Chemistry of Cereal Grains - Sourdough Biotechnology - page 16

On cooling granule remnants that are enriched in amorphous amylopectin
become incorporated into a continuous amylose matrix. Amylose molecules that are
dissolved during gelatinization reassociate to local double helices interconnected by
hydrated parts of the molecules, and a continuous network (gel) forms [ 27 ] . As
amylose retrogradation proceeds, double helix formation increases and, finally, very
stable crystalline structures are formed, which cannot be melted again by heating.
Amylopectin retrogradation takes several hours or days and occurs in the granule
remnants embedded in the initial amylose gel [ 27 ] . Crystallization mainly occurs
within the short-chain outer A- and B1-chains of the molecules. The amylopectin
crystallites melt at ca. 60 °C and, therefore, aged bread can partly be “refreshed” by
heating. This so-called “staling endotherm” can be measured by DSC to evaluate
amylopectin retrogradation. Amylopectin retrogradation is strongly influenced by a
number of conditions and substances, including pH and the presence of low-molecular-
weight (LMW) compounds such as salts, sugars, and lipids [ 26 ] .

2.2.1.4 Interaction with Lipids

Amylose is able to form helical inclusion complexes in particular with polar lipids
and this can occur in native (starch lipids; see below) as well as in gelatinized starch [ 28 ] .
During gelatinization amylose forms a left-handed single helix and the nonpolar moi-
ety of the polar lipid is located in the central cavity [ 16 ] . The inclusion complexes
give rise to a V-type X-ray diffraction pattern. The presence of polar lipids strongly
affects the retrogradation characteristics of the starch, because amylose-lipid com-
plexes do not participate in the recrystallization process [ 26 ] . Complex formation is,
however, strongly affected by the structure of the polar lipid [ 29 ] . For example,
monoglycerides are more active than diglycerides and saturated fatty acids more
active than unsaturated ones, because inclusion complexes are preferably formed
with linear hydrocarbon chains and with compounds having one fatty acid residue.
In addition, lipids, in particular lysophospholipids (lysolecithin), are minor con-
stituents of cereal starches in amounts of 0.8-1.2% [ 30 ] . As so-called starch lipids
they are associated with amylose as well as with the outer branches of amylopectin
[ 28 ]. These lipid complexes lead to a delay of the onset of gelatinization and affect
the properties of the starch especially in baking applications.

2.2.2 Nonstarch Polysaccharides (NSP)

Polysaccharides other than starch are primarily constituents of the cell walls and are
much more abundant in the outer than in the inner layers of the grains. Therefore, a
higher extraction rate is associated with a higher content of NSP. From a nutritional
point of view NSP are dietary fiber, which has been associated with positive health
effects. For example, cereal dietary fiber has been related to a reduced risk of chronic