Lubrication and Sensory Properties of Emulsion Systems and Effects of Droplet Size Distribution

Abstract

The functional and sensory properties of food emulsion are thought to be complicated and influenced by many factors, such as the emulsifier, oil/fat mass fraction, and size of oil/fat droplets. In addition, the perceived texture of food emulsion during oral processing is mainly dominated by its rheological and tribological responses. This study investigated the effect of droplet size distribution as well as the content of oil droplets on the lubrication and sensory properties of o/w emulsion systems. Friction curves for reconstituted milk samples (composition: skimmed milk and milk cream) and Casein sodium salt (hereinafter referred to as CSS) stabilized model emulsions (olive oil as oil phase) were obtained using a soft texture analyzer tribometer with a three ball-on-disc setup combined with a soft surfaces (PDMS) tribology system. Sensory discrimination was conducted by 22 participants using an intensity scoring method. Stribeck curve analyses showed that, for reconstituted milk samples with similar rheological properties, increasing the volume fraction of oil/fat droplets in the size range of 1-10 µm will significantly enhance lubrication, while for CSS-stabilized emulsions, the size effect of oil/fat droplets reduced to around 1 µm. Surprisingly, once the size of oil/fat droplets of both systems reached nano size (d₉₀ = 0.3 µm), increasing the oil/fat content gave no further enhancement, and the friction coefficient showed no significant difference (p > 0.05). Results from sensory analysis show that consumers are capable of discriminating emulsions, which vary in oil/fat droplet size and in oil/fat content (p < 0.01). However, it appeared that the discrimination capability of the panelist was significantly reduced for emulsions containing nano-sized droplets.

Keywords: droplet size distribution; food emulsions; food sensory; oral lubrication; oral tribology.

Figure 1

An illustration of the experiment setup ‘Soft Texture Analyzer Tribometer’.

Figure 2

Droplet size distributions from the Malvern Mastersizer for reconstituted milk and thickened samples: (A) reconstituted milk homogenized under a shear homogenizer, (B) thickened reconstituted milk homogenized under a shear homogenizer, (C) reconstituted milk and thickened samples homogenized under HPH, and (D) droplet size accumulation curve.

Figure 3

Flow curves for reconstituted milk (A) and thickened samples (B) with fat mass fractions of 0.3, 0.7, 2.0, 3.5, and 7.5 wt.%.

Figure 4

Stribeck curves for reconstituted milk and thickened samples, (A,B) with fat mass fractions between 0.3 and 7.5 wt.%, (C,D) with constant fat mass fraction of 7.5% and varied fat droplet size distribution. Lubrication tests were conducted at 25 °C and under a constant surface load of 0.0394 N.

Figure 5

Stribeck curves for reconstituted milk and thickened samples with a constant fat droplet distribution and fat mass fractions between 0.3 and 7.5 wt.%. Lubrication tests were conducted at 25 °C and under surface load of 0.0394 N.

Figure 6

Flow curves for CSS stabilized emulsions with oil mass fractions of 0.3, 0.7, 2.0, 3.5, 7.5, and 10 wt.%.

Figure 7

Stribeck curves for CSS-stabilized o/w emulsions with oil mass fractions between 0.3 and 10 wt.% with the same fat droplet distribution (A), with constant oil mass fractions 7.5% and varied oil droplet size distribution (B). Lubrication tests were conducted at 25 °C and under surface load of 0.0392 N.

Figure 8

A schematic diagram showing the range of ‘lubricating fat droplets’ and its friction curves. Figures with capitalized letter shows the droplets distribution of emulsion sample with constant oil mass fraction but varied droplets size (A–C), figures with minuscules letter shows its friction curve (a–c).

Figure 9

Ranking of selected sensory attributes obtained for samples with constant droplets size but with different fat content (A,C), or with same fat content but various fat droplet size (B,D). * Significance at 5%, ** Significance at 1%.