conditions [29,61,108,110]. Lipids

conditions [29,61,108,110]. Lipids have not been found in the leached material
[29]. Some examples of the leaking as a function of temperature are given in
Figure 10.5. The material solubilized during gelatinization can be characterized
by staining with iodine, either in solution or in preparation for microscopy. The
material leached from undamaged potato and cereal starches in the temperature
interval 50 to 70°C is mainly composed of amylose. The solubilized material
increases in molecular weight and becomes more branched with increasing
temperature [28,115,116]. In an oat starch suspension heated to 90°C, amylose
was found outside the granules, forming a network structure around them [59].
In barley starch, on the other hand, granules were found to still be stained
heavily blue at 90°C, whereas at 95°C not much blue staining was found inside
the granules. Demixing of amylose and amylopectin was observed in the microscope for potato starch [61]. If the starch contains a proportion of enzymatically
or mechanically damaged starch, this will influence the nature of the solubilized
material. Extraction of damaged granules with cold water preferentially leaches
out amylopectin of low molecular weight [117].
It has been suggested that in maize and wheat starches most of the amylose
will be solubilized before leaking of amylopectin begins [110]. In oat starch,
on the other hand, a concurrent leaking of amylose and amylopectin seems to
occur [111]. In case of oat starch, it was suggested that instead an intermediate
material (molecules less branched than amylopectin) is coleached with amylose [112].
FIGURE 10.5Leaking of amylose during heating of potato (
), wheat (), maize
(
), and oat () starches. (Adapted from Eliasson, A.-C., J. Text Stud., 17, 253, 1986;
Eliasson, A.-C., Starch/Stärke, 37, 411, 1985; Doublier, J.-L. et al., Cereal Chem., 64,
21, 1987.)
40
30
20
Solubility (%)
10
0
02040
Temperature (°C)
60 80 100
© 2006 by Taylor & Francis Group, LLC
Starch: Physicochemical and Functional Aspects 411
The solubility has been found to depend on the heating rate for wheat
starch, but not for maize starch [114]. A prolonged holding time at 96°C also
increases the solubility [111]. If the mechanical treatment is severe, this will
also change the nature of the solubilized material, and the solubility will
increase [111,118]. In normal starches (e.g., normal maize) an increased proportion of amylopectin is found in the solubilized material when the stirring
increases [84]. Increased stirring increases the leaking also from high-amylose
varieties [84].
Not all amylose leaches out during heating. After heating at 90°C, pea
starch was found to contain 16% amylose; wheat, 8.3%; and maize, 8.0%
[108]. The solubilized material corresponds to only 6 to 9% of the total starch
in high-amylose starches but to 60 to 76% in normal starch [84]. The parts of
the granules remaining after gelatinization (ghosts) contain mainly amylopectin without any crystalline order [116]. A decrease in amylose content from
27.4% in native barley starch to 6.2% in a ghost preparation was reported.
As will be described later, the leaking of amylose is necessary for gel
formation, but in many cases the leaking of amylose causes problems, as is
the case for pasta or potato flakes. To avoid free amylose that causes stickiness,
monoglycerides or other emulsifiers might be added to form a helical inclusion
complex with amylose.
10.3.3 MEANS OFINFLUENCINGSTARCHGELATINIZATION
There are many means of affecting the gelatinization temperature, both on
purpose and unintentionally. This can be achieved through the addition of
other food components to the starch, by using genotypes differing in amylose/amylopectin ratios, by chemical modification, and by annealing or other
Gelatinization is brought about as a consequence of certain combinations
of heat and water content; however, when inspecting Figure 10.2 it is evident
that water and heat might influence the behavior of starch without causing
gelatinization. Here, two different possibilities are investigated: heat–moisture
treatment and annealing. In fact, this is a rather artificial classification because
the underlying mechanism is more or less the same; however, it has become
customary in the literature to refer to treatments at low water contents and
temperatures above the gelatinization temperature range as heat–moisture
treatments and to treatments involving high water contents at temperatures
below the gelatinization temperature range as annealing. A third possibility is
the extrusion process, where high temperatures and low water contents are
combined with high shear forces due to mechanical treatment. The following
discussion is limited to systems without the application of shear forces.
© 2006 by Taylor & Francis Group, LLC
types of heat treatment. In this section, treatments corresponding to Figure
10.2are described, and other approaches are described in subsequent sections.