Results (
Thai) 1:
[Copy]Copied!
On the basis of X-ray diffraction spectrum, three crystallinitytypes of starch are distinguished: A, B, and C.Starch polymorphism results from a different length of lateralamylopectin chains and from the degree of order ofdouble helices. In A-type starch, double helices of chains,usually 10–12 glucose residues in length, crystallizing in ahexagonal system, are densely packed, with a small share ofcrystallisation water (4 water particles per 12 glucoseresidues). The B-type crystals with a pseudo-hexagonal systemare formed by rather loosely arranged double helices ofchains, 13–18 glucose residues in length, with the share ofa considerable number of water particles (36 per 12 glucoseresidues), grouped mainly in the centre of the crystal “cell”.The C form is considered a mixture of A and B forms[Gernat et al., 1990]. Type A crystallinity appears in starchof multiple cereals (wheat, maize, oat, rice) and of someroot plants (tapioca, sweet potato, taro). Type B is typical ofroot and tuber-bearing plants (potato, jam) and some cereals(high-amylose: barley, maize, rice). Type C crystallinityhas been observed, among other, in a number of leguminousplants. In starch of different maize species, containingfrom 0% to 84% of amylose, an inverse correlation has beenobserved between amylose content and a degree of crystallinity.Low-amylose starches form crystalline structuresof chains with an average polymerization degree of 20 glucoseresidues, with short chains (10–13) predominating, and
are characterised by a high degree of type A crystallinity.
On the contrary, high-amylose starches with a low degree of
crystallinity form type B crystals made of long chains with
35 glucose residues on average. Along with increasing
starch hydratation (10–30%), its crystallinity is also observed
to increase [Cheetham & Tao, 1998].
In plant tissues starch occurs in the form of structures
composed of a high number of particles. Those structures,
called granules, demonstrate a less or more regular, plain or
complex, variety-specific shape. Their size (average) fluctuates,
depending i.a. on the botanical origin, from 0.5 µm for
amaranth to over 100 µm for canna. The regularity of starch
chain ordering in a granule is reflected by its properties,
namely the above-mentioned X-ray spectrum and the phenomenon
of anisotrophy. The latter consists in the appearance
of luminous granule sections in the polarised light in
the microscopic image, taking the shape of the Maltese
Cross.
In the light passing under the microscope, spherical lamination
– the so-called “growth layer” – can be observed on
starch granules. It results from different refraction of light
in alternating crystalline and amorphous layers. The granule
surface is characterised by the occurrence of numerous
irregularities and pores of a different diameter and inside-
-granule depth [Juszczak et al., 2003a, 2003 b]. The granule
surface features are determined by the botanical origin of
starch and, along with an increasing size of granules, they
affect the specific surface area of starch. The specific surface
area is diversified depending on the type of starch and
ranges from e.g. 0.243 m2
/g in the case of potato starch granules
with type B crystallinity to 0.687 m2
/g in the case of type
A maize starch granules [Fortuna et al., 2000]. The specific
surface area of starch granules and pore volume are correlated
with gelatinisation temperature and the viscosity of
pastes obtained [Fortuna et al., 2000]. The specific surface
area of starch granules, as well as the number and size of
pores, are also linked with the ability of starch to adsorb different
substances, including protein compounds and
enzymes, and with its susceptibility to the effects of multiple
external factors
Being translated, please wait..
