Dynamic investigation of maltodextrins surface properties by environmental atomic force microscopy

For the first time on food powders, environmental Atomic Force Microscopy (AFM) was used to probe single particle surface properties in real time by variating relative humidity (RH) and temperature. Low, intermediate, and high dextrose equivalent (DE) maltodextrins values were used as a model matrix. Humidity ramps from 20 to 80% at constant temperatures of 20 and 50 degrees C and temperature ramps from 20 to 50 degrees C at a constant RH of 20 and 80% were performed. Surface topography, roughness, and Young modulus distribution evolutions at the particle surface were studied under these conditions. It was observed that glass transition and RH are driving particle surface properties. Glass transition was always accompanied by a significant global surface smoothing, whatever the DE value. Surface smoothing phenomenon were also accompanied by a large decrease of the surface roughness with the increase of RH. Apart from the impact on surface topography, glass transition also impacted particle physics. Particles in the glassy state were relatively hard with a high and heterogenous Young modulus distribution. An increase in the RH made the particle progressively softer, whereas crossing the glass transition temperature leads to a really soft surface and to the homogenization of the Young modulus distribution. These results showed that glass transition significantly impacts particle surface properties and is promising to optimize food powder formulation and their shelf-life extension.