How does high temperatures cause higher alcohol levels?

It’s been repeated so often that its accepted as fact that hot years result in higher brix levels at harvest and therefore higher alcohol levels in finished wine. But thinking last night as I was walking my vineyard in this very hot year has me wondering if that is really true and if so why. I have two data points in the past 5 years that seem to contradict this belief at least to some degree.

In 2008, I encountered a problem with downy mildew that resulted in a significant defoliation of my Barbera vines. The grapes stopped accumulating sugar at about 17 brix but kept maturing resulting in grapes that were showing ripe tannins and no green flavors at all but at only 10% potential alcohol.

In 2010, I had a number of family issues that required me to be out of town during projected harvest dates. I had to make a decision to either harvest everything early or chance it and wait the 2-3 weeks until I could get back. Brix levels ranged from 18 to 22 so waiting seemed to present a risk for brix to reach 26 by harvest. The fruit had brown seeds and tasted fully ripe with no green flavors so pulled the trigger harvesting everything and co-fermenting it all together. The finish wine does not show any green flavors even though it was the Cabernet Sauvignon that came in at 18 brix. Thinking about 2010, I still had an afternoon shading issue that cut off direct sun at between 2:00 and 3:00 in the afternoon. I think this shading was responsible for keeping sugars low while the fruit showed mature flavor.

These two examples seem to me to indicate that sugar accumulation is primarily a function of photosynthesis and mostly dependent upon sunlight. My site is very hot (3700 degree days) with overnight lows often not dropping below 70. I don’t seem to have problems reaching maturity at lower brix with a harvest window generally running from late September for Barbera to mid-October for Cabernet Sauvignon and Aglianico.
Looking at my own grapes, I don’t see how rising temperature levels are responsible for increased brix at harvest. I am assuming that there has to be an efficiency contribution of heat to photosynthesis such that increased temperatures result in more efficient photosynthesis and higher sugar levels for the same time period. I am also assuming that my night-time lows being so high have are a contribution in accelerating grape maturity where areas that drop into the 50s overnight that shut down the vine for a few hours may mature more slowly even at same or higher daytime temperatures. I am now also wondering if I should not have taken down the trees that were providing the afternoon shade and if I will see elevated brix this year before the grapes reach maturity. If this turns out to be the case, not sure how I will determine if it was a due to the very hot year or the increase in direct sunlight.

Does anyone really understand the relationship at work or do I need to go back to the textbooks and see if this is covered in any detail?

It's very straightforward, Brian. There is one way to accumulate sugar, and that is photosynthesis, a process which shuts down pretty tightly around 100 degrees F (all biological systems have optimal temperatures for each of their processes). And there are two ways to concentrate sugars, one of which is the aforementioned photosynthesis, which adds to what is already there, and dessication, which removes water. With air movement in the canopies of healthy vines, one can limit dessication by judicious irrigation. However, rarely are things optimal. Therefore, hot days typically see dessication occurring. This often occurrs when the leaves suck out the water in the berries, as the leaves themselves become dessicated and lose turgidity. With defoliated vines, the only dessication occurring is typically a physical process through the skins of the grapes, which is much less rapid, but which results in real raisining.

That all makes sense Craig but I still think there is a second part to this (unless I am missing something) and that is what is driving phenolic ripeness. I see it generally discussed as an issue of hang time with limited regard to the climatic conditions. The answer to why alcohol levels have increased is generally longer hang time to reach phenolic ripeness. Yet from what I understand, the hang time needed can differ significantly from valley floor to mountain vineyard in Napa. I am sure similar examples exist elsewhere. I know that its never exactly the same number of days between verasion and 23 brix or to what I consider phenolic ripeness. Maybe the time to phenolic ripeness differs less than I think it does year to year. Your answer makes sense and explains the variables for the brix but what factors influence a faster or slower maturation of flavors?

my info is that vines shut down at 85 degrees F not 100 degrees.

factors influencing higher sugar levels seem to include:
length of season - not just seasonal accumulation. it takes time not just heat.
fruit to leaf area ratio as well as trellising and leaf removal - ie vineyard practices.
the market driven desire for earlier drinking wines which requires riper if not overripe grapes that are picked later.
and yes, dessication

there are probably others

That all makes sense Craig but I still think there is a second part to this (unless I am missing something) and that is what is driving phenolic ripeness. I see it generally discussed as an issue of hang time with limited regard to the climatic conditions. The answer to why alcohol levels have increased is generally longer hang time to reach phenolic ripeness. Yet from what I understand, the hang time needed can differ significantly from valley floor to mountain vineyard in Napa. I am sure similar examples exist elsewhere. I know that its never exactly the same number of days between verasion and 23 brix or to what I consider phenolic ripeness. Maybe the time to phenolic ripeness differs less than I think it does year to year. Your answer makes sense and explains the variables for the brix but what factors influence a faster or slower maturation of flavors?

Brian,
It has benn suggested that some folks hang fruit passed phenolic ripeness as they prefer "overripe" flavors.
Best, Jim

My understanding is that optimal photosynthesis occurs at 25-30 degrees C, on the average. 30 works out to 86 degrees F. Below those temperatures and working up towards those temperatures, there is increasing efficiency, and increasing temperature above 30 causes a decrease in efficiency, until the process stops altogether, often accompanied by other physiological factors. "Photosynthesis declines rapidly above 86 degrees F and falls to zero between 113 and 122 degrees F" (General Viticulture, Winkler et. al.)

Be that as it may, fruit, no matter what fruit, is simply a seed receptacle complete with nourishment source for maturation of the seed to viability. It takes time to do it, but as long as there is enough sugar in the fruit, physiological maturity can be reached.

There are two things that effect brix at harvest that were not mentioned. First is the amount of rainfall (or irrigation), which will dilute the grapes, thus lowering the brix. The second is the yield per acre. You could have fully ripe grapes at 17 Brix if you have 5 tons/acre and the same variety at 24 Brix at 2.5 tons per acre. However, with the higher brix, you will also have fewer, but more flavorful grapes.

The second is the yield per acre. You could have fully ripe grapes at 17 Brix if you have 5 tons/acre and the same variety at 24 Brix at 2.5 tons per acre. However, with the higher brix, you will also have fewer, but more flavorful grapes.

Howie, I think this depends on trellising and leaf to fruit ratio as David noted above. There have been some studies in Virginia lead by I believe Tony Wolf that showed it was possible to increase yield per acre using divided canopies without significant impact to resulting brix at harvest. I don't know where I have the paper or if one can search it out on the internet. I tried recently and could not find it but I only gave it a few minutes.

Howie, I think this depends on trellising and leaf to fruit ratio as David noted above...

True, but for the sake of argument, I was assuming all other things being equal. A friend, who owns a winery, posted pictures on his website a few weeks ago with bunches of grapes lying all over the ground after being cut off the vines to increase quality.

Gotcha. Agree.

Other things to consider when looking at the accumulation of sugar in grape berries besides photosynthetic capacity of the vine, is the rate of import (of sugar) into the berries, and the levels and activity of sucrose metabolizing enzymes.

Salute