Biotechnological production of citric acid

Enhanced Citric Acid Production through Aspergillus niger: Insights from Fermentation Studies Using Sugarcane Molasses

The production of citric acid, a vital agricultural commodity utilized across various industries such as food, beverages, pharmaceuticals, agriculture, detergents, and cosmetics, predominantly relies on microbial fermentation, with Aspergillus niger accounting for approximately 90% of global production. In this study, we aimed to optimize the key factors influencing citric acid production, with a focus on strains, fermentation techniques, and carbon sources, particularly sugarcane molasses. A. niger, sourced from the Botany department/Biotechnology laboratories at Govt. College of Science, Lahore, was employed for citric acid production. The process involved inoculum preparation through spore collection from 3 to 5 days of cultured PDA slants. The fermentation medium, comprising cane molasses with a 15% sugar concentration, was meticulously prepared and optimized for various factors, including magnesium sulfate, potassium ferrocyanide, time of addition of potassium ferrocyanide, ammonium oxalate, and calcium chloride. Our optimization results shed light on the significant impact of different factors on citric acid production. For instance, the addition of 0.4 g/L magnesium sulfate led to a maximum yield of 75%, while 2 g/L potassium ferrocyanide, added at 24 h, achieved a yield of 78%. Remarkably, ammonium oxalate, at a concentration of 10 g/L, resulted in a notable 77% yield. Conversely, the addition of calcium chloride exhibited negligible effects on citric acid production, with the control group yielding more at 78%. Our study underscores the potential for optimizing factors to enhance citric acid production by A. niger in submerged fermentation. These findings highlight the pivotal role of magnesium sulfate, potassium ferrocyanide, and ammonium oxalate in augmenting citric acid yields while emphasizing the minimal impact of calcium chloride. Ultimately, these insights contribute to advancing our understanding of microbial citric acid biosynthesis, providing valuable implications for industrial applications and future research endeavors.

Citric Acid: Properties, Microbial Production, and Applications in Industries

Citric acid finds broad applications in various industrial sectors, such as the pharmaceutical, food, chemical, and cosmetic industries. The bioproduction of citric acid uses various microorganisms, but the most commonly employed ones are filamentous fungi such as Aspergillus niger and yeast Yarrowia lipolytica. This article presents a literature review on the properties of citric acid, the microorganisms and substrates used, different fermentation techniques, its industrial utilization, and the global citric acid market. This review emphasizes that there is still much to explore, both in terms of production process techniques and emerging new applications of citric acid.

Enhanced production of glutaric acid by biocatalyst-recycled bioconversion process integrated with in situ product recovery by adsorption

Product inhibition caused by organic acids is a serious issue in establishing economical biochemical production systems. Herein, for enhanced production of glutaric acid by overcoming product inhibition triggered by glutaric acid, a whole-cell bioconversion system equipped with biocatalyst recycling process and in situ product recovery by adsorption was developed successfully. From the whole-cell bioconversion reaction, we found that both dissociated and undissociated forms of glutaric acid acted as an inhibitor in the whole-cell bioconversion reaction, wherein bioconversion was hindered beyond 200 mM glutaric acid regardless of reaction pH. Therefore, as the promising solution for the inhibition issue by glutaric acid, the biocatalyst-recycled bioconversion process integrated with in situ product recovery by adsorption was introduced in the whole-cell bioconversion. As a result, 592 mM glutaric acid was produced from 1000 mM 5-aminovaleric acid with 59.2% conversion. We believe that our system will be a promising candidate for economically producing organic acids with high titer.

Introducing molasses as an alternative feedstock into itaconate production using Ustilago sp

In this work, we established an efficient process for the production of itaconate from the regionally sourced industrial side-stream molasses using Ustilago cynodontis and Ustilago maydis. While being relatively cheap and more environmentally friendly than refined sugars, there are some major challenges to overcome when working with molasses. Some of those challenges are a high nitrogen load, unknown impurities in the feedstock, and high amounts of ill-favoured carbon sources, such as sucrose or lactate. We could show that the activity of the sucrose-hydrolysing enzyme invertase plays a crucial role in the efficiency of the process and that the fructose utilisation differs between the two strains used in this work. Thus, with a higher invertase activity, the ability to convert fructose into the desired product itaconate, and an overall higher tolerance towards undesired substances in molasses, U. maydis is better equipped for the process on the alternative feedstock molasses than U. cynodontis. The established process with U. maydis reached competitive yields of up to 0.38 g g-1 and a titre of more than 37 g L-1. This shows that an efficient and cost-effective itaconate production process is generally feasible using U. maydis, which has the potential to greatly increase the sustainability of industrial itaconate production.

Novel genetic tools improve Penicillium expansum patulin synthase production in Aspergillus niger

Since the first CRISPR (clustered regularly interspaced short palindromic repeats)-Cas (CRISPR-associated) system was developed for creating double-stranded DNA breaks, it has been adapted and improved for different biotechnological applications. In this issue of The FEBS Journal, Arentshorst et al. developed a novel approach to enhance transgene expression of a specific protein, patulin synthase (PatE) from Penicillium expansum, in the important industrial filamentous fungus Aspergillus niger. Their technique involved the disruption of selected genes with counter-effects on targeted protein production and simultaneous integration of glucoamylase landing sites into the disrupted gene locus such as protease regulator (prtT) in an ATP-dependent DNA helicase II subunit 1 (kusA or ku70)-deletion strain. Multiple copies of the PatE transgene expression cassette were introduced by CRISPR-Cas9-mediated insertion. The purified PatE was further used for structural and functional studies, and the technique laid the foundation for elevating the overall production of various proteins or chemicals in those industrially important fungi.

Production of Extracellular Enzymes by Lentinula edodes Strains in Solid-State Fermentation on Lignocellulosic Biomass Sterilized by Physical and Chemical Methods

Two methods of sterilization of lignocellulosic biomass were performed in this study. Eucalypt waste (EW) supplemented with rice bran (RB) was added in the proportions 80:20 and 90:10 in dry weight. The compositions were sterilized by physical method (autoclaving) and by chemical method (H2O2). The production of extracellular enzymes by Lentinula edodes strains was compared within the two methods. Inactivation of catalase present in RB was achieved with 250 mM H2O2. The use of H2O2, when compared by physical method, favored high production of hydrolytic enzymes such as endoglucanase (1,600 IU/kg), twofold higher, β-glucosidase (1,000 IU/kg), fivefold higher, xylanase (55,000 IU/kg), threefold higher and β-xylosidase (225 IU/kg), similar results. Oxidative enzymes, MnP and laccase, were produced within a different profile between strains, with shorter times for laccase (2,200 IU/kg) by SJC in 45 days and MnP (2,000 IU/kg) by CCB-514 in 30 days. High production of extracellular enzymes is achieved by the use of the chemical method of sterilization of lignocellulosic biomass; in addition to no energy consumption, this process is carried out in a shorter execution time when compared to the physical process. The use of H2O2 in sterilization does not produce toxic compounds from the degradation of hemicellulose and cellulose such as furfural and hydroxy-methyl-furfural that cause inhibition of microorganisms and enzymes.

Investigating the Role of Citric Acid as a Natural Acid on the Crystallization of Amoxicillin Trihydrate

This study investigates the use of environmentally friendly citric acid as the main player in the process, rather than as an additive, to remove impurities from amoxicillin trihydrate (AMCT) crystals, aiming to optimize their purity and yield. By manipulating the concentration of citric acid, mixing speed, crystallization time, and pH, the researchers conducted experiments using a full factorial design. The dissolution stage was analyzed in both batch and continuous crystallization processes, emphasizing the significance of citric acid in enhancing crystallization. HPLC analyses were performed on the resulting crystals, and the data were analyzed using the Multi-Vari Chart program. The findings demonstrated that higher citric acid concentrations positively affected the yield, while factors such as crystallization time, mixing speed, and pH also contributed to the increased yield. The crystals obtained exhibited desirable dimensions sought after in the pharmaceutical industry, eliminating the need for additional purification steps. This study showcased the potential of citric acid in AMCT crystallization, offering advantages in product design, purification, and synthesis. The optimized conditions included a citric acid concentration of 2.0 M, mixing speed of 1000 rpm, crystallization time of 120 min, and pH of 5.5. Notably, the developed process proved to be environmentally friendly by avoiding the use of harmful chemicals, serving as a green alternative for crystallization processes, and producing purer AMCT products. Overall, this research contributes to the existing literature by highlighting the efficacy of citric acid in impurity removal and the optimization of AMCT crystal purity and yield.

Chemical, Aroma and Pro-Health Characteristics of Kaffir Lime Juice-The Approach Using Optimized HS-SPME-GC-TOFMS, MP-OES, 3D-FL and Physiochemical Analysis

The study aimed to provide the chemical, aroma and prohealth characteristics of the kaffir lime juice. A procedure using solid-phase microextraction with gas chromatography (SPME-GC-TOFMS) was optimized and validated for the determination of terpenes of kaffir lime. Main physicochemical parameters: pH, vitamin C, citric acid and °Brix were evaluated. Micro- and macro elements were determined using microwave plasma optic emission spectrometry (MP-OES). The binding of kaffir lime terpenes to human serum albumin (HSA) was investigated by fluorescence spectroscopy (3D-FL). β-Pinene and Limonene were selected as the most abundant terpenes with the concentration of 1225 ± 35 and 545 ± 16 µg/g, respectively. The values of citric acid, vitamin C, °Brix and pH were 74.74 ± 0.50 g/kg, 22.31 ± 0.53 mg/100 mL, 10.35 ± 0.70 and 2.406 ± 0.086 for, respectively. Iron, with a concentration of 16.578 ± 0.029 mg/kg, was the most abundant microelement. Among the macroelements, potassium (8121 ± 52 mg/kg) was dominant. Kaffir lime binding to HSA was higher than β-Pinene, which may indicate the therapeutic effect of the juice. Kaffir lime juice is a source of terpenes with good aromatic and bioactive properties. Fluorescence measurements confirmed its therapeutic effect. Kaffir lime juice is also a good source of citric acid with potential industrial application. The high content of minerals compared to other citruses increases its prohealth value.

Metabolic engineering and fermentation optimization strategies for producing organic acids of the tricarboxylic acid cycle by microbial cell factories

The organic acids of the tricarboxylic acid (TCA) pathway are important platform compounds and are widely used in many areas. The high-productivity strains and high-efficient and low-cost fermentation are required to satisfy a huge market size. The high metabolic flux of the TCA pathway endows microorganisms potential to produce high titers of these organic acids. Coupled with metabolic engineering and fermentation optimization, the titer of the organic acids has been significantly improved in recent years. Herein, we discuss and compare the recent advances in synthetic pathway engineering, cofactor engineering, transporter engineering, and fermentation optimization strategies to maximize the biosynthesis of organic acids. Such engineering strategies were mainly based on the TCA pathway and glyoxylate pathway. Furthermore, organic-acid-secretion enhancement and renewable-substrate-based fermentation are often performed to assist the biosynthesis of organic acids. Further strategies are also discussed to construct high-productivity and acid-resistant strains for industrial large-scale production.

Lignocellulosic biorefineries: A multiscale approach for resource exploitation

Biomass can become the source for chemicals towards a sustainable production system. However, the challenges it presents such as the variety of species, their widespread and sparse availability, and the expensive transportation claims for an integrated approach to design the novel production system. Multiscale approaches have not been properly extended to biorefineryes design and deployment, due to the comprehensive experimental and modelling work they require. A systems perspective provides the systematic framework to analyze the availability and composition of raw materials across regions, how that affects process design, the portfolio of products that can be obtained by evaluating the strong link between the biomass features and the process design. The use of lignocellulosic materials requires for a multidisciplinary work, that must lead to new process engineers with technical competences in biology, biotechnology but also process engineering, mathematics, computer science and social sciences towards a sustainable process/chemical industry.

Microbial and Chemical Characterization of Natural-Style Green Table Olives from the Gordal, Hojiblanca and Manzanilla Cultivars

Microbial and biochemical changes in the brine during the spontaneous fermentation of Gordal, Hojiblanca and Manzanilla olive cultivars processed according to the natural style were monitored. The microbial composition was assessed through a metagenomic study. Sugars, ethanol, glycerol, organic acids and phenolic compounds were quantified by standard methods. In addition, the volatile profiles, contents of phenolic compounds in the olives and quality parameters of the final products were compared. Fermentation in Gordal brines was conducted by lactic acid bacteria (mainly Lactobacillus and Pediococcus) and yeasts (mainly Candida boidinii, Candida tropicalis and Wickerhamomyces anomalus). In Hojiblanca and Manzanilla brines, halophilic Gram-negative bacteria (e.g., Halomonas, Allidiomarina and Marinobacter) along with yeasts (mainly, Saccharomyces) were responsible for the fermentation. Higher acidity and lower pH values were reached in Gordal brines compared to Hojiblanca and Manzanilla. After 30 days of fermentation, no sugars were detected in Gordal brine, but residual amounts were found in the brines from Hojiblanca (<0.2 g/L glucose) and Manzanilla (2.9 g/L glucose and 0.2 g/L fructose). Lactic acid was the main acid product in Gordal fermentation, whereas citric acid was the predominant organic acid in the Hojiblanca and Manzanilla brines. Manzanilla brine samples showed a greater concentration of phenolic compounds than Hojiblanca and Gordal brines. After a 6-month fermentation, Gordal olives were superior compared to the Hojiblanca and Manzanilla varieties regarding product safety (lower final pH and absence of Enterobacteriaceae), content of volatile compounds (richer aroma), content of bitter phenolics (lower content of oleuropein, which resulted in less perceived bitterness) and color parameters (more yellow and lighter color, indicating a higher visual appraisal). The results of the present study will contribute to a better understanding of each fermentation process and could help to promote natural-style elaborations using the above-mentioned olive cultivars.

Recent advances and perspectives on production of value-added organic acids through metabolic engineering

Organic acids are important consumable materials with a wide range of applications in the food, biopolymer and chemical industries. The global consumer organic acids market is estimated to increase to $36.86 billion by 2026. Conventionally, organic acids are produced from the chemical catalysis process with petrochemicals as raw materials, which posts severe environmental concerns and conflicts with our sustainable development goals. Most of the commonly used organic acids can be produced from various organisms. As a state-of-the-art technology, large-scale fermentative production of important organic acids with genetically-modified microbes has become an alternative to the chemical route to meet the market demand. Despite the fact that bio-based organic acid production from renewable cheap feedstock provides a viable solution, low productivity has impeded their industrial-scale application. With our deeper understanding of strain genetics, physiology and the availability of strain engineering tools, new technologies including synthetic biology, various metabolic engineering strategies, omics-based system biology tools, and high throughput screening methods are gradually established to bridge our knowledge gap. And they were further applied to modify the cellular reaction networks of potential microbial hosts and improve the strain performance, which facilitated the commercialization of consumable organic acids. Here we present the recent advances of metabolic engineering strategies to improve the production of important organic acids including fumaric acid, citric acid, itaconic acid, adipic acid, muconic acid, and we also discuss the current challenges and future perspectives on how we can develop a cost-efficient, green and sustainable process to produce these important chemicals from low-cost feedstocks.

Sustainable citric acid production from CO2 in an engineered cyanobacterium

Citric acid is one of the most widely used organic acids in the world, with applications ranging from acidity regulation in food and beverages to metal chelation in hydrometallurgical processes. Most of its production is currently derived from fermentative processes, using plant-derived carbon feedstocks. While these are currently dominant, there is an increasing need to develop closed-loop production systems that reduce process carbon footprint. In this work, we demonstrate for the first time that an engineered marine cyanobacterium Synechococcus sp. PCC 7002 can be used as a sustainable chassis for the photosynthetic conversion of CO₂ to citric acid. Decreased citric acid cycle flux, through the use of a theophylline-responsive riboswitch, was combined with improved flux through citrate synthase and enhanced citric acid excretion, resulting in a significant improvement to citric acid production. While allowing citrate production, this strategy induces a growth defect which can be overcome by glutamate supplementation or by fine-tuning aconitase levels, resulting in an increase in production relative to WT of over 100-fold. This work represents a first step toward sustainable production of a commodity organic acid from CO₂.

Citric Acid: A Multifunctional Pharmaceutical Excipient

Citric acid, a tricarboxylic acid, has found wide application in the chemical and pharmaceutical industry due to its biocompatibility, versatility, and green, environmentally friendly chemistry. This review emphasizes the pharmaceutical uses of citric acid as a strategic ingredient in drug formulation while focusing on the impact of its physicochemical properties. The functionality of citric acid is due to its three carboxylic groups and one hydroxyl group. These allow it to be used in many ways, including its ability to be used as a crosslinker to form biodegradable polymers and as a co-former in co-amorphous and co-crystal applications. This paper also analyzes the effect of citric acid in physiological processes and how this effect can be used to enhance the attributes of pharmaceutical preparations, as well as providing a critical discussion on the issues that may arise out of the presence of citric acid in formulations.

A Novel Endophytic Trichoderma longibrachiatum WKA55 With Biologically Active Metabolites for Promoting Germination and Reducing Mycotoxinogenic Fungi of Peanut

Plant residuals comprise the natural habitat of the plant pathogen; therefore, attention is currently focusing on biological-based bioprocessing of biomass residuals into benefit substances. The current study focused on the biodegradation of peanut plant residual (PNR) into citric acid (CA) through a mathematical modeling strategy. Novel endophytic Trichoderma longibrachiatum WKA55 (GenBank accession number: MZ014020.1), having lytic (cellulase, protease, and polygalacturonase) activity, and tricalcium phosphate (TCP) solubilization ability were isolated from peanut seeds and used during the fermentation process. As reported by HPLC, the maximum CA (5505.1 μg/g PNR) was obtained after 9 days in the presence of 15.49 mg TCP, and 15.68 mg glucose. GC–MS analysis showed other bioactive metabolites in the filtrate of the fermented PNR. Practically, the crude product (40%) fully inhibited (100%) the growth and spore germination of three mycotoxinogenic fungi. On peanuts, it improved the seed germination (91%), seedling features, and vigor index (70.45%) with a reduction of abnormal seedlings (9.33%). The current study presents the fundamentals for large-scale production in the industry for the sustainable development of PNR biomass as a natural source of bioactive metabolites, and safe consumption of lignocellulosic-proteinaceous biomass, as well. T. longibrachiatum WKA55 was also introduced as a novel CA producer specified on PNR. Application of the resulting metabolite is encouraged on a large scale.

A sustainable approach on biomining of low-grade bauxite by P. simplicissimum using molasses medium

In order to find a sustainable and low-cost alternative route to the traditional recovery of aluminum, the filamentous fungus Penicillium simplicissimum was evaluated for aluminum recovery from low-grade bauxite ore. The oat-agar medium was carefully chosen as the foremost solid medium for fungal sporulation due to lower cost, ease in preparation, and high spore production in a short incubation time. To examine the acid production capability in submerged fermentation, P. simplicissimum was inoculated in a medium augmented with glucose and molasses as an energy source. High-performance liquid chromatography (HPLC) technique was used for the determination of the produced organic acids. Three different bioleaching approaches were evaluated using 1% bauxite pulp density. The culture containing P. simplicissimum spores grown in a medium supplemented with molasses leached 86.6% Al in the direct two steps on the fifth day, 56.5% in the direct one step on the fourth day, and 71.7% in the indirect bioleaching on the fourth day, while in the controlled sterile flasks, Al leaching was almost negligible. A maximal amount of Al was leached by the fungal strains using low-cost molasses as a substrate.

Advances in sustainable approaches utilizing orange peel waste to produce highly value-added bioproducts

Orange peel waste (OPW), a discarded part of orange fruit, is a rich source of essential constituents that can be transformed into highly value-added bioproducts. OPW is being generated in million tonnes globally and returns to the environment without complete benefit. Thus, a high volume of annually produced OPW in the industry requires effective valorization. In this regard, limited data is available that summarizes the broader spectrum for the sustainable fate of OPW to produce value-added bioproducts. The main objective of this treatise is to explore the sustainable production of bioproducts from OPW. Therefore, this review covers all the aspects of OPW, from its production to complete valorization. The review encompasses the extraction technologies employed for extracting different valuable bioactive compounds, such as: essential oil (EO), pectin, and carotenoids, from OPW. Furthermore, the suitability of bioconversion technologies (digestion/fermentation) in transforming OPW to other useful bioproducts, such as: biochemicals (lactic acid and succinic acid), biopolysaccharides (xanthan and curdlan gum), and bioenergy (biomethane and bioethanol) is discussed. Also, it includes the concept of OPW-based biorefineries and their development that shall play a definite role in future to cover demands for: food, chemicals, materials, fuels, power, and heat. Lastly, this review focuses on OPW-supplemented functional food products such as: beverages, yogurts, and extruded products. In conclusion, insights provided in this review maximize the potential of OPW for commercial purposes, leading to a safe, and waste-free environment.

A standardized gnotobiotic mouse model harboring a minimal 15-member mouse gut microbiota recapitulates SOPF/SPF phenotypes

Mus musculus is the classic mammalian model for biomedical research. Despite global efforts to standardize breeding and experimental procedures, the undefined composition and interindividual diversity of the microbiota of laboratory mice remains a limitation. In an attempt to standardize the gut microbiome in preclinical mouse studies, here we report the development of a simplified mouse microbiota composed of 15 strains from 7 of the 20 most prevalent bacterial families representative of the fecal microbiota of C57BL/6J Specific (and Opportunistic) Pathogen-Free (SPF/SOPF) animals and the derivation of a standardized gnotobiotic mouse model called GM15. GM15 recapitulates extensively the functionalities found in the C57BL/6J SOPF microbiota metagenome, and GM15 animals are phenotypically similar to SOPF or SPF animals in two different facilities. They are also less sensitive to the deleterious effects of post-weaning malnutrition. In this work, we show that the GM15 model provides increased reproducibility and robustness of preclinical studies by limiting the confounding effect of fluctuation in microbiota composition, and offers opportunities for research focused on how the microbiota shapes host physiology in health and disease.

CRISPR-Based Genetic Manipulation of Candida Species: Historical Perspectives and Current Approaches

The Candida genus encompasses a diverse group of ascomycete fungi that have captured the attention of the scientific community, due to both their role in pathogenesis and emerging applications in biotechnology; the development of gene editing tools such as CRISPR, to analyze fungal genetics and perform functional genomic studies in these organisms, is essential to fully understand and exploit this genus, to further advance antifungal drug discovery and industrial value. However, genetic manipulation of Candida species has been met with several distinctive barriers to progress, such as unconventional codon usage in some species, as well as the absence of a complete sexual cycle in its diploid members. Despite these challenges, the last few decades have witnessed an expansion of the Candida genetic toolbox, allowing for diverse genome editing applications that range from introducing a single point mutation to generating large-scale mutant libraries for functional genomic studies. Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology is among the most recent of these advancements, bringing unparalleled versatility and precision to genetic manipulation of Candida species. Since its initial applications in Candida albicans, CRISPR-Cas9 platforms are rapidly evolving to permit efficient gene editing in other members of the genus. The technology has proven useful in elucidating the pathogenesis and host-pathogen interactions of medically relevant Candida species, and has led to novel insights on antifungal drug susceptibility and resistance, as well as innovative treatment strategies. CRISPR-Cas9 tools have also been exploited to uncover potential applications of Candida species in industrial contexts. This review is intended to provide a historical overview of genetic approaches used to study the Candida genus and to discuss the state of the art of CRISPR-based genetic manipulation of Candida species, highlighting its contributions to deciphering the biology of this genus, as well as providing perspectives for the future of Candida genetics.

Non-conventional yeasts for food and additives production in a circular economy perspective

Yeast species have been spontaneously participating in food production for millennia, but the scope of applications was greatly expanded since their key role in beer and wine fermentations was clearly acknowledged. The workhorse for industry and scientific research has always been Saccharomyces cerevisiae. It occupies the largest share of the dynamic yeast market, that could further increase thanks to the better exploitation of other yeast species. Food-related 'non-conventional' yeasts (NCY) represent a treasure trove for bioprospecting, with their huge untapped potential related to a great diversity of metabolic capabilities linked to niche adaptations. They are at the crossroad of bioprocesses and biorefineries, characterized by low biosafety risk and produce food and additives, being also able to contribute to production of building blocks and energy recovered from the generated waste and by-products. Considering that the usual pattern for bioprocess development focuses on single strains or species, in this review we suggest that bioprospecting at the genus level could be very promising. Candida, Starmerella, Kluyveromyces and Lachancea were briefly reviewed as case studies, showing that a taxonomy- and genome-based rationale could open multiple possibilities to unlock the biotechnological potential of NCY bioresources.

Bioleaching metals from waste electrical and electronic equipment (WEEE) by Aspergillus niger: a review

In the twenty-first century, the increasing demand for electrical and electronic equipment (EEE) has caused its quick update and the shortening of its service life span. As a consequence, a large number of waste electrical and electronic equipment (WEEE) needs to be processed and recycled. As an environmentally friendly method, biometallurgy has received extensive attention in the disposal of WEEE in recent years. Aspergillus niger is an acid-producing fungus with a potential applicability to improve metals' recycling efficiency. This review article describes the latest statistical status of WEEE and presents the latest progress of various metallurgical methods involved in WEEE recycling for metal recovery. Moreover, based on the summary and comparison towards studies have been reported for bioleaching metals from WEEE by A. niger, the bioleaching mechanisms and the bioleaching methods are explained, as well as the effects of process parameters on the performance of the bioleaching process are also discussed. Some insights and perspectives are provided for A. niger to be applied to industrial processing scale.

Physico-Chemical Characteristics and Amino Acid Content Evaluation of Citric Acid by-Product Produced by Microbial Fermentation as a Potential Use in Animal Feed

The production of citric acid produces 70% waste product or by-product. This by-product is produced by microbial fermentation which could be used as an alternative raw material for animal feed because it still contains citric acid, which could help to reduce pathogenic bacteria. The objective of this study is to evaluate the physical and chemical value of citric acid by-product from rice (CABR) to compare the properties with those of rice bran and broken rice and to determine its potential as an alternative energy source in animal feed. The chemical composition of CABR was calculated using proximate analysis. The color of CABR was darker, and the bulk density value was 549.65 (g/L) (p < 0.05). With free flow, the angle of repose was 40°, and the particle size had less polygonal starch granules. CABR had a low pH of 4.77 and contained 19.80% crude protein, 11.97% crude fiber, and 4005.72 kcal/kg of energy. CABR had a higher crude protein value than broken rice and rice bran and a higher gross energy value than broken rice but less than rice bran. It also had a higher crude fiber value (p > 0.05). The results suggest that CABR could be utilized as an energy and protein source for animal feed formulations.

Optimization of integrated phytoremediation system (IPS) for enhanced lead removal and restoration of soil microbial activities

Improving phytoremediation efficiency in lead (Pb) contaminated soil through either bacterial or fungal inoculants have extensively been studied with different successes and limitations. In this study, co-application of bacteria and fungi have been investigated for development of an integrated phytoremediation system (IPS) for efficient Pb removal and restoration of soil microbial and enzymatic activities in degraded soil. For this purpose, Pb tolerant bacterial and fungal strains were firstly analyzed for antifungal and antibacterial activities through disc diffusion method. Afterwards, the co-inoculation studies were performed to investigate the effects on phytoavailability and uptake of Pb by Pelargonium hortorum through soil incubation and pot culture experiments, respectively. Results indicated significant (p < 0.05) antibacterial activity of Mucor spp. against bacterial species (Klebsiella variicola and K. quasipneumoniae). The highest significant increase in extractable Pb fraction (5.0-folds) was observed when soil was co-inoculated with Aspergillus flavus + Microbacterium paraoxydans compared to the control soil (un-inoculated soil) at 2000 mg Pb kg^-1 concentration. Similarly, uptake results also indicated significantly higher Pb uptake in plants inoculated with A. flavus + M. paraoxydans. Soil microbial results indicated significant decrease in microbial health indicators and enzymatic activities with increasing Pb concentration and exposure time, as compared to control soil. A relatively severe decline was observed in soil respiration and dehydrogenase (DEH) activities by 2.8- and 2.5-folds, respectively at 2000 mg Pb kg^-1 of soil. The optimized IPS was effective for restoring enzymatic activities in Pb contaminated soil and could be applied for sustainable restoration of Pb contaminated soil.

Modulation in isocitrate dehydrogenase activity under citrate enrichment affects carbon and nitrogen fixations in the cyanobacterium Nostoc muscorum Meg 1

The enzyme isocitrate dehydrogenase (IDH) converts isocitrate synthesized from citrate to α-ketoglutarate in the TCA cycle. In cyanobacteria, α-KG has an additional role where it donates its carbon skeleton for ammonium assimilation in the GS-GOGAT pathway thereby linking carbon and nitrogen metabolisms. Looking at this crucial function of IDH that makes α-KG available for both carbon and nitrogen assimilation, changes brought about in its activity under excess availability of citrate in a cyanobacterium was evaluated. Further, how these changes are transmitted downstream affecting carbon and nitrogen metabolisms were also evaluated. A 100 μM citrate supplementation induced IDH activity. Consequently, there was an increase in concentrations of photosynthetic pigments, D1 protein and RuBisCO as well as in PSII activity. Heterocyst differentiation was initiated and an upsurge in the activities of nitrogenase and GS were recorded. An enhancement in the total protein and carbohydrate content reiterated the positive influence of citrate enrichment on carbon and nitrogen fixation. The increase in the mRNA contents of IDH, D1 protein, RuBisCO, nitrogenase and GS indicated their induction at the genetic level. Finally, there was augmentation in total biomass production by ∼28%. Interestingly as citrate concentration was increased to 500 μM, both C- and N- fixations were highly compromised suggesting that even though citrate is an essential metabolite in the cells, it became toxic beyond a certain concentration to the organism. SEM and TEM studies showed no changes in the organism's morphology and ultra-structure in presence of 100 μM citrate while adverse changes were noticed in presence of 500 μM citrate.

Citric Acid Production of Yeasts: An Overview

Background

Citric acid, an intermediate product of the Krebs cycle, has a wide usage area in the food industry since it has some functions such as acidulant, flavouring agent, preservative and antioxidant. Although molds are the most commonly used microorganisms in the citric acid production, it is known that there are significant advantages of using yeasts.

Purpose and scope

The microbial citric acid production mechanism needs to be well understood to make production more efficient. In this study, the yeasts used in the production, fermentation types and the factors affecting production were reviewed with studies.

Methodology

Although production of citric acid can be produced by chemical synthesis, the fermentation is preferred because of its low cost and ease of use. More than 90% of citric acid produced in the world is obtained by fermentation.

Results

Yarrowia lipolytica, Candida zeylanoides and Candida oleophila are evaluated for citric acid production with substrates such as molasses, glucose, sucrose and glycerol. On the other hand, there is great interest in developing processes with new substrates and/ or microorganisms.

Conclusion

Although the microbial strain is an important factor, the factors such as carbon, phosphorus and nitrogen sources, aeration, the presence of trace elements and pH are also parameters affecting the production.

Draft Genome Sequences of Aspergillus and Penicillium Species Isolated from the International Space Station and Crew Resupply Vehicle Capsule

The draft whole-genome sequences (WGS) of 30 fungal strains isolated from the International Space Station and belonging to the Penicillium and Aspergillus genera were assembled. The WGS will allow for detailed genomic characterization to determine the possible applications and importance for space and biotechnological industries.

Enzymatic degradation of xyloglucans by Aspergillus species: a comparative view of this genus

Aspergillus species are closely associated with humanity through fermentation, infectious disease, and mycotoxin contamination of food. Members of this genus produce various enzymes to degrade plant polysaccharides, including starch, cellulose, xylan, and xyloglucan. This review focus on the machinery of the xyloglucan degradation using glycoside hydrolases, such as xyloglucanases, isoprimeverose-producing oligoxyloglucan hydrolases, and α-xylosidases, in Aspergillus species. Some xyloglucan degradation-related glycoside hydrolases are well conserved in this genus; however, other enzymes are not. Cooperative actions of these glycoside hydrolases are crucial for xyloglucan degradation in Aspergillus species. KEY POINTS: •Xyloglucan degradation-related enzymes of Aspergillus species are reviewed. •Each Aspergillus species possesses a different set of glycoside hydrolases. •The machinery of xyloglucan degradation of A. oryzae is overviewed.

Fungal bioleaching of metals from refinery spent catalysts: A critical review of current research, challenges, and future directions

Petroleum refining operations such as hydroprocessing and fluid catalytic cracking (FCC) generate huge quantities of spent catalysts containing toxic and valuable metals (Ni, V, Mo, Co, W, Al, etc.), the management of which is a serious environmental issue. Besides environmental concerns, the different metals present in the spent catalysts are also a valuable commodity to modern industries. Therefore, these spent catalysts also provide an opportunity to use it as a source of value to the refiners. In recent years, a biotechnological based leaching process 'bioleaching' has emerged as a promising eco-friendly technique for the extraction of metals from these refinery spent catalysts. Among various bioleaching agents such as archean, bacterial, or fungi, the process mediated by the fungi (Aspergillus niger, Penicillium simplicissimum, and many others) is gaining attention owing to the high metal extraction ability of the various fungal produced metabolites (organic acids) under moderately acidic conditions. Furthermore, the ability of these fungi to withstand wide process conditions (pH, spent catalyst concentration, substrate types, etc.), high metal toxicity and use of low-cost organic substrate make them an ideal candidate for bioleaching. In this review article, we shed light on the role and mechanisms of fungi involved in extracting different metals from spent hydroprocessing and FCC catalysts. Key process parameters that affect the efficiency of fungal based bioleaching are discussed. The techno-economic challenges associated with the process are elaborated, and the needed future research directions to promote its commercial applications are highlighted. Based on our analysis, it can be argued that the fungi bioleaching has potential, however, some challenges (slower kinetics, and health and safety) should be addressed before the process can be scaled up for the commercial application.

Unlocking Nature's Biosynthetic Power-Metabolic Engineering for the Fermentative Production of Chemicals

Fermentation as a production method for chemicals is especially attractive, as it is based on cheap renewable raw materials and often exhibits advantages in terms of costs and sustainability. The tremendous development of technology in bioscience has resulted in an exponentially increasing knowledge about biological systems and has become the main driver for innovations in the field of metabolic engineering. Progress in recombinant DNA technology, genomics, and computational methods open new, cheaper, and faster ways to metabolically engineer microorganisms. Existing biosynthetic pathways for natural products, such as vitamins, organic acids, amino acids, or secondary metabolites, can be discovered and optimized efficiently, thereby enabling competitive commercial production processes. Novel biosynthetic routes can now be designed by the rearrangement of nature's unlimited number of enzymes and metabolic pathways in microbial strains. This expands the range of chemicals accessible by biotechnology and has yielded the first commercial products, while new fermentation technologies targeting novel active ingredients, commodity chemicals, and CO₂ -fixation methods are on the horizon.

Fine-tuning mitochondrial activity in Yarrowia lipolytica for citrate overproduction

Yarrowia lipolytica is a non-conventional yeast with promising industrial potentials for lipids and citrate production. It is also widely used for studying mitochondrial respiration due to a respiratory chain like those of mammalian cells. In this study we used a genome-scale model (GEM) of Y. lipolytica metabolism and performed a dynamic Flux Balance Analysis (dFBA) algorithm to analyze and identify metabolic levers associated with citrate optimization. Analysis of fluxes at stationary growth phase showed that carbon flux derived from glucose is rewired to citric acid production and lipid accumulation, whereas the oxidative phosphorylation (OxPhos) shifted to the alternative respiration mode through alternative oxidase (AOX) protein. Simulations of optimized citrate secretion flux resulted in a pronounced lipid oxidation along with reactive oxygen species (ROS) generation and AOX flux inhibition. Then, we experimentally challenged AOX inhibition by adding n-Propyl Gallate (nPG), a specific AOX inhibitor, on Y. lipolytica batch cultures at stationary phase. Our results showed a twofold overproduction of citrate (20.5 g/L) when nPG is added compared to 10.9 g/L under control condition (no nPG addition). These results suggest that ROS management, especially through AOX activity, has a pivotal role on citrate/lipid flux balance in Y. lipolytica. All taken together, we thus provide for the first time, a key for the understanding of a predominant metabolic mechanism favoring citrate overproduction in Y. lipolytica at the expense of lipids accumulation.

Citric acid bioproduction and downstream processing: Status, opportunities, and challenges

Citric acid (CA) has been widely used in different industrial sectors, being produced through fermentation of low-cost feedstock. The development of downstream processes, easier to operate, environmentally friendly, and more economic than precipitation, is certainly a challenge in CA bioproduction. Large volumes of by-products generated in precipitation require treatment before disposal. Adsorption, extraction, and membrane separation have been shown to have a lower environmental impact than precipitation, but the technological maturity of these methods is still limited. However, reactive extraction and adsorption have great potential for industrial applications. This review shows that there is still much to be explored, both about the factors that are intrinsic to the techniques, but also in their combination for new processes' development. This review reports the most recent advances on CA bioproduction, with significant information about recovery and purification methods involving this highly industrially demanded organic acid.

Synthesis of high-titer alka(e)nes in Yarrowia lipolytica is enabled by a discovered mechanism

Alka(e)nes are ideal fuel components for aviation, long-distance transport, and shipping. They are typically derived from fossil fuels and accounting for 24% of difficult-to-eliminate greenhouse gas emissions. The synthesis of alka(e)nes in Yarrowia lipolytica from CO₂-neutral feedstocks represents an attractive alternative. Here we report that the high-titer synthesis of alka(e)nes in Yarrowia lipolytica harboring a fatty acid photodecarboxylase (CvFAP) is enabled by a discovered pathway. We find that acyl-CoAs, rather than free fatty acids (FFAs), are the preferred substrate for CvFAP. This finding allows us to debottleneck the pathway and optimize fermentation conditions so that we are able to redirect 89% of acyl-CoAs from the synthesis of neutral lipids to alka(e)nes and reach titers of 1.47 g/L from glucose. Two other CO₂-derived substrates, wheat straw and acetate, are also demonstrated to be effective in producing alka(e)nes. Overall, our technology could advance net-zero emissions by providing CO₂-neutral and energy-dense liquid biofuels.

Alka(e)nes with chain lengths in C5-C23 range are ideal fuel components. Here, the authors report that high-titer production of alak(e)nes in pathway engineered Yarrowia lipolytica, which is enabled by the finding that acyl-CoA is another substrate of fatty acid photodecarboxylase (FAP).

Comparative Gate-to-Gate Life Cycle Assessment for the Alkali and Acid Pre-Treatment Step in the Chemical Recycling of Waste Cotton

The development of textile recycling solutions is an area of intense research and commercialization. Chemical recycling solutions are becoming increasingly popular due to their ability to separate complex blends and retain or improve the value of the original fiber. The chemical recycling of cotton requires a pre-treatment step to reduce the degree of polymerization (DP). The DP can be reduced in a variety of ways, and here, the environmental footprints of two different pre-treatment approaches are examined using life cycle assessment (LCA); sodium hydroxide pre-treatment and sulphuric acid pre-treatment. We find that the acid pre-treatment has a significantly lower environmental footprint across all impact categories calculated. This is attributed to the lower treatment times required and the lower material and energy requirements for the manufacture of chemicals. The results were normalized to show the most significant impact categories for each pre-treatment, and further environmental implications of the pre-treatments are discussed. The findings will aid academia and industry in implementing the most environmentally benign processes in chemical cotton recycling.

Citric acid from Aspergillus niger: a comprehensive overview

Microbial citric acid has high economic importance and widely used in beverage, food, detergents, cosmetics and pharmaceutical industries. The filamentous fungus Aspergillus niger is a work horse and important cell factory in industry for the production of citric acid. Although in-depth literatures and reviews have been published to explain the biochemistry, biotechnology and genetic engineering study of citric acid production by Aspergillus niger separately but the present review compiled, all the aspects with upto date brief summary of the subject describing microorganisms, substrates and their pre-treatment, screening, fermentation techniques, metabolic engineering, biochemistry, product recovery and numerous biotechnological application of citric acid for simple understanding of microbial citric acid production. The availability of genome sequence of this organism has facilitated numerous studies in gene function, gene regulation, primary and secondary metabolism. An attempt has been also made to address the molecular mechanisms and application of recent advanced techniques such as CRISPR/Cas9 systems in enhancement of citric acid production.

Overexpression of Citrate Synthase Increases Isocitric Acid Biosynthesis in the Yeast Yarrowia lipolytica

Yarrowia lipolytica is a non-conventional yeast producing valuable compounds, such as citric acids, from renewable raw materials. This study investigated the impact of citrate synthase overexpression on the biosynthesis of citric and isocitric acid in Y. lipolytica. Two transformants of Y. lipolytica A101.1.31 strain (efficient citric acid producer), overexpressing CIT1 or CIT2 gene (encoding proteins with citrate synthase activity), were constructed. The results revealed that overexpression of either of these genes enhances citrate synthase activity. Additionally, the cit1 knockout strain was unable to use propionate as the sole carbon source, which proves that CIT1 gene encodes a dual activity protein–citrate and 2-methylcitrate synthase. In the overexpressing mutants, a significant increase in isocitric acid biosynthesis was observed. Both CIT1 and CIT2 overexpressing strains produced citric and isocitric acid from vegetable oil in a ratio close to 1 (CA/ICA ratio for wild-type strain was 4.12).

GEMs: genetically engineered microorganisms and the regulatory oversight of their uses in modern food production

Over the past several decades, the use of genetically engineered microorganisms (GEMs, often referred to as Genetically Modified Microorganisms or GMMs) has become widespread in the production of food processing aids and other food ingredients. GEMs are advancing food production by increasing efficiency, reducing waste and resource requirements, and ultimately enabling beneficial innovations such as the cost-effective fortification of food with essential nutrients, vitamins, and amino acids, and delivery of tailored enzymes to achieve unique food processing capabilities. Regulatory agencies, including those in the European Union, United States, and Canada review the safety of GEMs when evaluating food substances produced using GEMs to ensure that both the microorganism and the resulting food substance are safe. This paper provides a summary of historical and current use of GEMs in food manufacture, an overview of frameworks that regulate their use, and a description of the safety assessment of both GEMs and food substances produced with GEMs. The paper encourages regulatory agencies around the globe to take a more aligned approach to the safety evaluation and regulatory oversight of GEM-produced food ingredients and enzymes, a category of food substances that enables more sustainable consumer food choices.

Safety assessment of glutaminase from Aspergillus niger

Glutaminase (glutamine aminohydrolase EC 3.5.1.2) is used in the production of food ingredients rich in l‐glutamic acid that are added to finished foods for the purpose of enhancing or improving the savory flavor profile of food. The glutaminase enzyme preparation evaluated in these studies, designated as Sumizyme GT hereafter, is obtained by fermentation of Aspergillus niger strain GT147. The safety of Sumizyme GT was evaluated in a series of standard toxicological studies, including a 90‐day oral toxicity study in rats, an in vitro bacterial reverse mutation assay, an in vitro mammalian chromosome aberration test, and an in vivo alkaline Comet assay. Sumizyme GT was not mutagenic or genotoxic, and administration of the enzyme by gavage at doses up to 2,570 mg total organic solids (TOS)/kg body weight (bw) per day for 90 days was without any systemic toxicity. The no‐observed‐adverse‐effect level was concluded to be 2,570 mg TOS/kg bw per day, the highest dose tested. Considering that A. niger has an established history of safe use in the food industry and its safety in the production of food ingredients and food enzymes is well documented, the results of these studies provide further support of the safety of glutaminase from A. niger when used in food production.

Optimization of Citric Acid Production by Immobilized Cells of Novel Yeast Isolates

Citric acid is a commercially valuable organic acid widely used in food, pharmaceutical, and beverage industries. In this study, 260 yeast strains were isolated from soil, bread, juices, and fruits wastes and preliminarily screened using bromocresol green agar plates for their ability to produce organic acids. Overall, 251 yeast isolates showed positive results, with yellow halos surrounding the colonies. Citric acid production by 20 promising isolates was evaluated using both free and immobilized cell techniques. Results showed that citric acid production by immobilized cells (30-40 g/L) was greater than that of freely suspended cells (8-19 g/L). Of the 20 isolates, two (KKU-L42 and KKU-L53) were selected for further analysis based on their citric acid production levels. Immobilized KKU-L42 cells had a higher citric acid production rate (62.5%), while immobilized KKU-L53 cells showed an ∼52.2% increase in citric acid production compared with free cells. The two isolates were accurately identified by amplification and sequence analysis of the 26S rRNA gene D1/D2 domain, with GenBank-based sequence comparison confirming that isolates KKU-L42 and KKU-L53 were Candida tropicalis and Pichia kluyveri, respectively. Several factors, including fermentation period, pH, temperature, and carbon and nitrogen source, were optimized for enhanced production of citric acid by both isolates. Maximum production was achieved at fermentation period of 5 days at pH 5.0 with glucose as a carbon source by both isolates. The optimum incubation temperature for citric acid production by C. tropicalis was 32 °C, with NH₄Cl the best nitrogen source, while maximum citric acid by P. kluyveri was observed at 27 °C with (NH₄)₂ SO₄ as the nitrogen source. Citric acid production was maintained for about four repeated batches over a period of 20 days. Our results suggest that apple and banana wastes are potential sources of novel yeast strains; C. tropicalis and P. kluyveri which could be used for commercial citric acid production.