NW Winemaking Notes

Picking grapes for optimum quality

The Challenge

Let's assume:

• It is mid-September in the Okanagan and you have a well-tended field of grapes getting close to harvest.  You have done all you can to ensure moderate cropping, good canopy management, and enough, but not too much, water—all factors that affect early and even ripening.
• There are no practical problems to get in the way of selecting the best picking date. For example
:
• The weather is good (days are getting cooler but no rain is coming).
• Pickers are available.
• Shipping or storage is ready.
• You know the style of wine you want to make.

Now all you have to do is to decide when the grapes are at their optimum. This may not be an ideal optimum, just the best you can expect in this year in this field for this variety of grape and for the style of wine you want to make.

In making your decision you will probably consider three things:

1. Appearance.
2. Numbers (sugar and acid levels).
3. Taste.

Unless you are lucky (and in the Okanagan you are not likely to be lucky very often), you will discover that appearance, numbers, and taste do not point to the same picking time. You have to make some compromises. In the recent past, the compromises were usually biased in favor of sugar and acid numbers. Today, top winemakers are more likely to prefer taste and almost ignore numbers. But any bias or compromise will affect how you make your wine and the eventual quality of this wine.

Picking on Appearance

The development stages of grapes

Grapes go through several stages of development and changes in appearance during a growing season.  To better understand the final stages, a brief overview is helpful.

1. Flowering
• Early flowering is preferred as long as the threat of cold is past—best vintages are often associated with an early start and an early harvest.
• Weather can interfere with complete and even pollination.
2. Green berry growth
• Berries use their own chlorophyll to grow and accumulate acid and sugar.
• Canes and roots also grow rapidly.
3. Arrest of green berry growth
• Berries have reached more than half their final size, but are still green and hard.
• Berries have all the basic acid they will ever have, but only a small proportion of the sugar.
• Roots and shoots also stop growing. This is usually the recommended time for pruning of vines for canopy management and for thinning of clusters.
4. Beginning of véraison
• Berries soften and the color changes (véraison means turning, ripening).  The color change happens dramatically in individual berries (in 24 hours), but unevenly among and within clusters.  Even under ideal conditions it will take two weeks for a vineyard to turn completely. The date of véraison is based on an estimate of when half the grapes have turned.
• Véraison comes earlier with a low leaf-to-fruit ratio and with warm, sunny, dry weather—best vineyards tend to start véraison early
5. Sugar/water accumulation
• Berries gain size and weight, adding somewhere between 25% (a dry season) and 75% (wet season) of their green berry weight.  A 40-50% increase is normal for good quality grapes.
• Sucrose is manufactured by the leaves and transported to the berries where it is hydrolyzed and stored as glucose and fructose.
• Acidity declines because of grape growth and the dilution of the existing store of acid. Malic acid also declines because it is used in grape respiration.
• Flavors begin to accumulate, although most varietal flavor development comes later in the season.
• Grapes typically do best at about 25°C—temperatures below 15°C and above 35°C will stop the plant from working.  Warm (not hot), sunny, dry ripening seasons tend to produce the best vintages.
• Berries ripen unevenly among and within clusters.  Within clusters, berries at the top (next to the stem) are riper (more sugar, less acid) than those at the tip.  Larger berries have more seeds, and these ripen more slowly than small berries of the same variety.
6. Arrest of phloem transport
• Plants "shut down" and berries stop accumulating water and sugar—this happens naturally at the "end" of the season but can happen earlier from stress.
• Berries soften noticeably and are easily "deformed" with a squeeze. It is easier to separate seeds and pulp and to detach grapes from their stems
• Stems go brown (in many varieties) and seeds go from green to brown.
• Flavors continue to develop, because most flavors are synthesized in the grape.
7. Dehydration
• Berries can lose 10% or more of their weight through water loss over a period of a couple of weeks.
• Sugar levels rise because of water loss.  The rise in sugar can suggest that the plants are still working when they aren't.
• Acidity is reduced.  Tartaric acid level can rise a bit because of dehydration, but malic acid continues to be lost (grapes can lose almost all their remaining malic acid during this period). Potassium activity in the grapes can continue to reduce pH even though TA may not decline much.
• Flavor synthesis comes to an end and and flavor deterioration begins (these periods overlap in ways not well understood).
8. Raisining
• Berry flavors continue to deteriorate.

The pros and cons of picking on appearance

Pro: Grapes can be picked successfully on the basis of appearance. Biologically, a grape is finished ripening when the phloem (water, sugars) stops moving from the leaves to the grapes. People can tell when a grape is ripe just by looking at and touching it. There is full and distinctive color, the grapes are soft, and the the stem is brown.  In fact, the best picking practices depend on this ability to recognize ripeness. If labor costs are no object, a field of grapes is best picked two or more times.  (The best Riesling vineyards in Germany may be picked four or five times.)  Even when a vineyard is only picked once, pickers will often be instructed to leave behind the unripe or overripe clusters.  Pickers make the distinction solely on appearance. If pickers can make these distinctions, an experienced viticulturist and winemaker, who can also examine seeds, should be able to determine optimum ripeness quite well just by appearance.

Pro: There are several problems when grapes go beyond biologically maturity: (a) Sugar rises from dehydration (not always bad). (b) Malic acid continues to be lost (usually bad) and potassium activity continues to increase pH (always bad).  And (c) the grape continues to soften and become more susceptible to bruising, breakage, and bacterial activity (always bad), especially if the grape has to be shipped any distance. Picking on a ripe appearance can help avoid these problems.

Con: For economic reasons, fields are picked all at once. To understand what this will mean for the must, you need to measure average sugar and acid levels.  You may want a slight concentration of sugar through dehydration.

Con: Flavors can continue to develop after the physical maturity of the grapes. It can be particularly important to let some of the least ripe grapes catch up a bit to the riper ones.

Picking by the Numbers (sugar and acid)

Testing for numbers

In Napa and Sonoma you can drop off a sample of 100 berries at the ETS lab in St. Helena before 2:00 pm, pay $110 (US), and by early evening have the results for the following tests:

• °Brix (% by weight of soluble solids, mainly fermentable sugar). During the time they are testing, ETS says this commonly varies between 19 and 30, which means people start testing only well along in the growing season. For another $60 you can get a precise measure of fermentable sugar.
• Various acid measurements:
• pH—commonly varies between 2.90 and 4.20
• Titratable Acid or TA (g/100ml)—commonly varies between 0.35 and 1.20
• Tartaric Acid (g/L)—commonly varies between 1 and 11
• Malic Acid (g/L)—commonly varies between 0.5 and 11
• Potassium (mg/L)—commonly varies between 500 and 4000
• Two yeast available nitrogen (YAN) measures
:
• Ammonia (mg/L)—commonly varies between 50 and 400
• Alpha Amino Nitrogen (mg/L)—commonly varies between 20 and 400

The YAN numbers are chiefly useful to indicate how much nutrient you are going to have to add to the fermentation. However, the sugar and acid numbers can serve as a guide to picking the grapes.

Recommended numbers for wines

As grapes ripen, sugar levels rise and acid levels fall. Both sugar and acid are important. Higher sugar levels are associated with grape maturity and the higher levels of alcohol expected of superior vintages. Higher acid levels (especially a low pH) provide better wine-making circumstances. Thus, the trick in picking by the numbers is to get to the optimum cross-over point, where sugars are high enough and acid numbers still allow for good wine-making. The current wisdom suggests you should be picking your grapes when the following specs are realized.

Recommended sugar and acid levels for wine grapes

Wine Type	°Brix	TA	pH
Source: Boulton et al. (1977), Principles and practices of winemaking, as cited by van Schalkwyk and Archer (2000).
Sparking	18.0-20.0	0.70-0.90	2.8-3.2
White Table	19.5-23.0	0.70-0.80	3.0-3.3
Red Table	20.5-23.5	0.65-0.75	3.2-3.4
Sweet	22.0-25.0	0.65-0.80	3.2-3.4
Dessert	23.0-26.0	0.50-0.75	3.3-3.7

Sugar acid combination measures

Because sugar rises and acid falls as grapes mature, there can be a trade-off in desirable specifications. Do you wait for more sugar at the risk of losing acid or increasing pH? This dilemma has led people to propose various measures of optimal maturity in which both acid and sugar figure.

One such index relies on the square of pH to assess critical acidity, and consists of °Brix * pH². According to researchers who use it, the desirable range for this index is between 200 and 270. For example, if you took the mid-points of the recommended values for red wine in the previous table (Brix 22, pH 3.3), this sugar acid index value would be (22.5 * 3.3 * 3.3) = 245, or nicely in the middle of the desirable range. For whites, the index value based on mid-point numbers would be (21.25 * 3.15 *3.15) = 211.

Another approach to combining sugar and acid is to divide the °Brix by the TA in g/100ml. This is called the sugar acid ratio. Again it is possible to generate ideal scores by using the mid-points of the recommended ranges in the previous table. (NOTE: If you are reading European sources, don't get confused by their higher values for the Sugar-Acid Ratio. Europeans often measure acidity in the equivalent of stronger sulfuric acid.)

The two combination measures are illustrated in the following table.

Measures combining sugar and acid

Wine Type	B-pH Index	B-TA Ratio
Source: Van Schalkwyk and Archer (2000) discuss the Index; Bisson (2001) discusses both the India and Ratio.  Both these sources are drawing on the work of thers.
Formula	Brix * pH2	Brix ⁄ TA
Ideal White Table	211	28
Ideal  Red Table	245	33
Desirable Range	200-270	27-34

The numbers for our recent grapes

The problem with the numbers in the above tables is that we seldom see them in the grapes we get. Our 2003 Black Sage grapes illustrate the issue. The following table is based on typical (median) readings by Club members.

Black Sage numbers for selected 2003 grapes

Grape Variety	°Brix	TA	pH	Brix-pH Index	Brix-TA Ratio
Source: Typical (median) values measured by at least three Club members, usually on very small lots. Initials after Grape Variety are for the growers.
Chardonnay (JB)	22.5	0.74	3.39	259	30
Gewürztraminer (JB)	21.9	0.69	3.45	261	32
Pinot Blanc (JB)	21.0	0.47	3.74	294	45
Pinot Gris (RF)	19.8	0.55	3.71	273	36
Riesling (AMi)	22.4	0.69	3.27	240	32
Sauvignon Blanc (AMa)	22.9	0.64	3.66	307	36
Semillon (MW)	22.7	0.57	3.42	266	40
Cabernet Franc (LM)	22.4	0.54	3.64	297	42
Cab Sauv 15-169 (DC)	25.5	0.37	4.03	414	69
Merlot (RF)	23.1	0.43	3.76	327	54
Pinot Noir 115 (RF)	22.5	0.47	3.77	320	48
Pinot Noir 667 (RF)	20.0	0.49	3.71	275	41
Syrah (BF)	24.8	0.51	3.95	387	49

These parameters for 2003 Black Sage grapes suggest at least three observations:

• Only Chardonnay, Gewürztraminer, and Riesling meet most of the suggested specs, but even Chardonnay and Gewürztraminer have relatively high pH levels and this puts then at the high end of the acceptable index range, where reds are more to be expected.  Semillon also comes close to meeting the suggested specs.  Most people would consider pH more important than TA, and the Index value for Semillon seems to capture the quality of the Semillon better than the simple Brix/TA Ratio does.
• The general "numbers" problem with these Black Sage grapes is low acidity—a low TA and a high pH. Only the Cab Sauv and perhaps the Syrah are clearly overripe in terms of sugar. The phenomenon of low acidity without excessive sugar is a well-known consequence of overripe grapes or a warm climate because of the respiration of malic and other organic acids. This past summer in the Okanagan, high heat caused vines to "shut down," stopping sugar production but increasing acid loss.
• The Index and Ratio numbers are not that bad for Pinot Gris and Sauvignon Blanc, but these were the white grapes that arrived in poor condition with obvious VA problems.  Appearance does matter! 

The pros and cons of picking by the numbers

Pro: Great, long-lived wines almost always come from grapes with "good numbers" to start with. pH is particularly important for good winemaking, and it is something that is very difficult to correct even by adding acid later. No one wants to add water to an overly sweet must and thereby dilute the intensity and flavor of the wine.

Con: The right numbers may occur fairly regularly in classic wine areas like Bordeaux, but you will not get them in hotter, drier wine areas like the Okanagan (or South Africa or Australia). In such conditions, sugar development usually outpaces flavor development.

Con: Ultimate wine quality (as determined later by tasting) is not well correlated with the numbers at harvest.  This is particularly true of sugar levels. You can always adjust the numbers prior to fermentation, by adding water or sugar or acid or other means. In some commercial areas there will be restrictions on additions, but there are no restrictions on the home winemaker. You cannot correct for flavor that is not there.

Picking on Taste

Basic information about taste

Taste and its development in grapes and wines is not well understood scientifically, but there are some basic things we do know.

• There are hundreds of different molecules that contribute to the flavor and aroma of grapes and wine.  Some of these have a a sensory impact at concentrations of only a few parts per billion.
• Flavors develop over the course of grape ripening, but much of the typical varietal flavor and intensity comes very late.  In one Oregon test (Watson), Pinot Noir was made from the same grapes, but one picking was one to two weeks later than the other.  Later, when the finished wine was tested, more than twice as many flavor compounds were isolated from the grapes picked later—a rather astounding differences. Of course, more flavor elements do not necessarily mean a better flavor, but in this case the later picking was definitely judged to be better in overall flavor and aroma.
• Most flavor compounds are synthesized in the grape itself and this synthesis can continue after grapes are biologically ripe and phloem flow has stopped.
• Identifiable tastes change as grapes develop. The exact tastes differ according to variety, but the usual sequence is more vegetative tastes early and more fruit tastes later.  The sequence for the appearance of flavors in Cabernet Sauvignon goes something like this:
  1. Vegetation (plant matter)
  2. Herbaceousness (straw, herb, vegetal, tobacco)
  3. Unripe fruit (green apple, citrus rind)
  4. Red fruit (cherry, strawberry, raspberry, cranberry)
  5. Black fruit (plum, blackberry, black cherry)
  6. Jam (prune, date, raisin)
  This is not necessarily a sequence of tastes. Tastes deteriorate at different rates than new ones appear. It is possible to have herbaceous and jammy tastes at the same time. This leads Bisson (2001) to suggest that it may be better to taste for the absence of the "unripe" tastes you want to avoid rather than to taste for mature complexity.
• Many of the flavors that are most desired in a mature wine are not taste-able in the grape. They are locked-up with glucose in complex molecules that are technically known as glycosides and more popularly known as flavor precursors. Some of these flavor precursors can be unlocked with pectic enzyme, but some emerge only with fermentation or aging. What can be tasted most easily in the grape are the fruit flavors associated with a young wine.
• NOTE: There is a test called the Glycosyl-Glucose Assay (or G-G Assay) that is used to establish the total level of glycosides in juice or wine. At one point in the 1990s there was hope that this test would provide a more objective basis for establishing the flavor potential of grapes. It has not so far proved to be a magic bullet.

The pros and cons of picking on taste

Pro: There is no point in making wine with a green or stemmy taste. Even if you have to compromise your winemaking and settle for a wine that is not for the ages, it is better to have good taste and aroma than not. And while there is no science of taste, experience with winemaking, particular varieties, and specific terroir and vineyards can provide valuable information about when to pick grapes at their optimum.

Con: If taste comes at the cost of "overripe" grapes, which spoil easily during shipping and have a high pH and potassium level, you could end up with poor and short-lived wine, even though the taste potential was there. And there is always the risk that one is waiting for flavors that are not going to appear, so that more time just means more deterioration.

Con: Tastes are notoriously elusive and subjective at the best of times. And many of the tastes we want in a wine are not discernable in the grape—they only emerge later in the fermentation and aging.

Sources

Bisson, Linda (2001). "Optimal grape maturity," Practical Winery, July/August. Bisson is at UC-Davis. This in a good overview.  It is available at: http://www.practicalwinery.com/julaug01p32.htm

ETS Laboratories (2001). "Tools for Grape and Must Analysis." Useful explanation of grape testing terminology. the 2003 version of this paper is available at: http://www.etslabs.com/pagetemplate/blank.asp?pageid=195. There are other useful technical papers available through the ETS website.

Jackson, Ron S. (1994). Wine Science: Principles and Applications (Academic Press).

Peynaud, Emile (1984). Knowing and Making Wine, second French edition, trans. by Alan Spenser. John Wiley and Sons.

van Schalkwyk, Hanno, and Eben Archer (2000). "Determining Optimum Ripeness in Wine Grapes," Wynboer. http://www.wynboer.co.za/recentarticles/0500optimum.php3

Watson, Barney (2003) "Evaluation of Winegrape Maturity," in Edward H. Hellman (ed.), Oregon Viticulture, Oregon State University Press, pp.235-245. This excellent chapter is available as a download at http://winegrapes.tamu.edu/grow/maturity.pdf

Zoecklein, Bruce W. (2000). "Yield and Quality," Vintner's Corner, 15:5, September-October. Available at http://www.fst.vt.edu/extension/enology/septoct00.html

Prepared by Rod Church as a "backgrounder" for a meeting of the Nanaimo Winemakers on March 5, 2004.