Untargeted metabolomics provide new insights into the implication of Lactobacillus helveticus strains isolated from natural whey starter in methylglyoxal-mediated browning

Hard cheeses may occasionally show a brown discolouration during ripening due to multifactorial phenomena that involve bacteria and give rise to pyrazines arising from methylglyoxal. The present work aimed at developing a novel approach to investigate the role of natural starters in browning. To this object, 11 strains of L. helveticus were incubated in a medium containing 10 % rennet casein dissolved in whey, and then growth was monitored by measuring pH and number of genomes/mL. Browning was assessed through CIELab analysis, methylglyoxal production was determined by targeted mass spectrometry, and untargeted metabolomics was used to extrapolate marker compounds associated with browning discoloration. The medium allowed the growth of all the strains tested and differences in colour were observed, especially for strain A7 (Delta E* value 15.92 +/- 0.27). Noteworthy, this strain was also the higher producer of methylglyoxal (2.44 mu g/mL). Metabolomics highlighted pyrazines and beta-carboline compounds as markers of browning at 42 degrees C and 16 degrees C, respectively. Moreover, multivariate statistics pointed out differences in free amino acids and oligopeptides linked to proteolysis, while 1,2-propanediol and S-Lactoylglutathione suggested specific detoxification route in methylglyoxalproducing strains. Our model allowed detecting differences in browning amid strains, paving the way towards the study of individual L. helveticus strains to identify the variables leading to discoloration or to study the interaction between different strains in natural whey starters.

Automated summary

This study developed a novel approach to investigate the role of natural starters in browning of hard cheeses. By measuring growth, color changes, methylglyoxal production, and metabolomics, differences in browning among strains were detected, indicating the involvement of amino acids and proteolysis in the process.