Effects of low-methoxyl amidated pectin and ionic calcium on rheology and microstructure of acid-induced sodium caseinate gels

The influence of low-methoxyl amidated pectin on the rheological and microstructural properties of sodium caseinate gels (2%, w/v), made by acidifying with glucono-δ-lactone, has been investigated at 25 °C. In the absence of calcium ions, a substantial reduction in the time-dependent storage modulus was detected at pectin concentrations as low as 0.01–0.02% (w/v), and a significant increase in gelation time at pectin concentrations ≥0.05% (w/v). Complete inhibition of acid-induced caseinate gelation was noted at ≥0.8% (w/v) pectin. Confocal microscopy has confirmed that the presence of an increasing content of pectin prevents aggregation of caseinate particles into a coherent network. Consistent with the sensitivity of both these biopolymers to calcium ions, the addition of calcium chloride (2 mol Ca2+/mol caseinate) has a major influence on the interactions in the caseinate+pectin system. Whereas for low pectin content the modulus was reduced (as for the calcium-free system), there was found to be a large enhancement of the time-dependent modulus at higher pectin contents (≥0.2%, w/v). The combination of a gradual release of calcium ions from caseinate aggregates as the pH is lowered and the electrostatic adsorption of pectin onto the casein particles influences the balance of casein–casein, pectin–pectin and casein–pectin interactions in the final gel structure. Strong syneresis was clearly evident in calcium-containing systems with 0.2–0.6% (w/v) pectin. Microscopy confirmed the total absence of the characteristic acid–casein gel network connections at high pectin content (0.8%, w/v).