Changes in chemical composition of decayed Scots pine and beech wood

2.1 Wood samples and accelerated aging procedure

Sapwood of Scots pine (Pinus sylvestris L.) and beech (Fagus orientalis L.) 10×5×20 mm (radial×tangential×longitudinal) were machined from the logs obtained from Gumushane and Macka located in the northeast Black Sea region of Turkey. Ten replicate samples for each group were conditioned in a conditioning room at 22°C and 65% relative humidity (RH) for 4 weeks before accelerated aging procedure. Accelerated aging procedure was performed according to ASTM D1037-89 [33] standard. Samples exposed to total six cycles for 12 days. One cycle consisted of six treatment steps: (1) immersion in water at 49°C for 1 h, (2) steaming at 93°C for 3 h, (3) freezing at -12°C for 20 h, (4) drying at 99°C for 3 h, (5) steaming at 93°C for 3 h, and (6) drying at 99°C for 18 h.

Samples were conditioned at 22% and 65% RH for 6 weeks after accelerated aging procedure.

2.2 Decay test

Decay test was performed according to principles of EN 113 [34], with some modifications on sample dimensions, Kolle flasks, and total test period. Instead of Kolle flasks, plastic sterile Petri dishes (∅, 9 cm) were used. Malt extract agar of 4.8% concentration and samples were sterilized in an autoclave at pressure of about 0.1 MPa at 120°C for 25 and 45 min, respectively. Two brown-rot fungi, namely, C. puteana (Mad-515) and P. placenta (Mad-698-R), were inoculated to sterile malt extract agar medium in the Petri dishes. Two samples (unaged and aged) were placed on the growing mycelium in each Petri dish and then were incubated at 20°C and 65% RH for 8 weeks. At the end of the test, samples were removed from the Petri dishes, cleaned, and dried at a temperature of 103±2°C. Weight loss was calculated on the basis of oven-dry weight before and after decay test. Brown-rot fungi were chosen in the study because brown-rot wood decay represents a major problem in the storage and preservation of wooden structures [35] and causes a rapid destructive decay, which is the main reason of failure in wooden structures and buildings [1].

2.3 FT-IR ATR measurements

Scots pine and beech samples were ground in a Wiley mill with a mesh size of 0.5 mm (IKA MF10, IKA-Werke, Staufen, Germany) for the FT-IR ATR analysis. Finely ground wood flour was pressed into pellets with a diameter of 10 mm. FT-IR spectra were obtained from pellets using a Perkin-Elmer Spectrum 100 with a Universal ATR sampling accessory. Four accumulated spectra were collected in the wave number region of 650–4000 cm^-1, with a spectral resolution of 4 cm^-1. In this study, fingerprint region (650–1800 cm^-1), where the main structural changes occurred, was studied. The assignments of the peaks to structural components [25, 29, 36, 37] are shown in Table 1.

3.1 Weight loss of samples

Weight loss of samples after decay test is shown in Table 2. Weight loss was found to be 25.2–45% for Scots pine samples and 29.1–46.4% for beech samples depending on fungi and being aged and unaged samples, respectively.

Accelerated aging treatments generally increased weight loss of samples for both wood species, probably affected wood structure and natural durability of wood negatively, and made the wood more susceptible to decay. Kartal and Green [19] reported that accelerated aging treatments caused more weight loss in MDF and solid wood samples in comparison with the controls and found that the weight loss of aged pine and beech wood samples was 30% greater than controls after P. placenta attack for 12 weeks. Results were in accordance with the findings in this study. Accelerated aging treatments such as water immersion, boiling, steaming, freezing, and drying may result in leaching of the water-soluble substances and hence reduce decay resistance [19]. Poria placenta caused more weight loss in aged samples than C. puteana did. Probably, the degradation of the cell wood attacked by C. puteana occurs firstly on the wall surface, but in the case of P. placenta the destruction of cellulose proceeds deep inside the wall even after 1 month of fungal attack [38]. Aging treatments accelerated biodegradation of wood with P. placenta in comparison with C. puteana. This assumption is also confirmed by FT-IR spectra (Figures 1 and 2). In contrast to findings in aged samples, C. puteana caused more weight losses in unaged samples in comparison with the weight losses observed by P. placenta attack. This could be related to nature of enzymes. It is reported that P. placenta tends to degrade amorphous regions of cellulose more readily, while the crystalline regions remain less damaged due to P. placenta containing endoglucanases and glycosidases but not exoglucanases. Coniophora puteana is able to degrade both amorphous and crystalline regions due to Coniophoroid brown-rot fungi that produce the full enzyme complement [38]. Surprisingly, beech samples showed greater weight loss than Scots pine samples. It was reported that brown-rot fungi, Gloeophyllum trabeum and Laetiporus sulphureus, could equally decay softwood and hardwood species [23].

Figure 1

FT-IR spectral changes of Scots pine samples: (A) unaged samples, (B) aged samples, (C) unaged samples degraded by C. puteana, (D) aged samples degraded by C. puteana, (E) unaged samples degraded by P. placenta, and (F) aged samples degraded by P. placenta.

Figure 2

FT-IR spectral changes of beech samples: (A) unaged samples, (B) aged samples, (C) unaged samples degraded by C. puteana, (D) aged samples degraded by C. puteana, (E) unaged samples degraded by P. placenta, and (F) aged samples degraded by P. placenta.

3.2 Changes in chemical composition of wood samples due to aging treatments

FT-IR ATR spectra show the changes in chemical composition of aged and unaged samples exposed to brown-rot fungi in Figure 1 for Scots pine and Figure 2 for beech, respectively. The intensity of peaks at around 1730 cm^-1 (3), 1593 cm^-1 (5), 1455 cm^-1 (7), 1422 cm^-1 (8), 1370 cm^-1 (9), 1323 cm^-1 (10), and 1230 cm^-1 (12) were higher in control beech samples, while the peaks at around 1506 cm^-1 (6), 1261 cm^-1 (11), and 808 cm^-1 (17) were higher in control Scots pine samples. Beech as a hardwood species has higher xylan (1730 cm^-1) and carbohydrate content than pine, and Scots pine as a softwood species has higher lignin content (1506 cm^-1) than beech [29]. It was shown that guaiacyl-type lignin (softwood lignin) developed the peak at around 1261 cm^-1 (11) and 1230 cm^-1 (12) for Scots pine and that the syringyl-type lignin (hardwood lignin) developed the peak at around 1230 cm^-1 (12) for beech similar to findings in a study carried out by Pandey and Pitman [29].

Accelerated aging treatments seemed to have a slight role on chemical composition of Scots pine and beech samples. No marked changes were observed at the intensity of peaks, except for some slight changes in the following peaks responsible for lignin, cellulose, and hemicellulose. Peaks of aged pine samples at around 1730 cm^-1 (3), 1650 cm^-1 (4), 1506 cm^-1 (6), and 1370 cm^-1 (9) slightly decreased, while the peaks at around 1593 cm^-1 (5), 1422 cm^-1 (8), 808 cm^-1 (17), and 664 cm^-1 (18) slightly increased compared to peaks of unaged samples. Peaks of aged beech samples at around 1027 cm^-1 (15) slightly decreased, while the peaks at around 1730 cm^-1 (3), 1593 cm^-1 (5), 1506 cm^-1 (6), and 1422 cm^-1 (8) slightly increased compared to peaks of unaged samples. As can be seen from Table 1, changes in peak intensities of aged Scots pine were supposed to be related to some modifications in hemicellulose and cellulose after aging treatments. Heat applications and weathering factors (UV and rain) generally cause a decrease at the intensity of 1730 cm^-1 peak probably due to cleavage of acetyl groups [39–41] and leaching of functional groups [42, 43]. The small decrease of the water peak at 1650 cm^-1 for pine samples might be related to the decrease of the carbonyl peak [40]. The small decrease at 1370 cm^-1 peak for aged Scots pine and at 1027 cm^-1 peak of aged beech samples was related to the CH₂ bending in cellulose and hemicellulose and C-O stretching in cellulose and hemicellulose, respectively. The increase at the intensity of 1422 cm^-1 peak showed C-H deformation in lignin and carbohydrates after aging treatments for aged Scots pine and beech samples, and the increase at the intensity of 664 cm^-1 peak showed some modifications in cellulose for aged Scots pine samples. The lignin peak at 1506 cm^-1 decreased for aged Scots pine samples, while it increased for aged beech samples probably due to composition of lignin types in softwoods and hardwoods. Slight or negligible changes in lignin component of wood might be related to its greater resistant against heat applications compared to other wood components [41]. Lignin component of wood samples seemed to be increased after aging. This might be related to the degradation of wood carbohydrates relative to lignin component of wood. Changes in beech samples were considerably less than changes in Scots pine samples due to beech having greater density than Scots pine samples, which improved resistance against aging treatments. Heavy and strong woods are more resistant to weathering factors than softwoods [43].

3.3 Changes in chemical composition of wood samples due to decay

A broad band attributed to hydroxyl groups or absorbed water was seen at 3336–3342 cm^-1 (1) for both wood species, and following C-H stretching, absorption band (2) was observed. The carbonyl peak at 1730 cm^-1 (3) stayed nearly unchanged after C. puteana attack in aged Scots pine samples; however, the peak in aged samples decreased after P. placenta attack. Decrease in the peak intensity of unaged pine samples was recorded after both fungi attack. In the case of beech samples, intensity of carbonyl peak at 1730 cm^-1 (3) decreased compared to the initial peak before decay test, except for unaged samples that were exposed to P. placenta. The reduction on this peak related to the degradation of glucurono-xylanes connected with splitting of acetyl groups and 4-O-methyl glucuronic acids side units [21]. The presence of the carbonyl peak is due to residual xylan that still remains in decayed samples [29, 31]. Decrease in carbonyl absorption peak was in accordance with the increase in weight losses for both wood and fungi species.

The peaks responsible for wood carbohydrates at 1370 cm^-1 (9), 1150 cm^-1 (13), and 897 cm^-1 (16) decreased for both aged and unaged pine samples after fungi attack. The reduction at 1150 and 897 cm^-1 was greater in pine samples after P. placenta attack than C. puteana attack. The peak intensities at 1316 cm^-1 (10) and 664 cm^-1 (18) decreased in aged samples after P. placenta attack; however, those peaks showed an increasing tendency in unaged samples. In the case of beech samples, the peaks responsible for wood carbohydrates at 1370 cm^-1 (9), 1027 cm^-1 (15), and 897 cm^-1 (16) decreased for both aged and unaged samples after fungi attack. The reduction at 1370 and 897 cm^-1 was greater in aged beech samples after P. placenta attack than C. puteana attack. The peak at 1322 cm^-1 (10), 1100 cm^-1 (14), and 664 cm^-1 (18) increased in beech samples after fungi attack. The small increase in the peak intensity of samples at 1200–1000 cm^-1 after decay could be supposed to be related to higher glucan content compared to control samples [28]. In beech samples, the peak intensity at 664 cm^-1 (18) was greater after C. puteana attack than the peak intensity of samples after P. placenta attack. Peak intensities responsible for wood carbohydrates decrease with the increase in weight loss caused by brown-rot fungi attack [8, 29, 30]. In general, changes in the carbohydrate peaks were greater in pine and beech samples degraded by P. placenta and in samples exposed to accelerated aging treatments.

Changes in lignin shown at the peaks of 1650 cm^-1 (4), 1593 cm^-1 (5), 1506 cm^-1 (6), 1455 cm^-1 (7), 1422 cm^-1 (8), 1261 cm^-1 (11), and 1230 cm^-1 (12) resulted in an increase in the intensities and changes in lignin at the peak of 808 cm^-1 (17) resulted in a reduction in the intensity after fungi attack for both aged and unaged pine samples. Coniophora puteana caused more increase in the peak intensity at 1593 cm^-1 (5) and 1422 cm^-1 (8) but more reduction at 808 cm^-1 (17) than P. placenta did. The increase at the peak intensities of 1506 cm^-1 (6) and 1455 cm^-1 (7) was greater in pine samples exposed to P. placenta attack than C. puteana attack. The peak at 1261 cm^-1 (11) increased more in aged pine samples after P. placenta attack, while the peak increased more in unaged samples after C. puteana attack. The intensity at 1230 cm^-1 (12) increased in aged samples after fungi attack. In the case of beech samples, changes in lignin shown at the peaks of 1650 cm^-1 (4), 1506 cm^-1 (6), 1455 cm^-1 (7), and 1422 cm^-1 (8) resulted in an increase in the intensities, but changes in lignin at the peak of 1593 cm^-1 (5) resulted in a slight reduction in the intensity after fungi attack for both aged and unaged samples. The increase at the peak intensities of 1506 cm^-1 (6) and 1455 cm^-1 (7) was greater in aged beech samples exposed to P. placenta attack than C. puteana attack. Lignin and carbohydrate peaks in aged beech samples behaved well-adjusted with the weight loss of samples. Intensity increase at 1660 cm^-1 peak supports the proposed oxidative degradation [5] and suggested that brown-rot fungi promote the formation of new conjugated and unconjugated acid substructures in the side chain of the lignin [28]. The peaks at 1455, 1422, and 1230 cm^-1 have some contribution from carbohydrates. In general, changes in lignin peaks were greater in aged pine and beech samples than unaged samples, and P. placenta seemed to cause more changes in lignin for aged samples than C. puteana did.

Intensity of the carbohydrate peaks decreased, whereas the intensity of the lignin peaks increased as the brown-rot decay progressed [5, 6, 8, 22, 29, 30] and elevated levels of the syringyl moiety in beech and guaiacyl moiety in pine left [29]. An increase in the lignin absorption peaks in decayed wood was attributed to its oxidative modification by Fenton reaction-derived reactive oxygen species [5].