Oral tribology of dairy protein-rich emulsions and emulsion-filled gels affected by colloidal processing and composition

Abstract

Designing nutritious food for the elderly population often requires significant quantities of leucine-rich whey proteins to combat malnutrition, yet high-protein formulations can cause mouth dryness and increased oral friction. This study investigated how various colloidal processing methods and compositions impact the in vitro oral tribological properties of protein-rich emulsions and emulsion-filled gels. Oil-in-water emulsions with oil fractions from 1 wt% to 20 wt% were prepared, alongside emulsion-filled gels containing whey protein isolate (WPI), hydrolysed whey protein (HWP), or a blend of both (10 wt% protein content). Two processing approaches were employed: creating emulsions with an initial 10 w% protein content (M1) and initially forming emulsions with 0.1 wt% protein content, then enriching to a final 10 wt% concentration (M2). The hypothesis was that formulations with HWP or method 2 (M2) would offer lubrication benefits by inducing droplet coalescence, aiding in the formation of a lubricating boundary tribofilm. Surprisingly, the tribological behavior of high-protein emulsions showed minimal dependence on oil droplet volume fraction. However, both HWP-based emulsions and those processed with M2 for WPI exhibited significant friction reduction, which may be attributed to the presence of coalesced oil droplets, supporting our hypothesis. Substituting 50 wt% of WPI with HWP in emulsion-filled gel boli resulted in very low friction coefficients in the boundary lubrication regime, suggesting oil droplet release from the gel matrix. These findings provide insights into designing high-protein foods with improved mouthfeel for the elderly population, necessitating further validation through sensory studies.

Keywords: Bolus; Lubrication; Oral processing; Viscosity; Whey protein hydrolysate.

Graphical abstract

Fig. 1

Effect of oil concentration on friction behaviour of o/w emulsions stabilised by (a) whey protein isolate (WPI) and (b) hydrolysed whey protein (HWP) prepared using the two processing routes i.e. using 10 wt% protein content initially to form the emulsion (₁) (filled symbols), and using 0.1 wt% protein content, followed by protein enrichment to achieve a final concentration of 10 wt% (₂), (open symbols). Friction graphs of HEPES buffer are shown as reference. Data represent mean of triplicate measurements on duplicate samples (n = 2 × 3).

Fig. 2

Mean apparent viscosity (ƞ) as a function of shear rate (γ) (a), friction curves (b), friction coefficient as a function of product of entrainment speed × effective viscosity (Uƞ_eff) (c) of o/w emulsions (20.0 wt% oil, 10.0 wt% protein) stabilised by whey protein isolate (WPI), hydrolysed whey protein (HWP) or a mixture of whey protein isolate and hydrolysed whey protein (WPI/HWP) using 10 wt% protein content initially to form the emulsion (₁) (filled symbols), and using 0.1 wt% protein content, followed by protein enrichment to achieve a final concentration of 10 wt% (₂), (open symbols). Emulsion formulation and sample preparation method details are listed in Table 1. Data represent mean of triplicate measurements on three samples (n = 2 × 3). Statistics is shown in Table 3.

Fig. 3

Confocal laser scanning microscopy (CLSM) images with insets of mean droplet size (d₄₃, μm) of emulsions (20.0 wt% oil) stabilised by 10.0 wt% whey protein isolate (WPI) or hydrolysed whey protein (HWP) made using 10% protein content initially to form the emulsion (₁), and using 0.1 wt% protein content, followed by protein enrichment to achieve a final concentration of 10 wt% (₂). Images are taken before and after tribological measurements. Red channel shows the Nile Red signal coming from the oil droplets and green channel shows the Fast Green signal coming from the proteins. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

Effect of model saliva (MS) on frictional behaviour of o/w emulsion boli (20.0 wt% oil, 10.0 wt% protein) stabilised by whey protein isolate (WPI), hydrolysed whey protein (HWP) or a mixture of whey protein isolate and hydrolysed whey protein (WPI/HWP). Emulsion formulation and sample preparation method details are listed in Table 1. Data represent mean of triplicate measurements on three samples (n = 2 × 3). Statistics is shown in Table 4.

Fig. 5

Visual appearance (a) and mean frictional coefficients (b) as a function of entrainment speed of o/w emulsion stabilised by hydrolysed whey protein (HWP) (10.0 wt% protein, 20.0 wt% oil) using 10% protein content initially to form the emulsion (₁) as fabrication conditions before (unheated) and after heat-treatment (pH 7, 90 °C, 30 min). Data represent mean of triplicate measurements on three samples (n = 2 × 3). Statistics is shown in Table 4.

Fig. 6

Effect of model saliva (MS) on friction coefficient curves of o/w emulsion-filled gel boli (_gb) stabilised by whey protein isolate (WPI) or a mixture of whey protein isolate and hydrolysed whey protein (WPI/HWP). Data represent mean of triplicate measurements on duplicate samples (n = 2 × 3). Emulsion formulation and sample preparation method details are listed in Table 1. Statistics is shown in Table 4.

Fig. 7

Schematic illustration of importance of using whey protein hydrolysate or fabricating the systems using 0.1 wt% protein content, followed by protein enrichment to achieve a final concentration of 10 wt% in preparation of high protein oil-in-water emulsions and emulsion gels. The top drawing illustrates the friction behavior trend observed during tribological analysis of the emulsions and emulsion-filled gel boli. The bottom diagrams displays the lubrication mechanism for both systems. Blue and red lines depict the adsorption of whey protein isolate (WPI) and hydrolysed whey protein (HWP) films onto the surfaces. For the left drawing, an oil film coats the surface, corresponding to the coalescence observed in emulsions prepared using HWP, which contributes to enhanced lubrication. In contrast, the right drawing depicts the lubrication mechanism for emulsion-filled gel boli. Here, the reduced oil release from the gel particles results in increased friction behaviour. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)