This season has been like no other harvest in my memory, 45 of them. We had four of the longest and hottest heatwave ever, one a record setter at 109° and sustained over 100° for five days. When the temperature reaches 90° the vines shut down. The stomata on the undersides of the leaves close to prevent the vine from wilting. We had more days over 90° this year than ever. We started the harvest three weeks earlier than historic normal but with the sundowns the vines are now back to historic normal for harvest. The chemistry of the fruit however has changed. You can’t judge this year by past experience. The shutdowns changed the metabolism of the vines. What we are seeing as a result is ripeness at lower sugar, higher acids and lower pH.
The question is, what is ripeness? When grapes are ripe the seeds turn brown (mature), the skins get soft, the pulp separates from the seeds and the stems holding the bunches lignifies, which means the get harder and turn brownish and break easier. We picked a vineyard yesterday that is ripe at 24 Brix, low pH and high T.A. where we didn’t use any knives. The bunches where just pulled off.
As grapes get over ripe the pH goes up and acid goes down. Is this a good thing? Not in my opinion. Do you want a peach that is over ripe, or a strawberry? No. The texture and flavor in each start to change, becoming less varietal, less balanced, less juicy, and less interesting. You need to evaluate grapes using your senses along with the chemistry. If you do it regularly enough you don’t need the chemistry at all. We all know how to pick a ripe tomato or pear or strawberry. We don’t need a lab to tell us how. Wine grapes can fool you because of their high acid. High acid makes them taste less ripe. We therefore like to measure the sugar acid and pH because these are easy to check and give us a baseline. But it is only a baseline. It is not gospel.
Every vintage is different. This one is very different. The high acids are dominated by Malic acid. How this will play out in the wine is unclear. Malic is converted to Lactic. This could mean the wines will be more creamy, not more acidic. If the acid goes down with longer hang time I think you’ll loose Tartaric acid first. Then once the Malic has converted the acid that makes grapes taste like grapes - Tartaric, might be too low, robbing varietal characteristics from the wine.
pH is buffered. See: https://www.google.com/search?q=What+does+it+mean+when+pH+is+bufffered&oq=What+does+it+mean+when+pH+is+bufffered&aqs=chrome..69i57.10975j0j4&sourceid=chrome&ie=UTF-8
For the buffered pH to change there has to be very large changes in the chemical balance of the grapes. I do not believe that pH should ever be used to judge when to pick grapes. We shouldn’t even be testing for it from a viticultural point of view. You simply cannot tell ripeness by pH. However, you can tell stability by pH. This is really why we test for it. It is to inform the winemaker of the probably stability of the must and wine. We know we have to be more involved with higher pH juice and wine, than with lower. The lower the pH the more stabile the wine will be. Stable wine is a good thing.
Cecilia Valdivia, our production winemaker, says your last three field samples had the same sugar. While a heatwave will temporarily raise sugar in the grape, we have had a variety of weather during this sampling period. Cool weeks interspersed with hot weeks. Sugar would not normally be static during such periods. Another indication of ripeness is when the grapes stand still.
Lastly, vigor of the canopy will tell you a lot about conditions of ripening grapes. If the leaves are shutting down there will be no factories to make carbohydrates for the grapes. In fact the vine will start to abort the grapes.
I trust my senses more than the instruments.
- Robert Rex, 9/30/2017