Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
precipitates in less dense domains. Within the granules, the tendency for separation
increases with increasing percentage of lcb-glucans in a mixture of both types. No
instance of a homogeneous scb-/lcb-glucan blend in a starch granule is known.
In the native environment (plant cell), starch granules are hydrated, and thus in
a swollen state, similar to the conditions of industrially hydrated starch granules: in
both cases, the granules show increased order (birefringence) and reduced hilum
opening. In the swollen state, lcb-glucans diffuse out of amorphous regions, and
transfer of ionic compounds into and out of the granules is enabled. Extraction
procedures (leaching) and drying result in the formation of compact granules with
a cracked hilum.
Based on differences in swelling behavior in aqueous environment, there are
reasonable suggestions for preferred localizations of scb- (amylopectin) and lcbglucans
(amylose) within starch granules: (1) Waxy maize starch granules represent
an arrangement of more or less 100% scb-glucans (amylopectin) and no lcb-glucans
(amylose). The scb-glucans are closely packed in concentric layers in dry waxy
maize granules and expand on swelling in aqueous media. These concentric layers
of scb-glucans represent the framework for the majority of starch granules. (2) Potato
starch granules represent a mix of a major fraction of ∼80% scb-glucans (amylopectin)
and a minor fraction of ∼20% lcb-glucans (amylose). The lcb-glucans are
localized in distinct concentric layers alternating with scb-glucan layers. On hydration,
these granules swell due to the expanding layers and simultaneously reduce
the volume for amorphous lcb-glucans by encapsulation. From such granules, even
lcb-glucans (amylose) may be extracted by leaching processes, which similarly
reduce the amorphous layer fraction. (3) Amylomaize starch granules are composed
of a minor fraction of ∼30% of scb-glucans (amylopectin) and a major fraction of
∼70% of lcb-glucans (amylose), with a pronounced separation of scb- and lcbglucans.
On hydration, lcb-glucans of such granules are kind of diluted but granules
do not swell. From such granules lcb-glucans may be extracted by leaching processes.
22.4 MOLECULAR LEVEL
Investigations on the molecular level in most cases require dissolution of the materials
of interest. Although starch polysaccharides are rather polar, due to pronounced
H-bond-binding they can hardly be dissolved in neutral aqueous media. For most
technical purposes, they get dissolved or suspended in H-bond-“breaker” alkaline
media; however, for analytical purposes dissolution in aprotic polar medium such
as dimethylsulfoxide (DMSO) is preferable as problems with ionic strength variations
can be avoided. A number of chemical, enzymatically supported, and physical
techniques are available to investigate molecular-level characteristics of starch glucans.
However, due to the multiple and superimposed heterogeneities of these materials,
the most important are fractionation and separation techniques, in particular
precipitation–complexation and chromatography (Table 22.3). As the major quality
of starch polysaccharides, like any polysaccharides, is to fill up volume in a more
or less structurized way, any analytical approach has to qualify and quantify this
performance. Size-exclusion chromatography (SEC) is a perfect tool to obtain such