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Water activity measurements in seed testing

Craig McGill, Institute of Natural Resources, College of Sciences, Massey University, New Zealand


1. What is water activity?

2. Water in biological systems.

3. Relationship between water activity and moisture inseeds.

4. Possibilities for water activity in seed testing.

5. Challenges.

6. Where to from here.

Presentation Plan


What is Water Activity ?

• water activity (aw) is a method to monitor moisture in organicmaterials

• is related to the physical and chemical properties ofhygroscopic substances (e.g. foods and natural products) thatwill absorb and desorb water

• has used in the food industry for many years for food quality andsafety, e.g. if water activity in food is below a certain level thenmould growth is inhibited

• reactions such as oxidation of lipids that can result loss of foodquality are also a function of water activity.


What is Water Activity ?

• can be thought of as the amount of water that is available forhydration of other substances

• aw ranges from 0 to 1 where 0 means there are no water moleculesavailable for hydration and 1 is pure water

Substance aw Pure water 1.0 Bread 0.96 Cheese 0.97 Dried fruit 0.76 Dry pasta 0.50 Point at which cereals lose their crunch 0.65 Most moulds1 0.80


Typical water activity (aw) values

Data from http://www.lsbu.ac.uk/water/activity.html and 1http://www.csiro.au/en/Outcomes/Food-and-Agriculture/water-activity-in-food.aspx


Water in Biological Systems

Three states of water

1. a strongly bound monolayer where the water is tightly bound to othermolecules

2. less strongly bound multilayer water where water is deposited on themore tightly bound water in the first state – can contribute to seeddeterioration

3. weakly linked or free water – contributes to rapid seed deteriorationif not removed during drying, early germination events and micro-organism growth can occur


Relationship of water activity to moisture content can be described using an isotherm

from http://www.lsbu.ac.uk/water/activity.htm


Characteristics of the moisture sorption isotherm

• are specific i.e. seed of different species will have different isotherms

• also have intra specific variation i.e. may vary within a species fordifferent seed lots

• water activity is related to the composition of the seed (percentageof starch, oils (and fats) and proteins)


Relationship of Water Activity to Moisture Content in Green Ash Seed

Water activity aw

water activity can be related to the seed moisture content if required


How Does Measuring Water Activity differ from the current ISTA Oven Method?

Reproduced with kind permission from Colas, F., P. Baldet, M. Bettez, A. Rainville, C. Périé, P. Vuillaume & A. Mehamha, 2011. L'activité de l'eau: un outil innovant pour la conservation de la diversité végétale. Carrefour Forêt Innovations, 4-6 octobre 2012, Québec (Canada).


How is water activity measured?

Moisture probe covers the sample chamber

Sample (seed)

chamber

Meter

is estimated by measuring the equilibrium relative humidity, in a closedchamber that contains the seed sample


What are the advantages of water activity?

• is a simple and quick method that requires relatively little training

• is a non-destructive method and therefore particularly useful for smallsamples of seed with high commercial (e.g. hybrid seed) or geneticvalue

• can be used for coated/encrusted/pelleted seeds

bare and coated lotus seed


What are the advantages of water activity?

• provides information on the water status in the seed – there is a directrelationship between water activity and seed longevity

• meters for measuring water activity are relatively inexpensive

• meters are portable


What is the Purpose of Moisture Testing in ISTA?

• as a check to determine if the seed is ready for harvest and/orrequires drying

• as a pre-storage check for both short term and long term storage

• as a pre-treatment check before applying a dormancy-breaking orvigour test

• to allow the dry weight of seed to be calculated to determine theselling price of the seed


Can water activity fulfil some of these purposes?

A pre-storage check for both short term and long termstorage

• water activity can be used to determine if the moisture inthe seed is at a level safe for storage

• water activity is routinely used in many seed banks for thispurpose


As a pre-treatment check before applying a dormancy-breaking or vigour test

– would need to relate the test to water activity e.g. for theAA-test the pre-test seed moisture must be between 10 and14%

– how does this relate to the water activity of the seed lot?

Can water activity fulfil some of these purposes?


Can water activity overcome of the problems we face with the current moisture testing methods?

Testing of coated/encrusted/pelleted seed

the moisture content determined using the oven method is the average of the waterin the coating and seed, this normally leads to an underestimation of seed moisture


Practical Challenges For Accurate Water Activity Measurement

1. The time for the relative humidity of the sample chamber toequilibrate to that of the seed.

2. The volume of seed in the sample chamber.

3. Temperature at which the measurement is made.

Three key practical considerations:

From: Probert, R.J., Manger, K.R. and Adams, J. 2003. Non-destructive measurement of seed moisture. In Seed Conservation; turning science into practice. Royal Botanic Gardens, Kew.



1. The time for the relative humidity of the sample chamber toequilibrate to that of the seed

Depends on:

• temperature

• difference between the relative humidity of the air in thechamber and the seed

• the seed itself for example permeability of the seed coat – seedswith impermeable coats may need to be cut



2. The volume of seed in the sample chamber

The seed chamber needs to be as full as possible to reduceinaccuracies in water activity measured as result of the seedabsorbing or desorbing water from the atmosphere.

3. Water activity is dependent on the temperature the measurement ismade at. Need to have the meter in a temperature controlledenvironment

From: http://www.rotronic.co.uk/humidity-measurement-feuchtemessung-temperaturmessung/water-activity-wasseraktivitaet.html


Where to from here

1. Develop a method protocol for the measuring water activity inseeds. Key question - what will it be used for?

– seed storage (water activity as a standalonemeasurement)

– coated/encrusted/pelleted seed (water activityneeds to be related to seed moisture (oven test))

2. Validate that protocol to ensure that the method proposed willprovide accurate and repeatable measurement of wateractivity.


Where to from here

3. Use that validation protocol to support a Rules proposal for theinclusion of water activity as a method in the ISTA Rules.


Thanks

Harry Nijënstein and Hugh Pritchard for their comments on thepresentation

Patrick Baldet and Fabienne Colas for the use of their graphic andsuggestions for the presentation

Robert Karrfalt who is leading this work in the Moisture Committee


Thank you for your attention

Questions

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