- The present investigation was undertaken to characterize a
number of strains of Lactobacillus bulgaricus and Streptococcus
thermophilus intended for use by a commercial starter supply
company. Thorough characterization of each culture was required in
order to combine compatible strains so that their usefulness in
Mozzarella cheese manufacture would be maximized. In this regard,
cocci were assayed for formate and carbon dioxide production, rods
for proteolysis, and both types for salt and phosphate tolerance as
well as rate of acid production. In addition, certain combinations of
cocci and rods were assayed as mixtures for these characteristics.
Analyses of the various strains of lactobacilli and S.
thermophilus were performed. Proteolysis, as determined by the
Church method, for the rods (L. bulgaricus , L. helveticus and L.
lactis ) varied from as low as 11.3 to as high as 34.7 mM when
incubated for six hours. Proteolysis analyses for S. thermophilus
also revealed a wide range of values from a low of 18.5 to a high of
46.4 mM. However, when strains were incubated for 16 hours, rods
were shown to be nearly twice as proteolytic as cocci. When mixed
cultures were tested for proteolysis, results were dependent on
strain synergism. Values ranged from a low of 5.1 mM to 70.5 mM
in mixed cultures.
Various strains of S. thermophilus and mixed cultures were
assayed for formate production. The S. thermophilus strain values
were from a low of 4.2 to as high as 20.3 mg/L. Formate production
in mixed cultures varied from traces of formate in one culture to
quantities two and a half times that produced by the single S.
thermophilus strains tested.
Carbon dioxide production for the rods (L. bulgaricus , L.
helveticus , and L. lactis ) varied from as low as 0 μl to as high as 376
μl when incubated for six hours at 44 °C. Carbon dioxide production
for S. thermophilus ranged from 5 μl to 1259 μl. Also, S.
thermophilus strains produced significantly more carbon dioxide
than rod cultures, with only three exceptions. All mixtures were
weak producers of carbon dioxide.
Nine of 19 L. bulgaricus strains were stimulated by 0.1%
phosphate ion and one strain showed stimulation at 0.3% phosphate
ion. Thirteen of 19 strains were severely inhibited by 0.5%
phosphate. Three of 10 L. helveticus strains were stimulated by
0.1% phosphate and another three strains were unaffected. All
strains were inhibited by 0.5% phosphate. Two L. lactis strains
showed stimulation at 0.1% phosphate, but inhibition at 0.3% and
0.5%. Acid production by strains of S. thermophilus was inhibited in
11 of 13 cases at 0.1% phosphate. The two strains not inhibited were
slightly stimulated by 0.1% and 0.3% phosphate and unaffected by
0.5% phosphate. The mixed cultures of L. bulgaricus CR 14/ S.
thermophilus 2 and L. bulgaricus Ql S. thermophilus 2 were not
inhibited by 0.1% phosphate, but inhibition occurred at higher
concentrations. Mixed cultures of L. bulgaricus C, E/ S. thermophilus
7, 12 and L. bulgaricus C, G/ S. thermophilus 4, 12 were stimulated
by all three concentrations of phosphate salts tested.
Sodium chloride produced toxic effects on the rods at
concentrations ranging from 2.5% to 3.0%, and acid production was
stimulated 7 of 32 strains by low salt concentrations(0.5%). In
general, cocci were more sensitive to NaCl, with 6 of 13 strains
showing sensitivity at 0.5%. Sensitivity to salt was a more gradual
effect in the cocci as revealed by a gradual reduction in rate of acid
production as NaCl concentrations increased. Mixed cultures were
more tolerant to NaCl with no inhibition occurring at concentrations
of 1.0%. Culture L. bulgaricus C, GIS. thermophilus 4, 7 were
stimulated at concentrations through 1.5%. The synergistic
properties of the mixed strains increased NaCl tolerance.