Growth of propionibacteria in complex media was independent of the
initial number of cells; in contrast, growth of propionibacteria in milk and whey did
not occur if the initial level of cells was <106 cfu/ml. Addition of vitamins, minerals
or complex nitrogen sources to the milk or whey, or incubation under anaerobic
conditions had no effect on the lack of growth. Addition of freeze-dried whey,
prepared from skim milk reconstituted from powder, to a complex medium
prevented growth from low inocula in the complex medium, demonstrating the
presence of an inhibitor or inhibitors in the whey. The inhibitor(s) was heat stable,
had a low molecular mass and retained its activity for at least 4 weeks at 20 °C.
Pregrowth of some lactic acid bacteria, used as starter cultures in Swiss-type cheese
manufacture, in milk for 2 weeks at 20 °C removed the inhibition, which explains
how propionibacteria develop in Swiss-type cheese from low numbers even though
they are inhibited in milk.