Growth of Listeria monocytogenes on Different Muscle Tissues

Growth of Listeria monocytogenes on raw and cooked beef, chicken, catfish, and shrimp was compared. Samples were inoculated with L. monocytogenes and stored at 4°C for 11 days. Listeria monocytogenes and psychrotrophic populations were monitored. Growth of L. monocytogenes was faster and reached a higher population on raw and cooked catfish and shrimp than on beef or chicken. Psychrotrophic populations were greater on beef and chicken than on catfish and shrimp after 11 days regardless of whether the muscle was raw or cooked. To determine what factor or factors inherent in the tissues may contribute to this growth-rate difference, beef and shrimp tissues were used. The pH of each tissue type was adjusted to yield portions of beef and shrimp at pH 5.7 (pH of fresh beet) and 7.6 (pH of fresh shrimp). Portions were inoculated with L. monocytogenes and stored at 4°C for 11 days. Listeria monocytogenes grew significantly faster on beef adjusted to 7.6 than on beef at the normal pH and on shrimp at pH 7.6 than on shrimp adjusted to pH 5.6. Thus, some difference in growth rate is due to inherent pH differences of fresh tissues. However, on cooked tissues the growth rate on acidified shrimp was the same as that on shrimp with the normal pH of 7.6. Growth rates of Listeria on cooked beef at either pH were similar to those noted on the uncooked tissue. This indicates additional factors in shrimp exert inhibitory effects at reduced pH levels that may be heat related.