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Peptides Produced by Selected Lactose-Positive and Lactose-Negative Lactococci in a Model Cheese Ripening System¹

Department of Food Science and Nutrition, University of Missouri, Columbia 65211

ABSTRACT

Pasteurized skim milk was placed in a dilution bottle with buffer (pH 5.1 ± .1) that had been immobilized in agar. Rennet and lactococci (10⁹/ml) were added. Microbial growth was inhibited with penicillin, natamycin, and nalidixic acid. Following incubation at 25°C for 12 d, proteinase activities were quantified by the o-phthaldialdehyde assay and peptides produced were determined by reversed-phase HPLC. Four lactose-negative mutants were among the 14 strains tested. Lactose-negative mutants were significantly less proteolytic than lactose-positive strains with one exception, but all strains were proteolytic. Three of the lactose-positive strains were two to five times more proteolytic than the other seven. Peptide profiles from reversed-phase HPLC were separated by cluster analysis into four groups with similarities greater than .64. The most proteolytic strains, which were lactose-positive, produced numerous hydrophilic peptides with chromatographic retention times of 5 to 25 min, whereas the lactose-negative strains produced numerous hydrophobic peptides with retention times greater than 50 min.

¹ Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 10718.

² King Saud University, Riyadh, Saudi Arabia.