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Mold (US) or mould (UK) are multicellular fungi that grow in the form of filaments called hyphae that appear like long branch-like structures under a microscope. Although not a taxonomic designation, there are an estimated 10,000 - 300,000 of species of molds that live in a wide variety of environments [1]. In general, they cause biodegradation of foods and other natural materials (such as wood in buildings), which has a wide role of effects depending on the species of mold. For example, some species of mold can be the cause of food spoilage, and other species play an important role in food production such as in cheese and soy sauce production [2]. Certain species of mold are thought to contribute precursors that affect flavor production in the toasting of oak barrels [3] Molds have also been used in the pharmaceutical industry to produce antibiotics and other medicines. Certain molds can cause allergic reactions as well as diseases from mold growth within the body or from mycotoxin production in exposed foods or from breathing spores from mold growth in buildings [2].

There are many discussions about identifying mold and what to do about mold if it grows in beer in the Milk The Funk Facebook group. Please read this wiki page and search the MTF Facebook group for previous posts about mold before posting a question about it so that we can limit the number of repetitive posts about mold (click here to do a search; must be a member of the MTF Facebook group).

Mold in Beer and Health Concerns

Mold Growth in Beer

Although little studying has been done on mold growth in beer, it has been known to occur in barrel aged beers are fruited sour beers. Oxygen generally needs to be present in order for mold to grow. Mold growth on fruit that is in sour beer can occur if the fruit is floating above the surface of the beer, and enough oxygen is present (either from vacuum suck-back, the oxygen permeability of the vessel, or sampling). For this reason, many commercial sour brewers will "punch down" the fruit frequently with a dish-like tool.

Mold Growth in Wild Mead

It has been demonstrated that while honey is antimicrobial due to its high sugar concentration and low water concentration, which inhibits the growth of microbes, honey is not inherently pasteurized or sanitary. Low levels of molds, yeasts, and spore-forming bacteria can survive in honey and can begin to grow once the honey is diluted with water as it is in mead-making. Honey can start to ferment when it contains as little as 17% water, and at 19% water honey is very likely to ferment. It is thought that most of the microbes found in finished honey originate from the handling of the honey, and do not originate from the hive, although some microbes may be introduced from the bees [4][5], including botulism spores that can cause death to infants under 12 months old [6][7]. While normal mead making procedures limit the potential of mold growth due to pitching healthy yeast and a fast start to fermentation, it is possible that mold growth can occur when attempting to ferment mead spontaneously by not adding any laboratory yeast because of the lag time for fermentation to begin [8]. To prevent mold growth during spontaneous fermentation of mead (also known as "wild fermentation" or "native fermentation"; see Mead), purging the vessel with CO2 until the mead begins actively fermenting should help reduce the chances of mold growth. Lowering the pH to 4.5 will also help prevent mold growth, as well as a starting gravity at or below 1.070 because higher gravity must takes longer to start fermentation, which gives more opportunity for mold to grow before fermentation begins [9].

See also:

Preventing Mold Growth

  • If aging sour beer on whole fruit, use a punch-down tool frequently so that the low pH kills any mold that might be present before it begins to grow. CO2 may be helping the fruit to float, and punching-down the fruit will help to release that CO2. Homebrewers can gently swirl their carboys to accomplish the same thing. Purging the headspace with CO2 will also help prevent mold growth.
  • Limit oxygen exposure to the beer.
  • Use diligent sanitation practices.
  • Inspect barrels for mold prior to use; discard barrels that have mold growth, or disassemble the barrel and vigorously clean/sanitize the wood before use. See the Barrel page for details on storing and maintaining barrels.
  • Mold is tolerant of both hops and alcohol levels above what is in beer (most are killed at ~28% ABV, but some can survive up to 50-60% ABV), so do not rely on hops or alcohol to prevent mold growth [10].
  • Keep relative humidity levels under 60% and preferably between 30-50%. This level of relative humidity also falls into the preferred levels for oak barrel environments [11].

Health Concerns

A small number of molds can produce poisonous substances called mycotoxins, or cancer-causing poisons called aflatoxins. Some molds that do not produce toxins can still cause allergic reactions or respiratory problems [1]. Their effects are cumulative, rather than immediately toxic as in the case of pathogenic bacteria. A small amount of ingested mycotoxin won't be harmful right away, but it will increase the risk of gastric, intestinal, and colon cancer, as well as other health issues [12]. As a general rule of thumb, pigmented molds (non-white/grey) are more likely to produce poisonous substances (especially black mold which could be Aspergillus), however, there are exceptions to this guideline [13]. Some mycotoxins can survive boiling temperatures in wort to varying degrees, so if mold develops during processes such as kettle souring, it is possible for some percentage of mycotoxins to survive the wort boiling process and fermentation (one study showed that 20-30% of two mycotoxins were removed during boiling and fermentation) [14]. Only mold that is growing in contact with the beer itself is considered potentially dangerous; mold growing in airlocks or outside of vessels should not be a cause for concern as long as they do not come into contact with the beer itself. If only a small amount of mold has grown on the beer, then removing it quickly and purging the vessel with CO2 to prevent more mold growth will limit the potential for exposure to toxins. However, if a large amount of mold has grown and if it happens to be a mycotoxin-producing species, then there is a higher chance that more toxin could be in the beer [13].

Identifying mold based on what it looks like is not a reliable way to determine if a mold is a cause of concern for health [13]. There are mold testing labs that are available that might be able to identify whether the mold is toxic or not. If mold is present in beer, commercial breweries should remove the mold and send it to a lab to check if it is a toxic mold before allowing the beer to be consumed. Some homebrewers may decide to risk taking a chance that the mold will not cause illness, however, we recommend discarding beer that has come into contact with the mold, especially if serving to others. Soft fruits and vegetables with high moisture that grow mold on the surface can also be contaminated with mycotoxins or aflotoxins below the surface, therefore beer or wort that has mold growing on its surface could also be contaminated below the surface [1].

See these resources on mold and mycotoxin:

MTF threads:

Identifying Mold

As mentioned above, identifying the species of mold based on what it looks like with the naked eye is not a reliable way to identify it at the species level, and thus not a reliable way to know if the mold growth is safe. It is, however, usually easy to tell the difference between general mold growth and pellicle or yeast growth. Mold tends to look "fuzzy" or "hairy" and often has green, red, black, white, or grey colors, whereas yeast activity or pellicles tend to have evidence of CO2 production (bubbles) and look "creamy", "powdery", or "chalky" and is never green, red, or black in color (unless there is material in the beer that is discoloring the pellicle, such as fruit). Mold tends to grow in isolated spots on the surface, while yeast pellicles tend to grow throughout the surface. Sometimes the pellicle is not a solid pellicle, but instead just scattered across the surface in a spotty yet more widely distributed manner compared to mold which would grow in a more isolated area. Protein/carbohydrate coagulation can also be misidentified as mold, however, this also never appears "hairy" or "fuzzy" like mold. We recommend taking a picture through a hand lens when posting an image of what might be mold as this will make it easier to identify. Be sure that the color of the mold/pellicle in the photo is accurate to what it looks like with the naked eye (sometimes cameras make white substances appear more pigmented, which can throw off attempts to identify the organism online). Please make an attempt to identify whether the growth is mold or not on your own before posting in the group (these types of posts clog the stream!). See the images below for examples that should help. Mold can be removed and sent to a mold testing lab to possibly be identified as toxic or benign.

Mold vs Pellicle Examples

The following table shows examples of mold or pellicles from the Milk The Funk Facebook group, and the consensus on whether mold is likely present or not based on the image.

Also, search the Milk The Funk Facebook group for examples of identifying mold versus yeast activity.

Mold or Pellicle? Determination
Mold ex1.jpg Pellicle or yeast fermentation activity, not mold.
Mold ex2.jpg Undetermined, but the brewer shook the carboy and the mass went away, so probably not mold.
Mold ex3.jpg Undetermined; the small green area in the middle of the pellicle might be mold.
Mold ex4.jpg Occurred in a clean dubbel after adding prunes; pellicle, not mold.
Mold ex5.jpg Mold growth during kettle souring (Lactobacillus was the only microbe pitched into the wort).
Mold ex6.jpg Mold growth on top of blackberries.
Mold ex7.jpg Small amount of pellicle, not mold.
Mold ex8.jpg Green and fuzzy mold.
Mold ex9.jpg Mold growth on an oak barrel bung. Beer is ok if the mold is scrubbed and removed; recommended to use rubber or silicon for the bung instead.
Mold ex10.jpg Pellicle on top of green grapes, not mold.
Mold ex11.jpg Mold growth on fruit that is floating above the surface of the beer.
Mold ex12.jpg Partial pellicle on a kettle sour; probably a wild yeast contamination from the added fruits. Not mold.
Mold ex13.jpg Mold growth from under-pitching with only one bottle of commercial beer dregs.
Mold ex14.jpg Brettanomyces activity, not mold.
Mold ex15.jpg Probably mold or another potentially unwanted microorganism from an attempt at collecting wild yeast from dried camomile.
Mold ex16.jpg Partial pellicle, not mold.
Mold ex17.jpg Pellicle, not mold.
Mold ex22.jpg Fermentation from a turbid mashed spontaneous inoculation; not mold.
Mold ex18.jpg Image of mold through a hand lens; photo taken with an iPhone by Mark Trent.
Mold ex19.jpg Identified in a lab to be Penicillium (considered a mold).
Mold ex20.jpg Mold in a barrel that made the beer taste like soy sauce; potentially Aspergillus (some can be carcinogenic).
Mold ex21.jpg Mold growth on an agar plate from swabbing persimmon in an attempt to catch wild yeast; probably not a dangerous mold.
Mold ex23.jpg Mold growth during kettle souring. Possibly Penicillium or Aspergillus; neither are good.
Mold ex24.jpg Mold growth mostly in the neck of the carboy. According to the brewer, this was identified as mold by an environmental testing lab. Since most of the mold wasn't in contact with the beer directly, it can probably be racked from under the fruit and safe to consume.
Mold ex25.jpg Spotty pellicle growth evenly throughout the fermenter; not mold.

See Also

Additional Articles on MTF Wiki

External Resources


  1. 1.0 1.1 1.2 "Molds On Food: Are They Dangerous?" United States Department of Agriculture. Retrieved 07/15/2016.
  2. 2.0 2.1 Mold. Wikipedia. Retrieved 07/12/2016.
  3. The duration effect of natural seasoning of Quercus petraea Liebl. and Quercus robur L. on the diversity of existing fungi flora and some aspects of its ecology. N. Vivas. 1996.
  4. Microorganisms in honey. Jill A Snowdon, Dean O Cliver. International Journal of Food Microbiology. Volume 31, Issues 1–3, August 1996, Pages 1-261996.
  5. Microbiological and parasitological quality of honey produced in İstanbul. Emek DÜMEN, Hayrettin AKKAYA, Gülay Merve ÖZ, Funda Hatice SEZGİN. 2013.
  6. Incidence and Origin of Clostridium botulinum Spores in Honey C. N. HUBTANEN, D. KNOX, and B. SBIMANUK. 1981.
  7. "Infant and toddler health". Jay L. Hoecker, M.D. Mayo Clinic website. May 15, 2018. Retrieved 09/20/2018.
  8. Adam Johnson. Milk The Funk Facebook group post on preventing mold growth. 06/21/2018.
  9. Justin Amaral. Milk The Funk group thread on mold in spontaneously fermented mead. 06/23/2018.
  10. Dr. Bryan Heit of Sui Generis blog. Milk The Funk Facebook group. 04/21/2017.
  11. "Mold". US EPA website. Retrieved 08/08/2017.
  12. Dr. Bryan Heit Heit. Milk The Funk Facebook group thread on the health risks of mold. 06/19/2018.
  13. 13.0 13.1 13.2 "Fact or Fiction? Can Pathogens Survive in Beer - Mould Edition." Dr. Bryan Heit of Sui Generis Blog. 08/11/2016. Retrieved 08/11/2016.
  14. Stability of mycotoxins during food processing. Lloyd B. Bullerman, Andreia Bianchini. 2007.