Fundamentals of Water Activity: Governmental Compliance
The U.S. food, drug, and cosmetic laws are intended to assure the consumer that foods are pure, wholesome, safe to eat, and produced under sanitary conditions; that drugs and medical devices are safe and effective for their intended uses; that cosmetics are safe and made from appropriate ingredients; and that all labeling and packaging is truthful, informative, and not deceptive. Modern scientific methods are required to enforce the Federal Food, Drug, and Cosmetic Act. Laws to ensure the wholesomeness of foods and the safety and efficacy of drugs would be impractical without reliable methods of laboratory analysis to determine whether products are up to a standard.
Good Manufacturing Practice
The United States has one of the safest food supplies of any country in the world. The FDA's Good Manufacturing
Practice Regulations incorporates water activity guidelines in defining food safety regulations. GMP regulations
detail the specific requirements and practices to be followed by industry to assure that foods are produced
under sanitary conditions and are pure, wholesome, and safe to eat. Specific parts and paragraphs of applicable
GMP regulations from Title 21 of the Code of Federal Regulations use water activity in relation to control
measures and food safety. However, neither GMP's alone nor activities of regulatory agencies alone can guarantee
a completely safe food supply. Consequently, a science based system, Hazard Analysis and Critical Control Points
(HACCP), has been created to improve food safety and reduce the incidence of foodborne illness.
In-process Detection
HACCP is a way for industry to control and prevent problems, and ensure safe food by controlling the production
process from beginning to end, rather than detecting problems at the end of the line. It identifies where
hazards might occur in the food production process and puts into place actions to prevent the hazards from
occurring. For example, a target water activity must be established to prevent hazardous organisms from growing.
By controlling major food risks, such as microbiological, chemical and physical contaminants, the industry can
better assure consumers that its products are safe.
Potentially Hazardous Food
The term potentially hazardous food (PHF) was developed by the U.S. Public Health Service during the last half
of the twentieth century to regulate perishable food. Potentially hazardous food means a food that requires
time/temperature control for safety (TCS) to limit pathogenic microorganism growth or toxin formation. Foods
were considered non-PHF when their aw ≤ 0.85, which is below the water activity for
Staphylococcus aureus growth and toxin production or the pH ≤ 4.6, which is below the pH for proteolytic
Clostridium botulinum growth and toxin production.
New Science-Based Criteria
The 2005 version of the Food Code updates the definitions of PHF by using a scientific based criteria that
considers the interaction of water activity and pH in determining if a food is designated as a non-PHF/non-TCS food.
Interaction Tables A & B consider the interaction of water activity and pH under certain conditions of heattreatment and
packaging. The hurdle effect will control or eliminate pathogens that would otherwise be ineffective when used
alone. The effect of a heat treatment which destroys vegetative cells and the effect of packaging which prevent
recontamination is considered.
0.85 or Less water activity Will Not Support Pathogenic Bacterial Growth
The USDA and FSIS also use water activity in Generic HACCP Model 10 for Heat-treated, Shelf-stable Meat and
Poultry Products. The science states and verifies that all pathogenic bacteria stop growing at a water activity
of 0.86. The model states that "Manufacturers should not use the moisture protein ratio (MPR) as a measure of
proper drying for shelf stability or safety. It is product water activity that is best correlated to inhibition
of each pathogen's growth." Thus, if you produce jerky to a water activity of 0.85 or less, then the product
will not support the growth of any pathogenic bacteria. Drying your product to a water activity of 0.80 or less
does not make the product any safer. The product will have less consumer appeal (because it will be tougher and
chewier) and because jerky is sold on a weight basis, you will be losing profit.
Interaction Table A
Table A can be used to determine if a food which is heat-treated and packaged is 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, higher pH and aw can be safely tolerated.
Interaction Table B
Table B can be used to determine if a food which is not heat-treated or is 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.