Grape harvest season, sometimes called “crush,” is a magical time to visit Napa Valley. From August to October, winemakers and growers are anxiously watching the weather, sampling fruit and surveying the land—preparing for the perfect moment to send hundreds of hands to work, delicately picking and sorting the bounty, which will soon become the vintage of another year gone by.
To an outsider, grape harvest may seem as fundamental as picking seasonal vegetables, but for the winemakers and growers of Napa Valley, choosing when to harvest is the single most important decision they’ll make all year.
How Do Growers Know When to Harvest?
Knowing when to harvest is a complicated matter. Different grape varietals mature differently. Sauvignon Blanc, for example, is typically picked early during harvest season, while cabernet sauvignon will likely be picked toward the end.
During the weeks leading up to harvest, it’s common to see viticulturists, winemakers and growers perusing each section of the vineyard, sampling grapes and inspecting stems, leaves and seeds. They’re looking for several quantitative indicators to determine if the grapes are ready, most notably the Brix (or sugar levels), acidity (pH) and tannin.
A grape’s sugar level is commonly referred to in measurements known as Brix, named after the man who invented the system. The Brix can be deciphered through experienced taste or with a device called a refractometer, which measures light refracted in liquid. Generally, growers will wait for the grapes to reach >24 Brix before they harvest the grapes. Depending on weather conditions throughout the year, this can occur at various times during the harvest season. It can also vary depending on the type of grape.
By measuring the acidity or relative pH of sample grape juice, growers are further able to predict the ideal time to harvest. Winemakers also pay particular attention to the grape’s skin and seeds, which help evaluate the tannic compounds, known as phenolics. Winemakers are looking for balanced tannin composition, usually indicated when grape seeds change from green to brown.
The Day of the Harvest
Once a winemaker has made qualitative assessments and determined the readiness of the grapes, the real work begins. Before the sun rises, workers take to the vineyard under the shade of night to begin picking grapes. By picking the grapes at night, the temperature variance is less volatile for the grapes.
Sorting begins once the grapes are picked. Many boutique wineries hand-sort through hundreds of pounds of grapes, looking for imperfections, debris and inconsistencies that may alter the quality of the pressed juice. In addition to hand sorting, optical sorters use highly sophisticated cameras and lasers to quickly separate the bad fruit.
Not a Perfect Science
While much of the harvest season is determined using scientific analysis and tools, it still leaves considerable room for stylistic interpretation. This is what makes the harvest season so exciting—although many quantitative factors help growers and winemakers make informed decisions about when to harvest, there are qualitative differences that make each harvest at every vineyard unique and representative of the year.
Harvesting grapes is the first step of the long and intricate process of winemaking. The final product is highly influenced by the winemaker’s choice of barrels, when it is bottled and how it is stored.
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