Come September, or Settembre, it's harvest time in Colorado. If we've averted Colorado's spring frosts and hail. Received sufficient temperatures with optimal minimums and maximums throughout the growing season. Controlled pests, diseases and mildew pressures. Properly managed the canopy, yield, vigor, and irrigation. Then! It's time to begin considering the optimal time to harvest the fruit. Incidentally, this vintage looks fantastic. After veraison, the point at which grapes turn color, sugars begin accumulating in the berry and acids begin decreasing (predominantly through respiration of malic acid). Phenolics (including Anthocyanins: color, and Tannins: astringency / bitterness) are evolving, or ripening, throughout this time as well. The seeds of the berry transition in color from green to brown as do the stems. Eventually sugars cease to accumulate but continue to increase in concentration due to dehydration of the berry. You can taste the evolution. The pulp of a berry starts out tart/sour but become sweeter with time. If the birds think the grape bunches are ready, they're not! Wine grapes ready for harvest are VERY sweet. Chewing on the skin of a berry and chafing the outside of a seed allows one to evaluate the astringency and bitterness that may be imparted to the wine. If the viticulturist managed the vineyard in an ideal manner the fruit throughout is uniform. In reality sampling methodology is a critical to ensure the harvested fruit, as a whole, replicates the sample. Vineyards are often harvested, and fermented, in individual lots when variation exists.
So, how does one decide when it's time to harvest? The first and foremost consideration is obtaining sufficient sugar accumulation (glucose and fructose). The unit of measure typically used in the United States is degrees Brix, and is simply the % soluble solids per 100 g solution (weight/weight). Sugars represent 90-95% of the soluble solids in grapes so one often uses a hydrometer to measure the density (or specific gravity) of the must (crushed grapes, skins, and seeds). For red and white wines one typically harvests when the fruit is above 20 Brix. Fruit in excess of about 26 Brix is typically due to dehydration, as opposed to sugar accumulation. The other method commonly used to measure sugars is a refractometer. A simple hand-held refractometer determines Brix by the juices index of refraction (the amount light bends when passing through the grape juice is a function of how much sugar is present). The amount of sugars in the juice relates to the potential alcohol that could result in the wine (a coarse estimate is: Percent Alcohol By Volume ABV = 0.55 x Brix so a 14% ABV wine is harvested at about 25.5 Brix).
One wants to Balance the sugar, and final alcohol, with the organic acids present. As the sugar concentration is increasing in the berry the acids are decreasing. If the acids in the berry decrease too significantly the resulting wine will be flabby (due to the insufficient amount of acid present) it also may potentially be microbially unstable due to the high pH. One typically measures acids through the metric of Titratable Acidity TA which measures the concentration of acid in the juice by titration, or neutralization of a juice sample with a base. TA is important as it dominates taste and consequently balance. The strength of the acids in the wine is measured by pH: the amount of hydrogen ions free and available to react. TA and pH are not directly related due to buffering of acids by, for example, Potassium. pH is directly related to the reactions in wine, microbial stability, color, and the effectiveness of a sulfite addition in achieving molecular stability and prevent browning. Thus, the harvest decision becomes further influenced by the point where the desired accumulating sugar and decreasing acid levels are at an optimal balance.
There are three organic acids in grapes which have the dominant impact on TA and pH: Tartaric, Malic, and Citric. The ratio of Tartaric Acid to Malic Acid is of importance to final acidity in a wine which will go through Malolactic fermentation, a “secondary fermentation” that transforms Malic acid to Lactic acid and lowers TA. In addition to these organic acids in the grapes there are also Amino Acids, which include nitrogen compounds and Argenine. These amino acids can also be measured to help determine an optimal harvest time for the vintage.
There are many phenolics in grapes and the types present vary with cultivar (grape variety). With respect to harvest one often considers Anthocyanins (color) and Tannins (astringency and bitterness). Anthocyanins are the colored pigments, and in a red grape these will increase after veraison, peak, and eventually decrease. The Anthocyanins affects the final color of wine and thus another factor that may be brought into the harvest decision. Skin tannins are set very early after veraison and change little beyond that point. Seeds tannins decline after veraison and can change yearly. The tannins in the skin and seeds are different (in fact there are a dizzying array of phenolics present in grapes and wine). Simplistically, skin tannins lend astringency (a sensation) and seed tannins lend bitterness (a taste); I should note there are astringent tannins in the seeds as well, but the bitter Catechins are predominantly located in the seeds. As the season progresses one may be tasting for ripeness in the seeds, associated with a decrease in bitterness. Tannins and Anthocyanins in grapes can be measured, though this measurement is not routinely made for deciding harvest.
Grape flavor and aroma compounds, despite being in trace quantities, evolve and are critical to a wines final quality. Each variety has its own characteristic flavors, and thus we must also make harvest decisions on taste and aroma of the berry itself. A young berry, or one lacking optimal viticulture techniques, may taste vegetal. As the season evolves the berry flavor can progress into red fruits, black fruits, and finally jammy characters. These characteristics, in addition to spice, can be tasted in the skin of the fruit (if separated from the masking effects of the sugar in the pulp). Stems may lend flavors as well with a transition from spice and clove to tea. The aroma compounds, including the volatile Terpenes (both free and bound), will influence the aroma and eventually the bouquet of a wine. One typically attributes Aromas to that of the grape, and Bouquet to that of the vinification. Once again, we find taste being a key parameter that is monitored throughout the ripening process to determine the optimal harvest time.
There are several metrics that have been developed in an attempt to quantify the relation between these various components in order to select on an optimal harvest time. However, these are just small insights into true complexity of the decision at hand and evolutions to follow with vinification. One of my favorite stories is of the company that tried to deconstruct a prized wine in order replicate it molecule-by-molecule in the lab: the result tasted like mud. These scientific methods are just tools and should be used to help make the best decision but not dominate it. Both taste and measured quantities should be used in concert to help make the best possible harvest decision.
I believe the optimal harvest time varies by both the vintage and the winemakers style. By style, one must understand how the initial grape will evolve through the winemaking process. For each of my wine styles, I have ideals in mind for all of these various parameters. However, one of the fascinating features of wine is its reflection of the vintage, season, location, and interactions with its complete environment. In fine wine, vintage matters. There are bulk producers who blend a dizzying array of wines to produce what is essentially the same composite wine each and every year (think of the mass manufacturing of soda which has the same taste year after year). In my opinion: this production goal has lost connection with the vineyard and vintage; its less interesting. In practice a winemaker may be presented with fruit that varies from their original ideal. As it's often said: it is impossible to make great wine from poor grapes, but one can easily make bad wine from great grapes. Thus, it is the winegrowers and winemakers challenge, to harvest at a time that optimizes the potential of the grapes and to adapt his or her style to give this fruit its best possible showing. The winemaker has a plethora of choices: crushing method, time and temperature before fermentation, use and timing of sulfites, fermentation temperature and rate, how long is the wine left in contact with the skins and or seeds, how is the wine pressed, clarification methods, barrel aging regimes, racking methods, malolactic fermentation, barrel ageing times, and topping regime to name just a few. All of these factors can influence the evolution of ABV, pH, TA, color, astringency, bitterness, aroma, bouquet, and flavor. These choices can emphasize the vintage and terrior or suppress it.
Thus, in my opinion both the vintage and winemakers style need to be considered when choosing the optimal time to harvest. If the same batch of grapes is given to three different winemakers you will almost certainly get three different wines. And as such, the ideal harvest parameters likely vary from winemaker to winemaker as well.