Extractive Fermentation Processes: Modes of Operation and Application

Biotechnological 2-Phenylethanol Production: Recent Developments

2-Phenylethanol (2-PE) is a key flavor compound with a rose-like scent, used in the cosmetics, perfume, home care and food industries. This aroma compound can be obtained naturally from various flowers, however chemical synthesis is the most used route to meet market demand. The increasing interest in natural products has led to the development of more environmentally friendly alternatives for 2-PE production through biotechnological approaches. The most efficient approach involves the biotransformation of L-phenylalanine into 2-PE via the Ehrlich pathway, a process observed in different microorganisms such as yeasts and bacteria. 2-PE produced by this way can be considered as natural. However, due to the toxicity of the aroma to the producing microorganism, low production yields are typically obtained, motivating efforts to develop production processes that can overcome this bottleneck, enhance 2-PE yields and reduce the production costs. This review presents and discusses the latest advances in the bioproduction of 2-PE through microbial fermentation, in terms of producing strains, the optimization of cultivation processes, strategies to mitigate product toxicity, and the use of low value feedstocks. Novel applications for 2-PE are also highlighted.

Enhancement of paclitaxel production by Neopestalotiopsis vitis via optimization of growth conditions

Accessibility of paclitaxel and other taxoids from natural resources is restricted. Endophytic fungi are novel, rapidly growing resources for producing these compounds. Neopestalotiopsis vitis (N. vitis) has been recently isolated from Corylus avellana, and its ability to produce a variety of taxoids has been detected and confirmed by analytical methods. Simultaneous growth and high production of taxoids by application of different sorts and concentrations of carbon and nitrogen were targeted in the present research. These criteria were assessed in different acidities (pH 4.0-7.0), carbon sources (sucrose, fructose, glucose, mannitol, sorbitol, and malt extract), and nitrogen forms (urea, ammonium nitrate, potassium nitrate, ammonium phosphate, and ammonium sulfate) by testing one parameter at a time approach. The first analysis introduced pH 7.0 as the best acidity of the medium for N. vitis, where the highest paclitaxel yield was generated. Further analysis introduced 3% Malt extract as the best carbon-providing medium. In the next step, the effects of nitrogen forms on the growth rate, paclitaxel yield, alkaloids, and amino acid contents were evaluated. Based on the results of this experiment, 5 mM ammonium sulfate was selected as the best nitrogen source to obtain the maximum biomass and paclitaxel yield. Overall, the results introduce a medium containing 3% (w/v) malt extract and 5 mM ammonium sulfate at pH 7.0 as the best medium in which N. vitis produces the highest paclitaxel yield coincident with rapid and sustainable growth. The findings pave the way for industrial manufacturing of taxoids.

Comprehensive comparative study on n-caproate production by Clostridium kluyveri: batch vs. continuous operation modes

Recently, there has been notable interest in researching and industrially producing medium-chain carboxylic acids (MCCAs) like n-caproate and n-caprylate via chain elongation process. This study presents a comprehensive assessment of the behavior and MCCA production profiles of Clostridium kluyveri in batch and continuous modes, at different ethanol:acetate molar ratios (1.5:1, 3.5:1 and 5.5:1). The highest n-caproate concentration, 12.9 ± 0.67 g/L (92.9 ± 1.39 % MCCA selectivity), was achieved in batch mode at a 3.5:1 ratio. Interestingly, higher ratios favored batch mode selectivity over continuous mode when this was equal or higher to 3.5:1. Steady state operation yielded the highest n-caproate (9.5 ± 0.13 g/L) and n-caprylate (0.35 ± 0.020 g/L) concentrations at the 3.5:1 ratio. Increased ethanol:acetate ratios led to a higher excessive ethanol oxidation (EEO) in both operational modes, potentially limiting n-caproate production and selectivity, especially at the 5.5:1 ratio. Overall, this study reports the efficient MCCA production of both batch and continuous modes by C. kluyveri.

A review on the upstream production and downstream purification of mannosylerythritol lipids

Biosurfactants are natural compounds with remarkable surface-active properties that may offer an eco-friendly alternative to conventional surfactants. Among them, mannosylerythritol lipids (MELs) stand out as an intriguing example of a glycolipid biosurfactant. MELs have been used in a variety of sectors for various applications, and are currently commercially produced. Industrially, they are used in the pharmaceutical, cosmetic, food, and agricultural industries, based on their ability to reduce surface tension and enhance emulsification. However, despite their utility, their production is comparatively limited industrially. From a bioprocessing standpoint, two areas of interest to improve the production process are upstream production and downstream (separation and purification) product recovery. The former has seen a significant amount of research, with researchers investigating several production factors: the microbial species or strain employed, the producing media composition, and the production strategy implemented. Improvement and optimization of these are key to scale-up the production of MELs. On the other hand, the latter has seen comparatively limited work presented in the literature. For the most part traditional separation techniques have been employed. This systematic review presents the production and purification methodologies used by researchers by comprehensively analyzing the current state-of-the-art with regards the production, separation, and purification of MELs. By doing so, the review presents different possible approaches, and highlights some potential areas for future work by identifying opportunities for the commercialization of MELs.

Towards industrial biological hydrogen production: a review

Increased production of renewable energy sources is becoming increasingly needed. Amidst other strategies, one promising technology that could help achieve this goal is biological hydrogen production. This technology uses micro-organisms to convert organic matter into hydrogen gas, a clean and versatile fuel that can be used in a wide range of applications. While biohydrogen production is in its early stages, several challenges must be addressed for biological hydrogen production to become a viable commercial solution. From an experimental perspective, the need to improve the efficiency of hydrogen production, the optimization strategy of the microbial consortia, and the reduction in costs associated with the process is still required. From a scale-up perspective, novel strategies (such as modelling and experimental validation) need to be discussed to facilitate this hydrogen production process. Hence, this review considers hydrogen production, not within the framework of a particular production method or technique, but rather outlines the work (bioreactor modes and configurations, modelling, and techno-economic and life cycle assessment) that has been done in the field as a whole. This type of analysis allows for the abstraction of the biohydrogen production technology industrially, giving insights into novel applications, cross-pollination of separate lines of inquiry, and giving a reference point for researchers and industrial developers in the field of biohydrogen production.

Development of an ultrasound-assisted pre-treatment strategy for the extraction of D-Limonene toward the production of bioethanol from citrus peel waste (CPW)

Citrus is one of the world's most abundant fruits containing vitamins, pigments, and fragrances, making it vital for several industries. However, these fruits contain about 45-50% residues (peels), which often end up as waste and can be harmful to the environment if not properly treated. Bioethanol production from citrus peel waste offers a potential solution to this problem. Hence, this study explores the potential of using ultrasound-assisted pre-treatment method as a novel strategy to extract D-Limonene (essential oil in the residue), and further demonstrates bioethanol production. This was done by investigating ultrasonication's optimal effect on pre-treatment of the citrus residue, followed by bioethanol production. The results show that, optimum values for D-Limonene extraction were obtained at a temperature of 14.6 °C and an ultrasound intensity of 25.81 W/cm2 with a validation yield of 134 ± 4.24 mg/100 g dry CPW. With optimal ultrasonic parameters, the study went further to demonstrate the effect of the essential oil on bioethanol production which is hindered by the oils present. Key findings show better bioethanol yield once the essential oil was extracted (treated) from the citrus waste as opposed to it not extracted (untreated), with a 66 and a 29% increase when comparing simultaneous saccharification and fermentation (SSF) and sequential hydrolysis and fermentation (SHF) respectively. Based on this result, ultrasound-assisted extraction as a pretreatment method was found suitable for bioethanol production from citrus residue and could be utilized as a biorefinery pre-treatment approach to scale bioethanol production.

Effective hexanol production from carbon monoxide using extractive fermentation with Clostridium carboxidivorans P7

This study achieved high production of hexanol via gas fermentation using Clostridium carboxidivorans P7 by extracting hexanol from the fermentation broth. The hexanol extraction efficiency and inhibitory effects on C. carboxidivorans P7 of 2-butyl-1-octanol, hexyl hexanoate and oleyl alcohol were examined, and oleyl alcohol was selected as the extraction solvent. Oleyl alcohol was added at the beginning of fermentation and during fermentation or a small volume of oleyl alcohol was repeatedly added during fermentation. The addition of a small volume of oleyl alcohol during fermentation was the most effective for CO consumption and hexanol production (5.06 g/L), yielding the highest known hexanol titer through any type of fermentation including gas fermentation. Hexanol production was further enhanced to 8.45 g/L with the repeated addition of oleyl alcohol and ethanol during gas fermentation. The results of this study will enable sustainable and carbon-neutral hexanol production via gas fermentation.

The liquid-liquid extractive fermentation of L-lactic acid in a novel semi-partition bioreactor (SPB)

Fermentation technology is commonly used as a mature process to produce numerous products with the help of micro-organisms. However, these organisms are sometimes inhibited by the accumulation of these products or their by-products. One route to circumvent this is via extractive fermentation, which combines the fermentation process with extraction. To facilitate this, novel bioreactor designs are required, such as the semi-partition bioreactor (SPB) which has been recently proposed for in-situ extractive fermentation. The latter combines a fermentation and an extraction unit into a single vessel using a mixer-settler principle. Where the bioproduct is produced in the mixer and removed continuous in the settler. As the SPB functionality is a subject of interest, this study builds on demonstrating different process conditions in the production of a sample bioprocess (lactic acid (LA)) which is susceptible to product inhibition. The results showed a 34.5 g/L LA concentration was obtained in the pH-controlled condition. While LA production can suffer from product inhibition, neutralizing agents can be easily used to curb inhibitory problems, however, the LA fermentation is a simple (and well-studied) example, which can demonstrate an alternative route to avoiding product inhibition (for systems which cannot be rescued using pH control). Hence, to replicate a scenario of product inhibition, two different process conditions were investigated, no pH control with no extraction (non-integrated), and no pH control with integrated extractive fermentation. Key findings showed higher LA concentration in integrated (25.10 g/L) as compared to the non-integrated (14.94 g/L) case with improved yield (0.75 gg^-1 (integrated) versus 0.60 gg^-1 (non-integrated)) and overall productivity (0.35 gL^-1h^-1(integrated) versus 0.20 gL^-1h^-1(non-integrated)) likewise. This is the first demonstration of an SP bioreactor, and shows how the reactor can be applied to improve productivity. Based on these results, the SPB design can be applied to produce any product liable to product inhibition.