A method of preparing a variant α-amylase polypeptide from a parental α-amylase polypeptide, comprising introducing at least one combinable mutation into said parental α-amylase polypeptide at a productive amino acid position; wherein: (a) the combinable mutation produces a variant amylase wherein the minimum performance indices relative to the parental amylase for: (i) protein expression, (ii) activity, and (iii) detergent stability or thermostability are either (aa) greater than or equal to 0.9, and the PI for any one of (i), (ii), or (iii) is greater than 1.0, (ab) greater than or equal to 0.8, and the PI for any one of (i), (ii), or (iii) is greater than or equal to 1.2, or (ac) greater than or equal to 0.5, and the PI for any one of (i), (ii), or (iii) is greater than or equal to 1.5; wherein protein expression, activity, detergent stability and thermostability are each determined using an assay described in Example 1; (b) the productive position is an amino acid position that can be substituted with a plurality of different amino acid residues, each of which substitutions result in a variant α-amylase that meets the requirements of (i), (c) the productive position is F152, using SEQ ID NO: 3 for numbering, and (d) the combinable mutation is F152H, F152W or F152Y, wherein the parental α-amylase and the variant α-amylase have at least 80% amino acid sequence identity to the amino acid sequence of SEQ ID NO: 3 or SEQ ID NO: 4. (a) the combinable mutation produces a variant amylase wherein the minimum performance indices relative to the parental amylase for: (i) protein expression, (ii) activity, and (iii) detergent stability or thermostability are either (aa) greater than or equal to 0.9, and the PI for any one of (i), (ii), or (iii) is greater than 1.0, (ab) greater than or equal to 0.8, and the PI for any one of (i), (ii), or (iii) is greater than or equal to 1.2, or (ac) greater than or equal to 0.5, and the PI for any one of (i), (ii), or (iii) is greater than or equal to 1.5; wherein protein expression, activity, detergent stability and thermostability are each determined using an assay described in Example 1; (i) protein expression, (ii) activity, and (iii) detergent stability or thermostability are either (aa) greater than or equal to 0.9, and the PI for any one of (i), (ii), or (iii) is greater than 1.0, (ab) greater than or equal to 0.8, and the PI for any one of (i), (ii), or (iii) is greater than or equal to 1.2, or (ac) greater than or equal to 0.5, and the PI for any one of (i), (ii), or (iii) is greater than or equal to 1.5; wherein protein expression, activity, detergent stability and thermostability are each determined using an assay described in Example 1; (aa) greater than or equal to 0.9, and the PI for any one of (i), (ii), or (iii) is greater than 1.0, (ab) greater than or equal to 0.8, and the PI for any one of (i), (ii), or (iii) is greater than or equal to 1.2, or (ac) greater than or equal to 0.5, and the PI for any one of (i), (ii), or (iii) is greater than or equal to 1.5; wherein protein expression, activity, detergent stability and thermostability are each determined using an assay described in Example 1; (b) the productive position is an amino acid position that can be substituted with a plurality of different amino acid residues, each of which substitutions result in a variant α-amylase that meets the requirements of (i), (c) the productive position is F152, using SEQ ID NO: 3 for numbering, and (d) the combinable mutation is F152H, F152W or F152Y,