By: Aude Watrelot, PhD – Department of Food Science & Human Nutrition, Iowa State University
Temperature plays a critical role in winemaking, influencing everything from fruit cleanliness to fermentation smoothness and wine stability. Proper temperature management helps reduce microbial spoilage risks, extract phenolic compounds, and preserve aroma and color, all of which contribute to the overall quality of the wine. Cold temperatures can be strategically used in several pre-fermentation techniques, such as cold soaking (or cold maceration), cryomaceration, and freezing grapes, to optimize these outcomes.
Cold Maceration: Enhancing Color and Flavor
Cold soaking, particularly common for red grapes, involves storing harvested grapes in a tank or bin, where they remain in contact with the must at low temperatures (typically between 10°C and 15°C) for a few days (3-5 days). This process reduces the risk of oxidation and microbial spoilage while preventing the onset of alcoholic fermentation. During this period, sulfur dioxide is often added to further minimize microbial activity.
The primary goal of cold maceration is to extract phenolic compounds—such as anthocyanins and tannins—from the grape skins, seeds, and flesh. These compounds significantly impact the color, flavor, and overall quality of the wine. Anthocyanins, the red pigments in grape skins (and sometimes flesh, in “teinturier” varieties), are water-soluble and easily extracted into the must. Condensed tannins, which are chains of flavanols found in grape skins and seeds, play a key role in wine structure. They not only help create stable color pigments by interacting with anthocyanins, but they also contribute to astringency, serve as antioxidants, and improve the wine’s aging potential.
Although cold soaking is widely used, the impact on the final wine quality can vary depending on grape variety, soaking duration, and temperature. The degree of tannin and anthocyanin extraction differs based on these factors, so it’s important to tailor the technique to the specific needs of the grape variety.
Cryomaceration: Freezing Grapes to Maximize Phenolic Extraction
Cryomaceration takes cold soaking a step further by freezing grapes at temperatures between 0°C and 10°C. This freezing process disrupts grape cells, breaking open their vacuoles and facilitating the release of phenolic compounds. Freezing below 0°C, as seen in ice wine production, enhances this effect by further breaking down cellular structures and making the extraction more efficient.
While ice wine production typically focuses on white grapes, the freezing technique can also be applied to red grapes. After harvesting, grapes can be frozen and then thawed prior to fermentation. This process is especially useful for red grape varieties that tend to have less color stability, lower tannin content, or a limited aromatic profile. For example, cold-hardy red varieties like ‘Marquette’ and ‘Frontenac’, commonly grown in the US Midwest, Northeast, and Eastern Canada, have specific chemical characteristics that make them suitable candidates for this technique. These varieties tend to have high acidity (from their Vitis riparia genetic background), abundant anthocyanins, and lower tannin levels, which can make phenolic extraction challenging during traditional winemaking.
A Study on Freezing Grapes: Marquette and Frontenac Case Study
A recent study led by Iowa State University enology assistant professor Aude Watrelot explored the effects of freezing grapes before fermentation on the phenolic and color profiles of wines made from Marquette and Frontenac grape varieties. In the study, three batches of each variety were frozen at -10°C for five months, while control batches were directly destemmed, crushed, and fermented without freezing.
The results showed some interesting insights:
• Alcoholic fermentation took two days longer to be complete in the frozen grape batches, likely due to the higher availability of sugars released during the freezing process.
• The pH of musts right before fermentation was significantly higher in Frontenac control (pH of 3.38) than in Frontenac frozen condition (pH of 3.28), while the opposite was observed in Marquette musts (pH of 3.02 in control and pH of 3.05 in frozen condition). These differences were not observed in the wines at bottling.
• The titratable acidity (TA) was statistically different in musts and wines of Frontenac and Marquette. At bottling, the TA was twice higher in the frozen condition wines than in control wines.
• The phenolic concentration in the must was the same between the frozen and control batches, but the musts made from Frontenac grapes had higher phenolic concentrations overall compared to the musts made from Marquette grapes.
• The concentration of phenolics in the Frontenac wines was lower at pressing and bottling in the frozen samples compared to the control, likely due to phenolics being adsorbed onto broken cell wall material during freezing.
• Tannin concentrations were low in the must of both treatments, as a result of the low solubility of tannins in aqueous solutions. However, Frontenac control wines contained 50% more tannins at bottling than the Frontenac frozen samples. No significant difference in tannin levels was observed for Marquette wines.
• Color intensity was noticeably higher in the frozen must compared to the control, but this difference was not maintained in the final Frontenac wines at bottling. In contrast, Marquette wines at bottling still retained higher color intensity in the frozen samples.
These results indicate that freezing grapes can enhance color intensity early in the process, but the long-term impact on wine quality may be less promising without additional steps to balance tannin and anthocyanin interactions.
Conclusion: A Valuable Tool for Certain Varieties
Freezing grapes as a pre-fermentation technique can be beneficial in extracting phenolic compounds and enhancing color intensity in musts, particularly for cold-hardy red grape varieties like Marquette and Frontenac. However, its long-term effectiveness on the finished wine’s color and structure may not be as pronounced without additional treatments, such as the addition of tannins from other sources, to ensure stable pigment formation.
Winemakers could consider freezing as part of an integrated strategy for improving wine color and phenolic content, especially when working with varieties known for having less color stability or lower tannin content. It’s essential to tailor this technique to specific grape varieties and wine goals to achieve the best results.
About the Author:
Dr. Aude Watrelot is an Assistant Professor of Enology in the Department of Food Science and Human Nutrition at Iowa State University. Dr. Watrelot’s research area is fruit, grape and wine tannin and polysaccharide chemistry and their relationship with wine quality. Dr. Watrelot graduated with a PhD degree in Food Science from the French National Institute for Agricultural Research (INRA) and the University of Avignon in France. Following graduation, Dr. Watrelot moved to California both at CSU Fresno and UC Davis to keep conducting research into polyphenol-macromolecular interactions on red wine chemistry.
At ISU, she is currently conducting research and developing Extension programs on understanding viticultural and winemaking practices that could maximize phenolics extraction and improve wine quality. She has developed extension programs such as webinars, workshops, field days, articles to answer the growing wine industry in Iowa and in the Midwest. She is currently serving as the chair for the American Society of Enology and Viticulture – Eastern Section (ASEV-ES) and as the vice-president for the Groupe Polyphenols.
References:
1. Aleixandre-Tudo, J.L.; du Toit, W. Cold Maceration Application in Red Wine Production and Its Effects on Phenolic Compounds: A Review. LWT 2018, 95, 200–208, doi:10.1016/j.lwt.2018.04.096.
2. Cheng, Y.; Wimalasiri, P.M.; Tian, B.; Watrelot, A.A. Influence of Grape Flesh on the Retention and Composition of Polyphenols from Skins and Seeds. J Agric Food Chem 2024, doi:10.1021/acs.jafc.4c00612.
3. Sperotto, G.; Marçal, E.N.; Campos, F.M.; de Souto, V.O.; Comparin, S.J.; Nogueira, A.; Lazzarotto, M. Cold-Driven Strategies as Pre-Fermentative Techniques on Winemaking: A Review. Food Chemistry 2025, 463, 141504, doi:10.1016/j.foodchem.2024.141504.
4. Watrelot, A.A.; Delchier, N. How Does Extended Maceration Affect Tannin and Color of Red Wines from Cold-Hardy Grape Cultivars? Foods 2025, 14, 1187, doi:10.3390/foods14071187.
