Natto soybeans are very small-seeded, uniform lines that are cooked and fermented for the Asian food market. In determining soybean suitability for natto manufacturing, seed hardness testing must be conducted. Usually, a professional natto taster performs this subjective test on a finished natto line. However, because hardness testing is time consuming and labor intensive, there must be a more objective, reproducible, and faster method for testing seed texture. The objectives of this study were 1) to compare the precision and efficacy of different seed hardness testing methods; and 2) to determine the optimal amount of cooked seeds required to quantify hardness. Five food grade soybean varieties (V97-6490, MFL-552, Hutcheson, MFS-591, and Camp) varying in seed size were analyzed with the one-bite method using a TMS Texture System (TMS-2000) equipped with a multiple blade shear cell and a TA-XT2i food texture analyzer equipped with a blade, a 2-mm probe, and a 75-mm cylinder. Dimensions of dry, soaked (16 hr), and cooked seeds (20 min at 121°C and 1.5 kg/cm2), water absorption capacity, swell ratio (weight and volume change), and hardness were quantified. The results showed that 30 g of cooked seeds had the lowest coefficient of variation for hardness (16%) with multiple blade shear cell as compared to 20 g, 40 g, and 50 g of cooked seeds. Five seeds simultaneously evaluated with the single blade and 10 seeds with the cylinder had the lowest coefficient of variation for hardness (11%). The single probe method is not recommended for soybean seed hardness testing because of the high coefficient of variation (25%). The peak value and area of the compression tests were positively correlated with swell ratio and soaked seed dimensions (79% and 69%).
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