LUM LUMiSizer® The All-in-one Dispersion Analyzer
Dispersions are unstable from the thermodynamic point of view; however, they can be kinetically stable over a large period of time, which determines their shelf life. This time span needs to be measured in order to ensure the best product quality to the final consumer.
These destabilizations can be classified into two major processes:
1. Migration phenomena: whereby the difference in density between the continuous and dispersed phase, leads to gravitational phase separation:
2. Particle size increase phenomena: whereby the size of the dispersed phase (drops, particles, bubbles) increases
Multiple light scattering coupled with vertical scanning is the most widely used technique to monitor the dispersion state of a product, identifying and quantifying destabilization phenomena. It works on concentrated dispersions without dilution. When light is sent through the sample, it is backscattered by the particles/droplets. The backscattering intensity is directly proportional to the size and volume fraction of the dispersed phase. Therefore, local changes in concentration (creaming and sedimentation) and global changes in size (flocculation, coalescence) are detected and monitored.
The kinetic process of destabilization can be rather long (up to several months or even years for some products) and it is often required for the formulator to use further accelerating methods in order to reach reasonable development time for new product design. Thermal methods are the most commonly used and consist of increasing temperature to accelerate destabilization (below critical temperatures of phase inversion or chemical degradation). Temperature affects not only the viscosity but also interfacial tension in the case of non-ionic surfactants or more generally interaction forces inside the system. Storing a dispersion at high temperatures makes it possible to simulate real-life conditions for a product (e.g. tube of sunscreen cream in a car in the summer), but also to accelerate destabilization processes up to 200 times.
Mechanical acceleration, including vibration, centrifugation, and agitation, are sometimes used. They subject the product to different forces that push the particles/droplets against one another, hence helping in the film drainage. However, some emulsions would never coalesce in normal gravity, while they do under artificial gravity. Moreover, the segregation of different populations of particles has been highlighted when using centrifugation and vibration.
Faster development times have now reached the food industry. The consumer continuously demands new products with longer, more accurate shelf-life data. For this purpose, however, time-consuming tests must be performed that simulate various storage conditions. That is nearly impossible, however, because it contradicts the short development times. For this reason, more conservative shelf-life times are used, which promotes food waste.
Another problem with the shelf-life testing is that there are currently no clear rules and guidelines. Thus, a related standard in the pharmaceutical industry is often used in which the photostability of drugs is tested such as ICH guidelines. In ICH guidelines, testing is carried out when at least 98% of the active ingredient is present in the drug, which then results in an end to the expiration date.
For such photostability testing, there are various simulation chambers available with a specific light spectrum that meet these requirements.
Due to the lack of regulation, many manufacturers in the food industry have passed over real shelf-life testing and gone with accelerated testing, which is primarily carried out in such simulation chambers. The chambers then simulate an accelerated shelf-life through increased temperatures and higher humidity from which the actual shelf-life is then derived. However, such testing should only be supplementary and should not completely replace real testing.
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