Food Reseurch International 25 (1992) 15 l-1 58 APPLIED TECHNOLOGY Food preservation by combined methods L. Leistner Institute for Microbiology, Toxicology and Histology of the Federul Centre ,fkjr Meat Research, D-8650 Kuhnbach, Germany Foods preserved by combined methods remain stable and safe even without re- frigeration, and are high in sensory and nutritive properties due to the gentle processes applied. The concept is gaining ground in industrialized as well as in developing countries. Several topics will be discussed briefly: (1) water activity, (2) hurdle effect, (3) hurdle technology (4) shelf stable products. (5) intermediate moisture foods, and (6) perspectives. Keywords: combined methods, hurdle technology, shelf stable products. inter- mediate moisture food, water activity. 1 WATER ACTIVITY AND FOOD oratory has introduced a new instrument for the PRESERVATION accurate and quick measurement of u, in meats, which is based on the determination of the freez- As is well known, the stability and safety of food ing point (Rode1 et al., 1989). It is remarkable, that does improve if the water activity (a,) of the pro- with this instrument made by Nagy (Filderstadt, ducts decreases. Common methods for decreasing Germany) an a, measurement is done in about the a, of foods are drying, addition of salt, sugar or 15 min, and the same instrument can be employed polyols, and freezing. The a, of foods influences the for determining temperature, relative humidity, multiplication, metabolic activity, resistance and sur- pH and redox potential, of course, by using differ- vival of the organisms present (Leistner et al., 1981). ent sensors. Such multipurpose instruments are ap- If we want to intelligently influence the a, of propriate for quality assurance in food processing. foods, we have to know it. The a,,, of foods may be The stability and safety of many foods is not calculated by employing equations (Chirife et al., based soley on a,, but on the combined effects of 1980) or it is measured using suitable instruments. several factors. Therefore, the a, of foods should Several reliable instruments are today available, always be viewed in relation to other inherent fac- which measure a, by applying different principles. tors, and this is the topic of this paper. The mode We devised several years ago a simple hair hygro- of action of the combined factors used in food meter made by Lufft (Stuttgart, Germany) which is preservation should be studied, since they could reasonably priced and works quite well, if handled have an additive or even synergistic effect. with care (Rode1 et al., 1975). Widely used are elec- tric hygrometers made by Novasina or Rotronic (both Zurich, Switzerland), for measuring the a, 2 HURDLE EFFECT AND FOOD STABILITY of foods (Rode1 et al., 1979), these are more pre- cise, but also more expensive. Recently our lab- The hurdle effect is an illustration of the fact that in most foods several factors (hurdles) contribute This is an introductory lecture to a symposium on ‘Food to stability and safety. This hurdle effect is of fun- Preservation by Combined Methods’ held at the Annual Meeting of the Institute of Food Technologists, Dallas, damental importance for the preservation of food, Texas, l--S June, 1991. since the hurdles in a stable product control micro- Food Research International 0963-9969/92/$05.00 bial spoilage and food poisoning as well as the 0 1992 Canadian Institute of Food Science and Technology desirable fermentation. 151 152 L. Leistner There are many processes used for making foods organisms present cannot overcome (‘overjump’) stable and safe, e.g. heating, chilling, freezing, freeze these hurdles, thus the food is microbiologically drying, drying, curing, salting, sugar-addition, acidi- stable and safe. However, Example 1 is only a fication, fermentation, smoking or oxygen removal. theoretical case, because all hurdles are of the However, these many processes are based on rela- same height, i.e. have the same intensity. A more tively few parameters or hurdles, i.e. high temper- likely situation is presented in Example 2, since ature (F value), low temperature (t value), a,, pH, the microbial stability of this product is based on redox potential (Eh), preservatives, and compe- hurdles of different intensity. In this product the titive flora. In some of the preservation methods main hurdles are the a, and preservatives, while mentioned, these parameters are of major impor- other less important hurdles are storage tempera- tance, in others they are only secondary hurdles ture, pH and redox potential. These five hurdles (Leistner et al., 1981). are sufficient to inhibit the usual types and num- We introduced the hurdle effect some years ago bers of organisms associated with such a product. (Leistner, 1978) and it has since been modified If there are only a few microorganisms present at and extended several times (Leistner, 1986a, 1987). the start (Example 3), then a few or low hurdles The present concept is shown in Fig. 1, which gives are sufficient for the stability of the product. The eight examples. Example 1 illustrates the principle aseptic packaging of perishable foods is based on and represents a food which contains six hurdles this principle. On the other hand, as in Example 4, (i.e. F, t, a,, pH, E,, and preservatives). The micro- if due to bad hygienic conditions too many un- No. 2 No.5 Fig. 1. Illustration of the hurdle effect, using eight examples. Symbols have the following meaning: F, heating; t, chilling; a,, water activity; pH, acidification; I?,,, redox potential; pres., preservatives; K-F, competitive flora; V, vitamins; N, nutrients. Food preservation by combined methods 153 desirable organisms are initially present, even the can grow and due to the multiplication of these usual hurdles inherent in a product cannot prevent bacteria the redox potential of the product de- spoilage or food poisoning. creases, and this in turn enhances the E, hurdle, Examples 5 is a food superior in nutrients and which inhibits aerobic organisms and favours the vitamins, which foster the growth of microorgan- selection of the competitive flora, primarily lactic isms (‘trampoline effect’), and thus the hurdles in acid bacteria. Their growth and metabolic activity such products must be enhanced, otherwise they cause acidification of the product and thus an in- will be overcome. Example 6 illustrates the be- crease of the pH hurdle. This is of particular impor- haviour of sublethally damaged organisms in foods. tance for the microbial stability of quick ripened If, for instance, bacterial spores in meat products fermented sausages, which are not properly dried. are damaged sublethally by heat (as occurs in The hurdles of nitrite, E,, competitive flora and F-SSP, discussed later), then the vegetative cells pH diminish with time, because in ripened salami derived from such spores lack vitality, and there- the nitrite level and the count of lactic acid bac- fore are already inhibited by fewer or lower hurdles. teria decrease, while the E, and pH increase again. In some foods, such as fermented sausages and Only the a, is strengthened with time, and this raw hams, the microbial stability is achieved dur- hurdle is mainly responsible for the stability of ing processing by a sequence of hurdles. Example 7 long ripened fermented sausages. Certainly, also illustrates the sequence of hurdles in fermented in the processing and storage of some other foods, sausages, as will be discussed later. e.g. cheese, a sequence of hurdles is responsible Finally, Example 8 should illustrate an impor- for microbial stability, and it should be challeng- tant phenomenon, which deserves particular atten- ing to investigate this phenomenon in various foods. tion in foods preserved by combined methods, be- With fermented sausages we can differentiate in cause different hurdles in a food might not just Germany two groups; i.e. quick ripened products have an additive effect on stability, but could act and slow ripened products. Quick ripened pro- synergistically. A synergistic effect of hurdles is to ducts amount to about 80% of the production and be expected if the different factors (e.g. pH, cc,, the slow ripened products to only 20%. In quick E,,, preservatives) have different targets within the ripened products the a, is rather high, because microbial cell, and thus disturb the homoeostasis they still contain much water, and therefore are in several respects. This could make it difficult for less expensive. However, to compensate for this spoilage or food poisoning organisms to overcome high a, a low pH in such products is necessary the lag-phase, and if multiplication is delayed the for microbial stability. On the other hand, slow microorganisms eventually die. Therefore, employ- ripened products, which are more expensive due ing different hurdles in the preservation of a par- to the long drying period, have a low a,, and ticular food should be advantageous, because al- therefore these products can afford a rather high ready with gentle hurdles microbial stability could pH which makes them much more tasty. I mention be achieved. Certainly, the relationship between these differences between quick and slow ripened food preservation and the homeostasis of micro- salami, because they illustrate that the hurdles in a organisms deserves attention. food are somehow interchangeable. Emphasis could As Fig. 1 indicates, in fermented sausages we be given to different hurdles to achieve stability, with can distinguish five different hurdles which be- the consequence that the products have different come active and fade out in a sequence (Leistner, features related to their sensory properties and price. 1985, 1986b). These hurdles effectively inhibit food poisoning organisms (Salmonella ssp., Listeria mono- cytogenes, Staphylococcus aureus, Clostridium botu- 3 HURDLE TECHNOLOGY AND FOOD linum) as well as other bacteria which might cause DESIGN spoilage. On the other hand, this sequence of hurdles favours the selection of the desired competitive flora From the hurdle effect, the hurdle technology has (especially lactic acid bacteria), which contribute been derived (Leistner, 1985, 1986a), since an in- to the stability of the products.