Thermal Food Processing

Thermal Processing of Fishery Products 243
herring and mackerel by Rehbein. 34 According to this author, herring heated at 55
or 60°C could be differentiated by IEF from herring that was smoked at 65 to
70°C. Different protein patterns were also obtained by IEF for mackerel smoked
at 55, 60, 65, or 70°C. From these results, it can be inferred that IEF or coagulation
test can be used for determining temperatures of fish heating of up to 65°C.
The coagulation method was used for determining the endpoint temperature of
heated blue marlin meat in the range of 1 to 67°C. 35 From the analysis of proteins
by sodium dodecyl sulfate–polyacrilamide gel electrophoresis (SDS-PAGE) and by
studying the enzymatic activity, lactate dehydrogenase was found to be responsible
for coagulation at 67°C. The assessing of the endpoint temperature of heated fish
and shellfish using the coagulation method was also reported by Uddin et al 36 The
coagulation test could determine the endpoint temperature of shellfish meats
between 60 and 67°C, and the results were confirmed by SDS-PAGE and enzyme
activity determination. A thermostable protein with a molecular weight of 35 kDa
was detected in heated shellfish meats up to 108°C. This protein was found to be
a tropomyosin subunit in the scallop adductor muscle. According to the results
obtained from these authors, tropomyosin could be used as an indicator of the
endpoint temperature. However, the applicability of the enzyme activities can have
great difficulties because they depend on the physiological condition of the fish.
Differential scanning calorimetry (DSC) can also be used to investigate the
thermal stability of proteins and to estimate the cooking temperature of the seafood
products. The thermal stability of fish myosin of different species in the range of
20 to 80°C by DSC was reported by Ogawa et al. 37 The denaturation process was
different depending on the species. The DSC curves of myosin of sardine, stone
flounder, sea bram, and carp had two peaks; trout, bigeye tuna, and yellow tail
showed three peaks; and horse mackerel only one peak.
The thermal denaturation of hake myofibrillar proteins using DSC was studied
by Beas et al. 38 Two endothermic transitions with T max values at 46.5 and 75.3°C
and a shoulder at 51°C were obtained by these authors. Martens and Vold 39
reported similar transitions for cod muscle. The DSC of the exudative sarcoplasmic
fraction of the whole muscle showed three peaks at 45.2, 59, and 75.5°C
and contributed to denaturation peaks.
The protein denaturation during fish precooking implies texture and binding
changes, which allow for a better differentiation between red and white muscle
and an easier separation of muscle from bone. The most labile proteins are the
sarcoplasmic proteins and myosin, and their denaturation is sufficient to ensure
textural changes needed. The effects of the thermal protein denaturation and
moisture loss in skipjack tuna during steam cooking were reported by Bell et
al. 40 The DSC thermogram showed that the first peak at 52°C corresponds to
myosin denaturation, the second peak at 59°C to collagen, and the third peak at
68°C to actin. During the precooking of skipjack, the structural muscle proteins
decrease in dimension upon reaching their thermal denaturation temperatures and
shrinkage of the muscle fibrils and tissue occurs.
Sulfhydryl groups and disulfide bonds are important in maintaining structure
and functions of native proteins. At temperatures above 90°C the oxidation of