Methods To Predict the Physical Stability of FlavorâCloud Emulsion

Chapter 13

Methods To Predict the Physical Stability of Flavor—Cloud Emulsion 1

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Downloaded by UNIV MASSACHUSETTS AMHERST on May 14, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/bk-1995-0610.ch013

K. Y. Tse and G. A. Reineccius 1

Robertet Flavors, 640 Montrose Avenue, South Plainfield, NJ 07080 Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Avenue, St. Paul, MN 55108

2

The usefulness of particle size determination, centrifugation and elevated temperature storage for the prediction of the stability of beverage emulsions was evaluated. Based on our results, storing diluted beverage emulsions at elevated temperatures will give the most accurate prediction of emulsion stability. Storing an emulsion at 45 °C would be expected to increase the rate of loss of turbidity by factors of 2.5 to 4 over room temperature storage. While this approach gives an accurate prediction of emulsion stability, the analysis time is too long to be useful in quality control applications. Particle size and centrifugation methods were found to be inaccurate due to their inability to consider emulsion instability caused by coalescence orflocculation.It is suggested that methods to determine Zeta potential be evaluated in the future.

A common problem in the flavor industry is producing a cloud or flavor emulsion which remains stable over the desired shelf life. The particulate phase (e.g. flavor formulations or orange terpenes) generally has a specific gravity of about 0.84 while the aqueous phase will generally rangefrom1.0 to 1.06 depending upon the amount of solids (most often sugar) in the product. These emulsions are, therefore, inherently unstable once put into thefinalapplication (e.g. a beverage) and will tend to undergo phase separation resulting in decreased turbidity of the bulk of the product and the formation of a ring of flavor or clouding agent at the top of the product. The industry uses a combination of emulsifiers (e.g. modified starches or gum acacias), homogenization (to reduce particle size of the dispersed phase) and the incorporation of weighing agents (e.g. brominated vegetable oil or ester gum) to attempt to stabilize these emulsions. Despite the use of these approaches, the industry still commonly has a problem with the stability of products containing flavor or cloud emulsions.

0097-6156/95/0610-0172$12.00/0 © 1995 American Chemical Society In Flavor Technology; Ho, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

13. TSE & REINECCIUS

Physical Stability of Flavor-Cloud Emulsion

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There is substantial need to be able to rapidly determine whether an emulsion will have the desired shelf life in a particular application for quality control purposes. The most common method to evaluate emulsion stability is to measure particle size of the emulsion at the time of manufacture. The importance of particle size in determining emulsion stability is evidentfromStokes' Law: 2

2gr (d -d ) 1

2


V= 9TI Where V is velocity of phase separation, r is radius of the dispersed phase, d! and d are densities of the dispersed and continuous phase, respectively, g is the acceleration of gravity and r| is the viscosity of the continuous phase. One will note that the velocity of separation is related to the square of the radius so small changes in particle size will have profound effects on emulsion stability. Particle size may be determined using a light microscope and visually estimating particle size or determining particle size distribution using some instrumental technique such as light scattering or electrical resistance. Another means of rapidly predicting emulsion stability is centrifugation. Centrifugation increases the g term in the equation which speeds destabilization of the emulsion. The use of elevated temperature storage on emulsion stability is less evident. It is clear that increasing temperature will decrease the viscosity of the system but the effect is greater than that alone. Elevated temperature provides greater mobility to the system thereby enhancing flocculation and coalescence. Both flocculation and coalescence will occur during aging of an emulsion and greatly increase particle size - thus emulsion instability. The last method to be noted is Zeta potential. Zeta potential, simply put, is a measurement of the rate of migration of the particulate phase in an electric field. In an emulsion, the particles will inherently take on an electrical charge and this charge will influence whether the particulate phase will flocculate and/or coalesce. Riddick (/) stated that emulsions with a Zeta potential of at least -40 mV are stable and those with a Zeta potential of less than -15 mV are generally unstable. It is of interest that emulsions based on gum acacia have Zeta potentials of about -23 mV suggesting they are on the border of stability (2-3). One should note that neither flocculation nor coalescence are considered in Stokes' Law. Stokes' Law considers only creaming and thus any method for predicting stability which is based on Stokes' Law (e.g. particle size and centrifugation) may not be accurate since these other mechanisms of instability are not considered. Tan and Wu (4) have provided a most thoughtful discussion of emulsion stability and suggest Zeta potential as the best predictor of emulsion stability. The industry currently uses several methods to predict emulsion stability. While Zeta potential may be the best predictor of emulsion stability, the instrumentation to determine Zeta potential is expensive and less expensive alternatives are generally used. Particle size (by light microscope) and absorbance (400 or a ratio of400/700 or 800 nm) (5-7) are thus, very common methods. The purpose of this study was to evaluate several methods of predicting the stability of an emulsion using the more common methodology. 2

In Flavor Technology; Ho, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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Material and Methods


Materials. Cold pressed orange oil was purchased from Sunkist Growers, Inc. (Ontario, CA). Gum acacia (Spray gum 28830) and Emulgum (BV 25631) were supplied by Colloides Naturels Inc. (Bridgewater, NJ) and Purity gum BE was provided by National Starch Inc. (Bridgewater, NJ). Weighing agent (ester gum 8BG) was provided by Hercules Inc. (Wilmington, DE). Emulsion Preparation. Emulsifiers (gum acacia, Emulgum and Purity gum) were rehydrated in 60 °C distilled water. In order to get all hydrocolloids fully rehydrated, gum acacia solution was prepared and allowed to stand overnight at room temperature before use while solutions of Emulgum and Purity gum were prepared two to three hour prior to use. It should be noted that ester gum solutions were also prepared in advance since the ester gum took several hour to dissolve in orange oil. Orange oil (with or without ester gum) was added to the gum solutions and emulsified thoroughly using a Waring-type blender (3 min on high speed) or a Microfluidizer (Microfluidics Inc., Newport, Mass) . The Microfluidizer was operated at either low pressure (8,400 psig) or high pressure (12,600 psig). The formulae of emulsions used in this research (Table I) were based on information provided by the gum suppliers in order to optimize the emulsifying properties of each gum. Emulsion Stability Tests Preliminary experiments indicated that the data obtained on emulsion stability were very reproducible. Therefore, all data for emulsion stability tests were collectedfroma single experiment with duplicate determinations. Centrifugation Stability Tests. Diluted emulsions (0.3% w/v) were freshly prepared by adding 3 g of the original emulsion to a volumetric flask and adding water to make 1000 mL. This diluted emulsion was centrifuged (International centrifuge model UV) at 500 X g for the times noted. After centrifugation, the white cream layer at the top of the emulsion was discarded and sample was pipettedfromthe remaining emulsion. Emulsion absorbance was measured at 400 nm in a 1 cm path length optical glass cuvette (Bausch & Lomb) using a spectrophotometer (Spectronic 21, Bausch & Lomb) before centrifugation and after 5, 10, 20, 30 and 45 min of centrifugation. Distilled water was used as a blank for absorbance measurement. All data reported are the result of a single experiment with duplicate determinations. Shelf Life Storage Test Diluted emulsions (0.3 % w/v) were stored in small test tubes (12 cm x 15 mm) sealed with parafilm atfivedifferent temperatures (4, 13, 23, 35 and 45 °C). Samples for absorbance measurement were pipettedfromthe middle portion of the diluted emulsions. Absorbencies (400 nm in a 1 cm path length cuvette) were measured over a period of 36 days. All data reported are the result of a single experiment with duplicate measurements.




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Table I. Formulae of emulsions used in this study

Component

Composition (%) With Weighing Without Weighing

Gum acacia emulsion Gum acacia Orange oil Ester gum Distilled water

20.0 6.5 6.0 67.5

20.0 6.5 (or 12.5)

Emulgum emulsion Emulgum Orange oil Ester gum Distilled water

3.5 6.5 6.0 84.0

3.5 6.5 (or 12.5)

Purity gum emulsion Purity gum Orange oil Ester gum Distilled water

12.0 6.5 6.0 75.5

12.0 6.5 (or 12.5)

73.5 (or 67.5)

90.0 (or 84.0)

81.5 (or 75.5)

Particle Size Analysis. Particle size distributions and mean particle sizes of freshly prepared and one week old emulsions were analyzed using a particle size analyzer (Brinkmann Instruments, Inc.). The diluted emulsion was put in a 1 cm path length cuvette and a beam of laser light was passed through the emulsion. Mean particle size and particle size distribution were determined through light scattering diffraction and photo imaging. The entire analysis was computerized and the detected range in particle size wasfrom0.5 to 150 urn. Results and Discussion The purpose of this study was to determine the usefulness of absorbance, particle size analysis and centrifugation to predict the stability of flavor/cloud emulsions. The data which are presented and discussed in this paper are a part of a larger study which was done to determine the effect of using different emulsifiers, means of forming the emulsions (equipment) and the effectiveness of weighing agents (ester gum) on emulsion stability. The results of this larger study will be published elsewhere (Tse, K.Y.; Reineccius, G.A. Perfumer & Flavorist, submitted). Influence of Temperature on Emulsion Stability. The influence of temperature on the stability of diluted emulsions is represented by the data provided in Figures 1-4.



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FLAVOR TECHNOLOGY

i

10

- i —

15

i

i

i

i

20

25

30

35

40

Days Storage

Figure 1. The change in absorption of emulsions based on gum acacia during storage (low pressure Microfluidization).

Figure 2. The change in absorption of emulsions based on Emulgum during storage (low pressure Microfluidization).




0


1

!

I

I

0

5

10

15

I

- -

I

20 25 Days Storage

-I-

30

-

I

35

111

\

40

Figure 3. The change in absorption of emulsions based on Purity gum during storage (low pressure Microfluidization).

Figure 4. The change in absorption of emulsions based on Purity gum (no weighing agent) during storage (high pressure Microfluidization).



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These figures arefromemulsions based on gum acacia, Emulgum or Purity gum. Several observations can be madefromthe data in thesefigures.Thefirstis that emulsion stability is inversely related to storage temperature. This is not unexpected since, as noted earlier, higher temperatures result in decreased solution viscosity and enhanced rates of collision between emulsion droplets. Decreased solution viscosity increases the rate of creaming while increased incidence of collision enhances flocculation and coalescence. A second point relates to the effectiveness of increasing temperature to accelerate emulsion instability. It is obvious from thesefiguresand the data in Table II that temperature had a definite effect on the stability of the emulsions and that different emulsions are affected differently by increasing storage temperature. While the effect of temperature on solution viscosity is likely to be reasonably the same for all emulsifiers, the effect on coalescence andflocculationis likely to be different and this is probably why there is a difference in temperature effects for the different emulsifiers. On the average, increasing the temperature 10 °C increased the rate of loss of absorption (or rate of loss of turbidity) by a factor of 1.4, 1.68 and 1.96 for the weighted gum acacia, Emulgum and Purity gum emulsions, respectively. Thus one could make an assumption that the loss of turbidity observed when an emulsion is stored at 45 °C (ca. 20 °C above room temperature) for 1 month should be about equal to that observed for the same emulsion stored for 2.5 to 4 months at room temperatures (time equivalent depends on the emulsifier used). The data provided on an unweighted emulsion are quite similar to those obtained for the weighted emulsion except that the rates of loss of turbidity are 3-5 times greater (Table II and Figure 4). However, increasing temperature has about the same accelerating effect as observed for the weighted emulsions. The utility of using accelerated storage temperatures as a quality control procedure suffersfromthe excessive time required to obtain data. If one is interested in predicting whether a spray dried emulsion (e.g. a dry beverage mix) will last 1 week in the refrigerator following reconstitution, accelerated storage testing may be appropriate. However, if one is interested in determining whether an emulsion will last 6 months, then at least 1 month testing will be required to be assured that it will last the required time. The manufacturer can not keep an emulsion for 1-2 months before shipping to be assured the emulsion is good. A quicker method is required for quality assurance purposes. Centrifugation to Predict Emulsion Stability. As noted earlier, it is desirable to obtain results faster than can be obtained in accelerated temperature storage studies. The use of centrifugation to increase the rate of creaming appears to be a reasonable means of rapidly evaluating emulsion stability. A major concern is that centrifugation will only measure creaming and not coalescence orflocculationwhich both occur over time. Thus there is some question as to whether centrifugation will accurately reflect emulsion stability. The resultsfromthis centrifugation study will have some error due to the low level of change that occurred during centrifugation. Most ester gum-containing emulsions lost

Methods To Predict the Physical Stability of FlavorâCloud Emulsion

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