Kappa-carrageenan enhances the gelation and structural changes of egg yolk via electrostatic interactions with yolk protein Min Huang a, b , Yuzhu Mao a, b , Hongliang Li c , Hongshun Yang a, b, * a Department of Food Science and Technology, National University of Singapore, 117542, Singapore b National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou Industrial Park, Suzhou, Jiangsu 215123, PR China c Guangzhou Welbon Biological Technology Co., Ltd, Guangzhou, Guangdong 523660, PR China A R T I C L E INFO Keywords: Protein Polysaccharide Carrageenan Rheology Gelation Physicochemical property CLSM FTIR ABSTRACT The effect of κ-carrageenan (κ-C) on yolk over heat-induced gelation at natural yolk pH (6.2) and natural whole egg pH (7.5) was studied. The results showed the zeta potential values changed from -2.3 to -31.3 mV, from -8.6 to -28.6 mV for native pH yolk and pH 7.5 yolk because of the κ-C addition, respectively. These results indicated electrostatic interactions formed between protein and κ-C. The average area of holes formed by yolk gelation increased by κ-C addition. The addition of 1.0% κ-C decreased the gelling points from 62.1 to 54.4 ◦ C, from 64.5 to 61. 6 ◦ C for native pH and pH 7.5 yolk, respectively. A schematic model was established to show that κ-C enhances the yolk properties via electrostatic interactions. And the Fourier transform infrared (FTIR) spectros- copy verifed the formation of κ-C-protein interactions. This study provides a guidance for designing novel food systems containing yolk and κ-C. 1. Introduction Egg yolk, as a source of high quality protein, is widely used in food products such as cakes. It also presents superior functional properties such as emulsifying, coagulating, and gelling (Aguilar, Cordob´ es, Ray- mundo, & Guerrero, 2017; Slade, Kweon, & Levine, 2020; Zhang et al., 2019). Considering the composition, egg yolk comprises approximately 50% water, 31–35% lipids, 15–17% proteins, and 1% carbohydrates (Abeyrathne, Lee, & Ahn, 2013; Aguilar et al., 2017). On structural level, yolk is a complex system consisting of aggregates (granules) in suspen- sion in yellow fuid (plasma) with low-density lipoproteins (LDLs) and other proteins (Anton, 2013; Yang et al., 2020). Kappa-carrageenan (κ-C) is a natural sulphated polysaccharide extracted from red seaweeds and used as thickener, stabilizer, and texturing agent in food industry (Necas & Bartosikova, 2013; Yang, Gao, & Yang, 2020). The gelation of κ-C in aqueous solution involves two steps: the transition from initial random coil to helix structure and the double helices aggregation introduced by mono or double positive ions. The gelation temperature of κ-C can be affected by the types and con- centrations of cations, and the strength of the formed gels depends more on the concentration of κ-C and the types and concentrations of cations (Aguilar et al., 2017; Nguyen, Nicolai, Benyahia, & Chassenieux, 2014). However, the gel strength of κ-C can disappear when the pH values are below 5.5 in solutions and below 4.3 for heat-set systems due to auto- hydrolysis effect (Aguilar et al., 2017). Previous reports emphasised the importance of protein- polysaccharide interactions in designing novel food formulations (Doublier, Garnier, Renard, & Sanchez, 2000; Lopes-da-Silva & Mon- teiro, 2019; Wu, Lin, Singh, & Ye, 2020). The main non-covalent in- teractions between protein and polysaccharide include electrostatic interactions, steric exclusion, hydrophobic interactions, and hydrogen bonding (McClements, 2006). Protein-polysaccharide interactions play a critical role in fow, stability, texture, and mouth feel of food and other macroscopic properties. Furthermore, it can control the interactions between the biopolymers and then provide textural and sensory prop- erties through creating microstructure in food systems (Aguilar et al., 2011). The relative importance of these interactions in a specifc system hinges on several elements, including the polymers involved, the liquid mixture composition, and the conditions of the environment (McCle- ments, 2006). Egg yolk and κ-C are found together in many food products such as mayonnaise (Liu, Xu, & Guo, 2007). Indeed, the effects of κ-C on egg yolk properties were studied in previous literature (Aguilar et al., 2011, 2017). The network formed by heating in yolk/κ-C systems was * Corresponding author. E-mail address: fstynghs@nus.edu.sg (H. Yang). Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem https://doi.org/10.1016/j.foodchem.2021.129972 Received 13 November 2020; Received in revised form 5 April 2021; Accepted 25 April 2021