RS3
RS3 represents retrograded starch. Thus, in the formation of RS3,
the starch granule is completely hydrated. Amylose leaches from
the granules into the solution as a random coil polymer. Upon
cooling, the polymer chains begin to reassociate as double helices,
stabilized by hydrogen bonds (Wu and Sarko 1978). The individual
strands in the helix contain 6 glucose units per turn in a 20.8 A° repeat.
The models for the double helices are left-handed, parallelstranded
helices. A type A crystalline structure can be obtained if
RS is formed in gelatinized starch stored at high temperature (that is,
100 °C) for several hours (Eerlingen and others 1993a). It has a
dense structure and only few water molecules in the monoclinic
unit cell. Upon further retrogradation, the double helices pack in a
hexagonal unit cell. The B form with hexagonal symmetry is more
open. Water molecules (36 to 42 molecules per unit cell) in the B
structure are located in fixed positions within a central channel
formed by 6 double helices. The degree of polymerization (DP) of
amylose also affects the yield of RS3; it rises with DP up to 100 and
thereafter remains constant (Eerlingen and others 1993b). A minimum
DP of 10 and a maximum of 100 seems to be necessary to
form the double helix (Gidley and others 1995). Schematic presentation
of RS3 formed in aqueous amylose solutions depicted as miscelle
and lamella model is shown in Figures 7 and 8.
Structural features of in vivo RS (ingestion of retrograded highamylose
maize starch, complexed high amylose maize starch, bean
flakes, or potato flakes) were assessed using the ileal contents of 4
humans (Faisant and others 1993). For all samples, starch fractions,
which escaped digestion in the small intestine, were composed of 3
populations of -glucans with proportions differing according to
the substrate. Small quantities of oligosaccharides made up the 1st
population, illustrating a limitation of absorption in the small intestine.
The 2nd population, the main RS, consisted of retrograded
amylose of mean degree of polymerization (DPn) of about 35 glucose
units with a melting temperature of 150 °C and exhibiting a
type B pattern. Finally high-molecular-weight semicrystalline -glucans
were attributed to fragments of starch. This study showed that
some potentially digestible starch could reach the colon and that
crystalline fractions constituted only part of the starch that escaped
digestion in the human small intestine.