Differentiation of intracellular peptidases of starter and adjunct cultures using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Abstract

Selection of starter and adjunct cultures is important to minimize bitterness of Cheddar and Gouda cheeses. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry may be useful for rapid screening of cheese cultures for propensity to produce bitter cheese. The objective of this study was to demonstrate the application of MALDI-TOF for differentiating intracellular peptidase activities of starter and adjunct cultures on β-CN f193–209 under simulated cheese condition. Bovine β-casein was incubated with chymosin in 9.55 g/l citrate buffer (pH 5.4, 40 g/l sodium chloride) at 30°C for 24 h, followed by incubation with cell-free extract (CFE) of starter or adjunct culture. Mixed strains of Lactococcus lactis ssp. lactis and L. lactis ssp. cremoris designated as 56 and 105 were the sources of nonbitter and bitter starter cultures, respectively. Lactobacillus helveticus WSU-19 and W900R represented adjunct cultures having high and low debittering activities, respectively. The degradation pattern of β-CN f193–209 by CFE of WSU-19 indicates general aminopeptidase and endopeptidase activities, while degradation of the peptide by CFE of W900R, 56, and 105 are mainly from endopeptidase activity. The rates of β-CN f193–209 hydrolysis by CFE of WSU-19, W900R, 56, and 105 are 6.90, 0.38, 0.39, and 0.23 mg/l per h, respectively.