Abstract
We have studied the influence of the calcium ion concentration, [Ca 2+], and the pH on the storage ( G′) and loss ( G″) shear modulus at 1 Hz of low methoxyl pectin solutions and gels. Upon lowering the temperature in the presence of Ca 2+, G′ and G″ increase immediately followed by a further slow logarithmic increase with time. The immediate response increases with increasing [Ca 2+] and decreasing temperature. The ‘gel’ temperature ( T g) where G′ and G″ cross increases with increasing [Ca 2+]. However, G′ and G″ have a universal temperature dependence at all [Ca 2+] if plotted as a function of T− T g. The pH was reduced slowly by addition of GDL. Reduction of the pH to below 3 leads to the formation of strong gels for amidated pectin and much weaker gels for non-amidated pectin. For non-amidated pectin lowering the pH below 3 leads to a weakening of the gel formed by Ca 2+, while for amidated pectin the gel is reinforced.
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