Propionibacteria in Italian hard cheeses

Economic losses for Parmigiano Reggiano and Grana Padano, for which production is about 200 000 tons per year, can result from ''late blowing'' caused by propionic acid bacteria (PAB). PAB contents of 306 milk samples for Grana cheese were analyzed. The mean value was 726 cfu/ml; however, when the 8 samples with the highest PAB content (>5 000 cfu/ml) are not included, this mean decreases to 377 cfu/ml. Sixty strains of PAB were isolated, identified (65% P freudenreichii, 25% P jensenii, 5% P thoenii and 5% P acidipropionici) and tested under different incubation temperatures (30, 22, 15 degrees C), pH of the substrates (6.0, 5.5, 5.0) and concentrations of NaCl (0.5, 1.0, 1.5, 2.0, 2.5, 3.0%). The ''optimal'' condition was 30 degrees C, with considerable slowing of the growth at lower temperature. In the presence of NaCl, as the incubation temperature decreased, the number of the totally inhibited strains increased, while the total of those insensitive and those only partially inhibited remained the same up to 1.5% of NaCl and increased at higher concentrations. The strains resulted partially or strongly inhibited only at pH 5.0, while the total number of insensitive strains and those only partially inhibited increased as the temperature was lowered. An extreme adaptability of PAB even to very drastic con ditions was noted: anywhere combinations such as low temperature (< 22 degrees C), high salt concentrations (> 2%) and low pH values (< 5.5) could effectively delay their development during the ripening of Grana cheese without total inhibition.