Microbiological and Chemical Quality of Cheese and Cheese Products - April-June 1998

April - June 1998

OBJECTIVE

  • To determine the incidence of Listeria monocytogenes and other pathogens in refrigerated soft ripened cheeses available in the ACT
  • Determine the compliance of Gouda and Cheddar cheese to Australia New Zealand Food Authority Food Standards Code microbiological requirements.
  • Determine the compliance of cheese products relative to preservative permissions.
  • To compare the results of this survey with the last survey conducted in 1996.

Background

Introduction

Cheese is a food that contains high concentrations of calcium and protein. It is comprised primarily of the major and minor components of milk including casein, fat, soluble and insoluble salts, colloidal material, lactose, whey proteins, vitamins, minerals and moisture. Although there are more than 400 varieties of cheese described, in essence there are only about 14 distinct types of cheeses (Ref_3).

Cheese manufacture

In cheese manufacture, the essential solids of milk are reduced to a concentrated form. The milk is coagulated, usually by the addition of rennet or acid, and a bacterial culture is added to promote the curdling process. Excess moisture is removed by heating or pressing and the product is shaped. Additional salt may be applied to the surface (if not already added to the curd in the vat) and the cheese is stored in curing rooms to ripen under precisely controlled conditions of temperature and humidity. Thus perishable milk is transformed into a less perishable and more concentrated food product. The final cheese produced will vary according to the kind and composition of milk used, the specific agents added, the method of manufacture, and the curing conditions (Ref_3).

Listeria

Listeriosis, or listeria infection, is an illness that may result from eating food contaminated with bacteria known as Listeria monocytogenes. Soft ripened cheeses, milk, raw vegetables, pate and other manufactured meats have been implicated in outbreaks of listeriosis in Europe and the USA. As L. monocytogenes can proliferate under refrigeration conditions it is extremely important that the organism does not enter the product, either during manufacture or via retail handling.

This organism is of particular concern to pregnant women as various serotypes of L. monocytogenes have the potential to cause spontaneous abortion and stillbirth. It can also cause serious illness to newborn babies, the elderly and people who are immuno- compromised (Ref_5).

Listeriosis is a relatively uncommon disease. For the period July 1995 to December 1997 there were 3 reported cases of Listeriosis in the ACT. The Health Protection Service (HPS) was not able to determine if food was implicated in these cases. The last survey conducted on cheese products occurred from July 1995 to June 1996. This surveyed 60 samples of various cheeses and cheese products and comparative information is provided in the discussion section of this document.

Microbiological Standards

The Australia New Zealand Food Authority Food Standards Code (the Code) states that certain high risk foods be free of L. monocytogenes.

Standard H9 - Cheese and Cheese Products clause 1(h) states:

Microbiological Standards. When examined by the methods prescribed by clause (22) of this standard-

(i) a cheese or cheese product which has -

(a) a moisture content equal to or greater than 40%; and

(b) a pH equal to or greater than 5.0.

must be free from L. monocytogenes in 25 g of the food.

(ii) Cheddar cheese and Gouda cheese must be free from -

(a) Escherichia coli in 0.01 g of the cheese

(b) Coagulase-positive Staphylococcus in 0.1 g of the cheese.

All the cheese and cheese product samples were tested for Escherichia coli in 0.01 g, Coagulase-positive Staphylococcus in 0.1 g, L. monocytogenes in 25 g and Salmonella spp in 25 g of the food. There are no prescribed standards for Salmonella spp in these foods and its detection in this food is deemed unacceptable as it would have major food safety concerns.

Chemical Standards

Nitrate

There are various additives permitted in cheese manufacture to control bacterial flora and thus improve fermentation and cheese quality. These include hydrogen peroxide, which is a germicidal for anaerobes, and potassium nitrate to control early gassing effects (Ref_3).

For most common cheese types, the Code permits up to 10 mg/kg of nitrate (from sodium and/or potassium nitrate). Certain other cheeses, including Amsterdam, Edam, Gouda, Saint-Paulin and Tilsiter are permitted to contain as much as 50 mg/kg of nitrate, provided that lysozyme has not been added.

Sorbic Acid

Sorbic acid is permitted in cheese, at levels below 3000 mg/kg, if wrapped in flexible packing material. Sorbic acid is also permitted in various processed cheese products including Cottage cheese, processed cheese, processed cheese foods, cheese spread and Club cheese. Permitted concentration levels vary from 500 mg/kg in Cottage cheese to 3000 mg/kg for most other types of processed cheese products (Ref_1). At these concentrations, sorbic acid acts as an efficient fungicide inhibiting the growth of moulds and fungi (Ref_3).

Sulphur Dioxide

Club cheese is permitted to contain 300 mg/kg or less of sulphur dioxide as a result of the added dried fruit. Essentially, sulphur dioxide (and the sulfites) are used in dried fruit (with up to 3 g/kg permitted) to maintain colour, inhibit microbial growth and act as an antioxidant (Ref_4).

SURVEY

During the period April to June 1998, 72 cheese samples were collected by ACTGAL staff and submitted for analysis. These consisted of 41 "soft ripened cheeses", 11 Cheddar, 11 Gouda and 9 other cheeses including Australian Fetta, Esprom, and Mascapone. Irrespective of the standard criteria, all cheeses were tested for E. coli, coagulase positive Staphylococcus, Salmonella and Listeria monocytogenes.

Chemical Testing

A random selection of 48 samples where subjected to chemical testing. This included analysis for moisture, calcium, protein, sorbic acid, nitrate levels, and sulphur dioxide for Club cheeses.

Dairy foods are known to be a good source of calcium in the diet and cheese is no exception. This was the first testing of calcium in cheese undertaken by the laboratory. This survey provided the opportunity for method development and gave insight into the nutritional quality of cheeses, especially processed cheeses and how they compared through calcium enrichment.

The following table shows the breakup of cheese samples for chemical testing:

 

Cheese Product No of samples
Fetta, Camembert, Gouda, Cheddar, Brie

25

Cottage cheese

3

Ricotta cheese

3

Cheese slices

6

Cheese spreads & Cream cheese

4

Club cheeses

7

 

MICROBIOLOGICAL RESULTS

Escherichia coli

E. coli was detected in 4 (5.5%) samples. This included bulk Ricotta and one sample of Australian Fetta. E. coli was not isolated for any Gouda or Cheddar sample and these cheeses were found to comply with the Code.

Coagulase-positive Staphylococcus

Coagulase positive Staphylococcus were detected in five (6.9%) cheese samples. Two of these samples were Gouda and cheddar. As single samples were collected, and the products were not resampled using the ANZFA FSC sampling plan, the products were considered to have complied with the Code.

Salmonella

Salmonella was not detected in any cheese or cheese product sample.

Listeria monocytogenes

Listeria monocytogenes was isolated from 7 (9.7%) cheese samples. This included Esprom (coagulase positive Staphylococcus also isolated), Australian Fetta (E. coli also isolated), Ricotta (3 of the 4 samples were also positive for E. coli), and Gouda. L. monocytogenes was isolated from 4 Ricotta samples, 3 of which were sampled from the same outlet on the same day. Indications are that these samples may have been cross contaminated.

CHEMICAL RESULTS

Sorbic Acid

Of the 48 samples tested, 18 (38 %) contained sorbic acid preservative with an average amount of 825 mg/kg, and ranged from 182 to 1886 mg/kg. Sorbic acid was detected in samples of cheese slices, Ricotta, Club and Cottage cheeses. All samples passed their respective standard.

5 samples detected low concentrations of benzoic acid, another preservative which is not normally permitted in cheese. Its presence may be explained by contamination effects associated with other foods used in cheese manufacture, for example salt. It may also be a contaminant from the manufacturing equipment which adds benzoic acid to other foods. The levels detected were below 30 mg/kg, and at these concentrations are not an effective preservative.

Nitrate/nitrite

40 samples were tested for nitrate/nitrite content with 13 (33%) found to contain detectable amounts. The average was 16.45 mg/kg with a range of 2.6 to 64.1 mg/kg. 5 samples had concentrations above the allowable 10 mg/kg including cheese slices, Club cheese and Cream cheese. The two highest results were 64.1 mg/kg from a sundried tomato cream cheese and 31.9 mg/kg from a chive and garlic club cheese. It would seem likely that the levels of nitrate/nitrite in these products are attributable to the dried vegetable matter. The remaining 3 samples had levels marginally over the 10 mg/kg level and are not considered a concern.

Calcium

All 48 samples were tested for calcium. The average amount found was 0.50 %, with a range of 0.08 to 0.85 %.

 

Cheese Type

Average

(%)

Range

(%)

Fetta, Camembert, Gouda, Cheddar, Brie

0.59

0.16 -0.84
Cottage cheese

0.17

0.08 - 0.25
Ricotta cheese

0.25

0.20 - 0.32
Cheese slices

0.67

0.50 - 0.85
Cheese spreads & Cream cheese

0.43

0.20 - 0.67
Club cheeses

0.30

0.15 - 0.67

 

It was interesting to note that cheese slices, very popular with young children, tended to have higher % of calcium than average.

The correlation of moisture and calcium content was also investigated. It is known for whole milk, approximately 40 % of the calcium exists in a soluble form and during cheese production, moisture is removed. Although moisture is removed approximately 80% of the original calcium is retained in the final cheese product.

The following graph shows there is little correlation between moisture and calcium levels (correlation coefficient 0.025). However, this does mean little calcium is being removed from the cheese in the drying process.

 

graph shows there is little correlation between moisture and calcium levels

Protein

All cheese samples were analysed for protein content. The average protein level was 16.1 %, with a range of 1.32 to 26.0 %.

 

Cheese Type Average

(%)

Range

(%)

Fetta, Camembert, Gouda, Cheddar, Brie 19.4 1.3 - 26.0
Cottage cheese 11.2 9.1 - 12.6
Ricotta cheese 7.6 6.3 - 10.2
Cheese slices 21.3 18.8 - 24.3
Cheese spreads & cream cheese 11.5 6.3 - 13.6
Club cheeses 9.1 7.3 - 15.9

 

Again, the results are interesting when comparing the different products, with cheese slices having the highest average protein content. The protein content of cheese consists mainly of casein (Ref_3). The low result of 1.3 % protein is from a Mascarpone cheese. It is believed that due to the high moisture content of the Cottage, Ricotta and cheese spreads, that there is an inversely proportional relationship to their low protein level. For Club cheeses, the low protein level is probably more influenced by the addition of other low protein foods into the final product.

DISCUSSION

Microbiological comparison with previous survey

In the last survey L. monocytogenes was detected in a total of 3 (5%) cheeses at the levels of 4, 23 and 43 cfu/g. Two of these samples came from the same shop on the same day suggesting cross contamination had occurred. This survey was not designed to determine the level of L. monocytogenes from cheeses as the Code only prescribes a presence/absence standard. L. monocytogenes was isolated from a total 9.7% of samples in this survey which could be regarded as a slight increase on the last surveys isolation rate. Evidence of possible cross contamination of cheese with L. monocytogenes has occurred in both surveys and needs to be addressed.

The last survey did not test for Salmonella in any cheese so no comparison can be made for this pathogen and Campylobacter was dropped from the testing regime for this survey as it was not detected in the previous survey. The previous survey did not test Cheddar and Gouda cheeses for compliance to the Code.

The presence of coagulase positive Staphylococcus in Gouda and cheddar samples needs further investigation.

Chemical testing

The chemical testing conducted proved to be very successful. Much of the data collected was found to be in agreement with referenced information for the macro-constituents protein, moisture and calcium. The calcium analysis was developed without complications and will have wider application for dairy product testing in the future. The sorbic acid preservative levels found were all acceptable and compliant with standard permissions. Nitrate/nitrites levels were also, on the whole, acceptable with only a handful of samples exceeding the standard permissible levels, but were not considered to pose a health concern.

Conclusion

This survey identified low level bacterial contamination of certain cheese products available on the ACT market and highlighted one possible case of cross-contamination which requires further investigation. Chemical testing showed good compliance of samples with preservative and nitrate/nitrite levels. Major constituent testing of protein and calcium reinforced the nutritious qualities of cheese and cheese products.

RECOMMENDATIONS

Refer to recommendations document.

BIBLIOGRAPHY

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.

Webb B.H., Johnson A.H. & Alford J.A. (editors), Fundamentals of Dairy Chemistry, Second Edition, The AVI Publishing Company Inc, 1974.

Fennema O.R (editor), Food Chemistry, Second edition, Marcel Dekker Inc, 1985.

ANZFA, For Pregnant Women - dietary advice on listeriosis, pamphlet.

ACKNOWLEDGMENTS

Sampling Officers: Louisa Bartolome and Chris Wixon

Sample analysis: Microbiology and Food Chemistry, ACT Government Analytical Laboratory.

Report: Simon Christen, Geoff Millard and Simon Rockliff