One hundred and eighty-two raw, 112 pre-cooked and 750 cooked hamburgers composed mainly of beef or beef and pork were subjected to microbiological examination.
Raw hamburgers gave total bacterial counts from 105 to 108 per g, counts of Enterobacteriaceae from 104 to 108 per g, of Escherichia coli from 103 to 105, of group D streptococci from 102 to 104, of Staphylococcus aureus from 3 to 102 and of Clostridium perfringens less than 10 bacteria per g. Of the samples, 32 % contained salmonellas; the highest most probable number was 102 per g but most estimates were below 1 per g. Corresponding figures for the pre-cooked samples were 2–3 log cycles lower, and only one sample contained salmonella. Yersinia enterocolitica was not isolated from any raw or pre-cooked sample.
Three hundred and ninety-five of the cooked hamburgers were prepared by grilling raw hamburgers for between 2 and 5·5 min. These gave total bacterial counts from 105 to 107 per g, and counts of Enterobacteriaceae from 102 to 105 per g. Of the samples, 9·4 % contained salmonellas, always in numbers below 1 per g. The remaining 355 cooked hamburgers were prepared from samples pre-cooked for 10 min at 80 °C. Some were grilled and some fat fried. The total bacterial counts were from 103 to 105 per g, and counts of Enterobacteriaceae below 102 perg. Salmonellae, again in small numbers only, were recovered from 3·5 % of samples.
When hamburgers were artificially contaminated with Salmonella typhimurium it took 5·5 min on a commercial grill, 2·25 min frying in a frying pan and 1·75 min on a household grill to reliably reduce the salmonella count one hundredfold. This means that at many vending places hamburgers are often cooked for too short a time.
D-values were determined for S. typhimurium in hamburger meat at 50, 55, 60, 65 and 70 °C, these values were 7·1, 5·1, 1·2, 0·9 and 0·6 min respectively. It can be concluded that the heating action in the centre of the hamburgers will take place more slowly than in the hamburger as a whole, and that the time between cooking and consumption is very important in reducing the microbial load to acceptable levels.
Pre-cooking (10 min at 80 °C in a water bath) gives a reduction in the numbers of salmonella of about 4 × 103, after which cooking gives a further reduction as mentioned above.