THESIS

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