Technical difficulties and solutions of direct transesterification process of microbial oil for biodiesel synthesis

A review of lipid accumulation by oleaginous yeasts: Culture mode

Microbial lipids have attracted considerable interest owing to their favorable environmental sustainability benefits. In laboratory-scale studies, the factors impacting lipid production in oleaginous yeasts, including culture conditions, nutrients, and low-cost substrates, have been extensively studied. However, there were several different modes of microbial lipid cultivation (batch culture, fed-batch culture, continuous culture, and other novel culture modes), making it difficult to comprehensively analyze impacting factors under different cultivation modes on a laboratory scale. And only few cases of microbial lipid production have been conducted at the pilot scale, which requires more technological reliability assessments and environmental benefit evaluations. Thus, this study summarized the different culture modes and cases of scale-up processes, highlighting the role of the nutrient element ratio in regulating culture mode selection and lipid accumulation. The cost distribution and environmental benefits of microbial lipid production by oleaginous yeasts were also investigated. Our results suggested that the continuous culture mode was recommended for the scale-up process because of its stable lipid accumulation. More importantly, exploring the continuous culture mode integrated with other efficient culture modes remained to be further investigated. In research on scale-up processes, low-cost substrate (organic waste) application and optimization of reactor operational parameters were key to increasing environmental benefits and reducing costs.

Neochloris oleoabundans cell wall rupture through melittin peptide: a new approach to increase lipid recovery

OBJECTIVES

Microalgae cell wall affects the recovery of lipids, representing one of the main difficulties in the development of biofuel production. This work aimed to test a new method based on melittin peptide to induce a cellular disruption in N. oleoabundans.

RESULTS

Neochloris oleoabundans cells were grown at 32 °C in the presence of a high concentration of nitrate-phosphate, causing a cell disruption extent of 83.6%. Further, a two-fold increase in lipid recovery following melittin treatment and solvent extraction was observed. Additionally, it was possible to verify the effects of melittin, both before and after treatment on the morphology of the cells. Scanning electron microscopy (SEM) and confocal images of the melittin-treated microalgae revealed extensive cell damage with degradation of the cell wall and release of intracellular material.

CONCLUSIONS

Melittin produced a selective cell wall rupture effect in N. oleoabundans under some culture conditions. These results represent the first report on the effect of melittin on lipid recovery from microalgae.

Evaluation of cell disruption methods in the oleaginous yeasts Yarrowia lipolytica QU21 and Meyerozyma guilliermondii BI281A for microbial oil extraction

The interest for oleaginous yeasts has grown significantly in the last three decades, mainly due to their potential use as a renewable source of microbial oil or single cell oils (SCOs). However, the methodologies for cell disruption to obtain the microbial oil are considered critical and determinant for a large-scale production. Therefore, this work aimed to evaluate different methods for cell wall disruption for the lipid extraction of Yarrowia lipolytica QU21 and Meyerozyma guilliermondii BI281A. The two strains were separately cultivated in 5 L batch fermenters for 120 hours, at 26 ºC and 400 rpm. Three different lipid extraction processes using Turrax homogenizer, Ultrasonicator and Braun homogenizer combined with bead milling were applied in wet, oven-dried, and freeze-dried biomass of both strains. The treatment with the highest percentage of disrupted cells and highest oil yield was the ultrasonication of oven-dried biomass (37-40% lipid content for both strains). The fact that our results point to one best extraction strategy for two different yeast strains, belonging to different species, is a great news towards the development of a unified technique that could be applied at industrial plants.

Sustainable Production of Biodiesel Using UV Mutagenesis as a Strategy to Enhance the Lipid Productivity in R. mucilaginosa

The future of petroleum-based fuel is biodiesel. Biodiesel is an eco-friendly fuel that can be used in any diesel engine without any alterations. Researchers have focused on biodiesel that can be produced from microbial lipids extracted from high lipid-yielding microbes. In this study, microbial cultures were screened for high lipid-yielding capabilities and mutated using UV radiation at three different time intervals of 30, 75, and 90 min. The Nile red fluorescence method was used to analyze high lipid-yielding microbes. An outstanding increase in biomass and lipid productivity was noted when the microbes were exposed to UV for 30 min. For example, an M30-8 UV-mutated strain produced a lipid yield of 68.5%. The lipids produced from the wild and mutated strains were analyzed using GCMS and FTIR spectrophotometric analysis. Then, the lipids extracted from both wild VS3 and UV-mutated M30-8 strains were transesterified using a base catalyst and the produced biodiesel was analyzed using ASTM standards. The aim and objective of the research was to mutate high lipid-yielding microbes by using UV radiation and produce biodiesel from the lipids extracted from both wild and UV-mutated strains.

Using techno-economic modelling to determine the minimum cost possible for a microbial palm oil substitute

BACKGROUND

Heterotrophic single-cell oils (SCOs) are one potential replacement to lipid-derived biofuels sourced from first-generation crops such as palm oil. However, despite a large experimental research effort in this area, there are only a handful of techno-economic modelling publications. As such, there is little understanding of whether SCOs are, or could ever be, a potential competitive replacement. To help address this question, we designed a detailed model that coupled a hypothetical heterotroph (using the very best possible biological lipid production) with the largest and most efficient chemical plant design possible.

RESULTS

Our base case gave a lipid selling price of $1.81/kg for ~ 8,000 tonnes/year production, that could be reduced to $1.20/kg on increasing production to ~ 48,000 tonnes of lipid a year. A range of scenarios to further reduce this cost were then assessed, including using a thermotolerant strain (reducing the cost from $1.20 to $1.15/kg), zero-cost electricity ($ 1.12/kg), using non-sterile conditions ($1.19/kg), wet extraction of lipids ($1.16/kg), continuous production of extracellular lipid ($0.99/kg) and selling the whole yeast cell, including recovering value for the protein and carbohydrate ($0.81/kg). If co-products were produced alongside the lipid then the price could be effectively reduced to $0, depending on the amount of carbon funnelled away from lipid production, as long as the co-product could be sold in excess of $1/kg.

CONCLUSIONS

The model presented here represents an ideal case that which while not achievable in reality, importantly would not be able to be improved on, irrespective of the scientific advances in this area. From the scenarios explored, it is possible to produce lower cost SCOs, but research must start to be applied in three key areas, firstly designing products where the whole cell is used. Secondly, further work on the product systems that produce lipids extracellularly in a continuous processing methodology or finally that create an effective biorefinery designed to produce a low molecular weight, bulk chemical, alongside the lipid. All other research areas will only ever give incremental gains rather than leading towards an economically competitive, sustainable, microbial oil.

Evaluation and optimisation of direct transesterification methods for the assessment of lipid accumulation in oleaginous filamentous fungi

Background

Oleaginous filamentous fungi can accumulate large amount of cellular lipids and potentially serve as a major source of oleochemicals for food, feed, chemical, pharmaceutical, and transport industries. Transesterification of microbial oils is an essential step in microbial lipid production at both laboratory and industrial scale. Direct transesterification can considerably reduce costs, increase sample throughput and improve lipid yields (in particular fatty acid methyl esters, FAMEs). There is a need for the assessment of the direct transesterification methods on a biomass of filamentous fungi due to their unique properties, specifically resilient cell wall and wide range of lipid content and composition. In this study we have evaluated and optimised three common direct transesterification methods and assessed their suitability for processing of fungal biomass.

Results

The methods, based on hydrochloric acid (Lewis method), sulphuric acid (Wahlen method), and acetyl chloride (Lepage method), were evaluated on six different strains of Mucoromycota fungi by using different internal standards for gas chromatography measurements. Moreover, Fourier transform infrared (FTIR) spectroscopy was used for the detection of residual lipids in the biomass after the transesterification reaction/extraction, while transesterification efficiency was evaluated by nuclear magnetic resonance spectroscopy. The results show that the majority of lipids, in particular triglycerides, were extracted for all methods, though several methods had substandard transesterification yields. Lewis method, optimised with respect to solvent to co-solvent ratio and reaction time, as well as Lepage method, offer precise estimate of FAME-based lipids in fungal biomass.

Conclusions

The results show that Lepage and Lewis methods are suitable for lipid analysis of oleaginous filamentous fungi. The significant difference in lipid yields results, obtained by optimised and standard Lewis methods, indicates that some of the previously reported lipid yields for oleaginous filamentous fungi must be corrected upwards. The study demonstrates value of biomass monitoring by FTIR, importance of optimal solvent to co-solvent ratio, as well as careful selection and implementation of internal standards for gas chromatography.

Microwave-Assisted One-Pot Lipid Extraction and Glycolipid Production from Oleaginous Yeast Saitozyma podzolica in Sugar Alcohol-Based Media

Glycolipids are non-ionic surfactants occurring in numerous products of daily life. Due to their surface-activity, emulsifying properties, and foaming abilities, they can be applied in food, cosmetics, and pharmaceuticals. Enzymatic synthesis of glycolipids based on carbohydrates and free fatty acids or esters is often catalyzed using certain acyltransferases in reaction media of low water activity, e.g., organic solvents or notably Deep Eutectic Systems (DESs). Existing reports describing integrated processes for glycolipid production from renewables use many reaction steps, therefore this study aims at simplifying the procedure. By using microwave dielectric heating, DESs preparation was first accelerated considerably. A comparative study revealed a preparation time on average 16-fold faster than the conventional heating method in an incubator. Furthermore, lipids from robust oleaginous yeast biomass were successfully extracted up to 70% without using the pre-treatment method for cell disruption, limiting logically the energy input necessary for such process. Acidified DESs consisting of either xylitol or sorbitol and choline chloride mediated the one-pot process, allowing subsequent conversion of the lipids into mono-acylated palmitate, oleate, linoleate, and stearate sugar alcohol esters. Thus, we show strong evidence that addition of immobilized Candida antarctica Lipase B (Novozym 435^®), in acidified DES mixture, enables a simplified and fast glycolipid synthesis using directly oleaginous yeast biomass.

Surfactant-assisted in situ transesterification of wet Rhodotorula glutinis biomass

In situ transesterification of oleaginous microbes with short chain alcohol has been developed as a renewable process for the production of biodiesel. Dry biomass is often a requisite for the process to avoid the adverse effect of water on the productivity. As a consequence, large amount of energy consumption is required for prior biomass drying. In this study, the wet biomass of Rhodotorula glutinis, an oleaginous yeast, was used directly in in situ transesterification without biomass drying. The reaction conditions were optimized for the production of fatty acid methyl esters (FAME) and the effects of adding different surfactants were also studied. The highest FAME yield of 110% was achieved with a methanol loading of 1:100 at 90°C for 8 h as catalyzed by 0.36 M H₂SO₄, and the FAME content was 97%, which meets the 96.5% specified in both European biodiesel standards and Taiwanese biodiesel standards. The addition of 50 mM 3-(N,N-dimethylmyristylammonio)propanesulfonate (3-DMAPS, a zwitterionic surfactant) improved the FAME yield from 69% to 83%, which was obtained with a low methanol loading of 1:10 at 90°C for 10 h. Hence, the production of FAME with wet biomass under optimized reaction conditions was as effective as that with the dry form. This clearly indicates that using wet R. glutinis as the feedstock is feasible for the production of biodiesel by in situ transesterification.

Evaluation of Downstream Processing, Extraction, and Quantification Strategies for Single Cell Oil Produced by the Oleaginous Yeasts Saitozyma podzolica DSM 27192 and Apiotrichum porosum DSM 27194

Single cell oil (SCO) produced by oleaginous yeasts is considered as a sustainable source for biodiesel and oleochemicals since its production does not compete with food or feed and high yields can be obtained from a wide variety of carbon sources, e.g., acetate or lignocellulose. Downstream processing is still costly preventing the broader application of SCO. Direct transesterification of freeze-dried biomass is widely used for analytical purposes and for biodiesel production but it is energy intensive and, therefore, expensive. Additionally, only fatty acid esters are produced limiting the subsequent applications. The harsh conditions applied during direct esterification might also damage high-value polyunsaturated fatty acids. Unfortunately, universal downstream strategies effective for all yeast species do not exist and methods have to be developed for each yeast species due to differences in cell wall composition. Therefore, the aim of this study was to evaluate three industrially relevant cell disruption methods combined with three extraction systems for the SCO extraction of two novel, unconventional oleaginous yeasts, Saitozyma podzolica DSM 27192 and Apiotrichum porosum DSM 27194, based on cell disruption efficiency, lipid yield, and oil quality. Bead milling (BM) and high pressure homogenization (HPH) were effective cell disruption methods in contrast to sonification. By combining HPH (95% cell disruption efficiency) with ethanol-hexane-extraction 46.9 ± 4.4% lipid/CDW of S. podzolica were obtained which was 2.7 times higher than with the least suitable combination (ultrasound + Folch). A. porosum was less affected by cell disruption attempts. Here, the highest disruption efficiency was 74% after BM and the most efficient lipid recovery method was direct acidic transesterification (27.2 ± 0.5% fatty acid methyl esters/CDW) after freeze drying. The study clearly indicates cell disruption is the decisive step for SCO extraction. At disruption efficiencies of >90%, lipids can be extracted at high yields, whereas at lower cell disruption efficiencies, considerable amounts of lipids will not be accessible for extraction regardless of the solvents used. Furthermore, it was shown that hexane-ethanol which is commonly used for extraction of algal lipids is also highly efficient for yeasts.

Turbo thin film continuous flow production of biodiesel from fungal biomass

Direct biodiesel production from wet fungal biomass may significantly reduce production costs, but there is a lack of fast and cost-effective processing technology. A novel thin film continuous flow process has been applied to study the effects of its operational parameters on fatty acid (FA) extraction and FA to fatty acid methyl ester (FAME) conversion efficiencies. Single factor experiments evaluated the effects of catalyst concentration and water content of biomass, while factorial experimental designs determined the interactions between catalyst concentration and biomass to methanol ratio, flow rate, and rotational speed. Direct transesterification (DT) of wet Mucor plumbeus biomass at ambient temperature and pressure achieved a FA to FAME conversion efficiency of >90% using 3 wt/v % NaOH concentration, if the water content was ≤50% (w/w). In comparison to existing DT methods, this continuous flow processing technology has an estimated 90-94% reduction in energy consumption, showing promise for up-scaling.

Optimization of co-culture inoculated microbial fuel cell performance using response surface methodology

Microbial fuel cells (MFCs) are considered as promising technology to achieve simultaneous wastewater treatment and electricity generation. However, operational and technological developments are still required to make it as a sustainable technology. In the present study, response surface methodology (RSM) was used to evaluate the effects of substrate concentration, co-culture composition, pH and time on the performance of co-culture (Klebsiella variicola and Pseudomonas aeruginosa) inoculated double chamber MFC. From the statistical analysis, it can be seen that the performance of MFC was not influenced by the interaction between the initial COD and time, pH and time, pH and initial COD, time and initial COD. However, the interaction between the inoculum composition and time, pH and the inoculum composition, initial COD and inoculum composition significantly influenced the performance of MFC. Based on the RSM results, best performance (power density and COD removal efficiency) was obtained when the inoculum composition, initial COD, pH and time were about 1:1, 26.690 mg/L, 7.21 and 15.50 days, respectively. The predictions from the model were in close agreement with the experimental results suggesting that the proposed model could adequately represent the actual relationships between the independent variables generating electricity and the COD removal efficiency.

Recent developments of downstream processing for microbial lipids and conversion to biodiesel

With increasing global population and depleting resources, there is an apparent demand for radical unprecedented innovation to satisfy the basal needs of lives. Hence, non-conventional renewable energy resources like biodiesel have been worked out in past few decades. Biofuel (e.g. Biodiesel) serves to be the most sustainable answer to solve "food vs. fuel crisis". In biorefinery process, lipid extraction from oleaginous microbial lipids is an integral part as it facilitates the release of fatty acids. Direct lipid extraction from wet cell-biomass is favorable in comparison to dry-cell biomass because it eliminates the application of expensive dehydration. However, this process is not commercialized yet, instead, it requires intensive research and development in order to establish robust approaches for lipid extraction that can be practically applied on an industrial scale. This review aims for the critical presentation on cell disruption, lipid recovery and purification to support extraction from wet cell-biomass for an efficient transesterification.

Identification and Characterization of Diacylglycerol Acyltransferase from Oleaginous Fungus Mucor circinelloides

Acyl-CoA:diacylglycerol acyltransferase (DGAT) is a pivotal regulator of triacylglycerol (TAG) synthesis. The oleaginous fungus Mucor circinelloides has four putative DGATs: McDGAT1A, McDGAT1B, McDGAT2A, and McDGAT2B, classified into the DGAT1 and DGAT2 subfamilies, respectively. To identify and characterize DGATs in M. circinelloides, these four genes were expressed in Saccharomyces cerevisiae H1246 (TAG-deficient quadruple mutant), individually. TAG biosynthesis was restored only by the expression of McDGAT2B, and TAG content was significantly higher in the mutants with McDGAT2B expression than in a S. cerevisiae mutant with endogenous DGA1 expression. McDGAT2B prefers saturated fatty acids to monounsaturated fatty acids and has an obvious preference for C18:3 (ω-6) according to the results of substrate preference experiments. Furthermore, only the mRNA expression pattern of McDGAT2B correlated with TAG biosynthesis during a fermentation process. Our experiments strongly indicate that McDGAT2B is crucial for TAG accumulation, suggesting that it may be an essential target for metabolic engineering aimed at increasing lipid content of M. circinelloides.