**4.4.1 Resistance against decay fungi**

As gamma radiation causes break-up of cellulose to shorter chains, which are water-soluble, and that leads to an "opening of additional microcracks", in which water molecules can easily penetrate. Consequently, gamma irradiated wood is also more accessible to enzymes of wood decaying fungi.

Only ten days after incubation, it was clearly visible, that irradiated specimens were more overgrown, than control steam sterilised controls, which indicates a higher susceptibility to biodegradation of gamma irradiated wood compared to non-irradiated wood (Hasan, 2006a; Hasan *et al.*, 2008; Figure 13).

Fig. 13. Specimen pairs in testing flasks 10 days after exposure to brown-rot fungus *Gloeophyllum trabeum* (AC, KCC, KDC – non-irradiated controls, BC – specimen irradiated with 30 kGy, CC – 90 kGy, DC – specimen irradiated with dose of 150 kGy).

Changes in Selected Properties of Wood Caused by Gamma Radiation 297

Fig. 15. Appearance of irradiated and autoclaved specimens after 16 weeks of exposure to

**90 kGy** 

**0 kGy** 

The difference in ML between specimens irradiated with 30 and 90 kGy and autoclaved ones is slight and not significant after 4 weeks of incubation, while specimens irradiated with 150 kGy had significantly greater ML. During further incubation differences in ML between autoclaved and irradiated specimens increased but also between irradiated groups. During the all incubation period ML increased with radiation dose. Since *P. placenta* was more virulent and more aggressive fungus compared to *G. trabeum*, the obtained results are

ML [%]

Fig. 16. Mass loss (ML) of autoclaved and irradiated specimens during exposure to fungus *Poria placenta*: A) 4 weeks incubation; B) 8 weeks incubation; C) 12 weeks incubation; D) 16 weeks incubation; (irradiated specimens n=7; G=0 kGy - autoclaved controls n=21).

ML [%]

0 30 90 150


0 30 90 150

G [kGy]


**0 kGy** 

**150 kGy** 

G [kGy]

± s ± 95% confidence interval **D)**

± s ± 95% confidence interval **B)**

fungus *Poria placenta*.

**0 kGy** 

**30 kGy** 

logical (Figure 16).

ML [%]

ML [%]

0 30 90 150


0 30 90 150

G [kGy]


G [kGy]

± s ± 95% confidence interval **C)**

± s ± 95% confidence interval **A)**

#### **Resistance against brown-rot fungi**

Visible difference in appearance (irregular shape and darker colour) between irradiated and autoclaved specimen after 16 weeks of exposure to the fungus *G. trabeum* indicated a significantly higher decay of the irradiated specimens (Hasan, 2006; Hasan *et al.*, 2008). After 12 and 16 weeks of exposure to the fungus *G. trabeum*, average ML of irradiated and control specimens was more than 25 %, which proved that the fungus was virulent (EN 113: 1996). The difference in ML between irradiated and autoclaved specimens was slight and not significant after 4 weeks of incubation, except specimens irradiated with 150 kGy, whose ML was greatest. The difference in ML between controls and specimens irradiated with 30 kGy reached maximum after 8 weeks of incubation, while other two irradiated groups had no significant difference in ML comparing to control. During further incubation time the differences in ML between irradiated and control specimens decreased and after 16 weeks became insignificant (Figure 14).

Fig. 14. Mass loss (ML) of autoclaved and irradiated specimens during exposure to fungus *Gloeophyllum trabeum*: A) 4 weeks incubation; B) 8 weeks incubation; C) 12 weeks incubation; D) 16 weeks incubation; (irradiated specimens n=7; G=0 kGy - autoclaved controls n=21).

Fungus *P. placenta* causes brown rot with broad and deep cracks. Clearly visible difference in appearance (cracks and irregular shape) between irradiated and autoclaved specimens after 16 weeks of incubation has been shown in Figure 15.

Visible difference in appearance (irregular shape and darker colour) between irradiated and autoclaved specimen after 16 weeks of exposure to the fungus *G. trabeum* indicated a significantly higher decay of the irradiated specimens (Hasan, 2006; Hasan *et al.*, 2008). After 12 and 16 weeks of exposure to the fungus *G. trabeum*, average ML of irradiated and control specimens was more than 25 %, which proved that the fungus was virulent (EN 113: 1996). The difference in ML between irradiated and autoclaved specimens was slight and not significant after 4 weeks of incubation, except specimens irradiated with 150 kGy, whose ML was greatest. The difference in ML between controls and specimens irradiated with 30 kGy reached maximum after 8 weeks of incubation, while other two irradiated groups had no significant difference in ML comparing to control. During further incubation time the differences in ML between irradiated and control specimens decreased and after 16 weeks

ML [%]

Fig. 14. Mass loss (ML) of autoclaved and irradiated specimens during exposure to fungus *Gloeophyllum trabeum*: A) 4 weeks incubation; B) 8 weeks incubation; C) 12 weeks incubation; D) 16 weeks incubation; (irradiated specimens n=7; G=0 kGy - autoclaved controls n=21).

Fungus *P. placenta* causes brown rot with broad and deep cracks. Clearly visible difference in appearance (cracks and irregular shape) between irradiated and autoclaved specimens

ML [%]

0 30 90 150


0 30 90 150

G [kGy]


G [kGy]

± s ± 95% confidence interval **D)**

± s ± 95% confidence interval **B)**

**Resistance against brown-rot fungi** 

became insignificant (Figure 14).

0 30 90 150


0 30 90 150

after 16 weeks of incubation has been shown in Figure 15.

G [kGy]


G [kGy]

± s ± 95% confidence interval **C)**

± s ± 95% confidence interval **A)**

ML [%]

ML [%]

Fig. 15. Appearance of irradiated and autoclaved specimens after 16 weeks of exposure to fungus *Poria placenta*.

The difference in ML between specimens irradiated with 30 and 90 kGy and autoclaved ones is slight and not significant after 4 weeks of incubation, while specimens irradiated with 150 kGy had significantly greater ML. During further incubation differences in ML between autoclaved and irradiated specimens increased but also between irradiated groups. During the all incubation period ML increased with radiation dose. Since *P. placenta* was more virulent and more aggressive fungus compared to *G. trabeum*, the obtained results are logical (Figure 16).

Fig. 16. Mass loss (ML) of autoclaved and irradiated specimens during exposure to fungus *Poria placenta*: A) 4 weeks incubation; B) 8 weeks incubation; C) 12 weeks incubation; D) 16 weeks incubation; (irradiated specimens n=7; G=0 kGy - autoclaved controls n=21).

Changes in Selected Properties of Wood Caused by Gamma Radiation 299

rot and brown rot causing fungi. Significantly greater mass loss of gamma irradiated wood than autoclaved wood has been established in the beginning of incubation to whiterot fungi. During further incubation, the differences diminished. In contrast, higher irradiation resulted in higher degree of cellulose depolymerisation, what makes wood significantly more susceptible to biodegradation by brown-rot fungi. Degradation of gamma-irradiated wood is greater and faster due to easier accessibility of simpler

Part of the research on the influence of gamma radiation onto wood properties was supported by the COST Action E37 in the frame of a Short-Term Scientific Mission (STSM) awarded to Marin Hasan, PhD. Parts of the experimental work were carried out at the Federal Research Centre for Forestry and Forest Products (BFH), Hamburg, Germany, which was the STSM host institution, and whose hosting is gratefully acknowledged. The authors extend their particular thanks to BFH and to Ruđer Bošković Institute in Zagreb, Croatia for gamma radiation provision and to its scientists for sharing their wide-ranging

Ardica, S., Calderaro, E., Cappadona, C. (1984) Radiation pretreatments of cellulose

Bakraji, E. H, Salman, N, Al-kassiri, H. (2001) Gamma-radiation-induced wood-plastic

Bakraji, E.H., Salman, N., Othman, I. (2002) Radiation-induced polymerization of acrylamide

Bogner, A. (1993) Modifikacija površine bukovine radi poboljšanja lijepljenja. Doktorska disertacija. Šumarski fakultet, Zagreb. pp. 141. (in Croatian language). Bogner, A., Ljuljka, B., Grbac, I. (1997) Improving the glued joint strength by modifying the beech wood (*Fagus sylvatica* L.) with gamma rays. Drvna industrija 47(2): 68-73. Brischke, C., Rapp, A.O., Welzbacher, C.R. (2006a) High-energy multiple impact (HEMI) –

Brischke, C., Welzbacher, C.R., Rapp, A.O. (2006b) Detection of fungal decay by high-energy

Brischke, C., Welzbacher, C.R., Huckfeldt, T. (2008) Influence of fungal decay by different

multiple impact (HEMI) testing. Holzforschung 60 (2): 217-222.

materials for the enhancement of enzymatic hydrolysis – II. Wood chips, paper, grain straw, hay, kapok. Instituto di Applicazioni e Impianti Nucleari, University

composites from Syrian tree species. Radiation Physics and Chemistry. 61(3): 137-

within Okoume (Aucoumea klaineana pierre). Radiation Physics and Chemistry.

test – Part 1: A new tool for quality control of thermally modified timber. Document No. IRG/WP 06-20346, International Research Group on Wood

basidiomycetes on the structural integrity of Norway spruce wood. Holz als Roh-

carbohydrates to fungi.

**6. Acknowledgements** 

**7. References** 

141.

scientific expertise on gamma radiation.

of Palermo, Italy.

64(4): 277-281.

Protection, Tromsø, Norway.

und Werkstoff 66: 433-438.

#### **Resistance against white-rot fungi**

Considering the patterns and mechanisms of decay of white-rot fungi, in the beginning of decay they utilise simple carbohydrates – the ones incurred by gamma radiation. During further incubation, white rot fungi mainly utilise lignin, and they use radical mechanisms to degrade lignin. Broken (somehow modified) lignin accompanied with radicals incurred in wood by gamma radiation could negatively interfere with these decay patterns. Therefore no significant difference in ML caused by white-rot fungi between autoclaved and irradiated specimens was determined (Despot *et al.*, 2006; Hasan *et al.* 2006a).

In contrary to white rot, brown rot decay mechanisms, particularly decay mechanism of *P. placenta,* are less radical dependent, higher radiation doses do not influence the decay patterns of this brown rot fungus. In contrast, higher irradiation resulted in higher degree of depolymerisation, what makes wood significantly more susceptible to dacay (Despot *et al.* 2006, 2007; Figure 17).

Fig. 17. Correlation between Mass loss (ML) of autoclaved and irradiated specimens and incubation time to fungus: A) *Poria placenta*; B) *Trametes versicolor*.

However, it has to be taken into consideration that depolymerisation of wood components by gamma irradiation is one aspect that could explain differences in changed "natural" durability of pine wood and needs to be considered for wood durability testing.

#### **5. Summary**

Gamma radiation at a level of 30 to 150 kGy causes irreversible and permanent changes in chemical and mechanical properties of wood. Using the HEMI test method it was possible to detect small but significant changes in structural integrity and brittleness of wood caused by gamma radiation. With increasing radiation dose the total amount of water-soluble sugars increased linearly, while the maximum swelling seems to be unchanged.

The method of sterilisation has a considerable influence on the natural durability of pine wood. However, wood sterilisation by gamma radiation has different influence on white rot and brown rot causing fungi. Significantly greater mass loss of gamma irradiated wood than autoclaved wood has been established in the beginning of incubation to whiterot fungi. During further incubation, the differences diminished. In contrast, higher irradiation resulted in higher degree of cellulose depolymerisation, what makes wood significantly more susceptible to biodegradation by brown-rot fungi. Degradation of gamma-irradiated wood is greater and faster due to easier accessibility of simpler carbohydrates to fungi.
