Yo. Don’t ask why yo. Just yo. Yo—I gotta study for my WSET Diploma (D1) exam. One thing that really helped me pass my Level 3 exam was writing posts that simulated questions I could expect to see on the exam. So, that’s what the next (several) posts will be. Interspersed with wine notes (probably). My first exam is in one month. On my birthday no less. So…help me study? Thanks.

Let’s start with grape development…

The calyptra is shed and pollen is transferred from the anthers to the stigma fertilizing the flower.
The calyptra is shed and pollen is transferred from the anthers to the stigma fertilizing the flower.

I’m going to start with the ideal climatic/environmental conditions for flowering. FUN FACT: Flowering *typically* takes place 8 weeks after budburst. Note: Warmer conditions will lead to earlier flowering, whereas cooler conditions may lengthen the duration of the flowering process (the latter of which will affect the evenness of subsequent ripening). Vines are happiest to flower at around 17°C/63°F (so, like, a typically San Francisco afternoon.)

So, what is flowering anyway? Glad you asked. If you look at the above image, that is a depiction of an inflorescence—potential flowers that can become pollinated and, thus, become little fruits. Flowering is when those individual flowers open, the stamen becomes exposed, and the pollen at the tip of that stamen shed onto the stigma. If they’re able to germinate, the pollen creates a pollen tube that goes into the stigma, ovule, and ultimately the ovary where there is an egg. The sperm cells are able to slide down the tube and, well, you get the gist: Egg fertilized, grape forms. Fruit set is the term used to describe the transition from flower to grape.

Developing inflorescences
Developing inflorescences

 

SAD FACT: Not all flowers become grapes. According to my text book, only about 30% of flowers actually become grapes—but can actually range anywhere between 0-60%. The process of germination requires warmer temps (makes sense), somewhere in the range of 26-32°C/79-90°F. (So, basically like the weather we’ve been getting in Sonoma in May and June.)

Following fertilization, the fruit begins to form.
Following fertilization, the fruit begins to form.

Ok, so what does this mean for acid and tannin? When the grapes are young, they’re crunchy little green things. Have you ever picked a completely unripe berry and eaten it? Don’t. Well, if you’re studying for this test then, ok go ahead. Because if you’ve made that mistake (or choice) in the past, you’ll never forget it. The exterior is hard, bitter—there is palpable sugar or acid.

The ripening phase of grape berries begins just after fruit set. During this time (June through October here in the Northern Hemisphere), those hard grapes begin to grow and both tartaric and malic acid begin to accumulate within the pulp. Eat a grape at this stage, and it’ll still be overwhelmingly bitter because tannins are developing but have not softened. It’s also noted that aroma compounds and precursors begin to develop during this early stage as well, such as pyrazines. (Potential wine flavors…yum.)

Sunshine is super important at this phase because the process of photosynthesis helps create the sugars that will accumulate inside the grape berries. (You know, so your wine will actually taste good.) The grape is also taking in a lot of water from the root system as well. While excessive vine stress is not idyllic at this time, nor is excessive water (or nitrogen), as this will encourage the growth of green material and stunt the ripening phase of the grape (especially bad if you live in an area with cold, rainy autumn/winter harvest seasons).

In a perfect world, the vine experiences “mild” water stress. This can speed up the ripening process, produce smaller grapes, which results in a higher skin to pulp ratio which, if we’re talking red wine, may mean better quality — greater color, tannins and aroma compounds.

A cluster of grapes undergoing véraison.
A cluster of grapes undergoing véraison.

Anyone who follows any dual winery and vineyard is no stranger to the above picture. Véraison is when grapes begin to change their color. It’s most noticeable with the black varieties, as they change from green to reddish/purplish (depending on variety). White grapes will become more translucent and, I guess kind of yellow-y (again depending on grape variety). The 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.”

Ripening—which is the next phase, and not part of this post—is when the biggest changes in sugar and acid occur. We’ll save that for another time.


So how did I do? Did I answer the question ok? Forget anything? Let me know your thoughts. And thanks for studying with me!


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