Mr Wizard in BYO says below that mould on malt can be pretty nasty. Note that aflatoxins could potentially grow on malt, and the bad thing about them is they dont just make you sick, they potentially cause cancer as well. Not trying to stir things up, and I aint no microbiologist, just pointing out that while in many cases I might eat something after scraping the mould off, I wouldnt be too keen on brewing with a mouldy malt.
From BYO website
Dear Mr. Wizard:
I bought some malt extract in 10-pound pails. I recently noticed that mold has developed on top of the malt. Has this ruined my extract or will the mold be eliminated in the boil? Can I scrape the top layer off and use what is left? Will refrigeration of the malt prevent mold? Please help!
John O'Brien
Via e-mail
Mr. Wizard replies: Malt extract and damp malt will grow mold. Moldy grain certainly should not be used for brewing and I personally would not use malt extract with mold on the surface, though some brewers do. Removing the mold from the surface of the extract may completely remove the mold from the container but then again it may not. Mold is bad for two main reasons.
Moldy grain is a known cause of gushing in beer. Certain molds, for example Fusarium species, excrete proteins that act as nucleation sites for carbon dioxide break-out in finished beer. In simple terms this means that when a bottle of beer is opened, the carbon dioxide uncontrollably breaks out of solution and a huge foamy mess gushes from the beer bottle. This is why it is called "gushing." The same thing could possibly result from using moldy malt extract.
Another reason to avoid using moldy malt or malt extract is that certain molds produce mycotoxins (toxins from mold) when they grow. Although many mycotoxins are completely destroyed when heated, some mycotoxins become more toxic when heated, as is the case with certain types of aflotoxin. This same concern applies to eating moldy foods. Not all molds are bad and some add a very nice flavor to food, such as Penicillium roqefortii that is used to make blue cheese.
The mold growing on your malt extract is most likely an airborne mold that came into contact with extract when you first opened it for use. Refrigeration will certainly slow the growth of mold and will extend the shelf-life of pails that are opened and only partially used. However, molds will grow in the refrigerator given enough time. Mold growth can be prevented on grains by storing grain in a dry environment.
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