Rheological Characterization of Hydrogels Based on Gelatin/Κ-Carrageenan Mixtures

ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 26, 2018

Rheological characterization of hydrogels based on gelatin/κ-carrageenan mixtures

Daria S. Kolotova, Nikolay G. Voronko, Yulia A. Kuchina, and Svetlana R. Derkach

Murmansk State Technical University, Russia, Murmansk

ABSTRACT form polyelectrolytes complexes in aqueous In this study, influence of concentration solutions in the result of interaction between of κ-carrageenan on rheological properties charged groups of gelatin and macro ions of and thermal stability of gelatin/κ- polysaccharide1. One of the promising carrageenan hydrogels has been studied. It polysaccharides for creating functional food was shown that an increase of the products is κ-carrageenan which is non- concentration of κ-carrageenan leads to toxic and biodegradable. Numerous studies growth of the thermal stability of gelatin/κ- demonstrate that addition of polysaccharides carrageenan gels. The strength of gelatin effects on rheological properties of gelatin- gels increases with the increase of κ- based gels2. This is achieved by formation carrageenan concentration. The models of of polyelectrolyte complexes in aqueous gelatin/polysaccharide complexes and phase and, in so doing, the yield stress and mechanisms of their formation are viscosity of gels growth with increasing discussed. Properties of hydrogels, modified concentration of polysaccharide. by κ-carrageenan, change due to the This current work is aimed to study of formation of complexes caused generally by rheological properties of gels formed from electrostatic interaction between positively gelatin and κ-carrageenan mixtures at charged groups of gelatin and negatively different polysaccharide/gelatin ratio. The charged sulfate groups of κ-carrageenan. impact of κ-carrageenan on thermal and Structure of modified hydrogels was rheological properties of κ- studied. carrageenan/gelatin gels was investigated. The mechanism of interaction between INTRODUCTION components including formation of Native gelatin is widely used as a polyelectrolyte complexes is discussed. gelation agent in food production. The applicability of gelatin is limited by physical MATERIALS AND METHODS and chemical properties of gelatin gels, An alkaline gelatin (225 Bloom) from particularly, thermal stability and strength of bovine skin with average molecular weight the gels. In some cases, modification of of 1·105 g/mol was used (Sigma Aldrich, gelatin is required. The gelatin can be Germany). pH at the isoelectric point of modified by additives of surfactants and gelatin is 4.9. The sample of κ-carrageenan biopolymers. One of the most popular and from the red algae with average molecular perspective methods of gelatin modification weight of 6.8·105 g/mol was used (Sigma is using of polysaccharides as a co-gelation Aldrich). agents. The action of polysaccharides is to

D. S. Kolotova et al.

Aqueous solutions of gelatin and k- carrageenan/gelatin ratio. The sol-gel carrageenan were prepared separately. The transition temperature sharply increases at samples were preliminary swelled in the κ-carrageenan/gelatin ratio of 0.05. distillated water at 20 °C for 1 h. Then they Gelling temperature of modified gels were dissolved at 50 °C and 80 °C increased as κ-carrageenan/gelatin ratio respectively. After that, aqueous solutions of increased. The addition of κ-carrageenan biopolymers were mixed together in a given strongly affected the thermo-stability of κ- ratio Z = mcarrageenan/mgelatin. Concentration of carrageenan/gelatin hydrogels what can be κ-carrageenan was ranging from 0.01 to 1 related with formation of polyelectrolyte wt. %. Concentration of gelatin in mixtures complexes between positively charged was 1wt. %. Gels were obtained from gelatin and negatively charged κ- aqueous solutions at 14 °C for 5 h. carrageenan with following transformation Rheological properties of gelatin and of supramolecular structure of hydrogels. modified gels were determined using the Physica MCR 302 rheometer with a cone- plate measuring cell. The cone diameter was 50 mm and the cone to plate angle was 1°. The gap between the cone apex and plate was 0.100 mm. The amplitude dependencies were obtained at 14 °C at constant frequency of 1 Hz at amplitude ranging from 0.1 to 1000 %. Frequency dependencies were measured at constant amplitude of 1 % and at constant temperature of 14 °C in the -1 frequency range from 0.05 to 200 s . The melting and gelling temperatures of Figure 1. Dependencies of gelling gelatin/κ-carrageenan based gels were temperature (Tg) and melting temperature measured at a constant frequency of 1 Hz (Tm) of carrageenan/gelatin based gels on Z. and a constant applied strain of 1 %. The γ = 1 %, ω = 6.28 с-1. samples were heated from 14 to 50 °C and subsequently cooled to 5 °C at constant rate of 2 °C/min. FT-IR spectra of samples were registered with IR-Fourier spectrometer Nicolet 700 (Thermo Scientific, Madison WI, USA) in the middle range of 400-4000 cm-1 IR radiation. Gel microstructure was studied by the scanning electron microscopy (SEM) using a microscope S405-A (Hitachi, Tokyo, Japan) supplied with the SEM LEO- 420 software.

RESULTS AND DISCUSSION The "melt transition temperature" is Figure 2. Dependencies of the yield stress σγ determined as the crossover point of storage and the storage modulus G´ on the κ- and loss modulus. Figure 1 demonstrates the carrageenan/gelatin ratio Z. dependence of the melting and gelling temperatures of hydrogels on the κ-

72 ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 26, 2018

The measurements of the viscoelastic properties of the systems after gelation (at 14 °C) show that they are physical gels. The storage modulus practically does not depend on frequency in the linear domain of viscoelastic behavior. Similar behavior is characteristic the solid-like materials. This Figure 3. Micrographs of gels structure: (a) applies for both modified and native gelatin – 20 wt% of native gelatin, (b) - the same gels. The storage modulus of modified gels gel with adding of κ-carrageenan (C = 0.5 exceeds modulus of individual components car wt %) obtained at 1000x magnification. at all frequencies (Figure 2). When carrageenan was incorporated, These changes are initiated by interactions carrageenan/gelatin gels became more between helix zones in gelatin with resistant to applied force as shown by the polysaccharide due to the formation of κ- increased gel strength. carrageenan-gelatin complexes. One can Intermolecular interaction between believe that contacts with κ-carrageenan in gelatin and κ-carrageenan is confirmed by gels based on carrageenan-gelatin mixtures the FT-IR spectroscopy data. The shift of are stronger then hydrogen bonds formed by Amide I and Amide II lines to the low gelatin that leads to the formation of the frequency region appeared in the result of interactions between helix zones in interaction between positively charged macromolecules. This leads to the increase amide groups in a polypeptide chain of of viscoelastic properties. gelatin and negatively charged sulfate groups in κ-carrageenan with following ACKNOWLEDGMENTS formation of polyelectrolyte complexes. The The study supported by the Russian shift of the Amide I band into the low Science Foundation (project No. 16-16- frequency range suggests that the 00076). conformation state of gelatin macromolecules changes with an increase of REFERENCES the order structures as the result of complex 1. Voron’ko, N. G., Derkach, S. R., Vovk, formation. Evolution of the conformation M. A., & Tolstoy, P. M. (2016), "Formation state obliged to modification and an increase of κ-carrageenan–gelatin polyelectrolyte in intermolecular contacts might cause an complexes studied by 1H NMR, UV increase of the strength and viscoelastic spectroscopy and kinematic viscosity parameters of modified gels. measurements", Carbohydrate polymers, Supramolecular structure of native 151, 1152-1161. gelatin formed in the result of the self- 2. Derkach, S. R., Ilyin, S. O., Maklakova, organization process is characterized by A. A., Kulichikhin, V. G., & Malkin, A. Y. transversal collagen like helix (Fig. 3a). The (2015), "The rheology of gelatin hydrogels size of helix strongly depends on gelatin modified by κ-carrageenan", LWT-Food concentration. Network is formed by fibrils. Science and Technology, 63(1), 612-619. Furthermore, gel structure includes discrete zones distributed along fibrils inside a network, which presumably consist mainly of non-helical pieces of macromolecules. Addition of even small quantity κ- carrageenan leads to significant changes in the gel structure (Fig. 3b).