This year has thrown a lot of challenges towards us all, and the harvest season seems to be no exception.
Depending on the grapes’ growing location, there may be unexpected challenges winemakers are going to have to overcome in order to produce quality, commercially sound wine. Below summarizes the key observations I have seen to-date based on interactions with clients and colleagues.
Before reading only the region you live in, it’s important to consider if you are going to purchase grapes, juice, or concentrate from other regions. All raw materials (grapes, juice, concentrate) may be impacted by this year’s seasonal growing effects, which can ultimately find their way to your crush pad.
Northeast, Mid-Atlantic, and Southern Growing Regions
Many winemakers are reporting unusually high acids (titratable acidities, TA’s, at or over 10 g/L of tartaric acid), coupled with low pH and low Brix readings.
From a winemaking perspective, many winemakers will assume grapes are not ripe with low Brix and high acid numbers. Nonetheless, as disease pressure worsens (see below), picking decisions may be focused on predicted up-coming weather events instead of optimal ripening markers. Keep in mind that the floated ideal Brix concentration of 21°Brix may not always indicate optimal flavor ripeness for that grape variety.
Winemakers should remain calm in these situations and think through the wine grape variety’s projected production steps. Write them down if necessary. While this practice may seem unnecessary, it can help the winemaker clearly and logically make key pre-fermentation decisions that may get missed otherwise.
If the winemaker knows what style of wine they are trying to make, ask oneself if the fruit can realistically produce that wine style. If the answer is no, what are alternative outlets for that fruit? Consider thinking outside the box (e.g., using a red grape variety for a rosé or formula wine instead of a dry varietal red wine) and review options with those individuals that make business and sale decisions for the winery.
The starting sugar concentration can easily be manipulated with sugar or grape concentrate additions. Sugar adjustments usually result in higher potential alcohol concentrations than expected.
For table sugar adjustments, I typically rely on chaptalization calculators. VinoLab is one that I’ve used previously. Pay special attention to the dissolved solids other than sugar (DSOS) correction if you are using a hydrometer to measure Brix.
If using a grape concentrate to chaptalize juice, the winemaker can use a Pearson Square equation to determine the volume of concentrate required to increase the Brix of the juice. This equation was previously published in Butzke (2010), which published the equation from Rankine (1991):
Concentrate Volume (gets added to jucie) = [V(D – A)]/(C – D)
In which V = the volume of must/juice, D = desired Brix, A = initial or starting Brix, and C = Brix of the concentrate. There is a special note associated with this equation that 1°Brix = 1% sugar.
Avoid adding sugar to concentrations that reach high potential alcohol percentages. High alcohol wines can accentuate the high acid (high sourness) and immature tannin (astringency) perceptions typically associated with low Brix fruit.
High Acid (Low pH, High TA)
With acid, it’s, again, important to consider the desired wine style from that fruit. Dry red table wines, for example, rarely have TAs of 9 g/L. Even dry or semi-sweet dry white wines with final TAs of 11 – 12 g/L may be extreme for some wine styles and wine drinkers.
First, the winemaker should consider how the wine’s acid will be changed through the duration of production. In very high TA situations (TA equal to or >10 g/L), there are a number of combinations in which winemakers can address the acidity:
- Biological reduction with use of a high malic acid reducing yeast strain and malolactic fermentation.
- De-acidification of the juice and biological reduction (either through the use of a high malic acid reducing yeast strain, malolactic fermentation, or combination of both).
- Biological reduction (in any form as noted above) and de-acidification to the wine.
- De-acidification of both the juice and the wine and no biological reduction.
- De-acidification of both the juice and the wine, and some form of biological reduction.
It is helpful for winemakers to know the proportions of malic and tartaric acid individually in addition to the wine’s TA. This enological information can help a winemaker make production decisions that will be most relevant to the fruit. For example, if the wine has a high TA, but very low concentrations of malic acid, then using biological reduction of malic acid may not be “enough” to obtain the acid reduction desired by the winemaker.
Essentially, having the analytical information removes the guess work in winemaking and allows winemakers to make clear, logical, quality-driven decisions.
For high acid juice/musts or wines that require chemical de-acidification, consider adjustments prior to the start of fermentation. Options for de-acidification agents include:
- Potassium carbonate (0.6 g/L dosage rate will decrease the TA by about 1.0 g/L tartaric acid units and increase the pH of about 0.2 units)
- Potassium bicarbonate (0.9 g/L dosage rate will decrease the TA by about 1.0 g/L tartaric acid units)
- Calcium carbonate (0.67 g/L dosage rate will decrease the TA by about 1.0 g/L tartaric acid units)
Please reference “Deacidification by the AWRI” for more information on these dosage rates.
While the proprietary blend, Acidex, was traditionally used for the traditional double-salt technique, calcium carbonate can also be used for reducing tartaric and malic acid concentrations simultaneously in the same manner (Schneider and Troxell 2018; Steiner OSU).
The selection of which de-acidification agent to use will have an impact on wine aroma/flavor and mouthfeel. Juice/must de-acidification steps are usually recommended to preserve aromatics through fermentation. As Schneider and Troxell (2018) explain, reducing tartaric acid concentrations pre-fermentation also preserves potassium, leading to a fuller feeling wine.
Juice bench trials are strongly encouraged prior to treating an entire volume of wine. Make sure to evaluate changes on both the TA and pH. (For a review on why TA is important and how it differs from pH, please refer to “pH Explained.”)
Because de-acidification techniques are difficult to understand, I recommend becoming more comfortable with these additions and how they influence wine taste, aroma, and quality. Practice additions in bench trials prior to the start of harvest.
Another issue in these regions seems to be ongoing disease pressure. Sour rot and then the regular line up of grapevine diseases (i.e., Downy Mildew, Powdery Mildew, and Botrytis) seem to be challenges for the 2020 vintage.
For the regular disease line up (i.e., Downy Mildew, Powdery Mildew, and Botrytis), at about 20% disease coverage on the fruit, wine quality is impacted (Loinger et al. 1977) through off-flavor development or mouthfeel alterations. Botrytis presence, at or above 20%, will usually require the use of a glucanase enzyme on the wine prior to filtration to avoid filtration issues.
During crush, there are a few things the winemaker can do to reduce the influence of disease:
- Sort out diseased fruit.
- Do not use products that increase extraction (e.g., enzymes, rice hulls in the press).
- Shorten press cycles and keep press fractions separate.
- Whole cluster press over crush/destemming prior to pressing.
- Limit skin contact time for reds.
- Pre-fine the juice with PVPP (to minimize oxidative compounds).
- Use fermentation tannins which can help enhance wine quality in a number of ways.
- Have good fermentation nutrition strategies.
- Select yeast strains that will provide the most positive outcome for the fruit in front of you. Avoid using a yeast strain because you use it every year.
In the case of sour rot, this is one disease in which I, from a winemaking perspective, do not think longer hang time ends up benefiting the fruit. Sorting out sour rotted fruit is essential for maintaining wine quality. Testing the juice volatile acidity (VA) may be important to know if the VA is too high prior to the start of fermentation.
Midwest Growing Regions
Like the East and South, several of my Midwest clients have been discussing very short ripening periods leaving grapes with very high acids. Again, make sure to consider the fruit and wine style that is desired, as explained above.
In addition to following the content, above, for some varieties, yeast and malolactic bacteria choices may also be helpful. This is especially true for varieties with high malic acid concentrations. Look for high malic-reducing yeast strains in addition to inoculating for malolactic fermentation.
Midwest and West Growing Regions
At the time of this post, many Western states are dealing with the threat of smoke taint from surrounding wildfires.
Lingering smoke over vineyards carries aromatic volatile phenol compounds, including guaiacol, 4-methylguaiacol, o-cresol, p-cresol, and m-cresol (AWRI Dec. 2018). These aromatic compounds can potentially get absorbed by both the vine leaves and fruit at any stage of growth. The onset of veraison and post-veraison tend to be the highest risk time periods in the vine’s growing cycle to absorb these aromatic compounds (AWRI Dec. 2019).
As the vines and fruit absorb the volatile phenols, they bind to sugars forming non-volatile (non-gaseous) phenolic glycosides (AWRI June 2020), which are commonly referred to as volatile phenolic precursors. Non-odorous phenolic glycosides are most prevalent in juice and must of smoke tainted grapes (AWRI June 2020). Both phenolic glycosides and volatile phenols exist in wine produced from smoke tainted fruit (AWRI June 2020). Some of the phenolic glycosides are transformed and released as volatile phenols during fermentation, giving rise to smoke tainted wines.
Furthermore, the smoke can carry ash, which physically falls on the vines and berries. It is thought that the ash could potentially carry some of these aromatics as well. This indicates that exposure to the ash may also contribute to the smoke taint aromatics and flavors in a finished wine.
Again, anyone purchasing fruit, juice, or concentrate from regions under wildfire or smoke pressure should be aware of potential quality issues to the raw material.
The typical recommendation to assess smoke taint potential is to test fruit that has had smoke exposure by sending a berry or micro-ferment sample to an analytical lab that can test for concentrations of some of the volatile phenols. Usually, the winery would sample fruit about 2 weeks prior to harvest to assess the extent of smoke taint. Micro-ferments are used to evaluate juice treatment options before the commercial lot of fruit arrives at the cellar door.
However, as many wildfires surge through 2020, ETS Wine Labs (in the U.S.) has reported an unprecedented backlog of samples for volatile phenol assessment. Unfortunately, this leaves wineries guessing how to treat, ferment, and produce potentially smoke tainted fruit.
If you are a winemaker that has to ferment potentially smoke tainted fruit, there are options for reducing the impact of smoke taint or which both the AWRI and UC Davis have covered extensively.
It should be noted that treatments can help reduce the aroma/flavor impact, but no winemaking techniques have been found to completely eliminate the influence of smoke taint. Most suggestions revolve around limiting skin exposure during crush and fermentation. This can be challenging for red wines, which require skin contact for color extraction.
A full summary of processing suggestions can be found at the following AWRI Fact Sheet (Dec 2018), and additional detail can be found from Dr. Anita Oberholster’s recent FAQ release on smoke taint. Many of these options include:
- Avoid over-macerating fruit.
- Separate press fractions.
- Fine juice with carbon. (Winemakers are encouraged to check deactivated carbon recommendations from the supplier. Some carbons work better to remove phenolic glycosides in the juice/must while others are better at removing volatile phenols post-fermentation. A more complete summary of this suggestion is found, here.)
- Consider your enological product choices (e.g., yeast, yeast nutrients, exogenous tannins, and polysaccharides) after using strong fining agents like carbon.
- Consider use of oak chips and tannin additions during fermentation. Again, discuss options with your suppliers.
- Use of technologies like thermo-vinfication or reverse osmosis (RO) may help reduce the intensity of smoke tainted wines.
Again, the two links above detail all of these choices in addition to other options that may be reasonable for those winemakers dealing with potentially smoke tainted fruit.
Australian Wine Research Institute (AWRI). December 2018. Fact Sheet: Smoke taint – practical management options for grapegrowers and winemakers.
Australian Wine Research Institute (AWRI). December 2019. Fact Sheet: Smoke taint – entry into grapes and vineyard risk factors.
Australian Wine Research Institute (AWRI). June 2020. Fact sheet: Treating smoke-affected juice or wine with activated carbon.
Butzke, C. (2010) Winemaking Problems Solved. ISBN: 978-1-4398-3416-9
Loinger, C., S. Cohen. N. Dror, and M.J. Berlinger. (1977) Effect of grape cluster rot on wine quality. AJEV. 28(4): 196-199.
Rankine, B.C. (1991) Making Good Wine. ISBN: 978-1405036016
Schneider, V. and S. Troxell. (2018) Acidity Management in Musts & Wines. ISBN: 978-1-935879-18-3
Steiner, T. Acid Reduction Techniques in Must and Wine. OSU.