The perfect pinot problem

AS GRAPE varieties go, pinot noir is famously finicky. Got right, the thin-skinned grapes can produce some of the world’s finest wines. Central to that is plucking the grapes from the vine at the right time. Winemakers typically depend upon testing the level of sugar to determine if their berries are ready, but that is not terribly accurate. As pinot noir grapes reach late stages of maturity, the rate at which they gain sugars slows down just as the rate at which they accumulate the aromatic compounds that can grant wine a good “nose” goes up. And in wine, the aroma is a fundamental part of its appeal. Varying rainfall, temperatures and soil conditions all affect the rate at which aromatic compounds enter grapes, making it difficult for wineries to know whether they should harvest their grapes a few days or weeks after the increase in sugar tails off.

Michael Qian and Fang Yuan of Oregon State University had a local interest in the problem. The prime crop of the vineyards in Oregon’s Willamette Valley is pinot noir. And Oregon’s cool climate, short growing season and periodic heavy rains make determining whether grapes are ready for harvest particularly challenging. Curious to see if they could monitor directly the presence of aromatic compounds, the biochemists set up an experiment.

They collected ten bunches of grapes from the Stoller Family Vineyard in Willamette Valley on two separate dates, one early season and one late, during 2012 and 2013. The first date from each year was just two weeks after the berries had started to change colour from green to red, while the second was between five and six weeks after this milestone. Aside from monitoring the sugar levels, Dr Qian and Mr Fang also made use of a technique called aroma-extract dilution analysis to study aromatic compounds in the grapes. This was done using a gas chromatograph, in which compounds are separated out from a sample based upon their boiling points and chemical properties. The kit was also fitted with a sniffing port for researchers to smell the compounds as they were released.

Oh, the damascenone!

The researchers report in the Journal of Agricultural and Food Chemistry that they detected 49 aromatic compounds in the grapes from the two years during both the early and the late sampling periods. Most of these compounds remained at low levels throughout the growing period. However, four of them (ß-damascenone, which carries a floral and tea-like smell; vanillin, the key compound in vanilla; 4-vinylguaiacol, which smells like cloves, and 4-vinylphenol, which is reminiscent of spicy almonds) were found in much higher concentrations in the mature grapes than in the early-season ones.

Dr Qian and Mr Fang suggest these four aromatic compounds are commonly associated with a good pinot noir and therefore could provide another way to determine the optimum time to harvest. For their part, wine buffs will keep settling the argument the old-fashioned way, with a sniff straight from the glass.