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-delta-lactone, has been investigated at 25 degreesC. 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 greater than or equal to0.05% (w/v). Complete inhibition of acid-induced caseinate gelation was noted at greater than or equal to0.8% (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 (greater than or equal to0.2%, 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). (C) 2003 Elsevier Ltd. All rights reserved.