DipWSET Theory: Grape Ripening

[Information based on DipWSET D1 material]

Before getting started, make sure to check out my post on Flowering and Fruit Set.

A Brief Overview:

Ripening is the third stage of Grape Development. Starting from the top…

During grape berry formation—the first stage of Grape Development— malic and tartaric acids begin to accumulate, aroma compounds and precursors begin to form (also check out Aromatic Compounds post), tannins start to accumulate. But the grapes themselves are green and quite bitter—very little sugar has accumulated at this point.

Another interesting anecdote is that water flow is quite high during grape berry formation, but too much water and nitrogen (please also see Stuck Fermentation to learn about soil nutrients) can prolong this phase, as it will encourage green growth, not grape growth. So “mild” water stress is the way to go here.

Veraison is considered second stage of Grape Development. This is when grapes begin to change their color, skins become more supple, stretchy. But regarding the grape growth, it actually slows down. This phase is what many viticulturists refer to as a “lag phase.”

Now we can enter the third stage—ripening.

Ripening: This is when the fun stuff happens. Hopefully canopy growth has slowed and the vine is focusing its energy on the fruit. Grapes begin to plump, sugar and water levels rise, acids drop; tannins, aroma precursors and compounds all begin to develop (such as terpenes, a precursor) or fall (in the case of mehoxypyrazines, a compound).

Harvest marks the end of the ripening stage—typically around August through October in the Northern Hemisphere; February through April in the Southern Hemisphere. But there are several factors that influence the length of the ripening stage and, thus, when harvest actually occurs.

1. Grape Variety: Some grape varieties innately ripen earlier than others. For example, Chardonnay and Pinot Noir are typically early ripeners; Cabernet Sauvignon and Grenache are pretty late to the game. This is due to the range of different aroma compounds and precursors found in different grape varieties—the synthesis, degradation, or retention of these elements all respond to the amount of heat and light available to the grape in a different way.

It’s noted that it is indeed difficult to make a direct correlation between ripening conditions and aroma compounds, but through tasting, we know that some aromas are more commonly associated with warmer or cooler climates. For example, Chardonnay from a cooler climate will more typically display aromas of citrus and green fruits; warmer climate Chardonnay will move toward stone and tropical fruits.

In black varieties, the level of tannins polymerize throughout the grape ripening phase, causing them to become less bitter—grapes exposed to a greater amount of sunlight and warmth will experience a greater amount of this polymerization; tannins, consequently will be less astringent.

Similarly, anthocyanins increase during grape ripening—responsible for black varieties’ color pigments (red, purple, blue or black depending on variety).

2. Climactic Conditions: Sugar accumulation is quicker in warm, dry conditions; slower in cooler conditions. In general, sugar accumulation is most rapid at the beginning of the ripening stage, slowing toward harvest. To get a little science geeky: sugar is produced in the vines’ leaves via photosynthesis, which can take place at a maximum rate at temps between 64 and 91°F and at sunlight levels that are above one third full sunshine. That being said, too hot, and the vine will become water stressed and cease photosynthesis, impeding sugar accumulation. Similarly, excessively cold weather and/or constant cloud coverage will decrease the rate of photosynthesis and, again, cause a lag in sugar accumulation.

Climactic conditions also affect acid concentration in the grapes. Interesting Factoid: The total amount of tartaric acid in the grape generally does not change. However, because of the water and sugar accumulation, the concentration is affected. Interesting Factoid 2: Malic acid concentration is affected even further because malic acid is used in respiration during the ripening stage (as opposed to sugar, which is used previously).  It is because respiration is slower at cooler temperatures that wines from cooler climates tend to have a naturally higher total acidity. In warm climates, the rate of respiration is higher, utilizing more acid and accumulating more sugars faster/earlier in the season.

A Caveat: Diurnal Range—referring to the difference in night time and day time temperatures—can create an exception to the above rule. Cool night time temperatures will slow down the rate of respiration, meaning less of that malic acid is lost throughout the overall ripening stage. This is how seemingly hot climate areas (I’m thinking about Paso Robles, but other examples include Central Otago in N.Z. and certain areas of Washington State) can produce wines with a higher acidity level.

3. Management of the Vine and Vineyard: i.e. canopy management techniques

• Heavy Crop Load—lots of grape bunches means less concentration of sugars and flavor compounds and precursors; growers may want to consider a green harvest (pruning fruit bunches before veraison) to allow fruit left on the vine to become more concentrated
• Excessive Shading—leafy canopies mean grapes are not receiving the light and heat o’ the sun, thus impeding photosynthesis and sugar accumulation; growers will want to make sure they prune shoots and leaves in a way that grapes receive the amount of sunlight needed to fully ripen

4. Time of Harvest: this depends on both human and natural factors

• Human Factors:
  • wine style—picking early with less sugars and higher acids will result in a much different wine than waiting for higher Brix, lower acids, and higher pH
  • logistics—were you able to get your crew out there on time? did you have your equipment ready in time? was the crew/machine able to harvest fast enough, at the exact desired pick time, etc.
• Natural Factors:
  • weather—did you have to rush pick-date due to oncoming rain? did an excessive heat spike raisin some of your fruit before harvest?
  • onset of disease—this could also be a factor determined by the weather, with humid conditions bringing on mildew or hot dusty conditions bringing in pests; it could be a factor of that sexy Botrytis turning into the unsexy Grey Rot.Extra Ripening
• Extra Ripening

If grapes are left on the vine, they will eventually stop accumulating water and sugars and, thus, begin to shrivel, or raisin. BUT it will continue to transpire (aka lose water through the grape), thus the sugars that are in the grape will become concentrated. Some growers decide to do this on purpose to create a specific style, the success of which will be dependent on the weather as well as disease pressure.

How’d I do? Did I forget anything? Anything you want to add?

Thanks again for studying with me!

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