Mold and Slime in a Nugget Ice Maker

This question is asked constantly and answered almost nowhere. Search for it and you will find brand support pages for one machine, forum threads, and short videos. Nobody explains the mechanism, which is a shame, because once you understand it the prevention is obvious.

The four conditions, all of which you supply

• Water that stays put. A machine left with a full reservoir for a week is a small warm pond.
• Room temperature. The reservoir is not refrigerated. Only the freezing cylinder is cold; the water waiting to be frozen sits at whatever your kitchen is. The original Opal manual specifies operation up to 90F, and biofilm is happy anywhere in that range.
• Organic material. It gets in from hands, from scoops, from an open bin lid, from the air. It does not take much.
• Surfaces to grow on. Plastic tanks, tubing, the bin. Scratched plastic is worse than smooth, which is why abrasive cleaning makes the next occurrence more likely.

Slime versus scale

Removing it

1. 1. Stop using the ice

   Throw away everything in the bin. Do not rinse it and reuse it.

2. 2. Unplug and strip the machine

   Power off at the outlet, then remove the ice basket or drawer, the water tank, and the internal baffle if your machine has one. Frigidaire's troubleshooting section specifically lists a missing internal water tank baffle as a cause of a machine not making ice, so note how it comes out before you take it out.

3. 3. Wash every removable part

   Warm water and diluted detergent with a soft cloth, per Frigidaire's published instructions. Get into corners and around seals, which is where biofilm actually lives.

4. 4. Run a full cleaning cycle

   This is the part that matters, because it circulates cleaning solution through the tubing and pump you cannot reach with a cloth. Use the product your manual names and give it the dwell time the instructions specify.

5. 5. Rinse twice

   Fill with fresh water, run a full ice cycle, discard the ice. Then do it again. Two discarded batches is not excessive after a cleaning chemical has been through a recirculating water path.

6. 6. Dry it before it goes away

   Leave the lid open until the interior is dry. Putting a damp machine into a cupboard is how you get to do all of this again in three weeks.

The one habit that prevents most of it

Empty the machine when you are not using it. Drain the reservoir, empty the bin, leave the lid open.

That single habit removes the standing water, which is the condition everything else depends on. It is more effective than any cleaning product and it costs thirty seconds. Everything below is secondary to it:

1. Use filtered water rather than straight tap water where your supply is hard or heavily chlorinated.
2. Change the reservoir water rather than topping it up. Topping up means the oldest water never leaves.
3. Keep the lid closed while it runs, and keep hands and unwashed scoops out of the bin.
4. Clean monthly whether or not you can see anything. Biofilm is established before it is visible.
5. Descale on your own water's schedule, because scale gives biofilm more surface to hold on to.

If it keeps coming back

Recurrence within weeks of a proper clean usually means one of three things: the machine is being stored with water in it, the cleaning cycle is not reaching the whole water path, or something has been scratched and now holds biofilm however well you clean it.

The first two are fixable habits. The third is worth knowing about before you spend money on a fourth bottle of cleaner. And if the symptom is taste rather than visible growth, start with why the ice tastes wrong instead, because scale and stale water produce that too.