Correlation of Global Lactosylation and Site-Specific Lactosylation of β-Lactoglobulin in Heated and Processed Milk

Milk processing can reduce protein quality mainly by lactosylation. For quality control, global protein lactosylation can be determined by HPLC-DAD via furosine, formed during acid hydrolysis. Alternatively, lactulosyllysine can be directly measured by site-specific relative quantitation using UHPLC-ESI-MS/MS-sMRM. The study aimed to compare global furosine analysis with analysis of lactulosyllysine at K8, K14, K47, K60, K69/70, K75/77/83, K91, K100/101, and K135 of β-lactoglobulin to evaluate lactosylation. A linear positive correlation between the modification indices of all analyzed lysine residues and the furosine content in raw milk samples heated for 0, 3, 6, 9, 12, and 15 min at 120 °C was detected (r = 0.9763-0.9921). In commercial milk products (pasteurized milk, ESL milk, UHT milk, sterilized milk, condensed milk, and coffee cream), both methods reflected the different heat loads during production. The furosine value detected slight differences more selectively, whereas UHPLC-ESI-MS/MS-sMRM revealed some site-specific effects.