February 13, 2016

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Predicting Harvest Dates

Yesterday, I took a few pictures in the vineyards at the Horticulture Research Center near Victoria, MN. We are well behind schedule for fruit ripening – which is expected with the late spring we experienced this year. There is some hope that we could catch-up somewhat if the rest of the summer remains warm and sunny.

However, years like this highlight the fact that selecting the right grape variety is crucial if you want to plant a vineyard at the extremes of where they will ripen. The following three pictures were all taken on July 1st.  All three of these pictures come from different grape cultivars, although they are planted within steps of each other. In a year as cool as this year is turning out to be, it is easy to see that simply choosing the right grape variety to plant will have a huge impact on whether or not one will see those grapes ripen before the leaves fall off the vine!

The first two photos are from Vitis vinifera cultivars: Cabernet Sauvignon and Pinot Gris. Even though they are the same species of grapevine, one can see that the Cabernet Sauvignon is about 10 days behind the Pinot Gris in berry development. This is one point in the vine growth where this lag can be appreciated, thus these early stages of berry development (flowering and fruit set) are typically noted by winemakers in order to plan for harvest. For growers, it’s important to note these phenological stages in order to plan for disease management.

In V. vinifera, harvest occurs, on average, 120 days from flowering. This means that we should expect to harvest the Cabernet Sauvignon around Halloween this year. This may all be well and good if we were in California, but in Minnesota it could mean harvesting in snowshoes if the vines somehow make it through a frost by then.  The Pinot Gris could possibly be ripe by mid-October. This is still not news to bet the farm on (literally speaking), but there is some hope. The average first frost in some areas of southern Minnesota is between October 11 -20th. If the weather stays warm and the leaves don’t start to drop, we might see some ripe fruit. Most of Minnesota will see a frost by the first week in October.

Now take a peek at the third photo. Marquette was bred as a cultivar that flowers early and ripens quickly. The cluster at the bottom of the page shows fruit that is past flowering, with many of the bunches starting to point downward on the vine and showing peppercorn-sized berries. Marquette seems to ripen more quickly than European grapevines, so we are likely looking at harvest sometime between mid-September and the beginning of October. However, even if we look at harvesting at the beginning of October it could still be a safe bet for most vineyards planted in Minnesota if we see typical weather for the rest of the year.


CabernetSauvignon 7.1.13 flowering

Cabernet Sauvignon – beginning Flowering July 1st, 2013

PinotGris 7.1.13 Fruit Set

Pinot Gris – end of flowering/fruit set – July 1st, 2013

Marquette 7.1.13 peppercorn

Marquette – Fruit set to Peppercorn-sized berries – July 1st, 2013. The rain and cold weather over the past few weeks during flowering means many vineyards in the state will likely see poor fruit set like we do here.

Method for Sensory Analysis for Grapes

As grapes begin the process of maturation, evaluation of their ripening should be taken place in order to determine harvest date. While for many people, grape maturity is a numbers game (optimal sugar and acidity levels), in reality it is much more complex and subjective. While optimally grapes are harvested with a potential alcohol around 12%, in many places in the world they would be considered “ripe” at much lower or higher levels. I’ve had perfectly well-balanced and complex Riesling Kabinett at 7% alcohol, and I’ve even had wine at 15-16% alcohol that didn’t taste ‘hot.’

Sugar and acidity only give us a small portion of the picture (the technological maturity), and while labs are capable of advanced measurements of grape phenolics, aroma development, and color development, you can get a good sense of grape ripeness by using your taste buds. This is especially important for grapes destined to become red wine, as the maceration of the wine on the skins is what gives red wine its color and structure. Phenolic and aromatic ripeness can be determined in the vineyard. As your sugar levels increase, tasting the grape berries can give you a good idea of the phenolic ripeness and aroma development. And, if you are methodical about the way you taste grapes, you can have a record from vintage to vintage in order to compare the flavors of the grapes from your vineyard from year-to-year.

Here is good method for Grape Sensory Analysis that was developed by the Bureau Interprofessionnel des vins de Bourgogne.

Groups of 3 berries are examined simultaneously, with 3 main areas of focus:
Visual and tactile examination
-Tasting of the pulp and the skin
-Visual and sensory analysis of the seeds

Maturity is determined from Twenty descriptors and scored from 1 to 4 on a scale corresponding to increasing stages of maturation.

Part 1: Visual and tactile Examination of the Berries

1) Press the berries gently, evaluate their ability to crush :

1- Hard berry, splits with strong pressure
2- Berry indents slightly under fingers, very elastic, will return to its initial shape
3- Berry indents easily, slightly plastic, slow to return to its initial shape
4- Berry is soft, splits completely under light pressure

2) Ability to shell

Facility with which the berry detaches from the pedicel

1- Berry is strongly adhesive, detaches with difficulty from the pedicel, tearing its skin
2- Berry is adhesive, pedicel detaches with difficulty while retaining a large part of the pulp
3- Berry detaches pretty easily, pedicel retains a little pulp
4- Berry detaches very easily, pedicel retains very little pulp attached to the pincher

3-Berry Color
Zone of insertion to the pedicel

Part 2: Tasting the Pulp

Place the berries in your mouth, extracting the pulp of each of the three berries by pressing successively each berry between the tongue and the roof or the mouth. Retain the pulp from the 3 berries in your mouth. Spit the skin and seeds, save them.

4) Adherence of the pulp to the skin – film of pulp adhering to the skin is detected by observation or by sliding the skin between your fingers or between your teeth

1 – Pulp adheres strongly to the skin

2 – Film of pulp adheres to the skin

3- Film of pulp hardly visible, but juice is freed when skins are chewed

4 – No visible film of pulp, and no juice freed when skins are chewed

5) Sucrosity of the pulp

1- Pulp is slightly sweet

2- Pulp is medium-sweet

3- Pulp is sweet

4- Pulp is very sweet

(6) Acidity of the pulp

1- Pulp is very acidic

2- Pulp is acidic

3- Pulp has medium acidity

4- Pulp slightly acid

7) Aromas of the pulp – This parameter depends on the variety, and doesn’t evolve in a regular fashion over the course of maturation





8) Intensity of the dominant aromas of the pulp
1- Weakly intense

2- Medium intensity

3- Intense

4- Very Intense

Part 3: Tasting of the Skins

After spitting out the juice and the pulp, place the 3 reserved skins back in the mouth while saving the seeds in your hand.    Chew 10 to 15 times. Next, rub the homogenized skins over the roof of the mouth (palate) then over the cheeks between your gums. After spitting out the macerated skins, pass your tongue over the palate (tannin intensity). Slide your lips over your gums (Astringency). Note the following observations:

9) Ability of the skin to lacerate
1- Skin is hard, presence of large fragments after chewing

2- Skin is hard, presence of small fragments after chewing

3- Skin is easily lacerated, forming a nearly homogeneous paste

4- Skin is pretty easily lacerated, crushed skins form a homogenous paste fairly quickly.

10) Tannin intensity of the skins
1- The tongue slides without effort on the palate

2- The tongue sticks slightly

3- The tongue slides with difficulty

4- The tongue slides with great difficulty

11) Acidity of the skin
1- Skin is very acidic

2- Skin is acidic

3- Skin has moderate acid

4- Skin has little acid

12) Astringency of the skin  – Determined by passing the chewed skins between the lips and incisors, and noting the facility in which your lips slide over your gums after the skins are removed. This parameter depends more on the variety of the grape rather than the level of maturity. Astringency of the grape correlates well with that of the wine.
1- Lip slides easily over the gums

2- Lip sticks slightly against the gums

3- Lip slides over gums with difficulty

4- Lip slides over gums with great difficulty

13) Dryness of the tannins – determined from passing the chewed skins over the roof of your mouth (palate)
1- The tongue slides without effort over the palate, no difficulty with re-salivating, feeling fine-grained and silky

2- The tongue sticks slightly, difficulty with re-salivating, feeling of medium grains

3- The tongue slides with difficulty, difficulty with short re-salivation, large grains

4- The tongue feels almost stuck against the palate, difficulty with re-salivation for more than 5 seconds, aggressive feeling on the palate

14) Aromas of the skin




15) Intensity of the dominant aromas of the skin
1- Weakly intense

2- Medium intensity

3- Intense

4- Very intense

Part 4: Visual and gustatory examination of the seeds
If green traces remain in the seeds, there is no need to taste them. They will be bitter and astringent.

16) Appreciation of the color of the seeds
1- White – yellow/green

2- Chestnut/Green

3- Brown-Grey

4- Deep Chestnut

17) Ability of the seed to break
1- Presence of soft peripheral matrix, and need of strong pressure from the incisors to break the seed.

2- Presence of a fine peripheral matrix; seed is still moist and breaks with strong pressure

3- Peripheral matrix almost absent; seed is still hard, slightly crunchy

4- Absence of peripheral matrix; seed breaks easily

18) Aromas of the seed

1- Seed unfit to taste

2- Green, herbaceous

3- Toasted

4- Roasted

19) Tannin intensity of the seeds – This is evaluated in the same manner as for the skins.
1- Tongue slides without effort over the palate.

2- Tongue sticks slightly.

3- Tongue slides with difficulty.

4- Tongue slides with great difficulty

20) Astringency of the seeds  – again, this is evaluated the same way as for the skins
1- Lip slides easily over the gums

2- The lip sticks slightly

3- The lip slides with difficulty

4- The lip slides with great difficulty

Now that you’ve made note of the the the maturity of the berry, it’s time to interpret the results. You can summarize the data into 4 main categories: technological maturity, aromatic maturity of the pulp, aromatic maturity of the skin, and maturity of the tannins. While all of these interpretations are varietal-specific, you can start to get a good sense of the overall maturity of your grapes in order to make important decisions concerning winemaking. This is where the old adage “you can’t make a good wine from bad grapes” comes in. What is really meant by this statement is that if you start with poor-quality fruit, you can’t expect to make a wine capable of aging for 30 years. If, however, you understand that your grapes are under-ripe or of poor-quality, you can adjust some of the winemaking parameters in order to make the best possible wine for that fruit. The true talent of a winemaker is seen in how well he or she handles bad grapes. So, for example, in under-ripe fruit you want a shorter cuvaison time in order to minimize the extraction on undesirable tannins. In higher-quality fruit, you may want to extract as much of the quality tannins as possible. With experience, you will begin to learn what works best for your particular vineyard site.


The grapes at the HRC are just hitting véraison. This is also known as the “onset of ripening” in grapes, and it’s a exciting point in the physiology of the grapevine. At this point in time, the vines start diverting all their energy from photosynthesis into the grape. It’s also exciting for winemakers, because we can start to see that the date of harvest isn’t too far off. Historically, at least for V. vinifera varieties, harvest is about 45 days after 50% of the grapes have changed color. Good vineyard management ensures that the grapes will accumulate sugar from photosynthesis and burn up some of the acids that were stored during the vegetative growth phase.

During this second period of growth, grapes become a sink for nutrients. Glucose and fructose are the main sugars present in grapes. In green (unripe) grapes, glucose represents nearly 85% of the sugars. The ratio being about 5:1 glucose to fructose. At véraison, this ratio decreases to 2:1, and at harvest the ratio is closer to 1:1.

The second most important phenomenon for winemakers is the fact that malic acid (malate) that has accumulated in the grape is now “burned up” through the vine’s respiration. Malate is biologically a very important compound in various biochemical pathways. At véraison, the vine increases respiration, and malic acid takes on the role of energy vector for the vine. At night, or at low temperatures, excess imported malic acid is transformed to glucose by gluconeogenesis. This is one of the reasons why regions with high diurnal temperature variation produce higher quality grapes. At harvest, tartaric to malic acid ratio should be around 2:1. This ratio varies according to grape variety and seasonal differences, however malic acid has been observed to be between 40 and 60% of the total acidity in most wine regions. Overripe grapes contain little malic acid (this is normally not a problem in northern climates).

Potassium is one of the few minerals that continues to accumulate in the grape berry after véraison. It plays an important role in the pH of the juice/wine, so this is why monitoring the pH as you approach harvest is important. While pH rises in part due to the decrease in total acidity, as potassium uptake continues you will see the pH increase at a more disproportional rate to your total acidity. In warm climates where fruit is typically overripe, high pH is usually a problem.

Other important compounds such as nitrogen uptake, and the evolution of grape phenols and aromas are also an important part of the maturation process from a winemaking standpoint. Nitrogen is consumed by yeast during fermentation. Juice should contain a minimum of 160 mg/L of yeast available nitrogen (YAN) for a proper fermentation (it can be supplemented with a maximum 300 mg/L Nitrogen if deficient).

As sugar and acid levels begin to look like they are reaching the right levels for harvest, it is important to begin evaluating the tannins and aromatic development of the grapes by tasting them. Chew on the skins and seeds to get an idea of the phenolic ripeness.