Emulsion Stability Determination

MONITORING DROPLET SIZE DISTRIBUTION AND CONCENTRATION DURING STORAGE 

The so-called storage stability test is a standard test that is used across many different fields—e.g., the pharmaceutical, cosmetic, and food industries. The test is popular because it yields precise information about the long-term shelf life of emulsions. In this test, emulsions are stored under conditions that are applicable to those encountered in the actual production/consumption situation. It should be noted that the presented test protocol is time consuming and requires sampling over an extended period of time. In the 

Additional reagents and equipment for measuring droplet size distribution (unit D3.3) and droplet concentration (see Support Protocol) 

For accurate stability evaluations, at least 10 to 12 samples should be withdrawn. 

A logarithmic time interval may be chosen if the emulsion consists of an oil that has a substantial solubility in the aqueous phase (e.g., aromatic or flavor oils see Background Information, discussion of Ostwald ripening). 

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D3.4 Emulsion Stability Determination Basic Protocol 1 Monitoring Droplet Size Distribution and Concentration D3.4.1... 

Ultrasonic methods infrared scanning for emulsion stability determination, 597-598 spectrometry, emulsion droplet size determination, 581 velocimetry, to measure fat, 572 Ultraviolet (UV). see also Spectrophotometry protein analysis, CD, 219-243 protein concentrations by... 

Figure 3 gives the results of emulsion stability determinations. It was found that the proportions observed after 3 h remained constant for a further 2 days. These data substantiate those found for emulsion capacity in that hot water grinding significantly reduces the ability to soy proteins to form a stable emulsion. 

Phase Separation. An approximate estimation of phase separation may be obtained visually. In general, creaming, flocculation, and coalescence have occurred before phase separation is visible, thus sometimes making quantitative evaluations more difficult. Accelerating the separation by centrifugation followed by appropriate analysis of the specimens may be useful to quantitatively determine the phase separation. Details on mechanisms of creaming and phase separation as well as some advances in the monitoring techniques of emulsion stability have been reviewed by Robins [146]. 

Emulsion stability of SEDDS is usually good because the droplets are small and have narrow size distributions. However, stability can be measured by determining the flocculation rates, degree of separation, or changes in the diameter of droplets formed on dilution over time during storage under various conditions. 

Figure 3.5 Demonstration of correlation between the stickiness of protein-coated droplet pair encounters in shear flow (left ordinate axis) and viscoelasticity of concentrated emulsions (right ordinate axis) with the strength of protein-protein attraction as indicated by the second virial coefficient A2 determined from static light scattering , percentage capture efficiency (0%) A, complex shear modulus (G ) for emulsions stabilized by asl-casein or (3-casein (pH = 5.5, ionic strength in the range 0.01-0.2 M).

This is a subjective test to determine the storage stability of an emulsion. A sample of the liquid emulsion before spray-drying is used to fill a 16 oz. tall glass jar. The jar is capped and stored in an oven for 16 hours at 50 C. When storage is complete, the jar is removed from the oven and evaluated. Surface oil layers on the emulsion indicate poor emulsion stability performance by the carrier. 

The samples (ca. 2500 g for each sample) were spray dried in a Niro Utility drier with the inlet temperature at 200 C and outlet at 100 C. The drier temperatures were allowed to stabilize before samples were collected for analysis. The dried samples were analyzed for total oil, surface oil, moisture, emulsion size and emulsion stability. Samples were also stored at an elevated temperature for shelf-life determination. Sensory analysis of rehydrated powder from the coarse and Microfluidized emulsions was performed to determine if differences in emulsion size affects the perceived flavor intensity. 

Emulsion Stability. The stability of the emulsions was determined by measuring optical density of the solutions following centrifugation. A 0.2% solution of each spray dried powder was prepared in water and the optical density read at 400 nm in a Coleman spectrophotometer. A 0.16% solution of carrier (gum arabic) was used as a blank. This is based on a carrier to flavor ratio of 4 1. The initial optical density of each solution was read and then the solutions were centrifuged in an lEC International Centrifuge at 500 x g for 5, 10, 15, 30, 45 and 60 min. The optical density was read after each time period. 

Determination of Emulsion Stability and Oil Droplet Size. A combination of centrifugation and spectrophotometric techniques was used for the evaluation of emulsion stability. Solutions... 

Emulsion Stability. The samples were examined for emulsion stability using centrifugation and light scattering techniques (oil droplet volume mean diameter). In general, these tests were used to determine stability of the emulsions to creaming effects. The results of the analyses are presented in Table I and Figure 1. 

Figure D3.4.3 Suitable storage container for emulsion stability tests to determine changes in droplet size distribution and concentration during storage.

Later we discover another parameter, the phase inversion temperature(PIT), which helps us to predict the structure of emulsions stabilized by nonionic surfactants. The PIT concept is based on the idea that the type of an emulsion is determined by the preferred curvature of the surfactant film. For a modern introduction into the HLB and PIT concepts see Ref. [546],... 

Figure 8.9. Critical flocculation density of a 0.04 wt % PEHA emulsion stabilized with 0.03 wt % PS-6-PFOA as determined by DLS from the average hydrodynamic radius (Yates et al., 1997).

A.N. Gurov, M.A. Mukhin, N.A. Larichev, N.V. Lozinskaya and V.B. Tolstoguzov, Emulsifying properties of proteins and polysaccharides, I, Methods of determination of emulsifying capacity and emulsion stability, Colloids Surfaces 6 (1983) 35-42. 

An assessment of emulsion stability involves the determination of the time variation of some emulsion property such as those described in the physical characteristics section above. The classical methods are well described in Ref. [9]. Some newer approaches include the use of pulsed nuclear magnetic resonance or differential scanning calorimetry [294]. 

Liquid emulsions are inherently unstable to a varying degree. It is important to understand, therefore, the mechanisms that contribute to emulsion stability. Before the solidification step, instability of an emulsion can arise due to either phase separation or phase inversion (Mulder and Walstra, 1974). It is evident that the likelihood of phase inversion will increase as the fraction of dispersed phase increases. The vast majority of literature references are concerned with the stability to phase separation as coalescence or creaming in oil-in-water emulsions (Hailing, 1981 Jaynes, 1983). In addition, a method for determining the stability of water-in-oil emulsions to inversion has not been reported. It is usually assumed that certain aspects of oil-in-water emulsion theory apply in reverse to water-in-oil emulsions. 

It has to be clear that, once diluted and injected (or administered in ocular and other routes), the emulsion stability and fate are determined by three measurable parameters. The first is the partition coefficient of each emulsion component (including added drugs and agents) between the emulsion assembly and the medium. To some extent this partition coefficient is related to oil-water and/or octanol-water partition coefficients. For example, it was well demonstrated that per component of which logP is lower than 8, the stability upon intravenous (IV) injection is questionable [42,138], The other two parameters are kQff, a kinetic parameter which describes the desorption rate of an emulsion component from the assembly, and kc, the rate of clearance of the emulsion from the site of administration. This approach is useful to decide if and what application a drug delivery system will have a chance to perform well [89],... 

Emulsion Capacity and Stability. A 0.5 g sample of the freeze-dried protein fraction was redissolved in a minimum of 0.3 M citrate-phosphate buffer at pH 7.0 and mixed thoroughly with 50 ml of 1 M NaCl for 1 min in a Sorvall Omnimixer at 1000 rpm in a one pint Mason jar set in a water bath (20°C). Crisco oil (50 ml) was added to the jar and an emulsion formed by mixing at 500 rpm with simultaneous addition of oil at the rate of 1 ml/min until the emulsion broke. The endpoint was determined by monitoring electrical resistance with an ohmeter. As the emulsion broke a sharp increase (l KS2 to 35- 0 KSi) was noted. Emulsion capacity was expressed as the total volume of oil required to reach the inversion point per mg protein. This method is similar to that used by Carpenter and Saffle (8) for sausage emulsions. To establish emulsion stability the same procedure was used except that 100 ml of oil was added and a stable emulsion formed by blending at 1000 rpm for 1 min. A 100 ml aliquot was transferred to a graduate cylinder and allowed to stand at room temperature. Observations were made of the volume of the oil, emulsion and water phases at 30, 60, 90 and 180 min. 

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