In this blog we discuss why adapting harvest date to a warming environment is challenging …and why you may not have to trade off sugar concentration to get high color concentration.
Higher sugar concentrations at harvest are often associated with an increase in temperature. This is observed empirically and scientifically in the last decades throughout many wine regions. But, if climate is warming, why not simply harvesting systematically earlier? That way, fruit would be exposed to higher temperatures for a shorter period of time and maybe the amount of sugar would not rise as much?
Why harvesting with a high sugar concentration?
Australian researchers have demonstrated that, even small increases of berry temperatures uncouple the color content and sugar content with red varietals. It also affects other berry sensory traits, like aromas. To compensate for the delay in color accumulation versus sugar accumulation, winemakers have 2 options:
- Harvest according to sugar level. It means fruit color will be less than what it could be. Flavor will probably be on the vegetative side. Color incorporation into tannins will also be less.
- Harvest according to color accumulation. It means harvest decision will track color variations and not so much focus on sugars.
Isn’t sugar accumulating continuously?
The simple answer is “No”. We must dissipate this false belief.
Under arid climate (meaning no rain near harvest) it is not correct to think that: “the more you delay harvest, the more sugar accumulates”. Instead, the correct sentence is: “the more you delay harvest, the more sugar is susceptible to concentrate due to water loss”. Discussing the difference between berry sugar accumulation and concentration is beyond the scope of this blog. However, it has already been discussed here.
If you observe more elevated temperatures during ripening, then, you know that color concentration will reach a peak according to a time profile which does not track sugar accumulation. Technically, winemakers will say that colors and sugars are no longer synchronized. It means sugar accumulation is not “coupled” with color accumulation…and this should become the “fruit ripening norm” if warm vineyards are getting warmer. Thus, if you ask a “dry climate” winemaker or if you are one, it will not be a secret. Many good wines can be made from grapes reaching more than 28 brix (i.e. potential alcohol higher than 17%)…However, that does not mean that sugar accumulated continuously until harvest disrupted the process. How do we know that?
There is a limit to the maximal amount of sugar per berry
If you are a dry climate winemaker, and if you select option 2 for harvest, here is what often happens next. Postponing harvest leads to high fruit sugar concentration. This results in wines with high alcohol content after fermentation, which can be hard to sell.
But here is the good news: if you understand the processes driving sugar accumulation, you are not doomed to harvest fruit at 28 Brix or higher to get more color and more complete ripening. Research has shed light on chemical changes observed in ripening grape berries and you can use it to your advantage. If you understand what drives berry susceptibility to concentrate sugar, then – with the right practices- you can keep a low brix with a higher color content. All it takes is understanding microclimate and vine transpiration effects on berry shriveling susceptibility. Time profile of sugar concentration is the result of three phenomenons occurring simultaneously at the berry level:
- Sugar import,
- Sugar metabolism,
- Changes in water quantity.
From this complex interplay, the ripening growth period is divided into 2 phases :
- Phase A: Sugars are unloading from phloem until berry sugars concentrations reach values near 1.1 M (source). During that phase sugar import is 5 to 10 times more important than sugar metabolism or water budget variations (source).
- Phase B: Sugars susceptibility to concentrate by berry shriveling is due to evapotranspiration.
How is that useful to reduce brix near harvest?
Assuming you will not harvest during phase A (color is too low, flavors are not ripe), then you should focus on what causes sugar concentrations to increase during phase B if you want to harvest at a lower brix. During phase B, sugar import has stopped. Your attention is thus solely on berry sugar metabolism and water budget. And here again the researchers have found that water shortage becomes the main factor responsible for increasing sugar concentration during phase B. Water shortage lowers berry water quantity and sugar concentration increases as a result.
In simple terms: water shortage is the culprit driving your brix to higher values during phase B. In practice you should minimize anything causing a water shortage during phase B if you want to keep your brix low.
The last question is: what causes a berry water shortage during phase B? At this point you are left with 3 main drivers: temperature and humidity within the cluster zone and the amount of water the vine can provide to the berry until harvest day.
Ultimately, controlling those 3 drivers until the day of harvest is key to reach a full maturity while keeping the brix lower, even under warmer conditions.
If you are a dry climate winemaker, make wine in the vineyard :
- Exert a tighter control over evapotranspiration conditions during phase B. By monitoring and acting on microclimate variations and vine water flow to the berry, you can maintain a low brix until harvest.
- Use alternatives instead of reducing wine alcohol content in the cellar. Manipulating microclimate and irrigating pre harvest is an essential part of winemaking operations. If climate keeps warming up in dry areas, this will be soon your best option. It saves on cost and time in cellar operations. But, more importantly, it will be your only way to harvest a mature grape which is not also a sugar bomb.
Winemakers often postpone harvest weeks after maximal sugar content per berry has been reached, particularly under semi arid climates. One of the rewards is to get more color extraction from the berries. But why would that be important?….
in a future blog, we will discuss why “Color is not only useful for color” … and why delaying harvest in hope to gain more color also affects other wine sensorial properties…stay tuned!