Managing pH and acid composition to assess microbial ecology of wine fermentation

Andrew Tyler Williams

This experiment was conducted to evaluate the impact of adjusting juice pH on microbial growth and metabolism. Different acids and a high acid wine were used to acidulate high pH Merlot juices grown in Washington State. Each treatment (~140L) was carried out in triplicate for a total of 45 fermentations. Tartaric, DL- 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 or 3.75. Additionally, the sequence of secondary fermentation was varied within each treatment (co-inoculation, sequential inoculation, and none). Acid adjustments overall had little impact on primary fermentation rate or alcohol production (14.9 ±0.2 % alcohol (v/v)), though bacterial non-inoculated control tanks (pH 4.0) did result in significantly lower ethanol content (14.52 ±0.15 % (v/v)). In addition, acetic acid was maintained below 0.4 g/L, though co-inoculated wines resulted in the highest values (0.35 ±0.03 g/L). Genetic material extracted and sequenced from wine samples indicate that regardless of inoculation regime, commercially-inoculated O. oeni bacteria dominate bacterial ecology within a timeframe no greater than 5-6 days, and this dominance of approximately 96-99% of the prokaryotic population lasts through the measured timeframe of the study to the xompletion of malolactic fermentation. The same degree of dominance by O. oeni was also recorded in non-inoculated regimes, though it required a greater amount of time to attain. The data indicate that the timing of inoculation was more important than acid adjustment with respect to microbial dynamics. Additions of L-malic acid were not seen to increase the rate nor onset of malolactic fermentation. However, a 20-28% increase of L-malic acid influenced the population dominance of several bacteria genera in the early stages of wine processing, including populations of Sphingobacteriaceae, Micrococcineae, and especially Sphingomonas, rising from approximately 2.5% to a 4.78% prokaryotic dominance with a 28% increase in L-malic acid. These and other under-reported bacterial genera in wine were recorded, but apparently only have the opportunity to flourish before the eventual domination by O oeni.

Managing pH and acid composition to assess microbial ecology of wine fermentation

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