Jesse Aplin* and Charles Edwards 
*Washington State University, 1405 NE Merman Dr., Apt. C241,
Pullman, WA 99164 (jesse.aplin@wsu.edu)

Harvesting grapes at higher maturities requires modified winemaking procedures to reduce resultant alcohol levels without compromising sensory properties. Procedures employing non-Saccharomyces yeasts capable of respiring sugars without ethanol production have been proposed. However, excessive aeration that encourages these yeasts can impart off-flavors and increase volatile acidity. Non-Saccharomyces yeasts isolated from Washington vineyards were grown in high-sugar (310 g/L 1:1 mixture of glucose/fructose) Merlot grape juice, with or without 0.025 mL O2/mL/min aeration, for six days prior to inoculation with Saccharomyces cerevisiae D254. While many strains reduced ethanol under aeration conditions compared to wines made with Saccharomyces alone, some produced unacceptable levels of acetic acid (>1.2 g/L). However, five species (Metschnikowia chrysoperlae, Mt. pulcherrima, Meyerozyma guillermondii, Pichia kluyveri, and P. membranifaciens) produced lower concentrations of alcohol and volatile acidity under aerobic conditions. In a second experiment, Merlot grape juice (24.3 Brix) was inoculated with these five species and additional strains obtained from other culture collections. Fermentations were capped with a gas-porous stopper to provide semi-aerobic conditions and after three days, inoculated with S. cerevisiae D254. Inoculation of Mt. pulcherrima yielded wines with significantly less ethanol, ~2.0% v/v less than wines inoculated with S. cerevisiae alone (p < 0.05). Additional isolates of My. guillermondii and Mt. pulcherrima reduced the alcohol concentration by 1.4 and 1.6% v/v, respectively. Additional trials were conducted in fall 2016 with Merlot grapes fermented in 300 L open-top fermenters and these strains. Ethanol, organic acids, and various odor/flavor compounds were quantified by HPLC and GC-FID.

Funding Support: Wine Advisory Committee