Comparative analysis of modern and ancient buried Phoebe zhennan wood:surface color,chemical components,infrared spectroscopy,and essentialoil composition

Comparative analysis of modern and ancient buried Phoebe zhennan wood:surface color,chemical components,infrared spectroscopy,and essentialoil composition

Jiulong Xie?Jinqiu Qi?Xingyan Huang?Ni Zhou?Yao Hu

We compared the chemical components and essential oils of ancient buried Zhennan(Phoebe zhennan) wood with those in samples from living trees.After removal of the carbon layer the recovered Zhennan exhibited a dark green color,which differed from the yellow color of the living samples.Low molecular weight components(including hot-water and toluene-alcohol extractives),hemicellulose,and 1%NaOH solubility in the recovered wood were greatly degraded.Degradation of cellulose was minor.Moreover,the ancient wood had somewhat more klason lignin than the modern wood. Fourier transform infrared(FTIR)analysis gave further evidence on the differences in chemical components. According to the GC–MS results,naphthalene derivatives were detected in the essential oils from both the modern and recovered wood.The delicate fragrance of the modern and recovered wood may be attributed to the aromatic constituents as identified by GC–MS.

Phoebe zhennan·Ancient buried wood· Chemistry·Essential oil

Introduction

Phoebe zhennan(Family:Lauraceae,Category:Phoebe) commonly called Zhennan is an endemic species in Sichuan Province,China and is well known for its durable wood,and termite and insect resistance.Other characteristics of Zhennan wood thatmake itpopular in China are its special fragrance and attractive surface color,usually with a visible golden tint.Because of its outstanding features, Zhennan wood was widely used in the construction of the Forbidden City since the Ming Dynasty.However,due to deforestation in recent centuries,Zhennan timbers of large diameter are increasingly scarce,resulting in sharp increases in the prices of the high quality products(structural and decorative materials and furniture)manufactured from them.In mainland China,Zhennan had been listed as a category II protected plant in the‘List of Wild Plants Under State Protection’,which is also attributed to the high prices of currently available Zhennan wood products.

Timber buried by floods and earth quakes has recently been recovered from the channel of the Qingyi River in southwestern Sichuan Province,China(Qi et al.2008). Most of the buried trees were identified as Zhennan. Because of the high quality and attractive color and aromas of Zhennan,the recovered timber has been processed for use in the manufacturing of art and craft products associated with traditional Chinese culture.These goods,which represent a kind of Chinese wood culture,are called‘Chinese Magical Wood’by foreigners.With the development of the emerging cultural industry,the unearthed Zhennan wood has earned a large fortune for residents,and the price of the ancient buried Zhennan has increased rapidly.For example,the annual sales of these products made from ancient Zhennan wood in LuShan,a smalltown in Sichuan,was about 1 billon RMB,and the unit price forthe raw materialwas about50,000 RMB ton-1(8,000 USD ton-1).

The main factors influencing the price of the recovered Zhennan include surface color,fragrance,and timber diameter.However,it is now very difficult for people to evaluate these characteristics because there are no referenced standards.This has resulted in trade disputes in the last few years.Hence,standards were needed to consistently evaluate the properties of ancient buried wood in terms of its commercial value.Though selected properties (physical and mechanical properties)of modern Zhennan woods(Li et al.2013)and the characteristics of several species of ancient buried woods(Blanchette et al.1991; Pan et al.1990;Wayman et al.1971)have been reported, information on the properties of recovered Zhennan has been ignored.Thus,the objective of this study was to compare the differences in chemicalcomponents,chemical structural groups,and essential oil composition between modern and ancient buried Zhennan wood.

Materials and methods

Materials

Recovered Zhennan was provided by a wood art museum located in Ya’an,Sichuan Province,China.The samples were obtained from a 90-year old Zhennan tree that had been buried for approximately 3,000 years.Modern Zhennan samples were obtained from an 84-year old living Zhennan tree growing in an old temple in Yingjing countryside(Ya’an,Sichuan Province).Both the recovered and modern Zhennan wood samples were placed in room temperature conditions for air drying.Allchemicals used in this investigation were of reagent grade and obtained from commercial sources.

Surface color measurement

Color measurement was conducted using a color meter. The lightsource was D65and the diameter of the measuring window was 8 mm.Specimens were placed directly at the measuring window.Color parameters(L*,a*,b*) according to the International Commission on Illumination were obtained directly from the color meter and were used for color evaluation.The brightness difference(ΔL*),difference of a*component(Δa*),difference of b*component(Δb*),chroma difference(ΔC*),hue difference (ΔH*),and color difference(ΔE*)were calculated using the following formulas:

where,L*was lightness with values varying from 0(black) to 100(white).The parameters a*and b*described the chromatic coordinates on green–red and blue-yellow axes, respectively.C*was chroma,i.e.,C*=[(Δa*)2+(Δb*)2]1/2.L0*,a0*,and b0*are the reference values obtained from the recent wood specimens.Five replicates were used and the average value was calculated for color analysis.

Chemical components analysis

Air-dried recovered and modern wood samples were reduced to particles using a Wiley mill.The particles were placed in a shaker,and fractions with particle sizes of 40 to 60 mesh were collected.The alpha-cellulose,Klason lignin contents,hot-water extractive,alcohol-toluene extractive, and 1%NaOH solubility of the wood samples were determined according to ASTM standards D1103-60, D1106-96,D1107-96,D1110-96,and D1109-84,respectively.The hemicellulose contentwas determined using the method reported by Zhang et al.(2012).

FT-IR analysis

The FT-IR analysis was performed using a Nicolet Nexus 670 spectrometer equipped with a Thermo Nicolet Golden Gate MKII Single Reflection ATR accessory.A small amount of sample was applied directly on the diamond crystal.Data collection was performed with a 4 cm-1spectral resolution,and 32 scans were taken per sample.

Essential oil extraction and identification

Essential oil was extracted from both the modern and recovered Zhennan samples according to a referenced hydro distillation method(Charles and Simon 1990).

The generalprofiles ofthe essentialoilsamples were analyzed using electron ionization mass spectrometry(EI-MS). Analysisofthe productwasconducted on a mass spectrometer (Agilent 5975C VL MSD),and the products were separated into their components using a gas chromatograph(Agilent 7890A)equipped with a fused capillary column(HP-5MSHP-5MS,5%PhenylMethylSilox,L=30 m,i.d.0.25 mm,film thickness 0.25μm)with 5%phenyl and 95%dimethylpolysiloxane as the stationary phase.The carriergas was helium ata flow rate of 1.5 mL min-1.

Condition for analysis:injection mode was split at split rate 35;the column was held at 50°C for 3 min and then heated to 150°C atthe rate of 5°C min-1,for 10 min,and thereafter heated to 250°C at 10°C min-1,for 5 min. Essential oil constituents were identified by using total ion chromatograms as well as fragmentation patterns.

Statistical analysis

Statistical analysis was carried outusing SAS(version 9.1, SAS Institute,Cary,NC).Analysis of variance(ANOVA) was used to assess differences in means(atα=0.05) between the parameters tested for modern and recovered wood samples.

Results and discussion

Surface color

Differences in surface color between the modern and recovered wood were characterized based on CIE LAB color parameters(Table 1).The L*,a*,b*values for the modern and ancient samples were 58.0 and 43.6,13.5 and -10.0,30.7 and 36.6,respectively.Wide color variation was observed,indicated by the large hue difference(24.3) and color difference(29.1).

a*is the value on the red-green axis:it proved reasonable and easier to evaluate color differences by comparing a*values.The a*value for the modern sample was positive and for the ancient sample a*was negative,meaning that the ancient wood exhibited a green color while the surface color of the modern wood was yellow.The significant variation in surface color between the recent and ancient Zhennan wood might be associated with differences in the chemical components.

Chemical components

The main chemical compounds of the modern and recovered Zhennan wood are listed in Table 2.The concentrations of hot-water extractive and alcohol-toluene extractive compounds for modern wood were significantly higher than for ancient wood.

Differences in carbohydrate portionsbetween the modern and recovered samples were represented by values forαcellulose and hemicellulose.Generally,α-cellulose represents the undegraded,high-molecular-weightcellulose.For comparison,α-cellulose in modern wood was insignificantly higherthan in ancientwood,indicating thatcellulose wasnot significantly degraded by long-term burial in soil.In contrast,hemicellulose in modern wood was significantly higher (by about 10%)than in ancient wood.Different from cellulose,the depolymerization of hemicellulose was greater in ancient wood.This result is consistent with the fact that cellulose is much more stable in response to acid hydrolysis than are hemicelluloses.However,our findings contrast those of Iiyama et al.(1988),who found that cellulose in ancient buried hardwoods was degraded to a greater extent than was hemicellulose,and thisphenomenon was attributed to chemicalagents and microorganisms.Based on the results of Iiyama etal.(1988),it could be concluded that degradation of carbohydrates in ancient Zhennan wood was mainly induced by chemical agents rather than by anaerobic microbiologicalattack.

The change in Klason lignin was opposite to that of the carbohydrate portions and extractive compounds.Lignin content in modern wood was about 7%lower than inancient wood(Table 1).This result might be attributed to chemical degradation,mainly to the degradation or weight loss of cellulose,hemicellulose,inorganic elements and extractives,and thereafter the residue was enriched, resulting in the(relatively)higher lignin content.

Table 1 Surface color of recent and ancient Zhennan wood samples

Table 2 Chemical components of recent and ancientburied Zhennan wood

In modern wood,1%NaOH solubility was significant higher than in ancient wood.The change in 1%NaOH solubility was similar to the changes in extractives,cellulose,and hemicellulose.Accordingly,the wood solubility in 1%NaOH contained mostly extraneous components, such as acid-soluble lignin,tannins,lipids,low molecular weight hemicellulose and degraded cellulose(Pettersen 1984).Therefore,it could be concluded that the change in 1%NaOH solubility between the modern and recovered wood could be used as an indicator for the degradation degree of the latter.

FT-IR analysis

The transmittance spectra of modern and ancient wood are shown in Fig.1.According to Lietal.(2011),a broad peak at around 3,416 cm-1can be assigned to–OH groups and the peak ataround 2,920 cm-1can be related to methyland methylene stretching.The absorbance peak at 1,740 cm-1was assigned to CO stretching vibration of the carboxyl and acetyl groups in hemicellulose.The absorbance at 1,594 and 1,510 cm-1arising from the aromatic skeletal vibration,the absorbance at 1,454 cm-1assigned to C–H deformation combined with aromatic ring vibration,and the band at 1,235 cm-1corresponding to methoxyl groups were attributed to the groups of lignin.The methylene groups at 1,422 and 1,365 cm-1.CH2rocking vibration at 1,322 cm-1,and C–O band 1,108 cm-1and CH deformation at 893 cm-1represented different functional groups of cellulose.

Fig.1 FTIR spectra of(a)modern Zhennan wood and(b)ancient Zhennan wood

Significant differences in absorption bands (1,740 cm-1)of hemicellulose in the spectra were found between the modern(spectrum a)and ancient(spectrum b) wood(Fig.1).The peak at 1,740 cm-1in the spectrum for modern wood was much more intense compared to that for ancient wood.This result gave further evidence that hemicellulose degraded after burial,resulting in lower hemicellulose content in the recovered wood.The three characteristic absorption bands(peaks at 1,594,1,510,and 1,454 cm-1)of lignin in the spectrum for ancient wood were stronger compared to those for modern wood.This result was consistent with the relatively higher lignin content in the ancient wood.However,there were no differences in the bands(1,422,1,365,1,322,and 1,108 cm-1)corresponding to cellulose between the spectra for modern and ancient woods.

Composition of essential oil

Average essentialoil yield for modern and recovered wood was 0.97 and 1.51 mL/100 g,respectively.The components in the essential oils obtained from modern and recovered wood were analyzed comparatively by GC–MS. In the oils of modern wood,56 individual peaks were observed as compared to 40 peaks for recovered wood. Naphthalene derivatives were detected in the oils from both modern and ancientwood.Cadinol(4.49%)in the oilfrom modern wood and Cedrene(4.39%)in the oil from ancient wood,which were ascribed to aromatic/flavor constituents (Yan et al.2012),might be associated with the everlasting fragrance of the modern and ancient wood.The identified compounds(only compounds with more than 0.2%of total area were listed)are listed in Tables 3 and 4.Itcan be seen that significant differences were observed in the constituents identified in the essential oils.The main differences between the two oils were their constituentcompounds and their concentrations.

According to GC–MS analysis results,the major identified compounds in the essential oil from ancient wood were 1-(3-Methylbutyl)-2,3,4,5-tetramet hylbenzene (21.72%)and Caryophyllene oxide(8.62%),while the main compounds in the essential oil of modern wood wereγ-Cadinene(21.47%)and 1H-Cycloprop[e]azulene, decahydro-1,1,7-trimethyl-4-methylene-,[1aR-(1a.alpha., 4a.beta.,7.alpha.,7a.beta.,7b.alpha.)](22.47%).In the report on the characteristics of buried Chinese Fir wood, Lu et al.(2000)documented differences in essential oil constituents and argued that this may be related to changes in environment and climate.

Table 3 Compounds of essential oil from recent Zhennan wood

Conclusion

Modern and ancient Zhennan woods differed significantly in their chemical constituents.The concentrations of hemicellulose and extractives,and 1%NaOH solubility for modern wood were much higher than those for ancient wood.Ancient wood had higher Klason lignin content. Cellulose in ancient wood was slightly degraded but not significantly different from that in modern wood.The barely detectable peak corresponding to hemicellulose in the spectrum for ancient wood was consistent with great degradation/weight loss of hemicellulose.In terms of essentialoilcomposition,more constituents were identified in the essential oils from modern wood than from ancient wood.Differences in constituents and their quantities were observed between the essential oils.

Table 4 Compounds of essential oil from ancient buried Zhennan wood

ASTM D 1103-60(1971)Standard testmethod for alpha-cellulose in wood

ASTM D 1106-96(1996a)Standard Test Method for acid-insoluble lignin in wood

ASTM D 1107-96(1996c)Standard test method for ethanol-toluene solubility of wood

ASTM D 1110-96(1996b).Standard test method for water solubility of wood

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12 December 2013/Accepted:25 February 2014/Published online:27 January 2015

?Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

Project funding:The project was supported by‘Key Laboratory of Wood Industry and Furniture Engineering of Sichuan Provincial Colleges and Universities’.

The online version is available at http://www.springerlink.com

Jiulong Xie,Xingyan Huang and Ni Zhou have contributed equally to this work.

Corresponding editor:Yu Lei

J.Xie·J.Qi(?)·X.Huang·N.Zhou·Y.Hu

College of Forestry,Sichuan Agricultural University, Ya’an 625014,Sichuan,China e-mail:qijinqiu2005@aliyun.com