Differential Release of β-Casomorphins from A1 and A2 Milk During Standardized Gastrointestinal Digestion Quantified by CE–MS

β-Casein A1 and A2 (β-CN-A1 and β-CN-A2) are the two predominant β-CN proteoforms in bovine milk. β-CN-A1 has been associated with a greater propensity to release opioid peptides, such as β-casomorphin-7 (β-CM-7) and β-casomorphin-5 (β-CM-5), during gastrointestinal (GI) digestion, which may have adverse biological effects. This has stimulated growing interest in milk from cows carrying the β-CN A2A2 genotype (A2 milk), which requires reliable characterization methods. In this work, we developed a rapid, selective, and sensitive capillary electrophoresis–mass spectrometry (CE-MS) method for the accurate identification and quantification of β-CM-7 and β-CM-5 in milk hydrolysates from in vitro GI digestion of bovine milk. The method showed good linearity (R2 > 0.99, over 0.5–100 mg/L for β-CM-7 and 0.25–100 mg/L for β-CM-5), limits of detection (0.25 and 0.10 mg/L), and repeatability (<0.2% for times and <1.4% for areas), and tandem mass spectrometry (MS/MS) allowed confirmation. The method was applied to A1A1 and A2A2 milk digested using the standardized INFOGEST protocol, followed by solid-phase extraction. β-CM-7 was detected and quantified only in A1A1 digests (0.98 mg/L), whereas β-CM-5 was not detected (<0.10 mg/L). These results indicate a differential release of β-CMs from A1 and A2 milk and support the method’s suitability for β-CM profiling, which may help assess A2 milk quality control and β-CM health impact.