A method for making a mutated pullulanase enzyme that hydrolyzes an α-1,6-glycosidic linkage is provided. The method includes obtaining the amino acid sequence of a pullulanase enzyme having an amino acid sequence that is at least 95% identical to the amino acid sequence set forth in SEQ ID NO: 13; identifying an amino acid to be mutated in the pullulanase enzyme of step (1), wherein the amino acid to be mutated corresponds to the amino acid at position Phe476 of SEQ ID NO: 2; constructing a mutated amino acid sequence by substituting the amino acid to be mutated with another amino acid or deleting the amino acid to be mutated, thereby making a mutated pullulanase enzyme having the mutated amino acid sequence that has increased affinity for pullulan and that hydrolyzes an α-1,6-glycosidic linkage.