Using hurdles for safer, fresher food

Using hurdles for safer, fresher food

Over-processing products can cause losses in taste, texture, and ultimately profit. Hurdle technology (also known as combination techniques or barrier technology) is a valuable tool in the fight against over-processing. It combines different preservation factors or techniques to achieve mild but reliable preservation.

Working together for food safety

Hurdle technology deliberately combines existing and new preservation techniques to establish a series of preservative factors that microorganisms are unable to overcome. These hurdles can include temperature, water activity, acidity, redox potential, preservatives, competitive organisms, vitamins, nutrients, and more.

How hurdles work

In order to thrive and multiply, microorganisms need to maintain homeostasis–a stable and balanced internal environment. Hurdles attempt to disturb one or more homeostasis mechanisms, causing the microbes to become inactive or even die. The best hurdles combine to disturb several homeostasis mechanisms simultaneously. This multi-targeted approach is more effective than single targeting and allows hurdles of lower intensity.

The following figure shows how hurdles work together to limit microbial growth.

Figure 1. Illustration of how processes can be combined to create hurdles


Figure 1 shows several examples of combined processes. Each of the illustrations shows, by means of dotted lines and arrows, whether or not the processes are effective in stopping microbial growth. In number 3, for example, temperature alone wasn’t an effective control, but water activity plus temperature combined to prevent the growth of microorganisms. In example 4, the hurdles were not enough to prevent microbial growth. In this instance, the temperature hurdle would have needed to be increased by refrigeration.

Cooperation or competition

The effect of hurdles on each other must also be considered. Sometimes a second agent will simply add to the effectiveness of the first. Sometimes the agents act synergistically, making their combined effect even greater. However, one agent may also antagonize or negatively impact the effectiveness of the other, either partially or completely offsetting the effectiveness of one or both agents. These effects must be carefully researched before agents are used in combination.

Figure 2. Types of hurdle effects

Water activity as a hurdle

Water activity (aw) is one of the most useful hurdles, both alone or in combination with another hurdle. There are specific water activities below which certain microbes can’t grow and a water activity below which no microbes proliferate. These microbial growth limits apply to every type of food and, in fact, to every porous product.

Water activity and pH: synergistic hurdles

Water activity and pH work synergistically, allowing you to control microbial growth to a degree not possible using one of these factors alone. This synergistic effect is part of the FDA’s definition of potentially hazardous foods.

Table A can be used to determine if a food which is heat-treated and packaged is a potentially hazardous food (PHF),  Non-PHF, or Requires Product Assessment. Food must meet cooking requirements of Food Code section 3-401.11 (no partial cooks) to eliminate vegetative pathogens. Spore forming pathogens are the only remaining biological hazards of concern. Food is packaged to prevent recontamination. Therefore, a higher pH & water activity can be safely tolerated.

aw ValuespH: 4.6 or lesspH: >4.6 - 5.6pH: >5.6
0.92 or lessNon-TCS food*Non-TCS foodNon-TCS food
>0.92 - 0.95Non-TCS foodNon-TCS foodPA**
>0.95Non-TCS foodPAPA
Table A. Interaction of pH and aw for control of spores in food heat-treated to destroy vegetative cells and subsequently packaged (*TCS means time/temperature control for safety, **PA means product assessment required)

Water activity predicts microbial growth

Find out how water activity prevents microbial growth and see a table of microbial growth limits for many common microorganisms.

Table B can be used to determine if a food which is not heat-treated or heat-treated but not packaged is PHF, Non-PHF, or Requires Product Assessment. Food not heat-treated may contain vegetative cells and pathogenic spores. Food that was heat-treated but not packaged may become re-contaminated. pH values considered in Table B must include 4.2 because Staphylococcus aureus can grow at that level.

aw ValuespH: <4.2pH: 4.2 - 4.6
pH: >4.6 - 5.0pH: >5.0
>0.88*Non-TCS foodNon-TCS foodNon-TCS foodNon-TCS food
0.88 - 0.90Non-TCS foodNon-TCS foodNon-TCS foodPA**
>0.90 - 0.92Non-TCS foodNon-TCS foodPAPA
>0.92Non-TCS foodPAPAPA
Table B. Interaction of pH and aw for control of vegetative cells and spores in food not heat-treated or heat-treated but not packaged (*TCS means time/temperature control for safety, **PA means product assessment required)

Controlling water activity

Need to reduce water activity in your product? Learn about the unique relationship between the product’s water content and water activity using a moisture sorption isotherm.

pH microbial growth limits

Like water activity, pH limits the growth of specific microorganisms in well-defined ways. All organisms prefer a neutral pH, but most can grow in more acidic environments with most microbial growth stopping at a pH of 5. Though 4.6 used to be considered the limit for all microbial growth, there are a few microorganisms that can tolerate a pH as low as 4.2.

Clostridium perfringens5.5 -
Vibrio vulnificus57.810.2
Racillus cereus4.96 - 78.8
Campylobacter spp.4.96.5 - 7.59
Shigella spp.4.99.3
Vibrio parahaemolyticus4.87.8 - 8.611
Clostridium botulinum toxin4.68.5
Clostridium botulinum growth4.68.5
Staphylococcus aureus growth46 - 710
Staphylococcus aureus toxin4.57 - 89.6
Enterohemorrhagic Escherichia coli4.46 - 79
Listeria monocytogenes4.3979.4
Salmonella spp4.217 - 7.59.5
Yersinia enterocolitica4.27.29.6
Table C. pH microbial growth limits for specific types of bacteria

pH is often controlled by adding acid, such as vinegar, lactic acid, citric acid, or fruit juice, directly to a product. It can also be reduced by the addition of naturally acidic ingredients like tomatoes or through fermentation, which uses lactic acid produced by a specific bacteria to lower the pH and prevent the growth of other microorganisms.

In the following table, you can see how the water activity and pH of several common foods work together to control microbial growth.  Strawberry preserves have a very high water activity, but citric acid causes the pH to be low enough to prevent microbial growth.  Mustard also has a very low pH and a high water activity. These two products are preserved by pH, not water activity. Maple syrup, on the other hand, is preserved by low water activity. The sugar in the syrup is a humectant that keeps the water activity low.

TypeWater ActivitypH
Strawberry Preserves0.98743.7
Yellow Mustard0.97453.6
Hot Sauce0.96423.6
Mediterranean Italian Dressing0.96283.8
Ranch Dressing0.95613.9
Asian Toasted Sesame Dressing0.94884.1
French Dressing0.93443.4
Barbecue Sauce0.93333.8
Table D. Water activity and pH of common foods

Mayonnaise has a very high water activity, but vinegar keeps its pH at 4.1. The low pH means it won’t support microbial growth. However, because it’s high in oil content, it is susceptible to lipid oxidation. Mayonnaise is refrigerated, not to prevent microbial growth, but rancidity. Interestingly, there isn’t any direct relationship between water activity and pH. When you add acid to a product to lower its pH, it has a minimal impact on water activity.

Figure 3. Water activity vs. pH

Fermented sausage: hurdles at work

Salami-type fermented sausages are stable at ambient temperature for extended periods. A sequence of hurdles is important at different stages of the ripening process. The first hurdles used are salt and nitrate, which inhibit many of the bacteria present. Other bacteria multiply at this stage, use up oxygen, and cause a drop in redox potential, which inhibits aerobic organisms and favors the selection of lactic-acid bacteria. These bacteria proliferate, cause product acidification, and increase the pH hurdle. During the long ripening process of salami, the initial hurdles become weaker. Nitrite is depleted. The number of lactic-acid bacteria decreases. Redox potential and pH increase. As the salami dries, however, water activity becomes the main hurdle and preserves the sausage. The curing process must be managed carefully when producing fermented sausages.

An increasing list of hurdles

About 50 different hurdles have been identified in food preservation. Apart from the most important and commonly used hurdles such as temperature, pH, and water activity, there are many other potentially valuable options. Examples include ultra-high pressure, mano-thermo-sonication, photodynamic inactivation, modified atmosphere packaging, edible coatings, ethanol, Maillard reaction products, and bacteriocins.

Water activity and pH: working together for product safety

In this webinar, Dr. Brady Carter explains the theory and measurement of water activity and pH. He also describes how these tools can effectively be used in concert to achieve the highest level of product safety.

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