Low-dose irradiation as an additional treatment.

Ready-to-eat meals that are cooked and stored frozen at -18 C or less until consumed have become a well-established business for the food industry. In recent years, the market for chilled ready meals that are cooked, stored chilled at -3C and reheated prior to consumption has also shown substantial growth. This is largely due to the consumer perception that chilled foods are fresher than frozen foods. They are also considered to be more convenient than frozen foods, and more nutritious than fast foods.

The main problems and limitations with cook-chill ready meals include a relatively short shelf life, microbiological safety, reduced sensory quality and a decreased nutritive value. Various methods, such as modified atmosphere packaging, have been used to help preserve the sensory quality and to extend the shelf life of cook-chill foods. But many of these techniques are not designed to inactivate potential pathogens. Some studies have shown that cooked foods not subjected to strict temperature control during cooking and storage, or which are recontaminated after cooking, could represent a potential health hazard.

An alternative solution could be the use of low-dose irradiation to improve the microbiological safety and shelf life of chilled ready meals. Irradiation could be used as an additional preservation hurdle. Although extensive research has been carried out on the microbiological and organoleptic effects of irradiating individual uncooked items, little work has been reported on the irradiation of mixed food systems, such as ready meals.

With this in mind, researchers at The Queen's University of Belfast (Department of Food Science, Newforge Lane, Belfast, BT9 5PX, Northern Ireland, U.K.) undertook a study to determine the sensitivity of foodborne pathogens to irradiation in the components of a cooked, chilled roast beef meal. They also investigated the effects of irradiation and chilled storage on the microbiological and sensory quality of the meal and determined the effect of irradiation on the sensory quality of ready meal carrot and broccoli portions during chilled storage.

Investigators compared the sensitivity of five pathogens (S. typhimurium, L. monocytogenes, S. aureus, B. cereus and C. perfringens) to irradiation in a roast beef meal that included beef, gravy, cauliflower with white sauce, roast potatoes and mashed potatoes. The D10 values of the microbes ranged from 0.17 kGy to 0.65 kGy, depending on the organism and plating medium. When irradiated in gravy, the pathogens generally had lower D10 values than in any of the other components. In a subsequent study, the effect of irradiation (2 kGy and 3 kGy) on the microbiological and sensory quality of the roast beef meals was assessed at intervals during 15 days of storage at 2 C to 3 C.

A dose of 2 kGy improved the microbial quality of the meals, but sensory profiling techniques, using a trained panel of assessors, showed that there were significant differences between the non-irradiated and the irradiated meal components during storage. The changes were most apparent in the cauliflower and potato components, and most often occurred in color, appearance and textural attributes.

As these changes could have a limiting effect on the shelf life of the meals, the suitability of other vegetables for inclusion in such irradiated meals was investigated. Irradiation and chilled storage of carrots and broccoli did not significantly affect sensory quality when compared with the non-irradiated controls. Scientists determined that irradiation could be used to improve the microbiological safety and shelf life of certain chilled meals without significantly affecting their organoleptic quality.

Further information. E.M. Stewart; phone: [phone]; fax: [phone].