High-yield production of single-cell protein from starch processing wastewater using co-cultivation of yeasts

From microbial proteins to cultivated meat for alternative meat-like products: a review on sustainable fermentation approaches

The global demand for protein is rapidly increasing due to population growth and changing dietary preferences, highlighting the need for sustainable alternatives to traditional animal-based proteins. This review explores cultivated meat and microbial alternative proteins, focusing on their potential to meet nutritional needs while mitigating environmental impacts. It also examines the production of cultivated meat as well as various sources of microbial proteins, including mycoproteins, bacterial proteins, and microalgae, highlighting their nutritional profiles, production methods, and commercial applications. This includes an evaluation of the state of commercialization of mycoproteins and the innovative use of agricultural and industrial by-products as substrates for microbial fermentation. The integration of microbial protein production with the bioenergy sector is evaluated as a relevant alternative to attain a synergetic effect between energy and food production systems. Ultimately, this work aims to underscore the importance of microbial proteins in advancing towards a more sustainable protein production system, offering insights into current challenges and future opportunities in the field of fermentation to produce alternative proteins.

Solid-state fermentation of corn wet distiller grains and wheat bran with Trichoderma reesei and Candida utilis for improving feed value

BACKGROUND

Solid-state fermentation is one of the most effective methods for the high-value utilization of agro-industrial by-products. Co-fermentation of wet distiller grains and agricultural waste is an effective way to mitigate the feed shortage caused by corn consumption for bioethanol. It is still challenging to convert wet distiller grains and wheat bran to easily accessible carbon sources and adjust the balanced proportion of amino acids together by fermentation.

RESULTS

Fermentation time, strain ratio, and the addition of ammonium sulfate have been verified to be the important factors influencing the symbiosis of Trichoderma reesei (T. reesei) and Candida utilis (C. utilis) in a mixed system of wet distiller grains and wheat bran. The optimum conditions were fermentation for 8 days, 2:1 (T. reesei: C. utilis) strain ratio, and addition of 4% ammonium sulfate. After fermentation, the cellulose degradation proportion reached 39.1%, and the hemicellulose degradation proportion was 13.1%. The protein content improved by 29.6%. The lysine content increased by 126%, reaching 11.3 g·kg-1. The threonine content increased from 6.10 to 10.3 g·kg-1. The phytate content was decreased to 3.97 g·kg-1. The in vitro digestibility of dry matter and protein increased to 62.8% and 76.1%, respectively.

CONCLUSIONS

These results indicated the feasibility of improving the feeding value of wet distiller grains and wheat bran by the symbiosis of T. reesei and C. utilis. © 2024 Society of Chemical Industry.

Industrial Microbial Technologies for Feed Protein Production from Non-Protein Nitrogen

Due to the increasing global demand for feed protein, microbial protein has great potential of being able to feed sustainably. However, the application of microbial protein in the animal cultivation industry is still limited by its high cost and availability on scale. From the viewpoint of industrial production, it is vital to specify the crucial processes and components for further technical exploration and process optimization. This article presents state-of-the-art industrial microbial technologies for non-protein nitrogen (NPN) assimilation in feed protein production. Nitrogen sources are one of the main cost factors in the media used for large-scale microbial protein fermentation. Therefore, the available NPN sources for microbial protein synthesis, NPN utilization mechanisms, and fermentation technologies corresponding to the strain and NPN are reviewed in this paper. Especially, the random mutagenesis and adaptive laboratory evolution (ALE) approach combined with (ultra-) throughput screening provided the main impetus for strain evolution to increase the protein yield. Despite the underlying potential and technological advances in the production of microbial protein, extensive research and development efforts are still required before large-scale commercial application of microbial protein in animal feed.

Improving Cellular Protein Content of Saccharomyces cerevisiae Based on Adaptive Evolution and Flow Cytometry-Aided High Throughput Screening

Enhancing the protein content and production efficiency of Saccharomyces cerevisiae is crucial as an alternative protein source. This study screened nongenetically modified yeast strains with high protein content for food ingredient production and explored the underlying mechanisms. Yeast protein levels were found to correlate with RNA, leading to a high-throughput screening method using RNA fluorescence and flow cytometry. Four mutant libraries (∼200,000 cells) were generated through adaptive laboratory evolution in protein synthesis inhibitors, resulting in the high protein mutant content B1, with a protein content of 65.8 g/100 g dry cell weight in shake flasks. In a 45 L bioreactor using fed-batch fermentation with ethanol below 1.5 g/L, B1's protein content increased to 70.3 g/100 g dry cell weight, an 18.5% rise. Mannan and β-glucan levels in the cell wall decreased by 21.7 and 30.5%, potentially enhancing protein extraction for food production. Transcriptome analysis revealed that increased protein content results from down-regulating the cell cycle and meiosis-related genes. Validation of differentially expressed genes demonstrated that up-regulating SUT1 and down-regulating CNM67 are key for enhancing protein synthesis and accumulation. This study proposes a nongenetic screening method for high protein content S. cerevisiae strains, achieving the highest reported protein content.

Improved enzymatic hydrolysis of corn stover by a low-temperature and low-pressure holding post-treatment

Lignocellulose is one of the world's most abundant and underutilized biomass resources, and its proper treatment and utilization are critical to environmental issues and sustainable development. However, lignocellulose's inherently compact and intricate structure reduces enzymatic hydrolysis's efficiency, which is still an obstacle to overcome. A new pretreatment method with relatively low-temperature and low-pressure holding (LTLPH) after the traditional extrusion, pulp refining instrument (PFI), and instant catapult steam explosion (ICSE) was proposed to obtain a better output of corn stover saccharification. The chemical composition, SEM, swelling capacity of corn stover before and after treatment, and types and contents of mixed sugars were examined to explore the mechanism for the diversity of the enzymatic hydrolysis effect. It was found that the highest reducing sugar in the optimized compounding pretreatment method of ICSE-LTLPH (LTLPH: 70 kPa, 115 °C, 30 min) could reach 27.96 g/L, promoting more than 50%. The cultured single-cell protein content was 8.19% and 7.73% higher than those with glucose and simulated mixed sugar medium, respectively, promisingly replacing commercial sugars for Candida utilis (C. utilis) growth. Therefore, the developed pretreatment method of ICSE-LTLPH could exert better effects on the enzymatic hydrolysis of corn stover, providing a potential for cultivating C. utilis without detoxification of enzymatic hydrolysate.

Current trends and possibilities of typical microbial protein production approaches: a review

Global population growth and demographic restructuring are driving the food and agriculture sectors to provide greater quantities and varieties of food, of which protein resources are particularly important. Traditional animal-source proteins are becoming increasingly difficult to meet the demand of the current consumer market, and the search for alternative protein sources is urgent. Microbial proteins are biomass obtained from nonpathogenic single-celled organisms, such as bacteria, fungi, and microalgae. They contain large amounts of proteins and essential amino acids as well as a variety of other nutritive substances, which are considered to be promising sustainable alternatives to traditional proteins. In this review, typical approaches to microbial protein synthesis processes were highlighted and the characteristics and applications of different types of microbial proteins were described. Bacteria, fungi, and microalgae can be individually or co-cultured to obtain protein-rich biomass using starch-based raw materials, organic wastes, and one-carbon compounds as fermentation substrates. Microbial proteins have been gradually used in practical applications as foods, nutritional supplements, flavor modifiers, and animal feeds. However, further development and application of microbial proteins require more advanced biotechnological support, screening of good strains, and safety considerations. This review contributes to accelerating the practical application of microbial proteins as a promising alternative protein resource and provides a sustainable solution to the food crisis facing the world.

Co-production of plant- and microbial- proteins from waste tobacco leaves by optimizing alkaline extraction and strengthening pectin bioconversion

The production of alternative proteins is of great significance in the mitigation of food problems. This study proposes an integrated approach including protein extraction, enzymatic hydrolysis, and fermentation to produce both plant proteins and single-cell proteins as alternative proteins from tobacco leaves, a highly-abundant and protein-rich agricultural waste. Alkaline extraction of proteins before polysaccharide hydrolysis was found to be preferable for increasing the yields of plant proteins and mono-sugars. The combined use of pectinase-rich enzymes from Aspergillus brunneoviolaceus and hemicellulase-rich enzymes from Penicillium oxalicum achieved the release of 80.7 % of the sugars after 72 h. Cutaneotrichosporon cutaneum could simultaneously utilize multiple sugars, including galacturonic acid, in the enzymatic hydrolysate to produce single-cell proteins. Via this approach, 43.54 g crude proteins of high protein contents and rich in essential amino acids can be produced from 100.00 g waste tobacco leaves, providing a promising strategy for its valorization.

Enhancing extracellular monascus pigment production in submerged fermentation with engineered microbial consortia

In this study, we investigated the impact of microbial interactions on Monascus pigment (MP) production. We established diverse microbial consortia involving Monascus purpureus and Lactobacillus fermentum. The addition of Lactobacillus fermentum (4% at 48 h) to the submerged fermentation of M. purpureus resulted in a significantly higher MP production compared to that achieved using the single-fermentation system. Co-cultivation with immobilized L. fermentum led to a remarkable increase of 59.18% in extracellular MP production, while mixed fermentation with free L. fermentum caused a significant decrease of 66.93% in intracellular MPs, contrasting with a marginal increase of 4.52% observed during co-cultivation with immobilized L. fermentum and the control group respectively. The findings indicate an evident enhancement in cell membrane permeability of M. purpureus when co-cultivated with immobilized L. fementum. Moreover, integrated transcriptomic and metabolomic analyses were conducted to elucidate the regulatory mechanisms underlying MP biosynthesis and secretion following inoculation with immobilized L. fementum, with specific emphasis on glycolysis, steroid biosynthesis, fatty acid biosynthesis, and energy metabolism.

Single-cell protein: overcoming technological and biological challenges towards improved industrialization

The commercialization of single-cell protein (SCP) obtained from microbial fermentation in large-scale bioreactors emerged almost 50 years ago, with Pruteen marketed as animal feed in the 1970s and Quorn®, released for human nutrition in 1985. SCP holds great promises to feed the meanwhile doubled world population in a sustainable way, but its application is still limited by price and availability on scale. There is a need to optimize the underlying manufacturing processes with enhanced affordability and productivity. From the industrial perspective, it is crucial to identify key process components and prioritize innovations that best promote cost efficiency and large-scale production. Here, we present the state-of-art in SCP manufacturing and provide a comprehensive insight into recent techno-economic analyses and life-cycle assessments of different production scenarios. Thereby, we identified the most influential technical hotspots and challenges for each of the main production scenarios and evaluated the technological opportunities to overcome them.

A comprehensive report on valorization of waste to single cell protein: strategies, challenges, and future prospects

The food insecurity due to a vertical increase in the global population urgently demands substantial advancements in the agricultural sector and to identify sustainable affordable sources of nutrition, particularly proteins. Single-cell protein (SCP) has been revealed as the dried biomass of microorganisms such as algae, yeast, and bacteria cultivated in a controlled environment. Production of SCP is a promising alternative to conventional protein sources like soy and meat, due to quicker production, minimal land requirement, and flexibility to various climatic conditions. In addition to protein production, it also contributes to waste management by converting it into food and feed for both human and animal consumption. This article provides an overview of SCP production, including its benefits, safety, acceptability, and cost, as well as limitations that constrains its maximum use. Furthermore, this review criticizes the downstream processing of SCP, encompassing cell wall disruption, removal of nucleic acid, harvesting of biomass, drying, packaging, storage, and transportation. The potential applications of SCP, such as in food and feed as well as in the production of bioplastics, emulsifiers, and as flavoring agents for baked food, soup, and salad, are also discussed.

Valorization of spent coffee grounds through integrated bioprocess of fermentable sugars, volatile fatty acids, yeast-based single-cell protein and biofuels production

Recovering nutrients from waste for biological processes aligns with sustainability principles. This study aimed to convert spent coffee grounds (SCG) into valuable products, including fermentable sugars, volatile fatty acids (VFAs), yeast-based single-cell protein and biofuels. Alkaline pretreatment was conducted before enzymatic hydrolysis, in which the pretreated SCG was hydrolyzed with varying enzyme loadings (20-60 filter paper units (FPU)/g-solid) and solid loadings (3-15 % w/v). The hydrolyzed slurry was utilized for VFAs and hydrogen production, yielding high values of 0.66 g/g-volatile solids (VS) and 109 mL/g-VS, respectively, using an enzyme loading of 50 FPU/g-solid and a solid loading of 3 % (w/v). The derived VFAs were used to cultivate a newly isolated yeast, Candida maltosa KKU-ARY2, resulting in an accumulated protein content of 43.7 % and a biomass concentration of 4.6 g/L. This study highlights the conversion of SCG into essential components, emphasizing the benefits of waste utilization through cascade bioprocesses.

Efficient production of single cell protein from biogas slurry using screened alkali-salt-tolerant Debaryomyces hansenii

Production of single cell protein (SCP) by recovering ammonia nitrogen from biogas slurry shows great potential against protein scarcity and unsustainable production of plant and animal proteins. Herein, a high-alkali-salt-tolerant yeast strain, Debaryomyces hansenii JL8-0, was isolated and demonstrated for high-efficient SCP production. This strain grew optimally at pH 8.50 and 2500 mg/L NH4+-N, and it could efficiently utilize acetate as the additional carbon source. Under optimal conditions, SCP biomass of 32.21 g/L and productivity of 0.32 g/L·h-1 were obtained in fed-batch fermentation. Remarkably, nearly complete (97.40 %) ammonia nitrogen from biogas slurry was recovered, probably due to its high affinity for NH4+-N. Altogether, this strain showed advantages in terms of cell biomass titer, productivity, and yield. A cultivation strategy was proposed by co-culturing D. hansenii with other compatible yeast strains to achieve high-efficient SCP production from biogas slurry, which could be a promising alternative technology for biogas slurry treatment.

Production of single cell protein from brewer's spent grain through enzymatic saccharification and fermentation enhanced by ammoniation pretreatment

Brewer's spent grain (BSG) is a major low-value by-product of beer industry. To realize the high value application of BSG, this work proposed a strategy to produce single cell protein (SCP) with oligosaccharide prebiotics from BSG, via ammoniation pretreatment, enzymatic hydrolysis, and fermentation. The optimum conditions of ammoniation pretreatment obtained by response surface method were 11 % ammonia dosage (w/w), 63 °C for 26 h. Suitable enzyme and yeast were screened to enhance the conversion of cellulose and hemicellulose in BSG into sugars and maximize the SCP yield. It was shown that using lignocellulolytic enzyme SP from Penicillium oxalicum and Trichosporon cutaneum, about 310 g of SCP with 80 g of arabinoxylo-oligosaccharides were obtained from 1000 g of BSG. This process is low cost, high efficiency, and easy to implement, which has good industrial application prospects.

Production of single cell protein rich in potassium by Nectaromyces rattus using biogas slurry and molasses

Single-cell protein (SCP) is a vital supplement for animal protein feed. This study utilized biogas slurry and sugarcane molasses to ferment Nectaromyces rattus for the production of SCP. The optimal batch fermentation conditions were obtained in a 5L jar with a tank pressure of 0.1 MPa, an initial speed of 300 rpm, and an inoculum volume of 30%. The highest cell dry weight concentrations of the fed-batch fermentation without reflux and the fed-batch fermentation with reflux were 46.33 g/L and 29.71 g/L, respectively. The nitrogen conversion rates (47.05% and 44.12%) and the cell yields of total organic carbon (1 g/g and 1.17 g/g) of both fermentation modes were compared. The SCP contained 42.32% amino acids. Its high concentrations of potassium (19859.96 mg/kg) and phosphorus (7310.44 mg/kg) present a novel approach for the extraction of these essential nutrients from biogas slurry. The enrichment of K was related to the H+ efflux and sugar transport.

Valorization of Food Waste into Single-Cell Protein: An Innovative Technological Strategy for Sustainable Protein Production

The rapidly increasing population and climate change pose a great threat to our current food systems. Moreover, the high usage of animal-based and plant-based protein has its drawbacks, as these nutritional sources require many hectares of land and water, are affected by seasonal variations, are costly, and contribute to environmental pollution. Single-cell proteins (SCPs) are gaining a lot of research interest due to their remarkable properties, such as their high protein content that is comparable with other protein sources; low requirements for land and water; low carbon footprint; and short production period. This review explores the use of food waste as a sustainable feedstock for the advancement of SCP processes. It discusses SCP studies that exploit food waste as a substrate, alongside the biocatalysts (bacteria, fungi, yeast, and microalgae) that are used. The operational setpoint conditions governing SCP yields and SCP fermentation routes are elucidated as well. This review also demonstrates how the biorefinery concept is implemented in the literature to improve the economic potential of "waste-to-protein" innovations, as this leads to the establishment of multiproduct value chains. A short section that discusses the South African SCP scenario is also included. The technical and economic hurdles facing second-generation SCP processes are also discussed, together with future perspectives. Therefore, SCP technologies could play a crucial role in the acceleration of a "sustainable protein market", and in tackling the global hunger crisis.

Metatranscriptome analysis unveils the mechanisms of zero-valent iron enhancing reactivation of starvation hydrolysis acidification sludge by inducing high-level gene expression

Hydrolysis acidification (HA) is a promising method for wastewater treatment and resource recovery. However, the extended time required for bacterial reactivation after starvation or a change in living conditions often poses a challenge to the efficient operation of the system. Although the addition of zero-valent iron (ZVI) could enhance HA performance, its effects on sludge reactivation in the HA process are not fully understood. In this study, ZVI was employed to accelerate sludge reactivation and its involved genetic mechanisms were unveiled. The results demonstrated that ZVI addition activated the sludge within 35 days with stable HA performance. Sludge characteristics revealed that ZVI improved active biomass, enzyme activity (by 11.4 % ∼ 26.7 %), ETS activity (by 566 %), and cell viability, with a higher concentration of MLVSS, live cells, more microbial byproducts in EPS, and relative abundance of HA bacteria (63.41 %). Moreover, metatranscriptome analysis showed that ZVI upregulated the expression of genes related to key enzymes in carbohydrate degradation metabolism, biosynthesis of electron transfer media such as heme and ubiquinone, and biosynthesis of vital cofactors like vitamin B12 and folate during microbial growth and metabolism. These findings suggest that ZVI enhanced electron transfer, bacterial growth, and metabolism, resulting in effective starch conversion and VFAs generation. Overall, these results deepen our understanding of the mechanism by which ZVI enhanced HA sludge reactivation, providing valuable information for addressing sludge starvation issues in HA systems.

Coproduction of single cell protein and lipid from lignocellulose derived carbohydrates and inorganic ammonia salt with soluble ammonia recycling

Co-production of single cell protein (SCP) and lipid from lignocellulose-derived carbohydrates and inorganic ammonia offers a promising alternative for poultry or aquaculture feeds. An engineered oleaginous yeast Trichosporon cutaneum MP11 showed great potential for producing SCP and lipid from wheat straw and ammonia sulfate with minimum nutrient input. Trichosporon cutaneum MP11 showed stronger SCP and lipid fermentability using dry acid pretreated and biodetoxified wheat straw than using pure sugars. The residual ammonium sulfate in fermentation broth was recycled up to five times, resulting in ∼70% of nitrogen fixation into SCP. The overall yield of SCP and lipid from lignocellulose-derived sugars was 0.15 g/g and 0.11 g/g, respectively. This translates to the production of one ton of SCP (0.56 ton) and lipid (0.44 ton) from 6.6 tons of wheat straw, or one ton of SCP and lipid containing yeast cells (dry) from 4.8 tons of wheat straw.

Sugar Beet Pulp as a Biorefinery Substrate for Designing Feed

An example of the implementation of the principles of the circular economy is the use of sugar beet pulp as animal feed. Here, we investigate the possible use of yeast strains to enrich waste biomass in single-cell protein (SCP). The strains were evaluated for yeast growth (pour plate method), protein increment (Kjeldahl method), assimilation of free amino nitrogen (FAN), and reduction of crude fiber content. All the tested strains were able to grow on hydrolyzed sugar beet pulp-based medium. The greatest increases in protein content were observed for Candida utilis LOCK0021 and Saccharomyces cerevisiae Ethanol Red (ΔN = 2.33%) on fresh sugar beet pulp, and for Scheffersomyces stipitis NCYC1541 (ΔN = 3.04%) on dried sugar beet pulp. All the strains assimilated FAN from the culture medium. The largest reductions in the crude fiber content of the biomass were recorded for Saccharomyces cerevisiae Ethanol Red (Δ = 10.89%) on fresh sugar beet pulp and Candida utilis LOCK0021 (Δ = 15.05%) on dried sugar beet pulp. The results show that sugar beet pulp provides an excellent matrix for SCP and feed production.