Water activity measurement plays an integral role at the Tree Seed Centre in British Columbia.
The Tree Seed Working Group has four objectives related to promoting tree seed science and technology through:
• Seed research from bud initiation to seed use
• Identification of seed problems relating to seed improvement and forest management
• Exchange of information on seed related problems
• Consulting on implementation practices
Water activity continues to be an area of interest to the Tree Seed Working Group, especially with their genetic conservation collection.
Rotronic has become the standard for water activity measurement for many seed storage facilities throughout the world. Although water activity is a relatively new technology to the Tree Seed Centre, it already plays an important role. Water activity measurement devices from Rotronic offer quick, non destructive tools for evaluating the free moisture in a sample of seeds. The meter outputs a value between 0 and 1, which is equivalent to equilibrium relative humidity (eRH). Aw or eRH differs from a moisture content test in that it only measures the free water rather than the total water content in a sample of seed. Free water is often far more relevant to germination, spoilage and shelf life of seeds. The amount of free water in a seed will depend on its relative composition of lipids, starches, and proteins. Storing seed in the freezer with a water activity measurement that is either too high or too low will decrease the longevity of the seed. The ideal level ultimately depends on the species; however, most people now accept the concept of a universal value. At the Tree Seed Centre, they keep within a range of 0.35 ± 0.05”.
The role of water activity measurement
Currently, water activity is used primarily for testing seed collected for genetic conservation and research. The seeds are often rare and valuable; thus a non destructive method of assessment is imperative. Collections intended for the seedbank arrive at the Tree Seed Centre grouped by population (noted by location), with each population containing,on average, a sample of 10 parent trees. Populations are stored in the cooler at 2°C to await an initial water activity assessment. If the water activity is too high, the seeds are dried back and retested prior to placement in the freezer at-18 °C to maximise seed longevity.
In the UK we have worked closely with Kew Gardens at the Millennium Seed Bank where a bank of Rotronic devices with direct PC connection are used to screen and test all seeds that are stored in their highly secure underground bunker.
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