Exploring whey and faba bean protein interactions at the oil-water interface: A combined drop tensiometry and microfluidicsstudy

Abstract

The interfacial properties of model oil-water interfaces stabilized by faba bean protein isolate (FPI), whey protein isolate (WPI), and their mixtures were investigated. Two complementary techniques, microfluidics-based analysis and drop tensiometry were employed to study the impact of protein mixing on interfacial tension, adsorption kinetics, and droplet stability. A microfluidic platform was used for droplet generation and single droplet analysis, assessing the impact of protein blending on droplet size and shape eccentricity after droplet generation. Drop tensiometry complemented the microfluidics-based analysis by evaluating interfacial tension and viscoelastic properties of the various interfaces. The presence of FPI altered WPI interfaces; in mixed systems, antagonistic interactions between proteins resulted in a decreased elastic modulus and broadening of the shape eccentricity. Mixed systems resulted in smaller droplets with narrower size distributions and increased resistance to short-term coalescence with respect to emulsion droplets stabilized by FPI alone, and droplet stability increased proportionally with WPI/FPI ratio.

Keywords: Emulsifying properties; Faba bean proteins; Microfluidics; Oil/water interface; Protein adsorption; Protein mixing; Whey proteins.

Graphical abstract

Fig. 1

Overview of the microfluidic setup showing the droplet formation zone, coalescence channel, and observation zones 1 and 2.

Fig. 2

SDS-PAGE of the protein dispersions analyzed under non-reducing (lanes 2–6) and reducing (lanes 7–11) conditions: WPI (lanes 2 and 7), FPI (lanes 6 and 11), mixed systems with ratios of 25/75 (lanes 3 and 8), 50/50 (lanes 4 and 9), 75/25 of WPI/FPI (lanes 5 and 10) and molecular weights of the protein ladder (Lane 1). Bands are labelled according to literature (Badjona et al.; Madureira et al., 2007).

Fig. 3

Interfacial tension isotherms of the sunflower oil-water interface with 0.1 mg/mL of total protein for the WPI/FPI mixtures of: 100/0 (blue), 75/25 (black), 50/50 (dark grey), 25/75 (light grey) and 0/100 (green).

Fig. 4

Elastic modulus (black squares) and loss modulus (red circles) of various protein solutions at 5 % volumetric oscillations, based on their weight ratios, for both single and mixed (WPI/FPI) systems: Each value represents the weighted average of two independent replicates, with error bars indicating the combined standard deviation, reflecting the overall uncertainty of the weighted average.

Fig. 5

(A) Representative equivalent diameter histogram and Gaussian fitted peaks for droplets stabilized by 0.1 mg/mL WPI (solid line, blue) and FPI (dashed line, green) solutions. (B) Representative equivalent diameter histogram and Gaussian fitted peaks for emulsion droplets stabilized by WPI/FPI mixtures at different protein ratios, immediately after droplet formation (observation zone 1, Fig. 1). The short-dashed line (dark grey) represents a ratio (WPI/FPI) of 75/25, the dashed line (medium grey) represents a ratio of 50/50, and the solid line (light grey) represents a protein ratio of 25/75.

Fig. 6

Representative stacked histograms showing the distribution of droplet shape eccentricity values in observation zone 1 (immediately after formation), for emulsions stabilized by whey protein isolate (WPI) and faba bean protein isolate (FPI) mixtures at WPI/FPI ratios of 100/0, 75/25, 50/50, 25/75, and 0/100.

Fig. 7

Droplet equivalent diameter of the droplets after the coalescence channel transit (observation zone 2, Fig. 1). Samples prepared with WPI (grey) and 50/50 (red), 75/25 (orange) WPI/FPI hybrid systems. Inset shows representative microscopic images of small ($\sim$ 95 μm, left) and large ($\sim$ 130 μm, right) droplets stabilized by the 50/50 WPI/FPI mixed system, before and after passing through the coalescence channel. Scale bar, 60 μm.