By Somchai Rice
This article series was inspired by one of our workshop participants. She asked why the fruity aromas in wines diminish over time. The answer was complicated. Here’s an expanded explanation of what we think is going on, based on current research.
The aroma compounds that we smell in wine are present in the headspace and are always at equilibrium with the compounds dissolved in the wine. The compounds present in wine aroma include esters, terpenes, norisoprenoids, phenols, aldehydes and ketones. So what happens to these compounds during aging?
We’ll continue our discussion with terpenes in part 2. Terpenes are important in varietal character, particularly in Muscat and Muscat related cultivars. When these terpenes oxidize, there is noticeable loss of varietal character in the wines.
Concentration of monoterpene alcohols, such as geraniol (percepts: rose, geranium), linalool (percepts: flower, lavender), and citronellol (percepts: rose) decrease during aging. Linalool concentration in Riesling wines have been shown to decrease by 80 percent over three years, falling well below its detection threshold.
In contrast, monoterpene alcohol derivatives such as linalool oxides (percepts: flower, wood), nerol oxide (percepts: oil, flower), hotrienol (percepts: hyacinth), and alpha-terpineol (percepts: oil, anise, mint) concentrations increase with aging. You must be thinking to yourself, “These are all floral aromas. Why does it matter if the concentration changes?”
Odor activity values (OAV) for any given compound is the ratio between the concentration and the odor detection threshold (ODT) for that compound. Compounds with higher OAV are more important to the aroma profile of the wine. OAV is inversely proportional to ODT. In other words, as ODT increases, OAV decreases. Most monoterpene alcohol derivatives have higher detection thresholds then their respective monoterpene precursors. I’ll walk through linalool as an example below.
Figure 1. Comparison of odor activity values and aroma descriptors of 100 ppb of linalool and linalool oxide
As illustrated, 100 ppb of both compounds have very different OAV, due to the different detection thresholds. Linalool, on the left, is present in high concentrations in young wines. As the wine ages, linalool is oxidized to linalool oxide, which has a much higher ODT. You can see that the same concentration of each compound has drastically different impact on aroma profile.
I also wanted to point out that the compounds have qualitatively different aromas, lavender and wood.
We recently poured an Edelweiss wine for an event. It is a clean wine, but was several years old. Someone commented to me that they tasted a pine aroma when they were expecting a floral wine. Based on what you have learned about terpenes and aging, presence of pine aroma should not be a surprise. Tune in to Part 3 in the next newsletter, where we’ll explore more of what happens to wine aroma during aging.