Studying fat droplet particle size of food emulsions - such as dairy - plays a vital role in determining emulsion stability, flavor release, and mouth feel. Larger emulsion droplets cause a decrease in flavor release, a greasy mouth feel and poor emulsion stability due to creaming. Performing emulsification to reduce droplet size improves the taste and reduces creaming. However this process must be performed in a balanced manner, as reduction in particle size enlarges the available surface area, leading to flocculation when emulsifier concentration isn't controlled.
Structural characteristics of products like ice cream are defined by the particle size of the fat droplets. The air cells contained in whippable dairy products are stabilized by aggregated fat clusters, formed with a controlled destabilization of the fat emulsion. This implies that an insight into particle size is vital to define functional quality and taste of various food emulsion products.
Fine emulsion droplets and large coalesced or flocculated droplets in food emulsions can be effectively characterized using the Malvern Panalytical Mastersizer, thanks to its a wide dynamic range. This range also allows the measurement of large protein micelles, specifically casein, enabling the study of interaction between the protein and emulsified fat phase.
Figure 1 shows the assessment of the particle size of milk products performed using laser diffraction, which allows the changes in the fat-phase to be discovered. Results for full-fat (3.6% fat), semi-skimmed (1.7% fat) and skimmed milk (0.1% fat) are illustrated in the figure. As seen in Figure 1, two modes, related to fat phase and free casein micelles, are detected in each of the samples. The relative proportions in each of the modes change when shifting from full fat to skimmed milk, tracking the fat content reduction.
Figure 1. Recording of size distribution for full-fat, semi-skimmed and skimmed milk.
To reduce creaming during storage, milk emulsions are usually homogenized while being processed. Tracking the progression of the homogenization process is completed using laser diffraction (Figure 2). An increase in homogenization pressure leads to a reduction in particle size. However at higher pressures, the decrease is negligible because of fat cluster formation, a result of the bridging of casein protein in between fat droplets. This is due to an increased surface area of the fat droplets, to the extent that it is too big to be covered by available protein. The formation of fat clusters can be prevented by using a suitable “casein-dissolving” solution that scatters the fat clusters and reduces the particle size.
Figure 2. Variation of the D[3,2] with homogenization pressure for a standard milk emulsion and cluster-free emulsion containing the “casein-dissolving” solution.
The properties of dairy emulsions during storage can be related to particle size. The viscosity of cream liqueurs often increases, and they may gel during prolonged storage. Variations of Dv90 (particle size below which 90% of droplet volume exists) as a function of measured viscosity for various liqueurs are illustrated in Figure 3. Appearance of bigger particles can be detected by using variations in Dv90. As illustrated Dv90 is directly correlated to viscosity, where the particle sizes become coarser with an increase in viscosity. This is due to the establishment of a flocculated droplet network.
Figure 3. Variation in particle size observed during cream liqueur storage.
Milk Powder Re-Hydration
Before shipping and reconstitution, milk products are often spray dried. Reconstitution of the spray-dried powder is a key factor in producing different foodstuffs, and is followed by the laser diffraction procedure. Figure 4 illustrates the change in particle size of an aqueous solution with 5% w/v milk powder. The original size of the powder is larger (>10 µm). Periodical measurement of samples revealed a mode with a very fine particle size when the volume of larger powder mode decreased. The fine mode relates to the formation of a protein micelle upon re-hydration of the powder. Initial phase of hydration was found to be rapid; however, it reduced drastically, with the process taking several hours to complete. Here, no fat was detected as the skimmed milk powder was utilized.
Figure 4. Milk powder reconstitution using the Malvern Panalytical Mastersizer 2000.
The particle size of food emulsions, such as dairy products, is a vital factor to define sensory and structural properties. Measurements made with the Malvern Panalytical Mastersizer provide an understanding of the changes to particle size while producing and storing dairy products, which helps to gain insights into the link between product formulation and performance.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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