Structure evolution of carrageenan/milk gels: effect of shearing, carrageenan concentration and nu fraction on rheological behavior

Carrageenans, sulphated polysaccharides extracted from red seaweeds, are widely added to milk systems because of the gelation they promote and for their ability to recover a structure at rest after shearing. The gelation of iota-carrageenan is closely related to the helix-coil transition they undergo at approximately 48 °C in milk. Carrageenan gels (e.g. iota with and without nu precursors) were formed in milk or permeate by decreasing the temperature from 60 to 10 °C following various mechanical treatments, e.g. gelation and ageing of the gel at rest, gelation under shearing and ageing at rest. Gels were characterized rheologically during cooling and ageing, performed in the dynamic mode, to follow the mechanical recovery of the mixed systems at rest. In many cases, a strain-hardening behavior was observed using strain sweep measurements: above the upper limit of the linear domain, stress increased more than proportionally to the strain. Two types of iota-carrageenan were compared: iota with and without nu precursors. The content of the precursor affects the ability to form helical structures. Samples without precursor gave the highest level of helical structures. The carrageenan concentration effect was tested in the range 0.1–0.5 wt%. Differences in sheared and unsheared gel properties, particularly in large strain conditions, are discussed in terms of gelation process, phase separation, carrageenan/milk proportion and carrageenan type. The industrial interest in using either type of carrageenan is discussed.