In pulp and paper industry, the most important application of enzymes is in the prebleaching of kraft pulp. Xylanase enzymes have been found to be most effective for this purpose. Enzymes have also been used to increase pulp fibrillation and water retention and to reduce beating time in virgin pulps. With recycled fibers, enzymes have been used for deinking and to restore bonding and increase freeness. Specialized applications include the reduction of vessel picking in tropical hardwood pulps and the selective removal of xylan from dissolving pulp. Enzymes have also been investigated for removal of bark, shives, pitch, and slime and for retting of flax fibers.
Bleaching
The removal of lignin from chemical pulps is called bleaching, and it is necessary for aesthetic reasons and for improvement of paper properties. Present-day bleaching of kraft pulp uses large amounts of chlorine and chlorine chemicals. Byproducts from using these chemicals are chlorinated organic substances, some of which are toxic, mutagenic, persistent, and bioaccumulating and cause numerous harmful disturbances in biological systems. Enzymes provide a very simple and cost-effective way to reduce the use of chlorine, chlorine compounds, and other bleaching chemicals. Enzymes also offer a simple approach that allows for a higher brightness ceiling to be reached. So far, two enzyme-based approaches have been investigated. One uses hemicellulase enzymes, and the other uses ligninolytic enzymes.
Hemicellulase enzymes are used commercially in pulp bleaching. The main enzyme needed to enhance the delignification of kraft pulp is reported to be endo-β-xylanase, but enrichment of xylanase with other hemicellulolytic enzymes has been shown to improve the effect of enzymatic treatment. Ligninolytic enzymes, unlike xylanases, attack lignin directly, and hence are more effective. Several reports in the literature suggest that these enzymes could prove useful in bleaching of kraft pulps. White rot fungi are the main producers of ligninolytic enzymes. The most important lignin-degrading enzymes are lignin peroxidases, manganese peroxidases, and laccases.
Fiber Modification
Enzymatic modification of fibers aims at decreased energy consumption in the production of thermomechanical pulps and increased beatability of chemical pulps or improvement of fiber properties. In highyield mechanical pulps, most of the lignin and hemicellulose remains in the pulps. According to determinations of the medium pore width and immunololabelings of the untreated wood, it is evident that enzymatic modifications to the composition of mechanical pulps can be achieved only on the outer surface of the fiber. This was verified when xylan ases were applied to thermomechanical pulp. Even when using rather higher enzyme dosages, only about 1% of the pulp was dissolved. When combined with an alkaline pretreatment, the enzymatic treatment was substantially improved, and the amount of energy required for refining the thermomechanical pulp was decreased.
Deinking
Enzymatic deinking represents a new approach to convert secondary fibers into quality products. It has proven on a laboratory and industrial scale to be an effective and economical method of deinking waste paper. Drainage enhancement is known to be a secondary benefit of enzymatic deinking. The enzymatically deinked pulps possess superior physical properties, higher brightness, and lower residual ink compared to chemically deinked recycled pulps. More importantly, size distribution and shape of ink could be effectively controlled using the enzymatic process to maximize the efficiency of size-based flotation process. This can be accomplished by selectively varying enzyme composition, charge, and residence time as well as varying other additives and pH in the system to effectively dislodge the normally large, flat, and rigid ink particles into much fewer and nonplatelet forms. The enzymatic deinking process also improves freeness compared to chemically deinked pulps.
Removal of Pitch
Pitch is composed of fatty acids, resin acids, sterols, glycerol esters of fatty acids, other fats, and waxes and is usually defined empirically as the wood component that is soluble in methylene. It is less than 10% of the total weight of wood but causes major problems. Pitch reduction with enzymes is a very efficient biotechnological method. Different lipases have been used for removal of pitch. Enzymatic pitch control helps to reduce pitch-related problems to a satisfactory level. It reduces defects on paper web as well as the frequency of cleaning pitch deposits in the paper machine. At the same time, it also offers other advantages, such as ecofriendly and nontoxic technology, improved pulp and paper quality, reduction in bleaching chemical consumption, reduction of effluent load, and space and cost saving in a mill wood yard by using unseasoned logs.
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