Thermophysical properties of silver hake and mackerel surimi at cooking temperatures

Thermal diffusivity values obtained using a transient method both from heating and cooling information and apparent specific heat of surimi made from silver hake and mackerel as a function of temperature (60–110 °C) and added potato starch content (3% and 7%, w/w) were determined. The water content of all samples was kept constant at about 77% (w/w) in order to determine the effect of starch. Thermal diffusivity and apparent specific heat of all samples increased with temperature. The effect of temperature on diffusivity was found significant (p=0.001) in all samples at temperatures above 85 °C. This effect may be caused by structural changes as temperature is raised. Thermal diffusivity increased with temperature more rapidly in samples with high levels of starch than in those with no starch. However, thermal diffusivity of silver hake surimi preset at 40 °C for 30 min did not show a significant increase (p<0.05) over the temperature range studied. Thermal diffusivities calculated from cooling and heating data were significantly different (p<0.05).

The differential scanning calorimetry (DSC) thermograms of silver hake and mackerel surimi showed three endothermic peaks. When starch was added to surimi samples an endothermic peak having a large area was observed, which overlapped the third highest temperature peak of surimi samples. No significant shifts in the endothermic peaks of myofibrillar proteins were detected with increasing starch content. Transition temperatures for starch–surimi system were higher than those for starch–water system. There were deviations in the apparent heat capacity function calculated from DSC measurements in surimi samples containing starch. These are attributed to gelatinization of starch and modification of water structure.