Acid Tolerance of Brewer’s Yeast
In the last 15 years, American sour beer has grown from experiments tucked away in the sheds or corners of a handful of breweries, to dedicated producers and year-round offerings from the largest craft breweries. In some ways not much has changed: At their new Asheville, North Carolina brewery, Sierra Nevada positioned the tank for Otra Vez gose out back, closer to the spent grain silo than the “clean” fermenters. Much of this increased production is a result of the popularization of quicker, cleaner, more reliable techniques for acidification.
Too often, I still taste beers that are sour mashed with poor technique, resulting in an aroma I can only describe as blue cheese left in an old running shoe. This article details the step that follows, specifically how to ensure that your brewer’s yeast respond well to all of that lactic acid.
Luckily, brewer’s yeast is more acid-tolerant than even lactic acid bacteria! Acid washing (briefly acidifying harvested slurry to a pH of 2.0–2.5 to disrupt bacteria) is a useful technique for brewers who repeatedly repitch (although chlorine dioxide is gaining favor). This technique can even be used to help select wild yeast over bacteria without plating. However, Saccharomyces cerevisiae is not universally adapted for fermentation in highly acidic wort. Low pH stresses yeast, sometimes leading to incomplete fermentation and residual off-flavors like diacetyl and acetaldehyde.
Most of the science conducted on the tolerance of yeast fermentation in highly acidic media comes from the biofuels industry. There, acid is sometimes added to inhibit wild bacteria from stealing carbohydrates to turn into acid instead of allowing yeast to produce ethanol (for your gas tank). Finding or engineering acid-tolerant yeast strains that can complete high gravity fermentations in these treacherous conditions is essential for improved yield. However, there isn’t a concern for how these corn-based concoctions taste.
Biology
While Lactobacillus alone can make a tart interesting beverage, achieving the amount of alcohol we expect in beer requires yeast. While heterofermentative Lactobacillus produce ethanol, the amount of lactic acid inhibits them before they reach 0.5% ABV. If you achieve attenuation of more than a few points, there is almost certainly yeast contamination to blame.1
All microbes have optimal conditions. In brewing we generally consider temperature, gas (oxygen and carbon dioxide) saturation, free amino nitrogen (FAN), trace elements (zinc and copper), sugar density (osmotic pressure), and alcohol. pH is another factor, but it rarely is significant because brewer’s yeast are fine at the typical 5.0–4.0 pH range. However, sour beers are ten times more acidic at a pH of 3! One experiment examined the results of Safale S-04 fermentation at three different acidities. Compared to a standard starting pH of 5, pH 3 reduced head retention by 42.9%, ester production by 18.4%, and ethanol by volume by 45%.2
When we introduce yeast to pre-soured wort the yeast go through two stages. The first, acid shock response, halts growth giving the cells time to acclimate to the acidity. The second stage, acid adaptation, holds that response while growth resumes (if the pH isn’t too low to inhibit growth). An analysis of a particularly acetic-acid-tolerant strain identified two important genes (AFT1 and HAA1). Both genes encode transcription factors activated by treatment with weak organic acids.3
Brettanomyces tends to be acid-tolerant as well, and a 100% Brettanomyces fermentation is a good choice to follow kettle souring if you want some of the fruity and funky aromatics often associated with long-aged sour beers. However, many brewers kettle sour because they do not want to introduce non-Saccharomyces microbes to their cold-side equipment.
Acidic Fermentation Experiment
Yeast labs list traits for each strain including alcohol tolerance, flocculation, and attenuation, but acid tolerance isn’t a common criterion. Mostly what we have to go by is anecdotal; back in his Cabinet Artisanal Brewhouse days Terry Hawbaker, now of Intangible Ales and Pizza Boy Brewing, made the fantastic Farmer’s Cabinet Gose. He followed a high-temperature lactic souring (by rolling a Grundy tank into the boiler room) with a French saison strain. I contacted a few yeast labs for their suggestions on acid-tolerant yeast strains. Kara Taylor from White Labs suggested WLP510 (Bastogne Belgian Ale), WLP566 (Belgian Saison II), and WLP300 (Hefeweizen Ale). Lance Shaner of Omega Yeast Lab suggested their DIPA Ale and HotHead Ale.
To get a few real-world homebrew data points, I tested the acid tolerance of three brewer’s yeast strains from White Labs WLP001 (American Ale), WLP007 (Dry English Ale), and WLP566 (Belgian Saison II). I took 1.058 wort from a schwarz-dunkel-bock: 92% Weyermann Bohemian dark malt, 4% Weyermann Caramunich® II, and 4% Briess Blackprinz® mashed at 158 °F (70 °C) and split it three ways: 1.5 gallons (5.7 L) each as is (pH 5.10), acidified to pH 3.54 with 25 g of 88% lactic acid, and acidified to pH 2.99 with 75 g of lactic acid. I split each three ways again and pitched 2 tsp. (10 mL) of yeast directly from the package into each of the nine jugs. I measured the apparent attenuation after two, six, and 11 days to gauge progress. You can see the results of this in the chart below.
I hoped that the low-fermentable wort would pose a challenge to the yeast, and it did! The results suggest that the moderately acidified (3.54 pH) wort didn’t drastically slow any of the strains or result in any off-flavors. However, all attenuated significantly less over both two and six days when the pH dropped to 2.99, the bottom of the typical sour beer range. The WLP566 (Belgian Saison II) strain ended up at the lowest final gravity at each pH, but the most acidic wort clearly took its toll lowering the apparent attenuation by 10% compared to the control.
Adding an absurd amount of 88% lactic acid was clearly not an exercise in making delicious beer. However, the American and English ale yeast seemed to compound the issue with off-flavors at that low pH. Honestly I wouldn’t suggest any of these strains be used for fermentation at such a low starting pH!
Best Practices
In addition to selecting an acid-tolerant strain, set it up for success. Reduce the need for growth by pitching at a slightly elevated pitching rate; aim for 1–1.5 million cells per mL per °P (for this test I pitched considerably more). Kara Taylor suggested acclimating the yeast to the lower pH environment by acidifying the starter to a pH of 3.5 (this will shorten the acid shock response time). If you are using dried yeast, rehydrate the cells following the lab’s directions rather than sprinkling directly onto the wort.
Add a second dose of yeast nutrient after souring, replacing the FAN content that the Lactobacillus depletes. Proper aeration after souring promotes healthier cell walls and more resistance to alcohol and acid. As fermentation slows, warm the beer to the top of the yeast’s temperature range for a few days to ensure it finishes attenuation and cleanup. I warmed the WLP001 and WLP007 strains to 75 °F (24 °C) on day five.
In addition to the implications for this in primary fermentation, these results also might be beneficial for those looking to bottle condition long-aged sour beers. Although, I routinely use rehydrated wine strains because of their acid- and alcohol-tolerances and my affinity for simplicity.
Good Funk and Bad Funk
Acid tolerance of Saccharomyces strains is becoming a key feature as brewers look to produce sour beers on a shorter timeline upending the traditional progression of mixed fermentations. Rather than ethanol fermentation coming first, it now follows lactic acidification. The result of not selecting a strong low-pH fermenter and putting it in a difficult situation can be disastrous: Under-attenuation, off-flavors, dumped beer, and an injured ego. There are many other strains that may perform well in kettle soured beers; try borrowing a gallon (4 L) of wort from a batch and adding acid to test a favorite strain before risking a full batch on an unproven strain, and let the rest of us know what works and what does not!
References:
1 http://www.milkthefunk.com/wiki/100%25_Lactobacillus_Fermentation#Lance_Shaner.27s_Experiment
2 Lettisha Hiralal, Ademola O. Olaniran, and Balakrishna Pillay. (2013) Aroma-active ester profile of ale beer produced under different fermentation and nutritional conditions. Journal of Bioscience and Bioengineering, 1-8
3 http://aem.asm.org/content/78/22/8161.long
