Sour Power: The many ways to Lacto

The image is startling: You’re sitting at your computer one evening and run a quick Google search. You think to yourself, “How hard can this be?” Filled with the exciting unknown, you slowly type in L-a-c-t-o-b-, and immediately, autofill jumps in . . . Lactobacillus plantarum? Lactobacillus brevis? Lactobacillus helveticus? Lactobacillus delbrueckii? Lactobacillus acidophilus? Oh no! The list seems to go on forever and you’re realizing there are so many options. Where do you begin? What do you choose? How sour will your beer be with one versus another? Well, lucky for you, you have this magazine (with this article and many others!), and by the end of this quick read I’m hoping you’ll feel more confident in your choice of Lacto strain. Hopefully you’ll be excited to jump on your next sour brew and confident you’re making the best of this bacteria!

Before we dive in to all the nitty-gritty research, let’s take a step back and start with the basics. Lactobacillus is one of the most common souring organisms used in brewing today. It’s a Gram-positive bacteria that is shaped like a rod. While these facts have little relevance to brewers since it references the organism’s cell wall design, it does have strong correlation to its performance. More important to brewers, Lactobacillus can be separated into two separate groups depending on how they metabolize sugars: Homofermentative and heterofermentative. The difference between these are homofermentative Lactobacillus predominantly produce lactic acid as the end-product and do so using a biological process known as the Embden-Meyerhof-Parnas pathway. Heterofermentative Lactobacillus produce mainly a mixture of lactic acid and acetic acid utilizing a process known as the phosphoketolase pathway.¹ Lactobacillus is commonly seen as a spoilage organism but is a major component for the production of a properly soured beer.

There are various sources of Lactobacillus and different methods to produce a sour beer. You can get a supply of lactic acid bacteria from a laboratory, a bottle culture, from nature, yogurt, or it can even be found on unmashed grains (it’s found on malt husks). There are now even yeast strains, naturally occurring and bioengineered, available that produce alcohol and lactic acid at the same time during fermentation. However, sour beer production can often be broken down into two separate categories: 1. Traditional methods and 2. Quick souring methods. The traditional methods are widely known to be co-fermentation from spontaneous or cultured sources often with barrel/foeder aging. These methods typically have mixed cultures of Saccharomyces, lactic acid bacteria, and even Brettanomyces and Pediococcus. A positive of this wide spectrum of microflora is that it tends to produce a more complex, flavorful beer. However, it takes much longer, and some brewers don’t have that extra time or space to produce these. The other concern is consistency. It’s not always going to be the same as brews before. This is where the popular “quick souring” methods come into play. Typically, this is done by a mash- or kettle-souring process. Kettle-souring has become one of the more popular methods due to the quick turnaround times and typically consistent fermentations. However, these methods have also been noted to be a bit one-dimensional in aroma and taste in comparison to the flavor profiles of a co-fermented or barrel-aged sour beer.

Now, when we analyze sour beers, we tend to measure acidity by pH. However, pH does not give us the whole story or true picture of a beer’s acid profile. pH is roughly defined as a measurement of the concentration of positively charged free hydronium ions. It is calculated as the negative log of a solution’s hydronium ion concentration: pH = -log₁₀ [H₃O⁺]. In pure water, the concentration of hydronium is 1×10^-7 M, which if you put that in the equation, you get a pH of 7. Anything more qualifies as being acidic, and anything less would be basic.² Since pH is a logarithmic scale, the difference between each unit (let’s say shifting from a pH of 7 to 6) is actually a tenfold difference in change to a solution’s acidity.

However, sour taste cannot be explained solely due to the measured free hydronium ions. For example, various acids (e.g., acetic vs. lactic) at the same pH will give different levels of perceived acidity and flavors. Within beer, there are also weak acids in the system — which are an undissociated portion of hydronium ions — that plays a role in perceived sourness.
To fully explain acidity, we have titratable or total acidity (TA) as a tool to provide a clearer understanding of sourness. Unlike pH, there is a direct correlation between TA and perceived sourness in beers.³ TA, often provided in g/L, approximates the total amount of acid. It is the sum of free hydronium ions AND the ions bound to weak acids, which makes it a better indicator of how “sour” a beer is. To find directions on how to test TA, please note the American Society of Brewing Chemists (ASBC) method “Total Acidity Beer-8” offers instructions for free, but note that it does require specialty equipment and solutions not typically found in brewing.⁴ But if you have any winemaking friends, they may have the necessary tools in their wine labs.

Focusing back on Lactobacillus bacteria (what we’re all here for!), a study conducted by my colleagues, a research team at Lallemand Brewing, compared seven different Lactobacillus strains held at four different temperatures, and then ran a TA analysis on each. The goal was to find a bacteria strain that achieved 3.5 pH or lower in less than 48 hours, has high lactic acid vs. low acetic acid concentration, and to find what temperature would be best to do a kettle sour with each. We wanted to avoid high acetic acid as it can be perceived as harsh or vinegar-like. However, note that in barrel-aged or mixed fermentations, low levels of acetic acid can be found and can be seen as desirable. Fermentations were run at 20 °C (68 °F), 30 °C (86 °F), 40 °C (104 °F), and 50 °C (122 °F). We trialed two different Lactobacillus plantarum strains, two L. delbrueckii strains, a L. helveticus, a L. brevis, and a L. acidophilus strain. Each strain was pitched at 10 g/hL. Look at the four graphs following.

By a quick glance, it is obvious that the best temperatures for these Lactobacillus fermentations were 30 °C (86 °F) and 40 °C (104 °F). The two L. plantarum (A & B) strains were found to be more temperature-sensitive than the other Lactobacillus strains. By looking at the 30 °C (86 °F) chart, Graph B, you can see that the two L. plantarum strains fermented slightly faster than they did at 40 °C (104 °F). However, 40 °C (104 °F) was still a successful fermentation temperature for all the Lactobacillus strains. While temperatures higher than 40 °C (104 °F) resulted in very little change of pH for many of the strains, the two species that handled the higher temperatures slightly better than the others were L. acidophilus and L. helveticus. These are also the strains that produced the most acid out of the bunch (Graph B/40 °C), resulting in pH close to 3.0. Lactobacillus delbrueckii (A & B) and Lactobacillus brevis were the strains that produced the least amount of acid, ending up with a pH around 3.4–3.6. This data provided a strong understanding of what temperatures worked best for each strain, but in order to understand how sour they were and not rely on the pH to show us that information, we ran an acid analysis (Graphs E & F).

Graphs E and F show the levels of lactic acid and acetic acid (g/100 mL) produced by each strain at the different temperatures. A few interesting things to note are: 1. The lower temperatures commonly had the most acetic acid produced, 2. L. brevis had more acetic acid production at the 30–40 °C/ 86–104 °F range, which did not follow suit with the other strains, and 3. For most strains, 40 °C (104 °F) was the sweet spot where acetic acid was lower and lactic acid was high. Although these were all Lactobacillus strains, they all produced different levels of lactic and acetic acid. Even the strains that were the same species (L. plantarum A & B, and L. delbrueckii A & B), behaved differently.

We unfortunately do not have explicit qualitative sensory data for this study, but the conclusions from this were that the different Lactobacillus strains also had distinct and different flavors to them. As an alternative example, a study done by Escarpment Labs focused on co-fermentation of 16 different Lactobacillus strains with a Voss Kveik yeast strain presented at the 2020 World Brewing Congress (WBC 2020).⁵ They found that “Lactobacillus strain selection impacted yeast ester production,” and that “these results suggest that Lactobacillus-yeast combination may be a productive route to maximize flavor impact” of sour beers. With this combination, they found that, on average, L. delbrueckii was commonly associated with floral and red fruit flavors and L. brevis was associated with acetic flavors. There were also other strains of L. rhamnosus and L. paracasei that were found to be associated with fruity, banana characteristics. You can see that there are many Lacto strains available to brewers nowadays, and there is much more room for experimentation and research. As noted in the video presentation from the WBC, it would be interesting to see if other yeast strains provide other flavors or, turn the study around, and look at 16 different yeast strains with a single Lactobacillus strain.⁵ There is still lots to learn!

In summary, before you get overwhelmed by the options of Lactobacillus strains, know what you want your final product to be. How sour do you want your final product? What temperatures can your equipment/kettle hold and for how long? What flavor profile are you looking for? Answering these few questions with information provided should help make the decision of choosing the correct Lactobacillus bacteria a bit easier.

References

¹ Lewis, Michael. Young, Tom. (2002) Brewing: Second Addition. Kluwer Academic/Plenum Publishers. 319-338.
² Acids, Bases, pH, and Buffers. Kahn Academy. https://www.khanacademy.org/science/biology/water-acids-and-bases/acids-bases-and-ph/a/acids-bases-ph-and-bufffers
³ Neta, E. et al. (2007). The Chemistry and Physiology of Sour Taste – A Review. Journal of Food Science. 72(2) R33-R38. https://fbns.ncsu.edu//USDAARS/Acrobatpubs/P329-350/P346.pdf
⁴ Total Acidity Beer- 8. ASBC Methods. https://www.asbcnet.org/Methods/Methods/Beer-8.pdf
⁵ Preiss, Richard. (2020). Lactobacillus strain selection impacts sensory and analytical outcome in sour beer. World Brewing Congress . https://www.asbcnet.org/events/LiveWBC/OnDemand/Pages/TechnicalSessions.aspx

Written by Brittney Christianson