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SCIENTIFIC REVIEW Table 1: Examples of food-related hurdles (adapted from [19]) Hurdles Physical Examples Thermal (high, low temperature) Non-thermal (e.g. irradiation, ultrahigh pressure, pulsed electric and magnetic fields, mano-thermo- sonication, etc.) Packaging (films, edible coatings, modified atmospheres, aseptic packaging) Physicochemical Microbial Addition/alteration of: water activity (aw), pH, redox potential (Eh), salt, nitrites, nitrates, oxygen, carbon dioxide, ozone, smoke, ethanol, acids, spices, herbs, essential oils, lysozyme, Maillard browning compounds, etc. Background microflora, protective cultures, bacteriophages, bacteriocins their applied magnitude. Theoretically, applying more hurdles will produce a greater additive effect, but only if these hurdles have different modes of action capable of affecting, for example, microbial DNA, cell membranes and their ability to respond and adapt to environmental changes (termed homeostasis) [18]. Taking aim at multiple targets also improves the food’s capacity to control a more diverse range of different microbes. For a microbe to scale such a wall requires various physiological responses and modifications to occur which expend cellular energy [16]. Ideally therefore, a multiplehurdle-preserved food will deplete this energy to cause ‘metabolic exhaustion’. Even under conditions where no growth can occur, the greater the energy needed for the cell to survive, the faster the rate of death among the pathogen population. If initial microbial numbers are low, or sub-lethal injury has occurred as a result of a previous exposure or treatment, scaling this wall will be particularly difficult. If, on the other hand, high contamination levels are present, getting over the wall may be a slightly easier proposition. Whether genuinely synergistic relationships occur (as suggested by Leistner) to boost the additive effects of each hurdle is debatable. The growing list of potential hurdles that can be employed to preserve foods is summarised in Table 1. These are generally considered in three main groups - physical, physicochemical and microbial. Many, including heating, drying, salt and sugar addition, acidification, smoking, etc. have been used in food preservation for centuries but have typically been the major hurdle. It should of course be pointed out that producing a microbiologically stable food (i.e. where microbes are unable to proliferate) does not necessarily equate to a safe food. This will only occur when high quality (ideally pathogen-free) raw materials are used, or alternatively when any contamination present is below the threshold for disease to occur which in practice may be difficult to reliably achieve. Hurdles such as thermal and nonthermal processes capable of inactivating pathogens therefore play an important role in maintaining the safety of foods, particularly those of animal origin, and extending their shelf life. When applied in combination with other factors, often only sub-optimal levels of a hurdle may be necessary to create the desired effect. This offers a number of advantages, particularly in terms of the development of ‘consumer-friendly’ foods low in, for example, chemical preservatives, without radical changes in sensory characteristics. Additionally, eliminating the need for refrigeration or frozen storage and reducing the severity of thermal processes, potentially offer the additional bonus of cost savings as a result of lower energy requirements [18]. This has been exploited in particular by developing countries in Asia to manufacture shelf-stable yet safe traditional meat products [3, 13]. 16 Food New Zealand
Table 2: Examples of the diversity of foods preserved by hurdle technology Food Grated fresh coconut Shelf-stable seaweed Hurdles employed Refrigeration, sodium chloride, citric acid, sodium citrate, butylated hydroxyanisole Pasteurisation, water activity, pH, potassium sorbate SCIENTIFIC REVIEW Steamed bread (dumpling-like) Tortellini Fruit Shelf-stable guacamole Gnocchi Shelf-stable military bread Canned peas Fish Pickled vegetables Water activity (glycerol), mild heat treatment, packaging Water activity, mild heating, modified atmosphere packaging Mild heat treatment, pH, water activity, sorbate, sulphite Acids, antimicrobials, antibrowning agents, vacuum packaging, high pressure, mild heat processing Sorbate, lactic/citric acids, temperature Water activity and pH Heat treatment, pH and nisin Superchilling (core -1.5°C), natural additives, modified atmosphere packaging, refrigeration Acetic acid, salt, heat treatment While foods of animal origin have been the main focus of hurdle technology (and will likewise be in this review), a large variety of different food products has been developed based on this multifactorial preservation approach (Table 2). A couple of recent publications have made significant contributions to advancing the understanding of the mathematics behind hurdle technology. The gamma hypothesis suggests that the effects of different antimicrobial factors are independent and do not act in a synergistic fashion, and the authors analysis of the data tends to support this [1]. While some publications report significant statistical interactions, this paper cautions that statistical interactions may not be representative of biological interactions. Further experimental work is required to investigate this further. In the second paper evidence is presented to support the assertion that most of the inactivation occurring under growth-preventing conditions is determined by the temperature of incubation, as opposed to other factors such as pH and salt concentration [27]. These considerations are of importance to food safety as they indicate that the primary drivers behind pathogen reductions in foods that rely on hurdles for their safety are time and temperature. It is suggested that other factors, such as pH, may be manipulated to prevent growth but they have little effect on the rate of inactivation. Meat Products Shelf-stable salami-style sausages are probably the best recognised example of hurdle technology applied to the preservation of meats. Multiple hurdles are created during processing and ripening including the use of preservatives (salt, nitrites and nitrates), the addition of a competitive microflora (in the form of lactic acid bacteria cultures) which also lowers the pH (through the production of lactic acid) and the redox potential, and reduced water activity due to drying. Smoking may also be used in semi-dry sausage production, adding another hurdle. A fermented meat product manufactured in this way will be microbiologically stable when it has achieved both a pH value in the range of 4.6 - 5.3 and a water activity of
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