Steam treatment of Cabernet Sauvignon, Merlot, Pinot noir, and Muscat Bailey A grape cluster stems eliminated >95% of the extractable methoxypyrazines (MPs) from the stems. Steaming of the Cabernet Sauvignon grape stems increased the levels of extractable flavonoid phenolics, the molecular weight distribution and degree of polymerization of which, as estimated by a gel permeation HPLC analysis, were lower than those of nonsteamed stems in a model must solution. Red wines fermented with steamed stems added showed higher absorbance values at 520 nm than those fermented with nonsteamed stems added and also showed lower MP levels than those fermented with nonsteamed stems. The addition of steamed stems increased the level of flavonoid phenolics detectable in the wines. The levels of phosphoric acid (phosphorus), potassium, and calcium were increased as a result of the addition of stems.