UC Professor Dan Petersen Has a Scientific Approach to Winemaking


Illustration by Chris Danger


an Petersen is a board-certified toxicologist with expertise in plant toxins and toxic alcohols, and he’s applied his expertise to winemaking for the past 30 years.

Your professional and personal interests have had a funny way of intersecting.
They have. I’m a board-certified toxicologist in my day [job], and I have some expertise in plant toxins and in toxic alcohols, so I got involved with wine about 30 years ago. Chemistry is important in making great wine. My PhD is in biochemistry—it’s all about metabolic pathways and fermentation. UC approached me to teach wine courses—I teach Introduction to Wine, an intermediate wine course, and an advanced wine course. I’ve been doing that for about 15 years. There’s a natural overlap. I also teach physiology and plant pathology and the nutrition courses in the same horticulture program. So wine is just part of the plant sciences and part of my expertise.

How has this expertise helped you make great wine?
There are aspects of making good wine that require you to consider the events that happen in the vineyards. You can’t make great wine from bad grapes. So why is nutrition important? There are aspects of plant physiology and a lot of aspects of plant science that are concerned with making great grapes. And then there are aspects in terms of making great wine, like fermentation.

Browsing wines, how important is it that consumers look at the vintages and regions?
A lot of people pick wines by the label, but the most important factor is where the grapes came from. If you pick grapes from a warm place, they’re going to be fruitier. If you pick wine from a cooler place, they’re going to be earthier. If you pick a wine from Europe, they’re going to be earthier just because it’s further north. If you pick wines from a hot place, like Australia, you’re going to get some big old fruity flavors that are going to knock your socks off. So know where the grapes come from, and you know what you’re going to get.

Are vintages important?
Well, there are so-called vintage charts, which are produced by magazines like Wine Spectator. I carry one in my pocket and you can look up and see whether, you know, 2010 was a good vintage in Bordeaux or not. The really good restaurant guys, the sommeliers, have this in their head and they already know it. But in general, white wines are aged in the barrel for six months to a year and then released, so the 2018 vintage is coming out now and is on the shelves. The 2019s are obviously just being harvested and the 2017s are pretty much sold out. So usually, you’ve got a couple vintages available on the shelf in most shops that you would see, and it’s going to be pretty recent vintages. Sometimes people store wines and age them, if they like older wines, but in general, you know what you’re going to find in the market is 1-year-old whites and 2-year-old reds. You actually won’t have much of a choice of vintages.

Are aged wines better?
I would say in general whites don’t improve with age, but reds do. But they’re all going to turn brown in the end if you wait long enough. So for whites, most don’t improve much with time. There are some exceptions, but in general, they’re ready to go, and by the time 10 years passes, they’re pretty much gone. They’re starting to brown because there’s a tiny amount of oxygen that gets in through the cork, and they slowly oxidize with time and start taking on oxidized flavors. The same thing happens to red, but because they have more natural antioxidants from the skins in them, because of the way they’re made, they oxidize more slowly. They’ll improve for five to 10 to 20 years for the best made wines, but then they start that slow decline. What happens for red wines is they go from bright ripe fruit to dried fruit. Plums go to prunes. Grapes go to raisin. They start to acquire some of these dried fruit characteristics.

How can people who are shy to try reds ease into them?
The biggest, baddest red wines are pretty tannic and pretty acidic, but there are less tannic and less acidic reds that are a little bit easier to try, and those are from grapes that are thinner skinned. The tannins, these big flavor compounds, are all found in the skins. All grape juice is white, so the color comes from the skins, too. So if you have a thinner skin red grape like pinot noir or grenache some of these stronger, more bitter flavors are going to be absent.

How does price affect quality?
There’s a correlation between price and quality, but it’s a pretty imperfect correlation. Some [wine brands] think too highly of themselves, and others don’t think highly enough of themselves and underprice their wines. There are places in the world and grapes in the world that are great values. Like, for example, cabernet, which is very highly sought-after. Very prestigious grape. Grown everywhere, commands pretty high prices no matter where it’s grown. So, you know, the average cabernet out of Napa now is like $75 and the great ones from Bordeaux are over $1,000, and that’s just lunacy. But there are great ones from Washington State that are under $20. So it’s a question of finding a good location where there’s some value for the grape that you love.

How do you approach food pairings?
I think the old adage of like with like works as a place to start, so white wines with white food and red wines with red. For a beginner that’s probably the thing to stick with, if you’re serving red meat or some of the heavier veggies, roasted veggies. Caramelized mushrooms with red wine works pretty well, and you know, chicken or fish with white wine works really well. Now, that said, there are some other things like Champagne that can go with everything. If you wanted a multi-course dinner with Champagne at every course, you could make that work. Sweet Champagne with dessert and a dry white with your salad. If you’re serving something gamey like venison or lamb, a bolder wine works really well with that, like syrah. On the other hand, if you’re serving oysters, which are more subtle, a chardonnay from the north of Burgundy is a particularly good pairing. Chardonnay tastes a little like oysters because it’s growing in limestone, which is fossilized sea floor where the oyster beds are. It’s about marrying the wine with the food and bringing out similarities in flavor.

Do you have a varietal that you consider to be like the most underrated?
Cincinnati is a chardonnay, cabernet town. Those are the two major grapes that are sold here, but my favorite grape might be syrah, because it tastes differently depending on where it’s grown.

And overrated?
I think chardonnay. It is one of these grapes that shows the hands of the winemaker perhaps more than any other grape. If you put it in an oak barrel, it’ll be toasty and oaky. If you caused it to go through something called a malolactic fermentation, it’s going to be more buttery and creamy. If you pick it at late harvest, like, say the first of October versus the first of September, it’s going to be less acidic and more full flavored, like red apples. You’re painting. The one that you’re going to end up with is like a painting and you’re painting from a palette of different flavors. There are just so many things available to the winemaker to steer chardonnay in the direction that they want. It’s less about the fruit and more about the winemaker. White wine makers love that but a lot of them tend to overdo it.

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