Mold and pink slime in a commercial ice machine
Pink or black slime inside a commercial ice machine is common, and it is not a mystery: it is microbial growth in the wet, dark parts of the machine. Under the food code, it is also a sign the machine has not been cleaned often enough.
Restaurant, bar and hotel ice machines — Hoshizaki, Manitowoc, Scotsman, Ice-O-Matic and similar. Countertop and refrigerator ice makers have the same problem but a different cleaning process: follow their manufacturer's instructions.
What you're looking at
| What you see | What it usually is |
|---|---|
| Pink, orange or red slime | A bacterial biofilm, most often Serratia marcescens |
| Black or dark green spots or slime | Mould |
| White, chalky crust | Scale (minerals) — not biological, but it gives growth somewhere to hold on |
Gwinnett County's water department describes Serratia marcescens as an airborne bacterium found in dust and soil, which grows "in any moist location where phosphorous containing materials or fatty substances accumulate". It's the same pink film people find in showers and pet bowls. It doesn't come from a bad water supply; it lands from the air and grows where it isn't cleaned away.
Where it grows
Not in the ice. It grows on the wet surfaces that stay above freezing: the water curtain, the trough and distribution tube, the bin walls and door gasket, the ice chute on a dispenser, and the drain. Check the underside of the curtain and the inside of the chute first — they're where it usually shows.
Is it dangerous?
Gwinnett County says Serratia marcescens "is not known to cause any waterborne diseases", though it has been linked to infections in a small percentage of people, mostly in hospital settings. Mould is a different matter, and neither belongs anywhere near something people put in their drinks.
The practical answer for a business is the food code. The FDA Food Code, adopted by most states, requires the enclosed components of ice makers and ice bins to be cleaned at the frequency the manufacturer specifies, or, where it doesn't, often enough "to preclude accumulation of soil or mold" (Minnesota's adoption of section 4-602.11). Visible slime is evidence that hasn't happened, and it's exactly what an inspector looks for.
Getting rid of it
The order matters: clean first, then sanitise. Sanitiser can't reach organisms protected under a layer of slime or scale.
- Discard all the ice. Don't use it.
- Run the machine's descaling cycle with the cleaner your manufacturer specifies. Scale gives slime somewhere to anchor.
- Take out the removable parts — curtain, distribution tube, trough where possible — and scrub them with a soft nylon brush in cleaner solution. Biofilm has to be physically removed; soaking alone often leaves it.
- Scrub the places parts don't come out of: bin walls and corners, the door and its gasket, the chute, the area around the evaporator.
- Rinse the cleaner out completely.
- Run the sanitising cycle and wipe or spray every surface you scrubbed, including the bin, with the manufacturer's sanitiser. Never mix cleaner and sanitiser.
- Reassemble, return to ice making, and discard the first batch.
Brand-specific cleaners, amounts and cycle times are in how to clean a commercial ice machine. Don't use bleach as a descaler, and don't use ammonia-based cleaners at all — Hoshizaki warns they damage the machine. Hoshizaki's KM instructions do specify plain bleach, in a measured amount, as the sanitiser.
Don't forget the drain
A drain line connected directly to the sewer can let bacteria back into the machine — Manitowoc's manual says exactly that, and requires an open drain with an air gap. If slime keeps returning around the bin drain, check the drain is properly air-gapped. See ice machine leaking water for the drain requirements.
Why it keeps coming back
- Not cleaned often enough. The manufacturers' minimums are once or twice a year; a machine that grows slime needs more. Ice-O-Matic allows descaling as often as monthly.
- What's in the air. The organisms arrive from the room. 86 Repairs notes kitchens with flour, yeast or grease in the air need air filters cleaned more often, and the same air feeds growth inside the machine.
- Less chlorine in the water. Gwinnett County notes the pink bacteria "can be made worse by removing chlorine with an activated carbon filter". Ice machine filters do remove chlorine — Manitowoc's cuts it by about 95% — for taste. That's not a reason to remove a filter, which protects against scale; it is a reason a filtered machine still needs regular sanitising. Manitowoc's Arctic Pure filters include a silver-impregnated carbon intended to protect machine surfaces.
- Only the bin gets cleaned. Wiping the bin while the curtain, trough and chute stay dirty just reseeds it.
Prevention
- Clean and sanitise on a schedule, more often than the minimum if you've had growth. Keep a dated log.
- Wipe and sanitise the bin and door between full cleans, and keep the bin door closed.
- Handle ice properly: a scoop kept in a holder outside the bin, never a glass or hands.
- Consider a growth inhibitor. Manitowoc's LuminIce recirculates air in the food zone over a UV light to inhibit yeast and bacteria. Manitowoc recommends replacing the bulb annually, and presents it as reducing how often cleaning is needed — not replacing it.