ANR : Poudres alimentaires d'origine végétale : rôle de la surface des particules dans l'expression de leurs propriétés fonctionnelles

école doctorale

équipe

contexte

Many types of fruit and vegetables are processed to increase their shelf-life, year-round availability, or to increase their value, which integrates structure-enabling and preservation techniques. Minimal processing includes drying and/or grinding (powdering process) of fruits and vegetables and guarantee that such foods are as nutritious as the food in its unprocessed form (Fitzpatrick and Ahrné, 2005). However, fruits and vegetables powders contain a high quantity of low molar mass sugars with low glass transition temperature (Tg) (Fang and Bhandari, 2011). The direct consequence is that fruit powders are highly hygroscopic and sticky at high temperatures but also at ambient temperature if the water content is not well mastered. This feature causes the powder adhesion to surfaces and powder caking during the storage, which affects the quality of the final product. The Tg is one of the most important parameters to consider during powder storage. Indeed, the phenomenon of glass transition is the gradual and reversible transition of amorphous materials from a hard and 'glassy' state into a viscous and rubbery state as the temperature and/or the moisture content is increased. The glass transition is also linked to the water activity and water as a strong plasticizer decreases the Tg. Also, the glass transition temperature depends on the molecular mass as the Tg of monosaccharides, disaccharides, oligosaccharides and polysaccharides increases with increasing their molecular mass (Roos, 2002). If the glass transition is reached during powder storage, unexpected phenomena can happen that affect powder functionality such as reconstitution ability. An important structure-determining component in this context is the particle surface. It should be noted that particle surfaces, in the case of fruits and vegetables powders, are essentially constituted by broken structures. Because of various origins, their particle size and shape distributions, chemical composition, surface composition, and physical properties are highly variable. Therefore, more than one analytical technique is often required to obtain a full set of information about a given scientific question (Burgain et al., 2017). Among these questions, the powders flowability and reconstitution are of upmost importance for the industry considering that most powdered ingredients are transported and dissolved or infused before use. For the past few years, numerous powder surface analysis techniques were used to further understand the role of powder surface on functionalities impairments. For example, microscopy techniques such as Scanning Electron Microscopy (SEM), Confocal Laser Scanning Microscopy (CLSM) or even chemical composition techniques such as X-ray Photoelectron Spectroscopy (XPS) are already widely used (Burgain et al., 2017). However, AFM is currently a rising star in the food powder surface analysis field, mainly for its resolutive capacity. AFM is a versatile tool compared to other surface analysis techniques. For example, AFM allows to study particle surface topography and roughness, surface chemistry and nanomechanics. In a previous project, we were able to have a better understanding of surface modification after high temperature storage of whey protein isolate and micellar casein powders (Burgain et al., 2016a, 2016b). Surface hardening with the development of a poorly dispersible skin layer composed of aggregated micelles was evidenced to be the phenomenon responsible for the reconstitution impairment. However only punctual analyses were possible and the development of an environmental chamber around the AFM will allow for continuous measurements at nanometer scale. By controlling the temperature and RH, their variation during time in the neighbouring of the sample will provide new insight in the elucidation of mechanisms occurring during powders storage or transport under unfavourable conditions, in particular when they reach the glass transition. Even if AFM was already applied to dairy powder, application to fruits and vegetables powders is still missing while there is an important industrial stake with the growth of the plant products market and the development of vegetable formulations. First experiments on fruits and vegetables powders provided hopeful results showing that when approaching glass transition, patches at the surface were crystallising by nucleation and the rest of the matrix presented a decreased elasticity. The glass transition event is accompanied by a physical change at the powder surface (modified topography) with a change in the Young modulus (modified nanomechanical properties) (Palzer, 2007). Moreover, powder caking related to glass transition is promoted by moisture adsorption which create liquid bridges between hydrophilic groups at particle surface (modified physicochemical properties). The strength of the technique is that it is now possible to follow the evolution of the same area during dynamic variation of temperature and RH. These observations confirm the fact that a focus at particle surface is undeniably required to better understand macroscopic phenomenon such as powder flowability, caking or reconstitution. This is particularly true for fruit powders that are highly hygroscopic materials with low glass transition temperature and as a consequence easily affected by ambient temperature and RH. In a recent work, we evidenced that above Tg, a viscous layer around the particle limits water entrance and is the limiting step in the global reconstitution process (Gaudel et al., 2022). With AFM, surface structure and chemical properties will be correlated thanks to the combination of surface topography analysis and force measurements.

spécialité

Génie biotechnologique et alimentaire

laboratoire

LIBIO - Laboratoire d'Ingénierie des Biomolécules