Professor David A. Mills

Diana Stockton
Diana Stockton
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Peter J. Shields Endowed Chair in Dairy Food Science
Department of Viticulture & Enology and Food Science & Technology
University of California at Davis

This year‘s Wine Seminar and Annual Tasting in celebration of terroir led us to be intrigued by Advisory Board Member Carole Meredith‘s suggestion of David Mills at UC Davis for a possible panelist at a future seminar. Carole wrote that Dave‘s field was “the role of vineyard microbes on terroir.” When we called Professor Mills, he readily accepted an invitation to be interviewed for this REPORT.

Professor David Mills is from Green Bay, Wisconsin, where his father was a doctor. During the football season Dave and his brother would sit in the children‘s section at Lambeau Field to cheer on the Packers. Dave had imagined he‘d become a doctor like his father but instead he has pursued an academic career of research and teaching in microbiology. He is still a Packers fan, however, and his cheesehead is on the windowsill in his office at UC Davis.

During research for his doctorate, Dave discovered introns within plasmids in the cytosol of certain bacteria. Cytosol is the goo inside a bacterium; plasmids are bodies that cruise through the cytosol, loaded with code to generate certain new molecules of protein. Introns are bits of DNA within plasmids that do not directly govern the production of protein (they had not been known in bacteria before Dave‘s research). Rather, they are like punctuating a sentence. The position they take up in a chain of DNA can alter coding for proteins, and can cause a plasmid to produce something else. The introns Dave was the first-ever to recognize were those governing the metabolism of lactose within lactose bacteria—“a rather industrially relevant determination,” he adds—given our appetite for products cultured from milk: hard cheese, cream cheese, cottage cheese, yogurt.

In 1995, his doctorate followed by two years of postdoctoral research at North Carolina State, Dave was offered Professor emeritus Ralph Kunkee‘s former position in viticulture and enology at UC Davis (Ralph had retired in 1991). The university‘s reputation in viticulture and enology had inspired Dave to suggest a study of microbes associated with fermentation when he had applied. ’Winegrowing and winemaking—a microbial zoo!‘ Dave laughs. Keep in mind that we human beings are largely microbial, as much as 90%, by number of cells—referred to as our human microbiome.

The advent of new DNA-identifying tools and techniques, largely thanks to research support from the medical and food communities, had made the study of microbial diversity in nearly any site possible. Dave thought, ’Why not apply this innovative technology to wine?‘ And soon after he joined the faculty at UC Davis, winemaker Ashley Heisey suggested he begin his vinous microbial research right then at Dolce in Oakville, and sample the must for its botrytized late harvest wine. The two agreed such must was sure to be “loaded with microbes” (starring Botrytis cinerea, aka “noble rot”).

In vineyards, microbes arrive via plants, insects, water droplets, dust particles. Rain splatter can stir them up as well as farming practices. After the grapes for Dolce had been harvested, cleaned and, in this case, pressed, and were ready to ferment, Dave‘s samples were taken at the press pan. With the results from this initial sampling that confirmed the presence of various populations of bacteria and yeasts that had come in on the skins of the grapes, Dave knew much more research was in order and set about doing just that over the next ten years.

John W. Thorngate directs the Sensory Department at Constellation Brands. He agreed to work with Dave and UC Davis to further identify and describe the microbial ecology of wine musts. For two years, musts from eight different Constellation wineries from Paso Robles, Central Coast, Napa Valley, and Sonoma were sampled, 280 samples in all, of Chardonnay, Cabernet Sauvignon, and Zinfandel. And halfway through the research, new DNA sequencing techniques made a hundred-fold improvement in the number of samples that could be run. As many as 300 samples could now be sequenced. The prior maximum had been 20. Furthermore, the more samples one has to analyze, the truer one‘s results.

Weather, wine grape variety, °Brix, leaky berries—manifold conditions enrich different microbial populations. Dave and his research team found that at the start of fermentation, Chardonnay from Paso Robles and Chardonnay from Napa harbored different populations of microbes. And, the microbial populations of Napa Chardonnay and Napa Cabernet differed. Furthermore, in the two years of samples, Napa Cabernet was more similar year-to-year than a Napa Cab was to a Paso Robles Cab. Dave is convinced there is regionality in microbial populations, a MICROBIAL TERROIR.

Dekkera Yeast
Dekkera yeast
Photograph courtesy of UC Davis

His department has also been asked to sample and describe the microbial environments where sake and beer are brewed, cheese and Matsoon (an unsweetened milk drink Armenian in origin) are cultured, and hard cider is fermented. In the last few years, cocoa and coffee beans have also been thoroughly sampled as well as leaven bread dough, fish paste, and various salamis (the potential roster of fermented food and beverage microbial ID‘s is lengthy). With the advent of extraordinarily effective methods of DNA (and RNA) identification, it is now possible to sequence the DNA of an entire tree, a whole grapevine.

One of the primary contributions to winemaking by UC Davis in the 1960‘s, Dave asserts, was cleanliness. With better sanitation practices such as improved management of microbial populations by applications of sulphur dioxide, wine production became healthier and more consistent. Mind you, the microbial population involved in fermentation is by and large, inherently good, inherently benign, and vanishes from the wine. After fermentation, after racking and settling, after filtration and fining, the microbial population is gone. Lees are the husks of a vast population of microbes starved out by the yeasts that have eaten all the sugar or been vanquished by alcohol, which depolarizes cell membranes.

Sporobolomyces yeast
Sporobolomyces yeast
Photograph courtesy of UC Davis

Dave also mentioned the work of Trevor Phister, a former post-doctoral fellow now at Pepsi. In Dave‘s lab, Trevor was able to develop a quantitative test for Brettanomyces bruxellenis, (Dekkera), a most unwelcome microbe when it becomes a crowd. Trevor published his research in 2003. A test based on the work has since been used that can determine the presence and magnitude of Brett at any stage in winemaking.

Now that Dave‘s lab has identified and measured the different microbes that arrive at the winery and are in wine must, further research will allow them to follow the must into the winery. With the new DNA sampling techniques, the microbial factors that bring change during fermentation and aging can be ascertained: the microbiome of wine production, unique to vineyard and winery.

Dave says his lab is on the front edge of how to look at all this information. Research into why wine tastes the way it does is tantalizingly underway. Dave says he wants his students to have concrete evidence of what stimulates the sensory experience of wine, its taste and smell. Collaboration on the sensory effects of wine has begun with sensory scientist Hildegarde Heymann, who succeeded Professor emerita Ann Noble. Dave wants his graduates to make their own winegrowing and winemaking decisions based on the evidence of actual microbial profiles; he wants them to be able to assess consequences, where possible, with factual, measurable parameters rather than guesswork.

What a sommelier says she or he can detect, scientists want to see and know—not via a somm hypothesis. Dave seeks further proof of regionality and real proof of how wine tastes. He and his students are at work on determining the how as we continue to savor the wow.