Contents lists available at ScienceDirect Food Structure journal homepage: www.elsevier.com/locate/foostr Electrostatic hydrogels formed by gelatin and carrageenan induced by acidification: Rheological and structural characterization Mariane Gonçalves de Alcântara a , Nailene de Freitas Ortega b , Clitor Junior Fernandes Souza b , Edwin Elard Garcia-Rojas a,c, * a Pós Graduação em Engenharia Mecânica (PGMEC), Universidade Federal Fluminense (UFF), Av. dos Trabalhadores, 420, 27255-125, Volta Redonda, RJ, Brazil b Universidade Federal da Grande Dourados, Faculdade de Engenharia, Pós-graduação em Ciência e Tecnologia de Alimentos, P.O. Box 533, 79804-70, Dourados, Brazil c Laboratório de Engenharia e Tecnologia Agroindustrial (LETA), Universidade Federal Fluminense (UFF), Av. dos Trabalhadores, 420, Volta Redonda, RJ, 27255-125, Brazil ARTICLE INFO Keywords: Rheology Protein Texture Polysaccharide Viscoelasticity ABSTRACT The rheological, chemical and structural characteristics of electrostatic hydrogels formed by gelatin and car- rageenan (Gel:Car) were studied in this work. Hydrogels were formed at 1:1, 1:2, 1:4, 1:8, 2:1, 4:1 and 8:1 protein:polysaccharide ratios. The influence of the ratio between the biopolymers on the rheological char- acteristics of hydrogels was evaluated. The hydrogels were characterized by large and small deformation, water holding capacity and microstructural analysis. All hydrogels showed viscoelastic characteristics, and the Burger model was appropriate to explain the viscoelastic behavior of hydrogels formed. The hydrogel that presented the best viscoelastic and mechanical characteristics was the 1:2 Gel:Car showing that good amount of carrageenan associated with slow acidification results the formation of more junction zones due to complementary interac- tions between the gelatin triple helix and the carrageenan double helix. When carrageenan (1:4 and 1:8) or gelatin (2:1, 4:1 and 8:1) was in excess there was a suppression of the gelatin structure modification significantly reducing the density of the junction zones negatively impacting the flexibility and hardness of these hydrogels. All samples showed a good water holding capacity, retaining between 80 % and 90 % of water in the interior, making these hydrogels an attractive material for food applications because they can improve food texture, increasing water retention capacity, which will produce greater durability of food. However, its structure and its mechanism of formation can also be explored in protecting active molecules in its structure or in the manu- facture of new hydrogels through the slow acidification, such as in yogurt manufacturing. 1. Introduction The interactions between proteins and polysaccharides are physi- cochemical phenomena that play an important role in controlling the structure, the texture and the stability of foods, as well as the systems involved, such as coatings and packaging (Gulão, de Souza, da Silva, Coimbra, & Garcia-Roja, 2014). Protein-polysaccharide complexes mostly originate from electrostatic attraction between two oppositely charged biopolymers (Zhang, Hsieh, & Vardhanabhuti, 2013). In ac- cordance with compatibility conditions, the interactions of biopolymers in aqueous mixtures can result in the formation of electrostatic gels or coupled gels due to the involvement of two different molecules to form the junction zones (Le, Rioux, & Turgeon, 2017). These hydrogels may be obtained with a very low solid content with no heat treatment which enhance the possibilities of developing new functional products. Hy- drogels are three-dimensional solid networks formed by hydrophilic polymeric structures that can be physically or chemically cross-linked and can retain large amounts of water or other biological liquids in their network (Abaee, Mohammadian, & Jafari, 2017). Hydrogels may be formed from natural polymers, such as proteins and polysaccharides, by the gelling process, which can be induced by physical processes, such as high temperature or high pressure (Huang, Yao et al., 2016; Mession, Roustel, & Saurel, 2017), or can be induced by chemical processes, such as addition of ions (Feng, Kopplin, Sato, Draget, & Vãrum, 2016), enzymatic reaction, and acidification (Agoub, Giannouli, & Morris, 2009; Ehrbar et al., 2007). The δ-lactone of D-gluconic acid has a central position in biological systems. Since, its 6-phosphate derivative is the immediate oxidation https://doi.org/10.1016/j.foostr.2020.100137 Received 18 September 2019; Received in revised form 10 February 2020; Accepted 6 March 2020 ⁎ Corresponding author at: Laboratório de Engenharia e Tecnologia Agroindustrial (LETA), Universidade Federal Fluminense (UFF), Av. dos Trabalhadores, 420, Volta Redonda, RJ, 27255-125, Brazil. E-mail address: edwinr@id.uff.br (E.E. Garcia-Rojas). Food Structure 24 (2020) 100137 Available online 12 March 2020 2213-3291/ © 2020 Elsevier Ltd. All rights reserved. T