Water activity measurement:

demonstration of a single and

non-specific optimal storage

value for orthodox forest seeds

ISTA Congress, Köln June16-20

Session 3

Fabienne COLAS, Direction de la recherche forestière, Québec

Patrick BALDET, Cemagref, France

Michèle BETTEZ, Centre de semences for. de Berthier, Québec

1. Water activity : definition & concept

• aw is an image of the thermodynamic activity of water

– aw is the ratio between the relative vapour pressure of a given sample “ps” and the one of pure water “p”.

– aw ranges from 0 to 1 (pure water).

• aw is equivalent to “equilibrium relative humidity (ERH)” and a function of “water potential (Ψ)”.

Both aw and Ψ depict the energy status of water in plants, seeds and other hygroscopic matrixes.

Relation between Water Potential (Mpa)

and Water Activity (aw)

-400

-350

-300

-250

-200

-150

-100

-50

0

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

aw

MP

a

aw

MP

a

aw 0.35 ≈ -140 MPa water potential

Relationship between seed longevity and awor Ψ is linear and similar from species to species. (Roberts & Ellis, 1989)

1. Water activity : definition & concept

1. Water activity : definition & concept

Degradation reaction rates ↑ as aw ↑

Hyd

rolit

ic react

ions

+

-

Re

lative

re

actio

n r

ate

Water activity

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Lip

id o

xidatio

n

Mold growth

Yeast g

row

th

Bacte

ria g

row

th

Enzyme activity

Relation between depreciating agents and aw

Adapted from Labuza et al.

J. Food Technol, 1972

Exception for

lipids oxidation

occurring both at

low and high aw

levels.

1. Water activity : definition & concept

aw consists in measuring equilibrium relative humidity generated by a sample in a sealed vial

Advantages : rapid, reproducible and non destructive method.

© P. Baldet

Sample in vial

aw probe

Sample holder

Display unit

Probe and sample

holder assembled

1. Water activity : definition & concept

1. Water activity : definition & concept

• Water behaviour in seeds can be described with moisture sorption isotherms obtained experimentally.

• Each isotherm is specific of the tested sample. A given matter may have a significant intra specific variability implying replication of isotherms.

• For a given ERH or aw corresponds a resultingmoisture content.

Typical shape of moisture sorption isotherms

The three “states” of water

Water activity

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Sorption isotherm

-M

ois

ture

conte

nt

(%)

+

Monolayer

water

aw < 0,25

Multi layer

water

0,25 < aw< 0,75

Bulk

water

aw> 0,75

1. Water activity : definition & concept

Sorption isotherm construction (steps)

Seed re-hydration

Stabilization at different aw

aw measurement

Resulting moisture

content determination

Sorption isotherms interpretation Use of “third degree model” of isotherm data

Determination of inflexion point (f’’(x)=0) → aw of best stability

At inflexion point, aw is between structural water and bulk water

Inflexion point

Structural

water

y = 30.328x3

- 34.045x2

+ 22.128x + 2.3845

R2

= 0.9994

0

2

4

6

8

10

12

14

16

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Water activity

Mo

istu

re c

on

ten

t (%

)

y = 30.328x3

- 34.045x2

+ 22.128x + 2.3845

R2

= 0.9994

0

2

4

6

8

10

12

14

16

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Water activity

Mo

istu

re c

on

ten

t (%

)

Linear region =

aw best stability

Inflexion point

Sorption

isotherm with

3rd model and

inflexion pointBulk water

Sorption isotherms for Jack Pine

0.361

7.6

aw best stability = 0.361

Display of

“Isotherm program”

graphic interface

from Water analyser

series® software.

BET and GAB monolayer moisture contents

Sorption isotherm analysis with

Water analyzer series®

Model determination coefficient

Ends of linear region aw values (aw of best stability)

Sorption heat

Sorption isotherms – results (3rd degree model)

Species Samples aw best stab. Res. MC

Coniferous P. mariana (EPN)

P. glauca (EPB)

20

14

0.352

0.343

6.65 %

6.33 %

P. banksiana (PIG)

P. contorta (PLI)

20

8

0.361

0.383

7.61 %

8.06 %

P. menziesii (DOUG)

T. plicata (CW)

A. balsamea (SAB)

12

8

5

0.355

0.378

0.334

6.99 %

7.31 %

6.22 %

Broadleaved F. sylvatica (FAGUS)

F. exelsior (FRAX)

B. alleghaniensis (BOJ)

C. betulus (CARP)

15

10

10

5

0.312

0.323

0.369

0.315

7.68 %

7.83 %

6.97%

6.69%

aw of best stability and resulting moisture content for the 11 species studied

Sorption isotherms – results (3rd degree model)

PLI

CW

DOUG

FRAX

PIGFAGUS

Broadleaved Coniferous

CARP

Sorption isotherms – results (Water analyser ®)

Species Samples Moisture content @ Monolayer BET

aw linear region span

P. mariana (EPN)

P. glauca (EPB)

20

14

4.08%

3.91%

0.27 – 0.69

0.23 - 0.69

P. banksiana (PIG)

P. contorta (PLI)

20

8

4.65%

4.56%

0.27 – 0.69

0.24 – 0.69

P. menziesii (DOUG)

T. plicata (CW)

A. balsamea (SAB)

12

8

5

4.93%

4.27%

4.17%

0.22 – 0.68

0.18 – 0.69

0.18 – 0.68

F. sylvatica (FAGUS)

F. exelsior (FRAX)

B. alleghaniensis (BOJ)

C. betulus (CARP)

15

10

10

5

5.32%

6.02%

5.03%

5.67%

0.17 – 0.64

0.26 – 0.67

0.3 – 0.7

0.3 – 0.7

Con

ifero

us

Bro

adle

aved

Our hypothesis : there is an universal aw for

optimal storage of orthodox seeds

Mean inflexion point from all isotherm 3rd model analysis is aw 0.35 (+/- 0.008*, p = 0.01).

Moisture content @ aw 0.35 > BET & GAB monolayer moisture contents for all the tested species.

Resulting moisture contents @ aw 0.35 are relevant to the state of the art storage values for the tested species.

At aw 0.35, chemical and biotic degradation factors are minimum.

* : confidence interval @ 99%

Water activity map adapted from: Labuza, T.P. 1970. Properties of water as related

to the keeping of foods. In: Proceedings of the third Annual Meeting of the International Congress of

Food Science And Technology, pp. 618-635. Chicago: Institute of Food Technology.

- R

ela

tive r

eaction r

ate

+ +

Mois

ture

conte

nt -

Water activity

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Lip

id o

xidatio

n

Hyd

rolit

ic rea

ctio

ns

Sorptionisotherm

Mold

g

rowth

Yeas

t gro

wth

Bac

teria

gro

wth

Enzyme activity-

R

ela

tive r

eaction r

ate

+ +

Mois

ture

conte

nt -

Water activity

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Lip

id o

xidatio

n

Hyd

rolit

ic rea

ctio

ns

Sorptionisotherm

Mold

g

rowth

Yeas

t gro

wth

Bac

teria

gro

wth

Enzyme activity

Conclusion

The results obtained with 11 orthodox forest species show that 0.35 is an optimal aw value for orthodox seeds drying and conservation.

This value can be applied to all orthodox species without long moisture behaviour characterisation.

We propose that aw can be introduced in ISTA quality rules.

Acknowledgments

The authors would like to express their gratitude to the

“62ème Commission Permanente de Coopération entre la

France et le Québec” for supporting our cooperative project.

Mrs. Pelletier and Mr. Allard for the technical work.

Ministère des Ressources Naturelles et de la Faune du

Québec & Cemagref for the financial support.

Invitation- ISTA Workshop

Water activity applied to seed testing

October 13-15, Montargis (France)

ISTA Committees: Forest Tree and Shrub seed, and

Moisture

Lectures and practical aspects related to water activity measurement

will be considered in relation with gravimetric moisture content.