The microorganisms present in a food should not be able to overcome (ie, leap over) the hurdles inherent in this food. This is illustrated by the so-called hurdle effect (1), which is of fundamental importance for the preservation of intermediate-moisture foods (2) and high-moisture foods (3), since the hurdles in a stable product control microbial spoilage and food poisoning as well as desired fermentation processes. Leistner and coworkers acknowledged that the hurdle effect illustrates only the well-known fact that complex interactions of temperature, water activity, acidity, redox potential, preservatives, and so on are significant for the microbial stability and safety of most foods (3). From an understanding of the hurdle effect, hurdle technology has been derived (4), which allows improvements of the microbial stability and safety of foods by deliberate and intentional combinations of hurdles. However, the sensory quality of a food also is determined by positive and negative hurdles. To secure the total quality of a food, the safety and the quality hurdles in a food must be kept in the optimal range (5). By an intelligent combination of hurdles, the microbial stability and safety as well as the sensory, nutritive, and economic properties of a food are secured. For the economy of a food item, it is for example, important how much water in a product is compatible with the microbial stability of this food.
Some examples will facilitate the understanding of the hurdle effect and the application of hurdle technology in food preservation. Figure 1 gives eight examples.
Example 1 represents a food that contains six hurdles: high temperature during processing (F value), low temperature during storage (t value), water activity (aw), acidity (pH), redox potential (Eh), and preservatives (pres.). The microorganisms present cannot overcome these hurdles, and thus the food is microbiologically stable and safe. However, example 1 is only a theoretical case, because all hurdles are of the same height, that is, have the same intensity, and this rarely occurs.
A more likely situation is presented in example 2, since the microbial stability of this product is based on hurdles of different intensity. In this particular product the main hurdles are aw and preservatives, whereas other less important hurdles are storage temperature, pH, and redox potential. These five hurdles are sufficient to inhibit the usual types and numbers of microorganisms associated with such a product.
If there are only a few microorganisms present ("at the start"), then a few or low hurdles are sufficient for the stability of the product (example 3). The superclean or aseptic packaging of perishable foods is based on this principle. On the other hand, as in example 4 if, due to bad hygienic conditions, too many undesirable microorganisms are initially present, even the usual hurdles inherent to a product may be unable to prevent spoilage or food poisoning. Example 5 is a food rich in nutrients and vitamins, which might foster the growth of microorganisms (this is called the booster effect or trampoline effect), and thus the hurdles in such a product must be enhanced, otherwise they will be overcome.
Example 6 illustrates the behavior of sublethally damaged organisms in food. If, for instance, bacterial spores in a food are damaged sublethally by heat, then vegetative cells derived from such spores lack "vitality" and therefore are inhibited by fewer or lower hurdles. In some foods the stability is achieved during processing by a sequence of hurdles, which are important in different stages of a fermentation or ripening process and lead to a stable final product. A sequence of hurdles operates in fermented sausages (example 7), and probably also in ripened cheeses or fermented vegetables, and so on. Finally, example 8 illustrates the possible synergistic effect of hurdles, which probably relates to a multitarget disturbance of the homeostasis of the microorganisms in foods, which will be discussed subsequently.
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