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Microbial Cellulases: From a Rotting Cartridge Belt to Designer Jeans
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An Invisible Enemy

During the Second World War, the American military units stationed in the South Pacific jungles had to face a different kind of threat: deterioration of their clothing and equipment made out of cotton. A strain of Trichoderma viride, possessing strong cellulose-degrading abilities was isolated from a rotting cartridge belt during the studies that were carried out to understand the deterioration process.

This focused attention on Trichoderma strains, which, to this day are among the most efficient producers of extracellular cellulases.

A Jack of all Trades

Since early 1980s, cellulases have been used in many industrial applications including food and feed, wine and brewery, textile and laundry, pulp and paper etc. Nowadays, the focus is on cellulases due to their ability of converting cellulosic biomass into glucose, enabling the biofuel production. This article provides an overall summery of some of the current industrial applications and the biotechnological production of cellulases.

What are cellulases?

Cellulases are a family of enzymes including three major groups of members, namely, endoglucanases, exoglucanases (cellobiohydrolases) and beta-glucosidases (Cellobiase) that catalyses the hydrolysis of cellulose, i.e. cellulolysis.

Cellulase in the Textile and Laundry Industry

Cellulases are widely used in the production of denim blue jeans in a process called bio-stoning which gives the garment a softer texture and a fashionable “stone-washed” look. In this process, cellulases digest cellulose fibres on the cotton surface and loosen the indigo dye, giving the fabric a faded look. Cellulase based bio-stoning has many advantages over the traditional use of pumice stones in the stone-washing process; such as reduced wear and tear of the fabrics, short treatment times, less damages to the washing machines and improved quality of garments, etc.

Cellulase enzymes are also being used in textile processing for desizing and bio-polishing of the textiles. Desizing is the process of removing ‘size’ --an adhesive composed of starch, vegetable gum and water-soluble cellulose derivatives, which is added to reinforce the cotton threads-- prior to dyeing, bleaching and printing of the fabric. Enzyme treatment has replaced the conventional use of acids, alkali or oxidising agents in this process thereby eliminating the corrosion of the fibres. During bio-polishing, short cotton fibres that give a surface fuzziness to the fabric are digested by the enzyme, thus giving a smooth and glossy appearance with improved colour brightness.

In many commercial detergents cellulases are used to remove the dirt, restore the colour and soften the garments. This is achieved by removing the partially detached microfibrils on the surface of garments that traps the dirt particles.

Cellulases in the Food Industry

A complex cocktail of enzymes consist of cellulases , hemicellulases and pectinases — collectively called macerating enzymes—are used in the extraction and clarification of fruit and vegetable juices. These enzymes remove the water insoluble plant particles such as fibres, cellulose, hemicellulose, pectin, starch etc. which make the extracted fruit juices turbid. Removing these substances is essential to avoid further turbidity and precipitation and to improve sensory attributes.

In addition, cellulases are added during extraction of juices from particular fruits as black current and red grapes, to improve the release of colour compounds from the skins of the fruit. Furthermore, celluloses, together with pectinases liquefy the plant tissue making it possible to filter juice straight from the pulp without the need for pressing.

Another application of cellulases in food industry is in extraction of olive oil. Macerating enzyme mixture decomposes the cell wall allowing efficient maceration and extraction of oil from olives, increases the oil yield. Macerating enzymes increase the anti-oxidants such as vitamin E, in extra-virgin olive oil thus reducing the rancidity of oil.

Cellulases in the Wine and Brewing Industry

During the production of ethanolic beverages, glucanases are added to hydrolyse glucan which forms gels during the brewing process leading to poor filtration of the wort, low yields and the cloudiness of the final product. Microbial beta-glucanases, added either during mashing or primary fermentation reduce the viscosity of the wort and releases reducing sugars during primary fermentation improving fermentation efficiency, filtration and quality of beer. The commonly used beta-glucanases are from Penicillium emersonii, Aspergillus niger, Bacillus subtilis and Trichoderma reesei.

In the production of red wines, addition of glucanases in combination with other enzymes such as pectinases and hemicellulases, improve the skin maceration and extraction of colour from the grape skins during pressing consequently enhancing the quality, stability, filtration and clarification of wines.

Cellulases in the Paper and Pulp Industry

Cellulases are used widely in the paper and pulp industry for bio-mechanical pulping, de-inking i.e. partial hydrolysis of carbohydrate molecules and the release of ink from fibre surfaces.

Apart from these applications, cellulases are extensively being used in waste management, pharmaceutical industry, agriculture and in various research applications. With the recent developments in the biofuel researches, there will soon be increased applications of cellulases for converting lignocellulose biomass into ethanol.

Cellulase Producing Microorganisms

A large array of microorganisms including both fungi and bacteria produce cellulases. Among them most extensively studied cellulase producers include bacteria such as Clostridium thermocellum (an anaerobe) Cellulomonas (an aerobe) and fungal species like Trichoderma, Phanerochaete and Aspergillus.

Improvement of microbial strains for the production of elevated amounts of cellulases is of vital important as the amount of enzyes produced by the wild-type strains of microorganisms is too low for sustainable industrial operations. Researches carried out to improve the cellulase production by mutagenesis and selection of cellulolytic microorganisms have yielded mutants with enhanced characteristics such as increased efficiency and reaction rates, higher glucose tolerance, greater tolerance to elevated temperature and pressure conditions used in industrial processes etc.
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