NOVELTY - Method (M1) for converting a biomass substrate to a fermentable sugar, involves contacting the biomass substrate with beta -glucosidase polypeptide variant comprising (a) amino acid sequence having at least 70% identity with respect to wild type Azospirillum irakense beta -glucosidase, and/or (b) an amino acid sequence encoded by a nucleic acid that hybridizes under stringent conditions over substantially the entire length of a nucleic acid corresponding to a sequence.    USE - The method (M1) is useful for converting biomass substrate to fermentable sugar which is used in producing alcohol. The alcohol is ethanol and/or butanol, preferably ethanol (all claimed).    ADVANTAGE - The beta -glucosidase utilized in method improves the yield of fermentable sugar, is suitable for industrial production and maximizes hydrolysis of cellulosic substrate. The beta -glucosidase has improved beta -glucosidase activity, thermoactivity, thermostability, pH activity, pH stability, product specificity, specific activity, substrate specificity, altered temperature profile, and chemical stability and increased protein expression and resistance to substrate or end-product inhibition. The beta -glucosidase splits cellobiose, water-soluble beta -1,4-linked dimmer of glucose, into two unit of glucose, thus leads to complete conversion of cellobiose to glucose in cellulose hydrolysis process.    DETAILED DESCRIPTION - Method (M2) for converting a biomass substrate to a fermentable sugar, involves contacting the biomass substrate with beta -glucosidase polypeptide variant, where the beta -glucosidase polypeptide variant comprises: (a) a 690 amino acid sequence (SEQ ID NO:4) fully defined in the specification having at least 70% identity with respect to wild type Azospirillum irakense beta -glucosidase and comprising at least 322 substitution or deletion (SD) of an amino acid residue at several position chosen from Thr at position 2, Ala at position 3, Ile at position 4, Ala at position 5, Gln at position 6, Glu at position 7, Gly at position 8, Ala at position 9, Ala at position 10, Pro at position 11, Ala at position 12, Ala at position 13, Ile at position 14, Leu at position 15, Pro at position 17, Glu at position 18, Lys at position 19, Trp at position 20, Pro at position 21, Pro at position 23, Ala at position 24, Thr at position 25, Gln at position 26, Ile at position 29, Asp at position 30, Glu at position 34, Lys at position 35, Ala at position 39, Leu at position 41, Lys at position 42, Gln at position 43, Leu at position 44, Glu at position 47, Val at position 46, Gly at position 51, Gln at position 52, Val at position 53, Gly at position 56, Gly at position 59, Thr at position 60, Ile at position 61, Glu at position 64, Leu at position 66, Arg at position 67, Lys at position 68, Pro at position 70, Ser at position 73, Asn at position 79, Asn at position 83, and Gly at position 84, where amino acid position is numbered with reference to SEQ ID NO:4, or its C-terminally truncated variant, under conditions suitable for the production of the fermentable sugar, and/or (b) an amino acid sequence encoded by a nucleic acid that hybridizes under stringent conditions over substantially the entire length of a nucleic acid corresponding to a sequence chosen from (i) a polynucleotide sequence that is complementary to a polynucleotide that encodes a polypeptide having the amino acid sequence of SEQ ID NO: 4, and (ii) a polynucleotide that is complementary to a polynucleotide that encodes a C-terminally truncated variant of SEQ ID NO:4, where the encoded polypeptide has the one or more substitutions or deletions (SD), and amino acid position is determined by optimal alignment with SEQ ID NO: 4, under conditions suitable for the production of the fermentable sugar. An INDEPENDENT CLAIM is included for method (M2) for producing an alcohol, which involves (a) providing the fermentable sugar produced by the method (M1), (b) contacting the fermentable sugar with fermenting microorganism to produce alcohol and (c) recovering the alcohol.