PROPERTIES OF ACID CASEIN GELS MADE BY ACIDIFICATION WITH GLUCONO-DELTA-LACTONE - 2 - SYNERESIS, PERMEABILITY AND MICROSTRUCTURAL PROPERTIES

Syneresis of casein gels made by acidification with glucono-delta-lact one (GDL) was studied in relation to gel structure as derived from per meametry and confocal scanning laser microscopy (CSLM). Gels made at 4 0 degrees C exhibited 'spontaneous syneresis' with wheying-off almost immediately after gelation, while those formed at 30 degrees C exhibit ed little syneresis and only after it was initiated by wetting the sur face. Syneresis decreased with a reduction in the pH of gels (increasi ng amount of GDL added). Gels cooled to 5 degrees C (before initiating syneresis) and low pH gels exhibited 'negative syneresis', i.e. an in crease in the height of the gel after wetting. Addition of NaCl had li ttle effect on syneresis, except at pH values > 4.6, where gels with a dded NaCl exhibited stronger syneresis than those made without added N aCl. Higher gelation temperatures resulted in a far greater permeabili ty coefficient (B), indicating the presence of large pores in these ge ls. 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 la rge pores, many > 20 mu m. At low gelation temperatures, the pores wer e small, mostly < 5 mu m. Fractal aggregation theory was used to expla in some of the results, especially the rearrangement of aggregated par ticles at an early stage of the gelation process, i.e. over relatively short distances. It was concluded that gelation temperature had a lar ge effect on this rearrangement. At low temperatures (e.g. 20 degrees C), rearrangement did not occur, whereas it was already extensive at 3 0 degrees C, implying that the 'building blocks' of the fractal gel co nsisted of dense aggregates of, say, 25 casein particles. This resulte d in increased permeability. It was also concluded that the syneresis occurring at 30 degrees C is primarily due to consolidation of the gel network under its own weight, which is soon counteracted by the stres s induced by the deformation of this network. Gels formed at higher te mperatures may be unstable and show spontaneous syneresis, presumably because some of the strands in the gel network were weak enough to bre ak. No conclusive explanation could be given for the effects of NaCl c oncentration and pH on the properties of acid casein gels. (C) 1997 El sevier Science Ltd. All rights reserved.