Calcium and pH influence on orange juice cloud stability.

Background: Clarification of citrus juice is a severe quality defect related to pectin methylesterase (PME) activity. PME activity and calcium chelation of pectic acid as well as other physical interactions of cloud components influence cloud stability. The cloud stability and physical properties of pulp-free, fresh juice with and without ammonium oxalate (AO) at pH 4.0 and pH 5.5 was evaluated.

Results: The only juice to clarify in the 3-week study was the sample without AO at pH 4.0. Particle size analysis showed that the samples at pH 4.0 were larger than those at pH 5.5, and samples at pH 5.5 had a more negative zeta potential than samples at pH 4.0. Furthermore, cloud particle size increased and then decreased prior to the onset of clarification.

Conclusion: Ammonium oxalate prevented sedimentation via calcium pectate cross-bridges and subsequent clarification. Interaction of cloud constituents, change in particle size with pH and change in particle size with storage time suggest that, in addition to electrostatic attraction and calcium binding, cloud particles associate and dissociate via non-covalent, non-electrostatic interactions.