Remediating Off-Aromas from Frost Impacted Red Wines
Some wine growing regions can experience frost conditions during late harvest, depending on a few factors, like topography, season and the climate of the day. Low-lying blocks with poor air drainage are especially prone to frost and usually the first to have leaf damage. Picking before full maturity can lead to muted varietal character, and frost makes harvesting that block a priority.
When grape leaves are killed by frost, they make all sorts of interesting stress response metabolites that can pose a threat to wine quality [1,2,3,4]. Key odor-active and flavor-active compounds include alcohols (e.g. Ethyl heptanoate), terpenes (e.g. α-Terpeneol), norisoprenoids (e.g. β-Demascenone) and organic acid esters. The aromas are familiar if you’ve ever experienced potpourri, a mixture of dried leaves and flowers. Think dried roses; ‘herbal’, ‘floral’, and ‘woody’.
Material Other than Grape (MOG) is a common winemaking term for non-grape vineyard solids that get into a hopper or tank during harvesting. Leaves and stems make up a good portion of the MOG in a grape bin. Stems are less of an issue and are often included in winemaking to help with juice pressing in whites, or as an additional tannin source to boost structure in lighter red varietals.
Grape leaves and petioles, not only contain astringent tannins but also have large amounts of the aroma and flavor compounds previously mentioned, that are soluble in juice and wine and can seriously impact quality perception. Depending on the amount of leaf material you allow to be in contact the grapes, you can develop some unusual and distracting quality impacts [2,3,4].
Frost-killed leaves are the worst, and contribute distracting amounts of ‘herbaceous’, ‘dried rose’, ‘artificial fruit’ and ‘confectionary’ impacts to a wine, which sounds interesting, but their effects are disproportionate when you experience them yourself.
Recent research of the key compounds related to “frost taint” has shown that freeze-dried grape leaves can contribute similar aromas, tannins and flavors to experimental wines where the amount of grape leaf introduced was as low as 1/4 of a leaf for each pound of fruit (~ 1g/Kg). Which is about 250 leaves per ton of grapes (~ 1Kg/tonne). That might seem like a lot, but, depending on how it’s setup, a mechanical harvester can contribute as much as 5% MOG (as leaves) to the mix [1]. Worse still, mechanical harvesters shatter freeze-killed leaves into tiny fragments that start impacting the grapes immediately and are very difficult to sort out, even with advanced sorter technologies.
Flavor and aroma impacts from frost killed leaves in red wines might require excessive blending or additional cellar treatments to bring the wines back into varietal balance. This is where molecularly imprinted polymers (MIPs) can help.
Targeted filtration remediates elevated terpenes from frost impacted red wines
So, what’s a winemaker to do when they have potpourri aromas and flavors in their red wine? MIP technology from amaea has demonstrated some good results in remediating the overly-floral and herbaceous characters in red wines. In a recent trial with an industry partner, a Malbec with frost impact was run over an amaea RMx column and recovered the varietal character.
The winemaker was happy to have a blending element that could remain in the intended program for their brand.
First, a MIP-based bench trial was performed using different flow and dose rates to establish the range of remediation. Then, the winemaker chose a treatment based on their team’s sensory feedback, and a commercial-scale treatment was completed. The winemaker was happy to have a blending element that could remain in the intended program for their brand. The bonus, in this case, was that some of the green characters (pyrazines) were minimized as well, allowing more fruit to dominate the profile.
A study with a separate industry partner demonstrated that up to 80% of terpenes in Pinot noir can be selectively removed using amaea RMx at the highest dose/flow rate. While this was an experiment, and decreased some aromatics, it also gave us insights to the mode of action we see at play with frost impact. Tailoring the treatment dose and flow rate gives winemakers the ability to do remediation, based on sensory feedback, in the winery.
Remediation methods using MIP technology are easy to apply. They can be used to fine-tune or create new blending elements for your current programs, or for new product innovation. We continue to work on different applications that resolve issues for the industry. If you have frost impacted wines, or some other wine related impact, and want to do a trial, please contact us.
Author: Torey Arvik, VP of Applied Research
Interested in a tasting?
Come join us at this year’s WIN Expo to sample a frost impacted 2023 Paso Robles Malbec treated using our innovative molecularly imprinted polymers.
Tasting details
Event: WIN EXPO 2024
Date: 5 December 2024
Time: 2:00pm - 3:00pm
Location: Middle of the Hall of Flowers - Booth 616
Venue: Sonoma County Fairgrounds
1450 Bennett Valley Rd, Santa Rosa, CA 95404United States
References:
Freeze-Killed Leaf Material Causes Atypical Aromas and Astringency in Cabernet Sauvignon SC Frost, DJ Fox, M Keller, TS Collins, JF Harbertson Am J Enol Vitic. 2023 74:0740005; DOI: 10.5344/ajev.2022.22012
Effects of Frozen Materials Other Than Grapes on Red Wine Aroma Compounds. Impacts of Harvest Technologies. Yibin Lan, Jiaming Wang, Emily Aubie, Marnie Crombleholme, Andrew Reynolds Am J Enol Vitic. 2022 73:142-155; DOI: 10.5344/ajev.2022.22006
Effect of leaf content and mechanical harvest on wine “quality”. Noble AC, Ough CS and Kasimatis AN. 1975. Am J Enol Vitic 26:158-163
Cause and fate of certain C6 Compounds Formed Enzymatically in Macerated Grape Leaves During Harvest and Wine Fermentation. W. S. Joslin and C. S. Ough. Am. J. Enol. Vitic., Vol. 29, No. 1, 1978.