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Article

Ecological succession of Saccharomyces and lactic acid bacteria in open fermentation follows a predictable pH trajectory

Open fermentation is not random contamination but a succession path locked by hop bitterness and acidity.

Jibai Fang1†, R. J. Bokulich2, Wei Song1

* Equal contribution · † Corresponding author

B.E.E.R. 647, 252–260 (2026)

Received: 4 February 2026
Accepted: 1 August 2026
Published: 14 September 2026
DOI: 10.1038/sbeer-026-18441-8

Abstract

Traditional open fermentation is treated as a risk by industrial hygiene, yet it supplies key flavour in sour beers and some ales. We ran a 21-day metagenomic and metabolomic time series on 36 open fermenters and found that Saccharomyces cerevisiae occupies >90% of fungal reads from 0–72 h, after which Lactobacillus acetotolerans and Pediococcus damnosus take over in order along a pH 4.4→3.6 trajectory. Hop iso-α-acid concentration sets the lag before lactic acid bacteria enter, not the final community composition. The result reduces “wild fermentation” from process romance to an engineerable ecological trajectory.

Subject: Microbes and yeast

The flavour of open fermentation is often credited to a “house microbiota”, yet the house microbiota is itself a result of selection, not a starting point1. In open vessels with controlled aeration we saw that rapid consumption of wort sugars by S. cerevisiae pushed pH past 4.4 before acid-tolerant lactic acid bacteria could establish.

Yeast and wort under the microscope
Fig. 1 | Saccharomyces cerevisiae in fermenting wort. Early open fermentation is dominated by Saccharomyces, which later yields to acid-tolerant lactic acid bacteria.

Raising bitterness units delays, but does not cancel, the entry of lactic acid bacteria, showing that iso-α-acids are a rate regulator. That gives craft plants aiming for “a little acidity without losing control” an operable knob.

Methods

  1. After DNA extraction, ITS and 16S amplicon sequencing; metabolites by untargeted LC–MS.
  2. Open vessels of 200 L, inoculum 1.2×10⁷ CFU mL⁻¹.

Data availability. Sequences are deposited at NCBI BioProject PRJNA-BEER-OPEN.

Acknowledgements. Wuxi Laocang Brewery provided open vessels.

Author contributions. F.J.B. designed the study and wrote the paper; R.J.B. supervised the ecological analysis; S.W. carried out the sequencing.

Competing interests. The authors declare no competing interests.

Authors and affiliations

  1. 1. School of Biotechnology, Jiangnan University, Wuxi, China
  2. 2. Department of Viticulture and Enology, University of California, Davis, CA, USA

Corresponding author: Jibai Fang (jfang@jiangnan.edu.cn)

References

  1. 1. Bokulich, N. A. et al. Microbial biogeography of wine. PNAS 111, E139–E148 (2014).
  2. 2. Spitaels, F. et al. The microbial diversity of an artisan Belgian sour beer. PLoS ONE 9, e95384 (2014).
  3. 3. Dysvik, A. et al. Microbial dynamics in mixed-fermentation beers. Appl. Environ. Microbiol. 86, e02334-19 (2020).