Introduction

The chemical diversity of wine

How many choices does a consumer have when they buy a wine? In the United States, all packaged wines must have a Certificate of Label Approval (COLA) from the Alcohol and Tobacco Tax and Trade Bureau (TTB). In 2023, the TTB approved COLA requests for approximately 30?000 white table wines, 40?000 red table wines, 10?000 sparkling or carbonated wines (including Champagne), and tens of thousands of dessert, rosé, fruit, and flavored wines.1 Because many wines are vintage products with an identical label (other than harvest year, and thus not requiring review) for each year, and many wines produced internationally are not imported into the US, the true number of wines available at any instance is even greater. An online wine marketplace (Wine Searcher) listed 6.8 million different wines in 2015 [1], and this quantity has almost certainly grown. In contrast to commodities where producers strive for homogeneity (e.g., wheat flour and milk), variation in specialty products like wine is not merely tolerated - it is appreciated and celebrated. Consumers expect that wines with different labels should smell different, taste different, and look different; from a chemist's perspective, consumers expect wines to have different chemical compositions. The study of wine chemistry is the study of these differences, explaining how there can be hundreds of thousands, if not millions, of different wine compositions, and contributing to a winemaker's understanding of how the myriad of choices they are faced with can lead to such diversity.

What is wine?

While wine is often described in poetic terms, such as sunlight in a glass, a chemical description for dry table wine is a mildly acidic (pH 3-4) hydroalcoholic solution. The two major wine components are water and ethanol, typically accounting for about 97% on a weight-for-weight (w/w) basis. The remaining compounds - responsible for most of the flavor and color of wine - are typically present at <10?g/L total (Figure I.1), and many key odorants are found at part-per-trillion (ng/L) concentrations! Notably, none of these compounds appear to be unique to wine - compounds present in wine can also be found in coffee, beer, bread, spices, vegetables, fruits, cheese, and other foodstuffs.2 What distinguishes different wines from other products (and each other) is differences in the relative concentrations of compounds resulting in different flavors, rather than the presence of unique components.

Figure I.1 Composition of a representative dry red table wine (a) on a % w/w basis and (b) typical concentrations (mg/L) of major wine components excluding water and ethanol, that is, the main contributors to "Everything else". Key trace components (0.1?ng/L-10?mg/L) would not be visible and are therefore not included. Chemists: note that 11% w/w ethanol is approximately 13.9% v/v ethanol, with the latter units being almost exclusively used to express ethanol concentration as well as appearing on wine labels

Wine is produced by the alcoholic fermentation of grape juice or must (juice and solids), which results in the complete or partial transformation of grape sugars to ethanol and CO2. The use of other fruits must be declared, i.e., "Raspberry Wine". However, winemaking and wine storage result in many chemical changes beyond simply the consumption of sugars and formation of alcohol. This is readily exemplified by the greater complexity of volatiles in wine as compared to grape juice (Figure I.2). An array of volatile components can contribute to the aroma of wine, and these "odorants" are sometimes classified based on when they are formed; that is, in the grape (primary), during fermentation (secondary), or during maturation and storage (tertiary) (Table I.1). Many wine components do not neatly fall into only one category - monoterpenes like linalool ("floral" aroma; Chapter 8), for example, may be found in grapes (primary flavor compound) but may also be formed from precursors during fermentation (secondary). Complicating this further, non-volatile varietal thiol precursors from grapes release their odiferous forms during alcoholic fermentation (Chapter 23.2), and thus are considered "secondary" rather than "primary" odorants. However, the free compounds are called "varietal" thiols (Chapter 10) because they can be linked to specific, cultivar-dependent precursors.

Figure I.2 Comparison of GC-MS chromatograms for (a) a grape juice and (b) a wine produced from that grape juice. Every peak in the chromatograms represents at least one unique volatile compound. Note that the relative size of a peak does not necessarily relate to the importance of that compound to wine aroma - many important odorants might scarcely be seen on the baseline

Table I.1 Primary, secondary, and tertiary classifications of wine odorants and representative examples of each class

1. (i) Normal metabolism of sugars, amino acids, etc.
   
2. (ii) Transformation of grape-specific precursors

1. (i) Ethyl esters (Chapter 22.2), fusel alcohols (Chapter 22.3)
   
2. (ii) Monoterpenes (Chapter 8), varietal thiols (Chapter 23.2)

1. (i) Extraction from oak
   
2. (ii) Microbial spoilage or chemical tainting
   
3. (iii) Abiotic transformation of precursor compounds in wine

1. (i) Oak lactones (Chapter 25)
   
2. (ii) Trichloroanisole (Chapter 18)
   
3. (iii) 1,1,6-Trimethyl-1,2-dihydronaphthalene (TDN) (Chapter 23.1)

aCompounds may be members of more than one class.

Many non-volatile components are also extracted and reacted during winemaking, along with the suite of volatiles, with the number of compounds identified following advances in analytical technology. A survey from 1969 reported that wine and other alcoholic beverages contained 400 volatiles, while a later book from 1983 reported over 1300 volatiles [2]. A more recent analysis of wines using a state-of-the-art mass spectrometry system (FT-ICR-MS) was able to detect tens of thousands of unique chemical signals across a set of wines, and assign chemical formulae to almost 9000 components (mostly non-volatiles such as amino acids, carbohydrates, and phenolics) [3]. However, the advanced instrumentation in that report would not distinguish structural isomers - for which there may be millions for a condensed tannin consisting of 10 monomers [4] (Chapter 14). Although technology is still advancing, even the extra dimension of resolution offered by emerging techniques, for example, the coupling of mass spectrometry with ion mobility spectroscopy or 2-dimensional chromatography (Chapter 32), would struggle to improve much on the situation. Thus, the number of distinct chemical compounds in wine, like most natural products, is essentially uncountable.

With this in mind, the goal of a wine chemist is not to enumerate every compound in wine, but rather to identify compounds, or in many cases classes of compounds, that will directly or indirectly control key aspects of the wine, such as organoleptic properties (aroma, flavor, and appearance), safety, and stability, and what reactions alter that profile. Alternatively, compounds may be of interest because they can be used to detect the presence of fraud or understand the effects of terroir. These categories, and examples, are summarized in Table I.2.

Table I.2 Summary of major functional classes of interest to wine chemists. Note that compounds may fit into more than one category