The phase behavior of sodium type-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.