Andrew Tyler Williams,* Thomas Henick-Kling, Hailan Piao, James Harbertson, Tom Collins, Caroline Merrell, Richard Larsen, Mitch Williamson, Dru Seed, and Cary Wilton
*Washington State University, 58311 N Sunset Rd, Benton City, WA 99320 (andrew.t.williams@wsu.edu)

This experiment was conducted to evaluate the impact of adjusting juice pH on microbial growth and metabolism. In this study, different acids and a high-acid wine were used to acidulate high pH Merlot juices grown in Washington State. Each treatment (~140 L) was carried out in triplicate for a total of 45 fermentations. The initial grape juice was 25 Brix, 4.3 g/L TA, and pH 3.8. Tartaric, D-L malic acid, and the high-acid wine (15.2 g/L TA, 5.5 g/L Malic acid, pH 2.90, and 7.4 % (v/v)) were added to control juices (pH 4.0 adjusted using K2CO3) to lower juice pH to 3.5. Additionally, the sequence of secondary fermentation was varied within each treatment (co-inoculation, sequential inoculation, and none). The acid adjustments had no significant impact on primary fermentation rate or alcohol production (14.9 ±0.2 % alcohol (v/v)). In addition, acetic acid was unaffected by the sequence of malolactic fermentation (maximum 0.4 g/L). Coinoculated fermentations finished secondary fermentation in two weeks, sequential fermentations required three to four weeks, and non-inoculated treatments finished after 50 to 70 days. Based on the data collected thus far, it appears that neither the acid adjustment nor the type of acid used has an impact on whether or not secondary fermentation occurred. Primary fermentation progress was likewise unaffected and all tanks were maintained below a maximum of 108 CFU/mL. The data indicates that the timing of the inoculation was more important than the acid adjustment, with respect to microbial growth. It does appear that the rate of secondary fermentation for the coinoculated ferments was slightly accelerated by using acid additions that contained malic acid; however, the effect was trivial (~2 days). Future work will examine the impact of acid addition on the microbial populations and their metabolism. Additionally, the wines will undergo sensorial evaluation.

Funding Support: The Washington Grape and Wine Research Program