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Partitioning of Potassium during Commercial-Scale Red Wine Fermentations and Model Wine Extractions

¹ School of Food Science, Washington State University, Irrigated Agriculture and Extension Center, 24106 N. Bunn Rd., Prosser, WA 99350-8694.

^* Corresponding author (email: jfharbertson{at}wsu.edu; tel: 509 786-9296; fax: 509 786-9370)

Potassium was measured in the skins, seeds, and pulp of own-rooted Merlot grapes from the Columbia Basin, Washington. Commercial scale (7,892 kg) fermentations were carried out that varied the skin contact time and finished alcohol content. Comparison of treatments showed the control had statistically greater potassium concentration than the high ethanol and extended maceration treatments. Potassium was measured throughout the fermentations and was found to peak after crushing, diminish during the period of skin contact, and thereafter remained constant (up to 161 days). Comparison of grape and pomace potassium contents revealed that a small amount of potassium is extracted from the seed during fermentation but the opposite occurred in skins, which showed adsorption of potassium during the same period. Model wine extractions from grape skins showed that potassium extraction was increased by lowering the pH, suggesting that negatively charged polysaccharides in the grape skins may be forming ionic bonds with potassium. This theory was further reinforced by model wine extractions with pectic enzymatic treatment of grape skins soaked in potassium that showed greater percentages of skin potassium extraction than when soaked with potassium alone. Model wine extractions of grape skin showed that the combined effect of ethanol and potassium concentration both limited the extraction of skin potassium and increased potassium adsorbed to the skin than in original fruit. Thus, the model solution work both confirmed and demonstrated that the key factors in retaining and gaining potassium in skins were ethanol and potassium concentration in must/wine or model solution.

Key words: potassium, grape skins, fermentation, extended maceration