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 °C and temperature ramps from 20 to 50 °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.
Dehydrated products, especially food powders, are widely used in the food industry, mostly for their ease of use, transport, and extended shelf-life (Bhandari, 2013). Some powders, such as maltodextrins, can be found in many food products. These products can be used to target food functional properties and can be introduced as thickener agents, for flavor enhancement, for their film-forming properties, to prevent crystallization, or even as an encapsulant agent (Castro, Durrieu, Raynaud, & Rouilly, 2016; Descamps, Palzer, Roos, & Fitzpatrick, 2013; Li, Pan, Ma, Miao, & Ji, 2020). Maltodextrins are carbohydrates powders originating from starch and are produced by enzymatical processes. Depending on the hydrolysis degree, different polymer chains length can be obtained depending on their reducing sugar content. Thus, the wide range of produced hydrolysates can be classified thanks to the dextrose equivalent (DE) value. This value is comprised of between 0 and 100, the lowest corresponding to pure starch with no enzymatic treatment and the highest pure glucose, respectively (Avaltroni, Bouquerand, & Normand, 2004; Chronakis, 1998). The final DE value is directly linked to the maltodextrin’s properties. For example, low DE maltodextrins (i.e., long polymer chain) show high viscosity and high Tg, whereas high DE maltodextrins (i.e., short polymer chain) show low viscosity and low Tg (Castro et al., 2016; Rong, Sillick, & Gregson, 2009; Siemons, Politiek, Boom, van der Sman, & Schutyser, 2020). Thus, selecting the appropriate maltodextrin is a crucial step when it comes to consider the final use.
Despite their extended storage properties, food powders, especially carbohydrate-based powders such as maltodextrins, are highly sensitive to environmental variations. For example, they are susceptible to caking phenomenon at high relative humidity (RH), but they can also be subjected to the glass transition, which significantly changes powder surface properties. Indeed, at the glass transition temperature (Tg), the powder comes from a glassy to a rubbery state, impacting powder functionalities, such as reconstitution in aqueous media (Badin et al., 2022; Roos, 2002). At the microscopic scale, glass transition causes a significant decrease of the Young modulus at the particle surface, enhancing the caking phenomenon between particles and powder stickiness. Moreover, time-dependent phenomena, such as crystallization, can also occur for some food powders above Tg (Descamps et al., 2013; Palzer, 2007; Wang & Truong, 2017).
Knowing that environmental variations significantly impact powder surface properties, many techniques have been developed to characterize food powder surfaces. For example, chemical composition identification techniques, such as X-ray Photoelectron Spectroscopy are already widely used on food powders. Also, Scanning Electron Microscopy (SEM) or Confocal Laser Scanning Microscopy (CLSM) were also useful for powder surface characterization (Burgain et al., 2017; Murrieta-Pazos et al., 2012). Atomic Force Microscopy (AFM) is rising in the food powder surface characterization field. This technique presents many advantages, such as the ability to work in air or liquids without the need to work under vacuum. It also allows the study of samples without pretreatment, compared to the conductive coating sometimes required for Scanning Electron Microscopy (SEM), or the thin cross-section needed in Transmission Electron Microscopy (TEM). AFM belongs to the scanning probe microscopes family and is based on the use of a micrometer sized probe that can feels and reconstructs single food particles’ surface topography by sweeping the sample surface. This unique system gives AFM its signature high-resolution capacity, allowing to detect nanoscale height and roughness variations at the surface (Badin, Burgain, & Gaiani, 2023; Dufrêne, 2002; Eaton & West, 2010). However, the most valuable advantage about AFM is its versatility, as it also allows to characterize and map mechanical and adhesive properties of the sample through force spectroscopy experiments (Butt, Cappella, & Kappl, 2005; Gaboriaud & Dufrêne, 2007).
However, AFM is yet poorly used to probe food powder properties, and until now, studies were mostly performed on milk-based powders (Badin et al., 2023). For example, Murrieta-Pazos et al. (2011) used it to highlight differences between skimmed milk powders and whole milk powders, showing nanoscale roughness differences between both powders. Fyfe et al. (2011) studied the impact of storage on the surface structures of milk proteins concentrates powders and the effects on powder solubility. Burgain et al., (2016a) highlighted surface modifications during high-temperature storage of Whey Protein Isolate (WPI) powders, showing that hollow structure formation and surface hardening was related to improper storage conditions. In other studies, Burgain et al. (2016b) used hydrophobic AFM cantilevers to show that aged WPI powders presented local hydrophobic areas with cracks formation. In the same way, Gaiani et al. (2021) used functionalized cantilevers and nanoindentation experiments to study single WPI particles chemical and mechanical properties with different storage temperatures. More recently, Mishra et al. (2022) studied microstructural and topographical changes of high protein milk powders as a function of moisture sorption and the results were useful to probe lactose crystallization and they highlight folds formation at the nanoscale. Finally, maltodextrins surface properties were studied for the first time by Badin et al. (2022), highlighting the differences in surface roughness depending on the DE value and the effects of glass transition after powder storage at the glassy and at the rubbery state.
Thus, despite the fact that AFM is yet poorly used to probe food powder properties, it was shown that it could be a highly valuable tool to explore their properties at the particle scale (Badin et al., 2023). Also, investigations in dynamic and real time, i.e. while changing temperature and/or RH of the surrounding air, were never reported. In this work, a specifically designed AFM environmental chamber was used to probe food powders surface properties to be able to dynamically study maltodextrin particle properties in real-time by controlling the RH and the temperature.
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Three different maltodextrin powders were used in this study, with theoretical DE of 4–7 (Low DE) (Product n°419672); 16.-19.5 (High DE) (Product n°419699) (Sigma-Aldrich, Saint-Louis, Missouri, USA) and 11–14 (Intermediate DE) (Glucidex 12) (Roquette Frères, Lestrem, France). They were chosen to cover the more extensive possible range of DE values existing for maltodextrins.
Regis Badin, Claire Gaiani, Stephane Desobry, Sangeeta Prakash, Bhesh Bhandari, Jennifer Burgain, Dynamic investigation of maltodextrins surface properties by environmental atomic force microscopy, Food Hydrocolloids, 2023, 109081, ISSN 0268-005X, https://doi.org/10.1016/j.foodhyd.2023.109081.
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