Cinema advertising is less effective when audiences eat popcorn, according to a study from Cologne University.
Researchers found that viewers remember brands by simulating the pronunciation of a new name with their mouths.
But this "inner speech" can be disturbed by chewing, rendering adverts redundant, said the research, published in the Journal of Consumer Psychology.
It contrasts with previous studies, which suggested chewing gum could aid memory.
There is some evidence that the act of chewing gum improves blood flow to the brain, and can make you more alert over long periods of sustained concentration.
However, researchers from Cardiff University reported last year that people's ability to recall lists in a specific order is impaired by chewing gum.
To investigate the effects of popcorn on memory, researchers at Cologne University invited 96 people to a cinema to watch a movie, preceded by a sequence of adverts.
Half of the group was given popcorn, which was replenished throughout the screening, while the rest received a small sugar cube.
They were shown genuine commercials for existing products which were, crucially, unfamiliar to the German participants - for example, for the Scandinavian butter Lurpak.
A week later, the participants were invited to a laboratory and asked to rate a series of products, including some of those which had been advertised.
The sugar cube group displayed preferences for the advertised products, but the popcorn munchers did not.
In a second study, 188 people were shown adverts in similar circumstances, then given money to donate to charity.
Again, the sugar cube sample tended to give money to charities that had been advertised in the cinema, but the participants who had been eating during the screening showed no such preference.
"The mundane activity of eating popcorn made participants immune to the pervasive effects of advertising," said Sascha Topolinski, one of the researchers.
The study posits that repetition of brand names is essential in imprinting them in our consciousness
"Particularly for novel brands, excessive exposure and repetition is necessary to establish the brand name in the first place," wrote the authors.
"Remember your initial irritation upon encountering the names Yahoo, Google and Wikipedia for the first time; now they are imprinted in your brain."
Mr Topolinski goes so far as to suggest that advertisers may try to boycott popcorn.
"This finding suggests that selling candy in movie theatres actually undermines advertising effects, which contradicts present marketing strategies.
"In the future, when promoting a novel brand, advertising clients might consider trying to prevent candy being sold before the main movie."
Cinema-owners may not be so keen, though. On average, popcorn is sold at a 900% mark-up, according to economics professor Richard McKenzie at University of California - Irvine.