Evaluating the sensory qualities of grapes
In California, assessment of grape maturity has historically focused on sugar levels.
Determining when a grape has reached peak maturity is increasingly the job of field technicians trained to recognize the sensory characteristics of grapes, but Dr. Nick Dokoozlian, vice president, viticulture, with E & J Gallo Winery in Modesto, California, wants to give vineyard managers and winemakers a better understanding of what makes a grape ripe.
This would help avoid grapes with optimal sugar levels from being harvested before they’ve been on the vine long enough to develop the compounds that give them optimal flavor, Dokoozlian told growers attending the annual British Columbia Wine Institute viticulture conference in Penticton, B.C., in July.
“The chemists can tell us what’s in the fruit; they can’t tell us whether it’s good or bad,” he said. “We’re seeing a gradual shift occurring from using traditional parameters or metrics such as degrees Brix into something that is very hard to describe and very hard to quantify in lots of cases, and that is berry sensory evaluation.”
Trained field technicians can typically identify grapes with the flavor characteristics that will contribute the most positive traits to a wine, allowing harvesters to swing into action and have fruit on a winery’s crush pad and into production within 48 hours without extensive chemical analysis.
“Until we get really rapid analytical techniques, probably berry sensory evaluation is the way we’re going to go,” Dokoozlian said. “We don’t have a better real-time way of monitoring the fruit.”
Sharper analysis of fruit quality could allow growers to receive payment for producing fruit that best suits a winemaker’s needs.
“We currently do not pay growers based on the sensory value of their fruit, but eventually we will probably pay growers based on the chemical composition, so how are we going to do that?” Dokoozlian said.
He would like to find a reliable way to quantify what technicians taste in the field so that wineries can pay growers for the characteristics that make the best wines.
California growers’ assessment of grape maturity historically focused on sugar levels.
“The harvest targets were primarily based on sugar, and when you’re making port and sherry, it probably doesn’t matter all that much,” Dokoozlian said.
Sugar targets were typically “off the board” between 1945 and the early 1960s, but this changed as generic table wines came into production. Competition from Australian wines since the early 1990s has increased pressure on wineries to pay attention to a broader spectrum of traits when determining maturity.
“Today, I don’t want to say sugar’s irrelevant,” Dokoozlian said. “But from my perspective, once you get to a certain point, not only are sugars irrelevant, viticulture’s become irrelevant because the bottom line is [that] winemakers get into the field, and they make the call.”
But gauging which flavors in the raw fruit will produce the desired wine remains a challenge.
“We know that a small fraction of potential grape flavors are really detected during berry sensory evaluation,” Dokoozlian said.
While up to a quarter of the vegetative compounds responsible for so-called “green” flavors in wine are present in the fresh grape, the compounds responsible for the fruit characteristics are primarily released after crush.
A technician tasting grapes has to gauge the green of the fruit present to the senses against the maturity of the grape and the characteristics released after crush.
“Generally speaking, if the grapes are green, the wines are even greener,” Dokoozlian said. “This is where it really indicates the real need for hang time, because one of the things we absolutely cannot allow is to have any green characters in our wine.”
This is where Dokoozlian believes analytical tools can help, though he recognizes that formal chemical analysis independent of sensory perception isn’t realistic.
“Chemists can’t do this by themselves,” he said. “We try to match the chemical compounds that we’re monitoring to match what our tasters are perceiving in the field. So far, I think we’re doing a pretty good job, but we’ve still got a long way to go.”