Water activity measurement: demonstration of a single and non-specific optimal storage value for orthodox forest seeds

Transcription

1 Water activity measurement: demonstration of a single and non-specific optimal storage value for orthodox forest seeds ISTA Congress, Köln June16-20 Session 3 Fabienne COLAS, Direction de la recherche forestière, Québec Patrick BALDET, Cemagref, France Michèle BETTEZ, Centre de semences for. de Berthier, Québec

2 1. Water activity : definition & concept a w is an image of the thermodynamic activity of water a w is the ratio between the relative vapour pressure of a given sample ps and the one of pure water p. a w ranges from 0 to 1 (pure water). a w is equivalent to equilibrium relative humidity (ERH) and a function of water potential (Ψ).

3 MPa MPa 1. Water activity : definition & concept Both a w and Ψ depict the energy status of water in plants, seeds and other hygroscopic matrixes a w Relation between Water Potential (Mpa) and Water Activity (aw) aw 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0 Relationship between seed longevity and a w or Ψ is linear and similar from species to species. (Roberts & Ellis, 1989) a w MPa water potential

4 Relative reaction rate 1. Water activity : definition & concept Degradation reaction rates as a w + Relation between depreciating agents and aw Exception for lipids oxidation occurring both at low and high a w levels. Lipid oxidation Hydrolitic reactions - Enzyme activity Mold growth Yeast growth Bacteria growth Water activity Adapted from Labuza et al. J. Food Technol, 1972

5 1. Water activity : definition & concept a w consists in measuring equilibrium relative humidity generated by a sample in a sealed vial Advantages : rapid, reproducible and non destructive method. P. Baldet

6 1. Water activity : definition & concept a w probe Display unit Probe and sample holder assembled Sample holder Sample in vial

7 1. Water activity : definition & concept Water behaviour in seeds can be described with moisture sorption isotherms obtained experimentally. Each isotherm is specific of the tested sample. A given matter may have a significant intra specific variability implying replication of isotherms. For a given ERH or a w corresponds a resulting moisture content.

8 - Moisture content (%) + 1. Water activity : definition & concept Typical shape of moisture sorption isotherms The three states of water Monolayer water a w < 0,25 Multi layer water 0,25 < a w < 0,75 Bulk water a w > 0,75 Sorption isotherm Water activity

9 Sorption isotherm construction (steps) Seed re-hydration Stabilization at different a w a w measurement Resulting moisture content determination

10 Moisture content (%) Sorption isotherms interpretation Use of third degree model of isotherm data Determination of inflexion point (f (x)=0) a w of best stability At inflexion point, a w is between structural water and bulk water Sorption isotherm with 3 rd model and inflexion point Structural 4 water 2 Linear region = a w best stability y = x x x R 2 = Inflexion point Bulk water Water activity

11 Sorption isotherms for Jack Pine a w best stability =

12 Sorption isotherm analysis with Water analyzer series BET and GAB monolayer moisture contents Model determination coefficient Sorption heat Display of Isotherm program graphic interface from Water analyser series software. Ends of linear region a w values (a w of best stability)

13 Sorption isotherms results (3rd degree model) Species Samples a w best stab. Res. MC Coniferous P. mariana (EPN) % P. glauca (EPB) % P. banksiana (PIG) % P. contorta (PLI) % P. menziesii (DOUG) % T. plicata (CW) % A. balsamea (SAB) % Broadleaved F. sylvatica (FAGUS) % F. exelsior (FRAX) % B. alleghaniensis (BOJ) % C. betulus (CARP) %

14 Sorption isotherms results (3rd degree model) a w of best stability and resulting moisture content for the 11 species studied FRAX FAGUS CARP CW PLI PIG DOUG Broadleaved Coniferous

15 Broadleaved Coniferous Sorption isotherms results (Water analyser ) Species Samples Moisture Monolayer BET P. mariana (EPN) P. glauca (EPB) P. banksiana (PIG) P. contorta (PLI) P. menziesii (DOUG) T. plicata (CW) A. balsamea (SAB) F. sylvatica (FAGUS) F. exelsior (FRAX) B. alleghaniensis (BOJ) C. betulus (CARP) % 3.91% 4.65% 4.56% 4.93% 4.27% 4.17% 5.32% 6.02% 5.03% 5.67% aw linear region span

16 Our hypothesis : there is an universal a w for optimal storage of orthodox seeds Mean inflexion point from all isotherm 3rd model analysis is a w 0.35 (+/ *, p = 0.01). Moisture a w 0.35 > BET & GAB monolayer moisture contents for all the tested species. Resulting moisture a w 0.35 are relevant to the state of the art storage values for the tested species. At a w 0.35, chemical and biotic degradation factors are minimum. * : confidence 99%

17 - Relative reaction rate + Lipid oxidation Sorption isotherm Water activity Hydrolitic reactions Enzyme activity Mold growth Yeast growth Bacteria growth + Moisture content - Water activity map adapted from: Labuza, T.P Properties of water as related to the keeping of foods. In: Proceedings of the third Annual Meeting of the International Congress of Food Science And Technology, pp Chicago: Institute of Food Technology.

18 Conclusion The results obtained with 11 orthodox forest species show that 0.35 is an optimal a w value for orthodox seeds drying and conservation. This value can be applied to all orthodox species without long moisture behaviour characterisation. We propose that a w can be introduced in ISTA quality rules.

19 Acknowledgments The authors would like to express their gratitude to the 62ème Commission Permanente de Coopération entre la France et le Québec for supporting our cooperative project. Mrs. Pelletier and Mr. Allard for the technical work. Ministère des Ressources Naturelles et de la Faune du Québec & Cemagref for the financial support.

20 Invitation- ISTA Workshop Water activity applied to seed testing October 13-15, Montargis (France) ISTA Committees: Forest Tree and Shrub seed, and Moisture Lectures and practical aspects related to water activity measurement will be considered in relation with gravimetric moisture content.