Content and spatial distribution of dairy-related Clostridium spores in Grana Padano cheese during the ripening period

This study investigated the stress factors that affect the content and the spatial distribution of Clostridium spores in Grana Padano (GP) wheels and in GP samples purchased from the market. The cheeses analyzed were produced adding lysozyme to prevent late blowing defects (LBD). The highest NaCl amount was detected in the external part (5.2 ± 0.6%) while a lower level was observed in the core (2.8 ± 0.3%). The clostridial community differed in cheeses with different ripening times along with pH and salt changes that occur slowly over time. The spore level in the core (0.24 ± 0.19 and 0.16 ± 0.14 MPN/g after 11 and 20 months of ripening) was significantly lower than in the peripheral zones (0.48 ± 0.26 and 0.45 ± 0.31 MPN/g). Moreover, in the wheels aged 11 months, the presence of C. tyrobutyricum was higher in the areas with the lowest salt content, while after 20 months of ripening when a homogeneous concentration of salt and pH within the wheel occurred, C. sporogenes was the prevalent species. Our results showed that in the GP wheels changes in the presence of clostridia responsible for LBD still occur until the salt content in the center of cheeses is higher than threshold NaCl needed to inhibit spore germination.