Gelation properties of milk concentrated by different techniques

The rheological properties of three different concentrated yoghurts (similar to 23% total solids, pH 4.0) and a standard yoghurt (similar to 16% total solids, pH 4.3) were determined during incubation and storage. The high solids yoghurts were manufactured from milks concentrated by ultrafiltration (UF) to 9% protein, by reverse osmosis (RO) to similar to 6.8% protein and by direct reconstitution of full-fat milk powder to similar to 6.4% protein; the standard product contained similar to 4.3% protein. Development of the gel structures during incubation was monitored discontinuously using a stress-controlled oscillatory dynamic rheometer. The complex modulus (G*) and loss tangent (tan delta) were measured as functions of amplitude (torque range 10(-3)-10(-2) mNm at 0.25 Hz). In the RO- and direct reconstitution yoghurts, a typical pattern of gelation was observed, with the initial viscous nature of the milk being modified by rapid development of a gel structure and, finally, a 'stationary phase' with respect to rheological changes. However, the complex modulus (G*) of the higher protein system (UF-milk) continued to increase with time, i.e., further development of the gel structure occurred during storage. The relative rates of gel development were dependent on protein content. (C) 1999 Elsevier Science Ltd. All rights reserved.