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The Oxford Handbook of Job Quality Chris Warhurst
https://ebookmass.com/product/the-oxford-handbook-of-job-qualitychris-warhurst/
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Contributors
S. Ahmed Savitribai Phule Pune University, Pune, Maharashtra, India
F.C.T. Allnutt BrioBiotech LLC, Glenelg, MD, United States
N. Arul Manikandan Department of Chemical Engineering, Indian Institute Technology Guwahati, Guwahati, Assam, India
M. Ayadi National Engineering School of Tunis, Tunis, Tunisia
B. Bharathiraja Vel Tech High Tech Dr. Rangarajan Dr. Sakunthala Engineering College, Chennai, Tamil Nadu, India
J.F. Blais Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRSETE), Quebec, Canada
S.K. Brar Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRSETE), Quebec, Canada
E. Chaabouni Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRS-ETE), Quebec, Canada
M. Chakrabortty Assam Engineering College, Guwahati, Assam, India
M. Chakravarthy Vel Tech High Tech Dr. Rangarajan Dr. Sakunthala Engineering College, Chennai, Tamil Nadu, India
R. Chidambaram VIT University, Vellore, Tamil Nadu, India
B.K. Das Gauhati University, Guwahati, Assam, India
N. Dasgupta VIT University, Vellore, Tamil Nadu, India
R.K. Das Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRSETE), Quebec, Canada
A. Daverey Doon University, Dehradun, Uttarakhand, India
K. Dutta National Institute of Technology Rourkela, Rourkela, Orissa, India
A. Ghosh Indian Institute of Technology Guwahati, Guwahati, Assam, India
K.P. Gopinath SSN College of Engineering, Chennai, India
R. Goswami Rajiv Gandhi University of Knowledge Technologies, Nuzvid, Andhra Pradesh, India
B.Z. Haznedaroglu Yale University, New Haven, CT, United States; Bogazici University, Istanbul, Turkey
K. Hegde Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India; Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRS-ETE), Quebec, Canada
J. Jayamuthunagai Anna University, Chennai, Tamil Nadu, India
P. Kalita The Energy and Resources Institute, New Delhi, India
A. Kumar National Institute of Technology Raipur, Raipur, Chhattisgarh, India
R. Kumar VIT University, Vellore, Tamil Nadu, India
L. Lonappan Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRS-ETE), Quebec, Canada
B. Mahanty INHA University, Incheon, Korea
D.C. Maiti Vidyasagar University, West Bengal, India
S. Maiti Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRSETE), Quebec, Canada
C. Marques Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRS-ETE), Quebec, Canada; Ponta Grossa State University, Ponta Grossa/PR, Brazil
subjects, such as chemical/biochemical engineering, industrial biotechnology, biochemistry, industrial microbiology, environmental biotechnology, environmental engineering, and fermentation technology. In addition to university students and scientists doing regular academic research, the book readership includes professional researchers and technical staff working in the industries dealing with industrial chemical manufacturing or industrial biotechnology. The concept of renewable platform chemicals is relatively
new in large developing nations, such as India, Brazil, South Africa, and China; however, they have a vast potential for developing platform chemical biorefineries in the coming years. This book can also serve as a technical guide worldwide to potential entrepreneurs keen to develop biorefinery.
Satinder Kaur Brar Saurabh Jyoti Sarma Kannan Pakshirajan (Editors)
1
Platform Chemicals: Significance and Need
V.L. Pachapur, S.J. Sarma, S.K. Brar, E. Chaabouni
Institut national de la recherche scientifique Centre - Eau Terre Environnement (INRS-ETE), Quebec, Canada
TABLE 1.2 Conversion Pathways, derivatives, and Potential Applications of Bio-based Alcohols
Platform
Chemical Other Names Pathways
1 Xylitol IUPAC (2R,4S)-Pentane1,2,3,4,5-pentol
1,2,3,4,5-Pentahydroxypentane; Xylite
2 Butanol
Butan-1-ol[1]
Butalcohol
Butanol
1-Butanol
Butyl alcohol
Butyl hydrate
Butylic alcohol
Butyralcohol
Butyric alcohol
Butyryl alcohol
Hydroxybutane
Propylcarbinol
Aerobic fermentation
Anaerobic fermentation
Enzymatic transformation
Derivatives or Derivative Family Potential Applications References
Xylaric and xylonic acids
Polyols (propylene and ethylene glycols), lactic acid
Xylitol, xylaric, xylonic polyesters and nylons
Antifreeze, unsaturated polyester resins (UPRs)
New polymer opportunities Werpy et al. (2004)
Anaerobic fermentation 2-Methyl-2-butanol, 2-butanol As alternative fuel Cooksley et al. (2012)
TABLE 1.4 Microbial Production of Alcohols
1 Xylitol Candida tropicalis Corncobs hydrolysate Batch fermentation
C. tropicalis Sago bark hydrolysate Batch fermentation
C. tropicalis JH030 Rice straw hydrolysate Batch
Immobilized Debaryomyces hansenii Sugarcane bagasse Batch fermentation
Recombinant Saccharomyces cerevisiae Glucose
2 Butanol Clostridium acetobutylicum ATCC824
Hemicellulosic Hydrolysate Batch
Clostridium sp. Glucose Batch fermentation
C. beijerinckii ATCC 10132 Glucose Batch fermentation
C. acetobutylicum Cassava bagasse hydrolysate Fibrous-bed bioreactor
C. acetobutylicum
Glucose in the presence of biodiesel as an extractant Fed-batch fermentation
Cheng et al. (2009)
Kamal et al. (2011)
Huang et al. (2011a)
Prakash et al. (2011)
Oh et al. (2013)
Sun and Liu (2012)
Cheng et al. (2012)
Isar and Rangaswamy (2012)
Lu et al. (2012)
Yen and Wang (2013)
1.2
COMMERCIALLY IMPORTANT PLATFORM CHEMICALS: ORGANIC ACIDS
Organic chemicals such as organic acids can be used to synthesize plastic materials and other products. To meet the increasing demand for organic chemicals, more efficient, costeffective, and environmentally friendly production methods are being developed, which utilize raw materials.
1.2.1 3-Hydroxy-propionic Acid
3-HP acid (also called 3-Hydroxypropanoic acid, hydracrylic acid, and ethylene lactic acid) is a three-carbon carboxylic acid that has an interesting industrial potential and stands third on the list of the top 12 platform chemicals in the United States. It contains two functional groups with different properties that make it a suitable precursor for many applications, ranging from synthesizing optical active substances to acting as a cross-linking agent for polymer, metal lubricants, and antistatic agents for textiles. It has been included in the top value-added chemicals among renewable biomass products, as listed by the US Department of Energy (Gokarn et al., 2007; Raj et al., 2008). In fact, 3-HP can serve as a precursor for a number of commodities and specialties, such as acrylamide, 1,3-propanediol, acrylic acid, and methyl acrylate. Moreover, 3-HP can also be used to synthesize chemical intermediates such as malonic acid, propiolactone, and alcohol esters of 3-HP (Gokarn et al., 2007). Several chemical synthesis routes have been described to produce 3-HP, including oxidation from either 1,3-propanediol or 3-hydroxypropionaldehyde and hydration from acrylic acid. A global market opening at 3.63 million tons per year has been estimated for 3-HP (Raj et al., 2008). For commercial use 3-HP is produced by organic chemical synthesis, which is relatively expensive, and it is prohibited from being used for the production of monomers (Suthers and Cameron, 2005).
1.2.2 Lactic Acid
Lactic acid (2-hydroxypropionic acid) is a traditional chemical organic acid that is used as a natural preservative in many food products and widely used for specialized industrial applications (Maeda et al., 2009; Zhao et al., 2010; Taskin et al., 2012). It’s a raw material for 2,3-pentanedione, propanoic acid, acrylic acid, acetaldehyde, lactate ester, and as dilactide in chemical industries (Wee et al., 2004). Due to its properties such as optical activity, hydroxyl and carboxyl moieties are exploited for safe applications in the pharmaceutical, textile, and cosmetic industries (Wang et al., 2010a,b). For improved human health, many food products such as yogurt, Yakult, and bread contain lactic acid. Lactic acid is used as a feedstock for biodegradable polymers, oxygenated chemicals, plant growth regulators, environmentally friendly green solvents, and specialty commodity–chemical intermediates (Maeda et al., 2009). The global consumption of lactic acid is estimated to be around 130,000–150,000 metric tons annually and is expected to increase 7% per year until 2013 (Wee and Ryu, 2009; Djukic-Vukovic et al., 2012). The demand for lactic acid production is increasing continuously due to its extensive application as a precursor of polylactic acid, a promising biodegradable polymer (Wang et al., 2010a).
1.2.3 Fumaric Acid
Fumaric acid is a C4 unsaturated dicarboxylic acid that is widely used as a building block for a variety of chemicals and polymers. It is also used in the food, chemical, and pharmaceutical industries (Xu et al., 2010; Roa Engel et al., 2011). Fumaric acid is used in ruminal digesta to decrease methane formation and increase glucogenesis; it also increases milk yield in the agricultural industry (Wood et al., 2009). It serves as an important intermediate for esterification reactions and is identified as one of the top 12 building block chemicals by the US Department of Energy (Yu et al., 2012). It is used as an acidulant in foods, beverages, and industrial products, including lubricating oils, inks, and lacquers, and as a carboxylating agent for rubber. Fumaric acid is primarily produced through the catalytic oxidation of petrochemical hydrocarbons to maleic anhydride, followed by hydrolysis into maleic acid, and finally isomerization into fumaric acid. Due to increasing prices of petroleum oil and depleting fossil reserves, the bio-based production of fumaric acid has generated attention (Zhang et al., 2012).
1.2.4 Butyric Acid
Butyric acid, a four-chain short chain fatty acid, is an important specialty chemical with wide industrial applications in the chemical, foodstuff, and pharmaceutical industries. It is also used in manufacturing plastics, emulsifiers, disinfectants, and esters (Zhang et al., 2009; Song et al., 2010). It is used in the form of pure acid in food flavors, as additives for increasing fruit fragrance, and as aromatic compounds in perfumes. Its roles in health care as multiple bioactive and therapeutic compounds are diverse (Wei et al., 2013), and it is used in the treatment of hemoglobinopatheis, cancer, and gastrointestinal diseases (Huang et al., 2011b). In addition, butyric acid with direct hydrogenation in the presence of copper-based catalysts can produce promising fuel 1-butanol (Lim et al., 2013). It is synthesized commercially from petrochemical routes (Jiang et al., 2009a) by the oxidation of butyraldehyde through an oxoprocess using propylene and also by a novel synthesis method from maleic anhydride (Song et al., 2010). Due to the high demand and decreasing supply of world crude oil, the urgency of addressing the problem of increasing the production of butyric acid is becoming acute (Zhang et al., 2009).
1.3 COMMERCIALLY IMPORTANT PLATFORM CHEMICALS: ALCOHOLS
1.3.1
Xylitol
Xylitol, a five-carbon sugar alcohol, is an expensive polyol sweetener found in food products such as chewing gum, soft drinks, and confectionery (Sakakibara et al., 2009); it also has specific healthcare applications for oral health and parenteral nutrition (Rao et al., 2006). Xylitol is low-calorie pentitol. It is used as an anticariogenic and is an ideal sweetener for diabetics because its metabolism is not regulated by insulin and does not involve glucose 6-phosphate dehydrogenase (Cheng et al., 2009; Sakakibara et al., 2009). The industrial production of xylitol is through the chemical reduction of xylose derived from hydrolyzed plant materials, mainly birchwood chips, sugarcane bagasse (SCB), birch trees, and corn stalks
smaller uniform pellet formation, prevented excessive pH changes, was a rich nitrogen supplement, reduced medium costs, and benefited the environmental problem by the utilization of waste (Taskin et al., 2012). Jawad et al. (2012) have investigated the production of lactic acid from mango peels under ambient conditions and optimized the production using a factorial design. A maximum production of 17.48 g/L highlights the potential of mango peels as a lowcost option, and process optimization will make the production of lactic acid economically viable and sustainable (Jawad et al., 2012).
The cost-effective production of optically pure lactic acid from lignocellulose sugars is commercially attractive but challenging. Ca(OH)2 was found to be a better neutralizing agent than NaOH in terms of its giving higher lactic acid titer and productivity. From a kinetic point of view, SSF, a two-reactor fermentation system, and a one-reactor repeated batch operation increased lactic acid production.
1.4.3 Fumaric Acid
Fumaric acid is among the top 12 chemicals produced by industrial fermentation. Due to a scarcity of petroleum worldwide, fermentation routes for fumaric acid production are gaining importance (Roa Engel et al., 2011). The fermentative production of these acids from renewable resources has received extensive attention worldwide and can replace fossil-based production via maleic acid (Deng et al., 2012).
Xu et al. (2010) have investigated a novel two-stage corn straw utilization strategy by the well-known producer R. oryzae for fumaric acid production. The pretreatment of corn straw after acid hydrolysis resulted in a xylose-rich liquid to be used for fungal growth and a residual glucose-rich liquid to be used for fumaric acid production. This two-stage corn straw utilization strategy resulted in 27.79 g/L fumaric acid production at a productivity of 0.33 g/L h (Xu et al., 2010).
In order to further increase fumaric acid production, R. oryzae ME-F12 was isolated and mutated to increase the activity of glucoamylase to develop SSF from starch materials without commercial glucoamylase supplementation. About 39.80 g/L of fumaric acid were successfully obtained using the mutant with 1.28-fold as compared to the parent strain, suggesting a new avenue for the cost-effective fermentation of fumaric acid (Deng et al., 2012).
Strain improvements for increased fumaric acid production with laser irradiation on R. oryzae were carried out to induce mutations. Following exposure to the irradiation, the mutant strain FM19 exhibited a 56.3% increased titer to produce 49.4 g/L of fumaric acid from glucose. The mutant strain followed carbon and amino acid metabolism and provided new insights into the metabolic characterization of a high-yielding fumaric acid strain (Yu et al., 2012).
A novel immobilization device using net and wire for filamentous R. arrhizus RH-07-13 for fumaric acid fermentation was developed. Abundant mycelia grew on a large surface of the net and consumed glucose rapidly with a transit of nutrients across the net, resulting in rapid fumaric acid production. The result was around 32.03 g/L of fumaric acid production, in comparison to free-cell fermentation (31.23 g/L), and a further reduction in fermentation time from 144 to 24 h (Gu et al., 2013).
Fumaric acid production can be increased by using the well-known producer Rhizopus strain, possessing high glucoamylase activity and maintaining conditions for mycelia growth for increased fumaric acid production at a reduced fermentation time.
1.4.4 Butyric Acid
Due to the use of butyric acid as an ingredient in food, cosmetics, and pharmaceutical applications, there is a high demand by consumers for bio-based butyric acid production (Jiang et al., 2009a). Butyric acid produced by fermentation is favored over chemically synthesized acids, and the food products get labeled as a “natural preservative” (Zhang et al., 2009). With the catalytic reaction of butyric acid and hydrogen, the most promising biofuels can be synthesized from microbial fermentation (Song et al., 2010). The dominant platform for the biological production of butyric acid is by using Clostridium sp., and some studies have focused on fermentation techniques, which are recombinant techniques to improve the productivity and titer of butyric acid.
To minimize the high share of carbon sources in the media component cost, the exploitation of cheap, renewable carbon sources has been stimulated. Cane molasses, a by-product of the sugar industry containing 45–50% total sugar, using attractive characteristics of immobilization for in batch, repeated-, and fed-batch fermentation in a fibrous-bed bioreactor (FBB) was carried out by Jiang et al. (2009a) The feasibility and robustness of the FBB system for producing butyric acid using low-cost cane molasses pretreated with sulfuric acid resulted in a 55.2 g/L increased production in comparison to batch fermentation (34.1 g/L) using C. tyrobutyricum (Jiang et al., 2009b). Similarly, Song et al. (2010) have proposed empirical kinetic models to determine the optimal operational condition and develop a proper substrate feeding strategy for fed-batch fermentation of C. tyrobutyricum. A model-based fed-batch fermentation with semicontinuous glucose feeding resulted in 73.77 g/L of butyric acid production, much higher than batch fermentation. The predictions of the models reported match with the fermentation data, showed improvement in production, and may contribute to developing a cost-effective butyric acid fermentation process (Song et al., 2010). Considering the cost efficiency of fermentation production, Jerusalem artichoke (JA), a relatively cheap and widely available nongrain raw material, was acid-hydrolyzed to generate fructose and glucose for butyric acid production. To compete with the petroleum route of production, an FBB with immobilized C. tyrobutyricum in a repeated-batch fermentation was successfully performed. The feasibility and efficiency of the FBB system with a high butyric acid concentration of 60.4 g/L from acid-pretreated JA hydrolysate could be achieved to compete with the petroleum route of production (Huang et al., 2011b). In a similar approach, an FBB with immobilized C. tyrobutyricum in a repeated-batch fermentation using SCB hydrolysate produced around 20.9 g/L of butyrate concentration (Wei et al., 2013).
These works demonstrate the feasibility of using low-cost feedstock, JA, and SCB for the efficient production of butyric acid. More studies on butyric acid production have focused on fed-batch fermentation, including an FBB using immobilized Clostridium. With advancements in genetic engineering, a redox cofactor regeneration system in E. coli was developed for the production of butyric acid. With the native redox cofactor regeneration system, butyrate was the only final electron acceptor. The demand of a cofactor was fulfilled for cellular growth and enabled the efficient conversion of glucose into butyric acid, reaching 83.4% of the theoretical maximum yield (Lim et al., 2013).
The best approach for increased butyric acid production is fed-batch fermentation, which showed the highest maximum cell density, minimized substrate loss, maximized the final titer, increased the yield of the target product, and showed pivotal importance for butyric acid production.
1.4.5 Xylitol
The chemical process is very expensive because of the high working temperature, application of pressure for the hydrogenation of xylose, and extensive steps for separation and purification. The industrial-scale production contains less xylose and other sugars such as arabinose, mannose, galatose, and glucose as major impurities (Sakakibara et al., 2009). From the economic viewpoint, the biotechnological production of xylitol seems to be very attractive, with the use of low-cost crude hemicellulosic hydrolysate as a potential substrate (Rao et al., 2006).
Corncob, the most abundant agricultural material, was chosen along with Candida tropicalis W103, capable of producing 200 g/L xylitol from xylose as the sole carbon source. The pretreatment step of acid hydrolysis on corncobs was followed by detoxification to reduce volatile and phenolic compounds. The effect of glucose in the hydrolysate promoted the growth of C. tropicalis, while the inhibition of acetate was alleviated by adjusting the pH to 6 prior to fermentation. Under these optimum conditions, 68.4 g/L of maximal xylitol concentration was obtained, giving a yield of 0.7 g/g xylose and a productivity of 0.95 g/L h (Cheng et al., 2009). Detoxification methods have been carried out to convert inhibitors to inactive compounds or reduce their concentration. Powdered activated charcoal was mixed with the hydrolysate at 2.5% (w/v) and stirred for 60 min; it enabled a reduction of furfural (58%) and total phenolic (78%) compounds, and the maximum xylitol concentration obtained was 19.53 g/L with a higher xylitol yield. The detoxification process of using low-cost activated charcoal strongly suggests an economical and significant impact in xylitol production (Kamal et al., 2011). After substrate hydrolysis, detoxification steps are necessary to minimize the inhibition of hydrolysate to improve microbial fermentation. A newly isolated yeast strain, C. tropicalis JH030, a high inhibitor tolerant to nondetoxified lignocellulosic hydrolysates, was developed for xylitol production. The applicability of isolated yeast to nondetoxified lignocellulosic hydrolysates derived from SCB and rice straw resulted in 26 and 46 g/L of xylitol production. The high inhibitor tolerant yeast’s using nondetoxified lignocellulosic hydrolysates enhanced the xylitol production and showed a practicable capacity on various other raw materials, such as silvergrass, napiergrass, and pineapple peel (Huang et al., 2011a).
A pretreatment process of steam explosion on SCB along with newly isolated thermotolerant strain Debaryomyces hansenii immobilized over Ca-alginate was carried out for xylitol production. The Ca-alginate immobilized system produced 73.8 g of xylitol in comparison to 68.6 g/L by free cells. The steam explosion pretreatment approach and immobilized system were reused for five batches with steady bioconversion rates and yields (Prakash et al., 2011).
Various microorganisms have been developed to produce xylitol from xylose, but some organisms do not have a xylose metabolic pathway. Genetic engineering has been adopted to express xylose reductase in recombinant Saccharomyces cerevisiae to be overexpressed for xylitol production. In-vitro activity analysis confirmed the functional expression of both enzymes: acetaldehyde dehydrogenase 6 (ALD6) and acetyl-CoA synthetase 1 (ACS1). The best result of xylitol production, 91.3 g/L xylitol concentration, was obtained by ACS1 overexpression, relative to those of the control and ALD6-overexpressing strains. The modulation of ALD6 and ACS1 in fed-batch fermentation showed the best xylitol concentration and productivity in comparison to other strains (Oh et al., 2013).
The main step in xylitol production is the pretreatment step to obtain an increased concentration of xylose in comparison to other undesired impurities. The pretreatment step adds
on impurities, for which the detoxification step is optional if a high inhibitor tolerant yeast strain capable of using nondetoxified lignocellulosic hydrolysates can be used, or genetically engineered strains capable of the best xylitol production can be developed.
1.4.6 Butanol
The increasing price of oil has led to a resurgence of interest in the microbial-based generation of biobutanol, in particular replacements for liquid fuels ( Cooksley et al., 2012 ). The important benefit of producing biobutanol is that it can be produced from various low-cost substrates and does not require supplementation of external enzymes, as butanol-producing strains produce hydrolysis enzymes ( Qureshi et al., 2013 ). Biobutanol production largely depends on the availability of low-cost, abundant raw materials and an efficient process conversion into butanol production. Soon after substrate hydrolysis, detoxification treatment fails to remove inhibitors completely. A membrane-filtered sugar maple wood extract hydrolysate was used to produce butanol. Using nanofiltration, the membrane could remove all small molecular organic acids such as acetic acid and formic acid. The treatment significantly improved the butanol concentration to 7 from 0.8 g/L ( Sun and Liu, 2012 ).
Microorganisms having a high tolerance toward solvents are beneficial for butanol production and to avoid product inhibition. Highly efficient butanol-producing bacteria belonging to Clostridium sp. were isolated from sludge of a sewage treatment plant. The maximum butanol concentration of 12.4 g/L with the addition of 6.0 g/L butyric acid, the pathway for butanol production, was triggered with the titer significantly increased to 17.51 ± 0.49 g/L. Using a 5-L fermenter with a pressurized fermentation strategy enhanced the butanol concentration to 21.1 g/L, and this was accomplished by inhibiting hydrogen production (Cheng et al., 2012).
C. beijerinckii ATCC 10,132 during butanol production demonstrated the ability to accumulate rhodamine 6 G, accompanied by an increased expression of the chaperone, and showed a high tolerance to 25 g/L n-butanol under optimized conditions. The strain reported for a high titer of butanol of 20 g/L without resorting to solvent stripping or strain improvement (Isar and Rangaswamy, 2012).
The recovery of butanol from fermentation broth is necessary to avoid product inhibition. The use of the hyperbutanol-producing C. acetobutylicum strain in an FBB under continuous butanol recovery was studied using concentrated cassava bagasse hydrolysate. The stable production of n-butanol with a periodic nutrient supply resulted in 76.4 g/L of butanol production. With gas stripping, long-term stability, improved fermentation kinetics, and continuous butanol production, the process is attractive for industrial production (Lu et al., 2012).
Butanol-enriched biodiesel can improve the fuel properties of blends, with biodiesel as an extractant-enhanced butanol production. The in situ butanol removal by the addition of biodiesel resulted in a maximum total butanol of around 31.44 g/L and had no significant toxicity on the growth of C. acetobutylicum, showing great potential for commercial butanol production (Yen and Wang, 2013).
To increase butanol production, it is very important to use high solvent tolerant butanolproducing bacteria, a requirement of simple sugar with no addition of external hydrolyzing enzymes. The most important parameter is to continuously recover the produced butanol from fermentation.
two-phase system provides an effective and promising way to separate butyric acid from fermentation broth (Wu et al., 2010).
1.5.5 Xylitol
Xylitol produced during fermentation is always separated and purified by chromatographic methods, which tend to be expensive for industrial-scale processes. With liquid–liquid extraction and precipitation techniques, the solvents used make xylitol recovery difficult and expensive for large-scale purification. A strategy for xylitol extraction using an activated charcoal treatment step followed by a vacuum concentration and crystallization method was carried out. The activated charcoal treatment followed by 15.0 g/L of charcoal concentration at 30°C for 1 h with 10 times super saturation of the initial concentration resulted in clear, large crystals of xylitol. The crystallization temperature of −20°C for initiation and 8°C after four cycles of crystallization resulted in a 76.20% xylitol crystallization yield. The purity of the xylitol was 98.99%, suggesting a cost-effective, efficient, easy, less time-consuming, and environmental friendly procedure (Misra et al., 2011).
1.5.6 Butanol
The only way to increase the economic efficacy of butanol fermentation is to increase its concentration by eliminating the production of undesired by-products. C. acetobutylicum EA 2018 was disrupted with an acetoacetate decarboxylase gene (adc) into a hyper butanol-producing industrial strain using TargeTron technology. The undesired by-product acetone production reduced with butanol concentration increased from 70% to 80.05%. A simple approach of blocking acetone production by Clostridium demonstrates the industrial potential of this strain for butanol production (Jiang et al., 2009b). The performance of fermentative butanol production quantitatively depends on the tolerance of solvent-producing bacteria. With a Clostridial species-dominated bacterial consortium the maximal butanol production was 10.64 ± 0.60 g/L and with tolerant butanol the concentration level was 16 g/L (Chen et al., 2012). With artificial simulation of bioevolution (ASBE) based on the evolutionary dynamics and natural selection a high butanol tolerance to C. acetobutylicum was developed. The increase of butanol production from 12.2 g/L to 15.3 g/L using corn meal as a substrate suggested that the ASBE method of enhancing butanol tolerance increased butanol production (Liu et al., 2013).
1.6 CONCLUSION
Platform chemicals have an undeniable commercial importance, and at present they are mainly produced from petroleum-based raw materials. Owing to the finite nature of fossilderived feedstock as well as environmental concerns, the sustainable manufacturing of platform chemicals using biomass-based substrates is becoming inevitable. In this context, advances in renewable platform chemical manufacturing have been summarized in the present literature survey. The screening of less expensive feedstock, a process designed for maximum substrate utilization, the development of more efficient microbial strains, integrated downstream processing techniques, and green manufacturing are the major areas of platform chemical biorefinery where further research should be focused.
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Wee, Y.-J., Kim, J.-N., Yun, J.-S., Ryu, H.-W., 2004. Utilization of sugar molasses for economical l(+)-lactic acid production by batch fermentation of Enterococcus faecalis. Enzyme and Microbial Technology 35 (6–7), 568–573.
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