1. EXTRACTION OF WOOD-DERIVED CONGENERS INTO SPIRITS AS A FUNCTION OF AGEING TIME, TEMPERATURE, SPIRIT TYPE AND %ABV: A KINETIC STUDY Ivonne Wendolyne Gonzalez-Robles1, and David J. Cook1,*
Division of Food Sciences: The University of Nottingham; Sutton Bonington Campus, Loughborough,Leic. . LE12 5RD, UK. *Corresponding author:
1. SUMMARY
The extraction of wood derived maturation compounds into spirit-strength ethanolic solutions was investigated as a function of aging time, temperature, spirit type and alcohol content. Two series of samples were used; consisting basically of new make spirit samples and ethanolic solutions of a similar range in spirit strength (40-64% ABV), to each of which were added oak sticks at 5% w/v. Temperature and alcohol content had significant impacts on the rate of extraction and final concentrations of all 18 compounds studied (P<0.05). Not surprisingly, extraction rates increased with increasing temperature, however the trend in terms of alcoholic strength depended on the particular compound. For example, extraction of furfural, cis-whisky lactone and ellagic acid were enhanced as spirit alcohol content was increased, whereas the greatest concentrations of vanillin and syringaldehyde were derived at just 40%ABV. Furthermore, different non-mature spirits extracted differing profiles of compounds, however, this was mainly due to differences in %ABV, with only minor effects apparent based on the spirit matrix. Kinetic analysis showed that the extraction process followed a second order model. This study provides a better understanding of the factors influencing the extraction of key wood-derived maturation flavour-compounds and of their relative significance in determining spirit mature character.
2. INTRODUCTION
Pure aqueous ethanol (64% ABV)
New make whisky (63.5% ABV)
Wood ageing is a common practice in distillate spirits production. Aging thus contributes to the mature character of the spirit by extraction of volatile and non-volatile compounds (complex sensory interactions). Although humidity, temperature and time are the main factors determining the sensory characteristics of aged spirits, other factors are also important (e. g. raw material). Studies on the contribution of oak to the olfactory characteristics of spirits have shown that these are mainly influenced by compounds such as furfural, guaiacol, whisky lactone, eugenol, vanillin and syringaldehyde (1). In addition, rapid ageing techniques have been used as a tool to mature alcoholic beverages in a short period of time (2), however in the case of spirits these techniques are yet to find favour as an alternative to warehousing and long-term maturation. Therefore, the objective of this study was to investigate the extraction kinetics of wood-derived compounds from oak sticks as a function of ageing time, temperature, % ABV and spirit type.
Figure 1. Effect of temperature on the extraction of cis-whisky lactone and vanillin from oak wood chips into pure aqueous ethanol (64% ABV) and new make whisky (63.5% ABV).
3. MULTI-LEVEL FACTORIAL EXPERIMENTAL DESIGN
2 Independent experiments: new make spirits and ethanolic solutions
Factors: Temperature (10-40°C), %ABV (40-64, & ), & aging time (2-28 d)
Response variables: Extracted concentrations of 18 maturation congeners
4. METHODOLOGY
Sample
preparation
SPIRIT MODEL SYSTEMS
(50 mL)
Placed in Incubator for
temperature control
(10, 20, 30, & 40°C)
Oak Sticks 5 % w/v
(0.8 cm x 0.8 cm x 9 cm)
New make
Spirits:
Tequila
(40%ABV)
Bourbon
(57.8%ABV)
Malt whiskies (63.4&63.5%ABV)
LLE
performed in duplicate
Pure. aqueous ethanolic solutions
(40, 48,
56, 64%ABV)
VOC’s analysis
Cut down ~ 3 cm
GC-FID
Periodic interval sampling
(2,4,7,10,14,21,28 d)
Non-VOC’s
analysis
Place in Duran
bottles of 100 mL
2 x 4 x 4 x 7 =
224 spirit samples
HPLC-UV
5. RESULTS AND DISCUSSION
Evolution of mature-related congeners as function of ageing time, temperature and alcohol content.
Figure 2. Effect of alcohol content (%ABV) on the extraction of aging congeners into pure aqueous ethanolic solutions and new make spirits of Tequila and Malt whisky ‘B’.
Values are for the final time point (28d) at a maturation temperature of 20°C.
Temperature, %ABV and time significant (P < 0.05) affected the extraction of all compounds (data not shown).
Increasing maturation temperature (10-40°C) increased the rate of extraction of all compounds into all spirit systems (Fig.1)
Temperature showed a dependence of extraction at the highest alcohol content.
Figure 2 shows a small but inconsistent effects of the spirit matrix (similar %ABV), and the impacts of %ABV. Ellagic acid extraction increased as % ABV increased.
Extraction kinetics of wood-derived congeners into spirits as a function of ageing, temperature, spirit type and alcohol content
Temperature 10°C 5 10 15 20 25 30
Time (d)
0.5 1
1.5 2
2.5
Concentration (mg/L)
Temperature 40°C 5 10 15 20 25 30
Time (d) 1 2 3 4
Concentration (mg/L)
Pure aqueous ethanol (64% ABV)
New make whisky (63.5% ABV)
K= 0.10
h= 0.68
Csat= 2.59
R2= 0.979
K= 0.42
h= 4.77
Csat= 3.36
R2=
Ea ~ 0.98 – KJ/moL
Figure. 3. Estimated kinetic parameters (R2, K, h and Csat) obtained during the modelling (non-linear regressions) of the extraction of cis-whisky lactone from American oak sticks in a bourbon model solution (57.8%ABV).
Kinetic parameters; R2, coefficient correlation, h, initial extraction rate, k, extraction rate constant, Csat, concentration at saturation of cis-whisky lactone.
6. CONCLUSIONS
Extraction of wood-derived maturation compounds were favoured at elevated temperatures (40ºC) and higher alcohol content (> 63.4%ABV).
Vanillin and syringaldehyde were extracted in greater concentrations at 40%ABV. When comparing extraction into ethanolic solutions with that for non-mature spirits of equivalent strength, differences were more subtle and it was concluded that the spirit matrix had a minor effect relative to those of alcoholic strength or temperature. Kinetic analysis showed that the extraction process followed a second order model. This study provides fundamental understanding of the factors that affect the extraction of key maturation compounds into spirit-strength alcoholic solutions.
References: 1. Reazin, G. H. (1981). Chemical mechanisms of whiskey maturation. American Journal of Enology and Viticulture. 32: Arapitsas, P., Antonopoulos, A., Stefanou, E. and Dourtoglou, V.G. (2004). Artificial aging of wines using oak chips. Food Chemistry. 86: