THESIS
THESIS
THESIS
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5. Mechanisms of RS III formation during cooling, freezing and drying<br />
5.1 The mechanisms of RS III formation during cooling<br />
The rate of cooling of the gelatinized starch to the nucleating temperature<br />
should be as fast as possible and may be at least about 1°C per min on average,<br />
preferably at least about 3°C per min on average, most preferably at least about 4°C<br />
per min on average. By cooling to the nucleation temperature rapidly, the propagation<br />
of undesirable crystal forms, such as amylose-lipid complexes, is substantially<br />
reduced or eliminated. Also, rapid cooling generally promotes the generation of large<br />
numbers of small seed crystals, rather than fewer, larger crystals. After nucleation, the<br />
nucleated crystals of enzyme-resistant starch type III may be propagated or grown by<br />
raising the temperature of the gelatinized starch from the nucleation temperature to a<br />
crystal-propagating temperature (Eliasson and Kim, 1992).<br />
Propagation of the resistant starch type III crystals may be achieved at a<br />
temperature above the melting point of any of amylose-lipid complexes which may<br />
have been formed during nucleation. Thus, use of a crystal-propagating temperature<br />
above the melting point of the amylose-lipid complexes remelts the complexes,<br />
thereby making more amylose available to formation of resistant starch type III.<br />
However, the crystal-propagation temperature is maintained below the melting point<br />
of the desired crystals of enzyme-resistant starch type III to avoid melting or<br />
destroying them. The temperature is preferably raised from the nucleating temperature<br />
to the crystal-propagating temperature at a rapid rate to avoid any substantial<br />
propagation of undesirable crystals, such as amylose-lipid complexes (Yuan and<br />
Thompson, 1998).<br />
5.2 The mechanisms of RS III formation during freeze-thawed process<br />
Freezing is a physical treatment widely applied for preservation, drying<br />
and lyophilisation of starchy food (Ahmed and Lelievre, 1978). It is also used for<br />
sample preparation in granule structural investigations by means of many physical<br />
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