In the food industry and especially in the flavour industry, emulsions are normally oil-in-water type ; containing 10 to 20% of oily phase.

These emulsions must be stable in acidic medium, acidity needed for the aromatic feeling, and this stability must remains many months after dilution of the concentrated emulsions.

The process of these emulsions involves high mechanical energy treatment to insure the optimum fractionation of the oily phase. By extension, this type of emulsion is used for preparing liquid emulsions of oil soluble products such as oleoresins (paprika, taget, carotens) for water dispersible natural colouring agents and vitamins (A, K, E) emulsions.

It is of interest to note that after having made the liquid emulsions, we can dry them by spray or by freeze drying to obtain a hydrodispersible powder. Due to their film forming properties, acacia gums are able to protect the encapsulated product from oxidation and fat or moisture migration.

To explain the emulsifying and stabilising power of acacia gums, it has been proved that the fractions of the molecule having high proteins content play an important role. The glycoproteins and the arabinogalactan-proteins fractions are fixed at the interface oil/water. Glucuronic acids develops negative charges around each oil droplets and create electric repulsion forces, cancelling the Van deer Waals attraction forces.

We can put the charges in evidence by zeta potential measurement. Potential on range of -30 to -40 mV insures a good stability, destabilisation increasing when the potential moves to zero.

The target is to make a stable suspension of citrus oil containing a majority of terpenes in a water medium containing Acacia as stabiliser :

Formulation :

• Essential oil ......................... 15-20%
• Acacia gum ......................... 15-20%
• Water ............................ up to 100%

Acacia gum is dissolved in the water under agitation. We have to note that the heating treatment of the gum solution is of importance: too much thermical treatment denatures the proteins which flocculate, decreasing the emulsifying capability of the gum (temperature higher than 95°C is prohibited).

The oil fraction, having a specific gravity of 0.82, will be very often mixed with a weighting agent, increasing the density to almost 1.00. The best will be to reach 1.04, density of the finished drink containing 10% sugar. It is simply solubilised in oil at room temperature.

To make a stable emulsion, we must reduce the globule of oil down to 0.8 to 1 micron diameter. Two steps mechanical treatment are usually applied. First step is processed with a high shear mixer by adding slowly the oily phase into the water phase. The oil droplets size at that stage is around 3 microns. Quantity of energy needed to reduce oil globule diameter being proportional to the interfacial tension and inverse to the square of the oil droplet, it is necessary to use pressure homogenizer in a second step (3).

The pressure varies from 200 to 300 kg/cm² with a second stage at 50 kg/cm² to decrease possibility of flocculation. At such high pressures, the emulsion speed is 100 m -s with an emulsification speed of 10 -4 second.

After this homogenization treatment, the oil globule repartition is between 0.8 to 1 micron (average diameter in volume). This low viscosity emulsion (40 cP Brookfield LVF - 60 rpm) must be stable for 6 months.

Obtained by dilution of the concentrated emulsion

• Concentrated emulsion ............. 1-2 g
• Sugar ...................................... 100 g
• Citric acid / colours ..................... 2 g
• Carbonated water ......... up to 1000 g

The soft drink must be stable for 12 months

An unstable emulsion is characterised by two visible phenomena :

• White ring in the neck of the bottle of the soft drink or a cream layer at the surface of the concentrate.
  
• Clear phase at the bottom of the bottle of the concentrate and/or soft drink

These macroscopic phenomena are consequent upon different sources of instability:

• Creaming : separation in two emulsions having different oil concentrations, creaming is regulated by Stokes' law.
  
• Flocculation : formation of oil grapes from the dispersed phase, aggregates or clusters without coalescence appear. The film between each oil droplet has not been destroyed. Flocculation is generally due to diminution of electric charges. Flocculation is reversible after dilution.
  
• Coalescence : coalescence implies localised disruptions of neighbouring droplets in aggregate. Interfacial colloidal form has been destroyed. The end of the coalescence is the two phases formation.

These unstable phenomena could sometimes occur quickly even just after homogenization. In this case the whole balance of formulation has to be reconsidered. Most of the time, the « ringing » appears after few days or few weeks storage. Destabilisation is quicker in the soft drink than in concentrate.

To avoid destabilisation after soft drinks have been dispatched, we need to predict as quick as possible the storage stability by easy and reliable tests.

Following Stokes' law, the parameters involve in the stability and depending upon the process are the aqueous phase viscosity, the diameter of oil globules and the difference of specific gravity between the continuous and the dispersed phase.

Apart from that, quality of Acacia gums, composition of oily phase and quality of water must be checked. The control methods of emulsion are based on the size of the oil globules and on the evolution of these sizes during accelerated storage conditions.

The oil size distribution could be evaluated by various tests :

• Microscopic observation : quick qualitative method but with low accuracy.
  
• Turbidity measurement : determined on the diluted (1000 times) concentrate. The figure depends upon oil repartition. It is a good process control method.
  
• Absorbance determined on dilution emulsion : size index is given by the ratio of absorbance at 800 and 400 nanometers.
  
• Laser granulometer : probably the most reliable test for having size and percentage of the oil droplets distribution.

or accelerated storage test condition, we keep the concentrate in oven at 45°C and we realise oven/fridge (+5°C) cycles during 8 days.

Centrifugation of emulsion gives a good idea of the stability by measuring the creaming of the concentrate.

Dilution of the concentrate as per manufacturing conditions is also made but gives an answer only after few weeks of observation.

A more precise evaluation of the creaming evolution could be done by determination of the transmission and back scattering of a diluted emulsion. These analysis could be done during storage of the emulsion by scanning the tube every 40 microns.

The following curve is characteristic from an unstable emulsion :

CONCLUSION

Acacia gums, non-modified natural products have emulsifying and stabilising properties widely used in the food industry.

A better knowledge of botanical species, unique functional and nutritional properties associated to sophisticated methods of purification make acacia gums reliable hydrocolloids with regards to availability and quality.

BIBLIOGRAPHY

• Anderson D.M.W. and Mc Dougall, (1987), Food Add and Contam., 4 , 247.
  
• Connolly S., Fenyo J.C., Vandevelde, (1987), Food Hydrocolloids, 3 -(1), 65.
  
• Dickinson E., (1996), Les Colloïdes Alimentaires, 88-107.
  
• M. Osman, A. Menzies, P. Williams, G. Phillips, (1993), Carbohydrate Research, 246 , 303-318.
  
• L. Picton, I. Bataille, G. Muller (2000), Carbohydrate Polymers, 42 , 23-31.
  
• Thevenet F., (1995), Encapsulation and Controlled Release of Food Ingredient, S. Rich - G. Reineccius ACS n° 590 , 50-59.
  
• C. Michel, T. Kravtchenko, A. David (1998), Anaerobe, 4 , 257-266.