Study On The Autolysis Characteristics Of Antarctic Krill(Euphausia Superba) And The Defluoridation Of Its Hydrolysate

Antarctic krill(Euphausia superba)possesses huge biomass and contains large amount of high quality protein,which can be utilized as tasty substance.However,the value-added utilization of Antarctic krill is limited by its high fluoride content.Thus,the autolysis characteristics and microbial community structure of Antarctic krill at different temperature were investigated.Response surface methodology was applied to optimize the preparation of Antarctic krill protein hydrolysates with low fluoride content via synergistic hydrolysis effect of exogenous and endogenous enzymes.To meet the national standard of fluoride limitation,the defluordation of Antarctic krill protein hydrolysates were carried out and the comparative study of the nutrition value and antioxidant activities of Antarctic krill protein hydrolysates via different fluoride removal treatments were also studied.The effect of temperature on the autolysis characteristics and microbial community structure of Antarctic krill were investigated.The results showed that the lower the temperature,the higher the protein recovery and the degree of hydrolysis.After 12 h,the total acids content and total volatile nitrogen basic nitrogen of 35 ? autolysates were significantly higher than 45? and 55? ones.Histamine was not detected in all samples although biogenic amines varied both in types and quantities.The higher the temperature,the lower the total biogenic amines.PCR-DGGE profile revealed that temperature had significant effect on the microbial community structure.The microflora of 35 ? autolysates was rich and contained bacteria such as Lactic acid bacteria and spoilage organisms and pathogens,which may contribute to the total acids content and nitrogen compounds of autolysates throughout autolysis.Enterococcus were dominant microorganisms of the microflora of 45 ? autolysates.The microbial community structure of 55 ? autolysates consist of not only Enterococcus and Pseudomonas,but also Lactobacillus plantarum,Lactococcus lactis strain and Acinetobacter junii.The preparation of Antarctic krill protein hydrolysates with low fluoride content was optimized,and the best exogenous enzyme was selected from PTN6.0S,PR23,Alcalase,Acid protease,flavor protease,neutral protease,papain and Protamex.The results showed that hydrolysis at acid condition facilitated the fluoride dissolution,and PTN6.0S was the best exogenous enzyme considering the protein hydrolysis and fluoride dissolution.The longer the time and the higher the temperature,the higher the fluoride dissolved rate.Ratio of material to solution of 1:1,PTN6.0S dosage of 0.1%(E/S),hydrolysis temperature of 45 ?,initial pH 7.5 and hydrolysis duration of 7 h were the optimal parameters by response surface methodology.Under above conditions,the dissolved fluoride rate and protein recovery were 15.53% and 69.64%,respectively.The fluoride content of Antarctic krill protein hydrolysates was 24.90 mg/kg,which is far below FDA limit.Hydrolysates with molecular weight < 3000 Da accounted for 80.48% of the krill hydrolysates,while hydrolysates with MW 1000-3000 Da and MW <500 Da accounting for 47.97% and 20.11%,respectively.Antarctic krill protein hydrolysates contained high content of functional amino acids and antioxidant amino acids.All amino acids were in sufficient amounts to meet FAO/WHO/UNU requirements for adults except Val.The effect of fluoride concentration on fluoride removal efficiency of different materials including organic calcium and inorganic calcium was investigated,and comparative study of defluoridation both in fluoride water and Antarctic krill protein hydrolysates was carried out.The results showed that low fluoride concentration had adverse effect on defluoridation owing to the inefficiency of absorption or CaF2 precipitation.Peptides,free amino acids and inorganic salts affected the fluoride removal of hydrolysates.As to AKPHs,Poly Aluminium Chloride possessed best fluoride removal capacity with defluoridation rates of 93.96% while chitin had entirely no fluoride removal capacity.The type of calcium materials had significant impact on defluoridation.The fluoride removal efficiency of inorganic Calcium varied significantly for the defluoridaion of AKPHs,Ca(OH)2 and Ca O showed the highest defluoridation rates of 71.28% and 70.12%,respectively,while Ca HPO4 and Ca(H2PO4)2 only 2.17%.There were no much difference among Organic calcium,and their defluoridation rates were over 50%.Ca(OH)2 could reduced the fluoride of AKPHs to less than 2 mg/kg using phosphoric acid to adjust the reaction pH.Orthogonal experiment was performed and the optimal conditions were as follow: addition amount of 0.5%,phosphoric acid as acidulating agent,pH 6.5,reaction time of 60 min,reaction temperature of 40 ?.Under the above conditions,the defluoridation rate was as high as 99.27%,protein loss rate and amino nitrogen loss rate were 8.57% and 8.26%,respectively.After fluoride removal treatment,the fluoride content of AKPHs was 0.30 mg/kg,which is far below Chinese national standard of 2 mg/kg.Comparative study of the nutrition value and antioxidant activities of Antarctic krill protein hydrolysates via different fluoride removal treatments was also studied.The fluoride removal treatments materials included Ca(OH)2+phosphoric acid,nano hydroxylapatite(nHAP)and CaCl2.The results showed that fluoride removal treatments caused protein loss and amino nitrogen loss.The protein loss of hydrolysates via Ca(OH)2+phosphoric acid treatment was as high as 10.40%,while Ca Cl2 defluoridated hydrolysates lost 7.31% protein.No significant difference was observed among amino nitrogen loss of all the hydrolysates(P>0.05).After defluoridation,dark matter and color ratio decreased while red index and yellow index increased.Amino acid composition of hydrolysates changed.EAA/NEAA and EAA/TAA were increased to some extent and able to meet the FAO/WHO/UNU requirements,even though the TAA,EAA and NEAA of hydrolysates were decreased after fluoride removal treatment.In different evaluations,the defluoridation caused loss of antioxidant activities of hydrolysates,which could be related to antioxidant amino acids,aroma amino acids and peptides of 3000-1000 Da.