Belessi, C. I. A.; Papanikolaou, S.; Drosinos, E. H.; Skandamis, P. N.
Survival and acid resistance of Listeria innocua in Feta cheese and yogurt, in the presence or absence of fungi
Journal of Food Protection 71 (4), 2008, 742-749

Abstract 

Survival of Listeria innocua, alone or coinoculated with fungal isolates, during storage of Feta cheese (pH 4.43-4.56) and yoghurt (pH 4.01-4.27) at 3-15°C was examined. 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 yoghurt at all temp. 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 yoghurt, L. innocua reduced >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 approx. 4.5 log. In contrast to what was observed in Feta cheese, surviving populations of L. innocua in yoghurt were eliminated after subsequent exposure for 3 h to pH 2.5. These findings indicate that growth of fungi on the surface of Feta cheese and yoghurt 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.