Researchers from the University of Valladolid in Spain analyzed 40 brands of beer from all five continents and found that dark beer has more free iron than pale and non-alcoholic beers.
Dark beers have an average free iron content of 121 parts per billion compared to the 92 ppb in pale beers and 63 ppb in non-alcoholic beers.
Iron is an essential part of the human diet and also helps in oxidizing the organic compounds in beers, which provide the beverages with their taste.
"Although these quantities are very small, the differences are apparent and could be due to the production processes or raw materials used in manufacturing," said co-author Carlos Blanco, professor of Food Technology at UVa.
The researchers believe that the higher iron content in dark beers could come from the hop and malt extracts used produce it.
In contrast, pale beer production includes a filtering process that uses sedimentary rock of a porous nature to trap the iron in the beer, thus lowering the iron content.
Non-alcoholic beer undergoes vacuum evaporation processes to remove the alcohol. This operation also removes iron ions given that they are dragged by the volatile molecules.
To analyze 17 Spanish beer brands and 23 from other countries, with 28 pale, 6 dark and 6 non-alcoholic beers, the researchers used a differential pulse adsorptive stripping voltammetry technique developed by themselves, and refer to it as a "an ultra-sensitive, selective, rapid, reliable and cost-effective method".
The beers with the highest iron content were a dark Spanish beer (165 ppb) and a dark Mexican beer (130 ppb). Those that had the lowest levels of iron were from The Netherlands and Ireland (41 ppb and 47 ppb, respectively).
The study is published in the Journal of Science of Food and Agriculture.