During food preparation, structures are formed which give the food its characteristic properties. The addition of protein ingredients is meant to improve the structure and give a positive contribution to functional properties such as texture, and fat- and waterbinding. A functional characterization provides information on the behaviour of a protein in a food system. Factors to be considered in a functional characterization are discussed, and examples are given from studies of various protein ingredients. Waterbinding and texture characteristics generated by gel formation and their relation to changes in the microstructure are illustrated by results from studies of plasma protein gels. The results clearly show the difference in the gel characteristics as well as the importance of a proper choice of methodology and definition of the property to be studied. A comparison between microstructure and functional properties of commercially processed and mildly prepared soy protein isolates demonstrates differences caused by processing and their impact on the application of the protein. Results from a study on gluten show that other components than protein may be responsible for drastic changes in the functional properties of a protein ingredient during processing. © 1983 Martinus Nijhoff/Dr W. Junk Publishers.