Water activity is a key factor in determining the safety, quality and shelf life of cannabis products. Maintaining a low and consistent water activity through proper drying and storage is important to prevent microbial growth and chemical breakdown. Measuring the water activity using a reliable instrument such as a chilled mirror hygrometer provides valuable information about the energy status of water in cannabis and helps ensure the product meets regulatory requirements and quality standards.

1. Importance of Water Activity
for Cannabis Safety and Quality
By Dr. Brady Carter

2. Why Measure the Water Activity of Cannabis?
• Mold and other Microbes
• Chemical Breakdown
• Shelf Life
• Quality of Edibles
Drying and storing at low water activity is the key to success

3. • What is water activity?
• How is it different from moisture content?
• How do I measure it?
• What does the value mean?
What you should know about Water Activity

4. aw = f/fo = p/po
aw = ——————————————————
aw = ERH (%) /100
Vapor pressure of water above sample @ °C
Vapor pressure of pure water @ same °C
Correct Definition: Water Activity is a measure of the energy status of
the water in a system.
Old Definition: Water activity is the amount of “free” or “available”
water in a product as opposed to “bound” water.
Water Activity Defenition

5. aw = p/po
1. Equilibrium
2. Constant T & P
Water Activity Defenition

6. Factors that Control Water Activity
• Colligative effects or Solute interactions
• Matrix effects or Surface interactions
• Capillary effects
It is a combination of these factors in
Cannabis that reduces the energy of the water
as compared to pure water.
Water Activity Defenition

7. Each product has its own unique moisture
sorption isotherm – due to different interactions
(colligative, capillary, and surface effects) between
the water and the solid components at different
moisture contents.
0
5
10
15
20
25
30
35
40
45
50
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Water Activity
MoistureContent(%d.b.)
Icing Creme Filling Cake
Wood Pulp Granola Bar Milk Powder
Moisture Sorption Isotherm

8. • Hair or Polymer Hygrometers
• Freezing Point Depression
• Isopiestic Equilibration
• Electric Hygrometers
• Chilled Mirror Dew Point
Water Activity Measurement

9. • Advantages
• Very stable
• Less susceptible to contamination
• High Accuracy = ±0.003aw
• Meets ASTM Standard
• Relatively insensitive to volatiles
• Measures entire aw range
• Disadvantages
• Slower read times
Electrolyte Sensor

10. • Advantages
• Less expensive
• Accuracy = ±0.02 aw
• Does not meet ASTM Standard
• Measures entire aw range
• Disadvantages
• Lower accuracy
• Slower read times
• Hysteresis
• Poisoned by volatiles
Capacitance Sensor

11. • Advantages
• High accuracy ±0.003aw
• Meets ASTM Standard
• Rapid measurement <5 minutes
• Measures entire aw range (0.03 – 1.0aw)
• Disadvantages
• Need clean mirror
• Susceptible to contamination
• Needs daily verification to determine if contamination has occurred
• Readings affected by alcohol and propylene glycol
Infrared Sensor
Mirror
Optical Sensor
Fan
Sample
Chilled Mirror Sensor

12. • Advantages
• High accuracy ±0.005aw
• Meets ASTM standard
• Rapid measurement <5 minutes
• Not susceptible to any volatiles
• Disadvantages
• Susceptible to atmospheric pressure changes
• Unstable and unproven technology
• Needs daily verification to determine if contamination has occurred
Tunable Diode Laser (TDL)

13. • Calibration / Verification Standards
• Saturated Salt Slurries
• Unsaturated Salt Solutions
• Temperature
• Always report temperature of water activity test
• Standardize by running all tests at the same temperature
• Utilize instrumentation that controls temperature
Taking accurate Measurements

14. • Need representative sample
• For dried biomass, break into
small pieces and place in cup,
filling to half full
• Prevent moisture exchange
with environment
• Place in instrument and read
• For edibles, same process, but
may require different sample
handling
Preparing Sample

15. • Microbial spoilage making the cannabis unusable
and dangerous if inhaled
• THCA loss due to chemical reactions
• Edibles
• Microbial spoilage
• Chemical breakdown
• Moisture migration
• Loss of texture
The correct water activity is the key to
delaying all factors that end shelf life
What will end Shelf life?

16. • Scott (1953 & 1957) showed that microorganisms have a limiting
water activity level below which they will not grow.
• Water activity, not water content, determines the lower limit of
available water for microbial growth.
Scott,W.J. 1953. Water relations of Staphylococcus aureus at 30ºC. Aust. J. Biol. Sci. 6:549-564.
Scott,W.J. 1957. Water relations of food spoilage microorganisms. Adv Food Res 7:83-127.
Microbial Safety

17. aw limit Microorganisms
0.91 Gram Negative Bacteria
0.86 Gram Positive Bacteria
0.88 Yeast (practical limit)
0.80 Production of mycotoxins
0.70 Molds (practical limit)
0.60 Absolute limit for all growth
Microbial growth in Cannabis is determined by its water activity
Microbial Growth

18. aw = 0.95 aw = 0.90
Water moves out to
lower aw and turgor
pressure is lost
aw = 0.93
aw = 0.90
The microbe tries to adapt by
altering its membrane make-up or
by reducing its aw to maintain
turgor pressure. It will produce or
transport in small solutes to
reduce its aw.
AA’s
Polyols
Sugars
AA’s
Polyols
Sugars
aw = 0.93 aw = 0.90 Unable to reach equilibrium with its
surroundings, the microbe will remain in
the lag phase with no growth or will begin
sporulation and go dormant, which can
only be reversed by ideal environment
conditions.
Water Activity mode of action

19. Effect of various aw levels on the growth curves
and maximum stationary numbers of
Staphylococcus aureus.
Adapted from Troller, J. A. (1987). Adaptation and growth of microorganisms in environments with reduced water activity. In: Water
activity: Theory and applications to food Rockland, L. B. and Beuchat, L. R. eds. Marcel Dekker, Inc.New York p.101-117.
Microbial Growth

20. • Varies Depending State
• California – requires water activity testing on all samples
• CA Section 19344 – 5322 Water activity and moisture content
• Nevada – requires water activity only for edibles
• NAC 453A.654 Required quality assurance tests; submission of wet marijuana for testing.
• NAC 453A.550 “Potentially hazardous marijuana products and ingredients” defined.
• Oregon – must test water activity on usable marijuana intended for retail sale or for further processing
• OAR 333-007-0300 to 333-007-0500 and OAR 333-064-0100 to 333-064-0130
• Washington – must test water activity on usable marijuana intended for retail sale or for further
processing
• WAC 314-55-102 Quality assurance testing Part (i) Moisture analysis
Government Regulations

21. • ASTM D8196–18: Standard Practice for Determination of Water Activity (aw) in Cannabis
Flower
• Does not specify specific water activity method
• Recommended accuracy of 0.005 aw
• ASTM D8197-18: Maintaining Acceptable Water Activity (aw) Range (0.55 to 0.65) for Dry
Cannabis Flower
ASTM Standards

22. • THCA loss due to Decarboxylation
• Enzymatic Degradation
• For Edibles
• Maillard Browning Reaction
• Lipid Oxidation
• Nutrient Loss
Water Activity
ReactionRate
0.0 0.2 0.4 0.6 0.8 1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
mo
Chemical / Biochemical Stability

23. What will happen if Cannabis with different water activities are stored together or if the
components of a multi-component edible are at different water activities?
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1
MoistureContent(%w.b.)
Water Activity
Equilibrium aw
Moisture Migration

24. As is Water Activity
0.58 aw
Too Hard Good
Too
Soft
and
Sticky
Product Texture

25. • Packaging determines the change in water
activity experienced by the product under
different storage humidities
• Models can be used to predict the water
activity change of a product in different
packaging
• Models can also be used to determine what
type of packaging material is needed to
maintain water activity.
Fick’s 1st Law
Evaporation
H20
H20
aw in
aw out
P/Lo
P/Lo
Tracking Changes in Package

26. Water Activity Range Most Likely Product Failure
> 0.850 Microbial Spoilage
0.70-0.85 Microbial Spoilage
Chemical Instability
0.40-0.70 Chemical Instability
Texture Changes
Moisture Migration
0.20-0.40 Texture Changes
Caking and Clumping
<0.20 Chemical Instability
Water Activity vs. Product Failure

27. Questions?
??

28. Thank you for your
attention