Fødevarestyrelsen (Danish food ministry) says the ‘illegitimate' Chinese pine nuts have a different fatty acid profile than 'genuine' pine nuts and are responsible for the bitter metallic tastes.
Pine nuts are a popular addition to salads, and a crucial ingredient in pesto sauce.
But over the last few years, there have been reports of some consumers experiencing “strange metallic tastes” around one day after eating pine nuts.
The bitter metallic tastes seem to only affect a small group of consumers, but no explanation of how or why they are affected has ever been found.
‘Pine-mouth’ was reported by Fødevarestyrelsen to be related to certain species of pine from China - Pinus armandii (Chinese white pine) and Pinus massoniana (Chinese red pine).
Both species have smaller, round or triangular nuts that are only occasionally for exported pine nuts. However, with a growing demand for pine nuts, they reported that these species of nut are being increasingly mixed with 'real' pines.
They say since the European Commission have been aware of the problem, they have stopped Chinese exports of these 'illegitimate' pine nuts to the EU – however there may be smaller batches of the ‘counterfeit’ Chinese pine nuts on the market.
Reports of ‘pine mouth’ initially circulated in Belgium in 2001, prompting the Poisons Centre to conduct a comparison of affected and unaffected nut batches - no chemical differences were found.
Last year the UK’s Food Standards Agency began an investigation into such reports of lingering metallic tastes, sometimes lasting as long as two weeks.
Over recent years the ‘pine mouth’ problem has grown, with reports of incidents occurring globally.
Many reports were said to have followed consumption of nuts imported from China. However, until now there has been no official link between Chinese imports and the ‘metallic taste’ problems.
Earlier this year the Swedish Food Administration (NFA) decided to carry out an investigation into the reports, but failed to find any chemical reason for the problems associated with ‘pine-mouth’. Concluding that there were no significant differences in the content of pesticides or mycotoxines of nuts with and without the ‘metallic tastes’.
The statement from Fødevarestyrelsen claims that the Chinese pine nuts that can cause ‘pine mouth’ have a different fatty acid profile than 'genuine' pine nuts.
However, it is still uncertain whether the symptoms of ‘pine mouth’ are related to these differences in fatty acid content, or other substances.