One of the most common questions we get from customers is: "How do you know when is the best time to harvest your grapes?"
The short answer is either "experience," or "I'm not sure we ever really know that what we did was the best."
Neither is a particularly satisfying answer, so I thought I'd dive into the subject and try and offer some insights into how we go about making picking decisions.
The first thing to realize is that grape picking decisions are based upon the style of wine that a winemaker is trying to produce and the appropriate corresponding ripeness in the grape. What makes the decision hard is that there is a range of ripeness and style that are 'correct' and any point in that spectrum might be considered ideal for some given style. In general wine styles that are more plush and full will lean towards the higher end of the ripeness spectrum while a wine style that is more acid driven will skew towards the lower end of the ripeness spectrum. Both are "right" for their given styles. In many climates there isn't wide a choice about when to harvest due to season-ending rain or cold weather approach, but in most of California we have a pretty wide range that we can work with. As a result you can talk with a couple of winemakers and you will very quickly learn that one winemaker's 'ripe' is another's definition of 'unripe.'
Objective measurements of ripeness are sugar percentage, pH, and acidity. Sugar ripeness is measured in units called brix which roughly equals 1% sugar per volume of measurement. A value of 23 brix equals 23% sugar per volume of solution and an approximate potential alcohol of 13.8% alcohol in a finished wine. Red wines are harvested roughly between sugar ripeness ranges of 20 to 28 brix. White and rose wine grapes will be harvested at the lower end of that spectrum typically. We measure brix in the field with a hand held device called a refractometer or in the laboratory with a hydrometer. A typical pH range for the grape at harvest will be between 3.2 and 3.8 while total acidity (TA) will range from 4.5 to 8.5 gm/L of total acid in the grape must. We can measure pH in the field with a pH meter and can utilize a titration method in the laboratory to measure TA.
The subjective ripeness variables include visual inspection of the grapes and the canopy, seed color and maturity, tannin astringency, and 'taste.' Dark grapes can move from blue to a dark black as they ripen and whites will go from green to golden depending on the variety. Some varieties will lose volume and deflate a little bit as they approach ripeness while others will maintain their size. The canopy can be brown and brittle indicating that the plant has shut down for the season or in some cases the leaves can still be a healthy green indicating that photosynthesis and other elements of ripeness may still be occurring. After looking at the grapes we taste one and feel the texture of the skin, the pulp, and whether the seeds taste astringent and drying or if they are pleasant and crunchy. We look at the seeds to see if we still see some green or if they've turned fully brown. Outside of all these things the final element is simply 'taste.' This is the hardest thing to define. You basically have to taste grapes over a season, make the wine, age it, and then determine if you got the wine you wanted or if you need to pick at more or less maturity. After multiple vintages of the same fruit from the same vineyard you start to build a mental map of what 'taste' in a ripe grape equates with in a finished wine.
All these different variables are all contributing towards our picture of ideal ripeness, but they often move at different rates and are different from season to season. In one season 23 brix and 3.6pH may be what you pick at. The following season the grapes at 23 brix are only 3.25pH with green seeds and several more weeks of ripening before harvest. During that year you harvest at 26 brix and 3.5pH.
In both cases you always wonder "what if" and think about how you'll refine your picking decision the following year.