Effects of protein gel structure on oil uptake in deep-fat frying process

Abstract

Fish protein (surimi) gel was processed in two heating steps: setting at low temperature (35-80 °C) and then cooking at high temperature (90 °C). Different final gel strengths were obtained in treatments with different temperature/time combinations during the setting step. But in all cases, it was seen that the gel strength passed through a maximum at a time depending differently on the setting temperature. A competitive-consecutive first order model was outlined. The DSC thermograms showed that both endothermic and exothermic processes occurred when the temperature changed. Thus, the effect of the different temperature-time histories experienced by surimi paste during the setting step on the final protein conformation resulted in different gel strengths and networks. The gel strength change could be correlated with the network structure as measured by the volume fraction of solid. Higher gel strengths were obtained from higher solid fraction gels. In frying process at 180 °C for 3 minutes, the oil absorption mostly occurred during cooling period rather than frying period. More than 80% of total structural oil uptake occurred by the first minutes of cooling time. A low permeability crust on the surface of surimi gel was developed. It restricted the rate of water loss leading to high steam pressure developed inside the gel, thus resulting in the internal gel structure breakdown. Consequently, the voids occurred and were filled with steam. The heat pipe-like-heat transfer was evidenced. Subsequently, a mathematical heat conduction model of infinite cylinder system with stepping change of thermal conductivity was developed and was able to predict temperature distribution inside the gel during frying at 180 °C for 3 minutes in good agreement with the experimental data.