Volatile composition of oak and chestnut woods used in brandy ...

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(Chatonnet & Dubourdieu, 1998; Masson et al., 1995). In the analysis of all the different types of wood (Table 2), we found a similar wood discrimination based on the contents of cis-b-methyl-c-octalactone + 4-methylguaiacol. Concerning hexanoic acid, the variance analysis (Table 1) showed two different groups of wood, one constituted by CFL, CNE, CAST and CNG with the lowest contents and other formed by CNF, CFA and CAM with higher content. Others authors also found this acid in untoasted oak extracts (Boidron et al., 1988; Clímaco & Borralho, 1996) and chestnut extracts (Clímaco & Borralho, 1996). Similar wood discrimination could be found in the ANOVA, concerning all the different types of wood (Table 2). Based on the eugenol amounts, three homogeneous groups can be observed, by ordering them from the poorest to the richest: CAST = CNE = CFL = CNG = CFA < CNF < CAM. These results are similar to those obtained by Pérez-Coello et al. (1999), but quite different from the results observed by other authors (Chatonnet & Dubourdieu, 1998; Doussot, Pardon, Dedier, & De Jeso, 2000). The results obtained from the analysis of all the different wood types are different, they confirm that CAM is the richest wood in this compound, CFA is a poorest wood and CNF is the richest, among the Portuguese woods. For both analyses (Tables 1 and 2), wood origin effect on the levels of 5-methyl-furfural, octanoic acid, decanoic acid, dodecanoic acid, syringol and 4-allyl-syringol was not detected. According to the results obtained, we have chosen the compounds significantly affected by the wood origin, and we have submitted these variables to a multidimensional analysis (clustering and principal component analysis). Fig. 2 presents the phenogram of distances for the unheated wood types, which presented a cophonetic 1.67 1.25 0.83 Distance 0.42 CNE1 CNE2 CNG1 CNG2 CFL1 CFL2 CAST1 CAST2 CNF1 CNF2 CFA2 CFA1 CAM1 CAM2 0.00 Fig. 2. Phenogram of UPGMA clustering of unheated woods according to volatile compounds levels. The initial matrix is composed by 14 wood samples · 9 variables (1 and 2 indicate the replicate). I. Caldeira et al. / Journal of Food Engineering 76 (2006) 202–211 207 correlation coefficient of 0.88. The American oak wood (CAM) forms an individual cluster, a second cluster joins together Allier oak (CFA) and Portuguese oak wood (CNF), a third cluster joins Portuguese oak woods (CNE and CNG), Limousin oak wood (CFL) and chestnut wood (CAST). The principal component analysis for the 14 unheated wood samples was performed (Fig. 3). The first three principal components, which accounted 91% of the total variance, separate the different types of wood, which is in agreement with the clusters found in the phenogram. The first component, which acounted 61% of total variance, makes the major wood separation. It was possible to observe a wood cluster (CAST, CNE, CNG and CFL) with low levels of furfural, 4-hydroxy-2-butenolactone, hexanoic acid and guaiacol in opposition to the other woods analyzed (CFA, CNF and CAM) which present high levels for the same variables. Chestnut is not completely separated from other types of wood, based on the low levels of the variables. The second component seems to separate the CAM wood from other woods due to itÕs higher levels of cis-b-methyl-coctalactone and eugenol. However, the cluster analysis of all the different types of wood does not show clusters based on the wood origin (Fig. 4). These results, in agreement with ANOVA results, suggest that the toasting process affected the wood discrimination. In fact, the principal component analysis (Fig. 5) with all the different wood types shows that the first component, which accounted 50% for the total variance, divided the wood samples based on the level of toasting. Only the second component divides Comp. 2 (20%) 1.15 0.57 0.00 -0.57 CAST1 CAST2 CNE1 CFL1 CFL2 CNG2 CNG1 CNE2 CFA1 2 25 CNF1 CFA2 3 CNF2 7 CAM2 CAM1 -1.15 -1.15 -0.57 0.00 Comp. 1 (61%) 0.57 1.15 Fig. 3. Projection of unheated woods and variables in the space defined by the first and second components. Variable identification: (2) acetic acid; (3) furfural; (7) 4-hydroxy-2-butenoic acid lactone; (8) hexanoic acid; (9) guaiacol; (10) trans-b-methyl-c-octalactone; (12) cisb-methyl-c-octalactone; (17) eugenol; (25) vanillin. 10 12 17 9 8
208 I. Caldeira et al. / Journal of Food Engineering 76 (2006) 202–211 1.93 1.52 American oak wood (CAM) from the others based on the amounts of cis-b-methyl-c-octalactone + 4-methylguaiacol and eugenol. These results suggest that the analyses of certain volatile compounds found in wood could help cooperage industry to select or control wood quality, but it must be done before the toasting process. 3.3. Toasting effect 1.11 Distance CNEQ0 CFLQ0 CASTQ0 CNEQM CNGQ0 CFAQL CASTQL CNFQM CFLQL CNFQ0 CFAQ0 CNGQL CNGQM CNFQL CNEQF CNFQF CASTQM CASTQF CFAQF CFAQM CFLQM CAMQM CAMQL CAMQ0 CNEQL CNGQF CFLQF CAMQF 0.30 Fig. 4. Phenogram of UPGMA clustering of all the analysed woods according to volatile compounds levels. The initial matrix was composed by 28 woods · 15 variables. Comp. 2 (13%) 0.48 0.13 -0.22 -0.57 Table 3 shows that the toasting degree has a very highly significant effect on the quantity of compounds 0.71 CASTQ0 CASTQM CNEQF CNGQM 3 CNEQ0 CASTQF 23 CFAQM CFAQL CNGQ0 CNEQM CNFQF 6 CFLQ0 CNFQM 25 26 2 18 CASTQL 5 CNGQL CFAQ0 CNFQL 7 24 CFLQL CNFQ0 CNEQL 8 CAMQL CFAQF CFLQM 17 CAMQM CAMQF CNGQF 12+13 -0.92 CAMQ0 -0.96 -0.31 0.33 Comp. 1 (50%) 0.98 1.62 9 10 CFLQF Fig. 5. Projection of all woods and variables in the space defined by the first and second components. (2) acetic acid; (3) furfural; (5) propanoic acid; (6) 5-methylfurfural; (7) 4-hydroxy-2-butenolactone; (8) hexanoic acid; (9) guaiacol; (10) trans-b-methyl-c-octalactone, (12) + (13) cis-b-methyl-c-octalactone + 4-methylguaiacol; (17) eugenol; (18) syringol; (23) HMF; (24) 4-allyl-syringol; (25) vanillin; (26) acetovanillone. found in the wood matrix, and for the majority of these compounds, the evolution profile is similar: as toasting intensity increases their concentrations rise (from Q0 until QF), reaching the highest concentrations in woods strongly toasted (QF). With toasting intensity increase and consequent temperature raise (Chatonnet et al., 1989; Sarni, Rabier, & Moutounet, 1990), most of the woods components suffer physical, structural and chemical changes (Fengel & Wegener, 1989). The thermal degradation of wood polysaccharides originates the formation of furanic derivatives, namely furfural, 5-methylfurfural and 5-hydroxy-methylfurfural and acetic acid. As summarised by Fengel and Wegener (1989), HMF and 5-methylfurfural proceeds from hexoses, that are the main constituents of cellulose, and furfural derives from pentoses, the main constituents of hemicelluloses. The acetic acid proceeds from the acetyl groups present in the wood xylans, an important group of wood hemicelluloses. For this reason, it was observed a significant increase in the amounts of these compounds (acetic acid, furfural, 5-methyl-furfural and 5-hydroxymetylfurfural) with the increase of toasting level. It was possible to divide the woods from different toasting levels based on the quantity of these compounds. The higher contents of furanic aldehydes was detected in strongly toasted woods, which is in agreement with several authors (Artajona, 1991; Nomdedeu et al., 1988) but in disagreement with Chatonnet et al. (1989), who found the higher levels in medium toasted oaks. The hemicelluloses are the most thermosensitive wood polymer (Fengel & Wegener, 1989), they are preferentially degraded during the toasting process, which explains the high amount of furfural among the furanic aldehydes (Table 3). Concerning the amounts of 4-hydroxy-2-butenoic acid lactone, their behaviour with toasting is similar to the furanic aldehydes. Under temperature effect wood lignins are also affected (Fengel & Wegener, 1989), which could explain the presence of many phenolic derivatives namely vanillin, volatile phenols and acetovanillone. In fact, it was observed that the toasting level had a very highly significant effect on the quantity of eugenol, syringol and 4-allyl-syringol found. Higher values were found in strongly toasted woods, in disagreement with other authors (Artajona, 1991; Chatonnet et al., 1989). The results also show (Fig. 6) that the increase in the amounts of syringyl-type compounds (syringol and 4-alylsyringol) are higher than the ones of guaiacyl-type (guaiacol and eugenol) in oak wood, as described by Chatonnet et al. (1989) and Sarni et al. (1990). This fact could be explained by the dominance of syringil units in the hardwood (oak and chestnut) lignins and also by their higher thermal stability.
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