Dishwasher Hygiene: Three Targeted Measures for a Cleaner Dishwasher

It may seem surprising that the appliance designed to clean our dishes can itself become a breeding ground for germs. Studies show that bacteria, fungi and persistent biofilms colonise virtually every household dishwasher, particularly in rubber seals and hidden corners. However, three targeted steps can substantially reduce the microbial load.

When the dishwasher becomes a home for germs

Extreme conditions prevail inside a dishwasher: very hot water, harsh detergents, changing levels of moisture, a high pH, salt and pronounced temperature fluctuations. These conditions are too much for many microorganisms, yet a number of specialists cope remarkably well.

Researchers have looked more closely at precisely these survivors. A 2018 study examined 24 household appliances. Biofilms were found on the rubber seals in every one: slimy layers of bacteria, yeasts and fungi that form a protective shield over the surface.

Biofilms act like a fortress: within their slimy layer, germs withstand heat, detergents and short wash cycles far more effectively.

Species from the genera Pseudomonas, Escherichia and Acinetobacter appeared particularly often. These groups include so-called opportunistic pathogens. Among fungi, yeasts from the genus Candida predominated and were detected in all appliances examined.

Black yeasts: inconspicuous but exceptionally resilient

The rubber door seal provides an ideal surface for microbes: it is rough, continually damp and covered with traces of detergent and food. This is also where so-called black yeasts thrive. One well-known species is Exophiala dermatitidis, a fungus able to tolerate very hot, salty and alkaline environments.

Measurements indicate that up to millions of colony-forming units of these fungi can be present on one square centimetre of seal. The risk for healthy people is generally low. However, people with severely weakened immune systems should pay particular attention to hygiene inside the appliance and should not disregard very old, musty or visibly black deposits.

Why low temperatures make the problem worse

Many households now use almost exclusively economy programmes at 30 to 45 degrees to save energy. While this reduces electricity bills, it also makes it easier for biofilms to survive because they are not heated sufficiently.

At the same time, grease and food debris build up in filters, on the walls and in the spray arms. There, they supply microbes with food, allowing the slimy layer to grow and become more stable with every wash cycle.

• Low temperatures: clean dishes only to a limited extent and have little effect on biofilms.
• Long periods of dampness: encourage bacterial and fungal growth.
• Food residues: provide an ideal source of nutrients.

What happens when you open the hot door

After a cycle, many people open the dishwasher straight away while it is still steaming. A burst of hot air and fine droplets then escapes from the interior. Researchers refer to a possible “aerosol cloud” that may carry spores and bacteria into the kitchen air.

A 2024 study points to this spread of microorganisms, although the precise health benefit of stricter hygiene measures still requires further research. One point is clear: damp, warm surfaces, whether a sponge, washing-up brush or seal, harbour considerably more germs than materials that are allowed to dry out properly between uses.

Leaving the door slightly ajar and allowing the dishwasher to dry thoroughly deprives many germs of the conditions they need to survive.

Three targeted measures for a noticeably cleaner dishwasher

1. Clean seals thoroughly

The rubber door seal is the key hotspot. It traps everything that collects in crevices and folds. A simple but effective routine is as follows:

• Remove visible crumbs and deposits from the seal.
• Dip an old toothbrush into a mixture of hot water and clear household vinegar.
• Scrub the seal firmly with the brush, including the underside and corners.
• Wipe it dry with a cloth so that moisture does not remain in the folds.

Vinegar contains acetic acid, which attacks the alkaline film left by detergent residues and weakens fungal structures. Many people also use a little black soap or mild washing-up liquid to remove grease more effectively.

2. Clean the filter and base regularly

The base of the dishwasher, including the filter, is easily overlooked, yet it collects all the dirt from a wash cycle. Failing to clean it gives microorganisms an abundant food source.

• Remove the filter once a week and discard larger debris.
• Rinse the filter thoroughly under the hottest water possible, using a soft brush if needed.
• Remove streaks and deposits from the base of the tub.
• Finally, sprinkle a little bicarbonate of soda over the base; it absorbs odours and has a mild effect against fungal growth.

Anyone who regularly loads heavily soiled dishes should carry out these steps even more often. This also lowers the likelihood of unpleasant odours coming from the appliance.

3. Spray arms and a hot empty cycle

The spray arms ensure that water and detergent reach every area. Their fine nozzles can, however, become blocked easily by limescale, grease or small food particles. This creates dead zones inside the appliance where germs can persist undisturbed.

A useful monthly routine:

• Remove the spray arms from the appliance according to the instructions.
• Rinse them through under running water.
• Carefully clear blocked nozzles using a toothpick or thin wire.
• Then run an empty cycle at no less than 60 degrees, ideally with around six tablespoons of citric acid inside the appliance to descale the pipes and metal components.

The high temperature destroys many heat-sensitive bacteria. At the same time, citric acid removes limescale deposits where germs readily settle.

How often should each task be done? A quick guide

Who should pay particular attention to dishwasher hygiene?

For most healthy people, the germ layer in a dishwasher does not pose a dramatic everyday risk. Consistent maintenance is nevertheless worthwhile where young children, older people or those with serious pre-existing illnesses live in the household.

People with allergies or chronic lung conditions can also be more sensitive to high levels of mould and spores indoors. Visible black deposits on seals or a musty smell from the appliance should therefore be treated as a warning sign.

Anyone who treats their dishwasher as a household appliance “with a maintenance schedule” reduces germ levels without any specialist chemicals.

The role of detergent, salt and rinse aid

Modern tablets and powders generally clean reliably. Even so, dosage matters: too little detergent allows grease and deposits to adhere more easily, while too much leaves residues that can supply biofilms with nutrients.

Dishwasher salt keeps the built-in water softener working, prevents limescale layers and therefore removes niches for germs. Anyone with extremely hard tap water should not allow the salt reservoir to run empty. Rinse aid mainly produces sparkling glasses, but it also helps water drain away, which in turn speeds up drying inside the appliance.

What many people forget: behaviour after the wash cycle

What happens during the hours after a wash is at least as important as the cycle itself. If the dishwasher remains fully closed, warm, damp air is trapped inside, creating ideal conditions for microbes.

• Open the door slightly once the programme has finished so that steam can escape.
• Do not leave cutlery and dishes wet in the dishwasher for days.
• Scrape heavily soiled dishes first rather than washing whole food leftovers through the machine.

These simple habits significantly reduce the ecosystem inside. Many people notice the difference first by smell: the typical “dishwasher smell” usually disappears after just a few weeks of regular maintenance.

Dishwasher hygiene does not need to become a major undertaking. Keeping seals, filters and spray arms in mind, scheduling an occasional hot empty cycle and not closing the door tightly again immediately after washing takes away the comfort biofilms need and helps ensure the appliance delivers genuinely clean dishes.