Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 36

2 Recent Processing Methods for
Preparing Starch-based Bioproducts

George F. Fanta, Frederick C. Felker
and Randal L. Shogren

2.1 INTRODUCTION

The physical and chemical properties of starch and the commercial applications of starch
and starch derivatives have been extensively reviewed (Jarowenko, 1970; Whistler and
Daniel, 1997). Starch is a polysaccharide composed of repeating 1,4- -D-glucopyranosyl

units, often referred to as anhydroglucose units or simply AGU (Figure 2.1); it is isolated
from the seeds and tubers of living plants as discrete granules with diameters ranging from
about 1 to 100 μm, depending on the plant source. Corn starch is the most commonly used
starch of commerce and granules of corn starch have diameters in the order of 10-20 μm.
Starch granules typically contain both linear polysaccharides (amylose) and highly-branched
polysaccharides with 1,6-

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-D linkages at the branch points (amylopectin). The
amylose:amylopectin ratio of normal dent corn starch is about 25:75; however, high amylose
starch varieties with apparent amylose contents of about 70% are also commercially
available, as well as the waxy variety that contains essentially 100% amylopectin. The
molecular weight of amylose is in the order of several hundred thousand, whereas
amylopectin is over an order of magnitude higher in molecular weight.
Although starch granules are hydrophilic and absorb water readily, they are insoluble in
water at room temperature. Granule swelling is minimal due to hydrogen bonding and
crystallinity within the granule. Native starch granules are birefringent and show a Maltese
cross pattern when viewed microscopically with polarized light. Starch granules are partially
crystalline; the areas of crystallinity consist of double-helical clusters of amylopectin side
chains. Starch granules exhibit different X-ray diffraction patterns, depending upon the starch
variety. Cereal starches such as corn, wheat and rice exhibit the A-type pattern, while the
B-type is characteristic of tuber starches, such as potato. B-type diffraction patterns are also
observed for high amylose starch varieties. Some starches, such as tapioca, exhibit the C-type
pattern, which is a combination of the A- and B-type patterns. V-type patterns are observed for
helical inclusion complexes formed from amylose and lipophilic materials, such as the native
lipids normally present in concentrations of less than 1% in cereal starch granules.
Gelatinization takes place when starch granules are heated in water; during this process
the granules absorb water, swell irreversibly, and lose their crystallinity and birefringence.
Techniques such as differential scanning calorimetry (DSC), X-ray diffraction and light
microscopy have been used to study this process. Gelatinization typically occurs between

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