Confidence in cork: Checking for phthalate migration from cork stoppers into wine

Alternative cork stoppers

The use of cork stoppers for wine bottles have been on the decrease for a number of years now, following worries about the depletion of cork forests. Plastic stoppers and screw tops are gaining in popularity but there is still a cork-based alternative for those who prefer their wines to be more traditional. They are known as agglomerated cork stoppers and are made by gluing together natural cork granules recovered as waste from cork factories.

However, this alternative stopper might introduce its own unique problems, based on the presence of phthalate esters that are used in the adhesives. If they leach from the stoppers into the wine, they could have harmful effects on the consumer as certain esters are endocrine disruptors, carcinogens and teratogens.

Several research groups have studied the migration of phthalate esters into food from different types of packaging material like paper, card, printing inks, plastics and cork. Now, researchers from Spain and Portugal have devised a new method for agglomerated cork, based on HPLC-tandem mass spectrometry.

Juana Bustos from the National Food Center, Spanish Food Safety and Nutrition Agency, Madrid, and colleagues from the University of Santiago de Compostela and Animal Health And Livestock Services, Tragsega, Spain, and the National Institute of Health Dr. Ricardo Jorge, Lisbon, Portugal, revealed their method in the Journal of Separation Science.

Migration tests

To begin with, the team selected nine phthalate diesters and used a stock solution of the mixture to establish the optimum HPLC conditions. They were separated on a reversed-phase high-speed C18 column using a gradient of increasing methanol concentration in aqueous acetic acid.

The eluting compounds were analysed by multiple reaction monitoring following electrospray ionisation in positive-ion mode, the optimum precursor and product ions being selected by infusion of the individual esters.

Once the conditions were established, two glues based on the reaction products of toluene diisocyanate and polyethylene/polypropylene glycol were extracted with dichloromethane and tested for phthalate esters.

Real wine bottles were not analysed but the researchers used a 12% aqueous solution of ethanol as simulated wine. The stoppers were soaked in this solution which was then analysed by the LC/MS method to see if any esters migrated into solution, as might occur during storage of wine in bottles.

Phthalates in stoppers but not wine

All nine phthalates were separated within 17 minutes by HPLC giving good peak shapes and the most intense precursor ions were always the protonated molecules. However, only four phthalates were chosen to be measured in the glue and the migration tests due to a number of reasons.

There were always high background levels of bis(2-ethylhexyl) phthalate, even for blank samples, despite exhaustive cleaning of the glassware and LC system and the avoidance of plastic components where possible. So, this compound was dropped as accurate results could not be guaranteed.

The specific migration limits of dimethyl isophthalate and diallyl phthalate specified by European legislation are so low that they are below the detection limits of the current method. Diisopropyl phthalate and diphenyl phthalate are not included in the same legislation for plastics, so the research team assumed that they would not be used in the manufacture of glues or agglomerated cork, so they were also excluded.

That left dibutyl phthalate, butyl benzyl phthalate, diisodecyl phthalate and diisononyl phthalate for which the limits of quantification were 0.15, 0.24, 0.27 and 0.57 mg/L, respectively. These are sufficiently low to cover the specified limits in the EU regulations for plastics.

When the method was applied to the two glues, diisononyl phthalate was the only target phthalate found, at 2.9 and 10.4%, respectively. In the migration tests, none of the four phthalates were found in the simulated wine solution from a total of 21 agglomerated cork stoppers.

The LC-tandem-MS method will be suitable for the analysis of the glues, the cork stoppers, and for phthalate migration tests into wine and other foodstuffs with which agglomerated cork might come into contact.