In a study published online on September 3, 2013 researchers from the Fraunhofer Institute of Process Engineering and Packaging (IVV) in Germany investigated the contribution of different sources to the presence of per- and polyfluorinated compound in milk and other dairy products (Still et al. 2013). Still and colleagues chose their research design carefully as to discriminate between raw product contamination, processing influence, and food contact material (FCM) contribution from both pre-filling (storage) and post-filling contact (i.e. food packaging). The researchers found the highest levels of perfluorinated alkylated acids (PFAAs) in high fat products and identified enrichment during processing as the main contributor to PFAA concentrations. They hypothesize that the contribution of fluorotelomer-alcohols (FTOH) (PFAA precursors) to overall PFAA levels is likely to be small. Still and colleagues conclude that while the contribution of grease proof wraps to PFAA levels could easily be avoided, the problem of enrichment could only be addressed by reducing dairy cow exposure to PFAA.
Polyfluorinated alkylated substances (PFAS) include both PFAAs and FTOH. PFAAs consist of perfluorinated carboxylic acids (PFCAs) and sulfonic acids and are highly persistent in humans and the environment. They have been linked to cancer, reproductive and immunologic adverse effects. They are used as greaseproof agents in food contact materials. FTOHs are precursors to PFAAs and are also used as grease-proofing agents for paper. For their analysis, Still and colleagues obtained samples of milk, yoghurt, cheese, whey drink, butter milk, cream and butter from local supermarkets and a cooperating dairy. The researchers first screened finished and packaged products for all PFAA congeners, before testing the PFAA and FTOH content in butter during various processing steps prior to filling. The products from the different processing steps included raw milk, skim milk, cream, buttermilk and butter and were taken from the same batch of raw milk. After an optimization of the extraction method required by PFAA’s high affinity for proteins, the researchers obtained recovery rates between 65 and 90 percent compared to less than 30 percent for other methods. The analysis of the samples with a minimum sample weight of 10 g was carried out using liquid chromatography – electrospray ionization – mass spectrometry (LC-ESI-MS/MS) and reached a medium limit of detection (LOD) of 3 pg/g. Fluorotelomer-alcohols (FTOH) were analyzed using gas- chromatography with chemical ionization mass spectrometry (GC-CI-MS).
From the 14 screened samples, the highest levels of PFAAs were seen in butter (73.5 g PFAAs/g of which 13.4 pg were PFOA, 14.6 pg perfluorinated sulfonic acid (PFOS) and 11.4 g perfluorododecanoic acid (PFDoA)). The subsequent analysis of butter at the different production steps showed an up to 1.2-1.6 fold increase of PFAA concentrations in cream and a further 1.2-2.1 increase in butter. Based on a mass balance flow analysis, Still et al. observed neither a loss of PFAA to, nor a significant release of PFAAs from equipment surfaces during the butter production. Further, no loss of PFAAs occurred due to heating, as was previously hypothesized by Del Gobbo et al. (2008). The researchers concluded that the increased concentrations in butter can be attributed to the hydrophobic properties of PFAAs which leads to a PFAA enrichment* in the fatty phases during the processing steps. The researchers highlight that the opposing roles of the PFAA’s aforementioned protein affinity and the hydrophobic properties preempt the conclusive use of octanol-water coefficients (Kow). Migration modeling normally relies on a substance’s Kow.
Still and colleagues then analyzed the migration of PFAAs from the butter wrap into butter in order to assess the contribution of packaging to PFAA levels in the butter. FTOH levels were in the nano-range per dm2 (23.5 g of butter/dm2) and exceeded PFAA levels by a factor of 100. Significant migration was measured for all PFAAs other than PFOS and perfluoroundecanoic acid (PFUdA) after 45 days of storage. PFAA migration per dm2 of wrap (0.1 to 0.5 ng) was found to decrease with chain length. Migration of FTOH ranged from 0.5-0.3 µg/dm2. Still et al. state that the study design did not allow discrimination between different chemical sources of PFAA migration. However, they hypothesize that the contribution of FTOH to the overall PFAA load is low, because low microbial activity determines the transformation rate of FTOH into PFAAs.
In conclusion, the authors point out that the major contributor to increases in high carbon chain length PFAA concentration in dairy products is processing, resulting in an increase in PFAAs in the fattier products. Migration from processing equipment could be excluded. The presence of PFAAs with lower carbon chain length could be traced back to FCM migration during storage. The contribution of food packaging to PFAA levels, they contend, could easily be avoided by using packaging materials without a fluoropolymer coating. As the increase of PFAA concentrations during processing is a mere reallocation of compounds, the remaining PFAA contamination could only be reduced by reducing the PFAA exposure of dairy cows.
Reference
Still, M. et al. (2013). “Impact of Industrial Production and Packaging processes on the concentration of per- and polyfluorinated compounds in mild and dairy products.” Agricultural and Food Chemistry (published online September 3, 2013).
* With increasing chain lengths, PFAAs are more likely to orient themselves into the fat droplets contained in the dairy emulsions, preempting them from between removed with the protein-rich water
