Abstract:
The activity of chymosin, plasmin, and Lactococcus
lactis enzymes (cell envelope proteinase, intracellular
peptidases, and glycolytic enzymes) were determined
after 5-min exposures to pressures up to 800 MPa.
Plasmin was unaffected by any pressure treatment.
Chymosin activity was unaffected up to 400 MPa and
decreased at 500 to 800 MPa. Fifty percent of control
chymosin activity remained after the 800 MPa treatment.
The lactococcal cell envelope proteinase (CEP)
and intracellular peptidase activities were monitored
in cell extracts of pressure-treated cells. A pressure
of 100 MPa increased the CEP activity, whereas 200
MPa had no effect. At 300 MPa, CEP activity was
reduced, and 400 to 800 MPa inactivated the enzyme.
X-Prolyl-dipeptidyl aminopeptidase was insensitive
to 5-min pressure treatments of 100 to 300 MPa, but
was inactivated at 400 to 800 MPa. Aminopeptidase
N was unaffected by 100 and 200 MPa. However, 300
MPa significantly reduced its activity, and 400 to 800
MPa inactivated it. Aminopeptidase C activity increased
with increasing pressures up to 700 MPa.
High pressure did not affect aminopeptidase A activity
at any level. Hydrolysis of Lys-Ala-ρ-NA doubled
after 300-MPa exposure, and was eliminated at 400
to 800 MPa. Glycolytic enzyme activities of pressure-treated
cells were evaluated collectively by determining
the titratable acidity as lactic acid produced by
cell extracts in the presence of glucose. The titratable
acidities produced by the 100 and 200 MPa samples
were slightly increased compared to the control. At
300 to 800 MPa, no significant acid production was
observed. These data demonstrate that high pressure
causes no effect, activation, or inactivation of proteolytic
and glycolytic enzymes depending on the pressure
level and enzyme. Pressure treatment of cheese may alter enzymes involved in ripening, and pressure-treating
L. lactis may provide a means to generate
attenuated starters with altered enzyme profiles.
