14A-10 |
Inhibitory effect on the plasmin system caused by lactic acid bacteria |
C. D. MEJIA, S. S. Nielsen, and K. D. Hayes. Dept. of Food Science, Purdue Univ., 745 Agriculture Mall Dr., West Lafayette, IN 47907- 2009
The effect of lactic acid bacteria (LAB) on cheese characteristics during ripening has been well studied. Proteases produced by LAB hydrolyze milk proteins. However, the effect of these protein breakdown products on the native plasmin enzyme system important for cheese ripening is unknown. The objective of this research was to determine the effects on plasmin (PL), plasminogen (PG) and plasminogen activator (PA) levels in milk cultured by selected LAB.Lactococcus lactis subsp. cremoris SK 11, Lactobacillus paracasei R50-2 and Streptococcus thermophilus M50-2 (1% v/v) were incubated in sterilized 11% reconstituted nonfat dry milk at 23°C, 30°C, and 35°C, respectively, for fifteen days. The curd and whey fractions obtained after the bacterial acidification were separated using centrifugation and then evaluated for PL activity and PG activation by a direct plasmin assay and by a coupled assay. In these assays, the active plasmin reacts with a chromogenic substrate to produce paranitroanilide measured by absorbance. Results showed that the three strains studied do not produce proteases that by themselves have activity as PL, PG or PA. However, all three strains caused an inhibitory effect that reduced the activation of PG by five fold after fifteen days of incubation (P < 0.05). This suggests that the milk proteins breakdown products caused by LAB proteases can significantly reduce active plasmin that would contribute to cheese ripening.
Session 14A, Dairy Foods: General developments in dairy technology I
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