I firmly believe there is single most important job for a winemaker, deciding when to pick the fruit. The best a wine will ever be is the moment you pick it. All you can hope to accomplish is to lessen how much it degrades. The techniques you read about in the produciton notes of your favorite wine cannot improve the wine, they represent only our best efforts to fully capture and express what is within the fruit. Since determening the pick is such an important task, I'm surprised how little attention it gets.
The how and why of picking dates is different for each winemaker. The separate routes taken to this end represent most of what is the underlying character of the wines produced by different winemakers. The routes are plentiful due to the mass of information available and the varied interpretations of that information.
You can break up most of the information into two rough categories, vineyard observation and lab data. The safe thing for the modern winemaker to say is that both forms of information are equals. But from what I see, most people lean towards one or the other. On one end you have the winemaker who reads only spreadsheet charts and can track the exact optimal moment of balanced sugar, acid, polysaccharides, phenolics, anthocyanins, etc. If you have 5,000 acres of fruit you are responsible for, it is pretty impossible to go "walk the vineyard" and decide how it's tasting. On the other extreme you have the winemaker who eats the fruit in the vineyard and looks at the vine for changes in shoots, leaves and stems. This really works best when you have a single vine in your backyard. So where you ask do I find myself in this continuum?
Being an organic chemist by training and trade, one would expect me to be most smitten with the data avalanche of the well-stocked winery lab. At first, that made most sense to me. But the more harvests I undertake, the more and more the vineyard and tasting the fruit have started to dominate my decision-making. The main reason I have made this change is a newfound belief that this decision is what we humans are designed to do.
When I was working in biotech as a multi-step organic bench chemist, most of the PhD's working there were Soviet trained. With this Soviet training came some "old school" techniques. The PhD I worked most closely with often made decisions about the success or failure of a reaction base on smell. The sane thing to do is to run a sample in the GCMS. But this takes about 20 minutes start to finish in a busy lab, and smelling the reaction is instant to the trained chemist nose. In fact, the history of organic chemistry pre-dates the analytical tools of the lab because early chemists could look at, listen to, feel, smell and taste their reactions. These senses of ours amazingly powerful, they easily rival large instruments costing hundreds of thousands of dollars. And they are never more powerful then when being used for their actual purpose.
The tasting of ripening fruit is an example of one of those actual purposes. Think about your sense of taste for second. Why should you want to taste things, what's the point? Well, first and foremost it immediately tells you if something is good or bad to eat or drink. If you could not taste salt, how could you tell if water was fresh and drinkable, or like the vast majority of water available to you, salty and dangerous. Without sour, bitter, sweet and Umami (wait, what? I think I missed a memo.) how could you tell if fruit is going to be a nutritious snack or a belly ache waiting to happen? This to me is the whole point of our ability to taste, so we know when fruit is good to eat. Lucky me, to have such an amazing tool to help me with my job.
The whole process is incredible to me. The vine wants to spread its seeds around. It decides to surround its seeds in a tasty treat so that animals will eat them and the seed will travel with them. The danger for the plant is the animal wanting to eat the fruit before the seed is ready. So while the seed matures, the fruit needs to taste unpleasant to the animal. Thus the young grape is extremely sour and bitter with very little sweetness. Once the seed is ready though, the fruit quickly loses bitterness and acidic flavors and becomes more and more sweet. At a certain point there is a perfect balance that may not be the exact best moment to eat the grapes, but is the exact best moment to make perfectly balanced wine.
Standing in the vineyard, eating grapes in the morning, the tuned winemaker sense machine can accurately determine brix and pH along with skin tannin and color and many other chemicals. It is a fun test to make a best guess as to the pH and Brix when tasting fresh must. The winemakers I learned from would have no problem producing a number that was within 1 Brix of a hydrometer and 0.2 pH of a probe.
There are even some things that only tasting the fruit can give you. Like tasting the difference between crisp and mushy watermelon, the consistency of the pulp is one of those vital things that I have never seen determined in a lab. (Note to self, build a little spring tension device that produces a "firmness" number, sell to wine labs, retire.) Things like cane and stem lignifications and leaf color change can only be determined by actually visiting the vine. I cannot see how it is possible to be absolutely confident you are picking the fruit at an optimal moment without spending time in the vineyard using your senses. Because really, that is what they are there for.