Effect Of The pH On The Topography And Nanomechanics Of Whey Protein Microgel Particles Investigated By Atomic Force Microscopy
Résumé
Microgels display specific features attracting a high research interest in several applications such as bioactives, drug delivery or protein purification. Whey protein microgels (WPM) are obtained by promoting whey protein intramolecular cross-linking through heat treatment. Temperature-induced protein denaturation and aggregation involves several kinds of interactions such as electrostatic repulsions and disulfide bonds. In the present work, the WPM particles topography and nanomechanical properties were investigated at native pH (6.5) and acid pH (5.5 and 3.0) by atomic force microscopy (AFM) in contact mode in a liquid environment. Prior to AFM analysis, WPM particles were captured on a gold substrate via low energy interactions by means of specific monoclonal antibodies. The 2D AFM images clearly showed a swelling of WPM particles induced by pH decrease. At native pH, they displayed an average width and height of 192 ± 27 nm and 44 ± 9 nm, respectively. A decrease in pH to 5.5 led to a significant (p<0.05) increase in WPM particles width (282 ± 9 nm) and height (101 ± 1 nm). At pH 3.0, a further increase in WPM size (a width of 420 ± 25 nm and a height of 78 ± 1 nm). The AFM elasticity (E) data showed a significant (p<0.05) increase in stiffness at pH 5.5 (E: 199 ± 9 kPa) and pH 3.0 (E: 187 ± 12 kPa) compared to native pH (E: 12 ± 1 kPa). These findings indicate that the mechanical profiles of WPM network directly varied with pH decrease. The WPM topographic and nanomechanical changes induced by acidification were most likely due to substantial changes in the shape and the structure of WPM particles. These strengthened internally crosslinked structures, modified by acidification, could display interesting encapsulation properties, providing an additional proof for their use as nanovectors monitoring bioactives release to the desired target after ingestion.