Diagnostics & problems
Black grout is a diagnosis. The porous lines have absorbed grease, soil and water below the surface where mould takes root - and nightly mopping is often what drives it in.
The short answer
Grout goes black because it is porous - a hard, dry sponge that soaks up grease, food soil and dirty mop water and holds them below the surface, where mould then takes root. Mopping cannot lift it out; in fact nightly mopping is often what drives the staining in. It takes mechanical cleaning to pore depth, then sealing, to fix.
Why grout stains
Grout is softer and far more porous than the tile it sits between, and it sits slightly lower, so it acts as a channel that collects everything the floor picks up: splashed cooking grease, food debris, and the mineral content of hard water. Foot traffic then grinds those contaminants down into the open pores. Once grease is absorbed it goes tacky, traps dust and darkens, and the damp, organic pore becomes an ideal home for mould and odour-causing bacteria.
Here is the part that surprises operators: the nightly mop is part of the problem. A mop does not remove soil so much as spread it, like a paintbrush, and the dirty rinse water settles straight into the porous grout lines. Change the water too rarely and every mopping adds a little more grey. If every grout line in the room has gone dark - not just the corners - the mopping routine is usually the cause.
Why mopping cannot win
Left long enough, grease on a floor polymerises into a hard, caked layer that a mop simply glides over - it needs friction to break the mechanical bond. And the mould is not sitting on the surface: its roots and spores are down in the pores. That is why you can scrub a line, watch it lighten, and still see a grey shadow - the colony is below the reach of the brush. Surface cleaning treats the stain; the source is deeper.
A deep clean diagnoses which of these you have. It uses agitation - a stiff deck brush or a low-speed floor machine - with a heavy-duty degreaser given real dwell time, then lifts the slurry rather than mopping it back in. Where the grout has failed or the mould is entrenched, the honest answer is to re-seal with a food-safe sealer or regrout, so the pores stop drinking in the next spill. Sealing also starves the mould of the moisture it needs to return.
The limit
Mopping is necessary for daily upkeep but it is not sufficient - and by itself it is part of what turned the grout black. The staining is grease and mould ingrained into porous grout below the surface, and it only lifts with mechanical cleaning to pore depth followed by sealing or regrouting to stop the re-absorption. Read the black grout as a diagnosis: it is telling you the porous floor has been absorbing what the mop left behind, and it needs resetting properly before daily cleaning can hold the line.
Questions
Because mopping does not remove soil from porous grout - it spreads it and settles dirty rinse water into the low, absorbent grout lines. Grease, food debris and hard-water minerals soak in and mould takes root in the damp pores. If every grout line has darkened rather than just the corners, the mopping routine is usually driving the staining in rather than cleaning it out.
Much of it can be recovered with mechanical cleaning - agitation from a stiff brush or floor machine plus a degreaser with proper dwell time, lifting the slurry rather than mopping it back. Where the grout has broken down or mould is entrenched deep in the pores, regrouting or resealing is the honest fix. A survey tells you which you are dealing with rather than guessing.
Because the mould lives in the pores, not on the surface. You can scrub a line lighter and still see a shadow, because the roots and spores sit below the reach of the brush and grow back. The grease, too, may have polymerised into a caked layer a brush only skims. That is the signal that surface cleaning has hit its limit and the floor needs a proper deep clean and sealing.
Sealing a cleaned, dry grout line creates a barrier that stops the pores re-absorbing grease and water, which both keeps it cleaner and starves mould of the moisture it needs. It is not a substitute for a sensible cleaning routine, but it stops the floor working against you. Regrouting achieves the same where the existing grout has already failed.
It is both. Beyond looking neglected, ingrained grout holds grease, organic soil and mould, which feed odour-causing bacteria and can make a floor slippery and harder to keep clean. In a food environment that matters. Getting the grout properly deep-cleaned and sealed removes a genuine harbourage point, not just a stain.
Phoenix Duct Clean · by the numbers
We scrub grout to pore depth with agitation and degreaser instead of mopping soil back in, then advise on sealing or regrouting so the lines stop absorbing grease and mould.