Aude Watrelot* and David Carter
*Iowa State University, department of Food Science and Human Nutrition, 536 Farm House Lane, Ames, IA, 50011 (watrelot@iastate.edu)

There is increased interest in using grape stems in red winemaking, but their actual impact on wine quality is poorly understood. Grape stems are either not harvested by mechanical harvest, or are discarded after manual harvest and the destemming process. However, grapes stems are rich in phenolic compounds, which makes them a potential good source of tannins to improve red wine quality. In 2023, two cold-hardy grape cultivars, Marquette and Frontenac, were crushed, destemmed, and processed with different stem additions. Stems were collected and either kept fresh (FS), oven-dried (DS), or oven-dried and ground (DGS) prior to being added to the musts. Those conditions were compared to a control treatment to which no stem was added. Red wines were made in triplicate for each condition following a standard red winemaking procedure, using yeast strain ICV D254 and co-inoculated with VP41 malolactic bacteria. Basic chemistry, color parameters, and phenolic compounds of the resulting red wines were analyzed by UV-vis spectrophotometer and high-performance liqud chromatography diode array detection at bottling. The pH was higher in Frontenac than in Marquette wines, but pH and titratable acidity were not affected by the treatments. The color intensity was greatest in control Frontenac wines and least in FS Frontenac wines. This could be explained by the ability of anthocyanins to bind with cell wall material of fresh stems. However, no effect of stem inclusion on color intensity was observed in Marquette wines, which may be due to their different anthocyanins profile. The FS and DS Marquette and Frontenac wines contained significantly more tannins than the control wines. Surprisingly, the concentrations of phenolics and tannins were not improved by DGS, suggesting a lower impact on the surface area-to-juice ratio on tannin extraction.

Funding Support: This project was supported by the U.S Department of Agriculture’s (USDA) Agricultural Marketing Service through grant 23SCBPIA1187.