(B-granules) are spherical with a d

(B-granules) are spherical with a diameter below (roughly) 10 µm, and the
large granules (A-granules) are lenticular with a diameter around 20 µm.
The particle size distribution of a commercial starch might not reflect the
true size distribution in the botanical tissue; for example, depending on the
isolation procedure, a smaller or larger proportion of the B-granules may be
lost in wheat starch [12,15]. Moreover, separating the various size classes by
gravity seldom gives 100% A-granules and 100% B-granules [15a].
For most starches, the granule is formed inside the amyloplast, and one
starch granule is inside each amyloplast [16]. In some starches (e.g., oat),
many tiny granules (4 to 10 µm in diameter) form aggregates of a much larger
size (diameters of 20 to 150 µm) [17]. Also, the small granules in wheat, the
B-granules, are reported to have a tendency to agglomerate when separated
from the wheat kernel [17a].
Because of the characteristic morphological properties of the granules it
is possible to identify most starches from their appearance under a light
microscope [18]. The light microscope reveals other features of the starch
particle in addition to shape and size. The cereal starches wheat, rye, and
barley show an equatorial groove along the large granules [19]. Surface indentations are found in some starches, assumed to be the result of close packing
in the cell [20]. Enzyme attacks might be observed as pits in the surface
[18,21]. Surface pores or fissures have also been observed without excess
TABLE 10.1
Starch Granule Dimensions
Starch Diameter (µm) Ref.
Dent corn 10.3–11.5 8
Potato 37.9 9
Potato 50 10
Rice 5–6 10
Taro 2–5 10
Rice 6.8 11
Cassava 16.8 11
English wheat 4.5, 15.3 12
Canadian wheat 4.0, 14.5 12
Wheat 6.1–6.3, 18.2–19.3 9
Durum wheat 5.7–6.2, 18.3–25.0 9
Barley 3.1–3.7 13
15.0–19.1 13
Normal barley 5, 10–25 14
Waxy barley 5, 25 14
Tapioca 17.7 9
© 2006 by Taylor & Francis Group, LLC
396 Carbohydrates in Food
enzyme activity in rye starch [22], wheat, barley, maize, and sorghum starches,
but not in rice, oats, potato, tapioca, arrowroot, or canna [23,24]. It has been
suggested that these pores, with diameters of 0.1 to 0.3 µm, are openings to
channels that penetrate the interior of the granule, perhaps even into the hilum
[24]. Depressions on the surface of cereal starches have also been revealed by
noncontact atomic force microscopy (AFM) [24a]. The size of the depressions
depends on the botanical source of the starch. Differences in starch granule
surface features between potato and wheat starches were observed by AFM
[24b], and the wheat starch granule surface was smoother with fewer protrusions than the potato starch surface.
10.2.1.2 Composition
The polysaccharides amylose and amylopectin are the most abundant components and will constitute almost 100% of a typical starch. The ratios between
amylose and amylopectin differ between starches, but a typical value for a
some starches (e.g., maize, barley, rice), genotypes exist with either an increased
amylopectin content (waxy varieties) or an increased amylose content (high
amylose or amylostarches). Waxy varieties of wheat starch have also been
produced [29a]. With regard to potato starch, high-amylopectin starch has been
produced through genetic modification [29b,30,31].
The components present in addition to the starch molecules are usually
described as “minor components” because they are present in low amounts.
Although they are present at very low levels, they have a dramatic effect on
physicochemical properties. A protein content below 0.5% is typical, and the
lipid content, typical of cereal starches, is usually around 1% (Table 10.2).
Phosphate groups are typical of potato starch, and the phosphate content is of
the magnitude of 1 phosphate ester per 200 glucose units [32]. The values
shown in Table 10.2 are obtained from chemical analysis of extracted starch,
and some of the values (especially lipid and protein content) depend on how
efficient the washing procedure has been.
As discussed later in this chapter, amylose forms a helical inclusion
complex with polar lipids. It has been discussed whether such a complex
exists in the native starch granule or whether it is formed during gelatinization.
X-ray diffraction analysis has not been able to give an unambiguous answer
because if the crystalline domains are too small they will not show up in
analysis. Evidence has been obtained that the complexes exist before gelatinization, at least in some starches. The V-pattern can be found in high-amylose
starches, in starches containing genes such as the amylose extender gene, and
in dull or sugary starches [33]. When
13
C-cross-polarization/magic-angle spinning nuclear magnetic resonance (
13
C-CP/MAS NMR) was used to study the
© 2006 by Taylor & Francis Group, LLC
“normal” starch is 25% amylose and 75% amylopectin (see Table 10.2). In
Typical values of the composition of starch granules are shown in Table 10.2.