The physical behaviour of food protein’s supramolecular structures during freezedrying

Many proteins are able to self-assembly into well-defined supramolecular structures. We have previously shown that two food globular proteins with highly homologous tertiary structures and opposite electric charges, i.e. lysozyme and a-lactalbumin, interact under specific conditions of temperature and pH and generate spherical microparticles. The objective of this study was to investigate the stability of the lysozyme - a-lactalbumin microparticles during lyophilisation, one example of food processes. Solutions of protein microparticles without and with addition of protective molecules such as sucrose or matlodextrin were freeze-dried under conservative conditions in a pilot plant and samples were removed at each step of the process: after freezing, sublimation and desorption. The morphology and the particle size distribution of the protein microparticles were investigated by direct optical observation under a microscope and by using a Coulter counter. The freezing (and thawing) appeared to be the most damaging steps with the formation of clusters or agglomerates of microparticles. Addition of sucrose or maltodextrin sharply reduced the formation of particles agglomerates after freezing by limiting the cryoconcentration of the particles and trapping them in an amorphous matrix. Maltodextrin seemed to be a more efficient protective molecule than sucrose for preserving the stability of the protein microparticles during the freeze-drying process