Propionibacteria in Italian hard cheeses

Abstract

Summary - Economie losses for Parmigiano Reggiano and Grana Padano, for which production is about 200 000 tons per year, can result from Iate blowing caused by propionic acid bacteria (PAB). PAB contents of 306 milk samples for Grana cheese were analyzed. The mean value was 726 du/ml; however, when the 8 samples with the highest PAB content (>5 000 du/ml) are not included, this mean decreases to 377 du/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°C), pH of the substrates (6.0, 5.5, 5.0) and concentrations of NaCI (0.5, 1.0, 1.5, 2.0, 2.5, 3.0%). The optimal condition was 30°C, with considerable slowing of the growth at lower temperature. ln 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 sa me up to 1.5% of NaCI 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 conditions was noted: anywhere combinations such as low temperature « 22°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.