Effect of cheese water activity and carbohydrate content on the barotolerance of Listeria monocytogenes Scott A
Morales, P. | Calzada, J. | Rodriguez, B. | Paz, M de | Gaya, P. | Nunez, M.
High-pressure processing is an appropriate technique for improving the microbiological safety of packaged ready-to-eat foods. The effect of high-pressure treatment on Listeria monocytogenes Scott A inoculated into fresh Hispanico-type cheese and ripe Mahon cheese was investigated. A 3.8-log reduction in the counts of L. monocytogenes Scott A in fresh cheese was recorded after 3 min at 400 MPa and 12 degrees C, whereas 18 min under the same conditions was required to obtain a 1-log reduction in ripe cheese. Dry matter values were 48.96% for fresh cheese and 58.79% for ripe cheese, and water activity (a(w)) values were 0.983 and 0.922, respectively. In dehydrated fresh cheese (58.20% dry matter) in which 5% NaCl was added to achieve a 0.904 a(w) value, L. monocytogenes Scott A counts were lowered by only 0.4 log after treatment for 10 min at 400 MPa. On the other hand, in a 60:40 mixture of ripe cheese:distilled water with a 0.976 a(w) value, the reduction under the same conditions was 3.9 log. Within the a(w) range of 0.945 to 0.965, L. monocytogenes Scott A barotolerance was significantly higher in fresh cheese than in ripe cheese for equivalent a(w) values. Carbohydrate content was higher in fresh cheese than in ripe cheese. The addition of lactose at a concentration of 5 mg/g to an 85:15 mixture of ripe cheese:distilled water did not influence L. monocytogenes Scott A barotolerance during treatment for 10 min at 400 MPa. Galactose at a concentration of 5 mg/g had a protective effect during high-pressure treatment, and glucose at a concentration of 5 mg/g favored L. monocytogenes Scott A survival during refrigerated storage of pressurized samples at 8 degrees C for 5 days.
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