What Makes a Wine Distinctive?

This February, the Robert Mondavi Institute's Sips and Bites series welcomed two professors from UC Davis's Department of Viticulture and Enology. 

Dr. Andrew Waterhouse is a professor emeritus, holder of an honorary doctorate from the University of Bordeaux, and an ISI Highly Cited Researcher. Dr. Liang Chen is a newly appointed assistant professor whose résumé includes work at E. & J. Gallo Winery and the analysis of wines that traveled to the International Space Station.

Together, they led a crowd of curious wine lovers through six wines and the chemistry that makes each one distinct.

It All Starts With the Basics

Waterhouse began by laying the foundation. He explained every single wine is built on four pillars: alcohol (ethanol, if you’re a chemist), acid, glycerol, and residual sugar. 

Alcohol gives wine body and warmth. Acid keeps wine from tasting flat. Glycerol? "Only chemists pay attention to it," Waterhouse admitted. It quietly adds a little mouthfeel and not much else. 

Residual sugar (RS) is what's left when yeast finishes converting grape juice into wine. "Dry" wines aren't actually sugar-free; they get as low as about 0.1%. Winemakers deliberately adjust RS to shape the final product. Some Chardonnays, for instance, typically have a touch of added RS to boost characteristic fruitiness.

But these are just the scaffolding. The real magic, the reason a Sauvignon Blanc smells nothing like a Cabernet Sauvignon, lives in aroma compounds.

Whatever Happened to Sherry?

Here's something that might surprise you. In the 1960s, more than half of all wine sold in the United States was sherry. Today, it's nearly invisible on store shelves.

Sherry is a fortified wine from southern Spain whose modern form took shape in the early 19th century, driven in large part by British merchants' appetite for a consistent, age-worthy wine. 

Its key compound is acetaldehyde, which is formed when ethanol reacts directly with oxygen, or when the yeast converts ethanol to acetaldehyde. Acetaldehyde is intensely aromatic. At low levels, it smells faintly of green apple, and at higher concentrations, closer to roasted nuts. 

It’s considered an off-note in most wines, but it’s the compound that gives sherry its characteristic bouquet and gave post-Prohibition winemakers a way to make grapes that weren't particularly good taste consistent and drinkable anyway. 

The oxidation chemistry isn't just historical trivia. In red winemaking today, controlled micro-oxidation and barrel aging deliberately produce acetaldehyde which reacts with color pigments to lock in that deep ruby hue. Without it, red wines would fade fast.

There's a more personal dimension to acetaldehyde, too. In your liver, alcohol converts to acetaldehyde, and the accumulation of this compound is linked to many classic hangover symptoms. 

The Many Faces of Chardonnay

Dr. Chen took over to walk the room through Chardonnay, arguably the world's most versatile white grape. It originates from Burgundy, France, and is a cross between the ancient, unremarkable Gouais Blanc, a peasant variety that has faced banning attempts across France since the Middle Ages, and Pinot Noir. That humble lineage produced a grape now planted on over 494,000 acres worldwide, including more than 90,000 acres in California alone.

What makes Chardonnay so interesting from a chemistry standpoint is how dramatically winemaking choices shape its final character. Fermentation in oak barrels extracts a suite of aromatic compounds, including vanillin (the same compound as in vanilla extract), eugenol (which smells of cloves), and oak lactones that deliver woody, coconut-like notes. The level of barrel toasting determines how much furfural, a toasted caramel aroma compound, makes it into the wine.

The room tasted two California Chardonnays side by side, a 2022 double-oaked version and a 2024 unoaked expression. The difference was stark. The unoaked wine was bright and fruit-forward with pineapple, lime, and clean acidity. The oaked version carried the unmistakable notes of vanilla, a hint of clove, and that warm, woody richness that comes from months of oak contact. 

That richness can go even further. The buttery quality found in many Chardonnays comes from diacetyl, produced by lactic acid bacteria during malolactic fermentation, a secondary process that converts sharp malic acid into softer lactic acid. Temperature, timing, and how often the winemaker stirs the lees, the sediment at the bottom of the fermentation vessel, all influence how much buttery character ends up in the glass. It's another way the winemaker's hand shapes what you taste.

Tannins Are Not a Taste

Waterhouse made a bold move with the Bogle Cabernet Sauvignon. He added extra tannin to the bottle before guests arrived. One sip had the drying astringency of biting into an unripe persimmon. 

He used the wine to explain how tannins aren't technically a taste. They're a physical reaction. Large molecules bind to the proteins in saliva, stripping away lubrication and creating that drying, gripping sensation known as astringency. The bigger the molecule, the more proteins it binds, which is why young, tannic wines feel so aggressively drying. As red wines age and acid slowly breaks down tannin molecules, that harshness softens.

Tannins in red wine come primarily from grape seeds and skins. How long the fermenting wine stays in contact with them, a process called maceration, determines how much astringency ends up in the bottle. Extended maceration means more tannin, more structure, and more aging potential. 

For wines built to age, astringency is a feature, not a flaw. But winemakers who want to soften tannins before bottling can use protein-based fining agents like gelatin or isinglass, which is derived from the swim bladder of sturgeon, to bind tannins and drag them to the bottom of the tank. Many producers have switched to plant-based alternatives, such as potato protein, which work just as effectively.

The Unique Flavor of New Zealand Sauvignon Blanc

Perhaps the evening's most mind-bending segment concerned New Zealand Sauvignon Blanc and why it tastes so unmistakably different from its California counterpart. 

"The appearance of this style," Waterhouse explained, "was a historical accident."

The explanation lies in a chain of practical decisions shaped by New Zealand’s environment.  Labor shortages pushed the industry toward mechanical harvesting, which increases berry damage and promotes the formation of precursor compounds that allow for the formation of thiols, sulfur-containing compounds that are intensely aromatic even at trace concentrations. This is territory Chen knows intimately. His doctoral research at the University of Adelaide focused specifically on varietal thiols in Sauvignon Blanc. 

Another contributing factor is the country’s wet climate, which creates widespread mold pressure, leading growers to apply elemental sulfur heavily and routinely. During fermentation, residual sulfur on the berries is thought to contribute to the formation of 3-MH, the thiol most responsible for the wine’s distinctive tropical, passion fruit-forward character. 

New Zealand Sauvignon Blanc has about 5 times as much 3-MH as typical California versions. 

So, New Zealand Sauvignon Blanc isn't an expression of terroir in the traditional sense; it's the product of a specific, reproducible process. To emphasize this point, Waterhouse described a trial in which those same conditions were recreated using Pinot Gris. The result was a wine that tasted unmistakably like New Zealand Sauvignon Blanc. The grape, it turns out, may not matter as much as the process, which can dominate flavor formation.

Why the Science Matters

An evening like this is a reminder that winemaking is chemistry. Every oak lactone, every thiol, every gram of residual sugar is a decision made by nature, by yeast, and a winemaker who understands the science well enough to guide it.

This event was part of the Robert Mondavi Institute's Sips and Bites series. Events like this are made possible by the RMI Friends Program. Learn more about becoming a friend of the RMI at rmi.ucdavis.edu/friends.

Kaylianne Jordan is a senior studying Viticulture and Enology at UC Davis. She has a background in culinary arts and a passion for sustainable farming and enjoys exploring the connections between agriculture, winemaking, and community. Outside of college, she loves trying out new recipes, and discovering local food spots.