Valorization of apple pomace using bio-based technology for the production of xylitol and 2G ethanol

Xylitol production from passion fruit peel hydrolysate: Optimization of hydrolysis and fermentation processes

The passion fruit peel (PFP) has a high cellulose and hemicellulose content, which can be used to produce fermentable sugars. In this context, this study aims to optimize the release of xylose and the production of xylitol from PFP. The optimized conditions were 0.71 M dilute sulfuric acid and a 21.84-minute treatment, yielding 19.03 g/L of xylose (PFP-1). Different PFP hydrolysates were evaluated to improve xylitol production by the yeast Kluyveromyces marxianus ATCC 36907: PFP-2 (PFP1 treated with Ca(OH)2), PFP-3 (PFP-1 treated with Ca(OH)2 and activated carbon), PFP-4 (PFP-3 with biological elimination of glucose with S. cerevisiae, and concentrated at different xylose concentrations). The applied methods resulted in higher xylitol production (14.97 g/L), when PFP hydrolysate was detoxified with Ca(OH)2, treated with activated charcoal for 1 h, biotreated for glucose removal, and concentrated to 40 g/L of xylose.

Expanding sugar alcohol industry: Microbial production of sugar alcohols and associated chemocatalytic derivatives

Sugar alcohols are polyols that are widely employed in the production of chemicals, pharmaceuticals, and food products. Chemical synthesis of polyols, however, is complex and necessitates the use of hazardous compounds. Therefore, the use of microbes to produce polyols has been proposed as an alternative to traditional synthesis strategies. Many biotechnological approaches have been described to enhancing sugar alcohols production and microbe-mediated sugar alcohol production has the potential to benefit from the availability of inexpensive substrate inputs. Among of them, microbe-mediated erythritol production has been implemented in an industrial scale, but microbial growth and substrate conversion rates are often limited by harsh environmental conditions. In this review, we focused on xylitol, mannitol, sorbitol, and erythritol, the four representative sugar alcohols. The main metabolic engineering strategies, such as regulation of key genes and cofactor balancing, for improving the production of these sugar alcohols were reviewed. The feasible strategies to enhance the stress tolerance of chassis cells, especially thermotolerance, were also summarized. Different low-cost substrates like glycerol, molasses, cellulose hydrolysate, and CO₂ employed for producing these sugar alcohols were presented. Given the value of polyols as precursor platform chemicals that can be leveraged to produce a diverse array of chemical products, we not only discuss the challenges encountered in the above parts, but also envisioned the development of their derivatives for broadening the application of sugar alcohols.

Sustainable Approaches Using Green Technologies for Apple By-Product Valorisation as A New Perspective into the History of the Apple

The apple has been recognised as the most culturally important fruit crop in temperate land areas. Centuries of human exploitation and development led to the production of thousands of apple cultivars. Nowadays, the apple represents the third most widely cultivated fruit in the world. About 30% of the total production of apples is processed, being juice and cider the main resulting products. Regarding this procedure, a large quantity of apple by-product is generated, which tends to be undervalued, and commonly remains underutilised, landfilled, or incinerated. However, apple by-product is a proven source of bioactive compounds, namely dietary fibre, fatty acids, triterpenes, or polyphenols. Therefore, the application of green technologies should be considered in order to improve the functionality of apple by-product while promoting its use as the raw material of a novel product line. The present work provides a holistic view of the apple’s historical evolution, characterises apple by-product, and reviews the application of green technologies for improving its functionality. These sustainable procedures can enable the transformation of this perishable material into a novel ingredient opening up new prospects for the apple’s potential use and consumption.

Kluyveromyces marxianus as a microbial cell factory for lignocellulosic biomass valorisation

The non-conventional yeast Kluyveromyces marxianus is widely used for several biotechnological applications, mainly due to its thermotolerance, high growth rate, and ability to metabolise a wide range of sugars. These cell traits are strategic for lignocellulosic biomass valorisation and strain diversity prompts the development of robust chassis, either with improved tolerance to lignocellulosic inhibitors or ethanol. This review summarises bioethanol and value-added chemicals production by K. marxianus from different lignocellulosic biomasses. Moreover, metabolic engineering and process optimization strategies developed to expand K. marxianus potential are also compiled, as well as studies reporting cell mechanisms to cope with lignocellulosic-derived inhibitors. The main lignocellulosic-based products are bioethanol, representing 71% of the reports, and xylitol, representing 17% of the reports. K. marxianus also proved to be a good chassis for lactic acid and volatile compounds production from lignocellulosic biomass, although the literature on this matter is still scarce. The increasing advances in genome editing tools and process optimization strategies will widen the K. marxianus-based portfolio products.

Integrated bioinformatics, modelling, and gene expression analysis of the putative pentose transporter from Candida tropicalis during xylose fermentation with and without glucose addition

The transport of substrates across the cell membrane plays an essential role in nutrient assimilation by yeasts. The establishment of an efficient microbial cell factory, based on the maximum use of available carbon sources, can generate new technologies that allow the full use of lignocellulosic constituents. These technologies are of interest because they could promote the formation of added-value products with economic feasibility. In silico analyses were performed to investigate gene sequences capable of encoding xylose transporter proteins in the Candida tropicalis genome. The current study identified 11 putative transport proteins that have not yet been functionally characterized. A phylogenetic tree highlighted the potential C. tropicalis xylose-transporter proteins CtXUT1, CtXUT4, CtSTL1, CtSTL2, and CtGXT2, which were homologous to previously characterized and reported xylose transporters. Their expression was quantified through real-time qPCR at defined times, determined through a kinetic analysis of the microbial growth curve in the absence/presence of glucose supplemented with xylose as the main carbon source. The results indicated different mRNA expression levels for each gene. CtXUT1 mRNA expression was only found in the absence of glucose in the medium. Maximum CtXUT1 expression was observed in intervals of the highest xylose consumption (21 to 36 h) that corresponded to consumption rates of 1.02 and 0.82 g/L/h in the formulated media, with xylose as the only carbon source and with glucose addition. These observations indicate that CtXUT1 is an important xylose transporter in C. tropicalis. KEY POINTS: • Putative xylose transporter proteins were identified in Candida tropicalis; • The glucose concentration in the cultivation medium plays a key role in xylose transporter regulation; • The transporter gene CtXUT1 has an important role in xylose consumption by Candida tropicalis.

An overview of the metabolically engineered strains and innovative processes used for the value addition of biomass derived xylose to xylitol and xylonic acid

Xylose, the most abundant pentose sugar of the hemicellulosic fraction of lignocellulosic biomass, has to be utilized rationally for the commercial viability of biorefineries. An effective pre-treatment strategy for the release of xylose from the biomass and an appropriate microbe of the status of an Industrial strain for the utilization of this pentose sugar are key challenges which need special attention for the economic success of the biomass value addition to chemicals. Xylitol and xylonic acid, the alcohol and acid derivatives of xylose are highly demanded commodity chemicals globally with plenty of applications in the food and pharma industries. This review emphasis on the natural and metabolically engineered strains utilizing xylose and the progressive and innovative fermentation strategies for the production and subsequent recovery of the above said chemicals from pre-treated biomass medium.

Xylitol Production by Candida Species from Hydrolysates of Agricultural Residues and Grasses

Xylitol is an industrially important chemical due to its commercial applications. The use of xylitol as a sweetener as well as its utilization in biomedical applications has made it a high value specialty chemical. Although several species of yeast synthesize xylitol, this review focusses on the species of the genus Candida. The importance of the enzyme xylitol reductase present in Candida species as it relates to their ability to synthesize xylitol was examined. Another focus of this work was to review prior studies examining the ability of the Candida species to synthesize xylitol effectively from hydrolysates of agricultural residues and grasses. An advantage of utilizing such a hydrolysate as a substrate for yeast xylitol production would be decreasing the overall cost of synthesizing xylitol. The intent of this review was to learn if such hydrolysates could substitute for xylose as a substrate for the yeast when producing xylitol. In addition, a comparison of xylitol production by Candida species should indicate which hydrolysate of agricultural residues and grasses would be the best substrate for xylitol production. From studies analyzing previous hydrolysates of agricultural residues and grasses, it was concluded that a hydrolysate of sugarcane bagasse supported the highest level of xylitol by Candida species, although corncob hydrolysates also supported significant yeast xylitol production. It was also concluded that fewer studies examined yeast xylitol production on hydrolysates of grasses and that further research on grasses may provide hydrolysates with a higher xylose content, which could support greater yeast xylitol production.