Conclusions and Recommendations

MSc Project > Conclusions & Recommendations

This post is part of a series detailing the findings of my MSc research project which looked at the effects of different fermentation parameters on wort souring with Lactobacillus. If you haven’t already, take a look at the MSc project page for a full overview.

The objective of this project was to determine the effect that different fermentation parameters had on the performance of Lactobacillus during lactic fermentation of wort. A series of lactic fermentations were performed using dry malt extract as a malted barley based substrate with independent variables of temperature, wort gravity, pH and inoculation rate examined. It was found that all the parameters tested produced differences in the measured total acidity (TA) following fermentation, but for the most part these only translated into small deviations in pH.

The following is a brief summary of the main findings from each chapter and my thoughts on what this means practically when wort souring.

Yeast Contamination

While not part of the research goals I thought it was worth mentioning that yeast contamination during lactic fermentation was discovered and found to be problematic for wort souring. It was shown that yeast contamination, aside from direct observation, could be reliably detected by monitoring the specific gravity during fermentation. A combined mean apparent attenuation (AA) of 4.2 ± 1.2 % or an SG drop of 1.0018 ± 0.0006 SG (0.47 ± 0.15°P) after 72 h was found for all LF1-4 fermentations (calculated from 36 independent fermentations) using an isolated culture of L. brevis WLP672. This indicated that only a small proportion of the available sugars were utilised by the bacteria.

In practice
If a change in wort gravity during lactic fermentation greatly in excess of ~1.002 SG (0.5°P) is observed within 24-72 h, it might be worth checking for contamination.

Impact of temperature (LF1)

Fermentation temperature was shown to have a large influence on the performance of L. brevis WLP672. The optimal temperature for acid production after 72 h was found to be 37°C but pH and TA were also very close at 30°C. At 20°C however, a very noticeable decline was seen in both the rate and amount of acid production along with a much slower drop in pH. The results suggested that this strain could produce good souring performance across a relatively wide temperature range around its optimum. Outside this range however, acidification was drastically hampered.  

In practice
Determine the optimal temperature range for a LAB strain and stay close to it. Be aware the acidification will be significantly slower at temperatures too far below the optimal range while too high will kill them.

Impact of wort SG (LF2)

Wort specific gravity was found to have a profound effect on the amount of acid produced. The results showed that as wort gravity increased, higher final TA values were obtained. It was postulated that wort buffering capacity and essential nutrient concentrations may have been important factors. In the highest gravity wort tested (1.061 SG) it was also thought that osmotic stress may have had an increasingly important inhibitory effect.

In practice
It’s useful to know a low gravity beer won’t reach the same acid concentration as a higher gravity beer. If you want to make a low gravity beer more sour then consider ways to increase wort buffering capacity, such as using an extended proteolytic rest or adding an external buffer e.g. citric acid. I highly recommend reading the excellent study by Peyer et al. on the effects of buffering capacity on acidification by LAB.1

Impact of Wort pH (LF3)

The initial wort pH was found to have very little effect on the final pH after lactic fermentation. Aside from the pH 4.84 wort, which yielded substantially more acid, results for initial pH worts from pH 3.76-5.03 were surprisingly close. It therefore appeared that the threshold for significant inhibition of this strain wasn’t reached even in pH 3.76 wort. My suggestion for further work would be to consider a wider pH range, measuring both individual acid concentrations and bacterial growth kinetics. This would help determine with greater certainty how much acid was produced and at what point the starting pH or acid concentration had an impact on bacteria growth and metabolism.

In practice
The L. brevis WLP672 strain appeared to be largely resilient to the starting wort pHs used in these trials but this may be a strain dependant trait. From the limited results here and without more information, I don’t think pH is worth worrying about or adjusting if it’s in a normal pre/post boil range, as LAB don’t seem that sensitive to it.

While it deserves a more in-depth discussion, I’ve come across various recommendations in brewing circles to pre-acidify wort, prior to lactic fermentation to help with beer head retention. If this process is working for you then at least these results suggest it probably isn’t detrimental to LAB acid production.

Impact of Bacterial Pitch Rate (LF4)

After 72 h of lactic fermentation the largest dose rate of 4×107 CFU/ml yielded the lowest pH and highest production of acid. This was in contrast to the lowest dose rate of 2×106 CFU/ml, which had the highest pH and lowest acid production. In these fermentations the 24 h results for pH and TA were not as straightforward, for reasons unknown.

It would have been interesting to see if the trend of larger pitch rates giving rise to greater acid production continued but unfortunately it wasn’t possible practically. As unmodified wort was used for culturing Lactobacillus the cell density and therefore inoculum size would have quickly become too large to accommodate in the 50 ml centrifuge tubes used as fermentation vessels.

In practice
Use the largest pitch rate possible without the volume of supernatant causing problems. For reference the largest pitch rate here (4×107 CFU/ml) was approximately 8% of the total fermented volume. Wort is an excellent growth medium for LAB and could help reduce any flavour impacts from the supernatant.

pH, Total Acidity and Attenuation

The L. brevis WLP672 strain appeared to reach the stationary phase after approximately 24 h but continued to produce organic acids until monitoring of fermentation ceased at 72 h. Examining the results from all lactic fermentations collectively, it was shown that the relationship between pH and TA was quite weak, such that a narrow pH range represented a large spread of TA values.

In practice
pH is useful for indicating that a change has occurred during lactic fermentation but it doesn’t tell you, with any degree of accuracy, how much acid has been accumulated. TA is simple and quick to perform, making it the measurement of choice for determining the acid concentration in sour beers. The equipment and reagents required however, make it impractical for most homebrewers. I’d be interested to see the accuracy of the human palate compared with TA measurements, to see how good we are at discerning sourness.

Closing thoughts

My hope is that these findings can help, in some small way, towards developing a better understanding of the wort souring process with LAB. I’d also be delighted if anyone is able to use them to improve their sour beers.

I think the more knowledge we as brewers have about the effects of different variables on lactic fermentation, the greater control we can exert over the process. An awareness of which parameters to adjust and their impact on lactic fermentation provides the means to more effectively tailor our techniques to achieve the desired results.

Thank you for taking the time to read about my research and if you have any questions or want to discuss something I’ve written, feel free to get in touch.

References

  1. Peyer, L. C., Bellut, K., Lynch, K. M., Zarnkow, M., Jacob, F., De Schutter, D. P., Arendt, E. K. (2017) Impact of buffering capacity on the acidification of wort by brewing relevant lactic acid bacteria, J. Inst. Brew., 123, 497-505. https://doi.org/10.1002/jib.447

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