Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Method for the Quantification of β-Casein Fragment (f 193-209)

Bitter cheese extract was incubated with cell‐free extract of Lactobacillus helveticus strain WSU19. Matrix‐assisted laser desorption/ionization‐time‐of‐flight (MALDI‐TOF) analysis showed an intense peak at m/z‐1880 that was markedly reduced after a 24‐h incubation, as was bitterness. The m/z‐1880 peptide was identified as β‐casein (f193‐209). Synthetic β‐casein (f193‐209) was mixed with internal standards, asparagine or deuterated glutamine substitution in β‐casein (f193‐209). Concentration ratios of β‐casein (f193‐209) to internal standard were plotted against signal ratios. Plots were linear for both internal standards. This study shows that MALDI‐TOF is a rapid method for measuring a specific peptide in cheese and may be useful for assessing cheese ripening and screening cultures for debittering activity.