Abstract
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 stim- ulating 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 low- est 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 con- taining PAB growth; the reliability of the proposed method suggests further individualization of the most suitable factor levels to contain the late blowing defect. Propionibacterium / inhibition / indirect conductimetry
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