Topics Covered
Background
The
Importance of Particle Size of Fat Droplets
Emulsion Measurements
Characterising Different Milk Products
Using Laser Diffraction
Tracking
Changes During Milk Homogenisation Using Laser Diffraction
Behaviour of Emulsion During Storage
Milk Powder Re-Hydration and Laser
Diffraction
Conclusions
Background
The particle size of the fat droplets present in dairy and other food
emulsions is important in defining properties such as flavour release, mouth
feel and the emulsion stability. Large emulsion droplets can lead to poor
flavour release, a greasy mouth feel and poor stability due to creaming.
Emulsification to a smaller droplet size tends to reduce creaming and improve
the taste of a product. However, in doing this a balance is required, as
decreasing the particle size increases the available surface area, which in turn
can lead to flocculation if the emulsifier concentration is not controlled.
The Importance of Particle Size of Fat
Droplets
In other products, such as ice cream, the particle size of the fat droplets
is important in defining structural characteristics. Aggregated fat clusters are
known to be involved in the stabilisation of the air cells within whippable
diary products. The formation of these clusters can only be achieved by the
controlled destabilisation of the fat emulsion. Thus, a knowledge of the
particle size is important in defining the functionality and taste of different
food emulsion products.
Emulsion Measurements
The Malvern Mastersizer 2000 provides an excellent tool to the
food scientist for the characterisation of food emulsions. Its wide dynamic
range (0.02 2000 microns) permits both fine emulsion droplets and larger
flocculated or coalesced droplets to be characterised. This range also allows
for the measurement of large protein micelles, such as casein, enabling the
interaction between the protein and emulsified fat phase to be understood.
Characterising Different Milk
Products Using Laser Diffraction
The particle size of milk products can be easily assessed using laser
diffraction, allowing changes in the fat-phase to be detected. An example of
this is shown in figure 1 where typical results for full-fat (3.6% fat),
semi-skimmed (half-and-half, 1.7% fat) and skimmed milk (0.1% fat) are shown. As
can be seen, two modes can be detected in each sample, one relating to the fat
phase and one relating to free casein micelles. In moving from full fat to
skimmed milk the relative proportions in each mode changes, tracking the
reduction in fat content.
Figure 1. Size distributions recorded for Full Fat, Semi-Skimmed
(Half and Half) and Skimmed Milk.
Tracking Changes During Milk
Homogenisation Using Laser Diffraction
During processing, milk emulsions are normally homogenised in order to reduce
creaming during storage. Laser diffraction can be used to track the progress of
homogenisation, as shown in figure 2.
Figure 2. Variation of the D[3,2] with homogenisation
pressure for a standard milk emulsion and a cluster-free emulsion containing
the "casein-dissolving" solution.
During the homogenisation of a milk emulsion (red curve, figure 2), a
decrease in particle size is initially observed as the homogenisation 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. Adding this to the milk emulsion
disperses the fat clusters, yielding a smaller particle size (blue curve, figure
2).
Behaviour of Emulsion During Storage
The behaviour of diary 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. Figure 3 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 to the coarser particle sizes as the viscosity increases. This is caused
by the formation of a flocculated droplet network.
Figure 3. Variation in the particle size observed during
the storage of cream liqueurs.
Milk Powder Re-Hydration and Laser
Diffraction
Milk products are often spray dried before being shipped and reconstituted.
The process of reconstitution of the spray-dried powder is an important factor
in the production of many foodstuffs. This can be followed using laser
diffraction.
Figure 4 shows the evolution of the particle size of an aqueous solution
containing 5%w/v milk powder. The initial size of the powder was relatively
large (> 10 microns). Over time, samples of the solution were taken and
measured. As can be seen, a mode at very fine particle sizes was observed as the
larger powder mode decreased in volume. This fine mode relates to protein
micelle formation during the re-hydration of the powder. Hydration is initially
rapid but then slows down dramatically, with the process taking several hours to
complete. In this case skimmed milk powder was used, thus no fat was detected.
Figure 4. Milk powder reconstitution following using
the Mastersizer 2000.
Conclusions
The particle size of diary and other food emulsions is an important factor in
defining both structural and sensory characteristics. Measurements using the
Mastersizer 2000 can be used to understand the changes in size that occur during
production and storage of dairy products. This in turn can lead to a better
understanding of how product formulation and performance are linked.
Note: A list of references is available by
referring to the original document.
Source: "Measurement of Dairy and Food Emulsions
Using Laser Diffraction", Application Note by Malvern Instruments.
For more information on this source please visit Malvern Instruments
Ltd (UK) or Malvern Instruments
(USA).