Oscillatory rheological study of the effects of pH and salts on gel development in heated whey protein concentrate solutions

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Abstract
The effects of pH and added NaCl or CaCl2 on gel development in heated whey protein concentrate (WPC) solutions were studied by oscillatory rheometry using a Bohlin rheometer, and by qualitative visual and tactile observations. The storage modulus (G′) exhibited maxima at pH values of about 4 and 7, while the phase angle (δ) exhibited minima at the same pH values. At pH 5·5, which lies within the range of the isoelectric points of the whey proteins, G′ was a minimum while δ was a maximum. At pH 7 and 8 G′ increased to a maximum and then decreased as added ionic strength was increased. At pH 4 G′ decreased steadily but slowly with increasing added ionic strength. The effects of CaCl2 were essentially the same as those of NaCl, but occurred at much lower added ionic strength. All the results are discussed in physicochemical terms. The main conclusions were, firstly, that changes in pH and salt concentration both affected gel development by altering the balance between attractive and repulsive protein-protein interactions, and secondly, that the effects of pH, salt concentration and salt type were interactive. Interaction both partly explains the wide variation in gel character and rheological properties among commercial WPC and potentially provides means of tailoring the gelling behaviour of a WPC so that it functions in a desired way in a given food system.