New Technologies Of Deep Processing Of Antarctic Krill(Euphausia Superba) Proteins

The biomass of Antarctic krill is huge, and Antarctic krill are rich in proteins with high quality. But, the fluoride content of Antarctic krill is extremely high and Antarctic krill contains very strong proteolytic enzymes. Thus, based on these characters of Antarctic krill, the investigation of deep processing of Antarctic krill proteins is important to develop safty krill commodities wih high added value. Two sets of new technologies for deep processing of Antarctic krill proteins were investigated in this thesis. One was based on the preparation of proteins with low fluoride level from Antarctic krill, including the preparation of proteins with low fluoride level from Antarctic krill, the preparation of protein powders by using the proteins with low fluoride level, and the utilization of the byproducts of waste liquor produced during the preparation of proteins with low fluoride leve. Another was the preparation of hydrolysates with high amino acid nitrogen level by using the technology of two-stage krill autolysis, including the investigation of the technology of two-stage krill autolysis, and the investigation of the fluoride removal and volatile flavor compounds of hydrolysates obtained by two-stage krill autolysis. The main investigations and their results were as follows.1. The alkali solubilization and acid precipitation method for recovering proteins from Antarctic krill was systematacially investigated. Based on the investigation of the method, the fluoride removal technology of the acid-precipitated proteins obtained by the method was investigated, and the proteins with low fluoride level were prepared. Besides, the composition of the proteins with low fluoride level was analyzed. The optimal conditions for protein solubilization were determined to be pH11.5and4℃. The optimum method of protein solubilization was the proteins were solubilized two times; a water/krill ratio (mL/g) of6and a time of30min were used for the first step, whereas the second used a water/krill residue ratio (mL/g) of3and a time of30min. The dissolved protein rate by the optimum method of protein solubilization was97.23%. The optimum pH for protein precipitation was4.6. The protein recovery yield was increased by5%when transglutaminase was correctly applied during the acid-induced protein precipitation. The proteins with low fluoride level were finally obtained through an effective fluoride removal technology of the acid-precipitated proteins. The fluoride content of the proteins with low fluoride level was lower than2mg/kg. The protein yield of the proteins with low fluoride level was52.68%. The proteins with low fluoride level were composed of66.96%of crude proteins (dry weight) and33.01%of total lipids (dry weight). The fatty acids of the proteins with low fluoride level were rich in EPA(13.65%)、DHA(7.17%)和MUFAs (16:18.70%,18:120.61%). The proteins with low fluoride level contained all essential amino acids in sufficient amounts to meet WHO/FAO/UNU requirements for adults and infants.2. The preparation of protein powders by using the proteins with low fluoride level was investigated. The optimal conditions of some pretreatments of spray drying were determined as follows:the dry matter content of feeds (≥7.89%), the adjusted pH of materials (7.2), the high pressure of homogenization (40Mp), the temperature of feeds (60℃). The moisture content of the prepared protein powders was lower than5%, and the protein powders contained high levels of total lipids, total phospholipids and surface free-fat. During the spray drying process, a lower dry matter content of feeds resulted in a higher protein content on the particle surface after the liquid drops were solidified, which made the denaturation degree of protein powders increase, whereas an increased dry matter content of feeds led to more lipids existing on the particle surface and more proteins enclosed in the particle interior after the liquid drops were solidified. The prepared protein powders with low fluoride level exhibited a bad oxidation resistance when they were exposed to air, and their easy oxidation properties were well inhibited by nitrogen-filled packages. Under the condition of nitrogen-filled packages, the antioxidant properties of the protein powders with antioxidant addition were significantly (P<0.05) improved, and a combined antioxidant (L-ascorbyl palmitate+DL-a-tocopheryl acetate,1+1) exhibited the best antioxidant performance.3. The preparation of the proteins with low fluoride level resulted in two waste liquor byproducts, of which one was produced during the acid-induced protein precipitation, and another was produced during the fluoride removal of the acid-precipitated proteins. The utilization of the byproducts was investigated through the addition of Antarctic krill to the byproducts, followed by the preparation of hydrolysates with high amino acid nitrogen level using the technology of two-stage krill autolysis in this thesis. The main nitrogen-containing ingredients of waste liquor byproducts were small molecules of peptides or amino acids. Through the utilization of waste liquor produced during the acid-induced protein precipitation, the hydrolysates obtained by two-stage krill autolysis contained1.070g/100mL of amino acid nitrogen,1.97g/100mL of total nitrogen,18.85μg/mL of fluoride, meanwhile the ammonium salt to amino acid nitrogen rate and protein yield of the hydrolysates were21.18%and79.48%, respectively. Through the utilization of the mixture of waste liquor produced during the acid-induced protein precipitation and the fluoride removal of the acid-precipitated proteins, the hydrolysates obtained by two-stage krill autolysis contained1.026g/100mL of amino acid nitrogen,1.94g/100mL of total nitrogen,18.79μg/mL of fluoride, meanwhile the ammonium salt to amino acid nitrogen rate and protein yield of the hydrolysates were19.71%and77.32%, respectively.4. The technology of two-stage krill autolysis was investigated. During the investigation, distilled water was added to Antarctic krill, and the preparation of hydrolysates with high amino acid nitrogen level using the technology was investigated. Furthermore, the partial influence factors of one-stage krill autolysis were investigated.The investigation of the influence factors of one-stage krill autolysis indicated Na+, K+, Ca2+and Mg2+solutions at appropriate concentrations showed significantly (P<0.05) promoting effects on krill autolysis, respectively, and the promoting effect of Zn2+solution on krill autolysis was insignificant (P>0.05). Furthermore, the Zn2+solution at high concentration exhibited significantly (P<0.05) inhibitory effects on krill autolysis. The amino acid nitrogen content of hydrolysates obtained by one-stage krill autolysis was0.739g/100mL through the pretreatment of appropriate ultraviolet radiation.The technology of two-stage krill autolysis was investigated. The optimal conditions of the second-stage krill autolysis during two-stage krill autolysis were determined to be a liquor to material rate of2.23(g/g), an initial pH of7.24, a temperature of40.9℃, a time of12h. Under these optimal conditions, the hydrolysates obtained by two-stage krill autolysis contained1.026g/100mL of amino acid nitrogen and1.92g/100mL of total nitrogen, and each of them was in sufficient amount to meet Chinese National Standard GB18186-2000requirements for the special grade soy sauce of low salt-solid fermentation. The ammonium salt to amino acid nitrogen rate of the hydrolysates met the new requirement of Chinese National Standard GB18186(Draft for Comment) for fermented soy sauce, and the fluoride content and protein yield of the hydrolysates were18.60μg/mL and75.33%, respectively. The technology of two-stage krill autolysis had the advantages of high protein yield, simple treatment processes, low equipment investment, short processing time, and prepared hydrolysates with high amino acid nitrogen level. The prepared hydrolysates contained all essential amino acids required by WHO/FAO/UNU, of which Trp, Thr, His and Phe+Tyr were the limiting amino acids. The technology of two-stage krill autolysis could greatly improve the total flavour amino acid content of the hydrolysates, which was46.06mg/mL. The total flavour amino acid content of hydrolysates obtained by two-stage krill autolysis increased by23.71mg/mL compared to that of hydrolysates obtained by one-stage krill autolysis, and the increasing rate was106.09%.5. The fluoride removal of hydrolysates obtained by two-stage krill autolysis was investigated. Compared to calcium chloride and calcium acetate, calcium lactate exhibited the best fluoride removal efficiency when the calcium precipitation method was employed to remove fluoride from the hydrolysates. The solution obtained by the preliminary fluoride removal of the hydrolysates through the application of calcium lactate contained8.19μg/mL of fluoride,0.854g/100mL of amino acid nitrogen,1.72 g/100mL of total nitrogen, and the loss rate of total nitrogen was9.54%. Baed on the preliminary fluoride removal of the hydrolysates through the application of calcium lactate, a biological defluorination agent could efficiently remove the residuaful fluoride from the hydrolysates and make the fluoride content of the hydrolysates decrease to1.53μg/100mL. Therefor, the safty problem of fluoride in the hydrolysates was successfully resolved. The solution obtained by the deep fluoride removal of the hydrolysates through the application of the biological defluorination agent contained1.53μg/mL of fluoride,0.783g/100mL of amino acid nitrogen,1.67g/100mL of total nitrogen, and the loss rate of total nitrogen was14.06%.The volatile flavor compounds of hydrolysates obtained by two-stage krill autolysis were investigated. The main volatile compounds of the hydrolysates were identified as2-methyl-Butanal,3-methyl-Butanal,3-methyl-1-Butanol, Ethanol, and each of their relative abundance in the identified volatiles was24.56%,17.79%,17.70%and5.14%, respectively. The three compounds of2-methyl-Butanal,3-methyl-Butanal and3-methyl-1-Butanol are permited to use as food flavors, and their total relative abundance was60.05%.