This question is not about molds that affect coffee plants. I am asking about molds that grow in coffee that has been sitting for a few days or more. I did not find any scientific articles on point so I have begun to try to answer the question by observing; but I may have missed an article and someone else may know the answer already.

Roasting probably kills any mold in coffee beans and if molds grow in a cup of brewed coffee they come from the surrounding air. After leaving many partially-filled cups in various spaces and looking at molds that grew in them, Aspergillus, Penicillium, and Aureobasidium are the only molds I have seen under the microscope so far.

If someone has seen an authoritative article about this it would interest me. Thanks. (This is not a health issue, IMO. I'm just curious.)

  • I think (but do not know) that molds colonize coffee before bacteria and tend to suppress the growth of bacteria. That's why I am asking about molds and not bacteria.
    – daniel
    Aug 7, 2015 at 11:23

2 Answers 2


Though I don't have any articles on the matter, I can certainly say I've found cups of coffee that I had forgotten that are only maybe a week old with mold growing in them. I'd assume it gets contaminated so quickly, because of its sugar content. It might just be a coincidence, but it seems to happen more readily with coffee that I brew over ice. I press my areopress directly over ice, I've heard this reduces the acidity. Which would make sense, as acidity usually wards off mold. I leave all of my coffee black and unsweetened, so it is not dairy or added sugars that are causing this.

Perhaps try variating your brewing process. I'd be especially keen on the pH. Maybe there are some spores which will not grow in environments below a certain pH.

  • Low molecular-weight sugars in coffee are almost totally degraded by roasting (see Redgwell and Fischer, Coffee Carbohydrates, Brazilian J. of Plant Phys., 165-174, 2006 at 172). Also I think that the variation of acidity in brewed coffee doesn't deter molds any more than it deters people. You could change the pH but I am mainly interested in molds that grow in coffee that humans would find drinkable, and whether ones other than those I mentioned ever appear. As for the time to appear--I think you are right, about a week in warm weather.
    – daniel
    Jul 21, 2015 at 17:03

I am severely allergic to Penecillium Mold. Around a year and a half ago, I began drinking coffee on a daily basis. I had my french press brewing set up perfectly to have a nice mug every day. After about 6 weeks, I had an allergic reaction. Mold spores cause a reaction in allergic people whether they're alive or dead, so the previous answerer is wrong about being dismissive about the mold being killed in the roasting process.

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