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Silver Nanoparticles in Packaging Can Contaminate Dry Foods

A package of oranges in shrink-wrap plastic.
Credit: Wander Fleur / Unsplash.
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Incorporating silver nanoparticles (AgNPs) into plastic packaging has been proposed as a novel way to protect against foodborne disease and extend the shelf life of food.  


Despite their demonstrated promise for this purpose, nanoparticle-infused polymers are not yet authorized in the United States or European Union for use as food packaging. This is largely due to the need for further studies proving that silver nanoparticles and ions will not migrate into the food they contact.


New research led by scientists at the US Food and Drug Administration (FDA) suggests that AgNPs can leach out of plastic packaging and into solid foods. The study, published in ACS Food Science & Technology, found evidence of nanoparticle migration into dry goods (flour and ground rice), moist solid foods (cheese slices) and leafy greens (spinach leaves).

Understanding nanoparticle migration

Studying the migration of nanoparticles out of these plastic matrices is not a new endeavor. Early assessments concluded that nanoparticles larger than a few nanometers in diameter would be too large to diffuse through and out of plastic packaging under commercially relevant timescales.


However, subsequent studies using beverages, gelatinous foods (such as yogurt) and liquid food simulants have shown that AgNP-containing plastics can release dissolved silver ions into the food they contain. In some cases, these ions were found to induce silver nanoparticle formation in foods during long-term storage.


One significant limitation of past studies is that they have mainly assessed liquids or liquid food simulants rather than packaged foods like dry goods (grains, cereals) or moist solids (cheese slices, deli meat).


This is an important distinction to make, as solid foods will have different interactions with their packaging than a beverage or liquid simulant. There is normally less contact area between solid food and its packaging due to air bubbles, for example, which may make the direct migration of nanoparticles more difficult. Solid foods are also likely to have a harder time penetrating or degrading the polymer, which would otherwise increase the likelihood of nanoparticle migration or silver ion release.


In this latest study, the FDA researchers aimed to answer the question of whether AgNPs can transfer into solid foods from this kind of polymer nanocomposite (PNC) packaging. 

Silver nanoparticles can transfer without a liquid medium

The researchers performed three different experiments to test whether AgNPs might migrate into solid foods.


“This study is the first to broadly consider the question of whether inorganic NPs, including AgNPs, can migrate out of PNC packaging and into either solid foods or abiotic surfaces under commercially relevant conditions (long-term room temperature or refrigerated storage),” the researchers wrote.


“To answer this fundamental question, we conceived of a simple model system to test whether NPs embedded in a polymer can migrate to a “non-fluid simulant” when the two materials are held in contact with each other for an extended time.”


This first test took two discs of polymer — one plain and one embedded with luminescent nanocrystals — and pressed them together using a clamp. After being held together for some time, the two were separated and photographed under UV light. This revealed a luminescent glow on the plain polymer disc, proving that ultrasmall nanoparticles can pass from a “donor” polymer to an “acceptor” polymer through sustained contact under simulated long-term storage conditions.


The second test repeated this, but using AgNPs in place of the luminescent nanocrystals. As AgNPs do not react under UV light, any nanoparticle migration was assessed by testing the plain polymer disc using laser scanning confocal microscopy (LSCM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This revealed traces of silver in the plain polymer disc, proving that AgNPs can transfer into other solid materials.

Silver nanoparticles can migrate into cheese, flour, leafy greens

The third test involved placing real food samples — sliced cheese (to represent moist foods), all-purpose flour (to represent dry goods) and spinach leaves (to represent fresh produce) — into sachets made from AgNP-infused polymer.


Analysis by ICP-MS revealed a “significant migration” of Ag into all three food samples.


The researchers also investigated whether there was any difference between samples of spinach that were washed in clean water after being removed from the sachet versus unwashed leaves. While the amount of Ag on the washed leaves was significantly lower, “significant Ag migration” was still found in both the washed and unwashed leaves.


“The fact that washing did not remove all the Ag may indicate some penetration of Ag into the leaves’ interiors or that Ag may become trapped in pore spaces or other surface topological features that renders rinsing inefficient,” the researchers wrote.


To probe whether food particle size might also affect Ag migration, a further two sachets were prepared containing either fine or coarse ground white rice. Significantly higher migration was observed in the case of the finely ground rice. The researchers believe this indicates the importance of polymer-food contact efficiency in controlling Ag migration in food.


“The observation of Ag migration to foods that vary considerably in terms of their moisture and fat content, and particularly that the amount of migration can depend on the food particle size, processing characteristics, and washing strategy, implies that more work is needed to understand the factors that may influence NP migration from PNCs intended for use in packaging of solid foods, including food chemistry/structure (polarity, moisture content, surface topology, particle size), NP characteristics (size, composition), and polymer type (composition, crystallinity, processing conditions),” the researchers concluded.


Reference: Adhikari L, Todorov TI, Yang T, et al. Silver migrates to solid foods and abiotic surfaces from model plastic packaging containing silver nanoparticles. ACS Food Sci Technol. doi: 10.1021/acsfoodscitech.4c00813