Hydrolysis of A- and B-type crystalline polymorphs of starch by α-amylase, β-amylase and glucoamylase 1

Abstract

The hydrolysis of A-type and B-type spherulitic polycrystalline amylose (degree of polymerisation ∼ 20) by glucoamylase 1 ( Aspergillus niger), by β-amylase (sweet potato) and by α-amylase ( Aspergillus oryzae) has been examined by differential scanning calorimetry, X-ray powder diffraction and microscopy (polarised light and scanning electron). B-type structure is hydrolysed more slowly than A-type by all enzymes probably due to surface area effects. A-type shows no discernible spherulitic structure after partial hydrolysis, whereas the B-type spherulites retain a large amount of visible spherical structure. Glucoamylase l-treated B-type crystals retain their positive birefringence, appear smooth, exhibit a 40% decrease in melting energy and retain crystallinity. On the other hand, α- and β-amylase treatment of B-type spherulites results in spherulites that are not birefringent, show extensive small pits (β-amylase) or several very large pits (α-amylase), exhibit an 83% (β-amylase) and 87% (α-amylase) decrease in melting energy and lose a substantial amount of crystallinity. A-type crystals show no change in melting energy of the solid material after partial hydrolysis by any enzyme, and fully retain crystallinity of the remaining material. The results demonstrate that the pattern of enzyme attack depends on the crystal type, and that each enzyme has a specific mode of attack on B-type amylose even in chemically homogeneous spherulites of a single crystalline type.