The decision of when to pick a certain parcel of wine grapes carries much weight. Picking too early or picking too late both bring consequences to the resulting wine. Each decision can be detrimental to the finished wine in regard to its aroma, range of flavour, structure and overall enjoyment. It is not an easy skill to be able taste a grape in the vineyard straight off the vines and tell at what stage of physiological ripeness and where its ripening cycles will go from here, as a number of variables need be taken into account.
A wine grape is not physiological ripe for example if it is simply sweet and not acidic. In fact, some acidity is beneficial to a well-balanced wine. The goal is to find the right balance between sweetness and the varietals natural acidity.

 

Grape sampling takes place frequently - in the last few weeks of the season, to determine the current sugars, pH, and acidity of the grapes, as close monitoring of these changes is necessary in the days before harvest. Then for example a few too many days growing on the vine and too much sun exposure can make the numbers change all too quickly.
Also while sugars are accumulating inside grapes during the growing season, tannins and phenolic compounds are also ripening. This type of ripeness relates to the grape skins, seeds and even the stalks. The ultimate and ideal timing of readiness for when to pick is when all of these variables happen simultaneously. For this to occur, vines need just the right amount of sunlight, water, available soil nutrients and appropriate vineyard and canopy management.
Winemakers need to make a careful decision when to pick if they want their wines to be balanced. If they pick too soon, the acid levels will be too high, the sugars too low and the tannins too aggressive. If they pick too late, the sugar levels will be too high, the acids too low and the tannins will have evolved to a point where they will not provide the wine the required structure. In theory this all sounds simple, but the devil is in the detail.
The numerous chemical changes occurring during the ripening process take place in a complicated sequence, and are highly responsive to local conditions. In wine regions, where summers had just enough heat to ripen the varietals planted, the higher the sugar levels the better, because once the grapes have reached about 12 degrees potential alcohol, they would generally be ripe in all the other ways, (give or take small variations). Though because of the constant threat of early autumn rains, it would not pay to leave wine grapes on the vine much beyond this point: as there was a risk of severe quality loss if it started rain heavily while the grapes remain unpicked.
As vines began to be planted in new, warmer wine regions, where an average summer has more than enough capacity to ripen the varietals, the rules slightly changed. Picking by sugar levels resulted in harvesting grapes that made wine with distinctive green characteristics and unresolved tannins: the grapes were not fully ripe. This new concept became dubbed ‘physiological’ or ‘phenolic’ ripeness. In these warmer climates the process of sugar ripeness (with rising sugars and lowering acids) seemed to have become uncoupled from the process of physiological ripeness. So growers began leaving the grapes on the vines longer, leading to a concept dubbed as ‘hang time’. Today, several ‘new world’ winemakers often allow their fruit to hang a little longer to taste of riper fruit, meaning grape sugars will be much higher and with less natural acidity left in the grapes - which can result in an earlier drinking wine, but one that can not age as long.
This extra hang time was not such a risk in warmer climates, as rain is not a serious threat. This opened up possibilities to winemakers: picking became much more of a stylistic choice than a necessity. Winemakers found that the longer they left the grapes on the vine, the more some consumers and critics enjoyed the resulting wines. They became richer, with a sweeter fruit profile and softer tannins. They also became more concentrated with more texture and darker colour. The alcohol levels became higher, mainly because of increased sugar levels but also because the grapes started dehydrating on the vine as they were left to hang.
With vineyard improvements - we now have vertical canopies with good bud spacing, improved clones and rootstocks, good soil nutrition and water management - all leading to the highly desirable result of more rapid and complete ripening with more flavour and colour along with better acidity. For most vineyards and varietals, there is an ideal picking window (which does slide by a few days each growing season) when the grapes fall within acceptable parameters for ripeness. Where one chooses to pick is then a stylistic decision.
Pick too late, and the tannins will have already undergone lots of modification on the vine. The wine may taste soft and sweet, but there will be very little opportunity for the tannins to develop. It could well be that as well as making more complicated viticultural interventions to bring sugar levels down, a little compromise with early drinkability could result in wines that have more potential for ageing, and which are more interesting expressions.