Correlation between sagging and shear elasticity in pectin, gelatin and polyacrylamide gels H. Nielsen a,b , B.U. Marr b , S. Hvidt a, * a Department of Chemistry, Roskilde University, DK-4000 Roskilde, Denmark b Hercules Copenhagen, DK-4623 Lille Skensved, Denmark Received 25 April 2000; accepted 19 June 2000 Abstract The relationship between the height of gels determined by a sag test and their elastic shear modulus G 0 ) has been both investigated experimentally and simulated using a ®nite element analysis for the inhomogeneous deformation of gels due to gravity. It was assumed in the simulations that gels can be modeled as incompressible linear elastic materials. General relationships between the sag of gels and their elastic modulus were obtained from the simulations for slip and no-slip conditions. The relationships were tested experimentally on pectin, gelatin and polyacrylamide gels with a range of concentrations and rigidities. The good agreement between the predictions and the results shows that these gels can be modeled accurately as incompressible elastic materials. A standard 1508 SAG pectin gel, which sags 23.5% in the SAG test, has G 0 moduli of 429 and 379 Pa under slip and no-slip conditions, respectively. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Elastic modulus; Pectin; Gelatin; Polyacrylamide; Gel; Sag; Rheology 1. Introduction Pectin is a polysaccharide, which is widely used in the food industry as a gelling agent Thakur, Singh & Handa, 1997). Pectin is the dominant agent used in the industry to provide texture in jams and other sweet gels, e.g. confec- tionery gels Rolin, Nielsen & Glahn, 1998). The consu- mer's acceptance of a jelly is largely dependent on its gel strength which is due to the pectin network in the gel Crandall & Wicker, 1986). It is therefore important to be able to determine the gel strength of pectin gels. In 1959 the American Pectin Committee ®nished several years of research resulting in the development of the widely used ªSAG testº for determining the grade strength of pectins Baker et al., 1959). This test is based on measuring the sag, which is the height decrease due to gravity of a pectin gel made in glasses of very precise dimensions. A ridgeli- meter is used to determine the percent sag Baker et al., 1959; Cox & Higby, 1944). The conventional SAG test fails to give information about gel strength for many pectin applications, since it only describes the pectin grade at well- de®ned pH and soluble solid content conditions. The measured sag is slightly dependent on time, and it is deter- mined 2 min after inversion of the gel in the recommended SAG test Baker et al., 1959). A typical standardized jelly strength for a commercial pectin sample is 1508 grade SAG and this grade corresponds to 23.5% sag. In the standard SAG test the pectin and sugar contents are varied until the gel has the desired grade, e.g. 1508 Cox & Higby 1944; Ehrlich, 1968). A correlation between the pectin SAG grade, intrinsic viscosity and breaking strength has been proposed Christensen, 1954; Swenson, Schultz & Owens, 1953). They showed that the intrinsic viscosity allows a quick esti- mate of the SAG grade. Dunstan, Mahon, and Boger 1997) proposed a linear relationship between sag or normalized slump and G 0 . It has also been shown that high methoxyl HM) pectin SAG gels exhibit viscoelastic properties, and that an increase in the pectin concentration results in a decrease in percent sag Watson, 1966). In the same way in which the SAG test is a common test for pectin gels, gelatin is often graded by the Bloom test Bloom, 1925), which has been evaluated by Borker, Stefanucci and Lewis 1966). A 6.67% gelatin gel in an aqueous solution is prepared, and the gel strength is deter- mined with a plunger-weight system, where the load for penetrating 4 mm into the gel is determined. Gelatin gels are cast in special glasses and test results are only of comparative value Borker et al., 1966; Stevens, Wijaya & Paterson, 1997). Another simple laboratory test was proposed by Lockwood and Hayes 1931), but this test Carbohydrate Polymers 45 2001) 395±401 0144-8617/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0144-861700)00266-6 www.elsevier.com/locate/carbpol * Corresponding author. Tel.: 145-46-742477; fax: 145-46-743011. E-mail address: hvidt@ruc.dk S. Hvidt).