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G. Singh et al.
of eucalyptus kraft pulp with 31% reduction in chlorine consumption (Sindhu et al., 2006). Extracellular cellulase free xylanase produced from Bacillus subtilis C01 increased the brightness by 19% of banana pulp Ayyachamy and Vatsala (Ayyachamy and Vatsala, 2007). Puri fi ed alkali stable xylanase from Aspergillus fi scheri was immobilized on polystyrene that reduced the kappa number of paper pulp by 87% (Senthilkumar et al., 2008). A synergistic action of xylano-pectinolytic enzymes from Bacillus pumilus was evaluated for the prebleaching of kraft pulp; as a result 8.5% and 25% reduction was noticed in kappa number and chlorine consumption respectively (Kaur et al., 2010). Alkali stable and thermo tolerant xylanase from B. pumilus SV-85S showed (at pH 9.0, 55 °C for 2.0 h) the reduction in kappa number by 1.6 points and increased brightness by 1.9 points. The pretreatment of pulp with xylanase resulted in 29% reduction in chlorine consumption (Nagar et al., 2013). First report on a bacterial system involving direct growth of xylanase -producing B. halodurans FNP 135 on kraft (eu- calyptus) pulp under submerged fermentation conditions, showed 35% reduction in kappa number and 5.8% enhancement in brightness with 20% reduction in chlorine consumption (Gupta et al., 2015). Kumar et al. (2016) emphasized that signi fi cant application of thermostable xylanases is biobleaching in pulp and paper industry, where these en- zymes acted as delignifying agents, showing clear economic and en- vironmental advantages over chemical alternatives. After xylanases, laccases are the next extensively explored enzymes for biobleaching of pulp; these are oxidative biocatalysts that have in fl uenced the re- searchers by their numerous merits over any other bleaching enzyme (Singh et al., 2008; Singh et al., 2010; Singh et al., 2009; Singh et al., 2015). Laccases, together with mediators are able to delignify the pulp by the oxidation chain reaction leading to lignin oxidation without the degradation of cellulose. In India pioneering work on alkalophilic lac- cases was started by Bains et al. (Bains et al., 2003), through isolation of a novel strain named as γ -proteobacterium JB. An alkalophilic cellu- lase-free laccase from γ -proteobacterium JB was applied to wheat straw-rich soda pulp to evaluate its bleaching potential by optimizing the conditions statistically using response surface methodology based on central composite design in the presence of ABTS at pH 8.0 which enhanced the brightness by 5.8 and reduced the kappa number by 21% within 4 h of incubation at 55 °C. It was noticed that pre-bleaching of eucalyptus kraft pulp with xylanase or laccase individually avoided the ClO 2 by 15% and 25% respectively. When both enzymes were applied together at pilot scale (50 kg pulp), there was reduced organo-chlorine compounds consumption by 34% in bleach e ffl uent (Sharma et al., 2014). Tables 1, 2 shows the year wise isolation of new laccase and xylanase producing organisms and enzyme characterization, but there were very few enzymes either xylanase or laccase evaluated for bio- leaching of pulps. Recently, also many reports published on xylanases and laccases from Indian laboratories but none of them studied on deligni fi cation of biomass (Sharma et al., 2015a; Sharma et al., 2015b; Sharma et al., 2015c; Desai and Iyer, 2016; Nikam et al., 2017; Afreen et al., 2017; Dharmesh et al., 2017; Raj et al., 2018; Kumar et al., 2018; Ranimol et al., 2018).
3.5. De-inking of RCF
Recycled fi bers (RCF) dispersion or fl oatation pulping process is applied for the de-inking of the news papers/print papers. For de- inking, chemicals such as detergents, dispersants and foaming agents added and ink is separated from the pulp.
3.6. Pre-bleaching of pulp with enzymes
The term bleaching is generally referred to the removal of lignin from any kind of the pulp by use of chemicals/gases/steam etc. but prebleaching terminology is used for the enzymatic treatment of the pulp for removal of lignin. Prebleaching is an eco-friendly and cleaner process of lignin removal that can save the chlorine based and other chemicals 10 – 15% (Bajpai, 2004; Camarero et al., 2007; Garg et al., 2011). Prebleaching of pulp with enzymes is still under trial or at pilot scale in paper mills of India.
3.7. Chlorine bleaching of pulp
The process is used to remove the residual lignin in the range 5 – 10%. This process is followed by several stages of treatment of pulp with chlorine dioxide or hypochlorite to whiten the pulp. Bleaching process employed in most of the medium and small mills is based on elemental chlorine. However, few of the large sized wood based/agro based mills have introduced elemental chlorine free (ECF) bleaching process making use of chlorine dioxide ClO 2 .
3.8. Elemental chlorine free (ECF) bleaching
ECF bleaching technology is being practiced in few large mills of the country where it uses oxygen deligni fi cation (ODL), followed by ClO 2 to enhance the brightness of the pulp.
4. Eco-friendly bleaching enzymes (xylanases and laccases) studied by the Indian research laboratories
There are numerous commercially available enzyme cocktails are available, but due to the di ff erences in paper making process in the developed countries and in India, it has been felt to characterize en- zymatic pre-bleaching process indigenously with enzymes produced from locally isolated cultures or with commercially available enzymes that match with the interests of Indian pulp industries. One of the major di ff erences is the use of di ff erent sort of raw materials for pulp making in India (Sharma et al., 2015a; Sharma et al., 2015b; Sharma et al., 2015c; Dutt et al., 2009; Bajpai et al., 1994; Singh et al., 2008; Singh et al., 2010). Up to the 1980, there was no university or institute was associated in research and development (R&D) that can directly in- volved for giving the technical guidelines to Indian paper industry. R&D progress on enzymes for paper industry is still in its beginning and only single institute works in a direction to undertake industry related issues and emphasized on applied research, is Central Pulp and Paper Re- search Institute (CPPRI). There were only a few reports on xylanases for the biobleaching of pulp in country before 2000, e.g. treatment of eu- calyptus pulp with commercial xylanases such as Novozyme 473, and Cartazyme HS-10 reduced the chlorine consumption by 31% and in- creased the fi nal brightness by 2.1 – 4.9 points (Bajpai et al., 1994). Thermostable cellulase-free xylanase from Streptomyces sp. QG-11-3 was produced and applied for deligni fi cation of eucalyptus kraft pulp at pH 8.5 and 50 °C for 2 h. There was reduction in kappa number and increase in brightness of pulp by 25% and 20% respectively (Beg et al., 2000). Bajpai, reported, properties of many commercial xylanases make them unsuitable for the real process of pulp bleaching (Bajpai, 2004). So industries need xylanases which can function e ffi ciently in their existing papermaking processes. Xylanase from Bacillus megaterium showed 8.1% decrease in kappa number and 13% increase in brightness
5. Commercial use and availability of Indian patents on bleaching enzymes
R&D work on isolation and screening of microbial cultures, capable of producing low molecular weight xylanases was started initially at National Chemical Laboratory Pune in early 1990s. Later, IIT Delhi, Birla Institute of Scienti fi c and Industrial Research Jaipur and few other research and academic institutions began working on culture develop- ment for the production of alkaline thermo-tolerant xylanase enzymes. A national research laboratory CPPRI and a premier educational in- stitution in the country, Institute of Paper Technology (IPT) also in- itiated R&D on xylanase enzyme based pre-bleaching of the pulp. The fi rst ever mill trial of xylanase pre-bleaching in India was conducted in a pulp and paper mill of Ballarpur Industries Ltd. (BILT) in 1992 using
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