If you see white or brown spots on the appliance (heating element in the base, baskets, lid) or on your sterilised items this means scale has built up there.
This is normal and occurs more quickly in hard water areas.
Note: In the UK you can find hard water with the equivalent of 200mg of calcium carbonate (limescale).
In hard water you also find a high mineral content which could affect its colour.
Heating causes the dissolved calcium and magnesium bicarbonates to break down to form a solid carbonate limescale.
This is harmless to health but does not look nice and causes deposits in hot water systems.
Pure scale from hard water is white in colour, but scale can become coloured by traces of other minerals or metals.
For instance, traces of chopper in plumbing can turn it grey, green or blue while traces of rust from the water pipe network could turn it to brown or black.
Descale the sterilizer to ensure that it continues to work effectively.