Structure formation in sugar containing pectin gels – Influence of Ca2+ on the gelation of low-methoxylated pectin at acidic pH

A new method for the examination of the pectin gelation process is presented as a complementation of the most common determination of the gelling point (cross-over of G′ and G″) from oscillation measurements. It is based on the first derivation dG′/dt from oscillation measurements (named as structuring velocity), and defines an initial as well as a critical structuring temperature. These allow an exact determination of the start of structure formation and description of the structuring process also in gels with pre-gelation that showed no clear GP. Moreover, phases and mechanisms of gelation can be identified and structure developing rates can be calculated.The application of this method on the gelation of low-methoxylated pectin at pH 3 and 30% saccharose with different contents of Ca2+ was tested. The results show differences as well as similarities between the GP and the newly defined structuring parameters that could be partly explained by varying structuring mechanisms at different Ca-content. The initial structuring process started probably with ionic interactions (egg-box junction zones and random crosslinks) via Ca-bridges as well as hydrophobic interactions at temperatures ≥ 60 °C, it was nearly completed around 40 °C. Hydrophilic interactions (below 50 °C) and inter-dimer aggregations (below 25 °C) perhaps dominated the gelation during further cooling. In dependence on the Ca-content, two to three phases could be identified during the structuring process. The properties of the gels after cooling were tested by oscillation measurements as well as the USA-sag method. With increasing calcium content the elastic behaviour of the gels increased but they became also more and more brittle.

New structure development parameters for pectin gelation as calculated from the first derivation dG′/dt from an oscillation measurement. Gel of low-methoxylated pectin at pH 3, containing 30% saccharose and 0.62 mM CaCl2/100 g. IST = initial structuring temperature = first deviation of dG′/dt from 0; CST = critical structuring temperature = extrapolated first strong increase of dG′/dt; — = dG′/dt, --- = storage modulus G′; end = end level at 10 °C, X GP = classical gel point as cross-over of G′ and G″. I, II, III = phases of the structuring process.Figure optionsDownload as PowerPoint slide