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E. T. Contis et al. (Editors) Food Flavors: Formation, Analysis and Packaging Influences © 1998 Elsevier Science B.V. All rights reserved

615

Effect of ethanol strength on the release of higher alcohols and aldehydes in model solutions H. Escalona-Buendia, J. R. Piggott, J. M. Conner and A. Paterson Centre for Food Quality, University of Strathclyde, Department of Bioscience and Biotechnology, 204 George Street, Glasgow G l IXW, Scotland, UK

Abstract Headspace concentrations of homologous series of higher alcohols and aldehydes dissolved in aqueous solutions at different ethanol concentrations were analysed by gas chromatography-flame ionisation detection. For each volatile, activity coefficients at all ethanol strengths were estimated and statistically compared to evaluate the effect of ethanol strength. There was a significant reduction of the activity coefficients between 10% and 20% v/v ethanol for all the volatiles studied. The reduction of the activity coefficients between 10% and 15% v/v was significant only for decanol, dodecanol, octanal and dodecanal. This confirms that there is a change in the efiect of ethanol concentration on volatile flavour compounds in aqueous solutions at 15-20% v/v ethanol.

1. INTRODUCTION Higher alcohols, aldehydes and esters are important volatiles for the aroma and flavour of wines and spirits. According to studies carried out in wine model solutions (Voilley et al. 1990, 1991; Lubbers et al. 1994a, 1994b; Langourieux and Crouzet 1994) and previous studies of whisky flavour (Conner et al. 1994a, 1994b), the release of volatiles is affected by other components in the solution and, therefore, the flavour quality of the beverage may also be affected. In model wine solutions the presence of ethanol reduced the activity coefficients of isoamyl alcohol, octanal and ethyl esters (Lubbers et al. 1994a). These volatile compounds are more soluble in ethanol than in water and when they are in alcoholic aqueous solutions, an increase of ethanol concentration also increases their solubility and, therefore, reduces their release. However, Conner et al. (1997) reported a significant reduction of the release of ethyl esters from alcoholic solutions above 17% v/v ethanol, while at lower strengths the activity coefficients remained almost constant. This behaviour may be explained by changes in the structure of ethanol-water solutions which are modified by the

616 proportions of the mixture. According to D'Angelo et al. (1994), above about 20% v/v ethanol concentration there are non-polar interactions between the hydrocarbon chains of the alcohol molecules, forming agglomerates or "pseudomicelles", and interaction with other non-polar molecules in the system is possible. In order to evaluate the effect of ethanol strength on the release of higher alcohols and aldehydes, the activity coeflScients of homologous series of both groups of volatiles (C6, C8, CIO and C12) were measured in aqueous solutions at different ethanol strengths (10%, 15% and 20% v/v). As the activity coefficient is estimated as the slope of a linear relation between the activity and the concentration of the solute, effects on release were evaluated by a statistical comparison between the slopes.

2. MATERIALS AND METHODS 2.1. Model solutions Absolute ethanol was used to prepare model solutions at 10%, 15% and 20% v/v. Water was distilled and filtered using a Millipore-Q system. For each volatile, stock solutions at 10 mg mL'^ were prepared in absolute ethanol. The volatiles studied were 1-hexanol, 1-octanol, 1-decanol, 1-dodecanol and the analogous series of aldehydes, all of them at least 95% pure. For each ethanol concentration, series of solutions from 1 mg L'^ to 12 mg L^ for every volatile (at least 6 different concentrations) were prepared adding different aliquots from the stock to the model solution. 2.2 Activity coefficients determinations The activities of higher alcohols and aldehydes were obtained by chromatographic determinations of headspace concentrations (Grant and Higuchi 1990; Conner et al. 1997) and activity coefficients calculated as described by Conner et al. (1994a). Glass vials (20 mL), fitted with PTFE fined silicone septa in plastic screw caps, were filled with 10 mL aliquots of standard ethanolic solutions of the volatile. After equilibration in a 25 °C water bath for at least 30 min, a 2.5 mL sample of headspace was withdrawn using a 5 mL gas tight syringe, preheated to 50 °C. Only one headspace c-;- column injection was made per vial and samples were analysed in duplicate on a Carlo Erba™ Mega Series gas chromatograph using a flame ionisation detector. Peak areas were calculated using Chromperfect™ integration software. Cold, on-column injection used a 0.55 mm x 0.5 m ultimetal retention gap with an external gas tight septum. For hexanal, a 0.53 mm x 12 m B P l column (df = 1) was used with helium carrier gas at 20 kPa, holding the column at 30 °C for 1 min and increasing to 50 °C at 18 °C min"\ For all the other volatiles, a 0.53 mm x 12 m BP20 column (df = 1) was used with helium carrier gas at 30 kPa, holding the column at 60 °C for 1 min and increasing to 240 °C at 18 °C min"\ The temperature of the detector was 250 °C.

617 3. RESULTS AND DISCUSSION Figure 1 shows the behaviour of octanol in aqueous solution at 10%, 15% and 20% v/v of ethanol; this was the typical behaviour of all the alcohols and aldehydes studied. A linear relation between the activities and the mole fraction of the volatile in the ethanolic solution is observed, and a gradual reduction of the slope of the curves, which is the numerical estimation of the activity coefficient, as the ethanol concentration increased.

O.OOE+00

5.00E-07

l.OOE-06

1.50E-06

2.00E-06

Octanol (mole fraction) Figure 1. Activities of increasing concentrations of octanol in 10%, 15% and 20% v/v ethanol aqueous solutions calculated from headspace concentration at 25 °C.

Table 1 shows the activity coefficients obtained for each volatile at every ethanol concentration. Statistical comparison of the slopes was carried out by calculation of the 95% confidence interval (Mead and Curnow 1983), considering a difference to be significant when there was not overlapping of the intervals. For all the volatiles, activity coefficients were significantly reduced between 10% and 20% v/v ethanol. This behaviour was expected according to the results reported by Conner et al. (1997) for ethyl esters, and agreed with the concentration range reported by D'Angelo et al. (1994) required to start deagglomeration of the alcohol molecules in ethanol-water solutions. A continuing effect would be expected at higher ethanol concentrations which are more favourable conditions for the formation of agglomerates. For the higher alcohols, the higher the number of carbons the higher is the activity coefficient. Figure 2 shows the semilogarithmic relation between the activity coefficient and the number of carbons in the alcohol. For all the alcohols there was a gradual reduction of the

618 activity coefficients as the ethanol concentration increased. However, for hexanol and octanol the difference between 10% and 15% was not significant (Table 1).

Table 1 Activity coefficients at 25 "C of higher alcohols and ethanol/water solutions. Ethanol strength Activity Standard error coefficient (% v/v) 566 29.4 Hexanol 10 497 25.0 15 393 13.2 20 3810 113.1 10 Octanol 3424 128.2 15 1926 87.8 20 81746 2372 10 Decanol 58990 3737 15 41221 1657 20 844331 67658 10 Dodecanol 544778 47660 15 270845 24098 20 1076 87.7 10 Hexanal 827 86.5 15 420 36.5 20 6360 279.3 10 Octanal 210.7 3829 15 2801 135.9 20 27195 1747 10 Decanal 1762 22281 15 16917 20 623 15614 251360 10 Dodecanal 181478 6153 15 147821 12258 20

aldehydes dissolved in 9 5 % Confidence interval 503-629 443-552 364-421 3570-4050 3138-3709 1735-2117 76578-86914 50765-67215 37667-44775 695417-993245 436964-652593 218340-323350 873-1278 628-1027 336-505 5738-6982 3380-4278 2489-3104 23389-31001 18355-26207 15545-18289 216570-286150 167935-195021 120509-175133

R^ 0.96 0.98 0.99 0.99 0.99 0.98 0.99 0.96 0.98 0.93 0.94 0.91 0.95 0.92 0.94 0.98 0.96 0.98 0.98 0.97 0.99 0.96 0.99 0.94

The relation between the activity coefficients and the number of carbons for the series of aldehydes studied is observed in Figure 3. There is the same semilogaritmic relation and similar behaviour in comparison to the higher alcohols. Activity coefficients for Hexanal and Decanal at 10% and 15% were not significantly different, and only Dodecanal, which is the less polar compound, had a significant difference between 10% and 15%, but no difference between 15% and 20%. The gradual reduction of the activity coefficients of the volatiles studied while ethanol concentration increases may be attributed to the increase of the

619 solubility of non-polar compounds by the presence of ethanol in the aqueous solution. The longer hydrocarbon chains of decanol, dodecanol, decanal and dodecanal may be more susceptible to hydrophobic interactions explaining the reduction of their volatility from 10% ethanol, which for hexanal, hexanol and octanol was from 15%.

8

10

12

Number of carbons in alcohol

Figure 2. Activity coefficients of alcohols in aqueous solution at 10%, 15% and 20% v/v ethanol.

-^10% -^15%

i 5

-i-20%

0)

o o ^

4

•r-H

1 bo o

3[ ,

8

10

12

Number of carbons in aldehyde

Figure 3. Activity coefficients of aldehydes in aqueous solution at 10%, 15% and 20% v/v ethanol.

620 4. CONCLUSIONS There was a reduction in the activity coefficients of all the higher alcohols and aldehydes studied when mixed in progressively higher ethanol concentrations. Activity coefficients of all volatiles were significantly reduced between 10% and 20% v/v ethanol; for hexanol, octanol and hexanal there was a significant reduction between 15% and 20% v/v. Thus not only the composition of the volatiles but also their interactions with the matrix must be taken into account to understand the aromatic properties of such alcoholic beverages.

5. REFERENCES Conner, J.M., Paterson, A. and Piggott, J.R. (1994a). J. Sci. Food Agric. 66, 4553. Conner, J.M., Paterson, A. and Piggott, J.R. (1994b). J. Agric. Food Chem. 42, 2231-2234. Conner, J.M., Birkmyre, L., Paterson, A. and Piggott, J.R. (1997). Headspace concentrations of ethyl esters at different alcoholic strengths. J. Sci. Food Agric. In press. D'Angelo, M., Onori, G. and Santucci, A. (1994). J. Chem. Phys. 100, 3107-3113. Langourieux, S. and Crouzet, J. (1994). Lebensmittel-Wiss. u Technol. 27, 544549. Grant, D.R. and Higuchi, T. (1990). Solubility Behaviour of Organic Compounds. John Wiley & Sons, New York. Lubbers, S., Voilley, A., Charpentier, C. and Feuillat, M. (1994a). Am. J. Enol. Vitic. 45, 29-33 Lubbers, S., Voilley, A., Feuillat, M. and Charpentier, C. (1994b). LebensmittelWiss. u Technol. 27, 108-114. Mead, R. and Curnow R. (1983). Statistical Methods in Agriculture and Experimental Biology. Chapman and Hall, London UK. Voilley A., Lamer C , Dubois P. and Feuillat M. (1990). J. Agric. Food Chem. 38, 248-251. Voilley A., Beghin V., Charpentier C. and Pe3n:*on D. (1991). Lebensmittel-Wiss. u Technol. 24, 469-472.

Acknowledgements The authors wish to acknowledge the UK Biotechnology and Biological Sciences Research Council and CONACyT-Mexico (Consejo Nacional de Ciencia y Tecnologia) for the financial support provided.