Water Activity – Food

Specimen Collection and Handling

Specimen Requirements

Test Requested Required Requisition(s) Specimen Type Minimum Volume Collection Kit

Shelf Stability1

Food – indicate if part of a meal, control, or follow up

25 grams

Sterile plastic sample bag with round wire closure

Submission and Collection Notes


Test selection is completed by the laboratory and is based on the information provided on the requisition.  Specific analyses may be requested using the ‘Comments’ section of the requisition.

Special Instructions

For sampling, follow the instructions found in Public Health Inspector’s Guide to Environmental Microbiology Laboratory Testing.

Shipping Instructions:

  • Ship all foods in containers with hard walls and lids secured in the closed position. 
  • Shipping containers must be labelled with the submitting organization, unique identifier and contents e.g. FOOD SAMPLES on the outside of the container. 
  • DO NOT SHIP WITH CLINICAL SPECIMENS.  Shipping containers used for food samples should be dedicated to food samples and not be used for other types of environmental samples. 
  • Ship dry foods and other shelf stable foods at ambient temperature in a closed container. 
  • Ship frozen foods in an insulated container with sufficient ice packs to maintain the frozen state. 
  • Ship perishable foods in an insulated container with sufficient cold packs to maintain a temperature as close to 4°C as possible. If ice is used, contain the ice in a manner that does not allow water contact with the samples.
  • Submit all food samples to the laboratory as soon as possible.

Requisitions and Kit Ordering

Test Frequency and Turnaround Time (TAT)

The water activity test is performed Monday to Saturday.

Turnaround time is up to 7 days.

Test Methods

The availability of water for microbial, enzymatic or chemical activity determines the shelf life of foods. This water availability is determined using a water activity meter. Water activity (aw) is the ratio of water vapor pressure of the food substrate to the vapor pressure of pure water at the same temperature. Water activity is measured on a scale of 0 to 1; pure water has an aw of 1.00 and the aw of a completely dehydrated food is 0.00. (Health Canada, MFLP-66)


For most foods to be considered safe to store at room temperature, they should have an aw ≤ 0.85. Bacterial foodborne pathogens are inhibited at this aw; however, for those microorganisms that can cause spoilage (e.g., yeasts and moulds), the aw should be < 0.60 to prevent food spoilage by inhibiting growth of these organisms.


Results are reported to the submitting health unit as indicated on the requisition, when all analyses are completed.

Food samples that show the presence of bacteria capable of causing food-borne illness are reported to the Medical Officer of Health as per the Ontario Health Protection and Promotion Act.

Additional Information

For outbreaks, contact your local Health Unit, your local Public Health Laboratory or the Public Health Ontario, Public Health Laboratory Customer Service at416-235-6556 or1-877-604-4567.

Mis à jour le 20 juill. 2020