X-ray diffraction. Three types of starches, designated as type A,
type B, and type C, have been identified based on X-ray diffraction
patterns. These depend partly on the chain lengths making up the
amylopectin lattice, the density of packing within the granules, and
the presence of water (Wu and Sarko 1978). Although type A and
type B are real crystalline modifications, type C is a mixed form. The
important features of the types of starches are as follows.
Type A. The type A structure has amylopectin of chain lengths of
23 to 29 glucose units. The hydrogen bonding between the hydroxyl
groups of the chains of amylopectin molecules results in
the formation of outer double helical structure. In between these
micelles, linear chains of amylose moieties are packed by forming
hydrogen bonds with outer linear chains of amylopectin. This pattern
is very common in cereals.
Type B. The type B structure consists of amylopectin of chain
lengths of 30 to 44 glucose molecules with water inter-spread.
This is the usual pattern of starches in raw potato and banana.
Type C. The type C structure is made up of amylopectin of chain
lengths of 26 to 29 glucose molecules, a combination of type A
and type B, which is typical of peas and beans.
An additional form, called type V, occurs in swollen granules. Xray
diffraction diagrams of these starches are shown in Figure 4.