A small charcuterie producer near Kraków, Poland, wants to cut the salt in her dry-cured sausage by a quarter for a retailer. The recipe has worked for decades: salt, nitrite curing salt, a starter culture, a few days of fermentation, then weeks of drying. Nobody ever wrote down why it is safe. Cut the salt and the sausage starts at a higher water activity, needs more drying to reach the same stability, and gives unwanted microbes a better chance in the first few days. That is hurdle technology at work: several modest barriers, none enough alone, that together keep a product safe.
In short
- Hurdle technology deliberately combines several preservation factors, each at a mild level, so that together they stop microbes growing with less damage to quality than one severe treatment.
- The concept was developed by the German scientist Lothar Leistner. Common hurdles are temperature, water activity (aw), acidity (pH), redox potential (Eh), preservatives and competitive microorganisms.
- Microbes spend energy keeping their internal conditions stable; several stresses at once can exhaust them, so a combination can achieve more than the sum of its parts.
- Salami, jam, mayonnaise, hard cheese and chilled ready meals are all hurdle products.
- Removing or weakening one hurdle, for example by cutting salt or sugar, can make a safe product unsafe, so every reformulation needs fresh validation.
What is hurdle technology?
Hurdle technology is the deliberate use of several preservation factors in combination, each set at a level the food can tolerate, so that microorganisms cannot overcome all of them. Each factor is a hurdle. One very high hurdle, such as heating a sealed can until no microbe can grow in it at room temperature (commercial sterility), can work alone but changes the food a great deal. Several lower hurdles can give the same safety with a fresher result.
The idea was developed in the 1970s and 1980s by Lothar Leistner, a German scientist working on meat products. He used the image of a hurdle race: a microbe that clears one barrier still has to clear the next. The “hurdle effect” describes what happens in a stable food; “hurdle technology” is the deliberate design of it. Our Introduction to Food Technology course introduces hurdles alongside heat, cold, drying, acidity, preservatives and fermentation.
Which hurdles are used to preserve food?
The main hurdles are high temperature, low temperature, water activity, acidity, redox potential, preservatives, competitive microorganisms and the packaging atmosphere. Leistner and later researchers described many more.
Water activity (aw) measures how much of the water in a food is available to microbes, from 0 to 1.0. Redox potential (Eh), measured in millivolts, describes how oxidising or reducing the conditions in a food are; little oxygen means a low Eh.
| Hurdle | How it holds microbes back | Examples |
|---|---|---|
| High temperature | Mild heat kills vegetative (actively growing) cells; severe heat also destroys spores, the tough dormant forms of some bacteria | Pasteurised milk, cooked ham |
| Low temperature | Slows growth; chilling at 5 °C or below, freezing at −18 °C or below stops it | Chilled ready meals, frozen fish |
| Low water activity | Water bound by salt or sugar, or removed by drying, is unavailable to microbes | Jam, dried fruit, salami |
| Low pH | Acid stresses cells; at pH 4.6 or below C. botulinum cannot grow | Pickles, yoghurt, mayonnaise |
| Low redox potential | Holds back aerobes such as moulds and pseudomonads, but not anaerobes | Vacuum-packed cooked meats |
| Preservatives | Nitrite, sorbate, benzoate, propionate, sulphites, smoke, vinegar | Cured meats, soft drinks, bread |
| Competitive flora | Lactic acid bacteria use up nutrients and produce acid and other inhibitory compounds | Salami, sauerkraut, cheese |
| Packaging atmosphere | Carbon dioxide inhibits many aerobic spoilage bacteria and moulds | Fresh pasta in modified atmosphere |
Why does combining hurdles work?
Combining hurdles works because microbes must keep their internal conditions stable, a state called homeostasis, and every stress costs them energy. A bacterium in an acidic food pumps out protons to keep its internal pH near neutral. In a salty or sugary food it accumulates compatible solutes to balance the osmotic pressure. Under several stresses at once the cell can run out of energy, stop growing and die, which Leistner called metabolic exhaustion.
Hurdles that hit different targets in the cell, such as the membrane, enzymes and internal pH, work better together than two that hit the same target. Microbes also adapt: cells exposed to mild acid or heat can become more resistant to later stresses, so a slow build-up of mild stress can toughen the survivors.
Published growth limits are measured with every other condition favourable. The table shows approximate values for some key pathogens.
| Organism | Minimum growth temperature | Minimum pH | Minimum aw |
|---|---|---|---|
| Clostridium botulinum, proteolytic | about 10 °C | 4.6 | 0.94 |
| Clostridium botulinum, non-proteolytic | about 3 °C | 5.0 | 0.97 |
| Listeria monocytogenes | slightly below 0 °C | about 4.4 | about 0.92 |
| Salmonella | about 5 °C | about 3.8 | about 0.94 |
| Staphylococcus aureus | about 7 °C | about 4.0 | about 0.86 (0.83 in air) |
In a real food, several factors each set some way inside these limits can together slow growth sharply or stop it. That is why combinations must be tested rather than assumed.
How does hurdle technology work in salami and jam?
Salami and jam show two classic patterns: hurdles that act in sequence, and a few strong hurdles acting together from the start. Leistner’s best-known example is fermented raw sausage such as salami:
- Salt and nitrite are added at the start. Salt lowers aw slightly; nitrite inhibits Salmonella and other bacteria in the first days, helps control C. botulinum and fixes the cured colour.
- Redox potential falls as the meat and microbes use up the remaining oxygen, which holds back aerobic spoilage bacteria and favours lactic acid bacteria.
- Lactic acid bacteria, usually added as a starter culture, multiply and outcompete other organisms.
- The pH falls as they ferment added sugar to lactic acid. Fast acidification matters: if it is slow, Staphylococcus aureus can multiply and form toxin, and drying does not remove that toxin.
- Water activity falls during drying and ripening and becomes the main long-term hurdle. In some long-ripened types the pH rises again and nitrite is used up, so low aw is what keeps the finished sausage stable.
Traditional full-sugar jam uses a few strong hurdles at once. Sugar, typically about 60 to 65 % soluble solids, lowers aw below the level most bacteria need (about 0.91). Fruit acid, often topped up with citric acid, gives a pH of roughly 3 to 3.5, which high-methoxyl pectin also needs to set. Boiling kills vegetative microbes, and hot filling into a sealed jar treats the jar and lid. Sugar-tolerant yeasts and moulds, some able to grow down to an aw of about 0.61, are what remain, which is why opened jam eventually goes mouldy.
| Product | Main hurdles | Weak point to watch |
|---|---|---|
| Mayonnaise | Low pH from vinegar or lemon juice, salt, sometimes sorbate, sealed jar | Too little acid |
| Hard cheese | Acid from starter cultures, salt, low moisture, competitive flora | Slow acidification |
| Chilled ready meal | Cooking, chilling at 5 °C or below, sealed pack, short shelf life | Temperature abuse |
| Fresh pasta in modified atmosphere | Slightly reduced aw, in-pack pasteurisation, carbon dioxide, chilling | Leaking packs |
How do you design a product with hurdle technology?
You design a hurdle product by identifying the organisms that matter, measuring each hurdle where it is weakest, closing the gaps and proving the result:
- List the pathogens and spoilage organisms relevant to the raw materials, process, pack and storage. For chilled, low-acid, packed foods this usually includes Listeria monocytogenes and non-proteolytic C. botulinum.
- Fix the storage conditions, the shelf life and the worst realistic abuse, such as a warm home fridge.
- Measure pH and aw in every component, not just the sauce. Large vegetable or meat pieces can take time to reach the pH of the liquid around them.
- Compare each value with the limits of the target organisms to see which hurdles are doing real work.
- Close the gaps with the hurdle that costs least in quality: a small pH drop, a preservative, a heat step, colder storage or a shorter life.
- Validate with predictive models, challenge tests (the organism is deliberately added under laboratory containment) and shelf-life trials, then set critical limits and monitor them under HACCP.
Worked example
A chilled, vacuum-packed cooked sauce has a pH of 5.6 and an aw of 0.98. It is cooked to kill vegetative pathogens, stored at 5 °C, and the retailer wants 28 days of shelf life.
Target: non-proteolytic C. botulinum. Its spores survive mild cooking, it grows without oxygen and it can grow from about 3 °C.
Check each hurdle: 5 °C is above its 3 °C minimum; pH 5.6 is above its limit of 5.0; aw 0.98 is above its limit of 0.97; the cook does not destroy its spores. No single hurdle stops it, and 28 days gives it time to grow.
Options: lower the pH to 5.0 or below throughout every component; lower aw to 0.97 or below; give a heat process of 90 °C for 10 minutes at the slowest-heating point, or equivalent, which targets a 6-log (million-fold) reduction of non-proteolytic spores; keep it at 3 °C or below throughout the chain; or prove another combination by challenge testing. Widely used national and industry guidance otherwise limits such products to 10 days at 3 to 8 °C. Rules differ, so check your national legislation.
Process lethality, non-thermal technologies and shelf-life engineering are covered in more depth in Food Technology for Industry Professionals.
What are the limits and risks of hurdle technology?
Hurdle technology fails when a hurdle is weaker than assumed, when one is removed, or when it is asked to do a job it cannot do. Common traps:
- Reformulation. Cutting salt, sugar or fat, or removing a preservative, can lower a hurdle without anyone noticing.
- Variation. A recipe averaging aw 0.94 may have batches at 0.96. Design for the worst batch.
- Temperature abuse. Above 5 °C the margin of a chilled hurdle product shrinks fast.
- Low oxygen can help pathogens. Vacuum packs and oil cover suppress the aerobic spoilage organisms that warn consumers, while anaerobes such as C. botulinum can still grow. Garlic in oil kept at room temperature is a known botulism risk.
- Dirty raw materials. Mild hurdles cope with low numbers of organisms, not heavy contamination.
- Toxins already formed. Hurdles stop growth; they do not remove toxin made earlier.
Frequently asked questions
Who developed hurdle technology?
The concept was developed by Lothar Leistner, a German scientist who worked on the microbiology of meat products, in the 1970s and 1980s, and was later extended with colleagues such as Grahame Gould and Leon Gorris. The principle itself is much older: salting, drying, smoking and fermenting all combine hurdles, and people used them long before anyone could explain why they worked.
What is the hurdle effect?
The hurdle effect is the observation that the stability and safety of a food depend on the combined action of all the hurdles in it. If microbes cannot overcome every hurdle, they cannot grow. Leistner also stressed that the hurdles needed depend on the starting load: a heavily contaminated raw material needs more or higher hurdles than a clean one, which is why good hygiene comes before any hurdle design.
Is hurdle technology the same as using preservatives?
No. A preservative is only one possible hurdle. Many hurdle foods contain no added preservative at all: traditional jam relies on sugar, acidity, heat and a sealed jar, and hard cheese relies on acid from starter cultures, salt, low moisture and competitive flora. Hurdle technology is about the whole combination, and it often lets a manufacturer reduce or remove preservatives by strengthening other hurdles.
Can hurdle technology make a low-acid food safe at room temperature?
Sometimes, but only with a validated combination. Sealed foods above pH 4.6 and water activity 0.85 that are stored at ambient temperature normally need a botulinum cook. Some shelf-stable products instead combine a mild heat treatment with reduced water activity or pH, but each combination must be proven by specialists, usually with challenge testing, and must meet the rules in your market. Check your national legislation before relying on one.
Why can reducing salt or sugar make a food unsafe?
Salt and sugar lower water activity, so cutting them raises it. A small rise can move a product past the growth limit of a pathogen or spoilage organism that was previously held back. Reduced-sugar jams, for example, usually need a preservative such as sorbate, refrigeration after opening, or both. Any reformulation, including a change of supplier for a key ingredient, should trigger a review of the hurdles and the shelf life.
Next step. Introduction to Food Technology explains why food goes off and how heat, cold, drying, acidity, preservatives, fermentation and packaging hold spoilage back, alone and combined as hurdles, before moving on to thermal processing, newer technologies and packaging. It ends with a proctored final assessment and an ASC certificate. You can also see all eleven food science and technology courses.
Sources. L. Leistner and G. W. Gould, Hurdle Technologies: Combination Treatments for Food Stability, Safety and Quality (Kluwer Academic/Plenum, 2002); ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens (Blackie Academic & Professional, 1996); James M. Jay, Martin J. Loessner and David A. Golden, Modern Food Microbiology, 7th edition (Springer, 2005); P. J. Fellows, Food Processing Technology: Principles and Practice, 5th edition (Woodhead Publishing, 2022); Codex Alimentarius Commission, Code of Hygienic Practice for Refrigerated Packaged Foods with Extended Shelf Life, CXC 46-1999.
This article is general guidance on hurdle technology and is not a substitute for the applicable standard, your national legislation or the advice of a qualified food safety professional.