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 conductimetric technique was employed and a medium containing yeast extract, Na acetate, L (+)-cysteine chloride and Na lactate was formulated to amplify the electric signal. Sixteen PAB strains isolated from milk for Grana cheesemaking were tested in different combinations of 4 parameters (temperature, pH, NaCl and species); data were expressed as percentage of growth delay compared to standard conditions (pH 6.1, 30 °C, no NaCl added). Decreasing temperature caused the most significant growth delay (131% at 22 °C and 438% at 15 °C), while PAB growth with decreasing pH and increasing NaCl concentration was less affected in the most restrictive conditions (236% at pH 5.2 and 222% at 2.5% NaCl respectively). A slight stimulating effect was observed at 30 °C and low NaCl content (from 193% at 0% down to 187% at 0.5%). The double combinations T × pH and T × NaCl further increased delay values up to 489% (T × pH) and to 482% (T × NaCl) in the most restrictive conditions, and temperature was always the most important factor; the stimulating effect due to NaCl was amplified at 30 ℧C for all concentrations and at 22 °C at 0.5%. A significant difference was found in the behaviour of the 4 tested species: P. thoeni was the most inhibited, while P. freudenreichii and P. acidipropionici underwent the lowest growth reduction. Strictly controlled temperature (under 22 °C), curd acidification (under pH 5.4) and brine salt concentration are the cheesemaking steps identified as the critical points for containing PAB growth; the reliability of the proposed method suggests further individualization of the most suitable factor levels to contain the late blowing defect.