Introduction
D u ring the wine ageing in wood barre l s , a number of phy- s i c o - chemical processes take place wh i ch affect the orga n o- l eptic ch a ra c t e ristics of the product. Fruity aromas and the ch a ra c t e ristic aromas of fe rm e n t ation are replaced by others induced by the wo o d : va n i l l a , spices and oak [1 , 2].
The oak species most commonly used in barre l - m a k i n g a re : Q u e rcus alba ( found in a number of states in the US and also known as A m e rican oak), Q u e rcus Pe t ra e a a n d Q u e rcus ro bu r, wh i ch are found in Euro p e, the most popu- lar being Fre n ch oak.
The sensory pro p e rties and chemical composition diffe- rences in the diffe rent types of oak wood used in cooperage [3 , 4] a re highly significant in commercial term s , and it is t h e re fo re important to know wh i ch ones yield most aro m a compounds to the wine.
Most of the authors who have studied the composition of oak species have used hy d ro-alcoholic solutions, o rga n i c age n t s , wines and distillates wh i ch have been in contact with wood chips [5 - 7], and the subsequent analysis of the re s u l- ting ex t ra c t s , either by GC (combined whit MS in many cases) or HPLC [8]. Of the nu m e rous vo l atile compounds i d e n t i fied in wood ex t racts obtained in these way s , some are of particular sensory importance because of their low thre- shold va l u e s : f u r f u ra l , oak lactones, vanillin and syri n ga l d e- hyde [9 , 1 0].
Experimental
Four oak wood samples we re ava i l abl e, t h ree samples of Fre n ch oak (Qu e rcus petra e a f rom A l l i e r, Vo s ge s , a n d C e n t ral France fo rests) and one sample of A m e rican oak ( Qu e rcus alba). Samples we re in the fo rm of wood ch i p s milled prior to analy s i s .
Direct thermal desorption (DTD) conditions
The ATD-400 is an automatic thermal desorption system d eveloped by Pe rkin-Elmer for ex t racting vo l atile substances f rom solid samples for injection onto a ch ro m at ograp h i c c o l u m n .
An amount of 20 mg of milled wood was placed toge- ther with internal standard in the desorption tube, p l u gged at both ends with silanized glass wool. The internal standard was 2 µL of a solution of γ- c ap rolactone (242 mg/L). Th e d e s o rption tube was heated to 180 ºC for 30 minutes. Th e vo l atile substances we re desorbed in a stream of helium at 50 mL/min and collected in a cold trap (–30 ºC) packed with Tenax. The trap was heated quick ly to 300 ºC and the vo l a- tile substances tra n s fe rred (split 1: 50) to the ch ro m at ogra- phic column through a line heated to 225 º C .
In the studies with synthetic solutions, 3 µL of a solu- tion in dich l o romethane at the concentration betwe e n 8 0 – 1 4 0 mg/L was injected into the empty desorption tube and subjected to the same DTD tre atment as wood samples.
Chromatographic conditions
The ap p a ratus was a Fisons model GC 8000 gas ch ro m at o- graph connected to a model MD 800 mass detector. Th e column employed was a 25-m SE-54 (5% diphenyl and 95%
d i m e t hyl poly s i l oxane) column with an i.d. of 0.5 mm and 0.25 µm of film thickness. The column temperat u re progra m was 60 ºC – (3 min) – 6 ºC/min – 250 ºC (40 min). Tra n s fe r line temperat u re was 225 ºC. Helium was the carrier ga s . Column head pre s s u re was 18.5 p s i .
Mass spectrum acquisition conditions (SIR mode) Ion production was by electron impact (EI) with an electro n e n e rgy of 70 e V. Selected ion masses we re : i n t e rnal standard
33
Original articles
Quantitative analysis of the principal volatile compounds in oak wood by direct thermal desorption (DTD)
and GC/MS*
E. García-Romero
1,**, M.S. Pérez-Coello
1, J. Sanz
2and M.D. Cabezudo
11Food Science Laboratory, Faculty of Chemistry, Castilla-La Mancha University, 13071 Ciudad Real, Spain
2Organic Chemistry Intitute, CSIC, Madrid, Spain
Abstract. This wo rk proposes one method of quantifying the principal vo l atile compounds in oak wood using the direct therm a l d e s o rption (DTD) technique coupled with GC/MS. These compounds have a particular sensory importance in wines aged in oak wood barre l s : f u r f u ra l , t rans and cis-β- m e t hy l -γ- o c t a l a c t o n e s , e u ge n o l , vanillin and syri n ga l d e hy d e. The described method re q u i re s p ra c t i c a l ly no sample handling and is precise and sensitive.
Key words. d i rect thermal desorption −vo l atile compounds −oak wood −wine age i n g.
Analusis, 1998, 26, 33-35
© EDP Sciences, Wiley-VCH
* Presented at In Vino Analytica Scientia, Bordeaux, 12-14 June 1997.
** Cor respondence and reprints.
Received July 28, 1997; r evised November 05, 1997; accepted November 19, 1997.
Article available at http://analusis.edpsciences.org or http://dx.doi.org/10.1051/analusis:1998107
(γ- c ap ro l a c t o n e ) : 85; furfura l : 95; cis and trans β- m e t hy l -γ- o c t a l a c t o n e s : 99; euge n o l : 162; va n i l l i n : 152; and syri n ga l- d e hy d e : 1 8 2 .
Ion collection time for all ions was 0.23 s , and lag time b e t ween the acquisition of two consecutive ions was 0.020 s .
Results and discussion
Sensitivity of the detector
With the mass spectrometer coupled to the gas ch ro m at o- grap h , it is possible to wo rk in either SCAN mode (scan- ning a ra n ge of selected masses) or SIR mode (just regi s t e- ring the ions of interest). The latter is more specific and p rovides more sensitive detection. Synthetic solutions of the compounds to be quantified in the wood samples we re ana- lysed by DTD/GC/MS in SIR mode. The ratio signal/noise per pg of injected compound was between 10 to 30.
Method precision
The proposed method precision was tested by perfo rm i n g nine consecutive DTD/GC/MS analyses of a synthetic solu- tion. Table I shows the distri bution of the values obtained and the results of the same synthetic solution injected d i re c t ly in GC/MS.
The standard dev i ation values from analysis of dire c t injection in GC/MS we re as ex p e c t e d, i n c reasing as the vo l a- tility of the diffe rent compounds declined; the ex c eption wa s f u r f u ral since its high vo l atility in the wo rking conditions m ay have been the cause of some discri m i n ation in the split injection. When DTD was coupled with the GC/MS system, p recision decre a s e d, although the values may be considere d good enough due to the fact that precision of the entire pro- c e s s : ex t raction and analy s i s .
Analysis of oak wood by DTD
Four samples of oak wood of diffe rent provenances we re a n a ly zed by DTD/GC/MS; results are shown in table II. Th e
ch ro m at ogram from the Central France oak wood sample of the individual traces of the six ions is shown in fi g u re 1.
The furfural concentration was mu ch higher than it had been rep o rted in the literat u re (0.01 – 28 µg/g) [1 1]. In the ex p e rimental conditions this result is pro b ably due to the fur- f u ral of the wood mat rix but also a produced furfural fro m the pentoses in the wo o d ’s hemicellulose [1 2] by therm a l hy d ro lysis occuring at desorption temperat u re (180 ºC).
Table II results show that wood from the three Fre n ch fo rests contains higher concentrations than A m e rican oak of all of the studied compounds ex c epting the c i s -β- m e t hy l -γ- octalactone and syri n ga l d e hyde found in the Allier oak. On the other hand, t h e re we re diffe rences among the thre e Fre n ch oak samples of the same species: the Vo s ges oak contained more furfura l , c i s -β- m e t hy l -γ-o c t a l a c t o n e, va n i l l i n and syri n ga l d e hy d e than did oaks from Central France and f rom A l l i e r.
Fi n a l ly, d i ffe rences we re found in the concentration rat i o
34
Original articles
Table I. Analysis of the synthetic solution by DTD (DTD/GC/MS) compared with direct injection (GC/MS).
GC/MS DTD/GC/MS
COMPOUND Mean SD SD Mean SD SD
value1 (%) value1 (%)
Furfural 1.39 0.05 3.6 1.39 0.11 7.8
trans-β-methyl- 0.71 0.01 1.8 0.65 0.02 2.7
γ-octalactone
cis-β-methyl- 0.81 0.02 2.2 0.72 0.03 4.4
γ-octalactone
eugenol 0.93 0.03 3.7 0.92 0.06 6.5
vanillin 1.15 0.08 6.7 1.19 0.11 9.6
syringaldehyde 1.36 0.10 7.3 1.33 0.14 10.5
1 Mean value of 9 injections (area of compound/area of Internal Standard ) .
Table II. DTD/GC/MS analysis of four oak wood samples of dif- ferent origin (µg compound/g wood).
OAK WO O D
C O M P O U N D U. S. VO S G E S C E N T R E A L L I E R
F u r f u ra l 8 8 . 1 1 7 3 . 4 9 7 . 8 3 8 . 0
t ra n s -β- m e t hy l - 0 . 3 1 1 . 8 1 3 . 7 9 0 . 3 9 γ- o c t a l a c t o n e
c i s -β- m e t hy l - 3 . 2 4 8 . 8 5 4 . 9 4 2 . 1 8 γ- o c t a l a c t o n e
e u ge n o l 0 . 0 8 0 . 1 7 0 . 4 7 0 . 0 8
va n i l l i n 4 . 0 6 8 . 9 8 7 . 7 3 4 . 6 9
s y ri n ga l d e hy d e 1 1 . 1 0 1 8 . 4 5 1 6 . 4 4 9 . 8 8
cis-lactone/ 1 0 . 4 5 4 . 8 9 1 . 3 0 5 . 5 9
t rans-lactone rat i o
Fig. 1. C h ro m at ogram of the vo l atile compounds in oak wood fro m C e n t ral France by DTD/GC/MS in SIR mode. 1: f u r f u ra l , 2 : γ- c ap rolactone (internal standard ) , 3 : t ra n s -β- m e t hy l -γ- o c t a l a c- t o n e, 4 : c i s -β- m e t hy l -γ- o c t a l a c t o n e, 5 : e u ge n o l , 6 : vanillin and 7 : s y ri n ga l d e hy d e.
of the oak lactone isomers in the diffe rent samples, wh i ch would support the proposal of this parameter as an indica- tor of oak wood ori gin [7 , 1 3], h owever more samples should be analysed to raise statistical concl u s i o n s .
Conclusions
The proposed method of quantifying vo l atile components of oak wood by DTD/GC/MS is both precise and sensitive and has the adva n t ages over classic ex t raction with organic sol- vents that it re q u i res only a short analysis time and pra c t i- c a l ly no sample handling.
This method is suitable for ch a ra c t e rizing oak wo o d s used in wines and distillates age i n g, and for discri m i n at i n g b e t ween woods from diffe rent ge ographic ori gins on the basis of their vo l atile composition.
Acknowledgement
This study was supported by project ALI95-0254-CO2-02 of Comision Interm i n i s t e rial de Ciencia y Te c n o l og í a .
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