Fact or Fiction: Beer and Moulds – More There Than You Think

I interrupt my Fifty Meter Beer Project for one of my “classical” posts on brewing contaminants. Moulds (molds for my American friends) are one of the more common microorganisms in the environment. They are the “universal decomposers” that ultimately dispose of pretty much all other lifeforms once they pass on. Because of this, moulds are commonly found in most environments – and that includes in the brewery.

Conventional brewing wisdom holds that moulds are only really an issue when brewing with fruits, or in sour beers where oxygen isn’t controlled sufficiently well. As it turns out, this isn’t correct, and moulds are more common than we expect.

What Are Mouds/Molds, and What Are Their Risks?

I’ve actually written a large article on this previously, so if you’re interested in this topic I’d suggest that you check it out: Fact or Fiction? Can Pathogens Survive in Beer – Mould Edition.

The Coles-notes version is that moulds are fungi that feed and grow by breaking down (decaying) organic matter. Many are harmless, but some can produce toxic metabolites called mycotoxins that can have a range of negative effects from driving allergy, to liver failure, to cancer. Even the harmless moulds are often unwanted guests in the brewery as, while harmless, they can produce compounds with unpleasant aromas and flavours that can taint a product.

How Common Are Moulds in Brewing?

You can only find what you look for is an old adage in microbiology. And, because brewers long assumed that moulds were obvious and only appear in barrel-aged and fruited beers, that they were a relatively rare occurrence.

Turns out that adage is correct. The application of modern genetic approaches which allow you to sequence identifiable portions of the genomes of everything present in a sample has really changed our perspective on what occurs in the brewery. I’ve written previously about how some of these studies have shown that wild ales are far more microbiologically complex than the tetral ensemble of Saccharomyces, Brettanomyces, Lactobacillus, and Pediococcus that are commonly associated with this style of beer. Unfortunately, many of these studies focus on the yeast and bacterial components of what they find, meaning that to find information on the moulds they detected often involves digging into the raw data – often even into the raw DNA sequences – which isn’t exactly practical.

Enter Piraine, Leite and Bochman, who in 2021 performed one of these sequencing experiments, and took the time to specifically analyze and quantify the moulds present in some wild ales. Turns out, moulds are pretty common in these samples, with over ten species found in their analysis. This is the first (and as far as I know, only) in-depth analysis of moulds in wild ales, but its finding is stark – they are present, in significant numbers. Even more surprising, they found at least one mushroom species in their analysis! It was probably a xylophagous (wood-eating) mushroom in the barrel, but bloody hell, there’s mushrooms in (some of our) beer!

Of the moulds, Penicillium was the most common genus found. This is not overly surprising, as the Penicillium are some of the more abundant food-contaminating moulds out there. They are also undesired – several make compounds which some people are allergic to, and they produce a number of metabolites that produce stale off-flavours. The authors didn’t dig into whether any of the fungi they identified produced mycotoxins, but they did find fungi form genera which have member species known to make mycotoxins, including Aspergillus and Capnodiales.

Outside of beer brewing, it is becoming increasingly aparent that moulds are present during alcoholic fermentation. Wines are often contaminated with mycotoxins from moulds which grow on grapes and which may be present in the fermenter. Other studies have found moulds present in fermenting wine, although they didn’t identify these down to the genus or species level.

But to come back to the question which started this section – how common are moulds in brewing – the answer is “they are there, but we don’t know how common they are”. Most of the work showing moulds as common place in the brewery come from the last decade, with most being under 5 years old. We’re still in the early days of this science, and so it is still unclear how common moulds are, and which moulds are most often present.

Should I Worry?

I imaging that most readers are unhappy to learn that many of their favorite beverages are likely contaminated with moulds. But should we be concerned? And is there anything we can do about it?

The concern: Two concerns are raised by these findings. The first is simply one of beer quality – moulds can damage beer, and their apparent common presence indicates that they may be causing more spoilage than previously thought. The second concern is the bigger one – are they negatively impacting our health? At this time we lack the information to have an informed opinion on this topic, but in my opinion there is a ray of hope – we don’t see mycotoxin-associated diseases as a co-morbidity to excessive alcohol consumption. In other words, even in those who consume unhealthy amounts of alcohol, we don’t see signs of mould-caused disease. That doesn’t mean that there isn’t a contribution of these toxins to negative health outcomes, but those contributions are likely small in the grand scheme of things.

What can be done about it? The good news is that you can limit fungal growth in the brewery. I discussed this in my previous post, so I won’t go into a lot of details here, but the short form is taking care to prevent moulds is always best practice:

  1. Limit oxygen ingress into your fermenters. Moulds are obligate aerobes – meaning they require oxygen to survive. While some can survive on surprisingly little oxygen, anything you do limit oxygen ingress will help limit their growth. If using a carboy or stainless fermenter, make sure your lids are sealed tight, your airlock is filled, and that your airlock is tightly seated. Barrels are more challenging as wood allows oxygen through, but most moulds will want to grow on the surface of the beer, so by minimizing headspace and ensuring you use a quality airlock, you can limit their growth.
  2. Remove opportunities for moulds to grow. Moulds usually grow at the air-liquid interface of fermenters, or on the walls of the fermented next to the top of the liquid. Keeping carboy-shaped fermenters filled to the neck limits this area, limiting mould growth. “Punching down” fruits (pushing them from the surface of the fermenter) can interrupt mould growth, especially if formerly “sunk” fruit replaces the portion you punch down on the top of the fermenter.
  3. Clean, clean, clean. No surprises here. Good cleaning and sanitation keeps the moulds away. Fermenters should be cleaned and sanitized as soon as they are empty, as this will kill any existing contaminants and deprive any new ones of a food source. Re-sanitizing before you fill the fermenter can add an extra degree of protection.
  4. Consider pre-sanitizing ingredients added to the fermeter. Ingredients such as whole fruits can be treated with a mild peroxide solution prior to processing and addition to a fermenter, which can reduce the risk of moulds entering the fermenter. See my previous post for a procedure for this.
  5. Don’t be an idiot. Mould growth, if excessive, can be fairly obvious. Your eyes and your nose will give away their presence. And if present at the level where they’re easily observed, you should dump whatever they’re growing in as it is likely unpleasant and may be unsafe.

Detecting Moulds in the (Home) Brewery

There is a lot of bad advice (and even infographics) out there on how to detect moulds in fermenting products. These myths are based on the idea that moulds tend to be intensely pigmented (e.g. coloured something other than white or beige) and form fuzzy colonies. While this is true of some moulds, it is not true of all. To make matters worse, the way a mould colony appears when growing in one condition (your fermenter, for example) may be very different than how the same species appears when growing on a petri dish. In reality, mould colonies can be fuzzy, mucoid (look effectively identical to lactobacillus/pediococcus colonies), oblate/fusiform colonies (which look like yeast rafts), discoid (appear like small “coins” floating on the surface), or can form plaques (thin and easily broken “crusty” material). Colours can range from clear/white (again, just like Lactobacillus and Pediococcus), to beige (just like yeast), to intense greens, reds, blacks, and other colours.

Because of this, you cannot always identify a mould based on the appearance of a pellicle or other growth in a fermenter. If its fuzzy it is most likely a mould, but if its not fuzzy…it may still be mould. Practically, you need a microscope to tell moulds apart from yeasts – and even then, you’ll only know that moulds are present, but you will not know which mould(s) are present and whether they are a risk to you. Also, toxic metabolites and mycotoxins can persist after moulds have died off, so even the absence of moulds is not a guarantee. Again, taking care to prevent moulds is always best practice.

The images above show some good example of what yeast can look like (left) versus what a mixture of yeasts, moulds and bacteria will look like. The image on the left was taken at 1000X magnification, while the one on the right is at 400X, so keep in mind that the smaller yeast cells on the right are the same size (and indeed, the same species) as the yeasts on the left. The moulds are the long, filament-like cells in the right image (and the small ovals are bacteria). The moulds are quite distinct from yeast – much larger, and in this case with large vacuoles in some cells. Large vacuoles are not diagnostic though – some yeast have them, and some moulds will lack them.

Here’s another example of a mixed population:

Yeast and Moulds

In this second image the moulds are even more obvious, as they are much, much larger than the yeast and some are growing as hyphae (chains of interconnected cells).

In Summary

To sum everything up, moulds are more common than brewers think. If you have a true pellicle (not just yeast rafts), there are probably some moulds present. At this time there is no direct evidence that this “base level” contamination is harmful…but don’t be an idiot. Fairly simple approaches can be used to limit mould growth, and if you think you’ve got a bad mould contamination, dump it!

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