Protein quality in cereals and pulses

Abstract

1. TheStreptococcus zymogenesassay procedure was modified and used in the estimation of available methionine and relative nutritional value in rice (Oryza sativaL.), sorghum (Sorghum vulgarePers.), barley and field beans (Vicia fabaL.). The results were assessed in relation to the content of tannins and ‘dye-binding lysine’, and to published information on the nutritional quality of some of the test samples.2. In grain of ten varieties of sorghum, for which other workers had reported a correlation (r— 0.82;P< 0.01) between tannin content and amino acid digestibility in chicks, the available methionine content ranged from 6.3 to 17.7 g/kg protein (nitrogen × 6.25) and was highly correlated with tannin content (r— 0.97;P< 0.001). The content of total methionine and dye-binding lysine varied little between varieties and was not related to tannin content.3. In nine samples of rice the availability of methionine was uniformly high, in accordance with literature values for true digestibility of the N.4. In field beans the presence of tannins in the seed coat was associated with a significant (P< 0.001) reduction in the availability of methionine. In eleven tannin-free varieties the coefficient of availability averaged 0.791, compared with 0.685 in eleven tannin-containing varieties. The standard deviation associated with both values was 0.04. There was no such difference between the corresponding dye-binding lysine values.5. Thirty-three samples of barley were examined, representing fourteen varieties, four of which had been grown at several widely different latitudes in England and Sweden. There was little difference in methionine content between samples. The average availability coefficient for eighteen samples grown in England was 0.995 ± 0.044, and for fifteen varieties grown in Sweden it was 0.851 ± 0.042. The difference was not associated with any difference in tannin content, which was uniformly low. The findings contradict published evidence for marked increase in tannin content with increase in geographical latitude of cultivation, associated with decrease in digestibility of N.