Provisional Report – 09/05/96

The purpose of this study was to establish the microbiological quality of cooked ready-to-eat burgers from burger bars, other take-aways, restaurants, cafes, public houses and mobile vendors in the United Kingdom and to determine whether ready-to-eat burgers at the point of sale were fit for human consumption and of the quality demanded by the purchaser. A further aim of the study was to make appropriate recommendations to food enforcement officers to assist them in their task of ensuring food safety. The survey was carried out during January and February 1996. Samples were collected by Local Authority Food Enforcement officers to assist them in their task of ensuring food safety. The survey was carried out during January and February 1996. Samples were collected by Local Authority Food Enforcement Officers and each sample had an aerobic plate count, and was examined for the presence of coliforms, Escherichia coli, Staphylococcus aureus, Clostridium perfringens and Salmonella species.

A total of 3440 samples were collected from retail outlets and investigated in 49 PHLS laboratories and 6 other laboratories, including those in Scotland and Northern Ireland. These laboratories represented 50 food liaison groups and the 10 PHLS laboratory groups. Approximately equal proportions (15-21%) of samples were collected from burger bars, other take-away outlets, restaurants, cafes and mobile vendors. The majority of burgers (87%) were meat based, 6% were vegetarian. Most burgers (52%) were cooked on a griddle, 18% were grilled. The majority of samples (66%) were fully frozen before cooking. There were a wide variety of methods to determine whether the cooking process was adequate. Half the burgers sampled had been produced by wholesalers and 34% were produced by a company’s national supplier.

When all the microbiological parameters were combined 3164 (92%) of burgers sampled were satisfactory according to the Microbiological Guidelines for Ready-To-Eat Foods published by the PHLS, Gilbert et al, 1996. A further 212 (6%) were of borderline microbiological quality, 53 (2%) were of an unsatisfactory microbiological quality and 1 sample (<1%) was unacceptable. Aerobic plate counts in excess of 10⁴ per gram accounted for 49/53 of the unsatisfactory samples. Three samples contained coliforms in excess of 10² per gram. No sample contained E. coli in excess of 10² per gram and all were satisfactory or borderline with respect to E. coli. No burger contained S. aureus in excess of 10² per gram and all were satisfacotory or borderline for this parameter. Clostridium perfringens in excess of 10² was isolated from six burgers and these were therefore of unsatisfactory quality. Salmonella was isolated from one sample which was therefore unacceptable/potentially hazardous according to the guidelines for ready to eat foods.

Aerobic plate counts were in general higher for fresh burgers than for those which were frozen or tinned. Meat based burgers made on the premises were less satisfactory than those produced at wholesale or by major distribution chains. In general the aerobic plate counts of vegetarian burgers were higher than those with a meat composition. The best microbiological quality was obtained from burgers which were served immediately or from a bain marie or hot cabinet.

Cooked burgers sampled at the point of sale are in general of high microbiological quality. The isolation of Staphylococcus aureus in small numbers from a small proportion of the samples tested (9/3440) is an indication of handling contamination, probably post cooking. The isolation of Clostridium perfringens in excess of 10² per gram from 6/3440 samples indicates that the cooking process was inadequate to eradicate these organisms from previously contaminated burgers. This is not surprising given the heat resistance inherent in these spore-bearing gram positive organisms and possibly suggests unacceptable levels of contamination of the ingredients used to prepare burgers.

Gilbert RJ, de Louvois J, Donovan T, Nichols G, Peen RN et al.

PHLS Microbiology Digest 1996 13:1; 41-43

For details of the subsequent burger survey carried out during 1999 .