39 SEED TESTING INTERNATIONAL APRIL 2026 • RULES DEVELOPMENT Inter-Laboratory Reproducibility of Water Activity Measurements Fiona R. Hay1, Jayanthi Nadarajan2, Deborah de Souza Vidigal3, Laura Bowden4, Ruud Nabben5, Marco Porceddu6, Gianluigi Bacchetta6, Maria Enrica Boi6, Siem Reijnders7, Céline Herbert8 and Paul Keijzer9 1Department of Agroecology, Aarhus University, Slagelse, Denmark 2New Zealand Institute for Bioeconomy Science Ltd, Food Industry Science Centre, Fitzherbert Science Centre, Palmerston North, New Zealand 3Bejo Zaden BV, Warmenhuizen, The Netherlands 4SASA, Edinburgh, UK 5BASF Nunhems, Nunhem, The Netherlands 6Sardinian Germplasm Bank (BG-SAR), Department of Life and Environmental Sciences, University of Cagliari, Italy 7Pedak Meettechniek, Heythuysen, The Netherlands 8GEVES, Beaucouzé cedex, France 9Rijk Zwaan BV, De Lier, The Netherlands THIS ARTICLE REPORTS THE RESULTS OF A SPECIAL PROJECT carried out by the ISTA Moisture and Storage Committees. Samples of three vegetable seeds equilibrated to four different levels of moisture were tested for water activity by six different laboratories. It was found that the results were both repeatable within a laboratory and reproducible between laboratories. These results are an important step towards introducing measurement of water activity into the International Rules for Seed Testing (ISTA Rules). A survey of ISTA accredited, genebank and research laboratories was also conducted as part of this Special Project. Water activity is already being measured by some laboratories, because of the advantages of this approach compared with moisture content determination. Wider adoption is limited because this method is not yet in the ISTA Rules. Introduction An ISTA Special Project carried out by the ISTA Moisture and Storage Committees, with the collaboration of ISTA accredited laboratories and genebank/research laboratories, aimed to evaluate the reproducibility of water activity measurements across different facilities and using different instruments, and to better understand demand for the introduction of water activity into the ISTA Rules or as an official ISTA method. Water activity (αw) is measured using a water activity meter and is usually expressed on a scale of 0–1, though it may also be converted to ‘equilibrium relative humidity’ (eRH) by multiplying by 100 to give a percentage scale (Hay et al., 2022, 2023). This method is widely used as an alternative to moisture content, in the food industry, in particular. It is considered more insightful than moisture content since it provides an indication of the availability of water for chemical reactions to take place, especially during seed development, germination and storage. Furthermore, it can be applied universally since the type and relative rate of reactions along the αw scale will be consistent for seeds of different species. Measuring αw also has the advantage of being non-destructive, relatively quick and simple, typically requiring from 7 to 30 min per sample. This Special Project was divided into two parts, where part one looked at the water activity measurements for three selected vegetable seeds (i.e. cabbage, carrot and onion), while part two involved a survey on the use of αw in seed laboratories. Materials and Methods Water Activity Measurements Seeds of cabbage, carrot and onion were provided by Bejo Zaden BV, The Netherlands. For each species, seeds were divided into four lots. Three of these lots were equilibrated to different moisture levels by placing them at 45, 65 or 70% relative humidity at 20 °C. The fourth lot of each species was used as provided (αw 0.30–0.35), hermetically sealed in a Mabfoil trilaminate-aluminium foil packet. Following equilibration, each lot was subdivided into 18 samples of approximately 5 g and vacuum sealed in Mabfoil trilaminate-aluminium foil packets. These packets were then sent to each of the six laboratories: GEVES (France), the Sardinian Germplasm Bank (Italy), SASA (UK), and BASF Nunhems, Pedak Meettechniek and Rijk Zwaan (The Netherlands). Thus, each laboratory received 36 packets (three species ´ four moisture levels ´ three replicates). Packets were randomly numbered to avoid any systematic error. Each laboratory was asked to measure the αw of the seeds at 20 °C and to measure the sample from each packet twice. They were then requested to determine the moisture content
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