Optimization of a Nanofiltration Desalination Process for Antarctic Krill Peptides Using Orthogonal Tests

Antarctic krill (Euphausia superba), an important group of marine zooplankton in the Southern Ocean, is the only fishery resource with extremely rich reserves and a low degree of development in the world. Antarctic krill is considered to be the greatest potential source of high-quality marine protein resources due to its abundant biomass and high protein content. Peptides prepared from Antarctic krill exhibit multiple physiological activities, including osteoporosis relief, glucose metabolism regulation, blood pressure amelioration, antioxidation, fatigue alleviation, and anti-aging activity. The production and development of Antarctic krill peptides has recently become an industry focus; however, existing research has been limited to the optimization of enzymatic hydrolysis processes, mainly involving the screening of suitable enzymes and the optimization of enzymatic hydrolysis conditions. Due to the high mineral content of Antarctic krill and the introduction of buffer salt in the process of enzymatic hydrolysis, current Antarctic krill peptides products have a high salt content, which leads to poor sensory experience and health risks. Hence, a process for desalination of Antarctic krill peptides is needed. Desalination methods for bioactive substances include dialysis, ultrafiltration, nanofiltration, electrodialysis, and macroporous resin adsorption. In the field of membrane separation, nanofiltration technology has been widely used in the purification, concentration, and desalination of food components owing to its advantages: low operation cost, no introduction of exogenous substances, no destruction of materials, and low rejection rate of monovalent ions. In order to improve product quality and ensure market expansion, the process of desalination of Antarctic krill peptides using nanofiltration technology was studied and optimized in this study.Defatted Antarctic krill powder was enzymatically hydrolyzed by alkaline protease to obtain Antarctic krill peptides for further use. The main factors affecting the desalination effect of Antarctic krill peptides (peptides concentration, nanofiltration pressure, and cycle times) were optimized by single-factor and orthogonal tests, using the desalination rate and protein loss rate as evaluation indexes. The experimental optimization ranges included peptides concentration of 1%~5%, nanofiltration pressure of 0.6~1.4 MPa and cycle times of 1~5. The salt contents of the samples before and after desalination were quantified using the silver nitrate titration method; the protein contents of the experimental samples were quantified using the Lowry colorimetric method. The quality indexes of the Antarctic krill peptides after treatment (including the basic nutritional composition: moisture content, protein content, ash content, salt content; amino acid composition; and molecular weight distribution) were systematically evaluated by the corresponding national standard methods. All experiments were performed in triplicate, and data were expressed as mean ± standard deviation. Excel 2016, IBM SPSS 20.0, and Origin 2018 were used for data analysis and chart drawing.Single-factor tests revealed that peptides concentration of 3%, nanofiltration pressure of 1.0 MPa and a cycle time of 2 could be selected as the design basis for the L9 (33) orthogonal test. The range value of the orthogonal test indicated that the degree of influence of the three factors on the desalination effect was as follows: peptides concentration > cycle times > nanofiltration pressure. The optimum conditions for desalting Antarctic krill peptides obtained by k value analysis were as follows: peptides concentration of 3.0%, nanofiltration pressure of 1.2 MPa and a cycle time of 3. Under the optimal condition, the desalination rate of the Antarctic krill peptides reached up to (86.35±2.11)%, and the protein loss rate was controlled at (9.10±0.35)%, demonstrating the feasibility of the process. The salt content of the Antarctic krill peptides after desalination was reduced to (1.14±0.12)% and the protein content was (92.73±2.29)%. The molecular weights of the Antarctic krill peptides after desalination were mainly distributed between 189 Da and 6500 Da, of which the proportion of peptides with molecular weight less than 3000 Da was (88.91±2.19)%, conforming to the molecular weight distribution range of bioactive peptides. The amount of essential amino acids in the Antarctic krill peptides after desalination accounted for (40.06±0.10)% of the total amino acids, and the ratio of essential amino acids to nonessential amino acids was (66.82±0.28)%. The amino acid compositions of the Antarctic krill peptides after desalination were ideal and met the standard stipulated by the FAO/WHO. The established nanofiltration desalination process presented good treatment effects, and the obtained peptides were of good quality and high nutritional value.The production of Antarctic krill protein-related products may be the next key development for the processing industry, since the sole high-value products of Antarctic krill at present are Antarctic krill oil and its derivatives. The established nanofiltration desalination process has practical application value and would provide technical support for the development of high-quality Antarctic krill peptides. This research provides scientific support for the efficient utilization of Antarctic krill resources.