Six barrels of each variation were included in the experiment.

In order to isolate the oak species as the key variable, all the barrels in this experiment were subjected to the same heat treatment and seasoning regimen.

Samples were taken from each barrel every 12 months and analyzed using Gas Chromatography-Mass Spectrometry (GCMS). All compound concentrations are displayed in graphs either as a percentage of the average concentration across all barrels in the experiment or concentration in mg/L.

Sensory analysis was also conducted as part of the experiment. This included a panel of six tasters who tasted the samples once daily for three days.

Results and Discussion

Lactone Concentration

Oak lactones are a major contributor to the overall flavor profile of barrel-aged spirits. The cis oak lactone has a distinctive coconut-like aroma and has long been thought of as a contributor to American oak’s tendency to deliver sweeter flavor profiles. Meanwhile, the trans oak lactone provides a grassy, celery-like aroma. Both lactones in higher concentration can lend fruity, citrusy notes to spirits. Extremely high concentrations can be perceived as woody or oaky.

There is well-known discrepancy in lactone concentration between different oak species and also a difference in the ratio of cis to trans isomers. American oak is known for having higher total concentrations of lactone and also a higher cis to trans oak lactone ratio when compared to French oak. Less known is how European oak compares to both French and American oak with respect to lactone concentrations. Figure 4.1 shows the GCMS results for lactone concentration after 12 months in barrel. Both lactone concentration and cis to trans lactone ratios are then presented in Table 4.1.

As expected, American oak displayed the highest concentration of lactones amongst the three species after one year in barrel. The ratio between cis to trans oak lactones fell in line with previous research from other sources¹. It should be noted that although oak lactones are generally considered to have a low threshold of perception, the differences shown in year one of this experiment are most likely not enough to cause any significant sensory differences. Previous research has shown that the threshold of perception for oak lactone can be below 1 mg/mL, but this is highly dependent on the base (spirits/wine). While lactone can be recognized at this low concentration, confidence is low and generally the characteristic coconut-like aroma from the cis isomer is more difficult to distinguish below 5 mg/mL.

Over the course of four years, concentrations of lactones increased across all three species and the cis to trans ratios changed slightly. The results for forty-eight months are shown in Figure 4.2 and Table 4.2.

Trends in ratios and concentrations remained largely the same from 12 to 48 months. Ratios tightened with the exception of French oak, and concentrations for all species increased as suspected. The percent increase in lactone concentrations for all three species can be found in Table 4.3.

French oak displayed the largest percent increase, while European oak displayed the least. This is likely due to the difference in density respective to each species.² At this point in the maturation period, the concentrations are nearing the point of high confidence (5 mg/L) sensory detection.

Lactone Perception

In addition to the GCMS analysis, an ISC sensory panel evaluated this experiment at 36 months. The results were interesting with respect to the oak lactones’ influence on the overall flavor profile.

GCMS results for lactone concentration at 36 months are presented in Table 4.4, and the sensory results are displayed in Figure 4.3.

The French oak sample, which displayed the lowest concentration of lactones over the course of the experiment, was perceived by the panel to be the sweetest of all three samples. This suggests that through the early years of maturation, lactones are not influencing the perception of sweet flavors and aromas to a significant degree.

Conclusions

This experiment confirmed the presence of oak lactones in all three oak species and provided insight into differences that could yield long-term nuances in barrel-aged spirits.

American oak yielded the highest lactone concentration of the three oak species measured, while French oak had the lowest total comparatively. American oak also displayed the highest cis to trans lactone ratio of the oak species tested.

The experiment combined chemical analysis with sensory analysis to determine if three years is sufficient to reach perceptible levels. Sensory analysis at three years of aging did not distinguish a key difference for the higher level of lactones identified in American oak; however, by four years, chemical analysis demonstrated the lactone levels for the spirit aged in American oak were nearing sensory detection. This suggests oak species should not be selected based on lactone potential unless the maturation period is longer than four years.

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¹ Lee, M., Paterson, A., Piggott, J. P., & Richardson, G. D. (1999). Perception of Whisky Flavour Reference Compounds by Scottish Distillers. Journal of the Institute of Brewing, 106(4). https://doi.org/https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2000.tb00058.

² This could be related to grain tightness, which was not a controlled variable in this study.