Indirect conductimetry in the study of propionibacteria inhibition

Temperature (T), pH and NaCl concentration are the parameters that control the rate of propionic acid bacteria (PAB) growth. The indirect conductimetr ic technique was employed and a medium containing yeast extract, Na acetate , L (+)-cysteine chloride and Na lactate was formulated to amplify the elec tric signal. Sixteen PAB strains isolated from milk for Grana cheesemaking were tested in different combinations of 4 parameters (temperature, pH, NaC l and species); data were expressed as percentage of growth delay compared to standard conditions (pH 6.1, 30 degrees C, no NaCl added). Decreasing te mperature caused the most significant growth delay (131% at 22 degrees C an d 438% at 15 degrees C), while PAB growth with decreasing pH and increasing NaCl concentration was less affected in the most restrictive conditions (2 36% at pH 5.2 and 222% at 2.5% NaCl respectively). A slight stimulating eff ect was observed at 30 degrees C and low NaCl content (from 193% at 0% down to 187% at 0.5%). The double combinations T x pH and T x NaCl further incr eased delay values up to 489% (T x pH) and to 482% (T x NaCl) in the most r estrictive conditions, and temperature was always the most important factor ; the stimulating effect due to NaCl was amplified at 30 degrees C for all concentrations and at 22 degrees C at 0.5%. A significant difference was fo und in the behaviour of the 4 tested species: P. thoeni was the most inhibi ted, while P. freudenreichii and P. acidipropionici underwent the lowest gr owth reduction. Strictly controlled temperature (under 22 degrees C), curd acidification (under pH 5.4) and brine salt concentration are the cheesemak ing steps identified as the critical points for containing PAB growth; the reliability of the proposed method suggests further individualization of th e most suitable factor levels to contain the late blowing defect.