By Arup Kumar Das and
Kapil Deb Nath


The combined use of several preservation methods, possibly physical and chemical, or a combination of different preservatives is an age-old practice. It has been commonly applied by the food industry to ensure food safety and stability. In smoked products, for example, combination treatment includes heat, reduced moisture content and antimicrobial chemicals deposited from the smoke onto the surface of the food. Some smoked foods may also be dipped or soaked in brine or rubbed with salt before smoking, to impregnate the flesh with salt and thereby adding a further preservative mechanism.

The microbial safety and stability of most of the food is based on an application of preservative factors called hurdles. Each hurdle implies putting microorganisms in a hostile environment, which inhibits their growth or causes their death (Leistner, 2000). Some of those hurdles have been empirically used for years to stabilise meat, fish, milk and vegetables. From an understanding of the effect of hurdles the technology has been derived with that name (Leistner, 1985). This has the goal not just to understand why a certain food is safe and stable, but to improve the microbial quality of the food by an optimisation and intelligent modification of the hurdles present. It employs the intelligent combination of different hurdles or preservation techniques to achieve multi-target, mild but reliable preservation effects. Hurdle technology has arisen in response to a number of developments. These are:
  1. Consumers’ demand for healthier foods that retain their original nutritional properties;
  2. The shift to ready-to-eat and convenience foods which require little further processing by consumers; and
  3. Consumers’ preference for more ‘natural’ food that requires less of processing and fewer chemical preservatives. Hurdle technology provides a framework for combining a number of milder preservation techniques to achieve an enhanced level of product safety and stability.
This sometimes leads to a completely different product with its own new taste characteristics. Examples of hurdles in marine products are salt (salted fish), smoke (cold or hot smoked fish), acids (marinated products, pickles), temperature (high or low), fermentative microorganisms (traditional Asian sauces) and more recently, redox potential (vacuum-packed products). These preservative factors have been studied for years, but a large amount of potential hurdles for food have been already described including organic acids, bacteriocins, chitosan, nitrate, lactoperoxidase, essential oil, modified atmosphere packaging as well as novel decontamination technologies such as microwave and radio frequency, ohmic and inductive heating, high pressure, pulsed electric field, high voltage arc discharge, pulsed light, oscillation magnetic field, ultraviolet light, ultrasound, X-ray, electrolyse NaCl water, ozone (Mahmoud et al., 2006). Hurdles that have a positive effect by inhibiting microorganisms may have a negative one on other parameters such as nutritional properties or sensory quality, depending on their intensity. As an example, salt content in food must be high enough to inhibit pathogens and spoilage microorganisms, but not so high to impair taste. In order to lower the preservative level, the hurdle technology concept has been developed (Leistner, 1985), consisting of using combined hurdles to establish an additive antimicrobial effect, and even sometimes a synergetic one, thus improving the safety and the sensory quality of food.

For fish products manufactured in industrialised countries, the hurdle technology has been identified as one of the most interest for two groups of products: convenience products based on traditional products, like rehydrated salt-cured or dried fish. The raw material is a preserved semi-finished product (PSFP) but as the preservative is removed during processing, surviving pathogens in the raw material may recover. Minimising the survival of pathogens in the PSFP is therefore, beside the hygienic process conditions, necessary to ensure product safety and lightly preserved fish products (LPFP) which are uncooked or mildly cooked products, with low level of preservatives (NaCl < 6% WP, pH >5), such as cold-smoked fish and slightly cooked shrimp.

LPFP are usually produced from fresh seafood. Their further processing involves one or a few additional steps that increase risk of cross contamination. The treatments are usually not sufficient to destroy pathogens, and, as several of these products are eaten raw, minimising the presence and preventing growth of pathogens is essential for the food safety. Some microorganisms that do not represent a health risk for consumer may sometimes be responsible for organoleptic damages such as off-odours and taste, pasty texture, visual defaults. Preventing the growth of those spoilers’ microorganisms is therefore also a challenge.

2.Principle of Hurdle Technology
There are many preservation methods used for making foods stable and safe, e.g., heating, chilling, freezing, freeze drying, drying, curing, salting, sugar addition, acidification, fermentation, smoking and oxygen removal. However, these processes are based on relatively few parameters or hurdles, i.e., high temperature (F value), low temperature (t value), water activity( aW), acidification(pH), redox potential( Eh), preservatives and competitive flora. In some of the preservation methods mentioned, these parameters are of major importance, while in others they are only secondary hurdles.

The critical values of these parameters for the death, survival or growth of micro-organisms occurring in foods have been determined in recent decades and are now the basis of food preservation. However, it must be kept in mind that the critical value of a particular parameter changes, if other preservative factors are present in the food. For instance, the heat resistance of bacteria increases at low aW and decreases in the presence of some preservatives, or a low Eh increases the inhibition of micro-organisms caused by a reduced aW.  The simultaneous effect of different preservative factors could be additive or even synergistic. Furthermore, as mentioned before, the microbial stability and safety of many foods is based on combined effects of hurdles. For instance, mildly heated canned foods (“half-preserved” or “three-quarter-preserved”) need refrigeration during storage, or fermented sausages are only stable and safe in both the aW and pH are in an appropriate range. Therefore, in food preservation, the combined effect of preservative factors must be taken into account.

3.Mechanism of hurdle technology
Microorganisms react homeostatically to stress factors. When their environment is disturbed by a stress factor, they usually react in ways that maintain some key element of their physiology constant. Micro-organisms undergo many important homeostatic reactions. Preservative factors functioning as hurdles can disturb one or more of the homeostasis mechanisms, thereby preventing micro-organisms from multiplying and causing them to remain inactive or even die. Therefore, food preservation is achieved by disturbing the homeostasis of micro-organisms. The best way to do this is to deliberately disturb several homeostasis mechanisms simultaneously, thus a combination of multiple hurdles (hurdle technology) could increase the effectiveness of food preservation. The success of hurdle technology depends on ensuring metabolic exhaustion. Most stress reactions of micro-organisms are active processes, and these often involve the expenditure of energy, e.g. to transport protons across the cell membrane, to maintain high cytoplasmic concentrations of ‘osmoregulatory’ or ‘compatible’ solutes. Restriction of the availability of energy is then a sensible target to pursue. This probably forms the basis of many of the successful, empirically derived, mild combination preservation procedures, exemplified by hurdle technology. As an example, if a food can be preserved by lowering the pH, then it is sensible also to include a weak acid preservative which will amplify the effect of the protons or to allow a milder, higher pH to be employed. It is sensible, if proton export is made more difficult by the additional requirement that cells be forced to regulate osmotic strength. Then, if the food can be enclosed in oxygen-free vacuum or modified atmosphere packaging, facultative anaerobes will be further energy-restricted at a time when the various stress and homeostatic reactions are demanding more energy, if growth is to proceed (Gould, 1995). However, environmental stresses can provide varying results because some bacteria may become more resistant or even more virulent under stresses through stress reactions such as synthesis of protective stress shock proteins (Leistner, 2000).

It has been reported that synthesis of protective stress shock proteins is induced by several stresses including those generated heat, pH, aw, ethanol, oxidative compounds, and starvation. And, although each stress has a different spectrum of anti-microbial action, those stress reactions might have a non-specific effect, since, due to a particular stress, microorganisms become also more tolerant to other stresses i.e. ‘cross-tolerance’ (Cheng et al., 2002). For instance, acid-shock or acid-adapted cells became tolerant to a range of other environmental stresses in several pathogenic bacteria including E. coli O157:H7, S. typhimurium, and L. monocytogenes (Leyer et al, 1995; Leyer and Jonson, 1993; Farber and Brown, 1990). Conversely, the heat shock response that follows mild heating can result in cells becoming more acid tolerance (Rowbury, 1995). Therefore, the various stress responses of microorganisms might hamper food preservation and could turn out to be problematic for the application of hurdle technology when hurdles are used consecutively. However, the use of different stresses at the same time (combination treatment) may also prevent the synthesis of those protective proteins because simultaneous exposure to different stresses will require energy-consuming synthesis of several or at least much more protective stress shock proteins which in turn may cause the microorganisms to become metabolically exhausted (Leistner, 2000). This anti-microbial action of combining hurdles is known as ‘multi-target preservation’ introduced by Leistner (1995). The concept of multitarget preservation increases the effectiveness of food preservation by using a combination of different hurdles which have different spectra of antimicrobial actions. It has been suspected for some time that combining different hurdles for good preservation might not have just an additive effect (explained below) on microbial stability, but they could act synergistically (Leistner, 1978). A synergistic effect (explained below) could be achieved if the hurdles in a food hit, at the same time, different targets (e.g. cell membrane, DNA, enzyme systems, pH, aw, Eh) within the microbial cells and thus disturb the homeostasis of the microorganisms present in several respects. Thus, repair of homeostasis as well as the activation of stress shock proteins become more difficult (Leistner, 1995). Therefore, simultaneously employing different hurdles in the preservation of a particular food should lead to optimal microbial stability. In addition, no one preservative factor is active against all the spoilage micro-organisms present in foods. An attempt is therefore made to compensate for this deficiency by combining various preservative factors having different spectra of action (Lück and Jager, 1997). Since from this multitargeted approach, hurdle technology could be more effective than single targeting, it allows the use of individual hurdles of lower intensity for improving product quality as well as for food preservation.

Table-1: Types of hurdles used for food preservation
(Ohlsson and Bengtsson, 2002)

Types of hurdles



Aseptic packaging, electromagnetic energy ( microwave, radio frequency, pulsed magnetic fields, high electric fields ), high temperatures ( blanching, pasteurization, sterilization, evaporation, extrusion, baking, frying), ionizing radiation, low temperature ( chilling, freezing ), modified atmospheres, packaging films ( including active packaging, edible coatings ), photodynamic inactivation, ultra-high temperature, ultrasonication, ultraviolet radiation.


Carbon dioxide, ethanol, lactic acid, lactoperoxidase, low pH, low Eh, low aw, Maillard reaction products, organic acids, oxygen, ozone, phenols, phosphates, salt, smoking, sodium nitrite/nitrate, sodium or potassium sulphite, spices and herbs, surface treatment agents.


Antibiotics, bacteriocins, competitive flora, protective cultures.

Table-3: Principle hurdles used for food preservation




High temperature



Low temperature


Chilling, freezing

Reduced water activity


Drying, curing, conserving

Increased acidity


Acid addition or formation

Reduced redox potential


Removal of oxygen or addition of ascorbate

Bio preservatives

Competitive flora such as microbial fermentation

Other preservatives

Sorbates, sulphites, nitrites

4.Basic Aspects
Food preservation implies putting micro-organisms in a hostile environment, in order to inhibit their growth or shorten their survival or cause their death.  The feasible responses of micro-organisms to this hostile environment determine whether they may grow or die. More research is needed in view of these responses; however, recent advances have been made by considering the homeostasis, metabolic exhaustion, and stress reactions of micro-organisms in relation to hurdle technology, as well as by introducing the novel concept of multi-target preservation for a gentle but to most effective preservation of hurdle-technology foods (Leistner, 1995a, b)

Homeostasis is the tendency towards uniformity and stability in the internal status of organisms. For instance, the maintenance of a defined pH is a prerequisite and feature of living cells and this applies to higher organisms as well as to micro-organisms (Haussinger, 1988). Much is already known about homeostasis in higher organisms at the molecular, sub-cellular, cellular, and systemic levels in the fields of pharmacology and medicine (Haussinger, 1988). This knowledge should be transferred to micro-organisms, important for the poisoning and spoilage of foods. In food preservation, the homeostasis of micro-organisms is a key phenomenon which deserves much attention, because if the homeostasis  of these micro-organisms is disturbed by preservative  factors (hurdles) in foods, they will not multiply, i.e., they remain in the lag-phase or even die, before  homeostasis is repaired (re-established). Therefore, food preservation is achieved by disturbing the homeostasis of microorganisms in a food temporarily or permanently. Gould (1988, 1995) was the first to draw attention to the interference by the food with the homeostasis of the micro-organisms present in this food, and more work in this direction is certainly warranted.

6.Metabolic Exhaustion
Another phenomenon of practical importance is metabolic exhaustion of micro-organisms, which could cause ‘autosterilisation’ of a food. This was first observed in experiments with mildly heated (950C core temperature) liver sausage adjusted to  different water activities by the addition of salt and  fat, and the product was inoculated with Clostridium  sporogenes and stored at 370C. Clostridial spores surviving the heat treatment vanished in the product during storage, if the products were stable (Leistner and Karan-Djurdjic, 1970). Later, this behaviour of Clostridium and Bacillus spores was regularly observed during storage of shelf stable meat products (SSP), if these products were stored at ambient temperature (Leistner, 1994b). The most likely explanation is that bacterial spores which survive the heat treatment are able to germinate in these foods under less favourable conditions than those under which vegetative bacteria are able to multiply (Leistner, 1992). Thus, the spore counts in stable hurdle-technology foods actually decrease during storage of the products, especially in unrefrigerated foods. Also during studies in our laboratory with Chinese dried meat products, the same behaviour of micro-organisms was observed. If these meats were contaminated after processing with staphylococci, salmonellae or yeasts, the counts of these micro-organisms on stable products decreased quite fast during unrefrigerated storage, especially on meats with a water activity close to the threshold for microbial growth. Latin American researchers (Alzamora et al., 1995; Tapia de Daza et al., 1996) observed the same phenomenon in studies with high-moisture fruit products, because the counts of a variety of bacteria, yeasts, and moulds which survived the mild heat treatment, decreased fast in the products during unrefrigerated storage, since the hurdles applied (pH, aw, sorbate, sulfite) did not allow growth.

A general explanation for this surprising behaviour might be that vegetative micro-organisms which cannot grow will die, and they die more quickly if the stability is close to the threshold for growth, storage temperature is elevated, anti-microbials are present, and the micro-organisms are sub-lethally injured (Leistner,1995a). Apparently, micro-organisms in stable hurdle-technology foods strain every possible repair mechanisms for their homeostasis to overcome the hostile environment. By doing this, they completely use up their energy and die, if they become metabolically exhausted. This leads to an auto-sterilisation of such foods (Leistner, 1995b). Due to auto-sterilisation hurdle-technology foods, which are microbiologically stable, become safer during storage, especially at ambient temperatures. For example, salmonellae that survive the ripening process in fermented sausages will vanish more quickly if the products are stored at ambient temperature, and they will survive longer and possibly cause food borne illness if the products are stored under refrigeration (Leistner, 1995a). It is also well known that salmonellae survive in mayonnaise at chill temperatures much better than at ambient temperatures. Unilever laboratories at Vlaardingen have confirmed metabolic exhaustion in water-in-oil emulsions (resembling margarine) inoculated with Listeria innocua. In these products listeria vanished faster at ambient temperature (250C) than under refrigeration (70C), at pH 4.25 > pH 4.3 > pH 6.0, in fine emulsions more quickly than in coarse emulsions, under anaerobic conditions, more quickly than under aerobic conditions. From these experiments it has been concluded that metabolic exhaustion is accelerated if more hurdles are present, and this might be caused by increasing energy demands to maintain internal homeostasis under stress conditions (P.F. Ter Steeg, personal communication, 1995). Thus, it could be concluded that refrigeration is not always beneficial for the microbial safety and stability of foods. However, this is only true if the hurdles present in a food inhibit the growth of microorganisms also without refrigeration. If this is not the case then refrigeration is beneficial. Certainly, the survival of micro-organisms in stable hurdle-technology foods is much shorter without refrigeration.

6.Stress Reactions
Some bacteria become more resistant or even more virulent under stress, since they generate stress shock proteins. The synthesis of protective stress shock proteins is induced by heat, pH, aw, ethanol, oxidative compounds, etc. as well as by starvation. Stress reactions might have a non-specific effect, since due to a particular stress micro-organisms become also more tolerant to other stresses, i.e. they acquire a ‘cross-tolerance’. The various responses of micro-organisms under stress might hamper food preservation and could turn out to be problematic for the application of hurdle technology. On the other hand, the activation of genes for the synthesis of stress shock proteins, which help organisms to cope with stress situations, should be more difficult if different stresses are received at the same time. Simultaneous exposure to different stresses will require energy-consuming synthesis of several or at least much more protective stress shock proteins, which in turn may cause the micro-organisms to become metabolically exhausted. Therefore, multi-target preservation of foods could be the key to avoiding synthesis of stress shock proteins, which otherwise could jeopardise the microbial stability and safety of hurdle-technology foods (Leistner, 1995b).

7.Multi target preservation
The concept of multi-target preservation of foods has been introduced recently by Leistner (1995a,b). Multi-target preservation of foods should be the ambitious goal for a gentle but most effective preservation of foods (Leistner, 1995b). It has been suspected for some time that different hurdles in a food might not have just an additive effect on microbial stability, but they could act synergistically (Leistner, 1978). A synergistic effect could be achieved if the hurdles in a food hit, at the same time, different targets (e.g., cell membrane, DNA, enzyme systems, pH, aw, Eh) within the microbial cells and thus disturb the homeostasis of the micro-organisms present in several respects. If so, the repair of homeostasis as well as the activation of stress shock proteins becomes more difficult (Leistner, 1995a). Therefore, employing simultaneously different hurdles in the preservation of a particular food should lead to optimal microbial stability. In practical terms, this could mean that it is more effective to employ different preservative factors (hurdles) of small intensity than one preservative factor of larger intensity, because different preservative factors might have a synergistic effect (Leistner, 1994a).

It is anticipated that the targets in micro-organisms of different preservative factors for foods will be elucidated, and that hurdles could then be grouped in classes according to their targets. A mild and effective preservation of foods, i.e., a synergistic effect of hurdles, is likely if the preservation measures are based on intelligent selection and combination of hurdles taken from different target classes (Leistner, 1995a). This approach is probably not only valid for traditional food-preservation procedures, but for modern processes such as food irradiation, ultrahigh pressure, pulsed technologies (Barbosa-Canovas et al., 1998) as well.

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About the authors:

The author, ARUP KUMAR DAS has completed Master of Fisheries Science in Fish Processing Technology from Central Agricultural University, Imphal  in the year 2012. He has the experience of working as Assistant Technology Manager and Block Technology Manager at Agricultural Technology Management Agency (ATMA), Department of Agriculture, Government of Assam.  Presently, Sri Das has been working as Senior Research Fellow at AICRP on Integrated Farming System, Assam Agricultural University, Jorhat, Assam. He has published a number of research articles in journals and magazine.

The author, Kapil Deb Nath has completed Master of Fisheries Science in Fish Processing Technology from Central Agricultural University, Imphal  in the year 2012. Presently, Sri Nath has been working as Subject Matter Specialist (Fisheries) at Krishi Vigyan Kendra, Assam Agricultural University, Silchar, Assam. He has published a number of training mannual, bulletin, research papers and articles  in journals and magazine.

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