Quality of Hamburger Meat - July-September 1997

July - September 1997


  • To determine the incidence of E coli in cooked hamburger patties available from ACT outlets compared with levels in the raw product;
  • Determine if enterohaemoragic serotypes of E. coli are present in the samples;
  • Compare the bacteria loading of cooked verse raw hamburger patties;
  • Determine the levels of sulphur dioxide preservative (Additive No. 220) in cooked and raw patties; and
  • To determine the source of meat being used, be it sausage mince and /or mince meat.


Outbreaks of Haemolytic Uraemic Syndrome (HUS) caused by E. coli serotype O157: H7 have occurred in the USA (ref_3) and recently in Japan. In 1995 a major outbreak of HUS occurred in South Australia. This was found to be caused by E. coli serotype O111 present in salami from a local manufacturer. This outbreak resulted in one death and several hospitalizations. These serotypes of E. coli have the capability of producing toxins and the ensuring illness may be asymptomatic or associated with a range of symptoms from diarrhoea, inflammation of the bowel with bloody diarrhoea, to HUS and possibly death. The term HUS is characterised by low circulating platelets (thrombocytopenia), anaemia (from intravascular coagulation and physical damage to red blood cells), and renal failure. The characteristic of these E. coli serotypes causing bloody diarrhoea has lead them to being grouped as Enterohaemoragic E. coli or EHECs. Research has showed that such serotypes of E. coli are present in comminuted meat products such as meat patties (ref_3) and there is a potential, in the case of partial cooking of these foods, for surviving pathogenic bacteria to cause food poisoning. E. coli O157 and O111 are particularly virulent, however there are many other strains which can also cause sporadic cases of serious food poisoning. In this regard, the addition of preservative such as sulphur dioxide, helps prevent bacteria from spoiling the food. However, they can not eliminate existing bacteria and thus, with improper cooking, the potential for food poisoning is ever present.


Microbiological Standards

There are no prescribed standards, however the Food Act 1992 states in no uncertain terms that food for sale must state correctly what it is, that it shall not cause injury, be unfit for consumption or be contaminated. Please refer to Section 23 of the Act for further details.

Microbiological Criteria

For this survey, the following bacteria were analysed from the hamburger patties:

  • E coli,
  • EHEC serotypes of E. coli,
  • Coagulase positive Staphylococcus,
  • Salmonella and
  • Standard Plate Count (SPC).

The SPC refers to the bacterial loading of the food and quantifies the number of aerobic bacteria which are responsible for spoilage and also includes potential illness causing types.

Tables 1 and 2 outline the acceptability criteria for raw and cooked hamburger meat used for this survey. The acceptability criteria were devised by the Health Protection Service (HPS) and based on ACT historical food data, W.A. microbiological guidelines (ref_5) and incorporate the level of concern based upon the food type and preservation procedures.

Table 1 Raw hamburger


Test Organism




E. coli


100 - 1000


Enterohaemoragic E. coli serotypes

Not detected



Coagulase positive Staphylococcus


100 - 1000


Salmonella spp

Not detected*

Detected* in a few samples

Detected* in many samples


Table 2 Cooked hamburger meat


Test Organism





<1 - 1000

1000- 10000


E. coli


2 - 100


Enterohaemoragic E. coli serotypes

Not detected



Coagulase positive Staphylococcus


50 - 100


Salmonella spp

Not detected*




# Units expressed in terms of colony forming units (cfu) per gram of food.

* Organism not detected in 25 grams of food.

Chemical Standards

Meat for hamburger patties is not specifically addressed by the Australia New Zealand Food Authority Food Standards Code (the Code) and there is no prescribed standard for this food type. Hamburger meat patties are formed from minced meat, sausage meat or more commonly, a mixture of the two. As individual food types, mince meat and sausage mince have prescribed standards for the addition of preservative. Mince meat is not permitted to contain preservative while sausage meat may contain a maximum of 500 mg/kg of sulphur dioxide. Consequently, hamburger meat patties made with compliant sausage meat should never exceed 500 mg/kg of preservative, and those made only with mince meat, should have no preservative.


The sampling was performed from mid July to the end of September and yielded 31 samples from different takeaway premises in Canberra. Each sample pair consisted of a raw and a freshly cooked meat patty.

Sample Collection Procedure

The initial methodology implemented was designed to ensure cooking techniques were not biased. Cooked hamburgers were collected by an Environmental Health Officer (EHO) who ordered the sample as a customer. The officer then identified themselves and requested a raw sample explaining the nature of the survey being undertaken. This procedure was amended after the first week of sampling as outlet proprietors and staff were upset at being taken by surprise and refused to supply the requested raw samples.

A subsequent methodology involved utilising two EHOs to purchase the samples, one purchased the cooked hamburger, the other identified themselves and purchased the uncooked meat.

After collection, the samples were iced and taken to the laboratory for bacterial and preservative testing. E. coli isolates which could not be serotyped by ACTGAL were forwarded to the Queensland Health Scientific Services.

At the conclusion of the survey, each participating proprietor was sent their results with an explanatory note attached.


Results Summary

The results are summarised in Table 3 below.

Table 3


Hamburger sample High Preservative levels High SPC E coli Coagulase positive Staphylococcus Salmonella Isolated
RAW Nil N/A 13/31 (41.9%) 2/31 (6.5%) Nil
COOKED Nil 3 Nil Nil Nil


Standard Plate Count (SPC) for cooked patties.

One sample pair of SPC results were not included in the survey as they were considered inaccurate. The remaining results shows that 3 (10%) cooked patty samples were found to have unacceptable SPC level and 9 (30%) were found to have a poor result.

Assessment of cooking of patties.


graph which compares the SPC of each paired raw and cooked patty

The graph above compares the SPC of each paired raw and cooked patty. An assessment of the effect of the individual cooking process can be gained from comparing the "log" reduction in the raw patty SPC with its cooked partner and Table 4 summarises the data in Graph 1 to give the percentage number of samples that fall into the stated log reduction categories. A one log reduction is equivalent to a 10 fold reduction in SPC for example. 1000 to 100 cfu/gram

Table 4 : SPC reduction raw versus cooked patty


4 log or greater reduction. 3 log reduction. 2 log reduction 1 log reduction
8 (26.6%) 9 (30%) 8 (26.6%) 5 (16.6%)


One can see from Table 4 and the above graph, there is wide variability in the cooking process ability to reduce the SPC. From a microbiological reduction point of view, approximately 40 % of patties appear to have not been cooked well enough to obtain a 2 log or 100 fold reduction in SPC. (See Discussion for more details concerning this.)

Escherichia coli.

Thirteen (41.9%) raw samples tested positive for E. coli at levels up to 49 cfu/gram while no cooked hamburger patty was found to be positive for E. coli. The raw patty consists of raw meat and it is not unexpected to find a low level of E. coli present in a considerable number of samples.

Enterohaemoragic Escherichia coli.(EHEC)

Possible E. coli EHEC isolates were screened against commercial polyvalent sera obtained from Seekin Denka and this screen resulted in six E. coli isolates being sent to Queensland for further analysis. Of the isolates sent to Queensland, 4 were not typeable by their scheme and therefore not likely to be EHECs The remaining two isolates were found to be serotypes O8 and found not to produce shiga-like toxin. The potentially hazardous EHEC serotypes O157 and O111 were not detected in either the raw or cooked samples.


Staphylococcus is a common food poisoning organism. Results from this survey are good as the bacteria was detected at low levels in only 2 raw samples, and not detected in any cooked samples.


The results for these tests is also reassuring, with no organisms detected in either the raw or cooked samples.

Sulphur dioxide preservative levels

No raw meat patties were found to have sulphur dioxide preservative levels in excess of the 500 mg/kg permitted in sausage meat. The results showed that upon cooking, the amount of preservative is reduced by approximately 30%.



% Sausage Meat

Results of preservative levels in raw meat patties indicates a wide range of formula being used in the market. Of the 31 samples of raw meat patties, 23 (74 %) had preservative levels below 100 mg/kg. This means that mince meat is the major component used by proprietors to prepare meat patties. This conclusion is also supported by anecdotal evidence received from the health officers who liaised with proprietors.

The survey provided opportunities to educate proprietors as evidenced by the following interesting example. The particular case involved a proprietor using 100% sausage meat for the patties. The proprietor mistakenly believed preservatives prevented patties from browning, which lead to complaints from customers. The proprietor was informed that the lack of browning was due to a high fat content of the sausage meat and was not associated with the preservative level. Evidence was provided when the results from this sample showed 300 mg/kg of preservative.


Standard Plate Count (SPC)

The SPC gives an indication of gross microbial content of the food and the likelihood of spoilage if the food is not maintained under appropriate conditions. Due to the nature of extra processing, for example mincing, hamburger patties are likely to have higher SPC than whole cuts of meat. There is no prescribed standard for SPC, however the literature states (ref_4) a level of 1,000 colony forming units is seen as acceptable for cooked comminuted meat products (like meat patties).

The small number of high SPC levels in the cooked samples could be attributed to either contact with other parts of the burger, the raw patty having a gross bacterial contamination or most probably, due to inadequate cooking. Graph 1 and Table 4 give a good indication as to the variability of the cooking process. Just over half (56.6%) of the patties received a cooking process capable of reducing the bacterial load by 2 logs (100 fold) or greater reduction in SPC. The 3 cooked samples with an unacceptably high SPC had log reductions of one, one and three.

E. coli

Traditionally E. coli is used as a indicator of faecal contamination from either slaughtering evisceration or from further process handling/storage. E. coli was not isolated from any of the cooked patties. The fact that no EHEC serotypes were isolated is also pleasing however this needs to be considered in light of the small sample numbers tested.

General Microbiological Findings

As reassuring as the overall results are, it is important to note the variability in the effective reduction of SPC through cooking, and therefore the possibility of under cooking, leading to the potential for "ready to eat" foods to contain illness causing bacteria. Thorough cooking is the only means to ensure a safe reduction of bacterial loading and to eliminate associated risks of illness. Another area requiring investigation is the handling and storage of the patties prior to cooking. For example, are they stored refrigerated and not longer than a certain period before being discarded? Also the possibility of cross contaminating other ready to eat food with pathogens is apparent, with E. coli frequently detected in raw meat patties. The overall results indicate no obvious correlation between a particular supplier of raw patties with poor results.

Preservative levels

Sulphur dioxide is a very common preservative in sausage meat and would therefore be found in meat patties composed from sausage mince either in part or whole. The reduction of preservative level due to the cooking process varied from approximately 10 % to 60 % of the initial level (refer Graph_2 above). This would seem to indicate variable cooking times. A comparison was made of the reduction loss of preservative with the corresponding SPC results to see if they correlated. However no evidence of correlation was apparent between the two sets of data, reinforcing the concept of contamination during the cooking, handling or storage processes.


Results from this survey indicate that there are some public health and safety issues concerning this food. While no cooked samples were found to contain known food poisoning bacteria, there is evidence, from the unacceptable SPC results, that under-cooking of the patties is occurring which poses a potential threat to public health. Results of the preservative testing identified that all samples contained acceptable levels of sulphur dioxide.


Refer to recommendations document.


Australia and New Zealand Food Authority, Food Standards Code, incorporating amendments up to and including Amendment 38, April 1998.

Food Act 1992 (ACT), reprinted as at 31 January 1996.

Foodborne Microorganisms of Public Health Significance, 5th Edition, AIFST (NSW) Branch Food Microbiology Group, 1997.

Microbial Ecology of Food, Volume 2 - Food Commodities International Commission on Microbial Specification for Foods, 1980.

Food Watch, Microbiological Guidelines for Ready-to-Eat Foods, , Western Australian Food Monitoring Program April 1999.


Sampling Officers: Andrew Kaye with assistance from James Edis, Craig Davies and Susan Payne

Sample analysis: Helen Kivela, Andrew Rigg, Fiona Wojtas and the microbiology unit

E coli serotyping: Denise Murphy of the Queensland Health Scientific Services, Centre for Public Health Services.

Report: Andrew Kaye, Simon Rockcliff and Simon Christen.