Effects of Radio Frequency Tempering on the Temperature Distribution and Physiochemical Properties of Salmon (Salmo salar)

Abstract

Salmon (Salmo salar) is a precious fish with high nutritional value, which is perishable when subjected to improper tempering processes before consumption. In traditional air and water tempering, the medium temperature of 10 °C is commonly used to guarantee a reasonable tempering time and product quality. Radio frequency tempering (RT) is a dielectric heating method, which has the advantage of uniform heating to ensure meat quality. The effects of radio frequency tempering (RT, 40.68 MHz, 400 W), water tempering (WT + 10 °C, 10 ± 0.5 °C), and air tempering (AT + 10 °C, 10 ± 1 °C) on the physiochemical properties of salmon fillets were investigated in this study. The quality of salmon fillets was evaluated in terms of drip loss, cooking loss, color, water migration and texture properties. Results showed that all tempering methods affected salmon fillet quality. The tempering times of WT + 10 °C and AT + 10 °C were 3.0 and 12.8 times longer than that of RT, respectively. AT + 10 °C produced the most uniform temperature distribution, followed by WT + 10 °C and RT. The amount of immobile water shifting to free water after WT + 10 °C was higher than that of RT and AT + 10 °C, which was in consistent with the drip and cooking loss. The spaces between the intercellular fibers increased significantly after WT + 10 °C compared to those of RT and AT + 10 °C. The results demonstrated that RT was an alternative novel salmon tempering method, which was fast and relatively uniform with a high quality retention rate. It could be applied to frozen salmon fillets after receiving from overseas catches, which need temperature elevation for further cutting or consumption.

Keywords: fish; quality; radio frequency; salmon (Salmo salar); tempering.

Figure 1

Layout of RF system and placement of salmon fillet sample and fiber optic sensor.

Figure 2

Dielectric constant (ε′) and dielectric loss factor (ε″) of salmon as a function of temperature at 27.12 MHz.

Figure 3

Temperature–time histories of salmon samples during different tempering treatments. (RT: Radio frequency tempering; WT + 10 °C: Water tempering at 10 ± 0.5 °C; AT + 10 °C: Air tempering at 10 ± 1 °C).

Figure 4

Surface temperature distribution of salmon fillets after different tempering treatments when the sample center reached −4 °C. (RT: Radio frequency tempering; WT + 10 °C: Water tempering at 10 ± 0.5 °C; AT + 10 °C: Air tempering at 10 ± 1 °C).

Figure 5

Effects of different tempering methods on TBARS of salmon. (RT: Radio frequency tempering; WT + 10 °C: Water tempering at 10 ± 0.5 °C; AT + 10 °C: Air tempering at 10 ± 1 °C). Different letters on the bar (a,b,c) indicate significant differences (p < 0.05) among samples treated with different methods.

Figure 6

Transverse relaxation time (T₂) curves of salmon samples after tempering, evaluated by LF-MNR. (RT: Radio frequency tempering; WT + 10 °C: Water tempering at 10 ± 0.5 °C; AT + 10 °C: Air tempering at 10 ± 1 °C).

Figure 7

The relaxation time (T₂) corresponding to relative peak areas (P₂₁, P₂₂ and P₂₃) of salmon fillets after different tempering treatments. (RT: Radio frequency tempering; WT + 10 °C: Water tempering at 10 ± 0.5 °C; AT + 10 °C: Air tempering at 10 ± 1 °C).

Figure 8

Microstructure of salmon fillet tissues after different tempering methods under light microscope (magnification ×400). (RT: Radio frequency tempering; WT + 10 °C: Water tempering at 10 ± 0.5 °C; AT + 10 °C: Air tempering at 10 ± 1 °C).