UGA Extension Viticulture Blog

Monitoring maturity / deciding when to pick.

MONITORING MATURITY: Monitoring grape maturity is important in order to determine when to pick. I am not going to go into the rather subjective details of what defines maturity for making quality wine. Most industry practitioners use a combination of chemical and sensory evaluations to determine when to pick (… as well as the physical integrity of the fruit – more on that, below). The chemical and sensory benchmarks that define maturity will vary based on variety and winemaking goals. For example, if I wanted to make a sparkling rosé out of my Merlot, I would aim for lower sugars and higher acids; the opposite would be true if I wanted to make a still red wine from Merlot. Soluble solids (Brix) is perhaps the most widely used chemical maturity index; this is perhaps due to its historical value and the relative low cost/ease of measurement. Barring special winemaking goals of sparkling wines and rosés, most will harvest somewhere between 21-25 °Brix, and the accompanying acidity and pH will greatly vary by variety and growing region.  Keep in mind that high pHs (i.e. > 3.9) can put the wine at higher microbial spoilage risk unless post-harvest amendments are made in the winery. Many will supplement objective chemical measurements to aid maturity evaluation by using: (1) the color of the seed, (2) the flavor/aroma of aromatic white grapes like Sauvignon blanc, Muscat, and/or Blanc du Bois, and/or (3) the phenolic maturity and “mouthfeel” of red grapes.

SAMPLING: It is important to monitor maturity separately for each variety. It is also important to sample grapes in an unbiased fashion by sampling from all parts of the cluster, and within each section of vineyard that represents a separate vinification. For example, samples from blocks A and B should be kept separate if they will be harvested and vinified separately – even if they are the same variety. However, care should be taken to sample across an entire block to best characterize the average maturity. This may best be done by creating a randomized sampling scheme for each block that will be separately harvested and vinified.

Most of the above was taken from the “grape chores: list found in the Small Fruit Consortium’s quarterly newsletter, which can be found in-full here:

http://www.smallfruits.org/Newsletter/Vol17-Issue3.pdf

 

CURRENT SITUATION: Many have harvested PD-tolerant hybrids in southern and western GA.  If they haven’t started already, many in northern GA, NC, and VA will begin harvesting early whites, and begin harvesting for sparkling production over the next couple weeks.  Muscadine harvest has begun for fresh market table grapes (I just ate some really good Supreme muscadine grapes earlier this week).  For processing muscadines, harvest will gear up for Carlos (the most widely-planted white-skinned juice/wine muscadine grape in GA) in a couple weeks, which will be followed by Noble (the most widely-planted black-skinned muscadine grape in GA) in the weeks thereafter.

Here is a list of what we have sampled from our bunch wine grape research plots throughout northern GA over the last couple weeks (Brix range in parentheses; large range is a function of different vineyards and/or treatments imposed):

Last week: Chardonnay (12.8-15.0), Sauvignon blanc (13.3-15.1), Traminette (10.8-14.1)

This week: Petit manseng (10.1-13.0), Merlot (15.0-17.0), Cabernet franc (13.3-15.4)

 

CASE STUDIES: I’ll share three recent “case studies” from industry practitioners (the first two have not been harvested yet, and the last one was harvested last week) that run the gamut of examples for making harvest decisions.  Not one is more “sound” or “right” than the other.  They are just reported here to give blog readers an example of some thought processes to produce wines without the most “balanced” natural fruit composition in a challenging climate.

1. Chardonnay: The grower feels the natural acidity is an important part of their stainless steel-Chardonnay production and knows that Chardonnay is very susceptible to bunch rots (see Chardonnay picture, below, from more than a month ago).  Thus, they will be harvesting in the next two weeks at relatively low (i.e. maybe 18-20) Brix levels.  They will chaptalize (add sugar) the must to bring alcohol potential up to (hopefully) produce a crisp, but balanced white wine.

2. Bordeaux reds: The grower feels that varietal maturity (mouthfeel, aroma, and color compounds) is not reached at their site until beyond a pH of 3.9-4.0.  Thus, they will let fruit hang until these high pHs and (likely) higher Brix levels are reached, and will supplement with exogenous acid applications to maintain acidity and bring pH down.  This will (hopefully) produce a bold red wine that is full of varietal character and less susceptible to microbial storage than if no exogenous acids were added.

3. Blanc du Bois: This crop was harvested about 10 days ago in a very meticulously-managed vineyard.  The Blanc du Bois grower estimates he lost 30% of his crop due to a rainfall event that happened three weeks ago.  Berry splitting and sour rot ensued (see picture, below).  You now how the saying about hindsight goes… but if rainfall events like this happen again and sour incidence and severity quickly increase, then it may be best to pick early.  From a very practical standpoint, the humid region in which we grow fruit often gives us unwelcomed help in deciding when to pick.  Blanc du Bois can have very good flavor at relatively low Brix levels, and low Brix can be fixed in the winery.  I am certainly not a winemaker.  Nor am I promoting one winemaking tool over the other.  Nor am I making the argument here that natural sugars are equal to cane sugar.  However, I felt this grower’s pain when he said he lost 30% of his crop.   I would thus urge folks to not always aim for the same chemistry they achieved last year (which was an unusually dry year here in GA), but rather pick based on what is happening in the present vintage.  If rots start setting in fast, it may be best to pick immediately to avoid drastic crop losses.  Quality is very important.  But quantity is too.

 

 

Okay – this very well may be my last blog post until harvest is over.  I may post another update or two if anything is urgent and/or give an update or two as we see things come into the winery throughout the region.

Either way – I sincerely wish everyone a great harvest.  I sure hope the weather dries up – it can’t get much more wet than it has been.

Best of luck,

Cain