronic oral toxicity trials demonstrated no gross pathological changes with average weekly weight gain in female Wistar rats at high xylitol loading (LD₅₀ > 10,000 mg/kg body weight). The clinical chemistry analysis supported the evidence of no dose-dependent effect by analyzing blood biochemical parameters. The finding suggests the possible application of the crystals (> 98% purity) as a food-grade ingredient for commercial manufacture pending human trials.

Strategy to reduce acetic acid in sugarcane bagasse hemicellulose hydrolysate concomitantly with xylitol production by the promising yeast Cyberlindnera xylosilytica in a bioreactor

The yeast Cyberlindnera xylosilytica UFMG-CM-Y309 has been identified as a promising new xylitol producer from sugarcane bagasse hemicellulosic hydrolysate (SCHH). However, SCHH pretreatment process generates byproducts, which are toxic to cell metabolism, including furans, phenolic compounds, and carboxylic acids, such as acetic acid, typically released at high concentrations. This research aims to reduce acetic acid in sugarcane hemicellulose hydrolysate concomitantly with xylitol production by yeast strain Cy. xylosilytica UFMG-CM-Y309 in a bioreactor by strategically evaluating the influence of volumetric oxygen transfer coefficient (k_La) (21 and 35 h^-1). Experiments were conducted on a bench bioreactor (2 L volumetric capacity) at different initial k_La values (21 and 35 h^-1). SCHH medium was supplemented with rice bran extract (10 g L^-1) and yeast extract (1 g L^-1). Cy. xylosilytica showed high xylitol production performance (19.56 g L^-1), xylitol yield (0.56 g g^-1) and, maximum xylitol-specific production rate (μp_máx 0.20 g_xylitol·g^-1 h^-1) at k_La value of 21 h^-1, concomitantly slowing the rate of acetic acid consumption. A faster acetic acid consumption (100%) by Cy. xylosilytica was observed at k_La of 35 h^-1, concomitantly with an increase in maximum cellular growth (14.60 g L^-1) and reduction in maximum xylitol production (14.56 g L^-1 and Y_p/s 0.34 g g^-1). This study contributes to pioneering research regarding this yeast performance in bioreactors, emphasizing culture medium detoxification and xylitol production.