Selective enumeration of propionibacteria in Emmental-type cheese using PetrifilmTM Aerobic count plates added to lithium glycerol broth

Abstract

Propionibacteria derived from dairy products are relevant starter cultures for the production of Swiss and Emmental-type cheeses, and the monitoring of which is mandatory for proper quality control. This study aimed to evaluate an alternative procedure to enumerate propionibacteria, in order to develop a reliable and practical methodology to be employed by dairy industries. 2,3,5-triphenyltetrazolium chloride (TTC) inhibitory activity was tested against five reference strains (CIRM 09, 38, 39, 40 and 116); TTC at 0·0025% (w/v) was not inhibitory, with the exception of one strain (CIRM 116). Subsequently, the four TTC-resistant strains, three commercial starter cultures (PS-1, PB-I, and CHOO) and twelve Emmental-type cheese samples were subjected to propionibacteria enumeration using Lithium Glycerol (LG) agar, and Petrifilm™ Aerobic Count (AC) plates added to LG broth (anaerobic incubation at 30 °C for 7 d). Petrifilm™ AC added to LG broth presented high counts than LG agar (P<0·05) for only two reference strains (CIRM 39, and 40) and for all commercial starter cultures. Cheese sample counts obtained by both procedures did not show significant differences (P<0·05). Significant correlation indexes were observed between the counts recorded by both methods (P<0·05). These results demonstrate the reliability of Petrifilm™ AC plates added to LG broth in enumerating select Propionibacterium spp., despite some limitations observed for specific commercial starter cultures.