Water activity as a better approach than moisture content for forest seeds moisture management

Seed moisture is a key factor influencing seed quality and an continual challenge for forest seed industry. Moisture content (MC) is the most common procedure used to assess moisture of forest seeds; for a given species, the range of optimal water content for storage has been defined both by know-how and conducting tests. However, this approach is not always reliable because it does not take into account the chemical properties of water remaining in dried materials. On the contrary, the concept of activity of water (Aw) quantifies the vapour pressure generated by the moisture present in hygroscopic products like organic substances. It is known as the chemical potential of water. Aw has been widely used for decades by food industry. In this discipline it is very well-known that the reactivity and depreciating ability of hydrolytic reactions, enzymes activity, lipid oxidation or biotic agents growth are dependent on Aw rather than on MC. Consequently, Aw is more consistent to define stability moisture for organic materials preservation. The relationship between Aw and MC is pictured by the sorption/desorption isotherm. This connection is rather stable for standardized organic materials like sugars or flours where it is possible to use equally MC or Aw for moisture assessment. Forest seeds are picked from natural stands or seed orchards where a high genetic diversity is sought after. The combination of genetic and environmental factors results in unpredictable seed characteristics. The R&D project we carry out in cooperation with French forest seed companies Vilmorin and ONF aims to: a)the experimental construction of sorption isotherms of different seed lots to demonstrate the consequent variability among different seed lots for a given species, b)calculate the water activity of greater stability on the basis of desorption isotherms and in accordance with bibliographic references, c)demonstrate the effectiveness of water activity assessment versus moisture content evaluation d)finally, to transfer the Aw technique to ONF and Vilmorin for seed moisture control. Aw applied to forest seeds moisture management is now regularly being used by Vilmorin and on the way to be by ONF.