[Image above] Powdered mixes, such as for soup, often contain silica as a food additive, which serves as an anticaking agent. Credit: ponce_photography, Pixabay
When people hear the word ceramic, they often picture products that make up the built environment, such as tiles, crockery, and sanitaryware. But ceramics are also used in many products that go on and inside our bodies, be it for beauty, cleansing, or medical purposes.
Ceramics are used in these products because of their biocompatibility, meaning they do not provoke an adverse response in the host tissue. Ceramics can be further classified as either bioinert (provokes no response whatsoever) or bioactive (provokes a beneficial response), and the application determines which type of ceramic is preferred.
For ceramics used as food additives, bioinert ceramics are preferred. In this case, the ceramic passes all the way through the body rather than being localized to a specific region. So, a reactive ceramic could potentially trigger an unwanted and adverse response as it passes through these different environments.
In recent years, more and more materials are being designed and applied on the nanoscale. Compared to macroscale or even microscale materials, nanomaterials can demonstrate very different properties from their larger counterparts.
While this difference in properties has opened fascinating new pathways in various fields, it has also led to some concerns within the food industry because ceramics traditionally considered bioinert may no longer be so on the nanoscale.
For example, in 2021, the European Food Safety Authority published an updated assessment on the safety of titanium dioxide (TiO2) as a food additive. Unlike the previous assessment, this one did not rule out a concern for genotoxicity due partly to new data on nano-TiO2. This updated assessment led the European Commission to announce a ban on TiO2 as a food additive in 2022.
Now, a new study published in August 2023 shines light on another ceramic traditionally considered inert—silica.
Silica, or silicon dioxide, is a common food additive and popular cosmetics ingredient. It is used as an anticaking agent in powdered mixes, such as soups and coffee creamers, and it serves as a bulking or absorbing agent in skin care products, as well as an abrasive in scrubs.
Silica is found naturally in many plants, such as leaf vegetables and bell peppers. But the silica typically added to food and cosmetic products is synthetic mesoporous silica, or nanosized silica particles that are engineered to have pores with a diameter ranging from 2–50 nm.
In the new study, researchers at Stanford University explored whether these nanostructured silica particles retain the inertness that larger, natural silica particles are known for.
The researchers include postdoctoral scholar Yangjie Li and Richard Zare, the Marguerite Blake Wilbur Professor in Natural Science, as well as Kurt Kolasinski, former graduate scholar of Zare’s and now professor at West Chester University.
Li’s role as lead author on this study was fortuitous, considering her background in probing presumed properties of everyday materials. During her doctoral studies at Purdue University, Li showed how glass can, in certain circumstances, act as a catalyst and accelerate chemical reactions, even though glass is traditionally considered chemically stable.
For this study, the researchers purchased commercially available silica particles sold as a dry powder. They added this powder to watery solutions containing one of three thiol-bearing biomolecules, which are widespread in nature and in the human body.
Over the course of 24 hours, as much as 95% of the biomolecules in solution ultimately oxidized, while the control experiments without silica incubation showed minimal oxidation.
The researchers hypothesized that this reaction occurs because silica, upon contact with water, forms surface-bound silyloxy radicals, i.e., a silicon atom bound to an oxygen atom in a configuration that has an unpaired electron.
“When encountering the radicals, thiol biomolecules in the solution transfer hydrogen atoms (H) to the radicals. Accordingly freed of bonded H, the sulfur atoms in two thiol molecules then recombine to form the S–S disulfides,” a Stanford press release explains.
Based on these results, “we ought to know more about silica and its interactions with other materials,” says Zare in the press release.
The researchers are now exploring how varying sizes of silica particles influence chemical reaction rates.
The paper, published in Proceedings of the National Academy of Sciences, is “Silica particles convert thiol-containing molecules to disulfides” (DOI: 10.1073/pnas.2304735120).
As consumers become more aware of the potential health effects of nanoparticles, they may wish to identify which items contain these food additives. However, doing so is not an easy task due to the current labeling requirements set by the U.S. Food and Drug Administration. Learn more in the video below.