Many modern fridges have a small, shallow tray at the back or underneath, where the water from the fridge's own defrosting ends up. Normally the water simply dries away, but the tray collects dust and food crumbs over time, and the little channel that leads to it can clog. The result is a stale, sour smell that seems to come from nowhere, and sometimes a damp patch on the floor beneath.
Before you do anything, switch the fridge off at the wall and pull out the plug. Then slide the fridge forward slowly, taking care not to drag it across a delicate floor. Keep an old towel ready and ask someone to help if the fridge is heavy. Check the instruction booklet if you are not sure where the tray sits, because the position varies between models.
If the tray lifts out easily, carry it level to the sink and pour away any water. Wash it with warm water and a little washing-up liquid, rinse it well and dry it thoroughly with a clean cloth. If it does not come out without force, leave it where it is and wipe it as best you can with a damp cloth, rather than risk damaging a fitting that you cannot see.
Inside the fridge as well
Open the fridge and look at the back wall, low down. Many models have a small drain hole there, and it is the other half of the problem. Food residue can settle in it and block the flow, so the meltwater backs up and starts to smell. Clear the hole gently with a cotton bud or a pipe cleaner, then wipe the surrounding area with a damp cloth.
While the fridge is pulled out, brush or vacuum the dust from the back and the floor behind it. Wipe the door seal too, since spills there often go unnoticed. When everything is dry, put the tray back, push the fridge gently into place and leave a small gap at the back so the air can move freely.
Make this a habit whenever you have a general clean of the kitchen. A quick look now and then is much easier than dealing with a smell that has been building up for months, and the rest of the fridge will stay fresher too.
