Full-fat soybean origin and thermal processing affect protein secondary structure, in vitro digestion kinetics, and predicted standardized ileal amino acid digestibility in broiler chickens

Background This study evaluated the chemical composition, protein secondary structure, crude protein (CP) solubilization, and in-vitro amino acid (AA) digestibility of three Kenyan full-fat soybean (FFSB) samples (S1, S2, S3) with unknown processing histories and compared with Canadian samples: raw FFSB, extruded FFSB at 160.0 °C, and FFSB roasted at 136.7 °C, 151.7 °C, or 154.4 °C. Protein structure was characterized using attenuated total reflectance-Fourier transform infrared spectroscopy. A two-phase in-vitro digestion model was used to determine CP solubilization and AA digestibility. Observed in-vivo standardized ileal digestibility (SID) values from a study using FFSB roasted at 136.7 °C were regressed on corresponding in-vitro AA digestibility values to generate predicted SID values. Results Thermal processing reduced trypsin inhibitor activity (TIA) in Canadian FFSB from 17.42 TIU/mg in raw FFSB to 3.85 TIU/mg in extruded FFSB and to 4.77, 4.59, and 2.30 TIU/mg in samples roasted at 136.7, 151.7, and 154.4 °C, respectively. In contrast, Kenyan Samples 1, 2, and 3 had residual TIA values of 8.00, 6.00, and 12.00 TIU/mg, respectively. Protein secondary structure differed among samples ( P < 0.001), with the 136.7 °C-roasted sample having greater aggregated β-sheet and β-turn contents and lower random coil content than most samples. CP solubilization differed among samples at each digestion time point ( P < 0.001). At the end of the intestinal phase, processed Canadian samples had greater CP solubilization than raw FFSB but did not differ from one another, whereas Kenyan Samples 2 and 3 were comparable to Canadian raw FFSB. In-vitro AA digestibility differed among samples for all AA ( P ≤ 0.001). For mean total AA digestibility, the 154.4 °C and 151.7 °C-roasted sample had the highest digestibility; Kenyan Sample 1 was greater than Samples 2 and 3, and Sample 3 did not differ from raw FFSB. The regression model showed agreement at the mean level but larger prediction errors for lysine, aspartic acid, and proline, indicating AA-specific limitations. Conclusions Overall, processing conditions and ingredient origin influenced FFSB quality, with roasting at 151.7–154.4 °C producing favorable structural and digestibility responses. The variability observed among Kenyan samples highlights the need for routine quality assessment of commercial FFSB before feed formulation.