Veraison: it’s easily the most identifiable moment in the vineyard cycle, when the grapes turn from green to red. Marked on the surface by obvious color transformation, the color swap belies a complex physiologic shift within grape metabolism. Veraison is the fundamental metabolic shift that culminates in harvest. The vines reposition from vegetative growth to berry maturation and its accompanying features. To better understand this fascinating process, let’s backup and talk a little about berry physiology. If you were to graph berry growth rate over time, you’d connect the dots to make a double humped camel. This is known as the “double sigmoidal curve” of berry growth rate. The first hump is phase 1 characterized by rapid cell division and berry growth. This is followed by the phase 2 dip-between-the-humps known as lag phase. This is slow growth, over variable time, lasting 1-6 weeks. This is followed by phase 3, final berry growth to harvest. You can ball park it at 5-8 weeks, but I’m not taking bets on harvest dates just yet. Veraison identifies the transition from phase 2 to 3, as berries begin to truly ripen. It also marks the end of rapid shoot growth. In fact, shoot growth is fastest after bud break (phase 1) and ceases after verasion. Persumably, once grapes hit veraison, all of the vine’s cumulative energy is directed towards berry ripening and away from vegetative growth.
After veraison, leaves become the dominant sugar factories, directing their goods to the berries via the phloem. The phloem pumps sucrose into the berry, which is cleaved to fructose and glucose inside. Actually, after veraison, the xylem largely involutes. This limits xylem-restricted minerals like calcium, maganese, and zinc whereas potassium, transported by phloem, continues to accumulate.
What we see with the naked eye is a helter skelter checker board of red and green grapes. But beneath the pericarp, there is an accumulation of sugar and phenolics, an increase in pH, and a reduction in acid. The berries begin to soften, expand, and increase in dry weight. But the biochemistry is trupmed by the onset of anthocyanin synthesis- the color swap.
Listed above are the five main anthoycyanins found in grapes. You can mix and match that “R3” moiety to recreate the 20 anthocyanins that have been identified to date. But pinot only contains the big 5, so we won’t bother with ancillary hydroxylation reactions. By the way, can you guess from where the anthocyanin names derive (delphinidin, peonidin, petunidin)? The onset and accumulation of anthocyanin pigments is influenced by temperature, sunlight (as controlled by God and/or judicious pruning), irrigation, sugar accumulation, and genetics. The exact biochemical spark that ignites anthocyanin production is unknown. One hypothesis is sugar accumulation, with sugar being a necessary substrate for anthocyanin synthesis. Other triggers may include abscisic acid, potassium accumulation or diminishing calcium (note that whole xylem/phloem bit back in paragraph 3). Although the go signal is unknown, we do know something about the color regulatory genes.
In ripening grapes, color change is regulated by the MYB gene. The VvMYBA1 gene comes in two flavors: VvMYBAa and VvMYBAc. The Aa version is a nonfuctional allele whereas VvMYBAc is dominant for color (hint- that little “c” stands for color). OK, now time transport back to 7th grade biology - not to geeky braces and acne but the part about Gregor Mendel and the pea plants. Remember when Mendel crossed the red pea with the white pea to make pink - which has one dominant red allele and one recessive white one? Like the pea plant, each grape skin cell has two copies of the gene. Stay with me, now. White skinned pinot gris has TWO copies of the non-functional allele VvMYBAa. Pinot noir, in contrast, is heterozygus, posessing one VvMYBAa (no color) and one VvMYBAc (dominant for pigment). Voila - color is born! (Jackson, Wine Science, 2008)**
If you’re a proponent of Intelligent Design, red wine comes from co-fermenting white grapes with red crayons. But moving on…
Here is a shot of veraison at Torrey Hill:
Our blackberry bushes and tomato plants are in various stages of veraison, too:
In summary, veraison is manifest by both exterior and interior changes in the ripening berry. Sugar and phenolics go up, and acid drops. Next step – harvest!
**Full disclosure. The most recent evidence suggests that pinot gris actually has three alleles at the VvMYBA1 locus- two white alleles and one red one. What gives? It turns out that when you sequence pinot gris’ red gene, it’s got a giant hole in the middle. Molecular biologists call this a gene deletion. Anyhow, since the pinot gris red gene is damaged and won’t express color, you end up with white wine. (Furiya et al, Molecular Characterization of a Bud Sport of Pinot gris Bearing White Berries, Am. J. Enol. Vitic. March 2009 60:66-73.)