Friday, April 11, 2014

A high-level map of the sources of wine aroma

A while ago I had set out on a journey aimed at comprehensively describing all of the attributes of a quality wine and had developed the below framework as a series of guideposts towards this end. I had covered most of the intrinsic factors -- with the exception of aroma/flavors -- when I got sidetracked by items which piqued my interest along the way. I am now ready to return to this task, prompted in large part by some excellent research work on aromas that has come to my attention recently.

According to Robinson et al., (Origins of Grape and Wine Aroma. Part 1. Chemical Components and Viticultural Impacts, Am. J. Enol. Vitic, 65:1 (2014)), "The sensations of flavor occurs when odor-active molecules stimulate sensors in the mouth and nose, which the brain collates to produce a flavor perception." Olfactory, gustatory, and trigeminal assets cooperate in order to produce the perception of flavor but, according to the authors, smell plays an outsized role "in the overall perception of the product." Smell, as defined by the authors, "is a biological and electrophysiological process that converts the molecular information of an odorant into a perceptual response."

We continue with some baseline definitions. An odor is a volatile compound, or combination of volatile compounds, that stimulates the olfactory organ to register a smell. The odor threshold of a compound is the lowest concentration at which its smell can be detected. The perception threshold is the minimum detectable concentration for 50% of a group of tasters while the recognition threshold is the minimum concentration of that compound necessary for identification of the odor.

If we relate the foregoing to wine, quality wine can be characterized as having complex associations of aroma compounds that exceed the odor threshold; and, for a subset of tasters, exceed the recognition threshold. But what are the sources of these odor? The figure below captures those sources at a high level.

As the figure shows, wine odors are the sum of the odors from the grape, maceration, yeasts, alcoholic fermentation, malolactic fermentation, and aging. Additionally (discussed as an odor source by Robinson et al., but not described graphically above), "chemical changes associated with acid and enzyme-catalyzed modification of both non-aroma-active and aroma-active grape constituents" are also sources of wine odor. The Wine Institute characterizes these odors as shown in the table below.

Over the course of a number of future posts I will delve deeper into these sources and attempt to show how the interaction of the odor compounds aid in the perception of a quality wine.

©Wine -- Mise en abyme

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