Material shouldn't matter, but I'll offer an interesting anecdote.

For many years, I knocked into a circular plastic food storage container that lost its lid, so got repurposed as my knockbox. It was (a) not a very good knockbox; and (b) a mold magnet. I admit that I would wait until the thing was overflowing until I emptied it, and it *always* made for a pretty neat science experiment starting under the topmost pucks and going all the way down. I always do 2 doubles a day, so it might have went ~5-7 days between being emptied.

About 2 years ago, I had some gift cards to spend, so splurged and got myself one of the small Motta stainless steel knockboxes. It is about the same size to the makeshift knock box I had been using, and my habits remain the same (emptying about every ~5-7 days). Now, the Motta knockbox (a) is a pretty good knockbox; and (b) rarely has mold in the bottom, and if it does, it's a very small amount that shakes right out with the spent pucks.

I know material shouldn't matter, but my habits have remained the same, and I continue to live in the same place (Bay Area, with moderate humidity). So, why the difference? I don't know. Maybe the pucks fall in a way that leaves more space between them for air circulation; maybe spores adhere less to the stainless steel than they did to the plastic; maybe the Motta factory casts an anti-mold magic spell before shipment. Who knows. What I do know is that I rarely wash my knockbox now, and in fact only rinse it infrequently (mostly just shake it out), and my science experiment days are over.

Your mileage may vary.

Tom,

If you have a the bigger diameter one, maybe the extra space allows the pucks to dry out quicker to inhibit mold growth?

My knockbox is stainless steel but it is a drawer-style so probably trapping the moisture from the puck inside is a big contributor...

jrham12 wrote: If you have a the bigger diameter one, maybe the extra space allows the pucks to dry out quicker to inhibit mold growth?

It is the 11cm diameter model (i.e., the small one)*. I think there are two realistic possibilities (the magic spell not being one of them). First, the plastic container I previously used didn't have a knock bar in the middle, so the pucks might have unintentionally been pressed down more forcefully and therefore compressed into the container. With the narrow diameter and the knock bar, the pucks can only fall naturally into the Motta, which probably leaves some space for air circulation (I agree, a drawer-style box would prevent airflow and promote mold growth). The second possibility is that I used to scrub the plastic container with a scouring pad (to remove the mold growth). This might have contributed to a vicious cycle: roughing up the surface may have made it more conducive to trapping mold spores between washes. I haven't used a scrubby pad on the steel knockbox because I've never needed to -- maybe this is a good indication that I shouldn't.

Another thing I just thought of is that the new knockbox was purchased around the time I got a new machine (within a few months). It's possible that the pucks may be imperceptibly less wet with the newer machine, but the difference is enough to matter in terms of mold growth. Again, who knows...


[*before anyone interprets this as a recommendation for the small Motta knockbox, you should know there are two things I don't like about it. First: the edges are sharp around the top, but this is minor and an easy fix. Second: there is a foam pad on the bottom that is made with open cell foam. It absorbs any water, and would therefore be unsuitable for some surfaces. It could probably be scraped off and replaced, but I haven't done so.]

I have a Cafelat "classic" style SS knock box for maybe 10+ years. I dump it every 3-4 days, usually wash with soap and a dish brush (takes 20 seconds), turn it upside down to drain the excess water out of it, and put it back next to my espresso machine. Never had mold.

That's been pretty much my experience also except my knockbox has spent the last 12 years in a wooden drawer in my coffeebar. Even though we get plenty of humidity in the summer months I think I've only found mold on the pucks twice in all that time. The plastic bucket I empty the knockbox into gets dumped about once a week & that usually has a layer of the creamsicle mold growing on the pucks.