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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. [ABSTRACT FROM AUTHOR]

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The conditioning and extrusion are important heat treatment process that affects the feed quality in the production of fish feed. In the mechanism research and process parameter optimization of the above process, as one of the basic thermal characteristics, specific heat is an essential parameter. The existing research about animal feed processing, mainly explores the variation of specific heat with processing parameters such as temperature and moisture content. But there is fewer research on specific heat prediction models based on nutritional composition. In order to improve the adaptability and accuracy of the model, this research comprehensively explores the influence of processing parameters (temperature, moisture content) and nutritional composition (protein content, fat content) on the specific heat of fish feed. The objectives of this investigation were to explore the thermal property of fish feed during processing, to regulate the heat supplement during conditioning and extruding, and to explore the adaptability of the existing specific heat prediction model that applied for livestock and poultry feed to fish feed that had high fat and protein content. The four main ingredients of fish feed formula were chosen, including fishmeal, soybean meal, wheat flour and soybean oil. Soybean meal and wheat were grinded using a universal grinder equipped with the 1 mm sieve, and all raw materials were passed through a 40 mesh screen. Fish meal and soybean meal were used as protein sources, and soybean oil was used as fat source. There were 9 diets with 3 crude protein levels (30%, 40%, 50%) and 3 crude fat levels (5%, 11%, 17%) to represent fish feeds with different feeding habits. DSC (Differential Scanning Calorimetry) was used to measure the specific heat of 9 diets in the temperature range of 20-120 ℃ and the moisture content range of 20%-26%, the influence rule of the above 4 factors on the specific heat was analyzed, and specific heat prediction model for 4 factors was constructed. The results showed that the specific heat of fish feed was significantly affected by moisture content, temperature, crude protein content, and crude fat content. The main order of each factor was, moisture content, temperature, crude protein content, crude fat content. The specific heat increased linearly with the increase of the water content and the crude protein content, but decreased linearly with the increase of the crude fat content. The specific heat had a non-linear relationship with temperature, which was a quadratic regression relationship. In all experiments, the specific heat of fish feed varied from 1.70 to 2.70 kJ/(kg·℃). Based on experimental data, a multiple regression model (R² =0.991) of specific heat that in relation to processing parameters (temperature and moisture content) and nutritional composition (crude protein content and crude fat content) was established. It can improve the adaptability of specific heat for livestock and poultry feed in the existing research. The prediction model can effectively predict the specific heat of fish feed within the range of this study variable. The research results can provide basic data for the establishment of heat transfer mathematical models of fish feed during conditioning and extruding, and also provide theoretical guidance for the adjustment of heat supplement parameters in the productive process. [ABSTRACT FROM AUTHOR]

Copyright of Transactions of the Chinese Society of Agricultural Engineering is the property of Chinese Society of Agricultural Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)