Lactic Acid Bacteria and Whisky Fermentation

At about 70 hours, a whisky fermentation can enter a new bacterial phase. As vigorous yeast activity subsides, other organisms come to the fore, changing the chemistry of the wash and helping shape the spirit’s aroma.[1][5] Many malt whisky distillers encourage this late lactic fermentation. Timing is crucial: bacteria that contribute to flavour late in the process can damage yeast performance and reduce alcohol yield if their growth becomes excessive.[1]

Why lactic acid bacteria are in whisky fermentation

Scotch whisky fermentation is not sterile. The wort, the liquid entering fermentation, is unboiled before it reaches the washback, or fermentation vessel. That leaves a route for bacteria associated with the raw materials and distillery environment to join the fermentation.[1][6]

A Heriot-Watt study traced several sources. Some bacteria arrived with the malt and survived mashing; others were thought to have colonised the distillery plant. The yeast suspension brought bacteria too, mainly organisms identified in the paper as Lactobacillus brevis and Lactobacillus plantarum. The community in the washback came from several places, not simply the yeast supply.[1]

Finding bacteria, then, does not by itself diagnose a fermentation problem. What matters is which organisms are present, when they grow and what they do to yeast and flavour. Distillers distinguish between a microbial contribution they want and growth that disrupts fermentation.[1]

For the wider sequence, see how the wash is made. Here, the focus is on the bacteria working alongside yeast and after the main alcoholic fermentation, when the wash can still be undergoing important chemical changes.

The changing community in the washback

The bacterial cast changes as fermentation progresses. In the Heriot-Watt investigation, early and middle stages included L. fermentum, L. brevis, L. paracasei or L. casei, and L. ferintoshensis. A final phase began at about 70 hours, with later organisms related to L. acidophilus and L. delbrueckii coming to prominence.[1]

Japanese research captured a similar late shift. Lactobacillus helveticus, less abundant at the start, dominated after 70 hours. It grew after alcoholic fermentation and yeast cell autolysis, the breakdown of yeast cells. The end of vigorous yeast activity was not the end of the fermentation’s chemistry: a different part of the microbial community was taking over.[5]

Across successive fermentations, however, a distillery’s bacterial community can remain recognisable. A survey isolated 64 lactobacillus strains from 23 Scottish malt distilleries, with L. brevis, L. fermentum, L. paracasei and L. pentosus among the main species. At one distillery followed for 12 months, the bacterial range remained stable. Researchers attributed influences on that range to the malt supply and the annual maintenance closure.[2]

When bacteria help, and when they hinder

Late lactic fermentation is encouraged by many distillers for its perceived contribution to flavour. The same activity is not welcome at every stage. High concentrations of lactic and acetic acids can harm yeast viability, and the Heriot-Watt paper reports reduced ethanol yield when lactobacillus populations exceeded 10 to the power 6 cells per millilitre.[1]

The competition is direct: lactobacilli and yeast both draw on sugar in the wort. ScotchWhisky.com’s Whisky Professor also identifies poor cleaning as the most common cause of failed fermentation. Encouraging a useful late bacterial phase calls for control, not a relaxed approach to cleaning.[10]

Laboratory work offers a closer look at the yield risk. University of Saskatchewan researchers tested lactobacilli in wheat mashes for industrial ethanol production, rather than whisky. At about 10 to the power 6 colony-forming units per millilitre, three strains caused an approximately 2% decrease in final ethanol concentration. At populations up to 10 to the power 9, reductions were approximately 3.8 to 7.6%.[4]

The researchers identified competition for essential growth factors as a main cause of lost yield. Their homofermentative strains produced 1 to 1.5% lactic acid by weight per volume. Those figures describe the risk under the study’s conditions; they do not predict the loss at a particular whisky distillery.[4]

The useful distinction is timing and control, not a simple division into “good” and “bad” bacteria. A late phase associated with desired flavour chemistry is a different production situation from an early population competing strongly with yeast. Both involve bacteria, but their consequences for fermentation differ.[1][4]

How bacteria influence whisky flavour

Acids and esters

Lactic acid’s influence carries beyond the washback. The Heriot-Watt paper describes it reacting with ethanol during distillation to form ethyl lactate. It also reports that spirit from longer fermentations, greater than 55 hours in the cited discussion, tends to contain more esters. Longer fermentation is associated with that change, rather than being a universal route to better spirit.[1]

Other research describes lactic acid bacteria participating in the conversion of higher alcohols to acetate esters after alcoholic fermentation. The late bacterial phase can therefore do more than increase the wash’s acidity: it can change compounds relevant to the spirit’s flavour.[6]

There is no single bacterial flavour signature behind these findings. Different organisms work at different stages of fermentation, so “lactic” alone tells a reader little about how a finished whisky will taste. The contribution comes from the chemistry of the community, not simply the presence of bacteria.[1][5][6]

Spice and phenolic notes without peat

Clove, spice and medicinal notes can have a route into whisky that does not begin with peat. Barley supplies ferulic and p-coumaric acids, released during mashing. Microbial activity can convert them into 4-vinylguaiacol and 4-vinylphenol, compounds described in research as having clove or spice-like, medicinal or phenolic characteristics.[3]

Nor does this chemistry simply build ever stronger aromas. In laboratory work, bacteria and yeast together produced an overall reduction in 4-vinylphenol and 4-vinylguaiacol before distillation. Working together, they also reduced 4-vinylphenol to 4-ethylphenol more effectively than either microorganism alone. Formation and reduction are both part of the picture.[3]

Japanese researchers likewise found that a whisky strain of L. helveticus could convert p-coumaric acid into 4-vinylphenol. They described that ability as potentially beneficial for malt whisky quality: a useful biochemical capability, rather than a reason to assume that more of the resulting compound is always desirable.[5]

A species name alone cannot explain a finished whisky’s character. These studies describe bacteria interacting with yeast and the compounds available to them, sometimes forming flavour-related substances and sometimes reducing them. The important story is what the organisms do together during fermentation.[3][5]

Sour mash is a related but different idea

American sour mash introduces acidity by another route. The Oxford Companion to Spirits & Cocktails describes recycling backset, or thin stillage, into the cooker. Its acids lower pH and suppress spoilage bacteria. The entry also describes a separate tradition in which lactic acid bacteria are introduced into the yeast mash to produce lactic acid.[8]

Both approaches involve acidification, but neither should be confused with the late bacterial phase observed in malt whisky fermentation. Returning acidic backset and encouraging bacteria to grow are different operations, even though both can alter the conditions in which fermentation takes place.[1][8]

“Sour mash” names a production method, not a sour flavour in the glass. The term alone does not identify which lactic bacteria were used, or establish whether a particular producer deliberately soured its yeast mash. Those details belong to the producer’s account of its process.[8]

Common misunderstandings

• “Bacteria mean the fermentation has gone wrong.” Presence is not the same as damaging growth. Malt whisky research documents naturally occurring bacterial communities, including a late lactic phase encouraged by many distillers.[1]

• “More bacteria must mean more flavour.” Their effect on yeast matters too. High acid concentrations can impair yeast viability, while sufficiently large bacterial populations can reduce ethanol yield.[1][4]

• “Clove and medicinal notes must come from peat.” Those familiar sensory associations have another possible source: microbes transforming barley-derived phenolic acids during fermentation.[3]

• “All the bacteria arrive with the yeast.” The yeast suspension is one source. The Heriot-Watt research also identified bacteria that survived mashing and presumed that others had colonised the distillery plant.[1]

When comparing production accounts, look beyond a passing mention of bacteria. The revealing details are their source, the stage at which they grow and the effect the distiller intends. Together, those details explain the role of bacterial activity far better than a blanket claim that it is beneficial.[1][4][6]

Where to go next

• Mashing: How Grain Starches Become Fermentable Sugars: follow the preparation of the liquid that enters fermentation.
• Enzymes in Whisky: Malted Barley vs Added Enzymes: distinguish enzyme activity from the microbial activity discussed here.
• Washbacks: Wooden vs Stainless Steel: explore the vessels in which yeast and bacteria share the fermentation.

Sources

[1] van Beek, S. and Priest, F. G. Evolution of the lactic acid bacterial community during malt whisky fermentation: a polyphasic study. Applied and Environmental Microbiology, 68(1), 297 to 305, 2002. Heriot-Watt University. https://pmc.ncbi.nlm.nih.gov/articles/PMC126549

[2] Simpson, K. L., Pettersson, B. and Priest, F. G. Characterization of lactobacilli from Scotch malt whisky distilleries and description of Lactobacillus ferintoshensis sp. nov. Microbiology, 147(4), 1007 to 1016, 2001. Europe PMC abstract record. https://www.ebi.ac.uk/europepmc/webservices/rest/search?query=DOI:10.1099/00221287-147-4-1007&resultType=core&format=json

[3] van Beek, S. and Priest, F. G. Decarboxylation of Substituted Cinnamic Acids by Lactic Acid Bacteria Isolated during Malt Whisky Fermentation. Applied and Environmental Microbiology, 66(12), 5322 to 5328, 2000. https://pmc.ncbi.nlm.nih.gov/articles/PMC92463/

[4] Narendranath, N. V., Hynes, S. H., Thomas, K. C. and Ingledew, W. M. Effects of lactobacilli on yeast-catalyzed ethanol fermentations. Applied and Environmental Microbiology, 63(11), 4158 to 4163, 1997. University of Saskatchewan. https://pmc.ncbi.nlm.nih.gov/articles/PMC168732/

[5] Kido, Y., Maeno, S., Tanno, H., Kichise, Y., Shiwa, Y. and Endo, A. Niche-specific adaptation of Lactobacillus helveticus strains isolated from malt whisky and dairy fermentations. Microbial Genomics, 7(4), 000560, 2021. Tokyo University of Agriculture; strains supplied by Suntory. https://pmc.ncbi.nlm.nih.gov/articles/PMC8208680/

[6] Daute, M., Jack, F., Harrison, B. and Walker, G. Experimental Whisky Fermentations: Influence of Wort Pretreatments. Foods, 10(11), 2755, 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8624260/

[8] Smith, K. Sour mash. In Wondrich, D. and Rothbaum, N. (editors), The Oxford Companion to Spirits & Cocktails, online entry, publication year not supplied in the fact sheet. https://www.spiritsanddistilling.com/dictionary/id/acref-9780199311132-e-669

[10] ScotchWhisky.com, The Whisky Professor. Are wooden washbacks better than steel? Trade article, 16 May 2016. https://scotchwhisky.com/magazine/ask-the-professor/9275/are-wooden-washbacks-better-than-steel

This desk-researched article uses the supplied verified fact sheet, distinguishing whisky studies, general ethanol fermentation experiments and trade explanations.

Last checked: November 2026