Microbial stability and the safety of most foods is based on a combination of several factors (hurdles), which must not be overcome by the microorganisms present.
Hurdles include high temperature during processing, low temperature during storage, water activity, acidity, redox potential, preservatives, competitive flora, vitamins and nutrients.
Complex interactions of these hurdles are significant for the microbial stability of foods.
Hurdle effects are of fundamental importance for the preservation of foods, since the hurdles in a stable product control microbial spoilage, food poisoning and the desired fermentation processes (Leistner, 1978; Leistner, et al., 1981), and they secure the sensory, nutritive and economic properties of food.
Traditional Italian mortadella, an emulsion-type sausage storable without refrigeration, is an example of the empirical application of hurdle technology in an industrialized country.
This meat product harbors viable spores of bacteria because of a mild heat process (172 degrees F core temperature). However, the bacilli and clostridia are inhibited in genuine mortadella by a decreased water activity (below 0.95), and this adjustment (by salt, sugar, milk powder and drying) was done in the past without knowledge of the reasons.
But by applying traditional recipes, the water activity has been surprisingly well adjusted to the desired level of 0.95 by hand-down experience (Leistner et al., 1979; 1980).
IMF benefits
The stability and safety of intermediate moisture food (IMF), which has water activity in the range of 0.90 to 0.60, is often based on hurdle technology (Leistner and RÅ¡del, 1976).
If the water activity of an IMF is quite low, then the addition of high amounts of humectants (such as salt or sugar) is required, and this could be undesirable from a sensory and nutritive point of view.
Therefore, efforts are being made to improve the quality of such foods by decreasing sugar and salt addition, as well as by increasing the moisture content and water activity without sacrificing the microbial stability and safety of the products if stored without refrigeration.
This may be achieved by an intelligent application of hurdles.
A case in point is a Chinese novel dried meat product developed on the basis of hurdle technology.
The traditional product (rougan) had water activity below 0.70. The new product (shafu), with superior sensory properties, has water activity of about 0.79 (Wang and Leistner, 1993), and nevertheless is stable without refrigeration.
Another example of hurdle technology applied to an intermediate moisture food relates to Chinese sausage, which is called lup cheong. Traditional Chinese sausage is storable without refrigeration since it has a water activity of about 0.75.
Lup cheong is a raw, but non-fermented sausage with a pH of about 5.9 because the Chinese dislike a sour taste in meat products.
Thus, lactic acid bacteria must not grow up in lup cheong, and this is achieved by a fast decrease of water activity (Leistner, 1987).
In Taiwan, a variety of lup cheong is preferred, which is soft and has a higher water content and water activity (about 0.94). Such products may be spoiled by lactic acid bacteria (sour taste), and may be poisoned by the growth of staphylococcus aureus.
In the German Meat Research Institute, the stabilization of Taiwanese lup cheong has been studied. By adding 3.5 percent sodium lactate and 0.1 percent sodium acetate, the Taiwanese sausage remained tasty and was rendered stable and safe-even if stored without refrigeration (J.C. Kow, J. Dresel and L. Leistner, 1992; unpublished data).
High-moisture foods
Traditional and novel high moisture foods (HMF), with a water activity above 0.90 and storable without refrigeration, are also on the rise.
Through a sequence of hurdles, the food poisoning and spoilage organisms are inhibited in a product like fermented sausage (salami). Important hurdles in the early stages of the ripening process of salami are nitrite and salt, which inhibit many of the bacteria in the batter.
Other bacteria multiply, use up oxygen and cause the redox potential of the product to decrease. This, in turn, enhances the redox potential hurdle, which inhibits aerobic organisms and favors the selection of lactic acid bacteria.
They are competitive flora and flourish, which causes acidification of the product, and an increase of the pH hurdle.
This condensed report was taken from a new 62-page booklet entitled "Food Design by Hurdle Technology and HACCP," written by Lothar Leistner and published by Adalbert-Raps-Foundation of Kulmbach, Germany.