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Processing and distribution of cook-chill foods

The May 1999 issue of Food Safety & Hygiene discussed the thermal processing of cook-chill foods and the need to select the appropriate target organism. Since that time the retail and food service markets for this product category have grown although the retail market in Australia has not developed to the same extent as that which exists in the UK and USA.

One contributory factor to this relatively slow development of retail products is the short shelf life which most products exhibit. This is viewed as a barrier to the wide distribution of the product and to the time retailers have to move product through their stores. So there is considerable commercial pressure to find ways to extend the shelf life while retaining the high quality, lightly processed image of the category.

As we noted in May 1999, these products have an excellent safety record both here and overseas. However, it is important to emphasise the technological constraints which must be faced in the production and distribution of these foods.

In an article titled Extending the Shelf life of Refrigerated Foods: For Better or Worse (Food Technology 1998, 52 2 p.20), Doyle wrote that 'stringent temperature control is the most critical factor for ensuring the safety and quality of refrigerated foods. The second most critical factor is an acceptable (preferably short) product shelf life. Processors who increase the shelf life of refrigerated foods without including effective barriers to microbial growth greatly increase their risk of selling unsafe or poor quality product'.

What are cook-chill foods?

Cook-chill foods can be defined as perishable foods which have undergone a mild heat or pasteurisation process and which, to extend the time during which they remain wholesome, are intended to be kept in the temperature range of 0-4 ° C. Products include pre-prepared meals, pasta, rice, soups and sauces. The products may be assembled after separate cooking of individual components, chilling and then packing in the final container. Alternatively, the components may be cooked individually, packaged, sealed and chilled in the final container. A third possibility is for meal components to be vacuum packed, and then to receive a pasteurisation process which gives the potential for a longer shelf life under chill storage conditions because of the reduced risk of post-process contamination (the sous-vide process).

Codes of Practice

In 1992 the Australian Quarantine and Inspection Service (AQIS) developed a Code of Hygienic Practice for Heat-treated Refrigerated Foods Packaged for Extended Shelf Life.

The AQIS Code recognises two categories of heat treated food packaged for extended life:

i) those which have not received a 6 D process for non-proteolytic C. botulinum and/or which have been cooked before packing and the filling operation was not aseptic. The Code recommends that these products must be stored below 3°C and if this temperature cannot be guaranteed then this process option must not be used. However, the Code is ambiguous in that it also states that if product is stored between 3° and 5° it should be consumed within ten days of manufacture.
ii) those which have received a 6 D process for non-proteolytic Clostridium botulinum and, if cooked before packing, the filling operation was aseptic. The Code recommends that these products be stored below 5°C and the shelf life be limited to 10 days. (If the temperature is between 5° and 10°C the shelf life must be <5 days.)

By inference, if 3°C can be assured, then the lesser process of a 6 D reduction in the more heat labile pathogen, Listeria monocytogenes, is regarded as the minimum process (70° C for 2 minutes or its equivalent).

These recommended shelf life limits of 5 and 10 days are often not sufficient to meet retail demands.

The Campden & Chorleywood Food Research Association's Guideline No. 11, 1996, The Code of Practice for the Manufacture of Vacuum and Modified Atmosphere Packaged Chilled Foods divides vacuum packed and modified atmosphere packed products into two shelf-life categories:
i) those with a shelf life of 10 days or less at >3° to 8° C.
ii) those with a shelf life of greater than 10 days at >3° to 8° C.

These two categories determine, for the purposes of that Code, the specific manufacturing and preservation factors required for safety of these products based on the potential for growth of psychrotrophic C. botulinum during storage. This organism is the most heat resistant, food poisoning bacterium able to grow at temperatures below 10°C as well as potentially the most dangerous.

Both the 1996 Campden Code and the AQIS Code are based on recommendations made in a 1992 Report of the UK Advisory Committee on Microbiological Safety of Food (ACMSF). That Report recommended that vacuum packaged products with an intended shelf life of more than 10 days from point of manufacture should contain one or more specific controlling factors as well as a chill temperature below 10°C. The 10 days is the ACMSF assessment of the minimum time at temperatures below 10°C, a realistic initial population of psychrotrophic C. botulinum would require to produce toxin.

The Report and the Campden Code of Practice suggest no specific limitation on shelf life with regard to botulism hazard for those products maintained at or below 3°C. This is done on the basis that while such temperatures might be maintained in factory storage, they can't be maintained with certainty during distribution, retail display and consumer storage.

Special attention is paid to vacuum packed products because C. botulinum is an anaerobe and its growth, if viable organisms are present, will be encouraged by vacuum packing. However most cook-chill products, if not vacuum-packed, are lip-sealed in plastic trays which are usually oxygen impermeable with only a small air-filled headspace that won't avoid anaerobic conditions within the container. In general, none of the factors listed above applies to all components of cook-chill meals with the exception of the opportunity to apply the recommended heat process.

If this minimum heat process is not applied, and it clearly is not in at least some products on the market, and a shelf life of more than 10 days in chill storage is being nominated, then the recommendations of both Codes of Practice discussed are not being met. In other words, an excessive reliance is being placed on chill storage and the integrity of the cold chain to present to the public a safe and attractive food.

In 1999 the European Chilled Food Federation set up a specialist working party to consider safety with respect to C. botulinum, of chilled foods that had been mildly heated in hermetically sealed packages.

The working party concluded that for such foods safety can be assured by

However this group did admit the possibility of the appropriate heat process being applied outside the final container followed by packaging without recontamination. There is therefore some scope for product development and longer shelf life investigations in this category provided packaging can be carried out in a manner which will not provide an opportunity for recontamination and particularly not with psychrotrophic C. botulinum.

The 10°C figure is used by the European Group because that is the temperature below which mesophilic strains of C. botulinum, the spores of which would not be destroyed by the 6 D process nominated, could grow. However as these products would be designed to enter the normal distribution cold chain, they should meet the temperature recommendations of the Australian Cold Chain of 'never warmer than 4°C.' These lower temperatures would be necessary in any case to maintain product quality.

A 6 D process is one which will bring about 6 decimal reductions in an initial microbial population.

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Prepared by Keith Richardson and Beverley George
Food Science Australia
PO Box 52, North Ryde 1670. Tel +61 2 9490 8397 Fax +61 2 9490 8499
Email enquiries@csiro.au