Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
22.3 GRANULAR LEVEL
Plant-specific and environmental-conditions-induced activities of starch synthases
and branching enzymes result in individual distributions of degree of polymerization
and branching characteristics for any kind of starch glucans. For storage, the crystallized
insoluble form is preferred, and thus formation of starch granules starts.
Formation and growth of granules is a complex process and ends up with each
granule as an individual object. Nevertheless, glucans with different percentages of
proteins, lipids, water and charges, primarily phosphates, are the major components
of all types of starch granules. In terms of order, granules typically are 20–40%
crystalline, of irregular, however, plant- and variety-specific shape with diameters
of 1–120 µm and density of 1.5–1.6 g/cm3 ; white to creamy color; and internally
organized in layers, having dense layers [120–400 nm formed by ∼16 alternating
crystalline (5–6 nm) and semicrystalline (2–5 nm) rings] 6 and less dense layers with
higher content of water. 7 Undoubtedly, water needs to be considered as a fundamental
structural feature in the formation of starch granules. Although starch contains no
crystal water, H2O is not just another bulk material. All variations of crystallinity
contain more or less water and represent more or less compact order on a dominant
amorphous background. Different classes of starch granule crystallinity are typically
discriminated by x-ray diffraction patterns: (1) A-type: left-handed, parallel-stranded
double helices crystallized in a monocline space group B2; compact packing of
glucan-chains and low water content: 12 H2O molecules with 12 anydroglucose units
(AGUs); 6 AGUs per helix turn, 1.04-nm height for each turn; (2) B-type: double
helices crystallized in the hexagonal (space group P6; less compact packed, higher
water content: 36 H2O molecules with 12 anhydroglucose units (AGU); 6 AGUs per
helix turn, 1.04-nm height for each turn; (3) C-type: a mix of A- and B-type; however,
listed as distinct type; and (4) V-type: formed by 6 AGUs in a helical structure with
0.8-nm height per helix turn.
Enzymatically supported fragmentation analysis reveals that A-type starch glucans
additionally differ from B-type in their branching pattern, in particular in their
ratio of terminal (A-chains) and internal (B-chains) glucan segments. 9–12
Whereas short-chain branched (scb) glucans (amylopectin) are assumed to form
crystalline lamellae by parallel double helices with branching positions in amorphous
regions, nonbranched (nb) and long-chain branched (lcb) glucans (amylose) are
preferably located in the amorphous layers13 and are subject for complex formation
with lipids. Additionally, limited cocrystallization of scb- and lcb-glucans forming
small (∼25 nm) and large (80 to 120 nm) blocks was observed by scanning electron
microscopy (SEM) and atomic force microscopy (AFM).
13 C CP/MAS spectra support the idea of amorphous single-chain and ordered
double-helix glucans. 16 Thermal stress on B-type results in loss of water and transformation
into A-type; swelling of A-type in aqueous media and destruction of
crystalline structure yields B-type when recrystallyzing.
Starch granules are composed of a crystalline short-chain branched (scb) glucan
(amylopectin) framework and an amorphous long-chain branched (lcb) glucan (amylose)
fraction. Glucans of scb- and lcb-type are more or less incompatible: scbglucans
form compact layers of high order, whereas lcb-glucans form amorphous
14, 25