Effect of
Lactococcus lactis ssp.
lactis
inoculated in UHT skim milk on the proteolytic
activity of psychrotrophic strains was studied. The titration of a purified non nitrogenous
fraction from refrigerated UHT milk samples, previously inoculated by 10
6 psychrotrophic
strains, showed that
Pseudomonas fluorescens
and
P. putida were the
most proteolytic species. This result has also been confirmed using polyacrylamide gel
electrophoresis of casein fractions extracted from refrigerated and contaminated UHT
milk samples. All extracellular proteases of the studied germs revealed a decreasing
affinity towards κ, β, and α
s1-caseins. This bacterial proteolysis of the UHT milk caseins
samples, voluntarily contaminated by each of the studied psychrotrophic strains, was
significantly reduced following the addition of 10
6 CFU/ml
Lactococcus, which caused,
after 9 days of refrigeration, an appreciable reduction of the number of psychrotrophic
germs from a minimum of 100 to 1000 times, depending on the tested strain. Our
results suggest that the inhibitory effect of lactic acid bacteria on psychrotrophs is due
to acidification or/and bacteriocin like production.
