Extraction And Biological Ativityof The Main Componentsof Tuna Cooking Liquid

Tuna is mainly distributed in the middle and low latitude waters of the world.It is a warm-water migratory fish and has important economic value.It is generally used to make dried fish,canned fish and other food.Cooking is essential in the processing and utilization of tuna.For every 1 ton of tuna processed,about 0.5 tons of cooking liquid will be produced.Water consumption for cooking only accounts for about 3%of the whole processing process,but the chemical oxygen demand,ammonia nitrogen,total phosphorus,total nitrogen accounted for 52%,28%,13%,37%.The main components of cooking liquid are sugar,protein,lipids,free amino acids,peptides,etc.However,due to the large amount of cooking liquid is not easy to store,it is often dumped directly without proper treatment,which not only wastes good resources but also causes pollution to the environment.Therefore,the separate collection of cooking liquid for high-value development and utilization can not only reduce the discharge of high-nitrogen oil wastewater in the industry,but also extend the industrial chain and improve the added value of tuna production and processing.The main research results of this paper are as follows:1)Firstly,fish oil was extracted from tuna cooking liquid by ultrasonic-assisted ethanol method.The effects of solid-liquid ratio,temperature,time and ultrasonic power on the extraction rate of fish oil were studied.Response surface methodology was used to further optimize the experimental conditions according to the single factor experimental results.The extracted fish oil was characterized by physical and chemical indexes,fatty acid composition and antioxidant activity.The optimal extraction results showed that the extraction rate of fish oil reached 91.32%under the condition of alcohol precipitation for 60 min at 34?and ultrasonic power 154 W.The characterization results showed that the fish oil extracted by this method could meet the standard of crude fish oil,and some indexes such as acid value,iodine value and peroxide value could meet the standard of refined fish oil.The fish oil is rich in eicosapentaenoic acid(EPA)and docosahexaenoic acid(DHA),accounting for 40.49%;The fish oil had strong scavenging ability on DPPH free radical,and its semi-inhibitory mass concentration(IC₅₀)reached 6.13 mg/m L.It also had synergistic antioxidant effect with vitamin E and chlorogenic acid.2)Protein was extracted from tuna cooking liquid by ethyl alcohol precipitation method.The effects of solid-liquid ratio,extraction time,extraction temperature,p H and solid content on the extraction rate of protein were investigated,and the optimization factors of response surface experiment were determined based on these factors.The protein was characterized by ULTRAVIOLET scanning analysis,infrared spectrum analysis,amino acid analyzer and circular dichroism chromatography.The optimal extraction conditions were as follows:solid-liquid ratio of 1:2.4,36?and alcohol precipitation of 2 h.Under these conditions,the protein extraction rate of tuna cooking liquid was 76.83%.Full-wave scanning results show that the maximum absorption occurs at 218 nm,but there is no obvious absorption peak at 280 nm.Ft-ir diagram showed the characteristic absorption peak of gelatin.The amino acid content accounted for 18.79%,among which hydroxyproline accounted for 10.21%.The structural characterization results showed that the extracted protein was gelatin.3)The protein obtained in the second part was enzymatically digested by alkaline protease to prepare bioactive peptide.The effects of solid-liquid ratio,adding amount of alkaline protease,temperature,p H and enzymolysis time on the preparation of bioactive peptides were investigated.Response surface test(RSM)was used to optimize the preparation conditions combined with single factor results.The bioactive peptides were graded by ultrafiltration to study their antioxidant activity.The peptides with different molecular weights were characterized by amino acid composition analysis and FTIR analysis.The optimized results showed that the maximum enzymatic hydrolysis rate(24.63%)could be achieved at 50?for 6 h with the ratio of material to liquid 1:15 and the amount of enzyme 3%.After grading,the four groups of peptides had certain scavenging ability on·OH,DPPH and·O₂^-,and chelated to Fe²⁺,and there was a dose-response relationship.The IC₅₀of the peptide with molecular weight<1 k Da to DPPH radical was 0.54 mg/m L,and the IC₅₀of the peptide to Fe²⁺chelating capacity was 0.59 mg/m L.The results of amino acid composition showed that the contents of hydrophobic amino acids in the four groups were in descending order:3-10 k Da,>10 k Da,1-3 k Da,<1 k Da,the contents of aromatic amino acids were in descending order:<1 k Da,1-3 k Da,3-10 k Da,>10 k Da).The contents of histidine(His)and tryptophan(Trp)in small molecular weight peptide(<3 k Da)were the highest,and the antioxidant activity was the strongest.