Investigations Into the Mechanisms Responsible for the Yield Stress of Protein Based Foams

Abstract

Proteins function as natural surfactants in many industrial processes involving foam formation. Egg white proteins have traditionally served this role in the food industry, although substitution with other proteins, including those derived from bovine milk is becoming more prevalent. Above a critical gas phase, foams transition from viscous liquids to semi-solid materials that display a yield stress. Measurements of yield stress relate well to the empirical concept of foam robustness and more robust foams are generally desirable from a food science perspective, in order to withstand the rigors of processing, including pumping, heating, coating, etc. Accordingly, the general goal of this research was to investigate mechanisms responsible for foam yield stress on a fundamental level, in order to more efficiently utilize whey proteins (derived from bovine milk) as a foaming ingredient, with an emphasis on their capacity to regulate foam yield stress.

In the first study, the yield stress of whey protein isolate (WPI) foams as affected by electrostatic forces was investigated by whipping 10% w/v protein solutions prepared over a range of pH levels and salt concentrations. Measurements of foam overrun, protein adsorption kinetics at the air/water interface, and dilatational rheological characterization, aided data interpretation. Interfacial measurements were also made with the primary whey proteins, beta-lactoglobulin and alpha-lactalbumin. Yield stress of WPI foams was dependent on pH, salt type and salt concentration. In the absence of salt, yield stress was highest at pH 5.0 and lowest at pH 3.0. The addition of NaCl and CaCl2 significantly increased yield stress at pH 7.0, with equivalent molar concentrations of CaCl2 as compared to NaCl increasing yield stress to greater extents. Salts minimally affected foam yield stress at pH 3.0 or 5.0. Comparisons with interfacial rheological data suggested the protein's capacity to contribute towards yield stress was related to its potential at forming strong, elastic interfaces throughout the structure. Dynamic surface tension data for beta-lactoglobulin and alpha-lactalbumin were similar to WPI, while the interfacial rheological data displayed several noticeable differences.

In the second study, polymerized WPI (pWPI) was investigated for its potential as a functional foaming ingredient. Note that pWPI is a soluble complex of covalently bound whey protein formed via controlled heating. Foam yield stress displayed a parabolic response to increasing concentrations of pWPI to native WPI, peaking at 50%. Foam air phase volume steadily decreased with increasing pWPI content, whereas equilibrium surface tension steadily increased. Dynamic surface tension measurements revealed that native WPI adsorbed much more rapidly than pWPI, presumably due to the former's smaller size. Interfacial dilatational elasticity also displayed a parabolic trend with increasing pWPI content, peaking at 50%. This suggested that pWPI coadsorbs with native WPI, bolstering the dilatational elasticity of native WPI interfaces. However, too much pWPI caused a weakening of the network. A positive, curvilinear relationship between dilatational elasticity and yield stress was observed, consistent with earlier data, further suggesting a general link between these parameters.

In the third study, beta-lactoglobulin, which is the primary whey protein, was hydrolyzed with three different proteases and subsequently evaluated for its foaming potential. Two heat treatments designed to inactive the enzymes, 75 degrees C/30 min and 90 degrees C/15 min, were also investigated for their effects on foam functionality. All unheated hydrolysates improved yield stress as compared to unhydrolyzed beta-lactoglobulin, with those of pepsin and Alcalase 2.4® being superior to trypsin. Heat inactivation negatively impacted foam yield stress, although heating at 75 degrees C/30 min better preserved this parameter than heating at 90 degrees C/15 min. The previously observed relationship between dilatational elasticity and yield stress was generally confirmed for these hydrolysates. Additionally, the three hydrolysates imparting the highest yield stress not only had high values of dilatational elasticity, they also had very low phase angles (essentially zero). This highly elastic interfacial state is presumed to improve foam yield stress indirectly by improving foam stability and directly by imparting resistance to interfacial deformation.

In the final study, the foaming and interfacial properties of WPI and egg white protein (EWP) were directly compared. The highest dilatational elasticity and resistance to drainage were observed for standard EWP, followed by EWP with added 0.1% w/w sodium lauryl sulfate, and then WPI. Previously observed relationships between yield stress and interfacial rheological measurements did not hold across the protein types; however these interfacial measurements did effectively differentiate foaming behaviors within EWP-based ingredients and within WPI. Addition of 25% w/w sucrose to the solutions increased yield stress and drainage resistance of the EWP-based ingredients, but it decreased yield stress of WPI foams and minimally affected their drainage rates. These differing sugar effects were reflected in the interfacial measurements, as sucrose addition increased the dilatational elasticity and decreased the interfacial phase angle for both EWP-based ingredients, while sucrose addition imparted the exact opposite effects on WPI.