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| FIGURE 112. A refractometer is used to determine the soluble solids (ºBrix) content. |
The proper harvesting time for grapes depends on the cultivars, nature of the growing season, and the particular use of the fruit. Although proper harvest methods cannot make up for poor weather or cultural management, harvest can diminish a quality crop to a level of mediocrity, if not handled with care. The rate at which grape fruits mature, as well as the time of their ripening, is governed by several factors, some of which can be altered by the grower.
Select cultivars that inherently mature their fruits within the growing season common in the Midwest, with bloom in early June and harvest before a killing frost in mid-October.
An average growing-season temperature of 65°F to 70°F is necessary for proper functioning of the natural processes that mature fruits and vines. A minimum amount of heat units is necessary to mature a cultivar such as Concord. A heat unit is defined as the daily accumulation of temperature above a certain threshold value (generally 50°F) for a specific stage of plant development.
Vines carrying a normal, or less than normal, crop mature fruits earlier than those carrying heavy crop loads.
Vines trained to expose the highest percentage of leaves to light mature fruits and wood earlier than vines with less light exposure.
Grapes with leaves free from insects, disease, herbicide injuries, and nutrient deficiencies mature faster.
Over-vigorous vine growth delays maturity of fruit and wood; vines with low vigor generally mature wood and fruit earlier than normal vines, if the crop load is not excessive. Moderately vigorous vines normally mature their fruits earlier than highly vigorous vines. Therefore, over-vigorous vines with densely shaded areas may not mature their fruit or wood before autumn frosts kill leaves and tender shoot terminals.
Grapes intended for fresh eating, processing into juice, jam, jelly, or wine, may have different desired harvest parameters.
As the harvest season approaches, growers should try to pick the fruit at optimum maturity and quality for the specific use. Grapes do not improve in flavor, color, and sugar content after separation from the vine, so harvest should be delayed until desired levels are reached. On the other hand, if the grapes are left too long on the vine, the berries may shatter; damage from birds, insects, and rots may become significant; and yields will be reduced.
The first indication of physiological changes in the maturing fruit is the development of pigments in colored cultivars and softening of berry skins. The beginning of the ripening process is called veraison. Following veraison, berry sugars will rapidly increase, acid will decrease, and the berry will reach its final size. Approximately three to four weeks after veraison, or three weeks before harvest, regular berry sampling should begin to monitor fruit composition. It is critical that the sampling procedure be consistent each time to fully evaluate ripening.
Most cultivars change color long before they are mature enough to harvest. Human taste buds respond primarily to the sugar/acid ratio. Grapes are approaching an acceptable maturity for fresh eating at a sugar/acid ratio of 15:1 (15% soluble solids and an acid content of 1%). As the sugar content increases and the acid content decreases, this ratio may rise to as high as 25:1 or 30:1. For table use, grapes should be picked when both color and flavor have reached their peak.
The average length of time between bloom and optimum harvest date for a cultivar can be helpful in determining the time of harvest but may vary considerably from year to year and from region to region. The section on grape cultivars contains information on approximate date of maturity to use as a guide.
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| FIGURE 112. A refractometer is used to determine the soluble solids (ºBrix) content. |
Grapes for commercial processing into juice or wine generally are harvested at the state of maturity desired by the processor. The primary index of maturity for this purpose for juice grapes is the soluble solids (°Brix) content. (Brix is determined with a refractometer. See Figure 112.) In addition to soluble solids, total acid content and pH also are important indicators of maturity, especially for wine grapes.
When grapes are grown for processing, the entire vineyard is harvested at one time to facilitate processing. Therefore, it is extremely important to determine the fruit composition in the vineyard prior to harvest. Berry samples should be taken from exposed fruit and shaded fruit in different parts of the canopy, on opposite sides of the row, and in a manner that as close as possible represents the balance of fruit in the vineyard. Berries at the tip of the cluster ripen last, so it is important that the sample include berries from all parts of the cluster. Avoid sampling from vines at the ends of rows or from odd vines. Although under-developed secondary clusters are best removed at veraison, if they are to be harvested (as with mechanical harvesting), they must be included in the sample. For the sample to accurately represent maturity, a minimum of 100 berries should be collected from multiple clusters on at least 20 vines.
Total acidity (often called titratable acidity) and pH are important harvest parameters because they affect both flavor balance and wine stability. Titratable acidity measures the quantity of grape acids (mostly tartaric and malic), while pH is a measure of the strength of the acidity. Normally, titratable acid decreases during ripening, and the decrease is greater under warm than cool conditions. Titratable acid level is measured by titrating against a standard base using a pH meter or color change indicator. Juice and wine with lower pH tend to have better color, microbial and chemical stability. If pH is too high, wine may taste at and lack crisp, fruity avors. A meter with a pH sensitive electrode is used to measure juice pH. Growers may work with a winery to perform tests of titratable acidity and pH, but the procedure is not difficult if the grower has access to the proper equipment and supplies.
A good reference on wine and juice analysis, including pH and TA measurement, is available from Southwest Missouri State University. See Micro Vinification: A practical guide to small-scale wine production, Dharmadhikari, M. R. and K. L. Wilker. 2001.
Guidelines for fruit harvest parameters vary according to the intended wine style. Generally, for dry white wines, fruit is harvested when the pH is 3.1 to 3.2 and the titratable acidity is about 0.75%. For dry red wines, fruit is harvested when the pH is 3.4 to 3.5 and the titratable acidity is about 0.65%. In both cases, winemakers generally prefer fruit at about 0.15% higher acid levels for wines they intend to finish sweeter, with 2 to 3% residual sugar.
In most cases, soluble solids should be 20% or above when the pH and titratable acidity are in the appropriate range. However, some cultivars, particularly labrusca types, tend to be lower in soluble solids when other parameters are optimal. Additionally, some winemakers prefer more or less ripe fruit of specific cultivars.
There are notable exceptions to these general guidelines, but they should help growers understand the importance of fruit composition to wine quality. Flavor, seed maturity, skin color, and other factors may also be important with certain cultivars and for discriminating winemakers. Working with the processor to determine the optimal level of ripeness is the best approach to success.
Fresh-market grapes and those for table use must be hand-picked and handled carefully. To achieve optimum quality and extend the length of the season, growers may sometimes harvest a vine more than once, selecting fully ripened clusters determined by color and taste. Some cultivars tend to crack at maturity, particularly if there has been excessive rain. In such cases, fruit rots can develop and spread rapidly. If fruit rots start to develop, harvest immediately to avoid losing the entire crop.
While grapes for wine making are often hand harvested, the majority of grapes for juice are harvested mechanically (Figure 113). Most of the harvesters rely on high-frequency vibration to shake both individual grapes and intact clusters from the vines. Several commercial models are available for both single- and double-curtain trellises. Some winemakers prefer machine harvesting because it facilitates rapid harvest and processing, especially on larger acreages and where labor may be limited.
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| FIGURE 113. Mechanical harvesters are used with juice grapes and some wine grapes, especially where acreages are large and labor may be limited. |
All grape growers should develop a marketing plan. Time and money spent cultivating a vineyard is wasted unless a suitable marketing plan is developed. There is not enough time to develop a market after grapes are ripe.
Successful marketing requires more than producing good fruit, although a quality product is necessary. The key is to develop a marketing plan that forces the grower to examine all the facets of grape growing as a business. It also forces growers to set realistic goals and devise methods to achieve them.
The first decision in formulating a marketing plan is identifying the target market. Growers cannot realistically try to sell grapes to everyone. They must aim for a segment of the total market. The wine producers or marketing association in each state can provide prospective growers with lists of wineries and other potential markets. A first step is to identify wineries and get input from them on what cultivar they need and whether a mutual relationship of supply and demand can be developed (Figure 114).
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| FIGURE 114. Retail sales room. |
Growers must be realistic about the potential yield from their vineyard, the cultivars adapted to their site, and the price those cultivars can bring. One of the best ways of ensuring a good business relationship is to operate under a mutually agreed-upon contract. Although it is best to consult legal advice to develop a contract, some characteristics that may be included are listed here.
Depending on the location of the vineyards, other types of markets such as pick-your-own, fresh-pressed juice, or produce distributors may be possibilities.