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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.

Dielectric properties are validated to be correlated with the changes of food quality during 4°C storage, thus it could be used as predictive indicators for the quality parameters of salmon (Salmo salar) during cold storage. An open-ended coaxial detection method was used to determine the dielectric properties (dielectric constant ε' and dielectric loss ε") of salmon at the frequency range of 20-2500 MHz, and to explore the relationship between dielectric properties and volatile base nitrogen (TVB-N), thiobarbituric acid reactants (TBARS), total viable counts (TVC), pH and K-value in salmon fillets during storage for 7 d. Results showed that the TVB-N, TBARS, TVC and K-value of salmon increased linearly with the increase of storage time, and the pH decreased firstly and increased subsequently. Also, the dielectric properties (ε' and ε") decreased with the increase of frequency, and increased with the increase of storage time. During 7 d storage, ε" increased the most at the frequency of 27.12 and 40.68 MHz, while at the frequency of 915 MHz and 2450 MHz, the increase of ε" was smaller. Results analyzed by Pearson correlation analysis showed that significant correlation (p<0.01) was found between ε" and TVB-N and TBARS at the frequency of 27.12, 40.68, 100.00, 300 and 915 MHz. The results of the partial least square (PLS) model showed that all the determination coefficients (R 2 pred ) were over 0.900. Especially for TBARS, the R 2 pred of PLS-ε' and PLS-ε" were 0.913 and 0.920, respectively, and the root mean square error of prediction (RMSEP) were 0.041 and 0.039, respectively. However, the highest R 2 pred for PLS-ε' and PLS-ε" both occurred in TVB-N. Also, the predicted values against measured values of TVB-N, TBARS and TVC of salmon presented a good linear relationship. The linear coefficient R 2 of TVB-N, TBARS and TVC for PLS-ε' were 0.937, 0.910 and 0.917 respectively, and 0.942, 0.917 and 0.933 respectively for PLS-ε". The results demonstrated the dielectric properties combined with PLS analysis can be used as a rapid and non-destructive method to predict quality parameters of salmon during 4°C storage and could be further applied to other aquatic products.

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To investigate the influences of modes and operating parameters on the radio frequency (RF) tempering uniformity of a pack of frozen pacific sauries, a 27.12 MHz RF equipment was employed for sauries tempering with different input power and electrode gaps under batch and continuous mode. Nine frozen sauries were placed in the RF chamber and tempered under different input power (200, 400, 800, 1,600 W), electrode gaps (7, 8, 9 cm) under batch and continuous tempering mode. With a target end‐point tempering temperature of −4°C. Results showed 200 W/9 cm resulted in the longest tempering time (16.44, 15.43 min) and 1,600 W/7 cm was the shortest (2.95, 3.54 min) under batch mode tempering, respectively. The processing conditions of 200 W/9 cm and 200 W/7 cm under batch mode, and 200 W/8 cm, 200 W/9 cm, 400 W/9 cm, 800 W/8 cm under continuous mode all produced a final sample surface temperature distribution within the range of −10°C to 20°C, which indicated a reasonable tempering process. The area of the high temperature zone (10°C < T ≤ 20°C) of tempered sauries under 200 W/9 cm decreased from 0.28% to 0.23% when switching from batch to continuous mode. This implies that continuous RF tempering could reduce overheating and improve tempering uniformity of sauries.Practical applicationsTempering rate, temperature distribution, and energy consumption are the most important concerns to researchers and the industry since tempering temperature and time are highly correlated to fish product quality. Results in this study showed both batch and continuous mode RF tempering could achieve reasonable tempering effect to a pack of frozen sauries. The application of continuous tempering system is more desirable than the batch ones, which can be applied to the tempering industry with higher throughput and connect to the following continuous processing unit operations. This study will broaden the knowledge of food engineers and provide an alternative technology of aquatic product tempering, and the parameters used in this study can assist technologists for the development of commercial equipment.