Identifying Pellicle: Brettanomyces vs. Lactobacillus

Identifying a pellicle as either Brettanomyces or Lactobacillus is critical for any brewer. Brettanomyces typically forms a thicker, more wrinkled, segmented film with distinctive funky aromas (horse blanket, barnyard) and results in very low final gravity. In contrast, Lactobacillus pellicles are generally thinner, smoother, often cloudier, and produce a clean, tart, lactic sourness with a more significant, rapid pH drop. Visuals, pH, and aroma are your primary diagnostic tools.

The Brewer’s Hook: When the Funk Kicks In (And You Didn’t Ask For It)

When I first started dabbling in wild fermentation nearly two decades ago, I made the classic mistake of underestimating the visual cues of microbial activity. I had a batch of a beautiful, robust stout sitting in a secondary fermenter, aging gracefully. Or so I thought. After about two months, I peeked in and saw what looked like a strange, greyish film spreading across the surface. My initial panic, fueled by visions of mold, quickly subsided as I recognized the distinct, leathery texture – a pellicle. But from what? The aroma was undeniably complex: earthy, a hint of something resembling cherries, and an undercurrent of horse blanket. That was my inaugural, accidental encounter with Brettanomyces.

Years later, I was experimenting with a quick souring method for a blonde ale. I pitched a known Lactobacillus culture, but a week in, I saw a much thinner, more cohesive film form. The aroma was a sharp, clean lactic tang, devoid of any funk. While both were pellicles, the subtle visual differences, combined with the profound aromatic and eventual flavor disparity, hammered home the necessity of precise identification. You can’t just guess; you need data, a systematic approach, and a lot of sensory memory. Every pellicle tells a story, and knowing how to read it is paramount to brewing excellence.

The Math: Quantifying Microbial Activity for Identification

Visual inspection and aroma are critical, but the numbers tell the undeniable truth. When dealing with unexpected pellicle formation, I rely on specific metrics to differentiate between Brettanomyces and Lactobacillus. The key lies in monitoring pH and attenuation over time.

Apparent Attenuation Index (AAI) for Brettanomyces

Brettanomyces is notorious for its ability to chew through complex sugars (dextrins) that standard brewer’s yeast leaves behind. This leads to a phenomenon known as “super-attenuation.”

• Initial Apparent Attenuation (Yeast Only): AAI_Y = ((OG – FG_yeast) / OG) * 100%
• Final Apparent Attenuation (Yeast + Brett): AAI_B = ((OG – FG_Brett) / OG) * 100%

If you’re seeing a pellicle and your gravity continues to drop significantly after primary fermentation has completed (e.g., FG below 1.005, or even below 1.000 for some strains), especially without an increase in perceived acidity, Brettanomyces is a prime suspect. For instance, if your beer finished at **1.010** with primary yeast and then, over two months with a pellicle, dropped to **0.998**, that’s a clear **0.012** point reduction well beyond normal. My typical observation is that a Brett-driven secondary attenuation can drop FG by an additional **0.005 to 0.015** points, sometimes more, depending on the available dextrins and Brett strain.

Rate of pH Change (ΔpH/day) for Lactobacillus

Lactobacillus, being a lactic acid bacterium, directly produces lactic acid, causing a rapid and often significant drop in pH. While Brettanomyces can also lower pH due to acetic acid production, it’s typically slower and less pronounced in the initial stages compared to Lacto.

To calculate the rate of pH change, I use a simple formula:

ΔpH/day = (Initial pH of Beer - Current pH of Beer) / Number of Days Since pH Shift Started

For example, if my beer initially finished fermentation at **pH 4.3**, and then after **7 days** with a visible pellicle, it measures **pH 3.7**, my `ΔpH/day = (4.3 – 3.7) / 7 = 0.6 / 7 ≈ 0.086 pH units per day`. A `ΔpH/day` value consistently above **0.05** pH units per day, especially coupled with a clean sour aroma, strongly suggests Lactobacillus activity. In contrast, Brettanomyces activity often results in a `ΔpH/day` of less than **0.03** in the early stages, gradually increasing over weeks.

Step-by-Step Execution: How I Identify a Pellicle

When I see an unexpected film on my beer, I follow a systematic approach. Precision and sanitation are non-negotiable.

1. Safety and Sanitation First

   • Always wear gloves.
   • Sanitize all tools that will come into contact with the beer or samples: pH meter probe, hydrometer, thief, sample jars. I use a **100 ppm** Star San solution or an iodophor solution at recommended strength.

2. Visual Inspection (Initial Clue)

   • Gently approach the fermenter. Avoid disturbing the beer surface or introducing oxygen unnecessarily if you suspect an unwanted infection.
   • Observe the pellicle’s morphology:
     • Thickness and Texture: Is it thick, leathery, and wrinkled, almost like brain coral (likely Brett)? Or is it thin, smooth, and gelatinous, like a veil or spiderweb (likely Lacto)?
     • Segmentation: Does it form distinct ‘islands’ or segments that are slightly separated? This is a strong indicator for Brettanomyces.
     • Color: Most pellicles are off-white, cream, or yellowish. Anything green, black, or dry/fuzzy indicates mold, which means immediate disposal.
   • Note its location: Pellicles often form at the liquid-air interface, where oxygen is most accessible.

3. Aroma Assessment (Crucial Indicator)

   • Carefully open the fermenter lid or airlock to take a gentle whiff. Do not plunge your nose directly into the headspace to avoid introducing contaminants.
   • For Brettanomyces: Expect distinct ‘funk’ notes. Think horse blanket, barnyard, earthy, leathery, cherry pie, or even a subtle medicinal (Band-Aid) character. These are 4-ethylphenol and 4-ethylguaiacol, classic Brett metabolites.
   • For Lactobacillus: Anticipate a clean, sharp sourness. Think yogurt, lemon, or green apple tartness. There shouldn’t be any ‘funk.’ Sometimes, a slight buttery (diacetyl) note can be present, especially from heterofermentative strains.

4. pH Measurement (The Telltale Sign)

   • Use a sanitized wine thief to extract a small sample (about **50 mL**). Avoid disturbing the pellicle more than necessary.
   • Measure the pH using a calibrated, sanitized pH meter.
   • Record the current pH. Compare it to your beer’s original post-fermentation pH.
   • A rapid drop to below **pH 3.5** (e.g., from **pH 4.3** to **pH 3.2** in a week) is a strong indicator of Lactobacillus.
   • A slower, more gradual drop (e.g., from **pH 4.3** to **pH 3.8** over a month), coupled with funk, points to Brettanomyces.

5. Gravity Monitoring (Attenuation Clues)

   • Take another small, sanitized sample (enough for your hydrometer or refractometer).
   • Measure the current specific gravity (SG).
   • If Brettanomyces: You might observe a continued, very slow drop in gravity, even after primary fermentation seemed complete. This often leads to an FG below **1.005**, sometimes even below **1.000**, over weeks or months.
   • If Lactobacillus: While Lacto consumes sugars, its primary effect is acid production, not necessarily further attenuation of complex sugars. You might see a slight drop if it’s consuming simple sugars, but usually not to the extreme levels of Brett.

6. Microscopic Examination (Advanced Confirmation)

   • If you have access to a microscope, this provides definitive confirmation.
   • Carefully take a minuscule sample of the pellicle. Place it on a slide with a drop of sterile water and a cover slip. Staining with Methylene Blue can help.
   • For Brettanomyces: Look for distinct ovoid, lemon-shaped, or apiculate (pointed at one end) yeast cells.
   • For Lactobacillus: Look for rod-shaped bacteria, which can vary in length and sometimes appear in chains.

Troubleshooting: What Can Go Wrong and How to Handle It

Identifying pellicles isn’t always straightforward. Here’s what I’ve encountered and how I’ve dealt with it.

• False Positives (Kahm Yeast): Sometimes, a thin, white, opaque film forms on the surface that looks like a pellicle but isn’t. This is often Kahm yeast (e.g., Candida, Pichia). Key differences: Kahm yeast often produces a yeasty, bready, or sometimes cheesy aroma, rarely sours the beer significantly, and doesn’t typically cause super-attenuation. If you observe a thin pellicle with no souring or funk, and the pH is stable, it’s likely Kahm. While generally harmless, it can contribute off-flavors over time.
• Co-Infection: It’s entirely possible to have both Brettanomyces and Lactobacillus (or other microbes) present. In such cases, identification becomes more challenging as characteristics can overlap. I focus on the dominant sensory notes (is it primarily funky or primarily sour?) and the rate of pH drop vs. gravity drop. A rapid pH drop points to Lacto, while a very slow, sustained gravity drop with funk points to Brett, even if both are present.
• Misidentification of Mold: This is a critical distinction. Mold (often green, black, or very fuzzy/dry patches) is a sign of severe contamination, often indicating oxygen ingress and compromised sanitation. Pellicles from Brett or Lacto are usually white, cream, or off-white, and wet/leathery. If you see mold, discard the batch immediately; it poses health risks.
• No Pellicle, Still Sour/Funky: These microbes don’t *always* form a visible pellicle. Many strains of Lactobacillus ferment entirely in suspension. Brettanomyces might only form a pellicle if exposed to sufficient oxygen. If you taste sourness or funk but see no pellicle, rely entirely on aroma, pH, and gravity data for identification.
• Prevention: The best defense against unwanted pellicles is meticulous sanitation. I cannot stress this enough. Every piece of equipment that touches the wort post-boil, especially after yeast pitch, must be absolutely spotless and sanitized. Pay extra attention to scratches in plastic fermenters. Using dedicated “sour” equipment is a common practice if you intentionally brew with these organisms. For more information on maintaining a sterile environment, visit BrewMyBeer.online.

Sensory Analysis: The Taste and Smell of a Pellicle’s Maker

After the visual inspection and data collection, the ultimate test is sensory analysis. This is where experience truly comes into play.

Pellicle from Brettanomyces

• Appearance: Beers with Brett often start hazy or cloudy, but can clear beautifully over time as the yeast flocculates. Carbonation can range from moderate to highly effervescent due to its continued fermentation.
• Aroma: This is the hallmark. Dominant ‘funk’ notes: horse blanket, earthy, barnyard, leather (4-ethylphenol). You might also get cherry pie, clove, or smoky notes (4-ethylguaiacol), or pineapple/tropical fruit esters. Some strains produce acetic acid, giving a vinegar-like aroma, especially with oxygen exposure.
• Mouthfeel: Typically very dry due to high attenuation. Can be thin-bodied initially but develops a pleasant roundness and complexity with age. High carbonation often accentuates the dryness.
• Flavor: Complex and evolving. Earthy, leathery, often with tartness (if acetic acid is present) but less pronounced than lactic sourness. Fruity esters (pineapple, strawberry, peach) can emerge. The finish is usually very dry, sometimes slightly tannic, with a lingering funk that can be highly desirable in certain styles.

Pellicle from Lactobacillus

• Appearance: Beers soured by Lactobacillus are typically hazy to cloudy, with a distinct sheen on the surface if a pellicle is present. They rarely clear up completely unless other fining agents or processes are used.
• Aroma: Cleanly sour and tart. Think fresh yogurt, lemon juice, green apple (malic acid conversion), or sometimes a distinct lactic sweetness. Diacetyl (buttery/popcorn) notes can be present, especially from heterofermentative strains, but are generally considered an off-flavor in purely sour beers. There is a complete absence of ‘funk.’
• Mouthfeel: Zesty, crisp, and refreshing due to the acidity. Can feel thin if the original beer lacked body, but the acidity can also contribute to a perceived fullness. High acidity can sometimes create a puckering sensation.
• Flavor: Dominant lactic sourness. Like a well-made lemonade or tart yogurt. The flavor profile is generally less complex than Brett, focusing primarily on clean tartness. The finish is sharp and clean, often without lingering bitterness.

Frequently Asked Questions

Is a pellicle always a sign of spoilage?

No, not always. While an *unwanted* pellicle indicates a contamination, pellicles are a natural and often desired part of complex sour and wild fermentation processes when working with *Brettanomyces* or *Lactobacillus* intentionally. In traditional Gueuze or spontaneously fermented lambics, a pellicle, often called a “mother,” is crucial for flavor development and protection.

Can Brettanomyces and Lactobacillus co-exist in my beer?

Absolutely. They often co-exist and collaborate, especially in many traditional sour beer styles. *Lactobacillus* typically rapidly lowers the pH by producing lactic acid, creating an environment that inhibits many spoilage organisms and conventional brewer’s yeasts. *Brettanomyces* then slowly ferments residual dextrins and produces unique flavor compounds over months or even years. This synergy is key to complex sour ales.

What’s the best way to prevent unwanted pellicle formation?

The best prevention is rigorous sanitation. Clean every piece of equipment that touches your wort post-boil with an alkaline cleaner, then sanitize thoroughly with a no-rinse sanitizer like Star San. Avoid plastic equipment with scratches, as these can harbor microbes. Use dedicated equipment (fermenters, hoses, bottling wands) for sour/wild beers, separate from your clean brewing setup. Minimizing oxygen exposure post-fermentation also helps, as many pellicle-forming organisms thrive with oxygen.

Do all sour beers develop a pellicle?

Not all sour beers will develop a visible pellicle. Many modern souring techniques, such as kettle souring with *Lactobacillus* or using specific *Brettanomyces* strains in closed vessels, are designed to produce sour or funky beers without a pellicle. Pellicle formation is often influenced by factors like oxygen exposure, specific microbial strains, and the presence of complex carbohydrates in the wort. The absence of a pellicle doesn’t mean the beer isn’t sour or intentionally wild fermented; it simply means the conditions weren’t conducive to pellicle formation by the microbes present.