Variation in the composition and content of in the heartwood of European oaks (Quercus robur and Q petraea). A comparison of two French forests and variation with heartwood age Jr Mosedale, B Charrier, N Crouch, G Janin, Ps Savill

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Jr Mosedale, B Charrier, N Crouch, G Janin, Ps Savill. Variation in the composition and content of ellagitannins in the heartwood of European oaks (Quercus robur and Q petraea). A comparison of two French forests and variation with heartwood age. Annales des sciences forestières, INRA/EDP Sciences, 1996, 53 (5), pp.1005-1018. ￿hal-00883112￿

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Variation in the composition and content of ellagitannins in the heartwood of European oaks (Quercus robur and Q petraea). A comparison of two French forests and variation with heartwood age

JR Mosedale B Charrier2 N Crouch3 G Janin4 PS Savill1

1 Department of Plant Sciences, Oxford Forestry Institute, University of Oxford, South Parks Road, Oxford OX1 3RB, UK; 2 École supérieure du bois, CP 3029, rue Christian-Pauc, 44087 Nantes cedex 03, France; 3 Dyson Perrins Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QY, UK; 4 Station de recherches sur la qualité des bois, Inra, 54280 Champenoux, France

(Received 3 March 1995; accepted 28 July 1995)

Summary - The concentration was measured in water extracts of different heartwood sections of Pressler cores from three Russian and one English Quercus roburtree. As the heartwood age increased, the concentration of total soluble ellagitannins showed a logarithmic decline, while individual ellagitannins varied in their response. A simple model relating the total soluble ellagitannins and heartwood age was calculated. In a second study two heartwood samples were taken from each of 20 oak trees (Q roburand Q petraea) from each of two contrasting (Limousin and Tronçais) French forests. Over 70% of the total variation in the concentration of water soluble ellagitannins and total phenolics extracted from the samples was attributed to differences between forests, while relatively little variation occurred between the two within-tree samples. Lower concentrations were found in more slowly grown timber from the Tronçais forest than in wood from the Limousin region. The different concentrations could not be explained solely by the greater heartwood age of Tronçais samples if one assumed that the rate of ellagitannin decline with heartwood age was similar in all trees. A correlation between wood colour, as defined by CIELab colour parameter hue, colour saturation and b* (representing colour along the blue-yellow axis), and total phenolics and soluble was also observed. The two forests differed in many regards, including environmental conditions, silvicultural practices and the dominant species.

Quercus robur / Quercus petraea / CIELab colour / ellagitannins / heartwood age

Résumé - Variation de la composition et de la teneur en ellagitannins dans le bois de cœur des chênes européens (Quercus robur, Q petraea). Comparaison de deux forêts françaises et variations en fonction de l’âge du bois de cœur. La concentration des ellagitannins a été mesurée dans les extraits acqueux des différentes parties du bois de cœur de carottes de sondage provenant de trois chênes (Quercus robur) prélevés en Russie et un prélevé en Angleterre. La concentration des ellagitanins solubles totaux présentait une diminution logarithmique au fur et à mesure que l’âge du bois augmentait, tandis que les divers ellagitanins présentaient des teneurs variables. Un modèle simple reliant la concentration en ellagitanins solubles totaux avec l’âge a été établi. Dans une étude ultérieure, deux échantillons de bois de cœur provenant chacun de 20 chênes européens (Q robur et Q petraea) issus chacun de deux forêts françaises très différentes (Limousin et Tronçais). Plus de 70 % de la variation totale de la concentration en ellagitanins solubles extraits de ces échantillons a été attribuée à la différence entre les forêts, tandis qu’une relativement faible variation pouvait être attribuée aux deux échantillons prélevés à l’intérieur de chaque arbre. Les teneurs en tanins solubles étaient plus faibles chez les arbres provenant de la forêt Tronçais que chez ceux de la région du Limousin, et ceci ne pouvait pas être expliqué seulement par les légères différences d’âge du bois de cœur des échantillons. Une corrélation entre la couleur du bois, mesurée dans le système CIELab avec les paramètres de teinte (h), de saturation (C) et la coor- donnée chromatique (b*), la teneur en phénols totaux et les tanins solubles a été aussi observée. Les deux forêts présentaient bien des différences du point de vue de l’environnement, des traite- ments sylvicoles et des dominances d’espèces.

Quercus robur / Quercus petraea / couleur CIELab / ellagitanins / âge du bois de cœur

INTRODUCTION oak such as Q alba L (Rous and Alderson, 1983; Quinn and Singleton, 1985; Miller et The hydrolyzable tannins have been estimated al, 1992). There is also a long tradition to comprise up to 10% of the dry weight of hear- within the wine- and brandy-making indus- twood of European oak (Scalbert et al, 1988). tries that the flavour imparted by oak casks Numerous studies have reported how the varies according to the geographic origins concentration of soluble tannins declines of the oak wood used in their construction. as the age of the heartwood increases, Although the role of the hydrolyzable tan- away from the sapwood boundary towards nins in influencing flavour is uncertain (Vi- the pith of trees (Peng et al, 1991; Klumper- riot et al, 1993), it is probable that the con- s et al, 1994; Viriot et al, 1994; Charrier et centration and composition of oak wood al, 1995). However, Viriot et al (1994) re- extract will influence flavour imparted by ported how the concentration of individual oak casks. However, as reviewed by Mose- ellagitannins responded in different ways to dale (1995), numerous factors may in- heartwood age. They proposed a series of fluence the extractive properties of oak reactions as occurring during heartwood wood. The few studies that have compared ageing. During the first 30 years of ageing, different species or origins of European oak there is conversion from monomeric to wood have generally failed to control other dimeric tannins. Hydrolysis reactions occur influencing factors sufficiently (such as throughout heartwood ageing at a slow rate wood age and storage conditions) or repli- estimated as 1 % of the total every 10 years. cation has been insufficient (eg, Puech, However, the polymerization of ellagitan- 1984; Miller et al, 1992; Marco et al, 1994). nins into larger is thought to be Studies of the variability of other wood the main cause of the decline in soluble properties, such as density, have generally tannins as heartwood ages. concluded that the greatest degree of vari- Few studies have examined the degree ation occurs between different trees within of variation in ellagitannin concentrations a forest and between provenances (Zobel that occurs between trees, populations and and Talbert, 1984). the two European oak species Q petraea The primary aim of this study was to (Matt) and Q robur L. Levels of tannins in examine the variation in soluble ellagitan- the heartwood of these two species have nins of European oak wood between and been reported to be greater than those within trees felled in two forest coupes. The found in the heartwood of American white forests were selected to correspond to two opposing types of French oak that are used One site was located in a forest near Tronçais, by the cooperage industry and frequently the other in the Limousin region of France. From each of these clear felled sites 20 se- claimed to have different effects on the fla- randomly lected trees, of suitable for vour of wine and To determine the quality cooperage, brandy. were chosen. the of and cut- relative of variation between and During splitting logs importance ting of bolts, two staves were removed and used within mature additional were trees, samples for this study. These staves were cut from the used to confirm the variation of soluble ellagi- outer heartwood, near the base of north and tannins with heartwood age. south facing sides of the bole. Therefore, for each site a total of 40 samples from 20 trees were examined. MATERIALS AND METHODS The 80 staves were stored for approximately 4 months before a hand-held plane was used to Materials remove shavings from their surfaces that would make up the inner face of a barrel. After removal Variation within trees of the frequently discoloured outer surface of the stave, shavings from the top 1-2 mm were taken A core was taken with a Pressler borer at breast and were then ground (Glen Creston type 14- from each of four trees of between 100- height 580 mill) and air-dried to reduce moisture con- 120 of Three of the trees came from years age. tent to approximately 4% of dry weight. an oak forest near Voronezh, Russia, having been collected in May 1993, while the other was from an isolated field boundary oak near Oxford, Methods taken in 1990. All the trees were Quercus robur and displayed regular and rapid growth throug- Determination of soluble ellagitannins hout the core The cores were cut into lengths. The concentrationof soluble ellagitannins ex- different sections according to the age of the tracted from samples of each French oak stave heartwood from the heartwood-sapwood and of each heartwood age zone from the four 11-20 so on in boundary: 0-5, 6-10, yearsand Pressler cores was measured. The ellagitannins of 10 to 40-70 steps years up years according were extracted from 50 mg of wood over a period to the tree. Wood from each zone were samples of 24 h at room temperature with 5 mL of the ground to less than 100 mesh and soluble ellagi- extracting solution: methanol/H2O/H3PO4 2/97/1 tannins measured. v/v/v, with 100 mg/L of pyrogallol used as the internal standard. After filtration the concentra- Variation between two forests tion of ellagitannins was determined by high per- Trees were compared from two forests that typi- formance liquid chromatography (HPLC). The fied contrasting types of French oaks used for solvent system allowed direct injection without the construction of casks (table I). The trees further analytical steps and was found to give felled were of suitably high standards for cooper- better separation of early peaks than solvents age. By the choice of two such contrasting sites containing higher proportions of methanol. it was intended to test the hypothesis that it is not Column: Waters reverse-phase C18; 260 x 4 mm; feasible to select for cooperage wood with signi- Spherisorb packing. Injection volume: 20 I. Detec- ficant differences in wood extractives. tion: at 230 nm (190-400 nm for dentification). Internal standard: pyrogallol (Aldrich) at Measurement of total phenolics 100 extraction solution. Gradient. The fol- mg/L Folin Denis reagent (AOAC, 1984, 1990; Scal- lowing solutions were used: H2O/H3PO4 99/1 v/v bert, 1992) was used to measure the total MeOH/H3PO4 99/1 v/v (solvent A); (solvent B). phenolics in the extracts of the 80 French oak The best of was ob- separation ellagitannins samples. One mL of Folin Denis reagent (Fisons tained using a linear gradient from 0 to 9% of diluted 1:4 with water), was added to 1 mL of the solvent B over 40 min. extraction solution followed by 1 mL of a 3% so- Identification and calibration dium carbonate solution. After agitation, the samples were placed in a water bath at 50 °C for the criteria Scalbert et al Using suggested by 20 min. After cooling for 5 min, absorbance at that have near identical ab- (1990), ellagitannins 760 nm was measured. Calibration of the spec- spectra with no maxima between 240- sorption trophotometer was performed for each batch of 400 nm but a shoulder around 280 nm, 12 samples using gallic acid (Aldrich) solutions and identified. possible ellagitannins were Acompari- the results were expressed as gallic acid equi- son of the relative retention times with results valents (GAE). Extract solutions were suitably described in earlier studies et (Scalbert al, 1988; diluted, typically by 1:5 with water. Viriot et al, 1994) allowed the identification of nine of these 12 ellagitannins (fig 1). Purified Insoluble ellagitannins and samples of vescalagin, , grandinin Insoluble in wood can be estimated roburin A Dr ellagitannins (kindly provided by Scalbert, INRA, in acid sol- allowed confirmation of their identification by degradation alcohol-hydrochloric Paris) utions measuring the resulting by and were used for calibration. HPLC or GC (Puech et al, 1990; Peng et al, RESULTS 1991; Scalbert, 1992).The concentration of inso- luble was determined in wood ellagitannins Variation of ellagitannins samples from one Limousin and one Troncais with heartwood stave. Three replicate extractions of wood age from each stave were carried out in Te- samples Due to the overlap of the peaks for flon tubes, using the solvent and conditions de- gallic acid and roburin B in some both scribed previously. After extraction the solvent samples, these were excluded from The was removed with a syringe fixed with a fine hy- analyses. podermic needle. The samples were air-dried variations in ellagitannin concentrations and re-weighed before 5 mL of MeOH/HCl 6M are illustrated in figure 2. The results indi- 9/1 v/v, containing 0.5 mg 1-naphthol (Aldrich) cate that as well as a general decline in was added to each of the residues. After heating ellagitannins, the individual tannins re- at 120 °C for 160 minutes, the solutions were spond differently during ageing. then filtered and analysed by HPLC to determine Vescalagin, the most abundant ellagitan- quantities of ellagic acid, which were expressed nin in outer heartwood, is seen to decrease as castalagin equivalents (Peng et al, 1991; Vi- riot et al, 1994). rapidly during the first 20 or 30 years of Column: Waters reverse-phase C18; ageing, after which the decline lessens or 260 x 4 mm; Spherisorb packing. Injection vol- even ceases. Castalagin, less abundant ume: 20 μL. Detection: at 280 nm (190-400 nm than vescalagin in outer heartwood, de- for identification). Internal standard: 1-naphthol clines at a slower and more constant rate, (Aldrich). Gradient: The following solvents were becoming the most abundant tannin in used: H2O/H3PO4 99/1 v/v and (solvent A) older heartwood. The other ellagitannins MeOH/H3PO499/1 v/v (solvent B) to run a linear show more diverse of variation. gradient from 0 to 100% solvent B over 30 patterns minutes with a flow rate of 1 mL/min. The dimer roburin D shows a similar pattern to vescalagin, which contrasts with the vari- Measurement of wood colour ation of roburins A and C. Roburin A in- and ring width creases in concentration during the first 10 years of ageing and roburin C over the first Ten measurements of wood colour were made 30 years, before each declines in across a cut transverse section again cleanly (radial older wood. Grandinin and roburin E show face) of each French oak stave. Mean ring widths were also determined. Colour was measured less clear patterns, but in general concen- with a Colorquest Hunterlab spectrocolourimeter trations remain approximately constant using the CIE standard illuminant D65 (corre- during the first 30 years before declining. sponding to daylight under an overcast sky) and The concentration of ellagitannins in each an observation angle of 10°. This measured the heartwood zone was then expressed as a percentage of reflected light at 32 wavelengths, percentage of that in the youngest hear- distributed at 10 nm intervals between 400 and twood (years 0-5). The means and stand- 710 nm. The reflectance spectrum was repre- ard error bars for all four trees are shown sented by the CIELab system, which has been widely used in previous studies of wood colour in figure 3. This displays a logarithmic de- (eg, Janin, 1987; Klumpers et al, 1994, 1993; cline with heartwood age. The following Charrier et al, 1995). The system represents col- simple linear model was fitted: our using L (lightness) and the chromatic coordi- nates a* (red-green axis) and b* (blue-yellow axis). Additional parameters used to describe colour may be derived from these variables. where a is the heartwood age; Ta is the These include the angle of taint or hue, h = arc- concentration of ellagitannins at age a and tan (b*/a*) and colour saturation : Toh where a = 0. This gives an estimate for b of -0.0219 with a standard error of 0.0007 and an2 R of 0.988. Therefore, if one knows the level between-tree and between-forest variation of tannins in the outermost heartwood, that are shown in table II. The two samples from in the heartwood of age a may be estimated each tree were treated as random repli- by Ta = Toh / e0.0219a. cates for this analysis. Due to one tree hav- ing only a single replicate, both this tree and Between- and within-forest variation the data of a random tree from the other site were removed from analysis, reducing the Figure 4 shows the concentration of ellagi- total degrees of freedom to 37 within each site. tannins in north -and south-facing staves of By calculating Spearman correlation each tree plotted on opposing axes. As well coefficients, highly significant correlations as illustrating the difference between the were found between all the individual el- two forests, the fact that most of the points lagitannins. The strongest correlation was lie approximately along a gradient of one that between total phenolics and total el- indicates that there were similar concentra- lagitannins, with an R value of 0.99. This tions in each of the staves from the same suggests that the Folin Denis method is an tree. The lower variation among samples effective means of comparing the tannin from the Tronçais forest than those from the contents of oak wood, supporting results Limousin is also apparent and the data described by Puech et al (1990). Viriot et al were log-transformed, resulting in more ho- (1995) reported that heartwood ellagitan- mogeneous variances. nin content determined by the Folin method A balanced, nested analysis of variance was less affected by heartwood age. Table II was used to compare the variation between shows that the difference between the forests and within trees and forests. The results and was only slightly lower for total phenols than the large proportion of variance explained by for total ellagitannins.

Wood colour and ring widths nins. A nested analysis of variance was car- ried out on arcsine-transformed percent- Of the three variables lightness (L*), a* and age data. A nonparametric comparison of b* used to define wood colour, lightness the two sites was also carried out by a Wil- varied most. However, analyses of vari- coxon two-sample test of rank sums (SAS ance found that b* and the derived only Institute Inc, 1985; Neave and variables hue and colour saturation varied Worthington, Both the and nonpar- between the two while 1988). parametric significantly forests, ametric tests found differences variation between trees and significant significant between the two sites for most of the ellagi- samples was found for all three variables. tannins (table IV). The most prominent dif- Variance components (see table III) show ference was the lower proportion of vesca- that the between-forest variation ac- lagin in the Tronçais samples. counted for a small amount of the relatively It has been observed that the total variation of wood colour. Greater be- previously proportion of vescalagin varies with hear- tween forest variation was found for ring twood The results suggest that the width, with the Limousin samples age. having are, on cut from much wider than those from Tronçais samples average, rings Tronçais older heartwood than the Limousin the wood colour (table II). Among par- samples. This is confirmed by the slower ameters, the variable b* (blue-yellow axis) growth, as indicated by narrower ring correlated most with tannin con- strongly widths, of Tronçais trees which results in tent both separately for each forest and the average heartwood age of these when the data for the two are grouped (R samples being greater than Limousin grouped = 0.640). Despite scatter this trend is in 5. Similar samples. perceptible figure Influence of wood correlations were found between total tan- age on soluble nins and both hue and colour saturation ellagitannins One could that the difference in tan- which correlate with b*. propose strongly nin concentrations between the two forests is due to the difference in heartwood Composition of ellagitannins simply age of the samples. Greater insolubulization or In order to test whether the composition of hydrolysis of soluble ellagitannins may have ellagitannins varied between sites, the per- occurred in the older samples from Tronçais. centage of each ellagitannin was calcu- lated in relation to total soluble ellagitan-

Two different approaches were used to ad- Measurement of insoluble tannins dress this possibility. in two samples

Table VI the mean concentrations of soluble tannin decline due gives Estimating insoluble in to heartwood age ellagitannins remaining samples of Limousin and Tronçais wood The heartwood age of samples from each after water extractions. The concentration forest can be estimated from ring widths. If of insoluble ellagitannins is probably one assumes that all staves are 7 cm slightly overestimated as the free ellagic across in transverse section with the near- acid present in the wood, formed from the est edge cut 2.5 cm from the sapwood natural hydrolysis of ellagitannins, was not boundary, one can calculate average hear- measured. Although slightly soluble in twood age assuming constant annual water the majority of free acid will have re- growth equal to the mean ring widths for mained in wood residues. Viriot et al (1994) each forest (see table V). calculated the concentration of ellagic acid Using this estimate of mean heartwood at the sapwood/heartwood boundary as age as a and the mean total ellagitannins representing 10% of the total ellagitannins, found in samples from each forest (table II) with 1 % of ellagitannins hydrolyzed into el- as Ta, the concentrations of tannins in the lagic acid every following 10 years. The dif- new, outermost heartwood Toh can be esti- ference in total ellagitannins between the mated using the model developed earlier, two samples is greater than that indicated where Toh = Tae(0.0219a). from the earlier calculations, suggesting that the earlier over-rather than under- Table V shows that there remains a large assumptions difference between the two forests in the estimated the difference in heartwood age. estimated soluble ellagitannins of new Overall, these results suggest that the dif- heartwood. ferences in tannin concentrations between the forests cannot be explained solely by a phenols due to the greater ’stress’ they difference in the heartwood age of the were under. However, the results reported samples, if one assumes that the rate of here give no support to these hypotheses, reactions which occur during ageing is with higher levels found in the more rapidly similar in all trees. grown Limousin wood. Rapid growth due to a more intense thin- DISCUSSION ning regime are characteristic of the mostly privately owned forests in the Limousin re- The increase in the concentrations of ro- gion. More slowly grown oaks will be felled burins A, C and possibly other dimers dur- at a later age and the stave wood from such ing the first 10-30 years of ageing are due trees will, on average, be older, even if cut to their synthesis from other tannins (Viriot from the same location within the tree. Due et al, 1994). This implies that the to the rapid decline in soluble tannins ob- monomers vescalagin and castalagin may served in the first 20 years of heartwood transform directly or indirectly to dimers. ageing, this might significantly influence The rapid decline in vescalagin corre- the average tannin content. Samples used sponds with an increase in concentrations in this study comprised between 12-50 of these dimers. Roburin D (vescalagin- years of heartwood growth, depending on castalagin) increases later than roburins A ring widths. However, the results and cal- and C possibly due to the slower decline in culations presented here strongly suggest castalagin. These results from four trees of that differences in extracted ellagitannins Q robur support those of Viriot et al (1994), cannot be explained solely by differences which were taken from measurements of in wood age. This supports the findings of single trees of Q petraea and Castanea other studies (Mosedale and Savill 1996; sativa. Mosedale et al, 1996) which have found A high degree of variation in the soluble significant differences between species, tannins is demonstrated between the two provenances and clones of young oaks in forests. The low within-tree variation was the total ellagitannin and phenolic content probably due to the restrictions placed on of outer heartwood of the same age. This the sampling, with samples taken of only is not to say that the differences may not be cask-quality wood from the outer hear- due to variation in the rates of reactions twood near the base of each tree. The which occur during heartwood ageing. The forests sampled were selected to corre- rate of oxidation reactions occurring during spond to contrasting criteria used within the ageing could, for example, be determined cooperage industry to select wood. There- by variation between trees and sites in the fore the relative proportion of variation ex- availability of metal cations. Figures 2 and plained by between-forest differences 3 show that even among the four trees used should be treated as an upper limit of the to calculate the model there was variation variation that could feasibly be selected for in the rate of ellagitannin decline with hear- by coopers. Singleton (1974) found that twood age. tannin concentrations were greater in the It has been previously observed that inner earlywood compared to the latewood of an- heartwood is darker and colour saturation nual rings and therefore proposed that oak more pronounced than outer heartwood, wood with narrow rings would contain and that tannins may play a role in the for- higher levels of tannins due to the higher mation of heartwood colour (Klumpers and proportion of earlywood. Hillis (1975) also Janin, 1992; Klumpers et al, 1993, 1994). suggested that slow growing trees would The results reported here support the no- contain higher levels of heartwood poly- tion of a relationship between wood colour and extractive content; however, there is no REFERENCES means of whether or not the determining AOAC Official Methods 14th ed. Sec- correlation between total and (1984) of Analysis, ellagitannins tions 9.110-9.112. Arlington, VA, USA the wood colour parameter b* is entirely ex- AOAC (1990) Official Methods ofAnalysis, 15th edition. plained by the different heartwood age of Section 952.03. Arlington, VA, USA samples. Charrier B, Janin G, Haluk JP, Mosedale JR (1995) Col- our and visual characteristics of moon rings in oak The in difference ellagitannin content may wood. Holzforschung 49, 287-292 be explained by different rates of the reac- Hillis WE (1975) Ethylene and extraneous material for- tions taking place during ageing, or by a mation in wood tissues. Phytochemistry 114, 2559- difference in the initial concentrations laid 2562 Janin G Mesure de la couleur du bois. Intérêt down in the heartwood. One could alterna- (1987) forestier et industriel. Ann Sci For 44, 455-472 tively propose that it is due to variation of Klumpers J, Janin G (1992) Influence of age and annual other heartwood constituents which in- ring width on the wood colour of oaks. 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