| The by-product of fish processing is a kind of agricultural waste rich in protein.For instance,carp scales have an extremely high protein content.Through acid or enzyme hydrolysis,the carp scale protein is broken down into amino acids,resulting in an amino acid containing water-soluble fertilizer that can improve soil quality and increase crop yield and quality.This approach not only mitigates the negative impact of fish processing byproducts on the environment but also supports sustainable agriculture.However,current research faces challenges such as slow rates and low degrees of protein hydrolysis.Furthermore,there is a lack of investigation into the hydrolysis of proteins from fish processing by-products,such as carp scales.This study aimed to screen the optimal conditions for acid and enzyme hydrolysis processes,in order to obtain an hydrolysate through a microwave-acid-thermal hydrolysis technology,that meets the Agricultural Industry Standard(NY 1429-2010),after neutralization and chelation.Verification of fertilizer effect revealed that a combination of urea and an appropriate proportion of amino acid-containing water-soluble fertilizer can enhance soil nutrient absorption,increase crop yield,and improve crop quality.The technical conditions established for amino acid-containing water-soluble fertilizer can be used as a reference for the resource utilization of other fish processing by-products,as well as for the market application of amino acid-containing water-soluble fertilizer.The results of this study are as follows.(1)The ideal conditions for acid-thermal hydrolysis of carp scale protein involved a sulfuric acid concentration of 4.5 mol/L,a liquid-solid ratio of 5.0 m L/g,an acid-thermal temperature of 100 ℃,and an acid-thermal time of 2.0 hours.These conditions resulted in a 53.5% conversion rate of amino acid-nitrogen and an 88.0% conversion rate of total nitrogen.For microwave-assisted acidolysis,the optimal conditions included a sulfuric acid concentration of 4.5 mol/L,a liquid-to-solid ratio of 5.0 m L/g,and a microwave power of480 W,with an acid-heat hydrolysis time of 5.0 minutes.Under these conditions,the concentration of amino acid-nitrogen in the hydrolyzed liquid was 13.74 g/L,with a 59.1%conversion rate of amino acid-nitrogen and a 92.8% conversion rate of total nitrogen.Combining acid-thermal hydrolysis and microwave-assisted acid hydrolysis,response surface optimized microwave-acid-thermal hydrolysis process conditions were microwave power of 480 W,microwave time of 5.4 min,acid thermal temperature of 96.7 °C,acidthermal time of 2.1 h,under these conditions,the concentration of amino acid-nitrogen in the hydrolysate was 19.72 g/L;the hydrolysate was neutralized and adjusted the p H value,and chelated with trace elements,the free amino acid content of the obtained hydrolysate was 26.0%,p H value was 3.18,the trace element content was 20.49 g/L,reaching the Agricultural Industry Standard(NY 1429-2010)for water-soluble fertilizers containing amino acids,thus the obtained fertilizer was called “self-made amino acid containing watersoluble fertilizer”.(2)The combination of flavor protease and neutral protease was the most suitable compound protease for enzymatic hydrolysis of carp scale protein.The optimal conditions for screened compound enzymatic hydrolysis were enzyme dosage of 4000 U/g,initial p H value of 6.5,enzymatic hydrolysis temperature of 45 ℃,enzymatic hydrolysis time of 6.0min.Under these conditions,the concentration of amino acid-nitrogen in the enzymatic hydrolysis solution was 3.15 g/L,and the conversion rate of total nitrogen was 74.4%.The optimal process conditions of the combined enzymolysis by response surface optimization were enzyme dosage of 4470 U/g,enzymolysis temperature of 44 ℃,the initial p H value of 6.6,and the hydrolysis time of 6.3 h.Under these conditions,the concentration of amino acid-nitrogen in the hydrolysis solution was 3.79 g/L.(3)Through the characterization and analysis of the hydrolysate and the residual solid phase of carp scales,it was found that the self-made amino acid-containing water-soluble fertilizer had a rich variety of amino acids.The hydrolysis degree of carp scale protein gradually increased in the order of compound enzymatic hydrolysis,acid-thermal hydrolysis,microwave-assisted acid hydrolysis,microwave-acid-thermal hydrolysis.Acid hydrolysis basically hydrolyzed all carp scale protein,and enzymatic hydrolysis only hydrolyzed part of carp scale protein structure,the content of free amino acid in acid hydrolysate was significantly higher than that in enzyme hydrolysate.(4)Taking “big and fast-growing” lettuce as the crop for fertilizer effect verification,the lettuce and lettuce planting soil’s indicators were analyzed,it was found that the additional application of water-soluble fertilizer containing amino acids could significantly promote the growth of lettuce compared with that of urea treatment alone.The lettuce yield,plant height,maximum leaf length,maximum leaf width and root length were all improved.It also promoted the absorption of lettuce to soil available nutrients,AN,AP,AK and OM,meanwhile effectively alleviated the increase of soil acidity and inhibited the increase of salinity,and increased the soil catalase,invertase,urease,acid phosphatase enzyme activity to varying degrees,increased lettuce soluble protein,soluble sugar,vitamin C contents,significantly reduced nitrate content.Self-made amino acid containing water-soluble fertilizer and “GROWS” brand amino acid containing water-soluble fertilizer had similar fertilizer effects,that is,self-made amino acid containing water-soluble fertilizer achieved the same fertilizer effect as the commercial amino acid containing water-soluble fertilizer.Comparing the fertilizer effects of fertilization treatments with different proportions of urea and amino acid containing water-soluble fertilizer,the fertilization treatment with proportion of 0.075 g N(urea)/kg soil + 0.0015 mg amino acid/kg soil amino acid containing water-soluble fertilizer achieved the best yield and quality of lettuce. |
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