Interactions and phase behaviors in mixed solutions of κ-carrageenan and myofibrillar protein extracted from Alaska Pollock surimi.

In the present work, we provide insight into electrostatic interactions and phase behaviors in mixtures of myofibrillar protein (MP) and κ-carrageenan (KC) of various pHs (8.0-3.0) and biopolymer weight ratios [R, from 1:1 to 20:1; total concentration=0.05% (w/w)] through turbidimetric analysis, dynamic light scattering (DLS) and zeta potential analysis, and optical microscopy. At R=1: 1, critical pH values (i.e., pHc, pHΦ1, and pHΦ2), which indicate phase transitions interrelated to the formation of soluble or insoluble MP-KC complexes, were observed at pH7.6, 6.8, and 3.6, respectively. As the ratio increased, the pHmax shifted from 4.9 to 5.8. A similar trend was observed for the isoelectric point of MP-KC mixtures, as determined by zeta potential measurements. The maximum interaction indicated by the highest turbidity occurred at pHmax=5.3, at a ratio of 5:1, whereas pHc remained constant during acidification. The changes in electrostatic interactions and transformations of phase behaviors accompanying the complex formation and disassociation processes were further supported by particle size distribution analysis and optical microscopic observations of MP-KC mixtures (R=1:1) at different pH values. This work fills the previous lack of studies on phase behaviors of surimi protein and colloidal polysaccharide in liquid system, and lay the foundation to provide a new way to solve problems of interactions between protein and polysaccharide during the processing of surimi products.