ABSTRACT
Summary Many studies have indicated that leaf protein prepared by green crop fractionation affords good potential as a protein supplement. It has been noted, however, that there were some marked differences between crops in the nutritional quality of leaf protein in spite of a similarity in amino-acid composition between leaf proteins made from different crops. We previously showed that brown casein, which was produced by the interaction of milk casein with phenolics undergoing enzymic oxidation, had lower nutritional value than original casein. In addition, it has been suggested that leaf proteins prepared from crops containing phenolics and polyphenolase were brown after extraction with acetone. It can therefore be presumed that the browning of leaf protein affects its nutritional quality. Italian ryegrass, red clover, sorghum, and alfalfa were used for leaf-protein preparation. Fresh leaves were pulped in the presence or absence of a reducing agent (sodium ascorbate or NaHSO3) and green juice was separated. Protein was coagulated by adjusting the pH of the juice to 4; then the juice was heated and washed with acetone. The biological evaluation of leaf proteins was carried out by the growth method with male rats weighing about 45 g. Italian ryegrass, red clover, and sorghum had o-diphenolic contents and polyphenolase activity. The leaf proteins from Italian ryegrass, red clover, and sorghum were brown when leaves were pulped in the absence of a reducing agent. On the other hand, alfalfa had neither o-diphenolics nor polyphenolase, and hence the alfalfa leaf protein did not brown during pulping even in the absence of a reducing agent. The brown leaf protein from Italian ryegrass had lower digestibility than the leaf protein protected from browning, although there were no differences in growth-promoting effect and protein-efficiency ratio (PER) between the two leaf proteins. The feeding of brown leaf protein from red clover resulted in the lowering of weight gain, digestibility, and PER, and all the measurements including diet intake were lowered by feeding the brown leaf protein from sorghum. In the case of alfalfa leaf protein, there was no difference in nutritional quality between the two leaf proteins made with and without an attempt to prevent browning. The results mentioned above indicate that the occurrence of phenolics and polyphenolase in a crop is responsible for the browning of leaf protein and that the browning of leaf protein causes its nutritional impairment.
