Choosing when to harvest a vineyard is easily the most stressful part of wine making.
If you pick too early the wine can be thin, too acidic, and overly herbal. Choose to pick too late and the fruit can 'jump the shark" and go from dark fruit flavors to an overly jammy and alcoholic wine with a heavy burn. Somewhere in the middle of those two extremes is the 'sweet spot' for ripeness and what we aim for.
How do we make this decision?
A grape accumulates sugar during veraison and those levels increase to a maximum at around 25 Brix (% sugar in the grape). After 25 Brix the sugar doesn't accumulate further in the berry, but the berry can lose water and further concentrate sugar by decreasing the overall volume of liquid. Ripeness for a red wine is generally thought to be around 23 Brix while lower values of 20-22 Brix for whites and rosé wines. Brix gets most of the attention as a component of ripeness that is measured as harvest approaches, but it isn't the whole story.
Acidity is an equally important aspect of ripeness that is critical to producing a balanced wine. Acidity decreases as the grape matures during veraison. Grapes contain malic, citric, and tartaric acid. Malic acid (it tastes like green apples) is metabolized and the pH of the grape increases as the season goes on. Hot temperatures can accelerate this metabolism and quickly decrease acidity. Hot California vineyards will often lose acidity and require fine tuning of the acidity through the addition of powdered acid to the must or finished wine. Cooler sites, like those with an ocean influence or higher altitude, will tend to hold more natural acidity over the course of ripening. An ideal pH range at harvest for red grapes destined for red wine is generally considered 3.4 to 3.6; with lower than 3.4 being a common target for white and rose wines.
Another component of acidity is also important. This is Total Acidity by volume or titratable acidity (TA). This measure of acidity is related to pH, but not directly correlated. This is a very confusing concept, but all you really need to know is that acidity falls over the course of ripening and that hot weather causes it to fall faster.
These three quantitative measurements comprise most of what we use to assess objective ripeness of the grape, but this doesn't tell the whole story.
Subjective assessments are equally important in determine ripeness. We use our vision to look at the grape stem for lignification (turning brown and woody), but this is a bit unreliable in Southern California. While grape stems may turn brown during ripeness in some cooler climates, we often harvest with bright green stems. The color of the grape is another important indicator or ripeness, but the final color depends upon the variety. Red grapes will proceed during veraison from green to pink, to blue, and then during peak ripeness to a very dark, almost black color. Again this varies by variety, but you can detect the right color after working with a variety and site year after year.
We use our sense of taste to roughly assess sugar in the grapes and flavor profiles that we believe have led to a particular type of wine. This is tricky because memory is imperfect and often we rate sweeter grapes better just due to this one element of ripeness. We will chew the grape seeds to assess astringency and tannin structure. Ripe grapes have fully brown seeds that are crunchy when chewed and have a neutral flavor that is neither pleasant nor unpleasant.
Finally, we use touch to assess the firmness of the grapes. Grapes soften as they become ripe. On the less ripe side of the ripeness spectrum the grapes are still very firm, while varieties like Syrah may actually deflate some at ripeness. Part of this tactile assessment is also observing how easily the grape detaches from the cluster as we pull it and also how easily the flesh detaches from the seed.
Now that we have all of these different aspects how do we bring them together to make a reasonably informed decision?
It first starts with our intended wine style. Higher natural acids and lower alcohols are found in earlier harvested grapes. Wines made from grapes with this profile will be fruitier in flavor with great acidity and freshness. Lower natural acids and higher alcohol levels are found in later harvested grapes. This profile can lend itself to more robust mouthfeel and a whole other set of flavors.
The vineyard sites we work with have shown us that red grapes in our area develop fully ripe flavor profiles starting around 24.5 Brix. Below this level the color and body generally aren't developed enough for the style of wine we aim to make. 26-27 Brix produce really exciting flavors, but unblended wines at this level can sometimes be too hot so we look at 26 Brix as the upper end of what we want. pH ranges from 3.4 to 3.65 at harvest with corresponding TA ranges of 0.5 to 0.72 balance out the profile that we find is ideal for our area.