How to Clean a Kitchen Sponge, What USDA Research Found

A kitchen sponge wipes counters, scrubs pans, and soaks up spills, and then sits damp by the sink between jobs. That makes it useful and also a good home for microbes. Scientists at the USDA's Agricultural Research Service (ARS) tested common household methods for cleaning sponges, and the results are a clear guide for a weekly kitchen routine.

What the USDA researchers tested

ARS microbiologists at the Food Technology and Safety Laboratory in Beltsville, Maryland, started by making sponges as dirty as possible. According to the ARS summary, they soaked sponges for 48 hours at room temperature in a solution made from ground beef and lab growth medium, reaching about 20 million microbes per sponge.

They then treated each sponge one of five ways, chosen because they are common in home kitchens:

1. Soaked for 3 minutes in a 10 percent chlorine bleach solution
2. Soaked in lemon juice for 1 minute
3. Soaked in deionized water for 1 minute
4. Heated in a microwave for 1 minute
5. Run through a dishwasher with a drying cycle

A sixth group was left untreated for comparison.

The results

The difference between the methods was not subtle. From the ARS summary:

ARS says the soaking methods left enough bacteria to potentially cause disease.

To put those percentages in sponge terms, start with the study's 20 million microbes:

• Microwave: 20,000,000 × 0.0000001 = about 2 bacteria left.
• Dishwasher: 20,000,000 × 0.000002 = about 40 left.
• Even an 87 percent kill from a soak leaves 20,000,000 × 0.13 = about 2.6 million.

Yeasts and molds followed the same pattern. ARS reports that sponges treated in the microwave or dishwasher kept less than 1 percent of their yeasts and molds, while 6.7 to 63 percent survived on sponges soaked in bleach, lemon juice, or water, or left untreated.

Why the bleach soak fell short

It surprises people that bleach did poorly. The published paper, by ARS scientists Manan Sharma, Janet Eastridge, and Cheryl Mudd in the journal Food Control, found that sponges soaked in 10 percent bleach had bacterial counts only slightly lower than those soaked in water or lemon juice. The authors suggest the lack of effect from bleach and lemon juice may have been due to insufficient contact time. A quick dip does not reach everything trapped inside a thick, food-soaked sponge.

How the winning methods were done

The paper gives details that matter if you want to match the results:

• Microwave: the sponges were heated for 1 minute at full power in a household microwave rated at 1.30 kW. The sponges in the study had been soaked in liquid and were wet, not dry, so the results describe wet sponges.
• Dishwasher: sponges went on the top rack through a normal cycle with the water temperature boost and heated drying cycle turned on. The researchers did not add detergent, and attributed the effect to hot water and the mechanical action of the wash.

The ARS summary describes both as simple, convenient treatments that can help keep contaminated sponges from spreading foodborne pathogens around a home kitchen.

Building it into a routine

A practical sponge routine based on the ARS findings:

Daily

• Rinse out food bits after use and squeeze the sponge as dry as you can.
• Let it dry somewhere with air circulation rather than sitting in a puddle by the sink.

Every few days, or after wiping up raw meat juices

• Option A: put the sponge on the top rack of the dishwasher and run a cycle with heated drying.
• Option B: microwave the wet sponge for 1 minute at full power. Let it cool before handling, since it will be very hot.

When to toss it

• Replace a sponge that is falling apart, smells bad even after cleaning, or has food residue you cannot rinse out. Cleaning resets the microbe count, but a sponge that cannot be rinsed clean will just collect more.

Sponge habits that reduce risk

• Keep a separate cloth or paper towel for raw meat juices. It is simpler to throw away a paper towel than to sanitize a sponge after every chicken prep.
• Do not rely on a quick bleach dip as your main cleaning method, given how poorly the short soak performed in the ARS test.
• Clean the sponge holder too, so a clean sponge does not go back into a dirty spot.

Key takeaways

• USDA researchers loaded sponges with about 20 million microbes and tested five common cleaning methods.
• Microwaving a wet sponge for 1 minute killed 99.99999 percent of bacteria, and a dishwasher cycle with heated drying killed 99.9998 percent.
• A 3-minute 10 percent bleach soak, lemon juice, or water killed only 37 to 87 percent, likely because of too little contact time.
• Microwave and dishwasher treatments also cut yeasts and molds to under 1 percent.
• Clean sponges every few days with one of the two winning methods, dry them between uses, and replace worn ones.