The development of advanced materials from proteins requires their modification to improve functional properties, mainly barrier properties. In order to obtain the improvement of properties desirable for some bio-related uses, such as food, pharmaceutical and biomedical applications, Maillard reaction, a natural and non-enzymatic reaction between proteins and sugars, can be promoted and controlled by sugar contents and heating treatments. In this paper, Maillard reaction improves the barrier properties of fish gelatin films, providing an excellent protection against UV light and decreasing solubility due to the cross-linking induced by heating and lactose addition. Furthermore, a fluorescence compound is formed during this reaction and can be used as an indicator of the reaction progress. The knowledge of reaction kinetics as well as the changes observed in the nanostructure as a consequence of the cross-linking between fish gelatin and lactose provides the scientific information needed to spread the field of application of the products that can be manufactured using these natural reactive polymers.