Using Laser Diffraction to Measure the Size and Stability of Dairy Emulsions

To improve its shelf-life, milk undergoes a homogenization process. This process produces fat globules of a uniform, small size.  During the homogenization process the size range of the fat globules is reduced from 0.1-15 μm in unprocessed milk to 1-2 μm in homogenized milk [1]. These smaller globules cannot form large enough clusters for creaming to occur, increasing the shelf-life of the milk. The creaming process is governed by Stokes’ Law and the relative densities of the fat and the other fluid components of the milk.

Milk also contains casein micelles, in the size range of 0.05-0.25 μm. The micelles play a role in stabilizing the fat globules, especially after the homogenization process. The size of the fat globules and the proportion of free casein micelles are important parameters for monitoring the homogenization process and can be measured simultaneously by laser diffraction.

The structure of milk

Homogenized milk is an oil-in-water emulsion containing:

• Fat globules (1-2 μm)
• Casein micelles (0.05-0.25 μm)
• Lactose
• Whey proteins
• Vitamins and minerals (low concentration, typically 5 g/l [2])

The fat globules and casein micelles contribute to the particle size distribution measured by laser diffraction {Figure 1}. The lactose, whey, vitamins and minerals are present in solution and so do not contribute.

Figure 1. From a laser diffraction perspective, milk consists of fat globules and casein micelles in a solution of whey protein and lactose.

Characterizing Different Milk Products

Three UK drinking milks with different fat contents {Table 1} were measured and the particle size distributions compared. A further experiment was performed by removing the casein from the milk to confirm how the casein micelles and fat globules contribute to the measured particle size distributions.

Table 1. Fat contents in different grades of drinking milk available in UK and USA.

The particle size distributions of the three different grades of drinking milk are shown in Figure 2 and can be interpreted in terms of the structure of the milk. The particles below 0.25 μm can be understood to be casein micelles and the particles at 1-2 μm can be interpreted as fat globules. The particles approaching 10 μm in the whole milk sample may be clusters of fat globules that have either survived the homogenization process, or formed subsequently. The proportion of fat globules, measured as the volume percentage above 0.25 μm, increases with the total fat content of the milk, with the casein micelles forming the remainder of the population {Table 2}. The largest particles are present in the whole milk, which has the highest fat content. The particle size distribution of the semi-skimmed milk represents an intermediate composition. The smallest particles are present in the skimmed milk, which contains only a trace of fat.

Figure 2. Particle size distributions of whole, semi-skimmed and skimmed milks. The particles below 0.25 μm are casein micelles, the particles at 1-2 μm are fat globules and the particles approaching 10 μm in the whole milk are clusters of fat globules.

Table 2. The volume below 0.25 μm compared with the fat content of the drinking milks.

There are trends in the data consistent with the presence of casein micelles below 0.25 μm and fat globules above 1 μm. To prove this interpretation, the casein in the milk was dissolved using a solution of EDTA [3]. In this experiment the milk was pre-dispersed using the EDTA solution before it was added to the dispersion unit. The particle size distributions obtained from whole and semi-skimmed milk with and without the casein are shown in Figures 3-4 and Table 3. In both cases, use of the EDTA solution increases the proportion of material above 0.25 μm. This supports the assertion that the material below 0.25 μm is casein.

Figure 3. Whole milk with and without casein.

Figure 4. Semi-skimmed milk with and without casein.

Table 3. Particle size statistics for drinking milks with and without casein.

Milk Homogenization

During processing, milk emulsions are normally homogenized in order to reduce creaming during storage. Laser diffraction can be used to track the progress of homogenization, as shown in figure 5.

During the homogenization of a milk emulsion (red curve, figure 5), a decrease in particle size is initially observed as the homogenization pressure is increased. However, at high pressures the observed decrease becomes less pronounced. This is due to fat-cluster formation caused by bridging of the casein protein between the fat droplets within the emulsion. This occurs when the surface area of the fat droplets becomes too large to be covered by the available protein. Formation of these fat clusters can be inhibited using an appropriate “casein-dissolving” solution [3]. Adding this to the milk emulsion disperses the fat clusters, yielding a smaller particle size (blue curve, figure 5).

Figure 5. Variation of the D[3,2] with homogenization pressure for a standard milk emulsion and a cluster-free emulsion containing the “casein-dissolving” solution [3].

Emulsion Storage

The behavior of dairy emulsions during storage can also be related to particle size. Often emulsions such as cream liqueurs are found to increase in viscosity and even gel during prolonged storage [4]. Figure 6 shows how the Dv90 (particle size below which 90% of the volume of droplets exists) varies as a function of the viscosity measured over time for different liqueurs. Changes in the Dv90 can be used to detect the appearance of large particles. As can be seen, a direct correlation is observed between the Dv90 and the viscosity, with a move the coarser particle sizes as the viscosity increases. This is caused by the formation of a flocculated droplet network.

Figure 6. Variation in the particle size observed during the storage of cream liqueurs [4].

Summary

Laser diffraction can be used to measure the particle size distribution of fat droplets and casein micelles in drinking milks. It can also be used to monitor changes in particle size during homogenization and to monitor flocculation processes in emulsions such as cream liqueurs.

References

1. Prof H Douglas Goff, Dairy Science and Technology Education Series, University of Guelph, Canada https://www.uoguelph.ca/foodscience/book-page/dairy-science-and-technology-ebook
2. CH McCrae & A Lepoetre, Characterisation of dairy emulsions by forward lobe laser light scattering application to milk and cream, Int Dairy Journal 6 (1996) 247-256
3. Muir, D.D., McCrae-Homsma, C.H. & Sweetsur, A.W.M., “Characterisation of dairy emulsions by forward lobe laser light scattering - application to cream liqueurs”, Milchwissenschaft (1991) 46: 691-694

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

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