A question on Cooking SE brought up the fact that:

One study showed that 91.7% of green coffee beans were contaminated with mold. This is before they were processed, which allows even more mold to grow. Another study showed 52% of green coffee beans and almost 50 percent of brewed coffees are moldy. SOURCE

The studies quoted by the answer indicated that there might be some toxins that make it to the ground used in brewing coffee but did not address if there were any problems with toxins actually in the coffee.

I thought the question may be better addressed by our learned folks that share their knowledge and research here at Skeptics :)

  • 3
    Green coffee is unroasted. Wouldn't several minutes of exposure to 400+F kill mold? At that point, the coffee is typically vacuum-sealed to preserve freshness. Dec 22, 2011 at 22:51
  • Apparently, the idea is that the mold generates toxic chemicals (which aren't destroyed by heat) before it is roasted. Dave Asprey seems to think it's a big deal. I'm not sure myself... Dec 23, 2011 at 2:51
  • and even if those chemicals exist in the roasted beans, that doesn't mean they survive the boiling heat when the coffee is brewed (or the chemical preservatives used to preserve the roasted beans during shipping).
    – jwenting
    Dec 23, 2011 at 7:03
  • @ChrisHynes that's exactly how botulism works
    – Sklivvz
    Dec 23, 2011 at 10:18
  • 1
    @Sklivvz Botox is quite heat sensitive; I thought the issue with C. botulinum contamination were the endospores which could survive cooking and cause problems in those with weak immune systems (babies and honey). Fungal toxins may be different though; it would really depend on their nature (protein vs. small molecule for one).
    – Nick T
    Dec 24, 2011 at 6:44

1 Answer 1


Well, the original study is behind a paywall, however from the abstract we can easily deduce that the incidence of toxins is only 33% and in those cases, it's within the EU safety limits.

In other words, there's nothing to worry about: you need a certain minimum concentration of mycotoxins before you can experience any effect.

Twenty samples (33.3%) of 60 were contaminated with the toxin at levels ranging from 0.2 to 7.3microg kg(-1). The average concentration was 2.38 microg kg(-1). All positive samples showed OTA levels below the limit suggested by the European Union (8 microg kg(-1)).

More sources:

  • EU legislation
  • FSA legislation
  • Major literature review on mycotoxins which specifies that chronic effects of aflatoxins (the mycotoxins specified in the claim) are associated with cancer, immunodepression and liver disease, but makes no mention of other effects:

    The data on aflatoxin as a human carcinogen are far more damning than the data implicating it in acute human toxicities. Exposure to aflatoxins in the diet is considered an important risk factor for the development of primary hepatocellular carcinoma, particularly in individuals already exposed to hepatitis B.

    It also mentions that, in industrialised countries, aflatoxin ingestion is "insignificant":

    In developed countries, sufficient amounts of food combined with regulations that monitor aflatoxin levels in these foods protect human populations from significant aflatoxin ingestion.

  • 1
    I don't think Wikipedia is a good source for the claim of a minimum concentration of mycotoxins before they create an effect. It's the core of the claim of Dave Asprey who get's cited by Chad that small amounts of mycotoxins do have an effect.
    – Christian
    Dec 27, 2011 at 15:01
  • @Christian added more sources
    – Sklivvz
    Dec 27, 2011 at 15:32
  • I don't follow the "if it doesn't reach a lethal dose" there no effect argument. Nobody claims that there are enough toxins in brewed to kill people directly. Dave Asprey claims that the toxins have an effect in reducing mental and physical performance.
    – Christian
    Dec 27, 2011 at 19:08
  • @Christian I provided a better quote, it wasn't clear :-)
    – Sklivvz
    Dec 27, 2011 at 19:54
  • @Sklivvz - But your source says that 20 of 60 "were contaminated with the toxin at levels ranging from 0.2 to 7.3microg kg(-1)". I think this answers the question with a yes, though they are low enough that they will not affect a healthy person.
    – Chad
    Dec 28, 2011 at 14:19

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