Inability of dairy propionibacteria to grow in milk from low inocula

Growth of propionibacteria in complex media was independent of the initial number of cells; in contrast, growth of propionibacteria in milk and whey d id not occur if the initial level of cells was < 10(6) cfu/ml. Addition of vitamins, minerals or complex nitrogen sources to the milk or whey, or incu bation under anaerobic conditions had no effect on the lack of growth. Addi tion of freeze-dried whey, prepared from skim milli reconstituted from powd er, to a complex medium Prevented, growth from low inocula in the complex m edium, demonstrating the presence of an inhibitor or inhibitors in the whey . The inhibitor(s) was heat stable. had a low molecular mass and retained i ts activity for at least 4 weeks at 20 degrees C. Pregrowth of some lactic acid bacteria, used as starter cultures in Swiss-type cheese manufacture, i n milli for 2 weeks at 20 degrees C removed the inhibition, which explains how proprionibacteria develop in Swiss-type cheese from low numbers even th ough they are inhibited in milk.