Hydrogen bonding enhances the electrostatic complex coacervation between κ-carrageenan and gelatin

• κ-Carrageenan/gelatin mixture exhibited a peculiar turbidity increase on cooling.
• It is attributed to the hydrogen bonding formation between the biopolymers.
• Hydrogen bonding could reinforce their electrostatic complex coacervation.
• Effects of pH, ionic strength, mixing concentration and ratio were clarified.

The phase behavior of sodium type κ-carrageenan (KC) and type B gelatin during cooling was studied using differential scanning calorimetry, thermal scanning rheology, turbidimetry, and confocal laser scanning microscopy. The turbidity of each single biopolymer solution and its mixtures stayed low above 40 °C, but that of the mixture began to increase steadily at 40 °C and then slowed down at 15 °C. Although it is already known that KC/gelatin mixture shows complex coacervation via electrostatic attraction, the dramatic increase of the turbidity below 40 °C is reported for the first time here. This peculiar transition is ascribed to the coacervation reinforced by hydrogen bonding between KC and gelatin. The second step of the turbidity increase below 15 °C is presumably related to the formation of KC helices that alters the hydrogen bonding with gelatin. The mechanism of hydrogen bonding reinforced complex coacervation was confirmed by mixing calorimetry and by examining the effects of urea and glycerol. Factors influencing the transition, including pH, ionic strength, mixing concentration and ratio, were discussed in relation to complex coacervation.

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