Lactic Acid

For a general description and information on lactic acid see the Wikipedia entry.

Effects on Yeast

Genetic Manipulation

It has recently been discovered that both L-lactic acid and D-lactic acid produced by lactic acid bacteria can manipulate a genetic trait in yeast that dictates how yeast ferments different types of sugars, including some strains (but not all) of Saccharomyces cerevisiae and Brettanomyces bruxellensis . Normally Saccharomyces and many other types of yeast will preferentially metabolize glucose when glucose is present and they ignore other sugars such as maltose and maltotriose until the glucose is completely consumed. This is called "glucose repression". Recently it has been identified that lactic acid produced by lactic acid bacteria in the presence of some yeast strains turns off this "glucose repression" in yeast, allowing them to simultaneously ferment all types of sugars. This has a side effect of limiting attenuation in wine, and has been one of the identified causes of stuck wine fermentations (it has been observed as far back as Louis Pasteur that stuck wine fermentations often contain lactic acid bacteria) .

The ability for yeast to bypass glucose repression and ferment multiple types of sugars simultaneously is controlled a by a protein-based genetic prion called [GAR⁺]. These genetic "prions" are not the same as DNA in genes, but are rather misfolded proteins contained in the cytoplasm of the cell. These proteins are dominant over [gar^-], and are passed to the offspring of the cell during cell division. This type of passing of genetic material from mother cell to daughter cell is much more frequent than genetic mutations, and probably exists to help yeast populations quickly adapt to rapidly changing conditions in their environment. Normally in brewers yeast only a small number of cells are [GAR⁺] if any at all. In the brewing environment where there is no competition from other yeasts, brewers yeast benefits from consuming glucose first. In the wild, however, many more strains have been found to be [GAR⁺]. This is thought to be an adaptive advantage for wild yeast depending on the environments in which they live; such yeasts can "hedge their bets" towards consuming other types of sugars, with the side effect of allowing bacteria to produce compounds such as lactic acid that may inhibit competing yeasts .

Not only do [GAR⁺] yeast have the ability to ferment other sugars at the same time as glucose, but they produces less alcohol (for example in the referenced study, the [gar^-] Saccharomyces cells fermented grape must into a 12% ABV wine, and the [GAR⁺] Saccharomyces cells fermented the same wine must into an 8% ABV wine ). Viability over time is also increased in yeast cells that are [GAR⁺] versus those that aren't. In wild fermentation of grapes, the wild [GAR⁺] S. cerevisiae strains thrived over the other types of fungi that were found on the wild grapes. While the induction of [GAR⁺] can occur in yeast without the presence of lactic acid, the presence of lactic acid greatly increases the occurrence of [GAR⁺] in yeast cells, with higher concentrations of lactic acid producing more occurrences of [GAR⁺] cells. The lactic acid merely has to be present for this to happen, and the yeast's ability to metabolize lactic acid or not does not have an effect. It is thought that this benefits both the [GAR⁺] S. cerevisiae and the lactic acid bacteria, which are often found together in the wild during fermentation of fruit. The bacteria isn't killed by higher alcohol levels, and the yeast has a broader food source. This effect that lactic acid bacteria have on yeast is known as cross kingdom chemical communication .

In a previous study by Jarosz et al. (2014), it was observed that only certain bacteria species had the effect of inducing [GAR⁺] in some strains of yeast. These bacteria included P. damnosus, Lactobacillus kunkeei, and species from genres Staphylococcus, Micrococcus, Bacillus, Listeria, Paenibacillus, Gluconobacter, Sinorhizobium, Escherichia, Serriatia. In this study, L. brevis, L. hilgardii, L. plantarum did not appear to induce [GAR⁺] in yeast . At the time of this study, it was not understood that lactic acid was an inducer of [GAR⁺]. Additionally, the method they used to discover this was simply to streak bacteria next to yeast on a plate, and see if it grew on a medium that would show whether or not they bypassed glucose repression. Therefore, it is possible that not enough lactic acid was produced, or that they didn't give the bacteria enough time to have an effect on the yeast. More work would need to be done to show that indeed all lactic acid bacteria that produce lactic acid have this effect on yeast, and also it is not clear whether other bacterial metabolites may influence this phenomenon .

In beer, this might explain other observations as well. For example, Yakobson reported higher attenuation with some strains of Brettanomyces bruxellensis (WLP650, BSI Drie, CMY001, and WY5526) and one strain of B. anomalus (WY5151) demonstrated a trend of increased attenuation with increasing concentrations of lactic acid . In mixed fermentations of beers such as lambic and American sour ales, attenuation is often slower, but typically eventually reaches a high degree of attenuation. Some strains of S. cerevisiae are more tolerant of acidic conditions than others. Although this might answer some questions, mixed fermentation is a complex thing with many other variables and more work needs to be done to identify whether all or just some strains of yeast/bacteria have the effect of inducing [GAR⁺], and how that might effect the fermentation profile of various types of beers .

See also this MTF thread by Richard Preiss of Escarpment Labs on speculating impacts of this in sour/wild beer.

See Also

Additional Articles on MTF Wiki

• Lactobacillus
• Pediococcus
• Saccharomyces Fermentation Under Low pH