Brix: the sweet spot for grape harvest
in Western Farm Press
|Grapes grow to their proper maturity on a vine in Sonoma County, Calif.|
The Cambridge English Dictionary defines ‘sweet spot’ as “the situation or combination of things that is the best or most effective possible.”
Grape growers have a slightly different definition of that same achievement apex and that is — when Brix hits its optimum, it signifies the height of harvest time.
Writing for Lodi Wine Growers, viticulturist Stan Grant puts it this way: “Soluble solids, i.e., sugars, are the principle criteria for scheduling harvest because fructose and glucose are at their most plentiful.”
Grape sugar development is an important phase in the reproductive life cycle of the vine, according to Calwineries Inc. who note: “Grape seeds contain the vines genetic material and its reproductive success is dependent on the survival of the seeds. Sugars are transported from the vine into the grape to make sure the seed has enough energy to live.”
Or as Grant writes: “Normally, a portion of the sugars produced each growing season is stored in woody vine tissues to support next season’s growth and sometimes these reserves contribute to accumulating berry sugars during fruit maturation.”
“As berries are developing,” according to the Calwineries explanation: “Sucrose (a glucose and fructose molecule bonded together) is transported into the grape to be metabolized. The availability of grape sugar, accumulated in the grape after veraison, provides the building blocks for alcoholic fermentation.
“Total sugar content of fully-ripened grapes is dependent on viticultural practices and environmental conditions which can make sugar content at maturity range from 12% to 28%.”
Which leads us to Brix — and how grape sugar content is measured.
Brix, determined at harvest time, is a measure of soluble solids content in grapes, mostly as sucrose, determined by the use of a refractometer, a hand-held instrument that measures dissolved sugar in a small juice sample in the field. Refractometers make it possible to determine ideal harvesting times of grapes.
Brix correlates to the potential alcohol content of a dry wine with each gram of sugar fermented turning into about half a gram of alcohol.
Brix measures sugar by weight
The Brix scale measures percentage of sugar by weight, giving vineyard managers an objective number to evaluate their crop conditions. At 25 degrees Brix, a grape has approximately 25 grams of sugar for each 100 grams of liquid.
While each vineyard block will have an optimum sugar level signifying the peak of harvest, as a generalization, a range of 19 to 25 degrees Brix is the target for sugar concentration coinciding with harvest time. Each degree of Brix equals 1 gram of sugar per 100 grams of grape juice.
As in all other aspects of growing, there are right and wrong ways to produce ample sugar content, a goal achievable by growing sufficient leaves and preserving their function.
Grant recommends starting with vines that have been tested for pathogenic viruses that will inhibit sugar accumulation. He advises a spacing-training-trellising-pruning system that will promote balanced fruit and foliage growth and allow for complete canopy development with maximum exposure of leaves to sunlight. “Complete canopy development typically involves 14 to 20 leaves for a shoot with two clusters.”
Adequate soil moisture to minimize stress is a given as are leaves in full sunlight. Leaves on the canopy exterior absorb about 90% of sunlight meaning greater photosynthetic activity and sugar production. “Even well-designed vineyard canopies may require fine tuning through shoot thinning, shoot positioning, or lateral shoot removal,” he notes. “For sugar production, it’s nearly as important to maintain healthy leaves as it is to grow the proper number of leaves.”