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I've got a 6 gallon plastic fermenter - one of those food-grade plastic buckets Midwest gives you in the starter kit. I guess after our last brew, my brother and I carelessly tossed the remains of our bag of bottlecaps in there without fully drying out the bucket. Brother just looked in there a few days ago and the caps are all rusty and there are some rust stains in the fermenter.

Obviously we can't use the caps now, but I'm more concerned about the fermenter. My gut tells me that if the stains do not come out with some water and a washrag they probably also won't come out into any beer we ferment in there, but I'm not sure. Should we just get another bucket just to be safe?

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    Have you tried getting the rust stains out yet? It should be possible, though you probably don't want anything too abrasive.
    – FrustratedWithFormsDesigner
    Oct 3, 2011 at 18:02
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    And by "probably," he means "definitely." If you scratch that stuff, it gives bacteria a place to hide from your chemical sanitizer. I've found magic erasers to be pretty good for getting stains out of these plastic buckets.
    – Dustin Rasener
    Oct 3, 2011 at 18:13
  • @FrustratedWithFormsDesigner - my brother is in possession of the fermenter now and I'm not sure how much he has actually tried cleaning it yet. Hopefully the stains will just come out easily and it won't be an issue.
    – Cassie Dee
    Oct 3, 2011 at 18:54
  • @DustinRasener: Would vinnegar work as well? I'd heard that the "magic erasers" contain some fairly toxic stuff, some apparently have formaldehyde in them. :(
    – FrustratedWithFormsDesigner
    Oct 3, 2011 at 18:58
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    @frustrated - soaking in vinegar with some aluminum foil in it can remove iron oxide stains to some extent (redox reaction), but I personally wouldn't bother. Same trick is used to polish siver-plated objects without removing the silver.
    – drj
    Oct 3, 2011 at 20:15

2 Answers 2

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Rust stains are from the iron oxide that formed (this much I assume you know) from the iron in the caps. These stains will not harbor bacteria or other "nasties" to a significant level, as these are large organisms when compared to the iron oxide molecule.

Trying to remove them by scrubbing can indeed scratch the plastic, but there is no scientific evidence that these scratches provide breeding sites for bacteria, contrary to popular myth, that can overcome the sanitizing action of iodine-based solutions (the most common base for brewing sanitizers). Any additional bacteria will be killed when sanitizing is done properly, as the chemicals will enter the scratches themselves.

Personally, I'd ignore the stains that might remain after a good cleaning with oxyclean and subsequent sanitizing with I-O-Safe or iodine tablets and use the bucket without scrubbing to remove the stains.

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  • Yeah I wasn't as much worried about bacteria as possible off tastes or other undesirable effects. I think you're probably right that anything that remains after a good cleaning is nothing to worry about though.
    – Cassie Dee
    Oct 3, 2011 at 20:05
  • I can't speak to off-tastes, but I know that I have used both glass and approved plastic (certified not to impart flavors to contents, specifically made for purified water) carboys and have noticed no discernible difference in flavors over a couple of years use.
    – drj
    Oct 3, 2011 at 20:09
  • I think I can safely say that where people get into trouble is when they do not sanitize properly before starting the fermentation.
    – drj
    Oct 3, 2011 at 20:11
  • You specifically mention iodine-based solutions. Can you speak to the efficacy of Star San with respect to scratches in plastic equipment?
    – Dustin Rasener
    Oct 3, 2011 at 20:19
  • I don't use StarSan, so I really can't. I do know that iodine-based sanitizers like I-O-Safe would easily kill any bacteria/yeast lodged in scratched in plastic. Most petri dishes used today are plastic and these are routinely sanitized in iodine solutions, when autoclaves are not available.
    – drj
    Oct 3, 2011 at 20:56
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I did the same thing a while back (actually a few times)

Just use a sponge to get the rust marks off then sanitize like normal.

I have brewed many batches since without ever getting sick.

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