Properties of acid casein gels made by acidification with glucono-δ-lactone. 2. Syneresis, permeability and microstructural properties

Abstract

Syneresis of casein gels made by acidification with glucono-δ-lactone (GDL) was studied in relation to gel structure as derived from permeametry and confocal scanning laser microscopy (CSLM). Gels made at 40 °C exhibited ‘spontaneous syneresis’ with wheying-off almost immediately after gelation, while those formed at 30 °C exhibited little syneresis and only after it was initiated by wetting the surface. Syneresis decreased with a reduction in the pH of gels (increasing amount of GDL added). Gels cooled to 5 °C (before initiating syneresis) and low pH gels exhibited ‘negative syneresis’, i.e. an increase in the height of the gel after wetting. Addition of NaCl had little effect on syneresis, except at pH values > 4.6, where gels with added NaCl exhibited stronger syneresis than those made without added NaCl. Higher gelation temperatures resulted in a far greater permeability coefficient (B), indicating the presence of large pores in these gels. Gels formed at low temperatures had a very low B. Addition of NaCl, at all gelation temperatures, markedly reduced B. Confocal scanning micrographs showed that gels made at high gelation temperatures had large pores, many > 20 μm. At low gelation temperatures, the pores were small, mostly < 5 μm. Fractal aggregation theory was used to explain some of the results, especially the rearrangement of aggregated particles at an early stage of the gelation process, i.e. over relatively short distances. It was concluded that gelation temperature had a large effect on this rearrangement. At low temperatures (e.g. 20 °C), rearrangement did not occur, whereas it was already extensive at 30 °C, implying that the ‘building blocks’ of the fractal gel consisted of dense aggregates of, say, 25 casein particles. This resulted in increased permeability. It was also concluded that the syneresis occurring at 30 °C is primarily due to consolidation of the gel network under its own weight, which is soon counteracted by the stress induced by the deformation of this network. Gels formed at higher temperatures may be unstable and show spontaneous syneresis, presumably because some of the strands in the gel network were weak enough to break. No conclusive explanation could be given for the effects of NaCl concentration and pH on the properties of acid casein gels.