Lactosylated casein phosphopeptides as specific indicators of heated milks

Abstract

Casein phosphopeptides (CPP) were identified in small amounts in milks heated at various intensities by using matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. CPP selectively concentrated on hydroxyapatite (HA) were regenerated using phosphoric acid mixed in the matrix. Unphosphorylated peptides not retained by HA were removed by buffer washing. This procedure enhanced the MALDI signals of CPP that are ordinarily suppressed by the co-occurrence of unphosphorylated peptides. CPP, belonging to the β-casein (CN) family, i.e., (f1-29) 4P, (f1-28) 4P, and (f1-27) 4P, and the α(s2)-CN family, i.e., (f1-21) 4P and (f1-24) 4P, were observed in liquid and powder milk. The lactosylated counterparts were specific to intensely heated milks, but absent in raw and thermized/pasteurized milk. Most CPP with C-terminal lysines probably arose from the activity of plasmin; an enzyme most active in casein hydrolysis. A CPP analogue was used as the internal standard. The raw milk signature peptide β-CN (f1-28) 4P constituted ~4.3% of the total β-CN. Small amounts of lactosylated peptides, which varied with heat treatment intensity, were detected in the milk samples. The limit of detection of ultra-high-temperature milk adjunction in raw or pasteurized milk was ~10%.