Homogeneity and stability studies during the preparation of a laboratory reference material of soy leaves for the determination of metals.

The homogeneity and stability of metals were tested in a candidate laboratory reference material of soy leaves. Inductively coupled plasma-optical emission spectrometry was used to quantify calcium, magnesium, manganese, iron, zinc, copper, and vanadium. A 6 kg amount of the material, which was dried, ground, and classified as mesh 60, was distributed among 100 bottles. The between-bottle homogeneity test was established by analyzing two subsamples from nine bottles. For the within-bottle test, five determinations of each element of a single bottle were performed. The stability test was performed at temperatures of -10, +27, and +40 degrees C, and after storage times of 4, 12, 24, and 52 weeks. The obtained results indicated that the material was homogeneous and stable under the conditions studied.

The evaluation of the protein quality of soybean products by short-term bioassays in adult human subjects.

The protein quality of soybean products was determined by a short nitrogen balance index (NBI) assay with 15 adult males. In the first study, a textured vegetable protein (TVP) was tested alone and in a 50:50 mixture with ground beef. Protein was fed at 0.2, 0.4 and 0.6 g/kg body weight/day with a constant energy intake. The protein quality (NBI) was then calculated by regression analysis of nitrogen absorbed to nitrogen retained. The NBI of beef (0.91) and of the mixture (0.87) were similar; however, the protein quality of TVP fed alone (0.77) was significantly lower. In the second study, carried out at an intake level of 0.6 g protein/kg/day, supplementation of the TVP with 0.5% DL-methionine improved nitrogen retention, but the values obtained did not reach statistical significance. Meat alone, or the 50:50 mixture fed at an equivalent protein intake, gave similar nitrogen retention values, both of which differed significantly from those obtained when TVP was fed alone. The addition of wheat gluten, used as a source of methionine, to the TVP: meat mixture, produced nitrogen retention values below those obtained with the 50:50 beef:TVP blend. Ten men participated in another trial, this time to determine the protein quality of a soybean protein isolate, which gave an NBI value of 0.91, indicating the high quality of the protein tested. Comparison of the protein quality values obtained by the short-term NBI assay revealed these were essentially the same as those reported by other workers using the conventional long-term approach.