Size characterization by Sedimentation Field Flow Fractionation of silica particles used as food additives

Four types of SiO₂, available on the market as additives in food and personal care products, were size characterized using Sedimentation Field Flow Fractionation (SdFFF), SEM, TEM and Photon Correlation Spectroscopy (PCS). The synergic use of the different analytical techniques made it possible, for some samples, to confirm the presence of primary nanoparticles (10 nm) organized in clusters or aggregates of different dimension and, for others, to discover that the available information is incomplete, particularly that regarding the presence of small particles. A protocol to extract the silica particles from a simple food matrix was set up, enriching (0.25%, w w⁻¹) a nearly silica-free instant barley coffee powder with a known SiO₂ sample. The SdFFF technique, in conjunction with SEM observations, made it possible to identify the added SiO₂ particles and verify the new particle size distribution. The SiO₂ content of different powdered foodstuffs was determined by graphite furnace atomic absorption spectroscopy (GFAAS); the concentrations ranged between 0.006 and 0.35% (w w⁻¹). The protocol to isolate the silica particles was so applied to the most SiO₂-rich commercial products and the derived suspensions were separated by SdFFF; SEM and TEM observations supported the size analyses while GFAAS determinations on collected fractions permitted element identification.