Why are people keeping an open jar of used coffee grounds in the fridge? The surprising reason

Your morning coffee can leave behind more than just a caffeine boost. Those used coffee grounds that usually end up in the bin may have another practical use around the kitchen. One simple household trick involves placing dried used coffee grounds in an open jar inside the refrigerator to help tackle lingering food odours. But how does it work, and is there any science behind the hack? Here is what you need to know.

Why put used coffee grounds in the fridge?

An open jar of used coffee grounds can act as a simple refrigerator deodoriser. The idea is not entirely based on an internet hack either. The US Department of Agriculture’s Food Safety and Inspection Service recommends placing fresh coffee grounds or baking soda in a large, shallow container inside a refrigerator or freezer when lingering odours remain.

Coffee grounds have a porous structure that can interact with compounds in the air. Research published in the Journal of UOEH examined the ability of used coffee grounds to adsorb volatile organic compounds, or VOCs. The researchers tested 14 VOCs and found that coffee grounds could adsorb these compounds on their surface. This helps explain why coffee grounds can have some odour-control potential.

How does the fridge hack work?

The key process is adsorption, where molecules adhere to the surface of a material rather than being absorbed into its interior. Used coffee grounds contain a complex porous material that provides surface area for certain volatile compounds to attach to. The Japanese study found that the amount of VOCs adsorbed by coffee grounds varied depending on the compound, with adsorption generally increasing as the boiling point of the VOC increased.

In practical terms, this means the grounds may help reduce some lingering smells circulating inside the refrigerator. However, coffee grounds do not remove the source of the odour. If a forgotten vegetable, spilled sauce or spoiled food is causing the smell, the underlying problem still needs to be dealt with.

Why should the coffee grounds be dried first?

This is the most important part of trying the hack. Freshly brewed coffee grounds contain considerable moisture. Leaving damp grounds sitting in an open container for an extended period can create conditions that encourage mould growth. The USDA notes that mould can grow even at refrigerator temperatures and says cleanliness is important for controlling mould in refrigerators. So, do not simply scoop wet grounds straight from the coffee maker into a jar and leave them in the fridge. Instead, spread the used grounds on a plate or tray and allow them to dry thoroughly. Once dry, transfer them to a clean, shallow, open container.

How to use used coffee grounds in the fridge

The process is straightforward:

- Collect the used coffee grounds after brewing.

- Spread them out on a plate or tray.

- Allow them to dry completely.

- Transfer the dried grounds to a small open jar, bowl or shallow container.

- Place the container inside the refrigerator, preferably somewhere stable where it will not spill.

- Replace the grounds regularly, particularly if they become damp or develop an unusual smell.

- Keeping the container open allows air from the refrigerator to come into contact with the grounds.

What if your fridge still smells bad?

Coffee grounds should not be used to mask a persistent bad smell. The FDA advises being alert to spoiled food and notes that mould can grow even under refrigeration. Food that appears suspicious should be discarded. The USDA similarly recommends disposing of spoiled or questionable food before attempting to remove lingering refrigerator odours. So, if your refrigerator suddenly develops a strong smell, check for:

- Expired or spoiled food

- Spills on shelves and drawers

- Food residue around seals

- Forgotten containers

- Mould growth

- Items that may have leaked

Clean the refrigerator first. The coffee grounds can then be used as an additional odour-control measure.

The bottom line

Keeping an open jar of completely dried used coffee grounds in the refrigerator can help with certain lingering food smells. There is scientific evidence that used coffee grounds can adsorb some volatile compounds, while the USDA also lists fresh coffee grounds as one option for dealing with persistent refrigerator odours.