Comparison of the protein quality of five new northern adapted natto soybean cultivars by amino acid analysis

The total protein and amino acid compositions of five newly released northern adapted natto soybean cultivars, namely Canatto, TNS, Nattosan, AC T2853, AC Pinson, and their recurrent parent, Nattawa, have been determined. Mean protein values ranged from 30.4% in AC T2653 to 31.0% (AC Pinson), 31.0% (Nattawa), 32.1% (TNS), and 32.3% (Canatto), compared to the protein content of Nattosan, which was 34.2%. The total nitrogen content was also variable among these cultivars, ranging from 5.15% to 5.92%. All five new natto soybean cultivars were similar in their essential amino acid (EAA) content, i.e., EAA(9) = 45.0-47.0%, compared to the FAO/WHO reference protein pattern value of EAA(9) = 33.9% for a 2-5-year-old child. Each of the six soybean cultivars was limited only in methionine and, to a lesser extent, in valine and isoleucine and had a protein digestibility corrected amino acid score of 91% for all cultivars, compared to the value of egg protein (97%). These results suggest that the most accurate evaluation of protein quality in natto soybeans, and possibly other legumes and cereals, is by the protein digestibility corrected amino acid score.