Analysis of Seed Amino Acids in Vegetable Soybeans Dried by Freeze and Thermal Drying

Vegetable soybean (Glycine max), known as edamame, has a high nutritional and market value. It is a relatively new crop in North America and Africa. The amino acid profile is important for the nutritional quality of edamame, and a challenge facing its genetic improvement is evaluating its amino acids rapidly. To explore a drying method suitable for the fast evaluation of edamame nutritional profiles, fresh seed samples of 20 soybean genotypes were dried using freeze, low- and high-heat drying methods, and their amino acid contents were analyzed by near-infrared reflectance (NIR) technology. Three-year results indicated that there were significant differences between the years of samplings and among genotypes for all amino acids. Significant differences existed between the drying methods for most amino acids except for leucine and the total amino acid. Low-heat drying at 65 °C and freeze drying showed similar results and were highly comparable to each other for ANOVA and repeatability estimation. The estimates of repeatability under the individual drying methods were 73–94%, except for tryptophan, cysteine and methionine; meanwhile, higher estimates (85–99%) were computed using the combined data of all three drying methods, with few exceptions. Two sulfur-containing amino acids were showed to be more sensitive to high temperature than the others. Six genotypes exhibited higher contents of all the 18 major amino acids and are recommended to be used for the nutritional quality improvement of edamame and other food-grade specialty soybeans. In conclusion, the low-heat drying method can serve as an alternative to freeze drying, and can be used in the large-scale drying of fresh edamame and in the evaluation of seed amino acids in research.