Biomedical Engineering Reference
In-Depth Information
9.2
Effects on Cereal Biopolymers
9.2.1
Starch
Dietary carbohydrate is the major source of plasma glucose. An increase in the
amount of rapidly digestible carbohydrate in the diet causes a rapid increase in
blood glucose levels and a large demand for insulin in the postprandial period. The
major carbohydrate sources in the Western diet contain rapidly digestible starch,
and many common starchy foods like bakery goods, breakfast cereals, potato prod-
ucts and snacks produce high glycaemic responses. There are strong indications that
the large amounts of rapidly available glucose derived from starch and free sugars
in the modern diet [foods with high glycaemic index (GI) and high insulin index
(II)] lead to periodic elevated plasma glucose and insulin concentrations that may be
a risk factor to health [
10
] .
Most processed starchy foods have low to medium moisture contents, thus
their digestion is basically a solid-liquid two phase reaction, and the enzyme
(particularly a-amylase) needs first to diffuse into the hydrated solid food matrix,
bind to the substrate, and then cleave the glycosidic linkages of the starch
molecules [
11
]. Factors affecting the binding of a-amylase to substrates [e.g.
inhibition by the hydrolysis products (maltose and maltotriose)] will slow down
the enzymatic reaction and thus digestion of starch. Other physiological factors
affecting starch digestibility include gastric emptying, enzyme inhibitors and
viscosity in the digestive tract [
12
] .
Macro- and microstructure of cereal foods has a profound influence on the digest-
ibility of starch, as reviewed by Singh et al. [
13
]. Especially, the characteristics of
starch per se are of crucial importance for glucose response. Amylose-rich starches
are more resistant to amylolysis than waxy or normal starches. The major intrinsic
factors affecting raw starch digestibility include the supramolecular structure (pack-
ing of crystallites inside the starch granule), the ratio of amylase and amylopectin,
the amylopectin fine structure, and the surface characteristics of starch granules
[
14
]. In vitro, native starches are hydrolysed very slowly, and to a limited extent, by
amylases [
15-
17
]. When starch is used in food processing, starch gelatinisation, i.e.
the process of disrupting starch crystalline structure with heat and moisture, usually
results in a decrease or loss of the slow digestion property of native cereal starches
[
18
]. Gelatinised starch will exist for example in bakery products in a partially or
completely amorphous state. Thus, the more gelatinised starch is, the more rapidly
it will be digested [
19
]. In many common starchy foods, such as in regular white
wheat bread, the starch is highly gelatinised and product structure very porous,
resulting in rapid degradation of starch in the small intestine and a very rapid rise of
blood glucose level (high GI).
There are several mechanisms leading to slow digestion of gelatinised starch
[
20
]. The first group of important factors is related to the state of starch in the food
matrix. Starch retrogradation, which is the reassociation of amylose and amylopec-
tin to form double helices and possible crystalline structures, promotes slow
