These results suggest that the TG reaction is most effective when applied during the early stages of the adsorption process, so that the TG is present to catalyse the reaction whilst the protein is adsorbing. If TG is added after adsorption is started, the strengthening effect of the TG reaction is slowed down and becomes smaller. The interaction with spread or adsorbed–perfused films appears to be much smaller, suggesting that there may be an interaction with non-adsorbed proteins. Therefore the protocol used for TG treatment of protein stabilised emulsions or foams is sensitive to the order and time of addition. The reaction may take preference whilst the protein is in solution just prior to adsorption, leading to a stronger network of cross-linked protein aggregates, or they may be an interaction between adsorbed and non-adsorbed proteins leading to a thicker, stronger protein film, however, the results shown in Fig. 5A, where further protein was added suggest that this effect may only be a minor component. The individual type of protein may also be important as the reaction requires the close proximity of certain amino acid side chains and different proteins will respond differently to the combined effects of adsorption and TG reaction, producing adsorbed films with different structure and physical properties. Further investigations of the interfacial structure, physico-chemical properties are required to reveal the true mechanism of TG induced crosslinking of interfacial protein films.