Bioengineering for utilisation and bioconversion of straw biomass into bio-products

Industrial Crops and Products 77 (2015) 262–274

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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop

Bioengineering for utilisation and bioconversion of straw biomass into bio-products Seyed Hamidreza Ghaffar a , Mizi Fan a,∗ , Bruce McVicar b a b

College of Engineering, Design and Physical Sciences, Brunel University, UB8 3PH, United Kingdom Stramit limited, United Kingdom

a r t i c l e

i n f o

Article history: Received 20 May 2015 Received in revised form 23 August 2015 Accepted 28 August 2015 Available online 11 September 2015 Keywords: Straw Biomass Bioengineering Biological pre-treatment Biotechnology Lignin biodegradation

a b s t r a c t This paper focuses on straw biomass and its main composition with the emphasis on the concept of bioengineering where agricultural biotechnology is discussed with references to success and limitations for the bioconversion of straw biomass to value added bio-products, mainly bio-energy and bio-composite. Biological pre-treatments on straw biomass have been reviewed where the function of biological enzymes and fungi have been discussed in details. Biological pre-treatments have been linked with fungi capable of generating enzymes that biodegrade lignin, hemicellulose and polyphenols. Lignocellulolytic enzymes are considered as prospective biomass degraders for industrial applications, although lignin which is the most recalcitrant constituent of straw biomass, links to hemicellulos and cellulose, thus turns into a barrier for enzymes and stops the infiltration of enzymes to biomass structure. White-rot fungi are identified to be responsible for effective lignin biodegradation in straw biomass decay procedures. It was found that biological pre-treatment in combination with mild chemical and or physical pre-treatments are the most critical process for the success of bioengineering. Bioconversion of biomass to bio-products e.g. bioethanol, biogas and bio-composites has achieved some success, however, several limitations such as long incubation times remain to be solved. The summarised information presented in this paper shall serve as scientific insights and directive fundamentals for researchers and industries for further developments and investments in biotechnology of straw biomass. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Main constituents of straw biomass for bioengineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Biological pre-treatment for straw biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 3.1. Combination of biological with chemical or/and physical pre-treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 3.2. Roles of microorganisms in biological pre-treatments of straw biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 3.2.1. White-rot fungi functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 3.2.2. Brown-rot fungi functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 3.3. Straw lignin biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 3.3.1. Role of laccase in lignin biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Bioconversion of straw biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 4.1. Bio-energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 4.2. Bio-composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Conclusions and future prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

∗ Corresponding author. E-mail address: [email protected] (M. Fan). http://dx.doi.org/10.1016/j.indcrop.2015.08.060 0926-6690/© 2015 Elsevier B.V. All rights reserved.

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1. Introduction Rising global energy demands have significantly increased the cost of fossil-fuel-based energy sources and petrochemical products. It is clear to many industry sectors that there is an enormous need to promote the use of biotechnology for biomass pretreatment and optimisation, many researchers have been working on these concepts e.g. (Agbor et al., 2011; Arora et al., 2002; Arora, 1995; Chang et al., 2012; Chen et al., 2013; Lopez-Abelairas et al., 2013; Maehara et al., 2013; Matsumoto and Taguchi, 2013; Petrik et al., 2013; Schilling et al., 2012; Shi et al., 2008; Singh et al., 2011; Taniguchi et al., 2005; Van Dyk and Pletschke, 2012; Zhang et al., 2007). Lignocellulosic residues e.g. from straw, are predominantly ample in nature which have a great prospective for bioconversion. However, very minor volume of the lignin, cellulose and hemicellulose produced as derivatives in biomass are utilised. It was calculated that 73.9 teragram (Tg) of dry wasted crops (e.g. agricultural residues) could possibly yield 49.1 gigaliter (GL) of bioethanol in the world (Kim and Dale, 2004). Asia could be the major potential producer of bioethanol from straw biomass, estimated to be up to 291 GL/year of bioethanol. Europe is second largest producer with a potential of 69.2 GL/year of bioethanol, mostly from wheat straw. In North America core feedstock is corn stover; that has a potential of producing 38.4 GL/year of bioethanol (Kim and Dale, 2004). Straw biomass is recognised as major origin of biofuels and bio-products. The use of such carbon sources that are accessible in big quantities and could be utilised in a carbon dioxide neutral way is a sensible solution to major sustainability problems of the society. The exciting combination of biology and engineering called biotechnology is becoming more real every day and is an emerging solution for sustainability issues faced by industries. Biotechnology offers state of art chances for sustainable emergence of new products. Biological microorganisms degrade and use cellulose and carbohydrates as sources of energy and carbon; other collections of filamentous fungi possess the capability of breaking lignin, the most recalcitrant constituent of biomass cell walls. Microorganisms which are able to completely and efficiently biodegrading lignin to CO2 and breakdown the lignin/carbohydrate complex are categorised as white-rot fungi. Further categories of fungi are brown-rot fungi that depolymerise and modify the lignin in biomass (Sánchez, 2009). The capability of fungi to biodegrade straw biomass efficiently and selectively is because of their effective enzymatic system and the mycelial growth routine that permits the fungus to carry nutrients, for instance nitrogen and iron into the less nutrient lignocellulose substrate that creates its carbon source. A variety of enzymes that have different functions are vital in biodegradation of constituents of lignocellulose (Banerjee et al., 2010; Saha, 2003a,b; Zhang and Lynd, 2004). Table 1 gives a summary of categories of enzymes which are essential to biodegrade compound lignocellulose constituents (Van Dyk and Pletschke, 2012). Bioengineering of straw biomass to bio-products requires accurate and efficient processes; the pre-treatment of straw biomass is one key step (mechanical, chemical, biological or combinational). The term bioengineering suggests the adoption of biological treatment or combination of mild chemical and physical treatments

with biological treatments. Maximising the synergistic effect of each pre-treatment or technologies whilst controlling the detailed parameters, therefore the disadvantages of each pre-treatment type or processing technology will be reduced to minimum. Several uses have been suggested for bioengineering of straw biomass; among them the production of bioethanol (Alvira et al., 2010; Binod et al., 2010; Kim and Dale, 2004; Maehara et al., 2013; Talebnia et al., 2010), biogas (Cheng et al., 2011; Kaparaju et al., 2009; Sapci, 2013; Zhong et al., 2011b) and bio-products (Ribbons, 1987) (e.g. organic acids, amino acids and vitamins (Sánchez, 2009)) has received much attention with sufficient success in the process. The exploration which is required for future agricultural biomass technology is of a comprehensive nature linking contributions from biochemistry, microbiology, biotechnology and biochemistry. The evidence for the success of bioengineering of straw biomass is discussed in more details along with further debate about the advantages and disadvantages later in the paper. This review delivers an overview of recent scientific reports on biotechnology concept and investigates its possible use for straw biomass bioconversion into bio-products, with main emphasis on biological pre-treatments and the use of various enzymes for lignin biodegradation, hence optimisation of straw biomass. Scientific insights and novel concept of biotechnology are identified and discussed. Clear and feasible strategies to solve the problems associated with the bioconversion of straw biomass and directions for further research in industrial application are proposed.

2. Main constituents of straw biomass for bioengineering The main component of straw biomass is cellulose, hemicellulose and lignin. Cellulose is a long chain of glucose molecules, connected to each other primarily by ␤ (1 → 4) glycosidic bonds; the non complicated structure of cellulose indicates that it could be biodegraded. Main difficulty in the bioconversion procedure is the crystalline nature of cellulose, therefore it must be pre-treated to expose the cellulose structure and change it to be more vulnerable to cellulase action (van Wyk, 2001). Hemicellulose just like cellulose is a macromolecule from different sugars but alters from cellulose in that it is not chemically homogeneous, is a polysaccharide which has lower molecular weight compared to cellulose. Key alteration among cellulose and hemicellulose is that the latter possesses branches with short lateral chains containing various sugars and cellulose contains easily hydrolysable oligomers. Hemicelluloses in straw biomass are made mainly of xylan, whereas softwood hemicelluloses have glucomannan. Lignin is connected to both hemicellulose and cellulose, founding a physical cover that is an impermeable wall in the plant cell wall. The existence of lignin in the cellular wall offers rigidity, impermeability and a confrontation to microbial attack. Lignin is a complex threedimensional polymer formed by radical coupling polymerisation of p-hydroxycinnamyl, coniferyl and sinapyl alcohols; these three lignin precursors’ monolignols induce the p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) phenylpropanoid units (Ghaffar and Fan, 2013). Lignin is biodegradable in nature unlike other synthetic polymers and is known to be one of the most durable biopolymers available. Huge quantities of lignin result as a by-product of

Table 1 The key enzymes essential for biodegradation of lignocellulose to monomers (Van Dyk and Pletschke, 2012). Component

Type of enzyme

Lignin Pectin Hemicellulose

Laccase, manganese peroxidase, lignin peroxidase Pectin methyl esterase, pectate lyase, polygalacturonase, rhamnogalacturonan lyase Endo-xylanase, acetyl xylan esterase, ␤-xylosidase, endomannanase, ␤-mannosidase, ␣-l-arabinofuranosidase, ␣-glucuronidase, ferulic acid esterase, ␣-galactosidase, p-coumaric acid esterase Cellobiohydrolase, endoglucanase, ␤-glucosidase

Cellulose

263

264

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Table 2 Compositions of straw biomass (% dry matter). Category

Lignin

Cellulose

Hemicellulose

Water-soluble

Wax

Ash

Others

Wheat straw Rice straw Rape straw Oat straw Rye straw Barley straw

14.1 12.3 21.3 16.8 17.6 14.6

38.6 36.5 37.6 38.5 37.9 34.8

32.6 27.7 31.4 31.7 32.8 27.9

4.7 6.1 – 4.6 4.1 6.8

1.7 3.8 3.8 2.2 2.0 1.9

5.9 13.3 6.0 3.1 3.0 5.7

2.4 0.3 0 3.1 2.6 8.3

the pulp and paper sector. It is apparent that the activation or the modification of lignin could make substantial attribution towards the utilisation of natural sources, e.g. the use of modified/activated lignin as natural adhesive for bio-composite production. The composition and quantities of these complexes differ between straw biomass which can however be averaged and summarised in Table 2 (Sun and Tomkinson, 2000).

3. Biological pre-treatment for straw biomass Biological pre-treatment is often used to increase biomass degradation which is a decisive stage in bioconversion of straw biomass to fermentable sugars and bio-products. In comparison to chemical pre-treatment, biological pre-treatment weakens the fractious of straw biomass with lignin biodegrading microorganisms and offers a sustainable and energy efficient pre-treatment. Fungi have potential for degradation of aromatic compounds. A biological process removes a substantial quantity of lignin from biomass, hence increases the enzymatic hydrolysis efficiency. When straw biomass are fermented by fungi, the biological pretreatment can achieve a greater sugar yield (20–65%) and lignin biodegradation (>15%) (Shi et al., 2009; Zhang et al., 2007). While the key benefits of biological pre-treatment are small energy input, no chemical obligation, environmentally friendly working style and minor environmental conditions, the drawbacks are the slow pre-treatment rate which requires a wary monitoring of the growth parameters (Chandra et al., 2007). The point that some of the carbohydrates segment of biomass is depleted by microorganisms could be a disadvantage. Biological pre-treatment is therefore not as striking on an industrial scale, and the introduction of some kind of catalyst is necessary that can speed up the process and also improve the efficiency. One way of improving the efficiency of biological pre-treatment is that it could be exploited

as an initial stage default pre-treatment in combination with one or more pre-treatment methods. 3.1. Combination of biological with chemical or/and physical pre-treatment Biological pre-treatment combined with mild physical/chemical or mechanical pre-treatments presents an efficient solution for straw biomass bioconversion to bio-products. It is very important for biological pre-treatment to be efficient in terms of time duration hence the mild chemical or physical pre-treatment prior to biological pre-treatment can make the pre-treatment times quicker. Combined pre-treatments can also potentially solve the issues related with physical and chemical pre-treatments, e.g. severe energy contribution for mechanical treatments and intense chemical loading for chemical treatments. Combined pre-treatment process should lead to a synergistic effect, enhancing the yields of finale products (Table 3). Two un-catalysed hydrothermal methods, hot water extraction and liquid hot water pre-treatments, were compared in an investigation for the synergistic effects of hydrothermal-fungal pre-treatment (Wan and Li, 2011). Hydrothermal pre-treatment enhanced the fungal biodegradation for soybean and wheat straw that seemed to be resistant to C.subvermispora degradation. The hot water extraction (85 ◦ C for 10 min) enabled fungal pre-treatment of wheat straw by eliminating water soluble extractives which wasn’t the case for soybean straw (Wan and Li, 2011). It has also been reported that the pressurised hot water pre-treatment (170 ◦ C for 3 min at 0.75 MPa) helped fungal biodegradation of soybean straw: the glucose yield of the collective liquid hot water and fungal pretreatment got to about 65% (Wan and Li, 2011). Yu et al. (Yu et al., 2010) showed that pre-treatment of corn stalks with Irpex lacteus could modify the lignin structure and facilitate lignin biodegradation and xylan elimination under mild alkaline environment (1.5%

Table 3 Combination of biological and physical/chemical pre-treatment for straw biomass a . Substrate

Physical/chemical treatment

Effectiveness

References

19 white-rot fungi tested

Alkaline (NaOH)

Hatakka (1983)

C.subvermispora

Hot water extraction

I. lacteus or P.subvermispora

Alkaline (NaOH)

Soybean straw

C.subvermispora

Liquid hot water pre-treatment

Corn stalk

Irpex lacteusc

Alkaline (NaOH)

Rice hull

Pleurotus ostreatus

Ultrasound

Pleurotus ostreatus

H2 O2

Strong alkaline pre-treatment masked the synergistic result of fungal pre-treatment for combined procedure. Hot water extraction enhanced delignification and the subsequent sugar produced No significant inhibitors of yeast growth improving significantly ethanol production. Liquid hot water pre-treatment enhanced delignification and the subsequent sugar yield. Fungal pre-treatment enhanced delignification and xylan elimination through mild alkaline pre-treatment. Ultrasound enhanced fungal delignification and improved cellulose and hemicellulose loss, leading to higher sugar yield in comparison to fungal pre-treatment on its own. H2 O2 enhanced fungal delignification, leading to sugar yield that is similar to that achieved from long term fungal pre-treatment.

Wheat straw

a

Fungi b

Wan and Li (2011) Salvachúa et al. (2011) Wan and Li (2011) Yu et al. (2010) Yu et al. (2009)

Yu et al. (2009)

Fungal pre-treatment is the following step of combined pre-treatment for all the rest of table. Of 19 white-rot fungi tested, Pleurotus ostreatus, Pleurotus sp. 535, Pycnoporus cinnabarinus 115 and Ischnoderma benzoinum 108 increased the vulnerability of straw to enzymatic saccharification. c Indicates that fungal pre-treatment is the first step of combined pre-treatment. b

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NaOH, 30–75 ◦ C for 15–120 min), the synergic result mainly rest on the harshness of alkaline pre-treatment. The less the harshness of the alkaline pre-treatment, the further the cellulose digestibility was enhanced by fungal pre-treatment. A fungal screening of basidiomycetes was implemented, combined with mild alkali washing (0.1% NaOH (5% w/v), at 50 ◦ C and 165 rpm for 60 min), for second generation bioethanol from wheat straw (Salvachúa et al., 2011). Arrangement of a biological pre-treatment by I. lacteus or P. subvermispora with mild alkali pre-treatment didn’t yield inhibitors for downstream processes, improving ethanol production (Salvachúa et al., 2011). Improvement of the digestibility acquired from biological pretreatment was achieved by optimising the operational settings of the mechanical and thermal pre-treatments for substrate conditioning that pave the way for the biological pre-treatment. Mechanical particle size reduction is essential to homogenise the substrate and thermal process is needed to reduce the presence of unwanted microorganisms. In optimised conditions, the rate of substrate consumption increased, due to increased alterations in straw structure from the mechanical and thermal pre-treatment, which increased the fungal accessibility of the substrate (LopezAbelairas et al., 2013). Mild alkali treatment (NaOH (25% w/v) per gram of wheat straw, 165 rpm and 50 ◦ C for 60 min) was performed prior to enzymatic hydrolysis as the fraction of the lignin that remains after pre-treatment is removed and hence enzymatic hydrolysis is improved. The digestibility of untreated wheat straw was 16% for cellulose and 12% for hemicellulose which improved to 21 and 17%, respectively, with the application of alkaline treatment, and further improved to 27 and 23%, respectively, when the autoclave sterilisation (120 ◦ C, 20 min without overpressure) of wheat straw before alkaline treatment was also carried out. The greatest improvement in hemicellulose digestibility was obtained with I. lacteus in optimised conditions, and the highest cellulose digestibility (81%) was obtained when the microorganism was changed from Pleurotus eryngii to I. lacteus under optimised conditions which was observed after 14 days of pre-treatment, that signifies an important reduction in treatment time (Lopez-Abelairas et al., 2013). In another study, by combining biological pre-treatment, using same fungus (I. lacteus), with an alkaline wash (0.25 M NaOH, for 30 min at 30 ◦ C); the biological process time was shortened to 15 days (Zhong et al., 2011a). Though, the NaOH dose used in this case is greater than that used in other studies. Combinatorial pre-treatment strategies are usually more effective in improving the biomass digestibility and reducing the time of biological pre-treatment. 3.2. Roles of microorganisms in biological pre-treatments of straw biomass White-rot and brown-rot fungi are responsible for lignocellulose degradation. Brown-rots, unlike white-rots and soft-rots, mostly attack cellulose and hemicellulose; while white-rots and soft-rots strike both cellulose and lignin. Typical microorganisms used in biodegradation of straw biomass in recent researches are summarised in Table 4. Due to the insolubility of main constituents of straw biomass the fungal biodegradation happens exocellularly, either in a link with the outer cell envelope layer or extracellularly. Fungi possess two categories of extracellular enzymatic processes; the hydrolytic and oxidative ligninolytic. The former process yields hydrolases that are liable for polysaccharide biodegradation; the latter system biodegrades lignin and opens phenyl rings (Sánchez, 2009). Fungal performance on biodegradation and the subsequent digestibility differs with feedstocks, fungi species and pretreatment times. For agricultural residue a saccharification yield of 36% was found from corn stover pre-treated with Cyathus ster-

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coreus for 29 days which was about 4 times that of the untreated (Keller et al., 2003). Wheat straw pre-treated with Pleurotus ostreatus yielded 27–33% cellulose digestibility during 72 h enzymatic hydrolysis (Hatakka, 1983; Taniguchi et al., 2005). A summary of previous researches on solid state fungal pretreatment with white rot fungi has been provided (Wan and Li, 2012) where critical pre-treatment factors influencing the efficiency of fungal pre-treatment, enzymes involved in biodegradation of biomass feedstock have been discussed. 3.2.1. White-rot fungi functions White-rot fungi have a high tolerance to toxic surroundings and they can endure elevated temperatures and an extensive range of pH. Lignin degradation is needed to increase the contact to cellulose and hemicellulose; white-rot basidiomycetes efficiently mineralise lignin, however different species cause different gross morphological patterns of decay (Blanchette, 1991; Blanchette et al., 1997; Daniel, 1994). Robust oxidative action and small substrate specificity of the ligninolytic enzymes in white-rot fungi make the lignin degradation very efficient through the action of secreted enzymes. The 3 main oxidative enzymes (ligninolytic enzyme) secreted by white rot fungi are phenol oxidase (laccase), lignin peroxide (LiP), manganese peroxide (MnP) (Eriksson et al., 1990). LiP and MnP were revealed in 1980s in Phanerochaete chrysosporium, labelled as lignases due of their great potential redox value (Martı´ınez, 2002). LiP and MnP oxidise the substrate by two successive one-electron oxidation steps with intermediate action radical formation, they require hydrogen peroxide (H2 O2 ) for their actions, although they are deactivated by high concentration of H2 O2 . LiP is capable to oxidise phenolic and non-phenolic lignin substructures, while MnP and laccase only biodegrade phenolic lignin substructures. An unsaturated fatty acid allows MnP to biodegrade non-phenolic lignin substructures. In fungal cultures not all of ligninolytic enzyme enzymes are detected. A study conducted by Guerra et al. (Guerra et al., 2002) on Pinus taeda biodegraded by Ceriporiopsis subvermispora indicated no sign of a link between oxidative enzymes and lignin biodegradation. The mineralisation of lignin happened after the lignin alteration, which indicates a sequence of reactions elaborate in lignin depolymerisation. The most extensively investigated white-rot organism is P. chrysosporium, which is an efficient producer as it possesses high cell growth rate and good efficiency for lignin degradation (Shi et al., 2009). P. chrysosporium strains at the same time biodegrade cellulose, hemicellulose and lignin, while C. subvermispora tend to eliminate lignin first. P. chrysosporium yields multiple isoenzymes of LiP and MnP however does not yield laccase. Several additional white-rot fungi produce laccase as well as LiP and MnP in varying arrangements (Hatakka, 1994). The white-rot fungus P. ostreatus assert several laccase genes encoding isoenzymes with altered characteristics (Piscitelli et al., 2011). Biodegradation of wheat straw by P. chrysosporium was studied (Singh et al., 2011) to discover the consequences of the lignin biodegradation process. The result showed about 30% loss of total lignin in 3 weeks of pretreatment. The analysis of lignin confirmed the important decrease of guaiacyl (G) units. The lignin from pre-treated wheat straw varied in its aromatic composition (Singh et al., 2011). It was revealed that pre-treatment of wheat straw by P. chrysosporium reduced the thermal degradation temperature of wheat straw significantly (Zeng et al., 2011). 3.2.2. Brown-rot fungi functions Brown-rot fungi successfully depolymerise and partially eliminate cellulose and hemicelluloses from straw biomass, and also oxidise lignin. They are in the category of basidiomycetous microorganisms (Enoki et al., 1988; Goodell, 2003; Kirk, 1975). Demethylation of methoxyl groups of phenolic lignin, along with

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Table 4 Examples of fungi used in biodegradation of straw biomasses. Type

Fungus

Reference

Wheat straw

Trametes sp. Lentinus crinitus Pleutotus ostreatus Pleutotus pulmonarius Ceriporiopsis subvermispora Bacillus polymyxa Trametes hirsuta Bacillus polymyxa Pleurotus ostreatus Trametes versicolor Phaerochaete chrysosporium Dichomitus squalens Cyathus stercoreus Ceriporiopsis subvermispora Irpex lactues Pleurotus ostreatus BP2 Echinodontium taxodii 2538 Ceriporiopsis subvermispora Aspergillus terreus Pleutotus ostreatus

Hossain et al. (2003) Hossain et al. (2003) Delfin-Alcala and Duran-De-Bazua (2003) Delfin-Alcala and Duran-De-Bazua (2003) Wan and Li (2011) Shah et al. (1982) Okamoto et al. (2011) Shah et al. (1982) Taniguchi et al. (2005) Taniguchi et al. (2005) Bak et al. (2009) Bak et al. (2010) Keller et al. (2003) Wan and Li (2011) Xu et al. (2010) Yang et al. (2010) Yang et al. (2010) Wan and Li (2011) Emtiazi et al. (2001) Hadar et al. (1993)

Rice straw

Corn stover

Soybean straw Hay and straw Cotton straw

aromatic hydroxylation is caused by brown-rot fungi. Therefore phenolic hydroxyl groups and the carboxyl content are high in brown-rotted lignin due to the oxidation of originally made catechol groups along with side-chain oxidation (Jin et al., 1990; Kirk, 1975). When brown-rotted lignin is pre-treated with a phenoloxidising enzyme it becomes suitable as adhesive for biomass bio-composites, as the increased phenolic hydroxyl content promotes radical development in lignin and would subsequently have an encouraging effect on adhesion. Brown-rot fungi also produce laccase (Lee et al., 2004) in some species. This then modifies the lignin in the brown-rotted biomass. Hence it is interesting to investigate the brown-rot fungi as possible method to produce reactive lignin from biomass which then could be used for adhesive and/or other applications. In a study for cleavage of lignin by a brown-rot basidiomycete, the complete solubilisation of lignocellulose determined by onebond 1 H 13 C correlation nuclear magnetic resonance (NMR) showed the brown-rot of spruce wood by Gloeophyllum trabeum lead to clear reduction of inter-monomer side chain linkages in lignin. Aromatic polymer residue after brown-rot was not recognisable as lignin (Yelle et al., 2013). The structural modifications which occurred in brown-rotted lignin were determined with the solubilisation method (Lu and Ralph, 2003) and the findings illustrated that polysaccharide signals diminished comparative to aromatic ones in the brown-rotted specimen. Numerical comparison of the NMR signals for C␣ in arylglycerol-b-aryl ether structures indicated, on a methoxyl basis, this major structure of lignin was diminished to 29% of its original amount after the brown-rot process. As previous judgment, that the brown-rotted lignin has an improved phenolic content (Kirk, 1975), it was shown by the only new structure in aromatic area, added acetoxyl substituents on the aromatic rings, which revealed after the brown-rot exposure. 3.3. Straw lignin biodegradation The comprehensive understanding of biological lignin biodegradation is useful for emerging an efficient pre-treatment procedure to break straw biomass so as to achieve sugars and lignin based bio-products. The significance of biological delignification has been established in many grounds of biotechnology; therefore in this section lignin biodegradation is examined in detail. Delignification of straw biomass improves their digestibility and also advances their nutritive worth. Up to 30–50% of the bonds in lignin polymers

is the arylglycerol-ˇ-aryl ether bond. Breaking down of this bond through biological pre-treatment represents a significant commercial interest. The initial step of lignin biodegradation is when the oxidative enzymes induce new functional groups into lignin’s macromolecular structure rendering lignin vulnerable to its consequent degradation by other enzymes. The natural structure of lignin has numerous different functional groups that could be specifically functionalised via oxidation (Zakzeski et al., 2010). Technologies pointing at the oxidative advancement of lignin via radical paths have been recently reviewed (Lange et al., 2013) with the focus on enzymatic catalyses i.e. laccase and peroxidase where important systematic aspects, and possibilities to further improve the industrial exploitation of these enzymes are highlighted, although this is not yet industrially feasible. Lignin biodegradation by white-rot fungi is oxidative where phenol oxidases are the main enzymes (Rabinovich et al., 2004; Sánchez, 2009). Raised oxygen levels raise the amount of lignin degradation via the production of H2 O2 as the extracellular oxidant and the following induction of ligninolytic activity (Faison and Kirk, 1983; Sánchez, 2009). The available values in terms of biodegradation of lignin through white-rot fungi are rather variable dependent on the strain, fermentation type and incubation period. Jalˇc (Jalˇc, 2002) in a review paper examining the findings from numerous researchers presented the values of wheat straw lignin biodegradation ranging from 2% to 65%. The results from a study on the enzyme complexes of whiterot fungi under solid state fermentation of wheat straw showed significant lignin degradation after an increase in the laccase activities (Dinis et al., 2009). Enzymatic complexes yielded by white-rot fungi influence extensively the cell wall structure of wheat straw as a result of an intensive action between various groups of enzymes. When it comes to the biodegradation of recalcitrant cell wall in straw biomass; fungal feruloyl and p-coumaroyl esterases which release feruloyl and p-coumaroyl units play a significant part (Kuhad et al., 1997). These enzymes perform synergistically with xylanases to break the hemicellulose and lignin link, without mineralisation of lignin (Borneman et al., 1990). Thus, hemicellulose biodegradation is essential before efficient lignin elimination. A detailed report on ligninolytic enzymes of several fungi and their lignin biodegradation capabilities, by means of wheat straw as a substrate, was reported (Arora et al., 2002). It was clear that either LiP or MnP production in combination with laccase causes high lignin degradation, as observed in Phlebia spp. and P. chrysosporium in the previous studies (Arora et al., 2002; Datta et al., 1991).

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267

Fig. 1. Enzymatic oxidation of phenol compounds (Batog et al., 2008).

Therefore no single ligninolytic enzyme might be liable for lignin biodegradation, as the distribution of LiP, MnP and laccase varies considerably in different fungi. The capability of white-rot, brownrot and soft-rot fungi separately and in combination with each other through semi solid biodegradation of wheat straw inspected by Arora (Arora, 1995) showed that Deadaleaflavida plus P. chrysosporium (white-rot fungi) was the greatest combination for a lignin loss of 36.27% in wheat straw. Selective biodegradation of lignin in rice straw was reported by white-rot fungi, P. ostreatus where the overall weight loss and the degree of Klason lignin diminished were 25% and 41%, respectively (Taniguchi et al., 2005). Primary moisture content of the lignocellulosic substrate is significant to the fungal growth which influences secondary metabolism in fungal pre-treatment (Reid, 1989); therefore it influences the quantity of lignin biodegradation. Earlier investigations showed that primary moisture between 70 and 80% was ideal for lignin biodegradation and ligninolytic activities of most white rot fungi. Shi et al. (Shi et al., 2008) noticed that after 14 days cultivation of cotton stalks by P. chrysosporium, lignin biodegradation of 27% was achieved at 75% moisture content, which was about 7% more than that at 65% moisture content. Apart from the use of fungi for biodegradation of lignin, other mediums have also been investigated. Hou et al. (Hou et al., 2012) carried out selective delignification of biomass using Cholinium amino acids ionic liquids ([Ch][AA] ILs), a novel type of bio-ILs that can simply be ready from sustainable biomaterials, showing a great potential for pre-treatment of biomass for environmentally friendly production of bio-products.The capability of bacteria to ˜ break down lignin is also investigated (Bugg et al., 2011; Vicuna, 1988), with potential for the usage of bacterial gene products for lignin biodegradation. The application of bacteria in lignin bioconversion to aromatic chemicals has been illustrated in the past by researchers (Gupta et al., 2001; Raj et al., 2007). For example, recent scientific publications indicate aromatic degrading soil bacteria, are capable of biodegrading lignin with extracellular peroxidase ˜ and laccase enzymes involved (Breen and Singleton, 1999; Vicuna, 1988; Wang et al., 2013). A lignin degrading bacterial consortium, “LDC”, was separated from the sludge of a reeds pond which is capable of biodegrading 61% lignin in reeds at 30 ◦ C in the conditions of static culture in 15 days (Wang et al., 2013). 3.3.1. Role of laccase in lignin biodegradation Laccase extensive substrate specificity leads to oxidation of different organic compounds that opens up opportunities of their utilisation in biotechnological applications. Fungal laccases play a key part in lignin degradation and modification processes. Laccases are versatile, phenol-oxidising enzymes found in white-rot fungi; when performing on lignin, they could show ligninolytic and polymerising (cross-linking) capabilities. Biotechnological applications of laccase have been widely provided in numerous review papers (Janusz et al., 2013; Mayer and Staples, 2002; Riva, 2006; Rodríguez Couto and Toca Herrera, 2006).

For industrial delignification applications laccase is the greatest encouraging ligninolytic enzyme, as it is more available and simpler to manipulate than LiP and MnP. Laccase has relatively low redox potential (≤0.5–0.8 V) and therefore its direct use for lignin biodegradation could be restricted to the oxidation of the phenolic lignin moiety as the non-phenolic structures possess a redox potential of >1.3 V. However laccase on its own is not suitable for lignin biodegradation since there are only a minor fraction of the phenolic groups in lignin structure; the capacity of the bulk laccase molecule to enter the cell wall is restricted. These boundaries are conquered through simulating the nature by means of redox mediators, which are able for realising the oxidation of nonphenolic lignin (Bourbonnais and Paice, 1990). Smaller mediator molecules are initially oxidised or activated by the laccase, and then the mediator breaches the dense lignocellulosic structure with ease and consequently realises the oxidation of lignin inside the substrate. The general delignification efficiency of the laccase mediator system depends on the laccase and mediator association. Efficient oxidations of the mediator, higher redox potential of the activated mediator for efficiently oxidising non-phenolic lignin, and ability to stop the laccase from being deactivated by the free radical form of the mediator are the key features for an efficient laccase mediator system (Du et al., 2013; Li et al., 1999). The role of laccase mediator 1-hydroxybenzotriazol (HBT), in laccase oxidation is to let radical mediated reactions that include the oxidation of side chains and oxygen addition in lignin (Crestini et al., 2003). Laccase in the presence of the mediators oxidises lignin via hydrogen atom transfer that differs from the electron transfer mechanism in direct laccase oxidation and typically leads to lignin degradation (Mattinen et al., 2008). The laccase enzyme could be optimised for bonding biomass materials by oxidation of phenol compounds (Fig. 1), laccase activates the phenolic substrates by catalysing oxidation of their phenolic hydroxyl group to phenoxy radical while dioxygen (O2 ) is condensed to H2 O (Leonowicz et al., 2001; Widsten and Kandelbauer, 2008b). Laccase redox ability oxidises a variety of aromatic substrates into phenoxy radicals under the catalysis of its copper ions by means of oxygen as the electron acceptor and making water as a by-product (Fig. 1). Lignin is not degraded under anaerobic conditions; the carbon–carbon and ether bonds joining subunits together should be separated with an oxidative mechanism. Laccase biodegrades ␤-1 and ␤-O-4 dimers via C␣ -C␤ cleavage, C␣ oxidation and alkyl-aryl cleavage. Aromatic ring cleavage could be identified subsequent to the action of lactase (Youn et al., 1995). Laccase and laccase-mediator pre-treatments using common redox mediators, like 2,2 -azinobis-(3-ethylbenzenthiazoline-6sulfonic acid) (ABTS), lead to fibre alterations, which could be witnessed by spectroscopic methods (Barsberg and Thygesen, 1999; Barsberg, 2002). The lignin in middle lamella is plasticised through the refining process which creates a coating on the external of the fibre. Examination of the pre-treated fibres in SEM showed

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Fig. 2. (a) Surface of a wood fibre made for bio-composite production. (b) Surface of a wood fibre incubated for 12 h with laccase (Hüttermann et al., 2001).

the initially existing crust of lignin (Fig. 2a) which was eliminated entirely by the enzyme pre-treatment (Fig. 2b). Indicating that lignin on the surface of fibre is vulnerable to reaction with laccase hence the crust on the surface is loosened by the laccase pre-treatment (Hüttermann et al., 2001). The oxidative polymerisation of different lignins by Trametes hirsute laccase revealed that all the technical lignins are activated and polymerised by laccase (Mattinen et al., 2008). The reactivity of laccase with different lignins was examined by the extent of oxygen consumption and clear oxidation of lignins by laccase was witnessed in 3 h of treatment on the basis of the oxygen consumption results. The factors which determine the activity of laccase include the pH of the medium, dosage of enzyme, substrate, temperature and incubation time period. The time of incubation should be long enough to activate the lignin. The pH level determines the structure of the enzyme and its active centre. The highest enzymatic activity is always observed at ideal pH level. Kharazipour et al. (Kharazipour et al., 1998) found that relatively short incubations of wood fibres with laccase are sufficient to activate lignin; the fibres were incubated for 3 h, pH 5 was identified as the ideal pH of laccase. 4. Bioconversion of straw biomass Bioconversion of main constituents of straw biomass, which make 20–90% of straw, is of great value as one of the emerging movements in bioengineering. Operative pre-treatment strategies, as discussed earlier, are necessary which lead to efficient energy processing or even further leads to the biodegradation of main components of straw biomass as they are highly resistant to biotransformation which has limited their use in an economically viable bioconversion process into value added bio-products. Bioconversion of retting process has effectively been utilised for the preparation of wheat straw fibres that could be utilised as reinforcing resources. Sain and Panthapulakkal (Sain and Panthapulakkal, 2006) isolated fungus from the bark of an elm tree for retting of wheat straw to produce fibres as reinforcing resource for bio-composites. Fungal retting was followed by mechanical defibrillation for the preparation of wheat straw fibres. The retted fibres were mechanically superior to the un-retted fibres, chemical distribution analysis showed the elimination of hemicellulose and lignin from the retted fibres. This study represents an example of bioconversion of straw biomass which aids the development of sustainable technologies. The bioconversion of biomass using enzymatic hydrolysis usually takes two methodologies, single enzyme or combinations of commercial mixtures. The use of single enzymes leads to bet-

ter evaluation of synergy and collaboration amongst enzymes to biodegrade a compound substrate such as lignin, while the usage of commercial enzymes could be a faster way to commercialisation. Several factors for instance enzyme ratios, substrate loadings, enzyme loadings, inhibitors, adsorption and surfactants are important for the bioconversion of biomass (Van Dyk and Pletschke, 2012). The processes and technologies available for bioconversion of lignocelluloses are reviewed (Kumar et al., 2008) with the discussion on their limitations and possible future developments. Despite progression in the field of bioconversion, the large scale conversion of biomass by biological fermentation to bio-products is still a problem as the speed of biological pre-treatment is slow for large scale industrialisation. Establishing a new industry requires fundamental applied work on methodologies to convert biomass to bio-products, such as extensive research on different essential parameters involved with the bioconversion of biomass; issues need be identified and overcome to make the process complete for industrial scale. Difficulties in the utilisation of biomass have been explored (Kumar et al., 2008). Main struggle in effective biomass utilisation is its crystalline un-reactivity and in specific its resistance to hydrolysis. Many types of pre-treatments have been examined for hydrolysis of straw biomass and a number of them have advanced adequately to evolve into technologies (Bisaria and Ghose, 1981 Kim et al., 1988). 4.1. Bio-energy The bioconversion of straw biomass to fermentable sugars has great prospective application in the area of bio-energy but the efficient and optimised process or technology available for bioconversion of straw biomass for bio-energy production is rare. Biofuels have become more important as potential alternative energy sources due to climate change and oil prices. In the hydrolysis of any biomass a mixture of hexose and pentose sugars are generated (Saha, 2003a, b). The efficient use of these sugars is crucial for the economical bioethanol production. Bioethanol is made via the fermentation of degradable carbohydrates, such as straw biomass. Fermentable sugars could be attained with acid or enzymatic pre-treatment of insoluble lignocellulosic substrate (Huber et al., 2006; Ragauskas et al., 2006). Straw biomass has large amount of constituents which can’t be transformed to ethanol by microorganisms. Another way could be to gasify straw biomass and to utilise the generated synthesis gas as a feed stock for the synthesis of ethanol and other valued bio-products (Henstra et al., 2007). Straw biomass as raw material is a good choice for ethanol and biogas production. Biogas biotechnology is a bioconversion methodology used in China. Biogas could be achieved in an

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269

Table 5 Biological bioconversion of straw biomass to oils. Carbon source

Microorganisms

Biomass conc. (gL−1 )

Lipid cont. (%)

Reference

Wheat straw Wheat straw Rice straw Rice hulls Corn stover

Cryptococcus curvatus Aspergillus oryzae Trichosporon fermentans Mortierella isabellina Trichosporon cutaneum

17.2 NA 28.6 5.6 15.44

33.5 36.6 mg/g dry substrate 40.1 64.3 23.5

Yu et al. (2011) Hui et al. (2010) Huang et al. (2009) Economou et al. (2011) Huang et al. (2011)

anaerobic environment from straw, human wastes, animal dung and organic wastes. Key constituent of biogas is CH4 (60–70%), which has a heat value of about 2.5 × 104 kJ/m3 , which is equal to 1 kg raw coal or 0.76 kg standard coal (Zeng et al., 2007). Hydrothermal pre-treatment (200 ◦ C for 10 min) was the initial stage in the procedure of turning wheat straw into second generation bioethanol (Petrik et al., 2013). Once the hydrothermal pre-treatment was done, the enzymes were added to the fibre mass, comprising mostly of cellulose and lignin, for bioconversion of cellulose to lower carbohydrates to enable the fermentation of ethanol in the following stage. Pre-treatment of straw for production of bioethanol is esteemed to account for 33% of the summed cost of bioethanol generation (Ibrahim, 2012); emergence of an economically suitable processing is the key for bioconversion of straw biomass into bioethanol. The ideal pre-treatment in terms of technical aspects would be to expose the cell wall constituents for enzymatic attack, increase the porosity and surface area of the substrate, diminish the cellulose crystallinity and disturb the heterogeneous structure of lignocellulosic biomass (Talebnia et al., 2010). Various pre-treatments (dilute acid, lime and alkaline peroxide) and enzymatic saccharification processes were assessed for the bioconversion of barley straw to monomeric sugars for bioethanol production (Saha and Cotta, 2010) in this study alkaline peroxide pre-treatment performed best followed by lime and dilute acid pretreatments in terms of sugar released. The yield of bioethanol by the mixed sugar using recombinant Escherichia coli strain FBR5 in 17 h was 11.9 g/L from 26.2 g sugars/L obtained from alkaline peroxide pre-treated barley straw (Saha and Cotta, 2010). It is interpreted from this study that the pre-treatments are not very sustainable and environmentally friendly, also for an industrial scale these pretreatments are not feasible, it could be interesting to see if a milder version of these pre-treatments in combination with some physical pre-treatments would yield better and/or similar results in terms of the bioethanol yield. Technologies for bioethanol production from lignocellulose are reviewed (Alvira et al., 2010) where numerous significant parameters that must be considered for low cost and advanced pre-treatment technologies, are discussed with recent advances for bioethanol production. Sustainable carbon sources including straw biomass have been used for microbial oils production (Xu et al., 2010). Straw biomass is interesting as a carbon source for production of microbial oils although its constituent’s heterogeneity make the industrial scale utilisation complicated. Some examples of straw biomass used for production of microbial oils are given in Table 5 (Xu et al., 2013). The overall economic feasibility in terms of processing needs to be assessed in more details, with the advances in biotechnology, microbial oil from several sustainable carbon sources will emerge as one of the feedstocks for production of biodiesel in the near future. Varieties of oleaginous microorganisms, typically yeast or fungi, have the abilities of applying hydrolyses of wheat straw, rice hulls, rice straw and corn stover for lipids accumulation (Xu et al., 2013). Liquid biofuels, such as ethanol, may well be made from straw biomass through fermentation of sugars extracted from cellulose and carbohydrates (Agbor et al., 2011).

Pyrolysis of straw biomass is an innovative procedure leading to the generation of charcoal, asphalt and other gaseous and organic products. These products are considerable alternative sources of energy; nonetheless the pyrolysis reaction of biomass is complicated due to formation of many intermediate products (Babu, 2008). Novel technology on lignocellulosic production of bio-hydrogen from feedstock pre-treatment to fermentation has been reviewed (Cheng et al., 2011). The integrated bio-hydrogen processes have the features of high chemical oxygen demand reduction, low CO2 emission and high hydrogen yield. When biogas is largely derivative of waste and residues, it is categorised as second generation biofuel. The biogas production is popular in some countries, and last few years it has been strongly implemented in Europe for generation of electricity from biogas. In Germany and Sweden, biogas is also utilised as transportation biofuel, after upgrading to biomethane. The benefit of first generation biofuel is the higher sugar or oil content of the raw materials and their easy bioconversion to biofuel (Cherubini, 2010). 4.2. Bio-composites Producers of medium density fibreboard, particleboard, plywood and oriented strand board must reduce dangerous formaldehyde emissions from the petroleum derived adhesives, and to recover product recyclability. Therefore, state of art methods to reduce the quantity of adhesives while ensuring product superiority is important. The lignin enzymatic activation for biomass bio-composite could contribute to the self-bonding characteristics of the biomass by oxidation of their surface lignin (Widsten and Kandelbauer, 2008a). The in situ cross-reaction of lignin via laccases were used to produce bio-composites, inclusion of hot pressing to this process further increased the crosslinking (Felby et al., 2002, 2004; Lund et al., 2000). The bonding mechanism of fibreboards produced from laccase pre-treated fibres could be related to phenoxy radicals on biomass surfaces that cross-link when the biomass is pressed into boards (Felby et al., 1997, 2002, 2004; Kharazipour et al., 1997a; Widsten et al., 2003). The condensation of hemicellulose degradation products, hydrogen bonding and molecular entanglement are also other contributions to the enhanced bonding quality effect (Felby et al., 2004; Widsten et al., 2003). In small scale trials by Felby et al. (Felby et al., 1997) and Kharazipour et al. (Kharazipour et al., 1997a, 1998), biomass fibres were incubated with laccase (Felby et al., 1997; Kharazipour et al., 1997a) or peroxidase (Kharazipour et al., 1998) at low consistency in H2 O medium at an appropriate pH or laccase solution was sprayed on wood fibres (Kharazipour et al., 1997a). According to European standard (BS EN 622-5, 2009), none of enzymatically bonded MDF obeys with the standards for mechanical strength and dimensional stability (Widsten and Kandelbauer, 2008a). Nevertheless some bio-composite of great mechanical strength and adequate dimensional stability were developed on laboratory/pilot-scale which passed the European specifications (BS EN 622-2, 2004; Kharazipour et al., 1993; Qvintus-Leino et al., 2003).

270

Table 6 Fibreboard produced from enzymatic activation of agricultural and forest biomass fibres. Enzyme

Fibre

Incubation parameters T, C

Laccase SP504 (Tramates versicolor) Laccase SP504 (Tramates versicolor) Peroxidase SP502

Tramates versicolor medium Laccase (Myceliophtora thermophila) Laccase (Trametes villosa) Laccase (Myceliophtora thermophila) Peroxide (4%H2 O2 + 1% Ferrous sulphate) based on dry wheat straw

t, h

Pressing parameters ◦

Fibreboard properties

Ref.

pH

Enzyme dosage U/g

Mat water content%

T, C

t, min

P, MPa

, g/cm

Thickness, mm

IB, MPa

MOR, MPa

MOE, GPa

24 h TS%

5

26,900

Dry fibres

190

5

1

0.78

5.4

0.95

–

–

23

Kharazipour et al. (1997b)

3

80% spruce/pine mixture with 20% beech Beech

35

12

20

1

4.5

–

12

200

5

n/a

0.90

3

1.57

44.6

3.36

19

Felby et al. (1997)

80% spruce/pine mixture with 20% beech Rape straw

RT

4

7

4200

Dry fibres

190

5

1

0.8

5

0.63

41.7

4.02

28

Kharazipour et al. (1998)

–

-

–

–

Dry fibres

-

–

–

0.8

–

0.35

20

–

50

Unbehaun et al. (2000)

Beech

50

0.5

7

24

11-13

200

5

n/a

0.85

8

0.93

46

3.95

46

Felby et al. (2002)

Beech

20

1

4.5

3

Dry fibres

200

5

n/a

0.85

3

–

40.3

4.08

–

Felby et al. (2004)

Beech (TMP)

RT

6

6.7

500

40

190

7

0.3

0.92

4–45

1.4

53

4.8

26

Petri et al. (2009)

Wheat straw

–

–

–

–

Wet fibres

200

1.3

0.5

1.0

6

0.67

23

3.5

85

Halvarsson et al. (2009)

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The physical and mechanical properties along with the processing parameters of bio-composites made through biological pre-treatment from forest and agricultural biomass are presented in Table 6. Particleboards using flax and hemp, treated by laccase enzyme and its mediators (ABTS, 1-hydroxybenzotriazole (HBT), 3 Hydroxyacetanilide (NHA)) were processed by Batog et al. (Batog et al., 2008). The particleboards treated by laccase had better strength than in the absence of laccase; the laccase mediators improved the enzymatic oxidation of lignin. Improvement of laccase activity was realised by adding of its mediators and processing medium (by using buffer and an organic solvent: dioxane solution). Halvarsson et al. (Halvarsson et al., 2009) used wheat straw for fibreboard production without synthetic resin. The bonding was triggered by activation of fibre surface by oxidative pre-treatment, Fentons reagent (ferrous chloride and hydrogen peroxide), during the defibration process. To summarise it is obvious that the surface is important in the bonding mechanisms of enzymatically activated biomass for biocomposites. For example the surface (fibre/particle) entanglement on a macro-scale and the level of contact which is induced by changes in surface morphology is vital in determination of bonding mechanism. The enzyme catalysis initiates stable lignin radicals, which face thermal decay by cross-linking across biomass fibres or particles during hot press procedure. These robust covalent bonds increase interfacial adhesion and are resistant to moisture so biocomposites should have a relative strength improvement (Felby et al., 2004). The enzyme pre-treatment could also produce radical decay products leading to improved level of carbonyl groups that induces robust Lewis acid–base bonding (hydrogen bonding) links.

5. Conclusions and future prospective The bioconversion of biomass into bio-products is crucial with emerging biotechnological methods. Emergence of the discipline called bioengineering which highlights the biological pre-treatments with the aid of other treatment technologies to achieve a synergistic effect, means benefiting from different aspects and reducing the disadvantages of each technology or pre-treatment type. So as to raise the digestibility of biomass, by increasing the availability of the cellulose and carbohydrates for following bio-processes, pre-treatment is an essential step in the bioconversion into bio-products. Designing the pre-treatment parameters for the specific application of the raw biomass requires the identification of the specific problems which is achieved by detailed characterisation of the overall properties of the biomass. Investigating the details of microstructural changes of biomass after the biological pre-treatment for better understanding the changes due to microorganisms, therefore optimising the pretreatment parameters for specific bio-products is an essential requirement for success in bioengineering. Qualitative and quantitative analytical techniques such as NMR, FT-IR, SEM, TGA, XPS and etc. provide valuable information in terms of chemical, morphological and thermal properties of biomass after biological pre-treatment. The problem that persists today in terms of biological pre-treatment of straw biomass is the rate of cultivation of microorganisms (i.e. fungi) and the sensitivity to growth parameters. For developing and up-scaling of biotechnology, it is essential to identify the optimal microorganisms, a good cultivation substrate and procedure for inoculum production as well as careful monitoring during biomass fermentation. Efficient utilisation of straw biomass as energy will meet the demands for energy as the economy grows, and similarly provides fundamentals for environmental security and sustainable growth.

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Necessary biological research is required for the upcoming expansion of the emerging biomass bio-products. Making market demand based on superior life-cycle value and adequate/enhanced properties is one of key challenges to be solved which must be addressed before full potential of bio-products can be recognised. The use of bioengineering for the bioconversion of biomass is in agreement with sustainability aspects of industries hence it requires a lot of attention to details to solve the associated problems with it in order to make the transition from lab scale to industrial scale successfully.

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