Cold-Pressing Sunflower Meal and Gastric pH Synergistically Improve <i>In Vitro</i> Nitrogen Solubility by Preserving Secondary Protein Structure, Outperforming Soybean Meal, Solvent-Extracted Sunflower Meal, and High-Protein Electrostatically Separated Sunflower

Protein digestion is influenced by gastric pH and enzymes accessibility to peptide bonds, both of which are constrained by the secondary protein structure. This study evaluated how processing methods influence nitrogen solubilization in sunflower meal (SFM) using an <i>in vitro</i> gastric-intestinal digestion model across physiologically relevant pH range (1.5, 2.5, 3.5 and 4.5). Four protein sources were evaluated: native solvent-extracted SFM (SFM_SE), electrostatically separated high-protein SFM (SFM_SEP), fully dehulled cold-pressed SFM (SFM_CP), and soybean meal (SBM). Cold-pressing preserved more native secondary protein structures, exhibiting a high α-helix/β-sheet ratio and reduced aggregation compared with SFM_SE. These structural features were correlated with greater nitrogen solubility, particularly at gastric pH 3.5 and 4.5, and enhanced release of low-molecular-weight peptides during the intestinal phase. SFM_SEP improved solubiity relative to SFM_SE, although it was less effective than SFM_CP. Overall, SFM_CP showed solubilization comparable to SBM while generating less resistant protein, demonstrating that processing and gastric pH modulate SFM protein solubility.