Uses of Waste Starch - The Economic Utilisation of Food Co-Products - page 67

to either A, B or C chains. Each amylopectin molecule contains one C chain,
which has a reducing end and is said to be the backbone of the amylopectin
molecule. 18
B chains within clusters are assigned as B 1 -B 4 and are designated according
to their chain length. Chain lengths of the corresponding chains A, and B 1 -B 4
are 12-16, 20-24, 42-48, 69-75 and 101-119, respectively, with the molar
distribution of A to B -chains, being 1 : 1 to 1 : 2, depending on starch type. 6
Native starches are semicrystalline structures. This concept has been
extended to describe the levels of structural complexity that are now regarded
as being important to the granule structure. The starch granule is composed of
alternating regions of ordered (crystalline lamella), and less-ordered
(amorphous lamella) regions. The ordered regions are composed of tightly
packed, parallel glucan chains, whereas the less-ordered regions are mainly
composed of branched B - chains. The size of each cluster is around 9 nm, with
the crystalline region being approximately 5 nm. 19
The crystallinity of these regions is classified according to X-ray diffraction
patterns, and assigned as having A- or B-type crystallinity, or possibly an
intermediate C type, 20 with A-type crystallinity being the most densely packed.
Cereal starches usually have an A-type X-ray diffraction pattern, while tuber
and other types of starches have B- or C-type crystallinity.
Crystallinity in starch occurs within the ordered segments of the
amylopectin molecule. When the chains of these molecules are 10 or more
glucose units long, they can intertwine forming a double helix arrangement.
These double helices can be formed from adjacent branches within the same
amylopectin cluster, or from chains of different clusters forming a three-
dimensional network. The way in which the double helices are intertwined,
and the length of the A-type amylopectin chains, determines its crystal
structure, and hence, what crystallinity category it belongs to, i.e. A, B or C.
Short A chains are associated with A-type crystallinity, longer A chains
display B-type crystallinity, and A chains of intermediate length show C-type
crystallisation. 18
The degree of crystallinity (15-45%) in starch, however, cannot explain its
relative stability, as there is simply not enough crystalline material within the
granule. It is therefore postulated, from studies on potato starch, that the
crystalline domains form continuous networks of left-handed helices, in which
the voids of these superhelixes are assumed to be empty, and to be
approximately 8 nm wide. 21,22
The interpenetrating superhelices are assumed to form a skeleton for the
granule to develop on. This model is only demonstrated for potato starch, but it
is assumed that a similar model is valid for other starches, as structural designs
in nature tend to be conserved.
Superhelices are believed to be ''blocklet'' structures, corresponding to the
framework of growth rings seen in starch granules. 23,24 These blocklets are
composed of crystalline and amorphous lamellae from amylopectin, and are
believed to be continuous throughout the granule. 25 The crystalline growth ring
consists of normal blocklets, while the amorphous growth rings consist of

d n 9 r 3 n g | 0

3 .