Biomedical Engineering Reference
In-Depth Information
60 and 70 °C, depending on the starch variety. Loss of birefringence and the endothermic
changes in enthalpy (
H) observed with DSC are two methods typically used to characterize
the gelatinization process. Mechanisms proposed to explain the gelatinization of starch have
been summarized by Jenkens and Donald (1998).
Heating aqueous starch dispersions above the gelatinization temperature causes the
starch granules to further swell and disintegrate, and increased amounts of starch (particularly
the amylose component) dissolve as the temperature is increased. At temperatures of about
80-90 °C, smooth aqueous dispersions or pastes are obtained. Although amylose dissolves
in hot water, complete solubility of amylopectin is not usually achieved unless an autoclave
or a high-pressure steam jet cooker is used to obtain temperatures greater than 100 °C. Thick
pastes or gels are formed when hot, aqueous starch dispersions are cooled due to the
retrogradation of dissolved amylose. Retrogradation also occurs with amylopectin, although
the process is much slower due to its highly-branched structure. Some additives, such as
sodium sulfate, sucrose and glucose, inhibit the gelatinization of starch; whereas other
additives, such as alkali metal hydroxides and urea, lower the temperature at which gelatini-
zation and solubility take place.
Treatment of starch with acids, enzymes or oxidizing agents (such as sodium hypochlorite)
are commonly used to reduce the molecular weight of starch and thus decrease the viscosity
and modify the gelling properties of aqueous starch dispersions. Starches that have been
partially depolymerized by acids or enzymes are commonly referred to as dextrins.
Conversely, the swelling and solubility of starch granules can be reduced by cross-linking.
The hydroxyl groups on starch provide reactive sites for the formation of ester and ether
derivatives, many of which are produced commercially. The number of hydroxyl groups
derivatized is defined by the degree of substitution (DS). Since the glucopyranose repeating
unit has three hydroxyl groups available for reaction, a DS of three indicates that derivatization
of all available hydroxyl groups has taken place. Partial derivatization is indicated by DS
values lower than three. Starch has also been derivatized by graft polymerizing vinyl and
acrylic monomers onto the starch backbone, and reviews on this subject have been published
elsewhere (Fanta and Bagley, 1977 ; Fanta, 1999 ).
There are numerous applications for starch in food products due to its thickening and
gelling properties (Jarowenko, 1970; Whistler and Daniel, 1997). Waxy varieties of starch,
which contain no amylose, are used in applications where gel formation due to retrogradation
would be undesirable. Conversely, high amylose starches that retrograde readily have been
added to food products for their enzyme-resistant properties after retrogradation.
Pregelatinized starches are used in products where thickening is required but cooking must
be avoided, for example, in instant puddings and pie fillings.
Starches and starch derivatives are also used for non-food applications (Jarowenko, 1970;
Whistler and Daniel, 1997). For example, pregelatinized starches are used as components in
oil well drilling muds and in cold water-dispersible adhesives. Acid modified and oxidized
starches are used in the textile industry as sizes and fabric finishes to increase yarn strength
and abrasion resistance; they are also used as adhesives and in the manufacture of paper.
A number of commercially-important products, such as glucose, ethanol and cyclodextrins,
are derived from starch by enzymatic and fermentation processing. There are also numerous
applications for starches that have been derivatized through reaction of the hydroxyl groups.
For example, hydroxyethyl and hydroxypropyl starch ethers have been used as paper
coatings, sizes and adhesives. Cationic starch ethers, where the ether substituents contain
tertiary amine or quaternary ammonium groups, are used in paper manufacture, principally
for fiber and pigment retention. These starch derivatives have also been used as flocculating
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