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. 2019 Oct 11;8(10):483.
doi: 10.3390/foods8100483.

Milk Emulsions: Structure and Stability

Katja Braun  1   2 , Andreas Hanewald  3 , Thomas A Vilgis  4
Affiliations

Milk Emulsions: Structure and Stability

Katja Braun et al. Foods. .

Abstract

The main aim of this research is to investigate the characteristics of milk and milk proteins as natural emulsifiers. It is still largely unclear how the two main fractions of the milk proteins behave as emulsifier in highly concentrated emulsions. The surface-active effect of these is determined experimentally for emulsions with a high oil content (φ > 0.7), in this case fully refined rapeseed oil. Recent publications have not yet sufficiently investigated how proteins from native milk behave in emulsions in which a jamming transition is observed. In addition, scientific measurements comparing fresh milk emulsions and emulsions of dried milk protein powders based on rheological and thermal properties are pending and unexamined. The emulsions, prepared with a rotor-stator disperser, are investigated by their particle size and analysed by microscopy, characterised by their rheological properties. The behaviour under shear is directly observed by rheo-optical methods, which enables the direct observation of the dynamic behaviour of the oil droplets undergoing a size selective jamming transition. For a better understanding of the contributions of the different emulsifying proteins, oil-in-water emulsions have been prepared by using whey protein isolates and sodium casinates. Their different role (and function) on the interface activity can be assigned to the droplet sizes and mechanical behaviour during increasing shear deformation. In addition, solid (gelled) emulsions are prepared by heating. It is shown that the cysteine-containing whey proteins are mainly responsible for the sol-gel transition in the continuous water phase and the formation of soft solids.

Keywords: casein; emulsions; jamming transition; microscopy; milk; rheo-optics; rheology; whey protein.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Stable emulsion of fresh milk containing 65%, 70%, 71%, 72%, 73% and 74% of oil (from left to right) after 6 days storage at 4 °C.
Figure 2
Figure 2
Left: Emulsion after 4 days: left: emulsion with 7% w/w whey protein isolate (WPI) solution; right: emulsion with 8% w/w WPI solution; oil content 73% w/w. Right: Emulsion after 4 days: left: emulsion with 7% w/w sodium caseinate solution; right: emulsion with 8% w/w sodium caseinate solution; oil content 73% w/w.
Figure 3
Figure 3
Oil drop size distribution of milk emulsions with varying oil proportions 70% w/w to 74% w/w and fresh milk.
Figure 4
Figure 4
Oil drop size distribution of reconstructed emulsions from different concentrations of sodium caseinate, Na-Cas (a) and whey protein isolate, WPI, (b) at given oil concentration of 73% w/w with respect to water.
Figure 5
Figure 5
Oil drop size distribution of emulsion consisting of water and 73% oil emulsified with Na-Cas (blue) and WPI (red).
Figure 6
Figure 6
Linear fit of the decrease of the droplet size with increasing protein concentration. Left, (a) WPI; right, (b) Na-Cas. The slope in the case of Na-Cas is much steeper.
Figure 7
Figure 7
A typical microscopy picture of the natural organic hay milk used for the milk emulsions in this paper. The magnification is 40×.
Figure 8
Figure 8
Emulsion with fresh milk with 65%, 70% and 73% oil (from left to right). The magnification is 40×.
Figure 9
Figure 9
Optical micrographs of reconstructed emulsions of water and 73% w/w oil with WPI (a) and Na-Cas (b). The magnification is 40x.
Figure 10
Figure 10
The storage (G’) and loss (G’’) modulus in amplitude sweeps of fresh milk emulsions from concentrations 70 to 74%.
Figure 11
Figure 11
The storage (G’) and loss (G’’) modulus in amplitude sweeps of WPI (a) and Na-Cas (b) in the concentration range from 6 to 8%, each, with 73% oil in water.
Figure 12
Figure 12
The storage (G’) and loss (G’’) modulus in frequency sweep of fresh milk emulsions from 70% w/w to 74% w/w oil at deformations γ = 0.5%.
Figure 13
Figure 13
The storage (G’) and loss (G’’) modulus in freuqency sweeps of WPI (a) and Na-Cas (b) in the concentration range from 6 to 8%, each, with 73% oil in water at shear deformations γ = 0.5%.
Figure 14
Figure 14
Storage modulus (G’) and loss (G’’) modulus and loss moduli of under temperature increase and decrease (4 °C to 80 °C to 4 °C) of reconstructed emulsions with 74% oil and increasing WPI (a), and Na-Cas (b) concentrations (from 5 to 8%).
Figure 15
Figure 15
Rheomicroscopy observations for the storage (G’) and loss (G’’) modulus from fresh milk emulsions (70% oil) and snapshots at different deformations. The micrographs Ⅰ–Ⅴ correspond to different shear deformations and are indicated by the arrows. Visible changes are indicated by the rectangles in the photographs.
See this image and copyright information in PMC

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