Hey all, spurred on by a discussion in The Mead Hall discord server (discussion starts here), I found a paper on malic acid degredation during fermentation in synthetic and Chardonnay musts via four yeast species: S. cerevisiae (71b), S. bayanus (EC1118), S. Pombe (strain F), and S. paradoxus (RO88) along with a spontaneous fermentation in the Chardonnay must. The paper can be found here, and I uploaded a copy to the discord server here.

There was some interesting info in it which I will briefly summarize in this post. To start the results of malic acid degredation in the synthetic musts with malic acid concentrations of 0.3% and 0.8% between three of the strains:

• EC1118 was able to reduce malic acid content by 5-8%
• 71b was 17-20%
• RO88 was 26-28%

This can be compared to the Chardonnay musts, with a measured malic acid concentration of 3.6 g/L:

• RO88 was 38%
• S. Pombe was 90%
• 71b was 18%
• Spontaneous fermentation was 10%

Interestingly the authors say that S. cerevisiae is generally unable to efficiently metabolize malic acid due to a lack of active transport mechanisms for that acid. This means that the yeast rely on diffusion processes through the cell wall and cell membrane to get malic acid inside, which is slow and inefficient. Additionally they mention that the fermentation environment inhibits the activity of malic acid enzymes within the cells, further inhibiting this process. On top of that the enzymes also have a relatively low affinity for malic acid in the first place. All of this combines together into a less efficient malic acid metabolism compared to other microorganisms.

My analysis of the biochemistry could be a bit flawed, since it is not my field, so if anyone notices mistakes please let me know so I can update this post for accuracy. I also recommend taking a look at the paper itself, as there is more data that is reported including total acidity data, other acid concentrations pre and post ferment, along with some comparison of fermentation rates between the strains. If for some reason you cannot get a copy of the paper from the sources I provided, you should be able to get it from sci-hub for free.

Edited for formatting

Edit: I just noticed that I misspelled "EC1118" in the title, I do mean EC1118.