Survival and acid resistance of Listeria innocua in Feta cheese and yogurt, in the presence or absence of fungi

Abstract

The objective of the study was to assess the survival of Listeria innocua, alone or coinoculated with fungal isolates, during storage of Feta cheese (pH 4.43 to 4.56) and yogurt (pH 4.01 to 4.27) at 3 to 15°C. The acid resistance of the bacterium during subsequent exposure to pH 2.5 for 3 h was also evaluated in samples stored at 3 and 10°C. In Feta cheese, L. innocua survived better than it did in yogurt at all temperatures. At 5, 10, and 15°C, the pH of cheese increased due to fungal growth, and this enhanced the survival of L. innocua more than during storage at 3°C. Moreover, during storage of Feta cheese, L. innocua was capable of surviving the subsequent exposure for 3 h in broth of pH 2.5, in contrast to cultures not inoculated in the product (control cultures; 24 h at 30°C in broth). In yogurt, L. innocua reduced more than 5 log within 15 days of storage at 5, 10, and 15°C, whereas extended survival was observed at 3°C until day 22, with total reduction of approximately 4.5 log. In contrast to what was observed in Feta cheese, surviving populations of L. innocua in yogurt were eliminated after subsequent exposure for 3 h to pH 2.5. The findings indicate that growth of fungi on the surface of Feta cheese and yogurt may compromise the safety of these products by enhancing survival of the bacterium. Particularly, when fungi increase the pH of Feta cheese, L. innocua demonstrates better survival and prolonged storage may raise concerns for the development of acid-resistant Listeria populations. Copyright ©, International Association for Food Protection.