6 Wood Discoloration

3.3 Wood Moisture Content 63
Table 3.5. Correlations between water activity (aw, relative vapor pressure p/p0), water potential (MPa), maximum water-retaining pore radius
(µm) within wood at 25 ◦C, wood emptiness class and microbial activity (compiled from Griffin 1977; Clarke et al. 1980; Siau 1984; Rayner and
Boddy 1988; Viitanen and Ritschkoff 1991; Schlegel 1992; Reiß 1997)
Water activity Water potential Pore radius Wood emptiness class Microbial activity
(aw, p/p0) (MPa) (µm)
1.0000 0 (free water) cell lumens, wood degradation and staining
large openings after decay
0.9999 −0.014 10.5
0.9998 −0.028 5.2
0.9993 −0.10 1.5 fiber saturation area, minimum for most wood fungi
0.9990 −0.14 1.1 pits and small openings
0.9975 −0.35 0.4
0.9950 −0.69 0.2
0.990 −1.4 0.1 half-maximum growth rate of
wood-decay Basidiomycetes on agar
0.980 −2.8 0.05 minimum for most bacteria
0.970 −4.2 0.035 minimum for mycelial growth and wood decay of
Serpula lacrymans
0.960 −5.6 0.026 optimum for growth and sporulation of Aspergillus niger
−6.0 no growth of S. lacrymans on agar
0.920 −11.3 0.013 minimum for sporulation of A. niger
0.900 −14.5 0.01
0.880 < 0.01 temporary or intermolecular minimum for growth of A. niger
0.840 openings in the cell wall minimum for germination of A. niger
and growth of Paecilomyces variotii
0.800 minimum for most molds
0.750 minimum for halophilic bacteria
0.650 minimum for A. repens
0.600 lower limit for microbial growth
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