Process Optimization and Effectiveness Verification of Liquid Amino Acid-Containing Fertilizer Prepared from Hydrolysis of Carp Scales

Fish by-products are a kind of agricultural waste rich in protein. As the current technology for their utilization is not mature, they are usually buried or discarded, resulting in resource waste and environmental pollution. Therefore, the in-depth development and utilization of fish by-products are of great significance for the comprehensive utilization of aquatic products and environmental protection. Fish scale is one of the main fish by-products, and it is usually utilized by acid hydrolysis to prepare liquid amino acid (AA) fertilizer, but there is a lack of acid hydrolysis process with high efficiency and low energy consumption. Here, we report on a method for preparing liquid AA fertilizer (LAAF) by microwave-acid-heat hydrolysis of fish scales. We first screened the optimal conditions of acid-heat hydrolysis and microwave-acid hydrolysis of carp scales, respectively, and then optimized the conditions for microwave-acid-heat hydrolysis by response surface methodology. Under the optimal conditions, the concentration and conversion of AA nitrogen in microwave-acid-heat hydrolysate were 19.72 g/L and 92.10%, respectively. After neutralization of the hydrolysate and supplementation with trace elements, the self-made liquid AA fertilizer (SMLAAF) showed a higher total AA content and superior composition compared with the commercial liquid AA fertilizer (CLAAF). In addition, the effect of SMLAAF on the yield and nutritional quality of lettuce was better than that of CLAAF, and SMLAAF has the potential to reduce the applied amount of urea. This study provides a reference for the resource utilization of fish by-products.