Drying kinetic model and hygrothermal characteristics of fish extruded feed during hot air drying

Abstract: Processing parameters during drying are highly critical to the quality and yield of extruded aquatic feed. Taking grass carp (adult fish) extruded feed as the research object, this study aims to explore the hygrothermal characteristics of fish feed in the process of hot air drying. An experimental box was also selected to simulate the temperature and moisture content in the extruder. The measured temperature of fish feed was 90.11±1.31 ℃, and the measured moisture content of dry basis was 24.02%±0.98%. Two experimental parameters were set, including the air temperature (60-100 ℃) and air velocity (0.5-1.5 m/s). The drying experiment was performed on a self-developed hot air dryer, where a load cell with the accuracy of 0.01g was selected to accurately record the moisture of fish feed during drying. An infrared thermal imager was used to observe the temperature distribution of fish feed. Low-field nuclear magnetic resonance analysis (LF-NMR) was also utilized to explore the change of water state in the fish feed. The samples were taken every 20 minutes for the LF-NMR during the test, where the main magnetic field strength was 0.5T, and the resonance frequency was 21 MHz. Among them, a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence was adopted to quantify the LF-NMR parameters. The results showed that there was a reduced rate drying stage in the drying process of feed in this experiment, rather than the typical constant rate drying stage. The drying rate of feed increased significantly, with the increase of air temperature (P<0.01) under the same air velocity. Similarly, the air velocity improved the drying rate at the same temperature as hot air. A Verma model was used to fit the obtained experimental data. The calculated values of R2, χ2, RMSE, and e showed that the model presented a high prediction accuracy for the experimental data of grass carp feed under different drying conditions. The Verma model parameters, k0, k1, and a, were achieved on the air temperature and air velocity, where R2 of the regression model was greater than 0.979. Transverse relaxation time (T2) spectra showed that the proportion of immobilized water in the feed was the largest in the initial stage of the drying process. Specifically, the peak area of T21 decreased gradually with the drying proceeded, indicating the proportion of immobilized water decreased significantly (P<0.01). Moreover, the whole transverse relaxation time tended to shift to the left, indicating that the combination of water and substrate was closer, and the free degree was lower. The total area of T20, T21, and T22 peaks was the smallest, while, the moisture content was the lowest at the end of drying. The infrared thermographs showed that the corner effect appeared at 5 min, particularly that some feed in the center presented the overheating at 15 and 20 min. The finding can provide a promising reference to optimize the processing parameters of grass carp extruded feed during hot air drying, together with a new idea for the study on the hygrothermal characteristics of aquatic feed.