THESIS

1.2.5 Degree of syneresis
The results obtained in this study suggested that the high degree
of pullulanase hydrolysis was closely related with high degree of syneresis of the RS
III production. It was obvious that by an enzyme hydrolysis with a pullulanase
enzyme improved the degree of syneresis as shown in Figure 17 and Appendix Table
E2. Degree of syneresis was improved from 30.02 to 51.45, 28.50 to 45.27 and 33.22
to 55.87 % for 0 to 48-hr debranched rice starches preheated at 75, 95 and 121°C for
30 min, respectively. This could be due to the pullulanase enzyme hydrolyze α-1,6-
glucosidic bonds, releasing a linear polymers linked by α-1,4-glucosidic bonds. These
fragments are linear polymers about 10 to 65 anhydroglucose unit, which were
formed by progressive re-association of starch molecules aged during short-term
incubation.
Additional, the freeze-thaw cycle was used to promote
syneresis of retrograde starch. Yuan and Thompson, (1998) revealed that the rate and
extent of retrogradation of starch paste during the freeze-thaw process was increased
by temperature reduction (greater nucleation rates). As the starch paste is cooled, the
starch chains become less energetic and the hydrogen bones become stronger, giving
a firmer gel. As a gel ages or if it is frozen and thawed, the starch chains have a
tendency to interact strongly with each other and thereby force water out of the
system (Figure 18).
The squeezing of water out of the gel was called “syneresis”.
Longer storage gives rise to more interaction between the starch chain and eventually
to formation of crystals. This process, called “retrogradation” is the crystallization of
starch chains in the gel. Because the crystalline areas differ from the non crystalline
areas in their refractive index, the gel becomes more rigid or rubbery, perhaps
partially as a result of crystallization and partially just from the interaction of the
starch chains (Hoseney,1998).
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