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AbstractThe results of this research were reported in two parts.
The first part was the study on postmortem physiochemical
changes of tilapia dorsal muscle in effecting meat qualities
during aging at different temperatures between 0℃and 40℃, just
below its muscle protein heat denaturation temperature. The
second part was the study on the dynamic changes of meat texture
in terms of muscle shortening and collagen-hydrolysis gapping
during immersion in water bath at various temperatures between
40℃ and 85℃ to monitoring the isothermal water-dipping cookery
of tilapia meat.In the first part of this research, the ultimate
RIbody and shortening (S%), the corresponding time required to
reach them, as well as the averaged RIbody and shortening
developing rates of postmortem Tilapia dorsal muscle being aged
at various temperatures were studied. Evidenced by the existing
of temperature dependent changes in RIbody and shortening
developing rates, cold shortening and heat promoting rigor did
occur for tilapia as expected, such that a minimum rigor rate
could be observed at an intermediate temperature around 20~25℃.
However, that the ultimate RIbody and shortening values were
found not the same for agings at different temperatures, and an
exceptional low valves were observed between 20℃and 25℃,
reflected not only that the apparent postmortem development
rigor might be interfered by the rapid occurrence of
tenderization at this temperature region, but also that these
two parameters should possess a limitation to be used as
universal rigor indices. Therefore, to establish a set of
reliable rigor indices based on the physiochemical changes was
desirable. It was found that to exclude the interference of
tenderization effect, the rigor-inducing actomyosin complex
formation could be determined quantitatively as an efficient
rigor index by measuring either the changes of muscle ATP
content or the changes of the heat inducing muscle shortening
temperature (Ts) of DSC thermograms. It was found that the ATP
contents and Ts values of muscle were fell from 4.0μmol/g-meat
and 49℃ of its prerigor state to 0.1~0.2μmol/g-meat and 30℃of
its full-rigor state, respectively. Whereas, the other possible
rigor-inducing factor, pH could only fell from 7.0~7.2 to its
ultimate value, 6.3~6.4, which was not low enough to induce the
pI effect. This excluded the pH to be one of the rigor mortis
inducing roles. In comparison of all above indices studied, it
was found that although the rate and the ultimate RIbody and S%
could not be used as reliable rigor indices, however, that the
time required to reach ultimate values of RIbody and S% were
well correlated to the time to allow muscle ATP contents and Ts
values to reach the level of full-rigor state, evidenced these
three parameters could be used as pure rigor mortis ( actomyosin
formation ) indices.The extent of tenderization of muscle could
be quantitatively evaluated by measuring the extractability of
salt soluble protein in using an insufficient homogenization
force to evaluate the extent of sacrolema damage or
tenderization during agings. The experimental result showed that
the rate of tenderization was increasing lineally with the
storage temperatures from 0℃ up to 40℃. By the comparison of
the tenderization development rate to the actomyosin complex
formation rate, it could be concluded that significant
tenderization developed faster than the actomyosin formation at
20~25℃, and the ultimate RIbody and S% were lower than those
other temperature. The profiles of muscle penetration force of
tilapia muscle being aged at different temperatures were
measured and used to reflect the meat texture and its
chewability when served in raw meat (Sashimi). An increasing
phase followed by a decreasing phase of the penetration force
was observed in all profiles. If was then explained as the
results of the competitions between rigor mortis ( actomyosin
complex formation ) and tenderization. In addition, the changes
of IMP contents and VBN/K-values were measured to denote the
sweat taste development and the criteria of freshness. All data
of these three indices combined could after and overall quality
of tilapia sashimi. It was then found that lower the aging
temperature, especially 0~5℃, could give a preferable quality
and preservation effects.In the second part of this research,
three representative raw meat samples of different qualities,
including a prerigor meat, a full-rigor meat with insignificant
tenderization (aging 24 hr. at 10℃), and a full-rigor meat with
significant tenderization (aging 24 hr. at 25℃), were used to
be studied on the changes of muscle shortening and the
occurrence of collagen gapping during dipping into an isothermal
water bath at a temperature between 0℃ to 85℃. It was found
that the heat-induced muscle shortening were occurred stepwisely
were highly correlative to the changes of DSC thermograms. A
brief water-dipping of prerigor below 50℃ could only induce a
rigor mortis shortening, whereas for water temperature above
50℃, rapid shortening caused by myosin and actin heat
denaturation were observed. All muscle samples treated with hot
water above 60℃, the muscle shortening of protein denaturations
followed by cooking gapping of collagen-hydrolysis were
observed. The raw meat with different extent of postmortem rigor
and tenderization development could give different cooking
responds. Their cooking thermoprofiles were reported and
discussed in great details.
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