Lactobacillus helveticus and Propionibacterium
freudenreichii are essential
starters in Swiss cheesemaking and the release of their intracellular enzymes
through
autolysis could significantly influence ripening. To provide evidence of
this lysis,
cheese made from microfiltered thermized milk inoculated with Lb. helveticus
ITGLH77, Prop. freudenreichii ITGP23 and
a commercial Streptococcus thermophilus
was assayed. Starter viability was determined and lysis was monitored during
ripening by protein analysis with SDS-PAGE of aqueous cheese extracts and
by
immunoblot detection of intracellular proteins: dipeptidase (PepD) for
Lb. helveticus
and methylmalonyl coenzyme A mutase for Prop. freudenreichii.
We verified that the
species specificity of these lysis markers was towards the cytoplasms of
all the species
currently used in Swiss cheese. Lb. helveticus exhibited an almost
complete loss of
viability (99·9%) from the beginning of ripening in the cold room;
concomitantly
PepD appeared in the cheese extracts and was detected until the end of
ripening.
Damaged Lb. helveticus cells were also visualized by scanning
electron microscopy. In
addition, free PepD was also successfully detected in commercial Swiss-related
cheeses. All these results clearly demonstrated the autolysis of
Lb. helveticus in Swiss
cheese. Prop. freudenreichii ITGP23 grew during warm room ripening
and no loss of
viability was detected after maximal growth (109 cfu/g
cheese). Free methylmalonyl-coenzyme A mutase was detected
at the end of ripening during cold storage, when
the cheese extracts were concentrated 20-fold, demonstrating that the autolysis
of
Prop. freudenreichii was tardy and limited.