Listeria monocytogenes (L. m. ) in shredded cheese was examined for its growth behavior during cold storage and cooking. During storage at 5°C, the concentration after inoculation decreased up to day 20 and thereafter increased. On day 77, the concentration was recovered to almost the same level as that at the time of inoculation, in shredded cheese inoculated with 104 or 106/g L.m. and increased by 1-2 orders in shredded cheese inoculated with 101/g L .m. . Also during storage at 10°, the inoculated L.m. decreased up to day 20 and thereafter increased, each bacterial concentration increased by 1-4 orders. Organoleptically, fungigenesis occurred from days 20 and 15 at 5° and 10°, respectively. The number of bacteria surviving in the pizza inoculated with 106/g L.m. was 3.6-23/100 g after heating at 200°C for 5 min, while heating at 250°C for 5 min caused the death of all the inoculated bacteria, that is 106/g. The change of the temperature at the center of the pizza while cooking tended to decrease with the increases in the temperature of storage and in the volume of ingredients, and these changes were also macroscopically recognizable. The above findings suggested that L. m. in shredded cheese dose not proliferate during the allowable period of storage before consumption and becomes extinct with general methods of cooking.
Home-made type mayonnaise and commercial type mayonnaise were inoculated with 105/g of 4 phage types of Salmonella Enteritidis (SE), isolated from unpasteurized whole eggs, and the behavior of SE was examined. At 30°C the number of SE in home-made mayonnaise decreased to less than 10/g after 5-6 days, and decreased to the same level after 1 day in commercial type mayonnaise. At 10°C the number of SE inoculated into home-made type mayonnaise hardly decreased after 9 days, and decreased to less than 10/g after 3-6 days in commercial type mayonnaise. The difference between the phage types was not so large. SE inoculated into a delicatessen plant type mayonnaise showed the same behavior as that inoculated into commercial type mayonnaise. In potato, egg, and crab salads containing 15% home-made type mayonnaise, in which SE was still surviving, SE grew rapidly at 25°C, but hardly grew at 10°C. The importance of using pasteurized yolk in commercial mayonnaise, and a low storage temperature for salads was recognized.
The survival and distribution of Bacillus thuringiensis in farmland soil were analyzed. First, the survival of B. thuringiensis in soil was examined after spraying with the BT-microbial pesticide formulation and spores of B. thuringiensis, Aizawai (H-7) or Kurstaki (H-3a3b3c) isolates from the formulation onto the surface of 1 m2 of fields for vegetables. The spore counts of B. thuringiensis increased more or less in any fields immediately after the spray. However, the count decreased by approximately log 1 within 1-4 weeks. Thereafter, the number of spores remained constant at the level of 104 cfu/g for 8-12 months. B. thuringiensis, Aizawai and Kurstaki each maintained their ability to form parasporal crystals during residence in soil. Secondly, the distribution of B. thuringiensis was examined in various types of soils collected from 257 fields in Japan. Whether those fields had previously been treated with the BT pesticides was unknown. More than 80% of total samples contained B. cereus/thuringiensis at the levels of 104-106 cfu/g. Of 1, 906 B. cereus/thuringiensis strains from soil samples, 40 (2%) were identified as B. thuringiensis, due to the presence of the parasporal crystals. B. thuringiensis isolates were serotyped by means of the tube agglutination test with anti H sera of 32 known serovars. Consequently, all 40 strains were typed; they consited serovars H-3a3b3c (15 strains), 3a3d (12), 5a5c (1), 6a6c (4), 8a8c (1), 11a11c (5) and 13 (2). Among these serovars, serovar H-3a3b3c (B. thuringiensis, Kurstaki) which is formulated usually in the BT pesticides was isolated from 14 (5%) of 257 fields.
The heat resistance of L. monocytogenes was experimentally examined by heating ground pork samples at 63°C for 30 minutes which is the standard pasteurization procedure for cooked meat products in Japan. Ground pork samples with and without curing salt addition were inoculated with 102/g or 107/g of serotype 1/2b, 1/2c or 4b strain and heated at 63°C for 30 minutes. Although a slight difference was observed in the heat resistance of L. monocytogenes inoculated at the level of 102/g depending on the strain and the presence or absence of curing salt, it perished within 4 minutes. In samples inoculated with 107/g, the bacterium was detected up to 5 minutes after heating regardless of the strain type. The 4b strain was detected at 10 minutes after heating in the samples containing the curing salts and the 1/2c strain at 20 minutes after heating in the samples without the salt addition. However, none of the strains survived after 30 minutes heating either with or without salt addition, indicating that the standard heating requirement is sufficient for pasteurization of L. monocytogenes.