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Depletion Flocculation of Beverage Emulsions by Gum Arabic and Modified Starch R

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Depletion Flocculation of Beverage Emulsions by Gum Arabic and Modified Starch R JFS: Food Engineering and Physical Properties Depletion Flocculation of Beverage Emulsions by Gum Arabic and Modified Starch R. CHANAMAI AND D.J. MCCLEMENTS ABSTRACT: The creaming velocity, apparent viscosity, and ultrasonic attenuation spectra (1 to 50 MHz) of 5 wt% n- hexadecane oil-in-water emulsions containing different droplet radii (r = 0.15 - 0.7 mm), biopolymer types (gum arabic or modified starch), and biopolymer concentrations (0 to 2.5 wt%) were measured. Depletion flocculation was observed in the emulsions when the nonabsorbed biopolymer concentration exceeded a critical concentration (CFC). The CFC increased with decreasing droplet radius for both biopolymers because the magnitude of the deple- tion attraction increases with droplet size. The CFC was lower for gum arabic than modified starch because it has a higher effective volume in solution. Depletion flocculation led to an increase in creaming instability and apparent viscosity of the emulsions. Flocculation could be nondestructively monitored by measuring the decrease in ultra- sonic attenuation of the emulsions. These results show that depletion flocculation by gum arabic and modified starch can have an adverse effect on the stability of beverage emulsions. Key Words: gum arabic, modified starch, beverages, emulsions, flocculation Introduction It consists primarily of amylopectin that has been chemically EVERAGE EMULSIONS ARE OIL-IN-WATER EMULSIONS THAT modified to contain nonpolar side-groups. These side- Bare normally prepared as a concentrate, that is, diluted groups anchor the molecule to the droplet surface, while the into finished products (Tan 1997, 1998). The emulsion in both hydrophilic starch chains protrude into the aqueous phase its concentrated and diluted form must have a high degree and protect droplets against aggregation through steric re- of stability. The oil phase usually consists of vegetable oil, fla- pulsion. Purity Gum is mildly anionic in aqueous solutions vor oil, and weighting agent, while the aqueous phase con- and has a surface activity that is almost as high as gum arabic sists of water, sugar, emulsifier, acids, and preservatives (Tan (Ray and others 1983; Tse 1990). 1997). Beverage emulsions are usually stabilized by am- Gum arabic and modified starch have relatively low sur- phiphilic polysaccharides, such as gum arabic or hydropho- face activities (compared to proteins or surfactants), and so bically modified starch (Ray and others 1995; Kim and others a large excess must be added to ensure that all the droplet 1996; McNamee and others 1998; Trubiano 1995; Garti 1999). surfaces are adequately coated (Phillips and Williams 1995). Gum arabic is the most commonly used biopolymer emul- For example, as much as 20% gum arabic or 12% modified Food Engineering and Physical Properties sifier in flavor beverage emulsions (Tan 1997, 1998). It is de- starch may be required to produce a stable 12.5 wt% oil-in- rived from the natural exudate of Acacia senegal and consists water emulsion (Tse and Reineccius 1995). As a result, there is of at least 3 high molecular weight biopolymer fractions. The a large excess of nonabsorbed polysaccharide in the aqueous surface-active fraction is believed to consist of branched ara- phase of emulsions prepared from them (Tan 1998; Garti binogalactan blocks attached to a polypeptide backbone 1999). Under certain circumstances, nonabsorbed biopoly- (Anderson and others 1985; Randell and others 1988; Phillips mers are capable of promoting droplet flocculation through a and Williams 1995; Jayme and others 1999). The hydrophobic depletion mechanism (Jenkins and Snowden 1996; Lips and polypeptide chain is believed to anchor the molecules to the others 1991; McClements 1999). Flocculation causes a number droplet surface, while the hydrophilic arabinogalactan blocks of effects that are detrimental to emulsion quality: (1) en- extend into the solution, providing stability against droplet ag- hanced creaming due to the increase in particle size, (2) de- gregation through steric and electrostatic repulsion (Phillips creased cloudiness due to the increase in particle size, and (3) and Williams 1995; Islam and others 1997; Jayme and others enhanced coalescence because droplets are brought into 1999). Gum arabic is an effective emulsifier because of its high close proximity (Dickinson and Stainsby 1982; McClements water solubility, low solution viscosity, good surface activity, 1999). The purpose of this study was to investigate the ability and ability to form a protective film around emulsion droplets of gum arabic and modified starch to promote depletion floc- (Glicksman 1983; Dickinson and others 1989). culation in model beverage emulsions and to determine their Problems associated with obtaining a reliable source of effects on the creaming stability and rheology of emulsions. consistently high-quality gum arabic has led many food sci- entists to investigate alternative sources of biopolymer emul- Materials and Methods sifiers for use in flavor beverages (Kim and others 1996; Tan 1997, 1998; Garti 1999). Hydrophobically modified starches Materials have been identified as 1 of the most promising replace- Experimental procedures. Polyoxyethylene sorbitan ments for gum arabic (Trubiano 1995). The modified starch monolaurate (Tween 20), a non-ionic surfactant, and hexa- used in this study (Purity Gum; National Starch, Bridgewater, decane were purchased from the Sigma Chemical Co. (St. N.J., U.S.A.) is an octenyl succinate derivative of waxy-maize. Louis, Mo., U.S.A.). Modified starch (Purity Gum) was ob- © 2001 Institute of Food Technologists Vol. 66, No. 3, 2001—JOURNAL OF FOOD SCIENCE 457 Depletion Flocculation of Beverage Emulsions . tained from the National Starch and Chemical Co. (Bridge- Coulter Corp., Miami, Fla., U.S.A.). This instrument measures water, N.J., U.S.A.). The average molecular weight of the Pu- the back-scattering of monochromatic light (l = 800 mm) as rity Gum was about 4 3 105 daltons, with a fairly broad dis- a function of sample height. Emulsions were placed into flat- tribution. Gum arabic was obtained from Importers Service bottomed, cylindrical glass tubes (100 mm height, 16 mm in- Corp. (Jersey City, N.J., U.S.A.). The major fractions of gum ternal dia) and stored at room temperature. The back-scat- arabic have been reported to have molecular weights of tering of light from the emulsions with height was then mea- around 2.5 to 10 3 105 daltons (Jayme and others 1999). Dis- sured. The extent of creaming was assessed by determining tilled and deionized water was used in the preparation of all the height (H) of the interface between the opaque, droplet- solutions. rich layer at the top of emulsion and the less opaque, drop- let-depleted layer at the bottom as a function of time (t). The Emulsion Preparation results are reported either as the full creaming profiles or as An aqueous surfactant solution was prepared by dispers- the initial creaming rate: dH/dt. ing 2.5 wt% Tween 20 in water. A 5 wt% hexadecane oil-in- water emulsion was prepared by weighing 100 g of hexade- Rheology Measurements cane and 1900 g of surfactant solution into a 2000 cm3 plastic The rheological properties of emulsions were measured beaker and blending with a high-speed blender for 1 min using a dynamic shear rheometer with a concentric cylinder (High Shear Homogenizing Container; Waring Laboratory, measurement cell (Constant Stress Rheometer, CS-10; Bohlin New Hartford, Conn., U.S.A.). The size of the emulsion drop- Instruments, Cranbury, N.J., U.S.A.). The dia of the rotating lets was then reduced further using a high-pressure valve inner cylinder was 25 mm, and the dia of the static outer cyl- homogenizer (Rannie 8.30R, Wilmington, Mass., U.S.A.). inder was 27.5 mm. Samples were placed in the temperature- Emulsions containing droplets with different sizes were ob- controlled measurement vessel and allowed to equilibrate to tained by withdrawing samples at different stages during the the required temperature (25 8C) for 5 min prior to making preparation procedure. The influence of biopolymer con- the measurements. The shear rheology of the samples was centration on the measurements was investigated by prepar- determined by preshearing them at a constant shear rate of ing a series of emulsions with the same droplet size distribu- 30 s-1 for 30 s, allowing a recovery period of 3 min, and then Food Engineering and Physical Properties tion and concentration and then adding different amounts of acquiring the apparent viscosity as a function of shear stress biopolymer and/or water to the aqueous phase to keep the (0 to 3 Pa). overall droplet concentration the same in each emulsion. Ultrasonic measurements Particle Size Determination by Light Scattering Ultrasonic attenuation spectra of emulsions were mea- The particle size distribution of the emulsions was mea- sured in the frequency range of 1 to 50 MHz using a custom- sured using a laser light scattering instrument (Horiba LA- built ultrasonic spectrometer. This spectrometer was based 900, Irvine, Calif., U.S.A.). This instrument measures the an- on the frequency scanning ultrasonic pulse echo reflectome- gular dependence of the intensity of light scattered from a ter described in detail elsewhere (McClements and Fairley dilute emulsion. It then finds the particle size distribution that gives the best fit between the experimental measure- ments and predictions made using light scattering theory. A refractive index ratio of 1.08 was used by the instrument to calculate the particle size distributions. Measurements are S 3 reported as the surface-volume mean radius: r32 = niri / S 2 niri , where ni is the number of droplets of radius ri. To prevent multiple scattering effects, the emulsions were dilut- ed with distilled water prior to analysis so that the droplet concentration was less than about 0.02 wt%. Each sample was analyzed 3 times, and the data are presented as the aver- age. The droplet size distribution did not change during the course of the experiments, which suggests that the emul- sions were stable to coalescence and Ostwald ripening.
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