ABSTRACT
Irradiation is a very effective preservation method that can improve the microbiological safety and quality of many foods. However, the dose which can be applied, and hence the extent of microbial kill, may be limited by undesirable changes in sensory quality including flavour, appearance and texture. This problem can be overcome by using irradiation in combination with other preservation techniques. The idea of superimposing a number of preservation methods has been described as the “Hurdle Concept”. It was originally used for the formulation of a number of shelf-stable meat products but the concept can be applied to a wide range of foods, particularly as minimally processed, low-additive products are perceived by consumers as being more “natural” and “healthy”. There are many examples of how the hurdle concept is used by the food industry to extend shelf-life and/or increase microbiological safety of foods as well as making them shelf-stable. This paper will describe a number of such applications involving the use of irradiation in combination with other preservation methods to enhance preservative action and/or reduce the severity of one or all the treatments. Some would argue that this approach is where the major benefits of food irradiation will eventually be seen.
One of the most common combination treatments is the use of irradiation in with refrigeration. For example, a dose of 2.5-3 kGy is sufficient to significantly reduce vegetative pathogens in poultry meat. Many spoilage organisms will also be killed, thus extending the shelf-life. However, the maximum potential of shelf-life extension will only be achieved if the meat is kept refrigerated after irradiation. This same principle also applies to milk pasteurisation, where the heat treatment is primarily aimed at inactivating vegetative pathogens and, as numbers of spoilage organisms are also reduced, the product has an increased shelf life. However, in order to achieve most benefit from the process the additional hurdle of refrigeration has to be used.
A combination of irradiation with mild heat has also been shown to be effective in controlling spoilage organisms and increasing the safety of certain foods. For example, an irradiation dose of 5 kGy is sufficient to sensitise clostridial spores to subsequent heating. This was found to be of value in heat-treated spiced meats, where the heat treatment required in the canning process could be reduced as the spores in the irradiated spices were heat sensitised. Irradiation can also sensitise vegetative cells to subsequent heating. This could be advantageous in irradiated foods, such as raw meats, which are cooked before consumption and in ready meals, which are reheated prior to consumption. Other physical treatments, such as modified atmosphere packaging can also act synergistically with irradiation to improve microbial kill.
72Successful combinations of chemical preservatives with irradiation have been reported by several authors. For example, the addition of sodium chloride or nitrite along with irradiation was found to delay or inhibit toxin formation by C. botulinum.
In conclusion, the preservative effects of radiation can often be advantageously combined with the effects of other physical or chemical agents. The resulting multi-hurdle treatments may involve synergistic or cumulative actions, leading to a decreased requirement for one or more of the agents. This in turn may result in improved sensory and microbiological quality of the products.
