Application of multisample analytical centrifugation for fast evaluation of long term stability and freeze/thaw stability of emulsions

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Contribution to the 4th World Congress on Emulsions Lyon 2006

Application of multisample analytical centrifugation for fast evaluation of long term stability and freeze/thaw stability of emulsions

T. Sobisch, D. Lerche

L.U.M. GmbH, Rudower Chaussee 29 (OWZ), 12489 Berlin, Germany

www.lum-gmbh.com, info@lum-gmbh.de

The development of emulsion based products as well as process development/monitoring and quality assurance require efficient and objective analytical tools for shelf life prediction and stability determination. Freeze/thaw stability is often used as measure of long term stability. On the other hand high freeze/thaw stability is often required as property of the final product. To this end the potential of multisample analytical centrifugation was evaluated as an accelerated test procedure.

A new multisample approach is presented using analytical centrifugation, which allows for an accelerated characterization of destabilization processes (creaming, sedimentation, coalescence, phase separation). The kinetics of these destabilizing processes can be traced simultaneously for up-to 12 different samples with temperature control in the range between 4 and 45 °C. No dilution is necessary, which otherwise could lead to changes of dispersion properties.

Analytical centrifugation measures in a rapid way the inherent stability of the samples. Therefore, alterations due to slow aging may be investigated combining first a shorted common accelerated stability test like freeze/thaw cycling and second a rapid evaluation by analytical centrifugation.

The wide application potential of this approach is demonstrated by examples of measurements on cosmetic creams and on beverages.

1 Introduction

The development of emulsion based products as well as process development/monitoring and quality assurance require efficient and objective analytical tools for shelf life prediction and stability determination. Freeze/thaw stability is often used as measure of long term stability. On the other hand high freeze/thaw stability is often required as property of the final product. To this end the potential of multisample analytical centrifugation was evaluated as an accelerated test procedure.

Multisample analytical centrifugation is efficient in characterization and measurement of the speed of destabilizing processes like creaming, coalescence and phase separation (1-7). It detects very small differences in stability and allows for an accelerated characterization of emulsions without dilution, thus avoiding changes of emulsion properties.

Analytical centrifugation traces the inherent stability, but cannot foresee destabilizing processes that will happen during storage of the sample. But, it will see the results of destabilizing processes far faster than visual observation. Therefore, to this end a combination with suitable stress tests is the method of choice. As a demonstration the evaluation of aging of cosmetic creams and of beverages is presented, which is important in relation to development of new formulations and in quality control of manufactured products. The stability of samples aged during storage at ambient conditions or by a freeze-thaw cycle (accelerated aging) was compared with the initial stability just after manufacturing.

2 Experimental

2.1 Materials and sample preparation
Two different commercial cosmetic o/w creams and two emulsion based beverage samples were used as received close to the date of production, after storage under ambient laboratory conditions over 2 month (cosmetic creams) and after one freeze/thaw cycle (24 h at -10 °C followed by 24 h at 25 °C).

2.2 Method of multisample analytical centrifugation

The multisample analytical centrifuge (LUMiFuge, LUM GmbH, Berlin Germany) used in this study employs the STEP-Technology, which allows to measure the intensity of the transmitted light as function of time and position over the entire sample length simultaneously (Measurement scheme see Fig. 1).

Fig. 1 Measurement scheme of the multisample analytical photocentrifuge. Parallel NIR-light is passed through the sample cells and the distribution of local transmission is recorded at preset time intervals over the entire sample length

The data are displayed as function of the position within the sample, as distance from the centre of rotation (transmission profiles, see Fig. 2). The progression of the transmission profiles contains the information on the kinetics of the separation process and allows particle characterization.

At the same time up to 8 different samples can be analysed simultaneously at constant or variable centrifugal speed up to 2300 g. The separation behaviour of the individual samples can be compared and analysed in detail by tracing the variation in transmission at any part of the sample or by tracing the movement of any phase boundary.

As for a given type of sample cells the position corresponds to a defined sample volume the relationship position – volume can be established by calibration.
For further details of the application of the method for characterization of emulsions see (1-7).

3 Results and Discussion

Fig. 2 Centrifugation of a cosmetic cream of the o/w type at 1100 g for 18 hours. Evolution of transmission profiles with time – first recorded profile undermost (red), last profile uppermost (green).

Figure 2 displays the transmission profiles obtained by centrifugation at 1100 g for the least stable cream analysed after just being manufactured.

The sharp drop in transmission at about 88.5 mm marks the filling height of the sample. The lowest transmission belongs to the first profile (lowest transmission). The destabilizing process, separation of an aqueous phase, starts from the bottom of the cell. The boundary water -emulsion is moving upwards (last profile highest transmission in the bottom region). That is, the separation process is characterized by creaming of oil droplets inside the continuous aqueous phase. From this it becomes obvious that the cream is an oil-in-water emulsion. The type of emulsion oil-in-water (o/w) or water-in-oil (w/o) can easily be deduced from the primary process of destabilization traced by the evolution of transmission profiles – creaming of oil droplets or sedimentation of water droplets.

It has to be noted that the cream has a high inherent stability, given the long time and high speed of centrifugation necessary to produce these changes.

The more stable creams exhibited only a small increase of transmission near the cell bottom. For a comparative investigation therefore the change in transmission of the bottom region was chosen as a measure of stability.

Fig. 3 Change of integral transmission (113 – 114 mm) during centrifugation at 1100 x g for 18 hours – effect of 2 month aging at ambient temperature

Figure 3 shows the effect of two month of aging at ambient temperature for the most and for the least stable sample. The just prepared samples show a distinct less pronounced demixing during centrifugation than the samples after 2 month aging at ambient laboratory conditions. The most likely cause is slow flocculation, which lead to a (temporary) stabilization against creaming due to an increase in viscosity. However, further flocculation is expected to cause irreversible destabilization. In contrast other aging processes like coalescence and Ostwald ripening would have led to a direct destabilization.

The effect of accelerated aging by a freeze-thaw cycle (thawing after storage at -10 °C, for 24 hours compared to the samples not subjected to the freeze-thaw cycle) is depicted in Fig. 4. It is readily obvious from this comparison that aging by one freeze-thaw cycle markedly enhances the differences in stability already inherent in the samples before freezing. As seen the separation of the continuous phase at the bottom is initially slow and speeds up only after a prolonged time of centrifugation. This indicates a stabilizing barrier against creaming (probably a flocculated network). The freeze-thaw cycle obviously reduces the stabilizing barrier causing a marked decrease of the induction period time.

In the case investigated the order of stability of the different creams does not change during aging (2 month, or after an additional freeze-thaw cycle), i.e. the inherent stability of the samples as prepared is altered during aging for all samples the same way. To trace slow destabilizing processes a combination with common accelerated aging tests is the method of choice.

Fig. 4 Change of integral transmission (113 – 114 mm) during centrifugation at 1100 x g for 18 hours – effect of a freeze-thaw cycle after 2 month aging at ambient temperature

Evaluation of long term stability is often complicated by slow diffusion based destabilization processes, which cannot be foreseen by analytical centrifugation. However, the result of these processes can be traced far more faster then by naked eye. An example is given in Fig. 5 for emulsion based beverage samples. In this case the stability was measured by the variation of turbidity during centrifugation. As can be seen the fresh samples exhibited a similar behaviour. However, they were markedly destabilized after only one freeze-thaw cycle. After this, sample 1 showed a substantial higher loss of stability. The same trends were observed after 6 week of storage but less pronounced.

Fig. 5 Effect of aging on emulsion based beverages, influence of 1 freeze-thaw cycle on speed of clarification measured as variation in transmission, centrifugation at 1100 g

4 Conclusions

The investigations revealed that analytical centrifugation was well suited for accelerated stability testing of commercial emulsions in case of slow destabilizing mechanisms. In one case analytical centrifugation traced differences in separation stability just after manufacturing, which were enhanced during only one freeze-thaw cycle. In the other case differences in stability were revealed after freeze-cycling only.

The multisample technique is ideally suited for screening purposes.

5 References

(1) T. Sobisch, D. Lerche, Rapid emulsifier selection and evaluation of emulsion stability by multisample analytical centrifugation, this conference - Topic: - 3 Behaviour, characterization and processing, Workshop: 3.1 New techniques of characterization

(2) T. Sobisch, D. Lerche, G.G. Badolato, F. Aguilar, H.P. Schuchmann, Evaluation of long term stability of model emulsions by multisample analytical centrifugation, this conference:

Topic: - 3 Behaviour, characterization and processing, Workshop: 3.1 New techniques’ of characterization

(3) D. Lerche, T. Sobisch, S. Küchler, Shelf life prediction of emulsions by based on multisample analytical centrifugation, this conference - Topic: - 3 Behaviour, characterization and processing, Workshop: 3.1 New techniques of characterization

(4) T. Sobisch, D. Lerche, Rapid characterization of emulsions for emulsifier selection, quality control and evaluation of stability using multisample analytical centrifugation, 2005,

SCI/RSC/SCS conference Cosmetics and Colloids, London 15 February 2005

http://www.soci.org/SCI/groups/col/2005/reports/pdf/gs3257_sob.pdf

(5) T. Sobisch, D. Lerche, Rapid selection of dispersants and evaluation of emulsion stability by analytical centrifugation, La Rivista Italiana delle Sostanze Grasse 82 (2005) 304 – 312

(6) T. Sobisch, D. Lerche, S. Küchler, A. Uhl, Rapid stability assessment for liquid and paste like foods by multisample analytical centrifugation, Paper Proceedings Euro Food Chem XIII, Macromolecules and their degradation products in food – physiological, analytical and technological aspects, Hamburg, Germany, September 19 – 23 2005, Eds. T. Eklund, M. Schwarz, H. Steinhart, H.-P. Their, P. Winterhalter, pp. 672 – 675, Gesellschaft Deutscher Chemiker e.V., 2005, ISBN 3-936028-31-1

(7) D. Lerche, T. Sobisch, S. Küchler, 2002, Stability analyser LUMiFuge 116 for rapid evaluation of emulsion stability and demulsifier selection, Chemistry Preprint Archive, Volume 2002, Issue 6, June 2002, Pages 195-201 http://www.sciencedirect.com/preprintarchive