The present invention provides an insertion, deletion and/or substitution mutein of wild-type Trichoderma reesei .beta.-mannanase having enhanced thermostability, proteolytic stability, specific activity and/or stability at low pH, a nucleic acid molecule encoding said mannanase mutein, a composition comprising said mannanase mutein; a method for its preparation, and its use for food and feed processing, for coffee extraction and the processing of coffee waste, as a supplement to food and feed, for enzyme aided bleaching of paper pulps, as bleaching and/or desizing agent in textile industry, for oil and gas well stimulation by hydraulic fracturing, as detergent, for removal of biofilms and in delivery systems, or for the processing of renewable resources intended for the production of biological fuels.