05-24-2013, 09:41 PM
(This post was last modified: 05-25-2013, 04:10 AM by Administrator.)
No Food? Eat Wood!
Plants give us food. But the question is can plants produce enough food to feed the growing global population that is estimated to reach 9 billion by 2050? Many statistics estimate they can’t. But a solution may not be far, predict the scientists. A new method of enzymatically converting cellulose, the structural polysaccharide in plants, into edible starch has been discovered by a group of researchers at the Virginia Polytechnic Institute.
Cellulose is the most abundantly occurring organic material in the biosphere. many kilogrammes of cellulose is said to be synthesized and degraded on earth each year. Finding a way to convert this cellulosic waste into utilisable forms has been the interest of many researchers for decades. However, due to the presence of strong binding forces between the cellulose molecules, hydrolysis of cellulose is relatively difficult compared to the breakdown of other polysaccharides.
This remarkable discovery, a bioprocess called ‘Simultaneous Enzymatic Biotransformation and microbial Fermentation’ (SEBF), provides a means of producing not only amylose, the unbranched form of starch, but also bioethanol and single cell proteins in a one-pot bioconversion process. Researchers declare that up to 30% of cellulose can be hydrolysed into starch while the remainder is converted into glucose which is then fermented into ethanol by yeast in the same bioreactor.
Following is a simplified summery of the process and the economic prospects of its products as depicted in the published report of the study in the journal PNAS on 15th April 2013.
Although the three processes; hydrolysis of cellulose, synthesis of amylose and production of ethanol occur simultaneously in the bioreactor, this process can be broken down into several steps for the sake of understanding.
Step 1 : Pre-treatment of cellulose biomass
The enzyme mixtures employed in this study were optimized for efficient hydrolysis of amorphous cellulose rather than of crystalline cellulose and hemicellulose- and lignin-containing biomass. Since natural plant biomass contains lignin and hemicellulose together with crystalline cellulose, interlinked in a hetero-matrix which is highly resistant and thus not efficiently degraded by enzymes alone, some kind of pre-treatment is necessary to make the biomass more accessible to enzymes.
In this experiment, insoluble Regenerated Amorphous Cellulose (RAC) was used as the source of cellulose. RAC was prepared by pre-treating Avicell (a commercially microcrystalline cellulose powder) with concentrated phosphoric acid.
Step 2 : Breakdown of pre-treated cellulose biomass into cellobiose
In this step, a combination of two enzymes, Bacillus subtilis endoglucanase (EG) and Trichoderma spp. Cellobiohydrolase hydrolyses (CBH) converts cellulose into glucose and cellobiose which is a disaccharide consists of two glucose molecules.
Among five cellulose enzymes tested in the study, the combination of Bacillus subtilis endoglucanase and Trichoderma spp. Cellobiohydrolase proved to produce higher cellulose degradation resulting in high cellobiose yields.
Step 3 : Synthesis amylose from cellobiose
Produced cellobiose is then polymerised into amylose, a linear polysaccharide made out of glucose monomers linked together with alpha-1,4-glycosidic bonds and alpha-1,6-glycosidic bonds. Two enzymes, Clostridium thermocellum cellobiose phosphorylase (CBP) and potato alpha-glucan phosphorylase (PGP) are used catalyse this reaction.
Here, cellobiose is first converted into Glucose-1-phosphate and glucose by CBP and then the resulting Glucose-1-phosphate is linked together by PGP, forming amylose chains.
Three αGPs, one from potato and the rest from two thermophilic bacterial species were tested. Among them, only the potato αPG (PGP) was able to facilitate the synthesis of amylose.
Step 4 : Fermentation of glucose into ethanol
Glucose produced in the steps two and four are then fermented into ethanol by the yeast strain Saccharomyces cerevisiae. The yeast biomass can be used as Single Cell Proteins.
Step 5 : Product recovery
At the end, ethanol can be separated by distillation. The precipitated synthetic amylose can be extracted with NaOH following precipitation by neutralization. The yeast cells and the biomass residues will remain in solid pellets.
The enzymes were recombinantly produced in Escherichia coli. The enzymes were co-immobilised on Avicel-containing nanomagnetic particles, enabling their recovery with the use of a magnetic field.
See the attachment for a simplified flow chart of the process.
Economic viability of the process
This method yields three products; amylose, Bioethanol, and yeast as single-cell proteins which are of immense commercial significance. Amylose can be used for various purposes including drug capsule materials for the pharmaceutical industry, biodegradable plastics, food additive, food-grade amylose, high-density hydrogen carrier etc. Amylose also can be converted into branched amylopectine by alpha-glucan–branching glycosyltransferase enzymes. Ethanol can be used for the production of biofuels and the SCPs together with the residual amylose can be used as animal feed.
Furthermore, the efficiency is enhanced because no energy or costly chemicals are required, the immobilised enzymes can be easily recovered and no glucose is wasted.
Further scaling-up
The researchers hope to further improve the process to ensure a sustainable industrial scale application of the process. Increasing the stability of CBP and PGP and decreasing their production costs, optimizing the enzyme mixture composition and ratio, improving process design and biomass pre-treatment conditions are some of the key areas they hope to look into.
Source:
You, C., Chen, H., Myung, S., Sathitsuksanoh, N., Ma, H., Zhang, X. Z., ... & Zhang, Y. H. P. (2013). Enzymatic transformation of nonfood biomass to starch. Proceedings of the National Academy of Sciences, 110(18), 7182-7187.
Plants give us food. But the question is can plants produce enough food to feed the growing global population that is estimated to reach 9 billion by 2050? Many statistics estimate they can’t. But a solution may not be far, predict the scientists. A new method of enzymatically converting cellulose, the structural polysaccharide in plants, into edible starch has been discovered by a group of researchers at the Virginia Polytechnic Institute.
Cellulose is the most abundantly occurring organic material in the biosphere. many kilogrammes of cellulose is said to be synthesized and degraded on earth each year. Finding a way to convert this cellulosic waste into utilisable forms has been the interest of many researchers for decades. However, due to the presence of strong binding forces between the cellulose molecules, hydrolysis of cellulose is relatively difficult compared to the breakdown of other polysaccharides.
This remarkable discovery, a bioprocess called ‘Simultaneous Enzymatic Biotransformation and microbial Fermentation’ (SEBF), provides a means of producing not only amylose, the unbranched form of starch, but also bioethanol and single cell proteins in a one-pot bioconversion process. Researchers declare that up to 30% of cellulose can be hydrolysed into starch while the remainder is converted into glucose which is then fermented into ethanol by yeast in the same bioreactor.
Following is a simplified summery of the process and the economic prospects of its products as depicted in the published report of the study in the journal PNAS on 15th April 2013.
Although the three processes; hydrolysis of cellulose, synthesis of amylose and production of ethanol occur simultaneously in the bioreactor, this process can be broken down into several steps for the sake of understanding.
Step 1 : Pre-treatment of cellulose biomass
The enzyme mixtures employed in this study were optimized for efficient hydrolysis of amorphous cellulose rather than of crystalline cellulose and hemicellulose- and lignin-containing biomass. Since natural plant biomass contains lignin and hemicellulose together with crystalline cellulose, interlinked in a hetero-matrix which is highly resistant and thus not efficiently degraded by enzymes alone, some kind of pre-treatment is necessary to make the biomass more accessible to enzymes.
In this experiment, insoluble Regenerated Amorphous Cellulose (RAC) was used as the source of cellulose. RAC was prepared by pre-treating Avicell (a commercially microcrystalline cellulose powder) with concentrated phosphoric acid.
Step 2 : Breakdown of pre-treated cellulose biomass into cellobiose
In this step, a combination of two enzymes, Bacillus subtilis endoglucanase (EG) and Trichoderma spp. Cellobiohydrolase hydrolyses (CBH) converts cellulose into glucose and cellobiose which is a disaccharide consists of two glucose molecules.
Among five cellulose enzymes tested in the study, the combination of Bacillus subtilis endoglucanase and Trichoderma spp. Cellobiohydrolase proved to produce higher cellulose degradation resulting in high cellobiose yields.
Step 3 : Synthesis amylose from cellobiose
Produced cellobiose is then polymerised into amylose, a linear polysaccharide made out of glucose monomers linked together with alpha-1,4-glycosidic bonds and alpha-1,6-glycosidic bonds. Two enzymes, Clostridium thermocellum cellobiose phosphorylase (CBP) and potato alpha-glucan phosphorylase (PGP) are used catalyse this reaction.
Here, cellobiose is first converted into Glucose-1-phosphate and glucose by CBP and then the resulting Glucose-1-phosphate is linked together by PGP, forming amylose chains.
Three αGPs, one from potato and the rest from two thermophilic bacterial species were tested. Among them, only the potato αPG (PGP) was able to facilitate the synthesis of amylose.
Step 4 : Fermentation of glucose into ethanol
Glucose produced in the steps two and four are then fermented into ethanol by the yeast strain Saccharomyces cerevisiae. The yeast biomass can be used as Single Cell Proteins.
Step 5 : Product recovery
At the end, ethanol can be separated by distillation. The precipitated synthetic amylose can be extracted with NaOH following precipitation by neutralization. The yeast cells and the biomass residues will remain in solid pellets.
The enzymes were recombinantly produced in Escherichia coli. The enzymes were co-immobilised on Avicel-containing nanomagnetic particles, enabling their recovery with the use of a magnetic field.
See the attachment for a simplified flow chart of the process.
Economic viability of the process
This method yields three products; amylose, Bioethanol, and yeast as single-cell proteins which are of immense commercial significance. Amylose can be used for various purposes including drug capsule materials for the pharmaceutical industry, biodegradable plastics, food additive, food-grade amylose, high-density hydrogen carrier etc. Amylose also can be converted into branched amylopectine by alpha-glucan–branching glycosyltransferase enzymes. Ethanol can be used for the production of biofuels and the SCPs together with the residual amylose can be used as animal feed.
Furthermore, the efficiency is enhanced because no energy or costly chemicals are required, the immobilised enzymes can be easily recovered and no glucose is wasted.
Further scaling-up
The researchers hope to further improve the process to ensure a sustainable industrial scale application of the process. Increasing the stability of CBP and PGP and decreasing their production costs, optimizing the enzyme mixture composition and ratio, improving process design and biomass pre-treatment conditions are some of the key areas they hope to look into.
Source:
You, C., Chen, H., Myung, S., Sathitsuksanoh, N., Ma, H., Zhang, X. Z., ... & Zhang, Y. H. P. (2013). Enzymatic transformation of nonfood biomass to starch. Proceedings of the National Academy of Sciences, 110(18), 7182-7187.
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