Casein hydrogels: Interaction with bioactive compounds and vegetable proteins

Hydrogels are three-dimensional networks able to entrap a high amount of water. They can be formed by a wide range of polymers alone or in combination and have different applications depending on their composition and rheological features. In the food industry, hydrogels are mainly designed to work as a carrier system of bioactive compounds or to tailor the texture, mouthfeel, and water retention of foods. The facility to modulate Casein micelles (CMs) structure and interactions by application of physical, chemical, or enzymatic treatments, makes it an excellent protein matrix for the hydrogel's formulation. Moreover, the use of casein hydrogels can be also a way to incorporate more plant proteins into human food. The mixtures of plant proteins with caseins have been viewed as a more sustainable alternative to a diet based mainly on animal proteins. Since, in the mixture, the drawbacks of pure plant protein producis, such as beany taste and low solubility, could be potentially diminished by the presence of caseins. Nevertheless, the CMs' interactions with micro molecules such as bioactive compounds or macromolecules such as proteins can alter the features of the gel. Thus, this study proposed the utilization of casein-based hydrogel in two distinct applications, |. in association with bioactive compounds extracted from Jabuticaba fruit with the use of transglutaminase for modulation of gels' microstructure and ii. in association with pea proteins (in different ratios) submitted to process conditions usually applied in the food industry such as thermal treatment and acidification, in addition, high-intensity ultrasounds also were applied. The addition of the bioactive extract in the gels decreased the gel elasticity and increase the pore sizes. However, these effects were contra-balanced by using transglutaminase as the crosslinking agent, which could modulate the release of the bioactive extracts from the gel. In the CMs: pea proteins systems, the heat treatment increased the elasticity of the systems with a higher impact in the systems with more pea protein. The network reinforcement is caused mainly by physical interactions between pea proteins, with disulfide bonds occurring only between proteins of the same source. During acidification, the replacement of 20 and 40% of CMs for pea protein disturbed the initial steps of CMs network formation, however, the final gel elasticity was higher than pure CMs gel due to the formation of the pea's network. In general, the proteins of different sources form independent protein networks even in high concentrations. Despite the reduced interaction between CMs and pea proteins, their distribution in the gel is responsible for modulating the final stiffness. In addition, the application of high-intensity ultrasound in the mixed suspensions increased the elasticity of the acid gels up to 10 times, depending on the protein ratio. This study shows that the association of CMs with bioactive molecules or pea proteins in gelled systems has the potential for the development of functional foods or foods with totally new rheological features.Keywords: Hydrogels. Casein. Transglutaminases. Bioactive compounds. Gelification.