Combining MAP with other preservation techniques 289 The principle of combined preservation has been well described by Leistner et al, and is often referred to as hurdle technology (Leistner, 1992; Leistner, 1995b, Leistner, 2002). Whilst the hurdle concept is widely accepted as a food preservation strategy, its potential, using MAP, has still to be fully realised. The intelligent selection of hurdles in terms of the number required, the intensity of each and the sequence of applications to achieve a specified outcome are expected to have significant potential for the future(McMeekin and Ross, 2002) Homeostatsis is the tendency towards uniformity and stability in the internal status of living organisms. For instance, the maintenance of a defined ph within narrow limits is a prerequisite and feature of all living cells, and this applies to higher organisms, as well as microorganisms. In food preservation the homeostasis of microorganisms is a key phenomenon because if homeostasis of these organisms is disturbed by some preservation methods in foods, they will not multiply, i.e. they will remain in the lag phase or may even die before their homeostasis is re-established. Therefore, in actual fact, the preservation of food is achieved by disturbing, temporarily or permanently, the homeostasis of microorganisms in the food. In most foods microorganisms are able to operate homeostatically in order to react to the environmental stresses imposed by the applied preservation procedures. Applying additional preservation will inhibit repair of disturbed homeostasis and this requires extra energy from the microorganisms concerned In MA products energy depletion increases as the intensity or concentration of preservation is increased and the restriction of the energy supply will inhibit the repair mechanisms of the microbial cells' factors and leads to growth inhibition or death 14.2.1 Preservation focused on specific groups of microorganisms If the true potential of some of the emerging preservation technologies, combined with MAP is to be realised, it will be important to develop systematic kinetic data describing their efficiency against key target microorganisms. The type and numbers of microorganisms in the raw material have a direct influence on the effectiveness of MAP in inhibiting both spoilage organisms and pathogens. When adding extra preservation to packaged food, it is therefore mportant to understand which part of the bacterial population is inhibited and which is not. The shelf-life extension obtained with ma does not always give the same extension in safety. Pathogenic bacteria may gain advantage when the competing flora is inhibited, e.g., Listeria monocytogenes increased in numbers on raw chicken in 72.5: 22.5: 5(CO2: N2: O2)atmosphere at 4C, irrespective of a decrease in the aerobic spoilage flora(Wimpfheimer et al., 1990). Many MA packaged products of meat, vegetable and sea-food origin have common key target organisms. For chilled products psychrotrophic pathogens are the target. while in heat-treated ready meals spore-forming Clostridium and Bacillus species are the target organisms. There are five food-borne pathogenic bacteria known to be capable of growth below 5C: Bacillus cereus, non-proteolytic Clostridium botulinum type E, B and F(group D), Listeria monocytogenes,The principle of combined preservation has been well described by Leistner et al., and is often referred to as hurdle technology (Leistner, 1992; Leistner, 1995b; Leistner, 2002). Whilst the hurdle concept is widely accepted as a food preservation strategy, its potential, using MAP, has still to be fully realised. The intelligent selection of hurdles in terms of the number required, the intensity of each and the sequence of applications to achieve a specified outcome are expected to have significant potential for the future (McMeekin and Ross, 2002). Homeostatsis is the tendency towards uniformity and stability in the internal status of living organisms. For instance, the maintenance of a defined pH within narrow limits is a prerequisite and feature of all living cells, and this applies to higher organisms, as well as microorganisms. In food preservation the homeostasis of microorganisms is a key phenomenon because if homeostasis of these organisms is disturbed by some preservation methods in foods, they will not multiply, i.e. they will remain in the lag phase or may even die before their homeostasis is re-established. Therefore, in actual fact, the preservation of food is achieved by disturbing, temporarily or permanently, the homeostasis of microorganisms in the food. In most foods microorganisms are able to operate homeostatically in order to react to the environmental stresses imposed by the applied preservation procedures. Applying additional preservation will inhibit repair of disturbed homeostasis and this requires extra energy from the microorganisms concerned. In MA products energy depletion increases as the intensity or concentration of preservation is increased and the restriction of the energy supply will inhibit the repair mechanisms of the microbial cells’ factors and leads to growth inhibition or death. 14.2.1 Preservation focused on specific groups of microorganisms If the true potential of some of the emerging preservation technologies, combined with MAP is to be realised, it will be important to develop systematic, kinetic data describing their efficiency against key target microorganisms. The type and numbers of microorganisms in the raw material have a direct influence on the effectiveness of MAP in inhibiting both spoilage organisms and pathogens. When adding extra preservation to packaged food, it is therefore important to understand which part of the bacterial population is inhibited and which is not. The shelf-life extension obtained with MA does not always give the same extension in safety. Pathogenic bacteria may gain advantage when the competing flora is inhibited, e.g., Listeria monocytogenes increased in numbers on raw chicken in 72.5:22.5:5 (CO2:N2:O2) atmosphere at 4ºC, irrespective of a decrease in the aerobic spoilage flora (Wimpfheimer et al., 1990). Many MA packaged products of meat, vegetable and sea-food origin have common key target organisms. For chilled products psychrotrophic pathogens are the target, while in heat-treated ready meals spore-forming Clostridium and Bacillus species are the target organisms. There are five food-borne pathogenic bacteria known to be capable of growth below 5ºC: Bacillus cereus, non-proteolytic Clostridium botulinum type E, B and F (group II), Listeria monocytogenes, Combining MAP with other preservation techniques 289