By-products of sugar factories and wineries as feedstocks for erythritol generation

Research progress on biosynthesis of erythritol and multi-dimensional optimization of production strategies

Erythritol, as a new type of natural sweetener, has been widely used in food, medical, cosmetics, pharmaceutical and other fields due to its unique physical and chemical properties and physiological functions. In recent years, with the continuous development of strategies such as synthetic biology, metabolic engineering, omics-based systems biology and high-throughput screening technology, people's understanding of the erythritol biosynthesis pathway has gradually deepened, and microbial cell factories with independent modification capabilities have been successfully constructed. In this review, the cheap feedstocks for erythritol synthesis are introduced in detail, the environmental factors affecting the synthesis of erythritol and its regulatory mechanism are described, and the tools and strategies of metabolic engineering involved in erythritol synthesis are summarized. In addition, the study of erythritol derivatives is helpful in expanding its application field. Finally, the challenges that hinder the effective production of erythritol are discussed, which lay a foundation for the green, efficient and sustainable production of erythritol in the future and breaking through the bottleneck of production.

Recent advances in biosynthesis mechanisms and yield enhancement strategies of erythritol

Erythritol is a four-carbon sugar alcohol naturally produced by microorganisms as an osmoprotectant. As a new sugar substitute, erythritol has recently been popular on the ingredient market because of its unique nutritional characteristics. Even though the history of erythritol biosynthesis dates from the turn of the twentieth century, scientific advancement has lagged behind other polyols due to the relative complexity of making it. In recent years, biosynthetic methods for erythritol have been rapidly developed due to an increase in market demand, a better understanding of metabolic pathways, and the rapid development of genetic engineering tools. This paper reviews the history of industrial strain development and focuses on the underlying mechanism of high erythritol production by strains gained through screening or mutagenesis. Meanwhile, we highlight the metabolic pathway knowledge of erythritol biosynthesis in microorganisms and summarize the metabolic engineering and research progress on critical genes involved in different stages of the synthetic pathway. Lastly, we talk about the still-contentious issues and promising future research directions that will help break the erythritol production bottleneck and make erythritol production greener and more sustainable.

Low-calorie bulk sweeteners: Recent advances in physical benefits, applications, and bioproduction

At present, with the continuous improvement of living standards, people are paying increasing attention to dietary nutrition and health. Low sugar and low energy consumption have become important dietary trends. In terms of sugar control, more and more countries have implemented sugar taxes in recent years. Hence, as the substitute for sugar, low-calorie sweeteners have been widely used in beverage, bakery, and confectionary industries. In general, low-calorie sweeteners consist of high-intensity and low-calorie bulk sweeteners (some rare sugars and sugar alcohols). In this review, recent advances and challenges in low-calorie bulk sweeteners are explored. Bioproduction of low-calorie bulk sweeteners has become the focus of many researches, because it has the potential to replace the current industrial scale production through chemical synthesis. A comprehensive summary of the physicochemical properties, physiological functions, applications, bioproduction, and regulation of typical low-calorie bulk sweeteners, such as D-allulose, D-tagatose, D-mannitol, sorbitol, and erythritol, is provided.

Multi-feedstock biorefinery concept: Valorization of winery wastes by engineered yeast

The wine industry produces significant amounts of by-products and residues that are not properly managed, posing an environmental problem. Grape must surplus, vine shoots, and wine lees have the potential to be used as renewable resources for the production of energy and chemicals. Metabolic engineering efforts have established Saccharomyces cerevisiae as an efficient microbial cell factory for biorefineries. Current biorefineries designed for producing multiple products often rely on just one feedstock, but the bioeconomy would clearly benefit if these biorefineries could efficiently convert multiple feedstocks. Moreover, to reduce the environmental impact of fossil fuel consumption and maximize production economics, a biorefinery should be capable to supplement the manufacture of biofuel with the production of high-value products. This study proposes an integrated approach for the valorization of diverse wastes resulting from winemaking processes through the biosynthesis of xylitol and ethanol. Using genetically modified S. cerevisiae strains, the xylose-rich hemicellulosic fraction of hydrothermally pretreated vine shoots was converted into xylitol, and the cellulosic fraction was used to produce bioethanol. In addition, grape must, enriched in sugars, was efficiently used as a low-cost source for yeast propagation. The production of xylitol was optimized, in a Simultaneous Saccharification and Fermentation process configuration, by adjusting the inoculum size and enzyme loading. Furthermore, a yeast strain displaying cellulases in the cell surface was applied for the production of bioethanol from the glucan-rich cellulosic. With the addition of grape must and/or wine lees, high ethanol concentrations were reached, which are crucial for the economic feasibility of distillation. This integrated multi-feedstock valorization provides a synergistic alternative for converting a range of winery wastes and by-products into biofuel and an added-value chemical while decreasing waste released to the environment.

Cell Immobilization for Erythritol Production

Nowadays, commercial erythritol synthesis is performed by free-cell fermentation with fungi in liquid media containing high concentrations of pure carbon sources. Alternative fermentation techniques, such as cell immobilization, could imply an economic and energetic improvement for erythritol-producing factories. The present work describes, for the first time, the feasibility of achieving cell immobilization during erythritol production. Cells of the fungus Moniliella pollinis were successfully immobilized on a cotton cloth which was placed inside a 2-L bioreactor, where they were fed with red grape must supplemented with yeast extract. They produced 47.03 ± 6.16 g/L erythritol in 96 h (yield 0.18 ± 0.04 g/g) over four consecutive fermentation batches. The immobilized cells remained stable and operative during a 456 h period. The erythritol concentration attained was similar (p > 0.05; Tukey HSD test) to the reference value obtained with the use of free cells (41.88 ± 5.18 g/L erythritol) under the same fermentation conditions. The comparable results observed for free and immobilized cells evidences the efficiency of the immobilization system. Therefore, the proposed method for erythritol bioproduction eliminates the need for the continuous preparation of fungal inocula before each fermentation batch, thus reducing the costs of the reagents and energy.

Use of carbohydrate-directed enzymes for the potential exploitation of sugarcane bagasse to obtain value-added biotechnological products

Microorganisms, such as fungi and bacteria, are crucial players in the production of enzymatic cocktails for biomass hydrolysis or the bioconversion of plant biomass into products with industrial relevance. The biotechnology industry can exploit lignocellulosic biomass for the production of high-value chemicals. The generation of biotechnological products from lignocellulosic feedstock presents several bottlenecks, including low efficiency of enzymatic hydrolysis, high cost of enzymes, and limitations on microbe metabolic performance. Genetic engineering offers a route for developing improved microbial strains for biotechnological applications in high-value product biosynthesis. Sugarcane bagasse, for example, is an agro-industrial waste that is abundantly produced in sugar and first-generation processing plants. Here, we review the potential conversion of its feedstock into relevant industrial products via microbial production and discuss the advances that have been made in improving strains for biotechnological applications.

Assessment of vine shoots and surplus grape must for succinic acid bioproduction

Vine shoots and surplus grape must were assessed as feedstocks for succinic acid production with Actinobacillus succinogenes and Basfia succiniproducens. After acidic and enzymatic hydrolysis, vine shoots released 35-40 g/L total sugars. Both bacterial species produced 18-21 g/L succinic acid from this hydrolysate in 120 h. Regarding grape must fermentation, A. succinogenes clearly outperformed B. succiniproducens. Yeast extract (a source of organic nitrogen and vitamins) was the only additional nutrient needed by A. succinogenes to grow on grape must. Under mathematically optimized conditions (145.7 g/L initial sugars and 24.9 g/L yeast extract), A. succinogenes generated 88.9 ± 1.4 g/L succinic acid in 96 h, reaching a succinic acid yield of 0.66 ± 0.01 g/g and a sugar consumption of 96.64 ± 0.30%. Substrate inhibition was not observed in grape musts with 125-150 g/L initial sugars, provided that an adequate amount of yeast extract was available for bacteria. Alternative nitrogen sources to yeast extract (red wine lees, white wine lees, urea, NH₄Cl, and choline chloride) were not suitable for A. succinogenes in grape must. KEY POINTS: • Vine shoots and surplus grape must were assessed for succinic acid bioproduction. • Succinic acid bioproduction was 21 g/L with vine shoots and 89 g/L with grape must. • Fermentation was efficient at high sugar loads if organic N supply was adequate.

One-pot fermentation for erythritol production from distillers grains by the co-cultivation of Yarrowia lipolytica and Trichoderma reesei

A co-fermentation process involving Yarrowia lipolytica and Trichoderma reesei was studied, using distillers grains (DGS) as feedstocks for erythritol production. DGS can be effectively hydrolyzed by cellulase in the single-strain culture of T. reesei. One-pot solid state fermentation for erythritol production was then established by co-cultivating Y. lipolytica M53-S with the 12 h delay inoculated T. reesei Rut C-30, in which efficient saccharification of DGS and improved production of erythritol were simultaneously achieved. The 10:1 inoculation proportion of Y. lipolytica and T. reesei contributed to the maximum erythritol production of 267.1 mg/gds under the optimal conditions including initial moisture of 55%, pH of 5.0, NaCl addition of 0.02 g/gds and DGS mass of 200 g in 144 h co-cultivation. Being compared with the attempts to produce erythritol from other raw materials, the one-pot SSF with DGS is proposed to be a potential strategy for efficient and economical erythritol production.

Utilizing yeasts for the conversion of renewable feedstocks to sugar alcohols - a review

Sugar alcohols are widely marketed compounds. They are useful building block chemicals and of particular value as low- or non-calorigenic sweeteners, serving as sugar substitutes in the food industry. To date most sugar alcohols are produced by chemical routes using pure sugars, but a transition towards the use of renewable, non-edible feedstocks is anticipated. Several yeasts are naturally able to convert renewable feedstocks, such as lignocellulosic substrates, glycerol and molasses, into sugar alcohols. These bioconversions often face difficulties to obtain sufficiently high yields and productivities necessary for industrialization. This review provides insight into the most recent studies on utilizing yeasts for the conversion of renewable feedstocks to diverse sugar alcohols, including xylitol, erythritol, mannitol and arabitol. Moreover, metabolic approaches are highlighted that specifically target shortcomings of sugar alcohol production by yeasts from these renewable substrates.

From the culture broth to the erythritol crystals: an opportunity for circular economy

Abstract

The reduction of sugar intake by adults has been stated by the World Health Organization as an important strategy to reduce the risk of non-communicable diseases. Erythritol is a four-carbon sugar alcohol that is considered as a highly suitable substitution for sucrose. This review article covers approaches for the separate stages of the biotechnological production of erythritol from cultivation to the downstream section. The first part focuses on the cultivation stage and compares the yields of erythritol and arising by-products achieved with different types of substrates (commercial versus alternative ones). The reported numbers obtained with the most prominently used microorganisms in different cultivation methods (batch, fed-batch or continuous) are presented. The second part focuses on the downstream section and covers the applied technologies for cell removal, recovery, purification and concentration of erythritol crystals, namely centrifugation, membrane separation, ion and preparative chromatography, crystallization and drying. The final composition of the culture broth and the preparative chromatography separation performance were identified as critical points in the production of a high-purity erythritol fraction with a minimum amount of losses. During the review, the challenges for a biotechnological production of erythritol in a circular economy context are discussed, in particular regarding the usage of sustainable resources and minimizing waste streams.

Key points

• Substitution of sucrose by erythritol can be a step towards a healthier society

• Biotechnological production of erythritol should follow a circular economy concept

• Culture broth composition and preparative chromatography are keys for downstreaming

• Substrate, mother liquor and nutrients are challenges for circular economy