Thermal behavior, microstructure and protein quality of squid fillets dried by far-infrared assisted heat pump drying

The effects of far-infrared assisted heat pump drying on the thermal behavior, microstructure and protein quality of squid fillets were studied. Heat pump drying (HPD) alone or in combination with far-infrared radiation (FIR) at the power of 100, 500 and 800 W, 40 °C and 2 m s−1 air velocity were compared. Nitrogen fractions, proteases activities, amino acid profile, microstructure, SDS-PAGE and protein quality of fresh and dried squid samples were examined. Results showed that glutamic acid, aspartic acid, lysine and isoleucine are the dominant amino acids in all samples. The total contents of essential amino acid and non-essential amino acid, essential amino acid index, estimated protein efficiency ratio and biological value were not significantly different among all squid samples (P > 0.05). The cathepsins B and L activity decreased along with increased power supplied to the FIR heater. Drying increased the denaturation enthalpy (ΔH) of myosin and actin as well as the thermal transition temperature (Tmax) of actin compared with the fresh, non-dried squid. HPD-treated samples had the highest Tmax of myosin and HPD + 1FIR-treated samples had the lowest Tmax of actin. Electrophoretic profiles showed the disappearance of 24, 57, 93, 105, 121 and 172 kDa bands, while the new concomitant bands were appeared at 30, 37, 102 and 154 kDa. Dried squid muscle had dense and firm microstructure, and high FIR intensity resulted in more compact and coherent structure of dried squid. Generally, HPD in combination with FIR did not induce significant loss of amino acid quality in the dried squid fillets.