The effects of acidification rate, pH and ageing time on the acidic cold set gelation of whey proteins

The gelation behaviour of 7% whey protein isolate (WPI) solutions under cold set conditions was studied. Gels were formed at 10 degrees C after the addition of different amounts of glucono-delta-lactone (GDL) to thermally denatured (80 degrees C/30 min) WPI solutions, leading to different acidification rates and final pH values. The theological properties of the systems were analysed under shear at incipient gelation and under uniaxial compression throughout the entire gelation process until reaching the equilibrium conditions of pH and mechanical properties. Different rheological techniques allowed for the distinction of two stages involved in the gelation process: the initial setting up of the gel network up to the gel point and the subsequent structure development through bond strengthening and/or local rearrangements. The acidification rate did not affect the rheological properties at the beginning of gelation, but the acidification conditions had a significant effect on the strengthening of the protein network after the gel point. An evaluation of the mechanical properties at specific pH values during gelation and under the equilibrium conditions permitted a qualitative distinction amongst the effects of the acidification rate, pH and structural organization on the gel properties. WPI cold set gelation induced by GDL hydrolysis showed a strong influence of the pH during acidification if low acidification rates were used, and when the final pH was achieved the properties remained stable. However, when the rate of acidification increased by adding larger amounts of GDL to the system, the effect of molecular rearrangements at the final pH was fundamental to network strengthening. (C) 2007 Elsevier Ltd. All rights reserved.