Fermentation as Hygienisation

Fermentation with lactic acid bacteria (LAB) has the capability to hygienise the raw materials used, for example fish and fish side-streams. Here's a closer look at how exactly this works.

Lactic fermentation hygienises by:

• Quickly acidifying the fish matrix so pH falls to arounds 4–4.2, where undissociated lactic acid diffuses into pathogens and collapses their internal pH;

• Driving anaerobiosis (low redox) that most enteric pathogens dislike;

• Producing other antimicrobials (acetic acid, bacteriocins, H₂O₂) while LAB out-compete spoilers.

In low-pH, strictly anaerobic conditions, pathogens such as Salmonella and E. coli are inactivated; many studies show fast die-off at pH≈4, though the exact rate depends on matrix and temperature.

Key issues to know

• Fast pH drop is the key “hurdle”: silage/ensiling work on diverse substrates (including fish) targets ≤4.5 in ~72 h for preservation; ≤4.2 provides an extra safety margin for pathogen inactivation.

• Pathogen kinetics at low pH: Salmonella becomes undetectable within ~6 h at pH 4.0 in broth at ambient temperature; complex fish matrices are slower, hence the conservative 7-day hold after reaching ≤4.2.

• Anaerobiosis matters: At low pH + no O₂, pathogens (incl. Listeria) die faster; with O₂, survival can persist even at pH 3.8–4.2.

• LAB temperature range (mesophilic) mean that the recommended environmental temperature is 20–35 °C.

Monitoring & release criteria

To claim “hygienised” in an EU fertilising product/biostimulant context, you should verify microbiology against EU limits used across fertilising product categories:

• Salmonella spp.: Absent in 25 g.

• Either E. coli or Enterococcaceae: < 1,000 CFU/g (fresh mass).

In-process controls:

1. pH log: Measure 0, 12, 24, 48, 72 h; then every 2–3 days. Do not release unless pH ≤4.2 is achieved and held for ≥7 days (documented). Aim for ≤4 in fish matrices.

2. Anaerobiosis check: Headspace O₂ strips or redox indicator — keep sealed. Listeria data show oxygen undermines low-pH lethality.

3. Microbiology tests (release): After the hold phase, test Salmonella (25 g) and enumerate E. coli/Enterobacteriaceae to confirm compliance.

Sources

• FAOHome: Paper 2.0: Silage fermentation processes and their manipulation - Stefanie J.W.H. Oude Elferink, Frank Driehuis, Jan C. Gottschal, and Sierk F. Spoelstra https://www.fao.org/4/X8486E/x8486e09.htm

• Ghosh, S.K., Reddy, R., Xavier, K.A.M. et al. Comparative Evaluation of Microbial Ensilaging of Fish, Vegetable and Fish-Vegetable Composite Wastes. Waste Biomass Valor 14, 1657–1666 (2023). https://doi.org/10.1007/s12649-022-01956-x

• Raquel Cristina Konrad Burin, Abelardo Silva, Luís Augusto Nero, Influence of lactic acid and acetic acid on Salmonella spp. growth and expression of acid tolerance-related genes. Food Research International, Volume 64, 2014, Pages 726-732 https://doi.org/10.1016/j.foodres.2014.08.019.

• Regulation (EU) 2019/1009 of the European Parliament and of the Council of 5 June 2019 laying down rules on the making available on the market of EU fertilising products and amending Regulations (EC) No 1069/2009 and (EC) No 1107/2009 and repealing Regulation (EC) No 2003/2003 (Text with EEA relevance) https://eur-lex.europa.eu/eli/reg/2019/1009/oj/eng