How Fermentation Begins in the Field

When it comes to winemaking, decisions made in the field are just as important as those made during fermentation. Great wine starts long before the grapes are harvested. 

“You can make good wine from good grapes, but it’s very hard to make good wine from bad grapes,” said George Zhuang, a viticulture advisor with the University of California Cooperative Extension in Fresno County. “If good wine is your goal, you need to understand viticulture and be involved in the vineyard from the very beginning.” 

As an instructor in UC Davis Continuing and Professional Education’s new online Viticulture Certificate Program, Zhuang teaches Vineyard Management and encourages winemakers and viticulturists alike to see grape growing and winemaking as one continuous process. 

“I was taught—and have always believed—that wine is made in the vineyard,” Zhuang said. “While fermentation technically begins in the winery, the foundation of wine quality is rooted in viticultural decisions. From site selection and canopy management to irrigation strategy and crop load, every choice shapes the fruit’s chemistry and, ultimately, the character of the wine.” 

Can you walk us through the process, step by step? 

First, start with your property. Assess the soil structure, chemistry and water availability. Based on those soil and water conditions and climate, choose your variety and rootstocks. Next, start thinking about which trellis system to use and how you’ll prune, manage the canopy and irrigate. 

Why is it important to think of fermentation as starting in the field and not just in the winery? 

It takes a lot of effort—and doesn’t have a high success rate—to make good wine from bad grapes. So, if you want to make good wine, you should start by producing high-quality grapes. Then you can follow the fermentation process according to each winemaker’s preference and create a high-quality wine from those grapes. 

What vineyard decisions have the biggest impact on wine quality? 

That’s a tough one. I’d say all vineyard decisions have some impact on the final wine quality. Any viticultural practice we do in the vineyard affects both the yield and the quality of the grapes at harvest—and that affects wine quality. 

But personally, I think the biggest impacts come from site selection, rootstock and variety choices, and trellis. These are the hardest factors to change once the vineyard is planted. You’d have to remove everything and start over, which takes time and money. 

Can you give an example of how something like canopy or irrigation directly affects the final wine? 

This is one of my favorite topics. I do a lot of research on different canopy and irrigation management practices to see their effects on yield, grape quality and wine quality. 

These aspects are important because they’re things we can change each year depending on the situation. Site, variety, rootstocks and trellises are hard to change. 

In my class, we go in depth on how you can use canopy and water management to achieve the wine style or characteristics you want. For example, practices like shoot thinning, leaf removal and hedging can have a direct impact on grape color, sugar, acidity and flavor—all of which carry over into the wine. 

Same with the water. Stressed or less stressed irrigation can lead to very different flavors and fruit chemistries at harvest. 

How do factors like crop load or harvest timing influence fermentation outcomes? 

This is another topic we cover in the Vineyard Management class. The reason crop load and harvest timing influence the fermentation outcome is because of a term called vine balance. That refers to the ratio between how much leaf you have and how much grape or crop you have at harvest. 

You want to avoid overcropping or undercropping the vines. You need an optimal ratio for ripening each cluster of grapes. For example, if you have overcropped vines, you’re going to have a huge crop and a smaller canopy, and potentially unripe grapes. You’d end up with low sugar, high acid and underripe, green flavors—not floral or fruity ones. Those berry characteristics affect aroma and chemistry, which have huge potential outcomes for the wine qualities after fermentation. 

What’s one viticulture mistake that can’t be fixed later in the winery? 

Most vineyard mistakes—site, variety, rootstock or even canopy and irrigation—are hard, or very costly, to fix in the winery. All winemakers want to start with clean, high-quality grapes because it means fewer headaches and fewer problems to correct later. 

If I had to pick the biggest mistake that can’t be fixed in the winery, I’d say rootstock, variety or trellis choices. These are not only hard to fix in the winery but also hard to fix in the vineyard. You’d have to remove everything and start from the beginning. So, the biggest mistakes happen at planting. 

How do you balance yield and quality in the vineyard? 

That’s a tough question because quality depends on the region, the winery and the production goals. Every winemaker has their own metrics for vineyard quality. 

To generalize: If you're farming premium wine grapes in California’s North Coast—like Sonoma or Napa—you may have a lower yield to increase quality. But in the Central Valley, you might have higher yields and achieve similar qualities. It really depends. 

In class, we’ll talk about how to balance yield and quality in cooler or hot climates so students can choose their own approach to achieving balance. 

How does the Viticulture Certificate Program help reframe the role of viticulture in winemaking? 

This program gives both general and in-depth viticulture knowledge. It’s a nice supplement to our Winemaking Certificate Program. For students who’ve gone through the winemaking track and want to learn more about how viticultural practices can impact their winemaking, the Viticulture Certificate Program can help. It's a very natural progression: start with winemaking, then deepen your knowledge with viticulture. 

This program reviews some basic concepts and goes much deeper into vineyard practices, so students walk away with a broader and more integrated understanding. 

How has the relationship between the vineyard and the winery changed over your career? 

Early in my career, there was very little communication between the vineyard side and the winery side. I’m not sure why it was that way, but there was a lack of collaboration. 

Now, with increasing demand for high-quality wine and high quality-grapes, we’re seeing much more collaboration and communication between vineyard managers and winemakers. Winemakers visit vineyards more often. They ask about canopy and irrigation practices, and they see how these affect the grapes. 

There’s also growing collaboration in response to climate events like wildfire smoke and extreme heat. Some of these issues—like smoke taint—are very hard to fix in the winery, so it makes sense to work together in the vineyard.