Process alternatives for bioethanol production from mango stem bark residues

Engineered yeasts and lignocellulosic biomaterials: shaping a new dimension for biorefinery and global bioeconomy

The next milestone of synthetic biology research relies on the development of customized microbes for specific industrial purposes. Metabolic pathways of an organism, for example, depict its chemical repertoire and its genetic makeup. If genes controlling such pathways can be identified, scientists can decide to enhance or rewrite them for different purposes depending on the organism and the desired metabolites. The lignocellulosic biorefinery has achieved good progress over the past few years with potential impact on global bioeconomy. This principle aims to produce different bio-based products like biochemical(s) or biofuel(s) from plant biomass under microbial actions. Meanwhile, yeasts have proven very useful for different biotechnological applications. Hence, their potentials in genetic/metabolic engineering can be fully explored for lignocellulosic biorefineries. For instance, the secretion of enzymes above the natural limit (aided by genetic engineering) would speed-up the down-line processes in lignocellulosic biorefineries and the cost. Thus, the next milestone would greatly require the development of synthetic yeasts with much more efficient metabolic capacities to achieve basic requirements for particular biorefinery. This review gave comprehensive overview of lignocellulosic biomaterials and their importance in bioeconomy. Many researchers have demonstrated the engineering of several ligninolytic enzymes in heterologous yeast hosts. However, there are still many factors needing to be well understood like the secretion time, titter value, thermal stability, pH tolerance, and reactivity of the recombinant enzymes. Here, we give a detailed account of the potentials of engineered yeasts being discussed, as well as the constraints associated with their development and applications.

Fruit residues as biomass for bioethanol production using enzymatic hydrolysis as pretreatment

The commercialization of fruits in markets generates a large amount of waste because they are perishable and have a short shelf life, so, they are discarded. This study aimed to provide a noble end to discarded fruits that have fermentable sugars. Banana, apple, mango and papaya residues were collected from supermarkets and underwent an enzymatic hydrolysis process. The ability of four pectinases, two amylases, one xylanase and one cellulase to release reducing sugars from fruit biomass before fermentation with two yeast strains (S. cerevisiae CAT-1 and S. cerevisiae Angel) for bioethanol production was investigated, obtaining a total of RS (Reducing sugar) of 268.08 mg/mL in banana residues. A fermentation with yeast S. cerevisiae CAT-1 resulted in 98% consumption of RS and the production of a total of 28.02 g/L of ethanol. Furthermore, fermentation with the yeast S. cerevisiae Angel, resulted in 97% RS consumption and 31.87 g/L ethanol production, which was the best result obtained throughout all the tests of hydrolysis, highlighting the banana residue as a promising biomass for the production of bioethanol.

Bioenergy production in Pakistan: Potential, progress, and prospect

Pakistan is a developing country with a rapidly growing population. It is currently facing serious economic and energy challenges. Pakistan's energy demand is increasing by the day, and it now stands at 84 MTOE. Currently, the use of fossil fuels dominates Pakistan's energy sector. Conversely, indigenous fossil fuel resources are rapidly depleting and will be unable to meet rising energy demands in the future. Therefore, to withstand its energy needs, the country will need to explore alternative energy production methods. Biomass is one of the alternatives that has enormous potential to help Pakistan combat its growing energy crisis. In this review, we first present an overview of bioenergy, biomass resources, and biomass conversion technologies. We then discuss in detail the current state of the energy mix of Pakistan. Subsequently, we show that annual production of about 121 MT of agricultural residues, 427 MT of animal manure, and 7.5 MT of MSW in Pakistan offer a variety of bioenergy options ranging from biofuels to bio-electricity production. Overall, these biomass resources in Pakistan have the potential to generate 20,709 MW of bio-electricity and 12,615 million m³ of biogas annually in Pakistan. Though these resources hold promising potential for bioenergy production in the country, however, there are some critical challenges that need to be considered, and some of which are extremely difficult to overcome for a developing country like Pakistan. This work is expected to provide a useful basis for biomass management and utilization in Pakistan to harvest eco-friendly and sustainable green energy locally.

Biotransformation of lignocellulosic biomass into industrially relevant products with the aid of fungi-derived lignocellulolytic enzymes

Lignocellulosic material has drawn significant attention among the scientific community due to its year-round availability as a renewable resource for industrial consumption. Being an economic substrate alternative, various industries are reevaluating processes to incorporate derived compounds from these materials. Varieties of fungi and bacteria have the ability to depolymerize lignocellulosic biomass by synthesizing degrading enzymes. Owing to catalytic activity stability and high yields of conversion, lignocellulolytic enzymes derived from fungi currently have a high spectrum of industrial applications. Moreover, these materials are cost effective, eco-friendly and nontoxic while having a low energy input. Techno-economic analysis for current enzyme production technologies indicates that synthetic production is not commercially viable. Instead, the economic projection of the use of naturally-produced ligninolytic enzymes is promising. This approach may improve the economic feasibility of the process by lowering substrate expenses and increasing lignocellulosic by-product's added value. The present review will discuss the classification and enzymatic degradation pathways of lignocellulolytic biomass as well as the potential and current industrial applications of the involved fungal enzymes.

Revalorization of bioactive compounds from tropical fruit by-products and industrial applications by means of sustainable approaches

Tropical fruits trade is on the rise due to the claimed health benefits related with their consumption. Functional activities are exerted by the presence of bioactive compounds which could be used for prevention or amelioration diseases. However, the occurrence of bioactive compounds is found mainly in non-edible fraction of tropical fruits which are usually discarded. Therefore, the revalorization of tropical fruits by-products as source of functional compounds is on the cutting-edge research. The implementation of this challenge not only allows the enhancement of the tropical fruits by-products management, but also the production of value-added products. This review compiles the latest comprehensive information about the revalorization of bioactive compounds from tropical fruits by-products. A revision of the sustainable green technologies used for the isolation of valuable compounds has been carried out as well as the current food, functional, cosmeceutical and bioenergetics industrial applications of bioactive compounds extracted from tropical fruits by-products.

Progress in the development of alkali and metal salt catalysed lignocellulosic pretreatment regimes: Potential for bioethanol production

Lignocellulosic biomass (LCB) is well suited to address present day energy and environmental concerns, since it is abundant, environmentally benign and sustainable. However, the commercial application of LCB has been limited by its recalcitrant structure. To date, several biomass pretreatment systems have been developed to address this major bottleneck but have shown to be toxic and costly. Alkali and metal salt pretreatment regimes have emerged as promising non-toxic and low-cost treatments. This paper examines the progress made in lignocellulosic pretreatment using alkali and metal salts. The reaction mechanism of alkali and metal chloride salts on lignocellulosic biomass degradation are reviewed. The effect of salt pretreatment on lignin removal, hemicellulose solubilization, cellulose crystallinity, and physical structural changes are also presented. In addition, the enzymatic digestibility and inhibitor profile from salt pretreated lignocellulosic biomass are discussed. Furthermore, the challenges and future prospects on lignocellulosic pretreatment and bioethanol production are highlighted.

Zirconium based metal-organic framework in-situ assisted hydrothermal pretreatment and enzymatic hydrolysis of Platanus X acerifolia exfoliating bark for bioethanol production

Metal-organic framework (MOF) assisted hydrothermal pretreatment and co-catalysis strategy based on UiO-66 MOF is developed for the first time. The Planetree exfoliating bark was pretreated with or without UiO-66 assisted hydrothermal method at a temperature ranging from 160 to 240 °C for 1-3 h residence. With the rise of pretreatment severity, the total reducing sugar (TRS) was increased till reached maximum, 180 mg g^-1, in the presence of UiO-66. The fitting models validate the optimal hydrothermal condition was at 180 °C and 1 h, which was characterized with high TRS and very low yield of furfural and HMF. The TRS from enzymatic hydrolysis reaches maximum, 391 mg g^-1, in the presence of MOF co-catalysis and the maximum ethanol yield achieved was 73%. Altered morphology, higher surface area and porosity are noticed after MOF assisted hydrothermal pretreatment. This study insights the MOFs' application in lignocellulose biomass processing.

Bioethanol production from sugarcane leaf waste: Effect of various optimized pretreatments and fermentation conditions on process kinetics

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Bioethanol kinetics was investigated under SSA-F, SSA-U, MSA-F and MSA-U conditions.

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Monod, logistic and modified Gompertz models gave R² > 0.97.

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SSA-U pretreated SLW produced 25% more bioethanol than MSA-U.

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No difference was observed between filtered and unfiltered enzymatic hydrolysate.

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SLW residue showed a suitable protein and fat content for animal feed.

This study examines the kinetics of S. cerevisiae BY4743 growth and bioethanol production from sugarcane leaf waste (SLW), utilizing two different optimized pretreatment regimes; under two fermentation modes: steam salt-alkali filtered enzymatic hydrolysate (SSA-F), steam salt-alkali unfiltered (SSA-U), microwave salt-alkali filtered (MSA-F) and microwave salt-alkali unfiltered (MSA-U). The kinetic coefficients were determined by fitting the Monod, modified Gompertz and logistic models to the experimental data with high coefficients of determination R² > 0.97. A maximum specific growth rate (μ_max) of 0.153 h⁻¹ was obtained under SSA-F and SSA-U whereas, 0.150 h⁻¹ was observed with MSA-F and MSA-U. SSA-U gave a potential maximum bioethanol concentration (P_m) of 31.06 g/L compared to 30.49, 23.26 and 21.79 g/L for SSA-F, MSA-F and MSA-U respectively. An insignificant difference was observed in the μ_max and P_m for the filtered and unfiltered enzymatic hydrolysate for both SSA and MSA pretreatments, thus potentially reducing a unit operation. These findings provide significant insights for process scale up.

Efficient production of succinic acid from duckweed (Landoltia punctata) hydrolysate by Actinobacillus succinogenes GXAS137

A novel process of enzyme pretreatment and semi-simultaneous saccharification and fermentation (SSSF) was developed in this work to improve succinic acid (SA) productivity from duckweed (Landoltia punctata) and achieve low viscosity. Viscosity (83.86%) was reduced by the pretreatment with combined enzymes at 50 °C for 2 h to a greater extent than that by single enzyme (26.19-71.75%). SSSF was an optimal combination with 65.31 g/L of SA content, which was remarkably higher than those obtained through conventional separate hydrolysis and fermentation (62.12 g/L) and simultaneous saccharification and fermentation (52.41 g/L). The combined approach was effective for SA production. Approximately 75.46 g/L of SA content with a yield of 82.87% and a productivity of 1.35 g/L/h was obtained after 56 h in a 2 L bioreactor. Further studies will focus on increasing the working scale of the proposed method.