Compression and impact strength of gels, prepared from fractionated whey proteins, in relation to composition and microstructure

Micromechanical Characterization of Hydrogels Undergoing Swelling and Dissolution at Alkaline pH

The swelling of polyelectrolyte hydrogels usually depends on the pH, and if the pH is high enough degradation can occur. A microindentation device was developed to dynamically test these processes in whey protein isolate hydrogels at alkaline pH 7–14. At low alkaline pH the shear modulus decreases during swelling, consistent with rubber elasticity theory, yet when chemical degradation occurs at pH ≥ 11.5 the modulus decreases quickly and extensively. The apparent modulus was constant with the indentation depth when swelling predominates, but gradients were observed when fast chemical degradation occurs at 0.05–0.1 M NaOH. In addition, these profiles were constant with time when dissolution rates are also constant, the first evidence that a swollen layer with steady state mechanical properties is achieved despite extensive dissolution. At >0.5 M NaOH, we provide mechanical evidence showing that most interactions inside the gels are destroyed, gels were very weak and hardly swell, yet they still dissolve very slowly. Microindentation can provide complementary valuable information to study the degradation of hydrogels.

Rheology of whey protein concentrate solutions as a function of concentration, temperature, pH and salt concentration

Rheological properties of whey protein concentrate (WPG) solutions were studied in steady shear, using a Bohlin VOR Rheometer, as a function of concentration, temperature, shear rate, shearing time, pH, salt type, salt concentration and solution age. At 22 °C and pH 7, the WPC solutions exhibited Newtonian behaviour up to a concentration of 10% total solids, pseudoplastic behaviour between 10 and 30% and time-dependent shear thinning at 35% and above. The apparent viscosity of solutions at 22 °C and pH 7 was linearly related to concentration up to 8%. The effect of temperature on apparent viscosity in the range 5–60 °C was closely described by the Arrhenius equation. The viscosities of WPC solutions were independent of solution age in the pH range 4–8 at all concentrations up to and including 20%, the precise pH range narrowing as concentration increased. At pH values above or below this range apparent viscosity became dependent on both pH and solution age, the age effect becoming more marked at higher WPC concentrations. Apparent viscosity at pH 7 increased markedly with both CaCl2 concentration and solution age at concentrations above 0·6 M-CaCl2, the age effect in this case increasing with CaCl2 concentration. In contrast, NaCl concentrations of up to 0·8 M-NaCl had little effect on apparent viscosity. The rheological behaviour of WPC solutions changed from time-independent to time-dependent shear thinning at high concentration, at extreme pH values, at high CaCl2 concentration (after ageing) and on heating to above ∼ 60 °C. This change is considered to be caused by the formation of structure in solutions; a 40% solution (at 22 °C and pH 6·75) exhibited classic thixotropic behaviour in a step–shear rate experiment.

Whey protein concentrates and isolates: processing and functional properties

Substantial progress has been made in understanding the basic chemical and structural properties of the principal whey proteins, that is, beta-lactoglobulin (beta-Lg), alpha-lactalbumin (alpha-La), bovine serum albumin (BSA), and immunoglobulin (Ig). This knowledge has been acquired in terms of: (1) procedures for isolation, purification, and characterization of the individual whey proteins in buffer solutions; and (2) whey fractionation technologies for manufacturing whey protein concentrates (WPC) with improved chemical and functional properties in food systems. This article is a critical review of selected publications related to (1) whey fractionation technology for manufacturing WPC and WPI; (2) fundamental properties of whey proteins; and (3) factors that affect protein functionality, that is, composition, protein structure, and processing.