Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant and Soil Sciences

First Advisor

Dr. Seth DeBolt

Abstract

Bourbon whiskey is a distilled spirit which is a uniquely American with several legal requirements including 1) the major grain in the mash is corn, 2) must be aged in a new charred white oak barrel, and 3) made in the United States of America. Kentucky is the whiskey historical home, producing most of the bourbon in the United States. Interest in whiskey has developed a need to understand more about the underlying chemistry of bourbon. The charred white oak barrel used in bourbon production is of substantial interest because the barrel has been implicated in giving American whiskey its flavor. White oak barrels are used in many different production systems and the variables between barrels have been documented in wine, spirits, and beer. Understanding the underlying factors of barrel variation could allow for the guidance of barrel production to more correctly meet the desires of distillers. The process of making a barrel involves several steps that are in some ways more art than science, but we explored how these steps may be interacting with the barrel as the composition is altered during production. Barrels undergo a thermal degradation event and/or charring during production which is similar to other biomass upgrading processes to pyrolysis or thermolysis.

The white oak wood used in barrels is composed of several biopolymers including cellulose, hemicellulose, and lignin. Each of these polymers has different conditions under which they break down and generate different products of interest such as volatile compounds that contribute to whiskey flavor. These volatile compounds are the same compounds that are of interest in biomass production as lignocellulose material is undergoing pyrolysis. We apply a novel strategy looking at factors that influence pyrolysis and wood chemistry to characterize how biopolymers in the wood break down and are altered during barrel production, and before and after whiskey maturation. We develop and document a system in which to survey barrel staves consistently as whiskey penetration in the barrel stave appears to be variable.

We found that cellulose structures are altered by the charring process in barrel production and this allows for whiskey to breakdown this otherwise resilient biopolymer. We found that hemicellulose content and composition were variable across all measured barrel staves but a general trend of degradation was found to occur with whiskey maturation. Through the application of a model of spirt solution and oak cell wall altering enzymes, we found that oak cellulose altered glucose content in a model spirit solution and that carbohydrates are not stable in a whiskey barrel environment. These results indicated that the starting barrel stave cellulose, hemicellulose and lignin composition would also influence the content of whiskey related volatile molecules from the wood.

Lignin is a resilient phenolic polymer whose content increased in the barrel staves after charring and whiskey maturation to become the dominant biopolymer in the innermost portion of the barrel stave. Using a model we developed, we tested how these compounds could be extracted during whiskey maturation by using homogenized charred material from the innermost portion of a new barrel. Not so surprisingly, we found that phenolic compound amounts were highly variable. We then used the model with toasted and untoasted wood chips from the same homogenized stock and found that heat increases the variation in the system. Further, we found that ash and copper content was elevated in the innermost portion in used bourbon barrels compared to other parts of the barrel stave.

Whiskey maturation is a complex chemical process with multiple reactions that occur in addition to direct extraction of the barrel wood. Compounds within the barrel can also react with each other over time- acid and alcohol resulting from the fermentation interact with each other and breakdown products extracted from the wood. This process, known as transesterification, gives rise to esters and fruity smells that occur in whiskey. While many acids are present in new whiskey, we focused on what the wood adds to the whiskey and how fatty acids are altered from cooperage and whiskey maturation. We found that like other compounds in the barrel, fatty acids were highly variable. Thus, we used a model to look at how the composition and content of fatty acids were altered by different alcohol contents in bourbon production.

Barrel composition overall was found to be different than the published values for virgin white oak wood, this appears to occur in part from the cooperage process. These shifts in composition seem to be the starting grounds for barrel variation. These composition issues are compounded by alkaline and alkaline earth metals which act like Insitu catalysis during pyrolysis (charring) and thermolysis (toasting). Remediation and alteration of ash content of biomass lignocellulose are effective at guiding breakdown products in other products . Using previously published methods to measure oak composition and ash content was ineffective and highly variable, so we utilized a model system in our studies. Overall the findings throughout this work show that barrel variation is due to a combination of factors beginning with differences in wood composition and amplified during the cooperage production process of the barrel. We found wood composition is altered during whiskey maturation and this appears to have potential as a target for altering flavors whiskey maturation.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.269

Funding Information

Kentucky National Science Foundation Experimental Program to Stimulate Competitive Research Suggest (2015-2020)

Available for download on Saturday, June 18, 2022

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