Increases in biodiesel production have led to a surplus of crude glycerol, which represents a major bottleneck in the biodiesel production chain and has created new challenges to its sustainable use. Although there is a wide range of potential uses for crude glycerol, they are limited by its degree of purity, which affects its physical, chemical and biological properties. This paper presents a review and analysis of patents related to the use of glycerol in the period from 1993 to 2015, showing the quantity and diversity of work related to the search for alternatives to add value to glycerol. It was found that 85.7% of the filed patents fell into four categories of applications: 1) manufacture of chemical products; 2) production of polymer compounds; 3) production of biofuels and biogas; and 4) purification and use of glycerol. In this regard, the development of new technologies for the use of glycerol, without the need for traditional purification steps, are fundamental. This paper portrays the efforts that have been made in this direction and the obstacles that still have to be overcome.

Glycerol from biodiesel production: technological paths for sustainability.

Unconventional and nonpathogenic Yarrowia lipolytica yeast has been addressed in various studies conducted in many research centers, and in recent years has been perceived as an especially attractive host for many applications of glycerol. In its initial paragraphs, this review article provides a short characteristics of Y. lipolytica ; followed by biodiesel production and brief characteristics of crude glycerol. Further on, this review summarizes relevant scientific research concerning the conversion of crude glycerol discharged after bio-diesel (fatty acid methyl/ethyl esters) manufacturing process into value-added products through biological methods with Y. lipolytica yeast. The feasibility of using Y. lipolytica biomass, rich in proteins and oils, as food and feed additives is described as well. Subsequently, different strategies employed to produce and improve yield and productivity of organic acids (citric, pyruvic and α-ketoglutaric acid) are presented. And, finally, the biosynthesis of new products, such as erythritol, mannitol and invertase, whose synthesis from glycerol by Y. lipolytica would be advantageous when compared with their production from common sugars, is evaluated. In conclusion, an actual wide range of compounds that can be produced from glycerol by Y. lipolytica are shown to be a valuable contribution to the development of the biodiesel industry as well as a cost-effective fermentation based on renewable resources.

Glycerol as a promising substrate for Yarrowia lipolytica biotechnological applications

Treatment of biowaste, the predominant waste fraction in low- and middle-income settings, offers public health, environmental and economic benefits by converting waste into a hygienic product, diverting it from disposal sites, and providing a source of income. This article presents a comprehensive overview of 13 biowaste treatment technologies, grouped into four categories: (1) direct use (direct land application, direct animal feed, direct combustion), (2) biological treatment (composting, vermicomposting, black soldier fly treatment, anaerobic digestion, fermentation), (3) physico-chemical treatment (transesterification, densification), and (4) thermo-chemical treatment (pyrolysis, liquefaction, gasification). Based on a literature review and expert consultation, the main feedstock requirements, process conditions and treatment products are summarized, and the challenges and trends, particularly regarding the applicability of each technology in the urban low- and middle-income context, are critically discussed. An analysis of the scientific articles published from 2005 to 2015 reveals substantial differences in the amount and type of research published for each technology, a fact that can partly be explained with the development stage of the technologies. Overall, publications from case studies and field research seem disproportionately underrepresented for all technologies. One may argue that this reflects the main task of researchers—to conduct fundamental research for enhanced process understanding—but it may also be a result of the traditional embedding of the waste sector in the discipline of engineering science, where socio-economic and management aspects are seldom object of the research. More unbiased, well-structured and reproducible evidence from case studies at scale could foster the knowledge transfer to practitioners and enhance the exchange between academia, policy and practice.

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Treatment technologies for urban solid biowaste to create value products: a review with focus on low- and middle-income settings

The exponential growth of biodiesel industries all around the world has produced a large amount of glycerol as a byproduct, which must be valorized for the sustainability of the biodiesel industry. Ketalization of glycerol with acetone to synthesize solketal-a potential fuel additive is one of the most promising routes for valorization of glycerol. In this article, state-of-the-art of glycerol ketalization is reviewed, focusing on innovative and potential technologies towards sustainable production of solketal. The glycerol ketalization processes developed in both batch and continuous reactors and performance of some typical catalysts are compared. The mechanisms for the acid-catalyzed conversion of glycerol into solketal are presented. The main operation issues related to catalytic conversion of crude glycerol in a continuous-flow process and the direct use of crude glycerol are discussed.

Catalytic conversion of glycerol for sustainable production of solketal as a fuel additive: A review

There is huge variability among yeasts with regard to their efficiency in utilizing glycerol as the sole source of carbon and energy. Certain species show growth rates with glycerol comparable to those reached with glucose as carbon source; others are virtually unable to utilize glycerol, especially in synthetic medium. Most of our current knowledge regarding glycerol uptake and its catabolic pathways has been gained from studying laboratory strains of the model yeast Saccharomyces cerevisiae. The growth of these strains on glycerol is dependent on the presence of medium supplements such as amino acids and nucleobases. In contrast, there is only fragmentary knowledge about S. cerevisiae isolates able to grow in synthetic glycerol medium without such supplements as well as about growth of non-Saccharomyces yeast species on glycerol. Thus, more research is required to understand why certain strains and species show superior growth performance on glycerol compared to common S. cerevisiae laboratory strains. This minireview summarizes what is known so far about the gene products and pathways involved in glycerol metabolism and transport in yeast and fungi as well as the regulation of these processes. This article is protected by copyright. All rights reserved.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/1462-2920.13617

Glycerol metabolism and transport in yeast and fungi: established knowledge and ambiguities.

Bioconversion of crude glycerol feedstocks into ethanol by Pachysolen tannophilus.

Pachysolen tannophilus is a non-conventional yeast, which can metabolize many of the carbon sources found in low cost feedstocks including glycerol and xylose. The xylose utilisation pathways have been extensively studied in this organism. However, the mechanism behind glycerol metabolism is poorly understood. Using the recently published genome sequence of P. tannophilus CBS4044, we searched for genes with functions in glycerol transport and metabolism by performing a BLAST search using the sequences of the relevant genes from Saccharomyces cerevisiae as queries. Quantitative real-time PCR was performed to unveil the expression patterns of these genes during growth of P. tannophilus on glycerol and glucose as sole carbon sources. The genes predicted to be involved in glycerol transport in P. tannophilus were expressed in S. cerevisiae to validate their function. The S. cerevisiae strains transformed with heterologous genes showed improved growth and glycerol consumption rates with glycerol as the sole carbon source. P. tannophilus has characteristics relevant for a microbial cell factory to be applied in a biorefinery setting, i.e. its ability to utilise the carbon sources such as xylose and glycerol. However, the strain is not currently amenable to genetic modification and transformation. Heterologous expression of the glycerol transporters from P. tannophilus, which has a relatively high growth rate on glycerol, could be used as an approach for improving the efficiency of glycerol assimilation in other well characterized and applied cell factories such as S. cerevisiae.

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Expression and functional studies of genes involved in transport and metabolism of glycerol in Pachysolen tannophilus.

Summary Plant‐derived Vitamin C (l‐ascorbic acid (AsA)) is crucial for human health and wellbeing and thus increasing AsA content is of interest to plant breeders. In plants GDP‐l‐galactose phosphorylase (GGP) is a key biosynthetic control step and here evidence is presented for two new transcriptional activators of GGP. AsA measurement, transcriptomics, transient expression, hormone application, gene editing, yeast 1/2‐hybrid, and electromobility shift assay (EMSA) methods were used to identify two positively regulating transcription factors. AceGGP3 was identified as the most highly expressed GGP in Actinidia eriantha fruit, which has high fruit AsA. A gene encoding a 1R‐subtype myeloblastosis (MYB) protein, AceMYBS1, was found to bind the AceGGP3 promoter and activate its expression. Overexpression and gene‐editing show AceMYBS1 effectively increases AsA accumulation. The bZIP transcription factor AceGBF3 (a G‐box binding factor), also was shown to increase AsA content, and was confirmed to interact with AceMYBS1. Co‐expression experiments showed that AceMYBS1 and AceGBF3 additively promoted AceGGP3 expression. Furthermore, AceMYBS1, but not GBF3, was repressed by abscisic acid, resulting in reduced AceGGP3 expression and accumulation of AsA. This study sheds new light on the roles of MYBS1 homologues and ABA in modulating AsA synthesis, and adds to the understanding of mechanisms underlying AsA accumulation.

Kiwifruit MYBS1‐like and GBF3 transcription factors influence l‐ascorbic acid biosynthesis by activating transcription of GDP‐L‐galactose phosphorylase 3

A draft genome sequence of the yeast Pachysolen tannophilus CBS 4044/NRRL Y-2460 is presented. The organism has the potential to be developed as a cell factory for biorefineries due to its ability to utilize waste feedstocks. The sequenced genome size was 12,238,196 bp, consisting of 34 scaffolds. A total of 4,463 genes from 5,346 predicted open reading frames were annotated with function.

Draft Genome Sequence of the Yeast Pachysolen tannophilus CBS 4044/NRRL Y-2460

The object of this work was to improve the temperature and salt resistance of filtrate reducers used in solid-free water-based drilling fluids at 200°C. This copolymer (PAD) was obtained by reacting pentaerythritol triallyl ether (PTE) with acrylamide AM, N-isopropyl acrylamide (N-AM), methacrylic acid (MAA), and 2-acrylamide-2-methylpropanesulfonic acid sodium (AMPS-Na) in radical polymerization with potassium peroxydisulfate (KPS) and sodium bisulfite as initiators. The molar ratio of each monomer shown as follow: nAm:nN−Am:nMAA:nAmps: = 1:1.6:0.9:1.5. The functional groups of each monomer were revealed by Fourier transformation infrared. Moreover, 13C nuclear magnetic resonance confirmed the PAD was synthesized by addition polymerization through these monomers. Scanning electron microscopy showed that the surface of the filter cake was smooth and dense, indicating that PAD can effectively block small pores and thus reduce filter loss. The analysis suggested the PAD produced a “ball” structure at high temperature and salt which controlled the filtration of drilling fluid. The maximum temperature resistance boundary of PAD was 210°C. PAD maintained filtration performance in nearly saturated sodium chloride, 1.4 g/cm3 HCOOK and 1.37 g/cm3 HCOONa, at 210°C for 16 h. 

Xiaoying Liu

Papers

Bioconversion of crude glycerol feedstocks into ethanol by Pachysolen tannophilus.

Expression and functional studies of genes involved in transport and metabolism of glycerol in Pachysolen tannophilus.

Kiwifruit MYBS1‐like and GBF3 transcription factors influence l‐ascorbic acid biosynthesis by activating transcription of GDP‐L‐galactose phosphorylase 3

Draft Genome Sequence of the Yeast Pachysolen tannophilus CBS 4044/NRRL Y-2460

Preparation and evaluation of high‐temperature filtrate reducer for solid‐free water‐based drilling fluid