Eighty-eight specimens of raw hamburgers obtained at retail markets in Saitama Pref. were examined for the presence of Listeria monocytogenes and other Listeria spp. Listeria spp. were isolated from 66 (75.0%) of the specimens. Thirty-two (36.4%) specimens contained L. monocytogenes, 39 (44.3%) specimens contained L. innocua, 6 (6.8%) specimens contained L. weishimeri, and 3 (3.4%) specimens contained L. grayi. The serotypes of 32 isolated L. monocytogenes were 11 of both 1/2b and 1/2c, 4 of 1/2a, 3 of 4b and the rest were not determined.
With the view of examining the bacteriological safety of raw hamburgers after cooking, 3 sizes of hamburgers (large 100 g, medium 75 g, small 50 g) innoculated with L. monocytogenes were examined for the change in central temperature during heating and viable bacterial count thereafter. Heating was done at 200°C and for 3 min (1.5 min each surface), and especially for large and medium-sized hamburgers, Heating was repeated for the same duration while covered. The change in central temperature with heating was represented by lower central temperature and greater time lag between heating duration and increase in central temperature for large size (average maximum temperature: large 69.7°C, medium 88.3°C, small 96.7°C). Viable inoculated L. monocytogenes were recognized in 2 of the 4 large hamburgers regardless of the inoculative concentration and 1 of 5 (inoculated at 103/g) and all 5 (inoculated o at 106/g) small hamburgers, compared with the medium-sized hamburgers having no viable inoculated L. monocytogenes at all. Similar tests performed 10, 10 and 5 min after the completion of heating for large, medium-sized and small hamburgers, respectively revealed no bacteriocidal effect of the remaining heat.
