Project Title: Impact of pH on the wine microbial ecology and wine quality
Project Duration: 2017-2020

Principal Investigator(s): Hailan Piao
Organization: Washington State University
Address: 359 University Drive, Richland, WA 99354
Phone: 509-372-7665
Email: [email protected]

Co-PI(s): Thomas Henick-Kling
Organization: Washington State University
Address: 359 University Drive, Richland, WA 99354
Phone: 509-372-7292
Email: [email protected]

Co-PI(s): Thomas Collins
Organization: Washington State University
Address: 359 University Drive, Richland, WA 99354
Phone: 509-372-7515
Email: [email protected]

Co-PI(s): James Harbertson
Organization: Washington State University
Address: 359 University Drive, Richland, WA 99354
Phone: 509-372-7506
Email: [email protected]

Summary:

Wine is a very complex alcoholic beverage as it contains a large number of components and its quality is affected by many environmental factors and diverse microorganisms. The growing environment and harvest maturity are key factors determining must and wine pH and acidity. The grape must pH in turn is a key determinant for the development of the yeast and bacteria populations during wine fermentation. The high pH and low grape must acidity affects many aspects of wine, such as, flavor and mouthfeel, color, and microbial stability. Typical Washington State red grape varieties have a high pH and low amount of malic acid. In high pH grape must, potential spoilage bacteria such as Lactobacillus sp. and Pediococcus sp. easily grow. L-malic acid and tartaric acid are the main organic acids of grape and wine and both acids play a dominant role in the wine acidity. Grapes grown in hot climates often show low acidity with high pH because most of the malic acid has been metabolized during grape ripening. Must and wine pH a critical physiological factor which significantly influences the yeast and bacterial populations and their metabolic activities in wine. Optimizing must pH to enhance the activity of beneficial wine microorganisms while to suppress growth of spoilage microorganisms are of vital importance to winemakers. Yeast and bacteria starter cultures are valuable tools in managing the microbial populations in high pH grape musts. In this project we evaluate the impact of acid additions such as a high acid wine, malic and tartaric acid, as well as the addition of yeast and bacteria starter cultures on fermentation rates, microbial populations, the metabolites and sensory profiles. We evaluated various acid and starter culture additions in two vintages, 2017 and 2018.

Yeast and bacteria population analysis by plating showed that native yeast and bacteria population from grapes were between 103-105, which population effectively reduced 10-100 fold with SO2 (30 -50 mg/L) addition. Alcohol fermentation (AF) was completed within 7 days after yeast starter culture addition. Spontaneous AF was also completed in 7 days when native yeast population reached to similar numbers of inoculated starter yeast. AF rates were neither affected by different targeting pH (4.0, 3.75, and 3.5) nor different acid adjustments (acid wine, tartaric acid, and DL-malic acid). Malolactic fermentation (MLF) was finished in about 10 days after adding ML starter culture in the coinoculated fermentation, while it was needed 5-7 weeks to completer MLF with native ML bacteria in the spontaneous and yeast only fermentations. MLF rates were not affected by different pH, however, MLF rate was slower in the tartaric acid adjusted musts in coinoculated fermentation.

Read more by downloading the full report above.