Rheol. Acta 19, 220-225 (1980) © 1980 Dr. Dietrich Steinkopff Verlag, Darmstadt ISSN 0035-4511 / ASTM-Coden: RHEAAK DepartmentJor Liberal Arts, Shizuokä University, Ohya, Shizuoka (Japan) Rheological properties of agarose-gelatin gels M. Watase and K. Nishinari With 10 figures and 2 tables (Received August 9, 1979) 1. Introduction Since gelatinous materials have been used in making sugar confectionary and for modifying texture in the food industry, their rheological properties have been studied by many authors, e.g. Glicksman (5), Sherman (13), Mitchell (7). Recently, the nature of the gelling processes generally has received considerable attention, cf. Flory et al. (4), and although much information on gels and gelling processes has been accu- mulated, the mechanism of gelation for many systems is not clear at present. Similarly, although the rheological properties of gelatin (Ward and Saunders (16), Veis (15), Watase and Arakawa (17) and agar-agar or agarose (Watase and Arakawa (18), Nishinari and Horiuchi (9)) have been investigated by many authors, the properties of mixed gels have received little attention. 2. Material and methods 2.1. Agarose Agarose has been shown to have a simple structure in which 3-1inked/3-D-galactopyranose residues and 4-1inked 3,6-anhydro-~-L-galacto- pyranose residues are arranged alternately, (Araki and Arai (3)). We used the preparative method of Akabane and Katsuura (1) which enables us to separate a large amount of pure agarose from agar-agar. Agarose was isolated from Gelidium amansii as follows: powdered Gelidium amansii, 1% by weight, was dissolved in water, and acrynol was added. Agarose was separated by filtration. The agarose gel so separated was melted by heating, mixed with ethyl alcohol, and centrifuged to separate the agarose. This purification with alcohol was repeated until the yellow color due to acrynol 587 vanished. The intrinsic viscosity of the purified agarose in KSCN aqueous solution (0.01 mol/1) was 3.5 (100 tal/g) at 35°C. 2.2. Gelatin A commercially-available powder (Wako Junyaku Kogyo Co.) was used. The intrinsic viscosity of the gelatin in NaC1 aqueous solution (0.15 mol/1) was 0.47 (100ml/g) at pH 7.0 and 55°C. The molecular weight as determined by Scatchard's equation (Scatchard et al. (12)) was 4.7.104. As is well known, the isoelectric point is determined by the pH value at which the velocity of electrophoresis becomes zero and the viscosity becomes minimum. The isoelectric point determined from the pH dependence of logt/r/c of 0.2wt% aqueous solution at 35°C was 4.7. The measurement of pH was made by use of a Toa Denpa pH meter. 2.3. Preparation of experimental samples The procedure for swelling and dissolution of the agarose and gelatin was the same as that in the previous papers (17, 18). Mixed gels of agarose and gelatin of various volume fractions were prepared: an agarose solution (3.0wt%) was heated at 100°C and gelatin solution (30wt%) heated to 80°C was added and quickly mixed, and poured into cylindrical moutds. The brief heating periods produced no perceptible drop in viscosity. Since each solu- tion was very hot and viscous, we must prepare a large amount of mixed solution (200 ml) to assure the exact mixing ratio. Cylindrical gels thus obtained were immersed in paraffin oil at 4°C for two weeks. The pH values of the gelatin and agarose solutions were within the range 5.6 to 6.2 (45°C).