The presence and growth characteristics of Listeria monocytogenes in blue crab (Callinectes sapidus) meat and the effectiveness of microwave energy in a pasteurization process
In this study, the incidence and occurrence levels of Listeria monocytogenes in fresh blue crab (Callinectes sapidus) meat and its behavior in the presence of L. innocua and competing microflora were determined. An attempt was made to develop a process using microwave energy to destroy, or significantly decrease, naturally occurring levels of L. monocytogenes in crab meat. Data for the growth of L. monocytogenes under refrigeration conditions were fitted to the logistics and Gompertz functions, to a developed modified nonlinear Gompertz function including the temperature effect, and to a developed extended modified Gompertz function that combined temperatures in the refrigeration range used.
Listeria was found in 10% of the 126 samples of fresh blue crab meat analyzed, with an 8% incidence of L. monocytogenes. The levels of occurrence were in all but one sample less than 100/g, the exception had an MPN of 1,100 /g. Growth curves were obtained for L. monocytogenes in pasteurized crab meat and generation times of 68.7, 31.4, and 21.8 h at 1.1, 2.2, and 5 °C respectively were observed. When L. monocytogenes was simultaneously inoculated with L. innocua in pasteurized crab meat, no significant effect (p > 0.05) was observed on their respective growth curves. This was observed for three mixture proportions of L. monocytogenes and L. innocua, during 15 days, at 4 and 36 °C. Fresh blue crab meat, classified according to the initial levels of microflora (APCs) into three different categories (< 15,000, 15,000-60,000 and, 60,000-100,000 cfu/g) was inoculated with L. monocytogenes at less than 50 cfu/g and incubated at 1.1, 2.2 and 5 °C for 21 days. A significant (p ≤ 0.05) inhibitory effect on the growth rate of L. monocytogenes was observed with decreasing temperatures. The effect of microflora population upon the growth of L. monocytogenes was observed after 13 days of storage, however, the levels of microflora were such that the meat was rendered inedible.
Fresh crab meat (454 g) was inoculated with L. monocytogenes (less than 50 cfu/g), packed in a polyethylene bag (18 x 20 cm) or in a polypropylene tub (11 cm top diameter, 9 cm bottom diameter, 7 cm height) and microwaved (2,450 MHz) for 3 min. L. monocytogenes was not detected in the product stored at O °C, 3 h or5d after microwave application. The best fit for the growth of L. monocytogenes in fresh crab meat during refrigerated storage was obtained by the developed modified nonlinear Gompertz function. Correlation of the estimates to the experimental growth values (log₁₀ cfu/g) were between 0.82 and 0.99. An extended modified Gompertz equation was developed for the estimation of growth of L. monocytogenes in the range of storage temperatures between 1.1 to 5 °C.