Water activity: a better moisture metric for pet food

Water activity: a better moisture metric for pet food


Moisture plays a key role in the quality of pet food products. For each product, there’s an ideal range of water activity values that maximizes profit, nutrition, and tastiness while keeping the product safe from microbes and mold.

For example, a bag of dog food kibbles has a moisture sweet spot of 0.55-0.65. Any lower than 0.55, and the dog food is unappetizingly dry and hard. Not to mention the seller is losing money because the product has been over-processed. Any higher than 0.65, and the product will mold in the bag, becoming unacceptable.

Formulate intelligently

Water activity has been used to guide the formulation of some of the most innovative–and commercially successful–pet food products. Complex pet food formulations like a simulated marrow-filled bone, a meaty cat food containing crunchy vitamin bits, and a shelf-stable, moist, meat-like product are all controlled by water activity.

Isotherms (curves showing how water activity changes as moisture increases and decreases in a specific product) are also a key formulation tool. They can be used to locate critical points where phase changes occur, to create mixing models, to investigate moisture-related issues, and in many other ways.

Know the moisture super-spec

Using water activity as a moisture super-spec can help you solve specific moisture challenges, determine shelf life, choose packaging, monitor incoming ingredients, prevent spoilage, and maintain taste, texture, and nutrition.

For example, there are a number of products on the market with multi-textured characteristics that use water activity. These products combine a hard, dry-baked pet food and a soft, moist pet food. The hard, dry component has the advantage of teeth cleaning, but is less palatable than a soft, moist food. Careful formulation using water activity keeps the crunchy kibble crunchy and the moist bits soft. The two can share a bag and a long shelf life without the kibble getting soft and the bits losing their moist texture.

Make every employee a moisture expert

Water activity is a test anyone can run to precise scientific standards. A certification course trains new users to measure water activity. In 20 minutes, nearly anyone can learn to run a reliable, repeatable, verified water activity test.


Not a “quick mode” reading, but a direct measure of water activity to 0.003. No calibration, no approximation

Moisture measurement with benefits

Pet food manufacturers use water activity to monitor their products because:

  • It accurately predicts safety: Water activity predicts whether microbes will grow on or in a product.
  • It also predicts quality: Water activity is correlated with many reactions (lipid oxidation, vitamin degradation, texture changes) that end shelf life.
  • It’s repeatable: Water activity is a test anyone can run to precise scientific standards. Measure water activity to ±0.003.
  • It’s standards based: Unlike other moisture measurements, water activity has a scale with a known zero, and measurements can be verified with known, NIST-traceable standards. Results can be compared between departments, factories, or companies no matter where they’re located or who makes the measurement.


Range of awMicroorganisms Generally Inhibited by Lowest aw in this RangePet Foods Generally Within this Range
1.00–0.95Pseudomonas, Escherichia, Proteus, Shigells, Klebsiella, Bacillus, Clostridium perfringens, some yeastsPedigree–Little Champions
Meow Mix–Tender Favorites
Pedigree–Little Champions
Purina–Beneful Prepared Meals
0.95–0.91Salmonella, Vibrio parahaemalyticus, C. Botulinum, Serratia, Lactobacillus, Pediococcus, some molds, yeasts (Rhodotorula, Pichia)Canine Carry Outs–beef
0.91–0.87Many yeasts (Candida, Tarulopsis, Hansenula), MicrococcusPurina–Moist & Meaty Dog Food
0.87–0.80Most molds (mycotaxigenic penicilla), Staphyloccocus aureus, most Saccharomyces (bailiii) spp., DebaryomycesCesar–Softies
Canine Carry Outs–bacon
0.80–0.75Most halaphilic bacteria, mycotoxigenic aspergilliPurina–Beggin Strips
0.75–0.65Xerophilic molds (Aspergillus chevalieri, A. candidus, Wallemia sebi), Saccharomyces bisporusMilk Bone–Trail Mix
0.65–0.40Osmophilic yeasts (Saccharomyces rouxii), few molds (Aspergilius echinulatus, Monascus bisporus)Iam's ProActive Health–MiniChunks
Natural Life Grain-Free–adult dog
0.60–0.50No microbial proliferationIam's ProActive Health–Original Cat
Purina–Alpo Variety Snaps
Table 1. Water activities that correlate to the growth of microorganisms in pet food. Water activity is directly related to mold and microbial growth and is an integral part of many HACCP programs. Chart adapted from L.R. Beuchat, Cereal Foods World, 26: 345 (1981).


  • Measure at the line with the scientific accuracy of an R&D or QA/QC lab

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  • Ensure that packaging will maintain product safety and quality over shelf life
  • Predict impacts of storing or shipping under abuse conditions
  • Set accurate sell by/use by dates
  • Determine storage requirements

Incoming ingredients

  • Test incoming ingredients for safety and quality
  • Ensure that ingredients to be mixed have compatible moisture (aw) levels


  • Maximize operating efficiency by pinpointing the right spec
  • Reduce waste in supply chain and manufacturing process
  • Ensure quality and profit by accurately measuring moisture at the right time


  • Eliminate variability with a standards-based moisture metric
  • Use a repeatable, reliable measurement
  • Run highly accurate, research-grade tests in 5 minutes or less

Research and development

  • Prevent multi-component moisture migration
  • Do accelerated shelf-life testing
  • Set a spec for product to avoid safety and quality issues such as mold, microbial growth, texture problems, off smells or flavors
  • Eliminate variability with a standards-based moisture metric
  • Use water activity to guide innovative formulations

Market snapshot: Pet food top competitors

We bought bags of kibble from a variety of retailers. We tested big brands alongside small niche companies. The result? A market snapshot of thirteen pet food brands. In this quick 15-minute webinar, Scott Campbell reviews the findings.


  • which brands came close to exceeding the 10% moisture limit.
  • what some big brands do much better than small ones.
  • which brands over-pack—and which ones don’t quite meet the minimum.
  • the projected costs of inconsistency in packaging and drying.

Watch webinar

Get the complete picture

Learn everything about how water activity can increase your profit and improve product quality and safety—all in one place.

Download The Food Manufacturer’s Complete Guide to Water Activity—>

Learn how to predict shelf-life using water activity.

Download The Food Manufacturer’s Complete Guide to Shelf Life—>

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Use the Vapor Sorption Analyzer to study kinetics of vapor sorption and determine how quickly your products take up and lose moisture.