Mean values for each characteristic followed by different letters within a column differ significantly (P = 0.05).
220
Ishfaq Ahmed et al
storage conditions. Amylose concentration
significantly influenced the RVA pasting properties
of fresh rice flour. On the other hand, stored paddy
showed significant difference in pasting properties
during the storage period. Peak viscosity (PV),
trough (T), final viscosity (FV) and setback (SB) of
KDML105 rice flour were lower than Kalasin11 rice
flour that were obtained from milled rice and stored
paddy, respectively. Furthermore, it was observed
that the changes in pasting properties of milled rice
were lower than stored paddy. It was noticed that
the paste viscosity decreased as the ageing process
proceed [44]. The structure of rice granules became
more organized during storage. Although, the
surface of granules characterizes the major barrier
to hydration, consequently, pasting may be affected
by the presence, nature and orientation of surface
proteins and lipids. Protein is basically composed
of amino acids and is hydrophilic in nature.
Any changes in the granules during storage and
processing could alter their hydrophilicity, which
would influence the hydration and swelling of
granules, thereby influencing the pasting properties
of rice flour [31, 42-43, 45]. Storage of rice at
cold temperature stopped alterations in the pasting
properties [31, 46].
Techawipharat et al. [47] observed that waxy
rice starch had much lower pasting properties than
normal rice starch. This is due to the lower amylose
concentration (< %) in waxy rice starch. Granular
interaction and amylose concentration significantly
affected the starch pasting properties. Similarly,
storage temperature and duration also influenced
the pasting properties of rice flour [31, 43, 45-46].
Inglett et al.[48] investigated the pasting properties
of blends of rice and wheat flours. It was observed
that the pasting properties of wheat flour increased
with the increase in the concentration of rice flour
in the blends. Nura et al. [13] evaluated the pasting
properties of rice flours with different particle sizes.
It was found that variation in particle size influenced
the pasting properties as shown in table 2. Pasting
temperature of rice flour samples was decreased
significantly with the reduction in particle size.
This could be attributed to the smaller particle
size, which provides greater surface area for rapid
hydration. The starch granules swell quickly leading
to gelatinization of starch at lower temperature. Rice
flour with smaller particle size attained the onset
gelatinization temperature earlier. Peak viscosity
(PV) shows the highest viscosity attained by starch
under given conditions during the gelatinization
process (Shuey and Tipples, 1994). Rise in PV
indicates the ability of starch granules to swell up to
the extent before physical breakdown occurs [49].
However, set back (SB) was comparatively higher in
all the flour samples evaluated. This might be due to
leaching of amylose in starch gel that caused quick
retrogradation and gave harder texture after cooling
[13]. The SB shows the retrogradation tendency
and is determined by the difference between PV
and HPV [5]. On the other hand, hot paste viscosity
(HPV) is affected by the formation of amylose-lipid
complex, granule swelling, amylose exudation,
and competition between remaining granules and
exuded amylose for free water [50].
Table 2. Pasting properties of rice flours with different particle sizes [13].
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