THESIS
THESIS
THESIS
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methods, for instance in scanning electron microscopy (SEM) or transmission<br />
electron microscopy (TEM) (Hayagava et al. 1998). It was also considered to cause<br />
some changes in the nutritional properties of starch (Guraya et al. 2001b).<br />
Freezing of starch solutions resulted in their cooperation and increasing<br />
retrogradation, while pregelatinised starch became less sensitive to retrogradation and<br />
stayed smooth after the process (Waigh et al. 1998) well as Donald et al. (2001)<br />
reported that some reversible structural disorder of starch granules occurred at subzero<br />
temperatures. Repeated freeze–thawing procedures slightly influenced the water<br />
solubility and the water holding capacity, but did not change the branching<br />
characteristics of starch granules (Szymonska et al. 2003). Retrogradation is<br />
enhanced when starch gels are subjected to freezing and thawing. Freezing of starch<br />
gel causes the phase separation upon ice crystal formation. Upon thawing, water can<br />
be expressed from the gel known as syneresis.<br />
Resistant starch type III is cooked cooled and retrogradation of starch,<br />
some of which is resistant to the enzymatic digestion in the human digestive system.<br />
In the previous studied retrogradation of starch gels (starch pastes) is often enhanced<br />
when they are subjected to freezing and thawing treatments. During freezing, the gel<br />
can separate into fractions by the formation of ice crystals, such that the starch is<br />
concentrated in a non-ice phase. When thawed, the ice crystals melt to form a mixture<br />
of water and gel. The freeze-thaw stability of starch gels has been evaluated by<br />
measuring the quantity of liquid that can be separated from a thawed gel after<br />
centrifugation (Hood and Scifried, 1974).<br />
Varavinit et al. (2002) investigated the freeze-thaw stabilities of three<br />
different rice flour gels (amylose rice flour with 28% amylose, Jasmine rice flour with<br />
18% amylose and waxy rice flour with 5% amylose) by first freezing at -18°C for 22<br />
h and subsequent thawing in a water bath at 30°C, 60°C and 90°C. The freeze-thaw<br />
stability was determined for five cycles. Starch gels thawed at higher temperature<br />
exhibited a lower syneresis value (percent of water separation) than those thawed at<br />
lower temperature. Amylose rice flour gels gave the highest syneresis value<br />
(especially in the first cycle). The jasmine rice flour gels gave a highest syneresis<br />
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