Preparation And Functional Properties Of Low-phosphorus Egg Protein Powders

Chronic Kidney Disease(CKD) is a disease of high mortality, high morbidity and poor prognosis, which gradually arouses public concern. Due to the fact that high content of phosphorus tend to cause some complications such as hyperphosphatemia, the dietary of CKD patients with dialysis treatment requires the food with high protein and low phosphorus. The egg protein, as high-quality protein, should be the first choice for CKD patients. However, the rich phosphorus poses the potential risk to patients with hyperphosphatemia. In this context, we try to develop a new method(i.e. alkaline protease auxiliary dephosphorization) to prepare the functional food, low-phosphorus egg yolk protein powders(LPEYP) and low-phosphorus compound egg protein powders(LPCEP), for CKD patients.The method of alkaline protease auxiliary dephosphorization consists of three steps: enzymolysis, dephosphorization with dilute alkali and removal of the phosphate using adsorbent mainly composed of Fe(OH)3. The effect of the degree of hydrolysis, the concentration of alkali liquor and the condition of adsorption on the dephosphorization rate were systematically investigated in our study. The results demonstrated the dephosphorization rate got the highest value when the yolk powder was firstly hydrolyzed with alkaline protease for 2 h [55oC, 1%(total mass of the sample), pH=9], then it was dephosphorized with 0.01 mol/L NaOH for 1 h, finally, the resultant was processed with the adsorbent for 30 min [1:1.5(samples: adsorbent), pH=6.5]. Under the optimal condition, the degree of hydrolysis, dephosphorization rate, protein content and P content were 7.42%, 45.17%, 74.74%±1.59% and 588±24 mg/100 g, respectively, while the solubility of LPEYP achieved 11.54%. In adiition, the dephosphorization rate of alkaline protease auxiliary dephosphorization was higher than that obtained using the conventional method(alkaline dephosphorization(0.10 mol/L NaOH), 38.36%).In addition, the physicochemical properties and emulsifying properties(color, surface hydrophobicity, microstructure, CD spectra,amino acid and functional properties) of LPEYP prepared with alkaline protease auxiliary dephosphorization method, were studied in detail and compared carefully with those of raw egg yolk powders, defatted egg yolk protein powders and LPEYP prepared with alkaline dephosphorization method. In comparison with defatted egg yolk protein powders, the color of LPEYP prepared employing the two methods were lighter, while the microstructures of LPEYP prepared by these two methods were looser and the random coil were all over 90%. The surface hydrophobicity of LPEYP were all relatively higher. AA content of LPEYP was 68.17 g/100 g and the FAO/WHO degree model was 0.958, higher than that of LPEYP(0.884) prepared by the alkaline dephosphorization method. The soluble protein content of LPEYP was 15.56%, higher than that of LPEYP prepared by alkaline dephosphorization(12.07%), defatted egg yolk protein powders(4.48%), and raw egg yolk powders(7.50%). However, the solubility was worse than that of raw egg yolk powders, and compared with defatted egg yolk protein powders and LPEYP prepared with alkaline dephosphorization, it was higher. The emulsifying activity index(EAI) and stability index(ESI) of LPEYP were 0.640 and 43.84, respectively, significantly lower than those of the raw egg yolk powders(48.28) and defatted yolk protein powders(49.25). The LPEYP had high nutritional value and good process characteristics, which could be adopted as a high quality protein source for CKD.Furthermore, In order to further decrease the P content, balance nutrition and improve the processing characteristics of LPEYP, a study of the effect of adding a certain percentage of egg white powder into LPEYP on these indexs was carried out. The results showed that in the complex ratio of 1:1, the essential amino acid content was higher, up to 44.62% and P content 390 ± 18 mg/100 g; Solubility and emulsifying properties were significantly improved. Solution of LPCEP was mushy, and emulsion particle size was normal distribution and a stable and uniform emulsion state was observed. On the basis, the optimum brewing parameters of LPCEP were obtained: time and temperature of water bath were 30 min and 50 oC respectively, ionic strength 0.3 mol/L, pH=8~9. Under this brewing condition, dissolvability of LPCEP prepared was the best one, hydration capacity 4.13±0.14 g/g, agglomeration rate 8.75±0.51 g/100 g, solubility 70.17%±1.35%, stability factor 95.38±3.73. In addition, by adding 0.6% sucrose esters, the dissolvability of LPCEP was improved remarkably. The results indicated that hydration capacity increased about 12.35%, while the agglomeration rate reduced about 9.60%, and solubility increased about 3.24%.