Protein quality of alfalfa protein concentrates obtained by freezing

Freezing protein curd was prepared from alfalfa juice and extracted with 2-propanol, either unwashed or after washing with water. To examine the effect of extraction on digestibility, in vitro digestibility was analyzed for the two extracted protein concentrates and the unextracted protein concentrate. Two methods, one direct, the pH-stat method, and one indirect, the dietary fiber method, were used to determine in. vitro digestibility. The protein quality of the unextracted freezing concentrate was also determined by bioassays. The in vivo digestibility of this concentrate (82.8) was lower than that of soybean meal, while the biological value (75.4) was similar. Thus, the lower net protein utilization (62.4) of this concentrate as compared to soybean meal can be ascribed to its lower digestibility. Extraction of the concentrate with 2-propanol lowered in vitro digestibility about 11 and 2% by pH-stat and dietary fiber methods, respectively, while washing the concentrate with water before extraction either had no effect on in vitro digestibility or lowered it slightly. There was an inversely proportional relationship statistically significative between the content of dietary and insoluble fiber in the concentrates and their in vitro digestibilities.