FABRICATION AND RHEOLOGICAL CHARACTERIZATION OF WHEY PROTEIN ISOLATE FLUID GELS

The production of colloidal gelled structures offers a broad range of material properties that can be modified to meet specific product needs. In this regard, fluid gels (FG) are suspensions of gelled particles whose structure and physical dimensions can be tailored to impart a wide range of rheological properties. The objective of this study was to evaluate the operational conditions of FG fabrication on their rheological properties. Whey protein isolate (12% w/w) at natural pH (6.7) was used to fabricate FG using a controlled stress rheometer with a stirrer geometry. FG were formed at 500 or 600 rpm by means of a thermal treatment consisting of three stages: (i) heating stage at 6.5 °C/min from an initial temperature of 25 °C, (ii) constant temperature stage at 80 °C for 6 or 7 min and (iii) cooling stage at 4.5 °C/min until a final temperature of 25 °C. The change in apparent viscosity was recorded during the FG formation by the rheometer. Additionally, ascendant (0.1 to 100 s-1) and descendant (100 to 0.1 s-1) flow curves at 25 °C were measured in the same equipment using a concentric cylinder configuration with a sandblasted probe after 30 min of FG formation. Oscillatory measurements were done in a oscillatory rheometer after 30 min of FG fabrication using plate-plate geometry (10 cm diameter) with a sandblasted upper plate, measurements were done between 0.1 and 100 Hz and at 10% of deformation, obtaining the storage (G’) and loss (G’’) modulus. Apparent viscosity at the end of the FG fabrication process was a function of the process parameters, varying from 170 +- 12 to 289 +- 40 mPa s. Increasing the time of the constant temperature stage and the agitation speed generated higher apparent viscosity values. Rheograms for all FG presented a yield stress (201 +- 5 to 313 +- 14 Pa), the phenomena of stress overshot (peaks between 206 +- 4 and 396 +- 8 Pa) and hysteresis (5023 +- 22 to 7851 +- 74 Pa s-1) between ascendant and descendant flow curves. Increasing the time of the constant temperature stage and the agitation speed produced higher values for these three rheological parameters. Under oscillatory test, FG showed a viscoelastic behavior with G’ values always higher than G’’ values, therefore, the elastic component was predominant over the viscous component. Storage modulus ranged between 42227 +- 9383 and 311060 +- 11494 Pa and loss modulus ranged between 7664 +- 1775 and 71194 +- 3359 Pa. Rheological properties of whey protein isolate FG can be tailored by modifying the process parameters (thermal treatment and agitation speed) during their fabrication allowing to obtain food materials for specific applications, like thickening ingredients or fat replacers.