Shelf Stability Based on pH and aw
Potentially hazardous food (PHF/TCS food) is defined in terms of whether or not it requires time/temperature control for safety to limit pathogen growth or toxin formation.
Heat-Treated Foods:
Spore Forming Pathogens
Table A below applies to foods which have been heat-treated, either in the final package, or heat-treated and subsequently packaged in a way which prevents re-contamination (they must be protected from contamination in an area with limited access).
Unless contamination by vegetative cells occurs during packaging, only spore-forming bacteria are of concern in these foods and the requirements shown in Table 1 apply.
Clostridium botulinum requires a pH above 4.6 to grow and produce toxin, Clostridium perfringens requires a aw above 0.93, and Bacillus cereus requires a aw above 0.92 for growth.
The requirements displayed in Table A are based upon the pH and aw requirements of these sporeforming pathogens.
Not Heat-Treated Foods:
Vegetative Pathogens
Table B below applies to foods which have not been heat-treated, or have been heat-treated but are either not packaged or are packaged in a way that allows re-contamination with vegetative cells.
Staphylococcus aureus can grow at aw values above 0.85, but toxin production requires a aw value above 0.88. Furthermore, Listeria monocytogenes (pH 4.3), Salmonella spp. (4.2) and Yersinia enterocolitica (4.2) can all grow at low pH values.
The requirements displayed in Table B are based upon the pH and requirement of these non-sporeforming pathogens.
TABLE A
Interaction of pH and aw for Control of Spores in Food Heat-Treated to Destroy Vegetative Cells and Subsequently Packaged
pH Value
aW Value
≤ 4.6
> 4.6 to 5.6
> 5.6
≤ 0.92
non-PHF
non-PHF
non-PHF
> 0.92 to 0.95
non-PHF
non-PHF
PA
> 0.95
non-PHF
PA
PA
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
pH Value
aW Value
< 4.2
4.2 to 4.6
> 4.6 to 5.0
> 5.0
< 0.88
non-PHF
non-PHF
non-PHF
non-PHF
0.88 to 0.90
non-PHF
non-PHF
non-PHF
PA
> 0.90 to 0.92
non-PHF
non-PHF
PA
PA
> 0.92
non-PHF
PA
PA
PA
PHF means Potentially Hazardous Food
TCS Food means Time/Temperature Control For Safety
PA means Product Assessment Required
Microorganisms require a minimum pH and aw in order to grow. Furthermore, there are two types of microorganisms of concern: vegetative (alive) microorganisms and spore-forming microorganisms. Vegetative cells are more easily killed via cooking while only specialized high-pressure, high-temperature devices are able to kill spores, as in the case with hermetically sealed canned goods.
- The progressive growth of all foodborne pathogens is considered whether slow or rapid.
- The term does not include foods that do not support growth (non-PHF) but may contain a pathogenic microorganism or chemical or physical food safety hazard at a level sufficient to cause foodborne illness or injury. For example, Salmonella spp. may survive in many foods which are non-PHF, even though the product does not allow it to grow.
- The definition of PHF/TCS food takes into consideration pH, aw, pH and aw interaction, heat treatment, and packaging for a relatively simple determination of whether the food requires time/temperature control for safety.
Using the food’s known pH and/or aw values, position the food in the appropriate table.
- Choose the column under “pH values” that contains the pH value of the food in question.
- Choose the row under “aw values” that contains the aw value of the food in question.
- Note where the row and column intersect to identify whether the food is “non-PHF/non-TCS food” and therefore does not require time/temperature control, or whether further product assessment (PA) is required. Other factors such as redox potential, competitive microorganisms, salt content, or processing methods may allow the product to be held without time/temperature control but an inoculation study is required.
Therefore, if the food is packaged after heat treatment to destroy vegetative cells and subsequently packaged in a way that prevents re-contamination, higher ranges of pH and/or aw can be tolerated because remaining spore-forming bacteria are the only microbial hazards of concern. (While these foods will need to be cooled slightly to prevent condensation inside the package, they must be protected from contamination in an area with limited access and packaged before temperatures drop below 135°F (57°C)).
In some foods, it is possible that neither the pH value nor the aw value is low enough by itself to control or eliminate pathogen growth; however, the interaction of pH and aw may be able to accomplish it. This is an example of a hurdle technology. Hurdle technology involves several inhibitory factors being used together to control or eliminate pathogen growth, when they would otherwise be ineffective if used alone. When no other inhibitory factors are present and the pH and/or aw values are unable to control or eliminate bacterial pathogens which may be present, growth may occur and foodborne outbreaks result. Cut melons, cut tomatoes, and cut leafy greens are examples where intrinsic factors are unable to control bacterial growth once pathogens are exposed to the cellular fluids and nutrients after cutting.
A combination product is one in which there are two or more distinct food components and an interface between the two components which may have a different property than either of the components alone. Determine whether the food has distinct components such as pie with meringue topping, focaccia bread, meat salads, fettuccine alfredo with chicken, or does it have a uniform consistency such as gravies, puddings or sauces. In products with distinct components, the pH and/or aw at the interphase, or in the separate components is important in determining if the item is a PHF/TCS food.
Some bacteria can resort to a spore form as a method of surviving unfavorable conditions. Think of it like a hibernating bear, it’s still alive even though it’s not really doing anything. Bacterial spores are extremely resistant, surviving extreme dryness, subzero temperatures, as well as high temperatures. Once conditions become favorable for growth again (temperature, pH, aw) the spores will come back to life and begin multiplying. Some spore-forming bacteria, such as Clostridium can subsequently cause food poisoning.
Many products will qualify as a non-potentially hazardous food based upon pH alone or aw alone, forgoing the need to determine both parameters.
Water activity is reduced in a product via two methods. The first involves processing the product in a way which drives off water, such as cooking, baking or dehydration. The second method involves tying up the free water by the addition of solutes, usually sugars or sodium chloride (salt). This creates an imbalance in osmotic pressure which draws the water from the cells.
The following categories are types of products which typically have low aw values: nuts, dried fruits; powders; spices; seeds; cereals and grain products; candy; snack foods such as cookies, crackers, chips, snack bars; some baked goods.
On the other hand, products with naturally low pH values, such as salsas, and other acidified foods may qualify as non-potentially hazardous based upon pH alone. Some foods may include:
Sauces, such as BBQ sauces, jams and jellies, may fit in one category or another, or may have to be analyzed for both pH and aw to determine their status. Due to wide variance in product formulations, some sauces may or may not have a low aw and/or pH value depending upon their concentration of solutes, and pH lowering ingredients.
Combination Products It may be best to analyze components separately in some multi component products. For example the frosting of a doughnut may be non-potentially hazardous while the doughnut is not. MBL can help make the determination if the interphase or each component should be analyzed for pH and/or aw. |
Regulatory bodies such as the Colorado Department of Public Health and Environment, or the Tri-County Health Department typically require analyses for one sample from each of three separate batches of a standardize recipe.
The samples should be representative of real-world product. There can be no variations in the day-to-day preparation of the food in question with respect to maximum pH or aw.
Some products should be analyzed as quickly as possible (i.e. same day), such as is the case with acidified coked sushi rice.
We request you have your samples to us by 1 pm M-F so that we may analyze them the same day.
Results are typically sent the next business day.
| Test | Cost per Test | Cost for 3 Tests |
| pH | $20 | $60 |
| aw | $25 | $75 |
