Understanding Colors in Emulsions - American Chemical Society

Chapter 26

Understanding Colors in Emulsions 1

2

Withida Chantrapornchai , Fergus M. Clydesdale , and D. Julian McClements 2

Downloaded by RUTGERS UNIV on March 12, 2016 | http://pubs.acs.org Publication Date: June 13, 2008 | doi: 10.1021/bk-2008-0983.ch026

1

Department of Product Development, Faculty of Agro-Industry, Kasetsart University, 50 Phaholyothin Road, Chatujak, Bangkok, 10900, Thailand Department of Food Science, University of Massachusetts, Amherst, MA 01003 2

Emulsion color depends on their scattering and absorption efficiency. The scattering efficiency is determined mainly by droplet characteristics (size, concentration, aggregation and relative refractive index), while the absorption efficiency is mainly determined by dye characteristics (absorption spectra and concentration). The lightness of an emulsion is correlated to the scattering efficiency of the droplets. The color of an emulsion (a- and b-values) is mainly determined by the dye type (red, green, blue) and concentration (0 - 0.1 wt%). Experiments showed that emulsion lightness increased steeply between 0 and 5 wt% oil, and increased slightly at higher concentrations (5 - 20 wt%). It also increased with decreasing droplet diameter (30-0.2 micron) and increasing refractive index difference between the two phases. The influence of droplet characteristics on the optical properties of emulsions containing different types of dye was similar. Droplet flocculation did have an impact on emulsion appearance.

364

© 2008 American Chemical Society

Culver and Wrolstad; Color Quality of Fresh and Processed Foods ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

365


Introduction Food color is an important factor in food consumer choice, since it not only influences taste thresholds, sweetness perception, food preference, pleasantness, and acceptability; but also determines food quality (1-5). Since color can be assessed more easily than taste, odor, and texture, consumers largely make their purchasing decisions by looking at and judging it by appearance. Many natural and manufactured food products exist in the form of oil-in-water emulsions, e.g., milk, cream, fruit beverages, salad dressings, mayonnaise, soups, sauces, and infant formulations (6-9). Lately, consumers have given more attention to their dietary intake; which has stimulated the development of reduced and low fat foods (10). However, it is difficult to get the same product quality, if the fat content has been removed or reduced. In addition, although consumers want foods with low or no fat, they also prefer them to taste, look, and give a texture as close as possible to the original ones. As a food component, fat provides many major attributes in foods, especially sensory and physiological benefits (11). One of the most important attributes is appearance. Still, surprisingly little work has been done on the factors that influence emulsion appearance. This article reviews the major factors that affect the color of emulsions.

Theory Physical Basis of Emulsion Color An emulsion is defined as two immiscible liquids with one of the liquids being dispersed as small spherical droplets in the other. In most food emulsions, the droplet diameters are in the range of 0.1 to 100 μιη. (6, 7, 9, 12, 13). Emulsions consist of two phases; the liquid that forms the droplets in an emulsion is called the "dispersed" or "internal" phase, while the liquid that surrounds the droplets is referred to as the "continuous" or "external" phase (13). The fundamental physical phenomena of all appearance attributes is determined by the proportion of wavelengths reflected, transmitted, absorbed and/or scattered by an object (5, 14-19). In an emulsion, these interactions are mainly governed by the characteristics of the droplets (concentration, size, and refractive index) and of any chromophores (type and concentration) present in it (16, 20-24). When a light wave that enters an emulsion encounters a droplet, part of the wave is transmitted and part of the wave is scattered (25-27). The fraction of the wave that is scattered and the direction that the scattered waves travel depends on the refractive index of the droplets and continuous phase, as well as on the size of the droplets relative to the wavelength of light. In a dilute emulsion, a light wave that travels through an emulsion may only encounter a



366 single droplet before emerging (single scattering). In a concentrated emulsion, a light wave scattered from one droplet may be scattered by a number of other droplets before it emerges from an emulsion (multiple scattering) (28). In a highly concentrated emulsion, a significant fraction of the incident light may travel back to the surface of an emulsion through multiple scattering events and emerge as diffusely reflected light (29). When chromophoric substances are present in either the continuous or dispersed phases some of the light wave is absorbed. The extent of absorption depends on the concentration and absorbtivity of the chromophores and on the wavelength of the light used. Some wavelengths are absorbed more strongly than others so that the color of the light emerging from the emulsion is no longer white. Consequently, emulsion appearance is dependent on the combination of light scattering and absorption. Scattering is largely responsible for the turbidity or lightness of an emulsion, whereas absorption is mostly responsible for the color (30).

Prediction of Emulsion Color In order to control optical properties of emulsions, it is important to understand the factors that determine them. However, it would be even more useful if we could predict the appearance of an emulsion from knowledge of its composition and structure. This would be of great advantage to food manufacturers due to savings in both time and cost. A mathematical model has been developed to predict emulsion color using light scattering theory (31). The overall prediction procedure is shown in Figure 1.

Step 1: Input the refractive index ratio of the dispersed and continuous phases into the Mie theory, and calculate the g and Q values as a function of χ (=2τιτη/λ). sca

Step 2: The wavelength dependence of the scattering and absorption characteristics (E and Σ ) of the emulsion are calculated from the predicted g and Q values for the appropriate droplet size and the measured absorption spectrum using Equations (1) and (2). s

α

sca

2

E = Nnr Q s

s


(1)

367

Measure Dye Characteristics: α(λ)

Calculate Characteristics of Individual Droplets: Q and g (Mie Theory) sca

\


Calculate Characteristics of Concentrated Emulsion Κ and S (Kubelka-Munk Theory)

Calculate Spectral Reflectance R (Kulbelka-Munk Theory)

Calculate L,a,b values

Figure 1. Color prediction procedure using light scattering theory

Σα = α

(2)

where, E and Σ are the scattering and absorption cross sections of the droplets, g is the asymmetry factor, a is the absorption coefficient of the emulsion, and Ν is the number of droplets per unit volume. Values of Q and g are calculated from the relative refractive index ratio and droplet size, e.g., using a computer program available on the Internet (32). s

α

sca

Step 3: The Kubelka-Munk scattering (S) and absorption (K) coefficients are determined from Σ and Σ using Equations (3) and (4). 5

α

(3)

Κ = 2Σ

α


(4)

368

Step 4: The spectral reflectance, R(À), is calculated as a function of wavelength (λ = 380-780 nm, at 10 nm intervals) using Equation (5).


s

*

ys S

+

2

(5)

where R is the spectral reflectance of the material, S is the scattering coefficient, and Κ is the absorption coefficient. Note that R, S and Κ are all functions of wavelength. The values of the coefficients can be calculated from diffuse scattering theory (33).

Step 5: The tristimulus coordinates Equations (6) to (15).

X =

a- and 6-values) are calculated using

kf^S(À)x(À)R(À) (6)

Y =

k^S(À)y(À)R(À) (7)

(8)

100

(9) where S(X) is the spectral distribution of the standard illuminant at wavelength λ, χ(λ),

y(À)

, ζ (λ) are the human response functions of the CIE color system,


369 R(X) is the spectral reflectance of the material. The Χ, Υ, Ζ values can be converted into Hunter L, a, b values (34), and so the color of an emulsion can be predicted from its spectral reflectance: X =k^S(A)x(A) n

(10)


(Π)

(12)

1 = 100 L

(13)

_X_ Y_ x Y„ m

(14)

Y_

z_ z„

b=K

t

(15) where X Y , Z are the tristimulus values of the reference white for the selected illuminant and observer. K and K are chromaticity coefficients or the expansion factors for the selected illuminant and observer, which are in this study illuminant D 5 and 10 degree observer. ns

n

n

a

b

6


370

Emulsion Preparation and Characterization


The experimental data presented in the following section is taken from recent studies on the influence of composition and microstructure on emulsion color (24, 30, 35, 36). The dispersed phase in these oil-in-water emulsion systems was rc-hexadecane due to its colorlessness. All dyes used in these experiments are water-soluble. Double distilled and deionized water was used to prepare all solutions and emulsions. The emulsions were then characterized:

Droplet Size Distribution Measurement A static light scattering technique (Horiba LA-900, Horiba Instruments Incorporated, Irving, CA) was used to measure the droplet size distribution of the emulsions. To avoid multiple scattering effects emulsions were diluted with distilled water prior to analysis so that the final droplet concentration was ~ 0.005 wt%. Droplet size measurements are reported as the "surface-weighted mean diameter": d Ση ά\ ΙΣη ά\ , where rt\ is the number of droplets with diameter d\ The droplet size distribution of the emulsions was measured at the beginning and end of the experiments to indicate whether coalescence or Ostwald ripening occurred during the experiments. All measurements were carried out before the dye was added to an emulsion to avoid complications in the droplet size analysis associated with the wavelength dependent absorption of the dye. =

32

χ

χ

Colorimetry The color of the emulsions was measured ising an instrumental colorimeter (LabScan II, Hunter Associates Laboratory, Reston, VA). The optical sensor used 0° incident light (filtered to closely approximate CIE Illuminant D ) on the sample plane. Viewing was at 45° through a ring of 16 fiber optic receptor stations. This geometry excludes specular reflection from measurement and essentially eliminates the effect of directionality. A fixed amount of emulsion sample is poured into the measurement cup, which is then surrounded with a black paper strip, and covered with a white or black tile before the measurement is carried out. The instrument reports the color of the samples in terms of the L,a,b color space system. 65


371 UV-Vis Spectrophotometry Absorbances of dye solutions, turbidity and reflectance spectra of emulsions were measured using a UV-visible spectrophotometer (UV-2101PC, Shimadzu Scientific Instruments, Columbia, MD). During the measurements, emulsions were contained in glass cuvettes with a 1 cm path length. Spectra were obtained over the wavelength range 380-780 nm using a scanning speed of 700 nm min" . Absorbance measurements were made using a standard double-beam arrangement, with the absorption of the dye solution being measured relative to that of a reference cell containing distilled water. Spectral reflectance measurements were made using an integrating sphere arrangement (ISR-260, Shimadzu Scientific Instruments, Columbia, MD). The spectral reflectance of the emulsions was measured relative to a barium sulfate (BaS0 ) standard white plate.


1

4

Factors Influencing Color of Emulsion

Effect of Droplet Concentration and Size Studies of the influence of droplet concentration and size on emulsion color have been investigated (24, 35, 36). Photographs illustrating these effects are shown in Figures 2 and 3. The most significant change in emulsion color (relative to the color of emulsions in the absence of droplets) occurred between 0 and 5 wt% oil (Figure 4f). The intensity of emulsion color tended to fade with increasing droplet concentration, except at low droplet concentrations (0-1 wt%) in dyed emulsions where there was initially an increase in chroma (Figure 4b,c). This initial increase has been attributed to the influence of multiple scattering on the effective pathlength that the light waves travel through the emulsion before being reflected back to the detector (36). The Z-value of the emulsions decreased with increasing droplet diameter (Figure 5a), whilst the chroma increased with increasing droplet size (Figure 5b,c). The most likely reason for the increase in the chroma of the emulsions with increasing droplet size is that the scattering efficiency of the droplets decreases and therefore the light beam can penetrate further into the emulsion, which increases the absorption. The color difference of the emulsions (relative to the emulsion containing the largest droplets) became increasingly small as the droplet diameter increased (Figure 5f) (24). The increase in emulsion lightness and decrease in emulsion chroma with increasing droplet concentration (0-20 wt%) and decreasing droplet diameters (d = 0.2 to 30 μπι) was confirmed by sensory analysis for emulsion containging blue dye (24). 32



372

Figure 2. A series of n-hexadecane oil-in-water emulsions with different droplet concentrations (0.1 to 20 wt%), but the same mean droplet diameter (du = 0.3 pm) and dye content (0.005 wt%). (See page 23 of color inserts.)

Figure 3. A series of n-hexadecane oil-in-water emulsions with the same droplet concentration (9.5 wt%) and dye content (0.005wt%), but different droplet diameters (d = 0.2 to 30 pm). (See page 23 of color inserts.) 32

The influence of flocculation on the optical properties of concentrated emulsions was investigated (37). The study showed that flocculation had a slight effect on emulsion appearance. As flocculation increased (depletion flocculation or electrostatic screening flocculation), the spectral reflectance and L-value (lightness) of the emulsions decreased, especially above the critical flocculation concentration. The influence was fairly similar in both the presence and absence of dye. Nevertheless, their impact was not as strong as that of the individual droplet size.


373 Effect of Refractive Index We have also investigated the influence of refractive index on the optical properties of oil-in-water emulsions (38). Experiments both with and without dye showed the same effect (Figure 6). Generally, the reflective index ratio can be varied by adding water-soluble solutes (such as sugars, polyols or salts) to the aqueous phase to increase its refractive index (Figure 7). In our study, we added different amounts of glycerol. As the n /n ratio approached unity, the emulsions became more transparent, and the spectral reflectance decreased. On the other hand, as the n /n ratio moved away from unity, due to the increase in the refractive index difference between the two phases, the scattering efficiency of the emulsion droplets increased and so the spectral reflectance increased (Figure 8). The spectral reflectance predicted by the theory showed similar trends to measured spectra. d

aq


d

aq

Effect of Dye Type and Concentration The optical properties of colored emulsions are determined by the characteristics of the droplets and dyes they contain. Experimental measurements of the influence of dye type and concentration on the color of oil-in-water emulsions containing different concentrations and sizes of droplets have been carried out (36). There are dramatic changes in the lightness and color of emulsions over the droplet concentration range 0 to 3 wt% (Figure 9). Around 0.5 μιη diameter, the Z-value of the emulsions had a slight maximum value and the emulsions were least colored because of the maximum in the scattering efficiency of the droplets. When droplet size increased, the emulsion lightness decreased and became more colored (Figure 10). This is because the scattering efficiency of the droplets decreased, and therefore the light waves could penetrate further and be more absorbed by the dyes. In addition, the influence of droplet size and concentration on the lightness and color of emulsions containing different types of dye was fairly similar (Figure 11). Besides the droplet characteristics, the optical properties of concentrated oil-in-water emulsions also depend strongly on the concentration of dye present. The impact of dye concentration on the color coordinates of /i-hexadecane oil-inwater emulsions is shown in Figure 12. As one would expect, emulsion lightness decreased and chromaticness increased as the dye concentration increased because more of the light was absorbed by the chromophores in the dye.

Comparison of Theory with Experimental Measurements Mostly, the values predicted by light scattering theory exhibit the same trend and agree qualitatively with the measured values (24, 26, 28). However, there





5

10

15

20

Oil concentration (%)

Ο

0,3 μιΐΐ 2 μίΐι ΙΟμιη

25

c)

I

ο i ο

10

30

40

50

8

12

16

Oil concentration (%)

4

20

0.3 μιη 2 μι» 10 μιη

Figure 4. Dependence ofL,a,b and color difference on droplet concentration for n-hexadecane oil-in-water emulsions (Reproducedfrom reference 24. Copyright 1998 American Chemical Society.)

-14

-12

-10

-8

-6

-4

-2

Ο

2

4


in


Ο

10

15

20

25

Particle size (μηι)

5

30

35

0

10

15 20

25


5


30

35

b)



Figure 5. Dependence ofL, a, b, and color difference on droplet size for 10 wt% n-hexadecane oil-in-water emulsions (Reproducedfrom reference 24. Copyright 1998 American Chemical Society.)



-a -a

378

Figure 6. Two series of 4 wt% n-hexadecane oil-in-water emulsions with the same median droplet diameter (1 pm) but a range of different nyn ratios (0.97-1.07), in the absence and presence of red dye (0.002 wt%). (See page 23 of color inserts.)


aq

0

20

40

60

100

80

Glycerol in water (%wt) Figure 7. Index of refraction of hexadecane, water, and glycerol in water at different concentrations (Reprinted from 38, with permission from Elsevier).

0 J 0.97

.

.

.

.

0.99

1.01

1.03

1.05

_ 1.07

Figure 8. Dependence of the reflectance (%) at 700 nm on n/n ratios (0.971.07) of a series of 4 wt% n-hexadecane oil-in-water emulsions with the same median droplet diameter (1 μηι), red dye concentration (0.002 wt%), comparing to a predicted one. (Reprinted from 38, with permission from Elsevier). aq



QJ caiœntiiitiai (°/

Understanding Colors in Emulsions - American Chemical Society

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