Rheological properties of laccase-induced sugar beet pectin gels

The chemical properties of two different sugar beet pectin preparations and their gel formation due to laccase was studied. The pectins differed in their molecular weight, acetylation and methylation degree. No gelling was observed without laccase addition with the two pectins. The response of laccase to the gelling was also different and gels made of sugar beet pectin SPB with the aid of laccase with or without calcium were significantly harder than that of RU-L sugar beet pectins. In the pH range 3.5–5.0, the hardest texture was measured at pH 4.0. The rate of cross-linking increased with increasing enzyme activity. However, at the highest enzyme activity levels the magnitude of storage modulus after 5.5 h was lower than that of gels containing 100 and 150 nkat laccase/g pectin. The hardness of gels was also measured at large deformation after 1 day storage time. The gel with 67 nkat/g pectin laccase gave hardest gel. At higher laccase activity (100–500 nkat/g) the gel was softer suggesting that gels formed at slower rate had better properties. In the presence of Ca the hardness of gels was independent on laccase activity in the range 67–500 nkat/g. Viscoelastic measurements suggest that Ca retards the rate of covalent cross-linking and improves the texture at high enzyme activities.