Water activity measurements in seed testing. Craig McGill, Institute of Natural Resources, College of Sciences, Massey University, New Zealand

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

2 Presentation Plan 1. What is water activity? 2. Water in biological systems. 3. Relationship between water activity and moisture in seeds. 4. Possibilities for water activity. 5. Challenges. 6. Where to from here.

3 What is Water Activity? water activity (a w ) is a method to monitor moisture in organic materials is related to the physical and chemical properties of hygroscopic substances (e.g. foods and natural products) that will absorb and desorb water has used in the food industry for many years for food quality and safety, e.g. if water activity in food is below a certain level then mould growth is inhibited reactions such as oxidation of lipids that can result loss of food quality are also a function of water activity.

5 What is Water Activity? can be thought of as the amount of water that is available for hydration of other substances a w ranges from 0 to 1 where 0 means there are no water molecules available for hydration and 1 is pure water

6 Typical water activity (a w ) values 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 moulds Data from and 1

7 Water in Biological Systems Three states of water 1. a strongly bound monolayer where the water is tightly bound to other molecules 2. less strongly bound multilayer water where water is deposited on the more tightly bound water in the first state can contribute to seed deterioration 3. weakly linked or free water contributes to rapid seed deterioration if not removed during drying, early germination events and microorganism growth can occur

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

9 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 for different seed lots water activity is related to the composition of the seed (percentage of starch, oils (and fats) and proteins)

10 Relationship of Water Activity to Moisture Content in Green Ash Seed Water activity a w 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.

11 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, L'activité de l'eau: un outil innovant pour la conservation de la diversité végétale. Carrefour Forêt Innovations, 4-6 octobre 2012, Québec (Canada).

12 How is water activity measured? is estimated by measuring the equilibrium relative humidity, in a closed chamber that contains the seed sample Moisture probe covers the sample chamber Sample (seed) chamber Meter

13 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 small samples of seed with high commercial (e.g. hybrid seed) or genetic value can be used for coated/encrusted/pelleted seeds bare and coated lotus seed

14 What are the advantages of water activity? provides information on the water status in the seed there is a direct relationship between water activity and seed longevity meters for measuring water activity are relatively inexpensive meters are portable

15 What is the Purpose of Moisture Testing in ISTA? as a check to determine if the seed is ready for harvest and/or requires drying as a pre-storage check for both short term and long term storage as a pre-treatment check before applying a dormancy-breaking or vigour test to allow the dry weight of seed to be calculated to determine the selling price of the seed

16 Can water activity fulfil some of these purposes? A pre-storage check for both short term and long term storage water activity can be used to determine if the moisture in the seed is at a level safe for storage water activity is routinely used in many seed banks for this purpose

17 Can water activity fulfil some of these purposes? As a pre-treatment check before applying a dormancybreaking or vigour test would need to relate the test to water activity e.g. for the AA-test the pre-test seed moisture must be between 10 and 14% how does this relate to the water activity of the seed lot?

18 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 water in the coating and seed, this normally leads to an underestimation of seed moisture

19 Practical Challenges For Accurate Water Activity Measurement Three key practical considerations: 1. The time for the relative humidity of the sample chamber to equilibrate to that of the seed. 2. The volume of seed in the sample chamber. 3. Temperature at which the measurement is made. From: Probert, R.J., Manger, K.R. and Adams, J Non-destructive measurement of seed moisture. In Seed Conservation; turning science into practice. Royal Botanic Gardens, Kew.

20 Practical Challenges For Accurate Water Activity Measurement 1. The time for the relative humidity of the sample chamber to equilibrate to that of the seed Depends on: temperature difference between the relative humidity of the air in the chamber and the seed the seed itself for example permeability of the seed coat seeds with impermeable coats may need to be cut

21 Practical Challenges For Accurate Water Activity Measurement 2. The volume of seed in the sample chamber The seed chamber needs to be as full as possible to reduce inaccuracies in water activity measured as result of the seed absorbing or desorbing water from the atmosphere. 3. Water activity is dependent on the temperature the measurement is made at. Need to have the meter in a temperature controlled environment From:

22 Where to from here 1. Develop a method protocol for the measuring water activity in seeds. Key question - what will it be used for? seed storage (water activity as a standalone measurement) coated/encrusted/pelleted seed (water activity needs to be related to seed moisture (oven test)) 2. Validate that protocol to ensure that the method proposed will provide accurate and repeatable measurement of water activity.

23 Where to from here 3. Use that validation protocol to support a Rules proposal for the inclusion of water activity as a method in the ISTA Rules.

24 Thanks Harry Nijënstein and Hugh Pritchard for their comments on the presentation Patrick Baldet and Fabienne Colas for the use of their graphic and suggestions for the presentation Robert Karrfalt who is leading this work in the Moisture Committee

25 Thank you for your attention Questions

Water Activity (a w. ) in Foods WHITEPAPER CONTENTS SUMMARY

Water Activity (a w. ) in Foods WHITEPAPER CONTENTS SUMMARY Water Activity in Foods Page 1 Safefood 360 is the provider of food safety management software for industry leading businesses. WHITEPAPER Water Activity (a w ) in Foods CONTENTS SUMMARY 1. Introduction