The Bee Glue That Made the Strawberries Mouldier

Department of Questionable Ideas experiment report: Raw Propolis Versus Six Unsuspecting Strawberries

The idea in one sentence: Put a small piece of untreated propolis from a beehive in a sealed tub with strawberries and see whether its suspected antimicrobial properties slow spoilage.

The short verdict

The idea did not work. More precisely, it did not work in the form actually tested.

Three strawberries were sealed in a control container. Three more were sealed in a second container with one roughly 1 × 1 cm lump of raw propolis taken from a beehive and given no prior treatment. Both tubs were left at room temperature. If the hypothesis had been right, the propolis tub should have remained fresher for longer.

Instead, visible fungal growth accelerated in the treated container compared with the control.

That is a splendidly questionable result: the material introduced to discourage spoilage apparently accompanied more of it. It failed as preservation, but succeeded as an experiment by contradicting the hypothesis.

The fair conclusion is narrow. In this trial, an untreated lump of hive propolis did not preserve three strawberries. The treated tub did worse. That does not prove all propolis encourages mould or explain what happened.

Why the idea made sense

Propolis is the sticky, resinous material bees make from plant-derived substances, wax and their own secretions. They use it around the hive to seal gaps and coat surfaces—the sort of building maintenance for which no bee appears to submit an invoice.

The original hypothesis was that propolis might contain antibacterial properties. That was reasonable: controlled research has found antimicrobial effects from some preparations. In one primary study, a defined propolis-extract dip helped preserve refrigerated pomegranates and reduced grey mould under some conditions (Kahramanoğlu and colleagues, 2018).

But three claims can easily become tangled:

• propolis can affect bacteria in a laboratory;

• an extracted preparation can inhibit certain fungi on contact; and

• a raw lump sitting near fruit can protect it through the air.

They are not the same. The strawberries mainly tested the third and boldest claim. The visible spoilage was fungal, not bacterial. Even a substance with antibacterial activity need not stop mould, and potentially useful compounds may need extraction, direct contact or a particular concentration.

What was actually done

Feature	Control	Propolis treatment
Strawberries	3	3
Added material	None	One untreated lump of beehive propolis, about 1 × 1 cm
Container	Sealed	Sealed
Storage	Room temperature	Room temperature
Result	Less rapid fungal growth	Accelerated fungal growth relative to control

The method has one great virtue: it is understandable. There is a control, a treatment and a visible outcome.

Do not retrofit sophistication that was not there. The propolis was not sterilised, ground, extracted into alcohol or made into a coating. There was no recorded weight, humidity measurement, microbial identification or numerical mould score. The experiment tested raw material as found.

That simplicity is legitimate. If the proposed household trick is “put bee glue in the tub”, then the effortless version is the right thing to test first.

The reversal

The expected story was irresistible: the control strawberries would surrender while the protected berries remained improbably photogenic. The headline could practically write itself: Could bee resin keep soft fruit fresh for weeks?

Reality objected.

The treated container developed fungus faster. The supposed preservative did not merely fail to create an obvious benefit; it sat on the losing side of the comparison. Any earlier teaser suggesting that the experiment kept fruit fresh should therefore be retired. The honest headline is better anyway.

This is why the control mattered. Without it, mould in the propolis tub could have been dismissed as the ordinary fate of a strawberry. With the untreated tub beside it, the direction of the result became visible. It was the opposite of the prediction.

Why might the treated tub have become mouldier?

Several explanations fit. This experiment cannot choose between them.

The propolis may have brought passengers

Raw propolis comes from a living hive; it is not a sterile pharmaceutical tablet. A primary investigation of propolis collected from seven Tunisian locations cultured 80 fungal strains across its samples, including members of Aspergillus, Penicillium, Cladosporium and Fusarium (Sallemi and colleagues, 2022).

That does not prove this lump introduced spores. It only makes it plausible. A natural material may contain compounds active against some microbes while also carrying living fungi.

The useful chemistry may not travel through the air

Encouraging studies commonly grind propolis, extract compounds into a solvent, define a concentration and apply the preparation directly. The pomegranate study dipped fruit in specified extract solutions and used cold, controlled storage. A grapefruit study applied ethanol extracts; its results changed with concentration, solvent and whether fungal infection was already established (Soylu and colleagues, 2008).

A nearby untreated lump may simply release too little active material to matter.

Propolis is not one standard substance

Its composition varies with geography and the plants available to bees. Samples from 12 US regions differed in both chemistry and inhibition of tested bacterial and fungal bee pathogens (Wilson and colleagues, 2015). Another laboratory study found raw Czech and Slovak samples generally showed less antifungal activity than some solvent extracts (Buchta and colleagues, 2011).

“Propolis has antimicrobial properties” is therefore a family of findings, not a guarantee for every untreated fragment.

The strawberries may simply have started unequal

One berry can begin with more invisible spores, surface moisture or damage than another. Three unlucky strawberries in the treatment tub could create the appearance of an effect by chance. Sealed, room-temperature fruit also gives condensation and mould plenty of encouragement.

The important limitations

Although there were three strawberries on each side, there was only one independent container per condition. The berries in a tub shared the same air, moisture, propolis exposure and spores. They were subsamples, not three separate repetitions. Six strawberries can create drama without creating a large sample.

The experiment also measured visible fungus, not antibacterial action. Nothing was cultured, counted or identified. It therefore cannot say whether bacteria were affected, which fungus grew, or what caused the difference.

It did not show that all propolis is useless, nor that it always accelerates mould. It showed that this raw-lump preservation method failed under these particular conditions.

The DQI verdict

This experiment deserves publication precisely because the prediction lost.

The starting thought was coherent: bees use propolis in the hive; some prepared propolis products show antimicrobial activity; perhaps a raw piece could protect soft fruit. The experiment asked that direct household question and received an awkward answer.

It does not condemn propolis. It condemns the tested trick—at least until a properly replicated trial rescues it. A lump beside fruit is not equivalent to a standardised extract or direct coating. Meanwhile, natural propolis can carry its own biology, strawberries vary, and two tubs leave coincidence with an enormous office.

So the idea was definitely questionable. Excellent.

Final status: failed as a preservation method.

The bees may still know exactly what they are doing with propolis. They simply never promised it could supervise six strawberries in Tupperware.