Shelf Stability pH & aw

Microorganisms require a minimum pH and _w 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, _w, pH and _w 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.

1. Choose the column under “pH values” that contains the pH value of the food in question.
2. Choose the row under “aw values” that contains the aw value of the food in question.
3. 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.

Colorado Retail Food Establishment Rules and Regulation (6 CCR 1010-2)