Structural characteristics of carrageenan gels: temperature and concentration dependence Yoshiaki Yuguchi a, * , Thanh Thi Thu Thuy b , Hiroshi Urakawa b , Kanji Kajiwara b a National Institute of Advanced Industrial Science and Technology (AIST), 2217-14 Hayashi-cho, Takamatsu 761-0395, Japan b Faculty of Engineering and Design, Kyoto Institute of Technology, Kyoto, Sakyo-ku, Matsugasaki 606-8585, Japan Received 16 August 2001; revised 22 October 2001; accepted 19 November 2001 Abstract k-and i-carrageenan in aqueous solution undergoes reversible sol/gel transition by cooling or increasing concentration. The small-angle X-rayscatteringpro®lesfromcarrageenanswereobservedatdifferenttemperaturesandconcentrations.Theresultswereanalyzedbymeans of molecular models of double helix or associated double helices. It was found that k-carrageenan formed two or three associated double helices by gelation, although gelation took place in i-carrageenan mostly by the transition from single chain to double helix without association. l-carrageenan exhibited the normal scattering behavior of polyelectrolyte in solution. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Carrageenan; Gelation; Small-angle X-ray scattering; Cross-linking domain 1. Introduction Carrageenan is a sulfated polysaccharide extracted from redseaweed(Rhodophyceae).Carrageenanisclassi®edinto threetypesaskappa(k-),iota(i-)orlambda(l-)carrageenan according to the number (one, two or three) of sulfate groups per repeat unit of disaccharide, respectively. As shown in Fig. 1, k-carrageenan consists of a repeating unit composed of the disaccharide, b-(1-3)-d-galactose-4-sulfate and a- (1-4)-3,6-anhydro-d-galactose. i-carrageenanpossessestwo sulfate groups in a disaccharide repeat unit; b-(1-3)-d- galactose-4-sulfate and a-(1-4)-3,6-anhydro-d-galactose-2- sulfate. l-carrageenan consists of b-(1-3)-d-galactose-2- sulfate and a-(1-4)-d-galactose-2,6-disulfate including three sulfate groups. It should be noted that the represented structure is an ideal one and the real samples contain some extent of different kind of sequences. In an appropriate condition, k-carrageenan and i-carrageenan in aqueous solutions undergo thermoreversible sol±gel transition, while no gelation takes place in l-carrageenan having more electrolyte groups. Carrageenan is widely applied to food industry as gel or viscous agents, and also exhibits some physiological effect such as anti-tumor activities. Suchawideapplicationandaninterestinsol±geltransition phenomenon have promoted many fundamental studies on the aqueous solutions of carrageenan. It is widely accepted that the gelation of carrageenan is based on the formation of double helix structure proposed by (Anderson, Campbell, Harding, Rees, & Samuel, 1969). The structural model of a double helix was ®rst deduced from the X-ray diffraction image of carrageenan ®brils, and other results, for example, the comparison of experi- mental results with the calculated optical rotation (Rees, Williamson, Frangou, & Morris, 1982), or the dimerization of carrageenan segments investigated by light scattering (Jones, Staples, & Penman, 1973), support this model in- directly. Carrageenan assumes a random coil conformation in sol state, and low temperature induces anhydro-galactose sequences to twist in a double helical manner. The further aggregation is also promoted among formed double helical parts. A part of hydrated sequences function as a helix breaker. Subsequently the aggregation of double helices forms a cross-linking domain and leads the in®nite network structure enough to complete gelation. Since the carrageenan repeat units possess the electrical charge in sulfate groups, the counter ions are found to play a role in gelation. Morris et al. suggested the modi®ed gelation model by including the counter ions mediated between double helices (Morris, Rees, & Robinson, 1980). In this study, the gelation of carrageenan is discussed from theresultsofsmall-angleX-rayscattering(SAXS)observedat various temperatures and carrageenan concentrations. Food Hydrocolloids 16 (2002) 515±522 0268-005X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII:S0268-005X(01)00131-X www.elsevier.com/locate/foodhyd * Correspondingauthor.Tel.: 181-87-869-3511;fax: 181-87-869-3550. E-mail address: y-yuguchi@aist.go.jp (Y. Yuguchi).