TECHNOLOGY FOCUS - BIOLOGY - Preparation (Claimed): Preparation of the mutant thermostable enzyme comprises culturing the host cell in culture medium under conditions suitable to produce the enzyme. Preferred Method: The contacting step is conducted at 50-55 degrees 0C, preferably 60-70 degrees C. The biomass, plant material, textile, soiled textile, paper pulp, or crystalline cellulose is pretreated prior to contacting. Preferred Components: The enzyme comprises: a lysine, alanine, glutamine, glutamic acid, or arginine at an amino acid which aligns with the amino acid 230 of SEQ ID NO. 1; a leucine or serine at an amino acid which aligns with the amino acid 113 of SEQ ID NO. 1; and a threonine or glycine at an amino acid which aligns with the amino acid 115 of SEQ ID NO. 1. The enzyme exhibits increased thermostability as compared to the wild-type enzyme after incubation at 50 degrees C for one hour. The enzyme has a specific activity of at least 0.5 mM GE/ mu M enzyme/hour after incubation at 50 degrees C for one hour. The enzyme is an endoglucanase or an exoglucanase. The enzyme is Trichoderma reesei endoglucanase I. The enzyme is derived from fungi or bacteria. The enzyme is derived from fungi including Candida, Kluyveromyces, Pichia, Saccharomyces, Schizosaccharomyces, Yarrowia, Acremonium, Agaricus, Alternaria, Aspergillus, Aureobasidium, Botryosphaeria, Ceriporiopsis, Chaetomidium, Chrysosporium, Claviceps, Cochliobolus, Coprinopsis, Coptotermes, Corynascus, Cryphonectria, Cryptococcus, Diplodia, Exidia, Filobasidium, Fusarium, Gibberella, Holomastigotoides, Humicola, Irpex, Lentinula, Leptosphaeria, Magnaporthe, Melanocarpus, Meripilus, Mucor, Myceliophthora, Neocallimastix, Neurospora, Paecilomyces, Penicillium, Phanerochaete, Piromyces, Poitrasia, Pseudoplectania, Pseudotrichonympha, Rhizomucor, Schizophyllum, Scytalidium, Talaromyces, Thermoascus, Thielavia, Tolypocladium, Trichoderma, Trichophaea, Verticillium, Volvariella, Xylaria, or Sporotrichum thermophile. The host cell is Escherichia, Pseudomonas, Proteus, Ralstonia, Streptomyces, Staphylococcus, Lactococcus, Bacillus, Saccharomyces cerevisiae, Schizosaccharomyces pombe, Yarrowia lipolytica, Hansenula polymorpha, Kluyveromyces lactis, Pichiapastoris, Aspergillus, Chrysosporium lucknowense, or Trichoderma reesei. EXAMPLE - No suitable example given.