Relative maximum fluorescence intensity after 1 h incubation at different pH, using unheated WPI, and 24 h pre-heated WPI with (<i>R</i>_NEM/WPI = 0.5) and without NEM.

<p>Data normalized with the fluorescence intensity of aggregates without NEM at pH 9.3. Final [WPI] = 0.03 wt%, [ANS] = 0.05 mM, excitation wavelength λ_ex = 295 nm, emission wavelength λ_em = 464 nm. Lines show the best-fit sigmoidal curves to determine the pK_a.</p

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

The diameter profiles of 1 wt% WPI 24 h aggregates at different pH (unbuffered) and room temperature, without NEM (empty symbols, dashed lines) and with NEM at <i>R</i>_NEM/WPI = 0.5 (solid symbols, continuous lines).

<p>Dotted line shows comparable results from the literature for 2 h preheated WPI aggregates at pH 12.3 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164496#pone.0164496.ref012" target="_blank">12</a>].</p

Regression parameters using two power-law regimes, Eqs 1 and 2, and using a percolation model, Eq 3.

<p>Uncertainties given are the standard error of the regression parameters.</p