Flow and viscoelastic properties of concentrated milk treated by high hydrostatic pressure

The present study analysed the effects of high isobaric pressure on the rheological and structural characteristics of concentrated milk, Milk concentrates receiving high hydrostatic pressure (HHP) treatment in the range of 276-690 MPa were studied. Apparent viscosity and storage modulus quantitatively reflected the influence of the applied high pressure treatment, and the pressure-induced gelation of milk concentrates led to the formation of a fine network that exhibited both viscous and elastic properties. The rheological behavior of milk concentrates was characterized as a function of solids content (129 to 471 g/kg) and HHP treatment. Steady and dynamic shear were applied to follow the rheological response of concentrated milk prior to and after the HHP process. The milk concentrates' consistency expressed by the apparent viscosity as a function of the concentration level was fitted by a single-term exponential model (ln mu(a) = A + bX), whereas as a function of die concentration and pressure it was modeled by a two-term logarithmic equation (ln mu(a) = beta(0) + beta(1)X + beta(2)P). The storage modulus, which reflected the effect of both variables very well, was plotted as a function of the shear frequency. Based on the rheological results exhibited by the milk concentrates subjected to 414 MPa, this treatment was selected as the most convenient pressure for further analyses. Scanning electronic microscopy allowed observation of the structural changes of concentrated milk due to the HHP and the differences among pressure treatments. HHP led to the disintegration of the native protein of milk concentrates, producing a gel of relatively uniform structure. Changes detected in viscosity, viscoelasticity and texture for concentrated milk could be attributed to the HHP process, which mainly affected the protein structure. (C) 1998 Academic Press Limited.