emulsifiers used in food and drink

7/30/2019 Emulsifiers used in food and drink

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Comparison of Gum Arabic, Modified Starch,and Whey Protein Isolate as Emulsifiers:Influence of pH, CaCl

2and Temperature

R. CHANAMAIAND D.J. MCCLEMENTS

ABSTRACT: The particle size and zeta potential of model beverage emulsions (0.01 wt% soybean oil-in-wateremulsions, d 1 mm) stabilized by gum arabic, modified starch, or whey protein isolate (WPI) were studied withvarying pH (3 to 9), CaCl

2concentration (0 to 25 mM), and temperature (30 C to 90 C). Temperature, pH, CaCl

2

strongly influenced emulsions stabilized by WPI because its stabilizing mechanism was mainly electrostatic repul-sion, but not those stabilized by gum arabic or modified starch because their stabilizing modes of action weremainly steric repulsion. This study may have important implications for the application of WPI as an emulsifier inbeverage emulsions.

Keywords: emulsions, stability, whey protein, gum arabic, modified starch

succinate derivative of waxy maize. It consists primarily ofamylopectin that has been chemically modified to containnonpolar side groups. These side groups anchor the mole-cule to the droplet surface, while the hydrophilic starchchains protrude into the aqueous phase and protect dropletsagainst aggregation through steric repulsion. Previous stud-ies have indicated that modified starch is mildly anionic inaqueous solutions and has a surface activity that is almost ashigh as that of gum arabic (Ray and others 1983; Tse 1990).

A wide variety of proteins are also used as emulsifiers infoods because they naturally have a high proportion of non-polar groups and are therefore surface-active (Dickinson1992; Damodaran 1996). Whey protein is one of the emulsifi-ers frequently used in foods because of its ability to facilitatethe formation and stabilization of oil-in-water emulsions(Phillips and others 1994; Huffman 1996; Dickinson 1997; Mc-Clements 1999). The ability of whey protein to form stableemulsions depends on emulsion composition (including pHand mineral content, salt, sugar, surfactant, and polysaccha-ride contents) and environmental conditions (temperatureand pressure) (Dickinson and Stainsby 1982; Kinsella 1984;Kinsella and Phillips 1989; Mangino 1989; Dickinson 1992;Dalgleish 1996; Dematriades and others 1997a, 1997b; Kul-myrzaev and others 2000; Singh and Ye 2000). Whey proteinsare therefore suitable for application in food emulsionswhere the composition and environmental conditions favora stable product, but not in those products where the condi-tions promote emulsion instability.

Beverage emulsions may have a variety of different com-positions and experience a variety of environmental condi-tions during their storage, transport, and consumption. It istherefore important for manufacturers of these products tounderstand the influence of these factors on emulsion stabil-ity. In this study, we examined the influence of pH, calciumion concentration, and temperature on the stability of diluteemulsion stabilized with different types of biopolymer emul-sifiers: gum arabic, modified starch, and whey protein iso-late. Ultimately, we aim to determine whether whey proteincan be used as a suitable alternative to polysaccharide-based

Introduction

BEVERAGE EMULSIONSARE OIL-IN-WATEREMULSIONSTHATare normally prepared as a concentrate that is dilutedinto finished products (Tan 1997, 1998). The oil phase usuallyconsists of vegetable oil, flavor oil, and a weighting agent,while the aqueous phase consists of water, sugar, emulsifier,acids, and preservatives (Tan 1997). This unique class ofemulsions must have a high degree of stability in both theconcentrated and the diluted form.

Beverage emulsions are usually stabilized by amphiphilicpolysaccharides, such as gum arabic or hydrophobicallymodified starch (Ray and others1995; Trubiano 1995; Kimand others 1996; McNamee and others 1998; Garti 1999).Gum arabic is the most commonly used biopolymer emulsi-fier in flavor beverage emulsions (Tan 1997, 1998). It is de-rived from the natural bark exudate ofAcacia senegal andconsists of at least 3 high-molecular-weight biopolymer frac-tions. The surface-active fraction is believed to consist ofbranched arabinogalactan blocks attached to a polypeptidebackbone (Anderson and others 1985; Phillips and Williams1995; Jayme and others1999). The hydrophobic polypeptidechain is believed to anchor the molecules to the droplet sur-face, while the hydrophilic arabinogalactan blocks extendinto the solution, providing stability against droplet aggrega-tion through steric and electrostatic repulsion (Phillips andWill iams 1995; Islam and others 1997; Jayme and others1999). Gum arabic is an effective emulsifier because of itshigh water solubility, low solution viscosity, good surface ac-tivity, and ability to form a protective film around emulsiondroplets (Glicksman 1983). However, the relatively high cost,large quantity required, and problems associated with ob-taining a reliable source of consistently high-quality gum ar-abic have led many food scientists to investigate alternativesources of biopolymer emulsifiers for use in flavor beverages(Kim and others 1996; Tan 1997, 1998; Garti 1999). Hydro-phobically modified starches have been identified as one ofthe most promising replacements for gum arabic (Trubiano1995). The modified starch used in this study (Purity Gum

BE; National Starch, Bridgewater, N.J., U.S.A.) is an octenyl


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F o o d C h e m i s t r y a n d T o x i c o l o g y

emulsifiers in beverage emulsions.

Materials and Methods

MaterialsSoybean oil was purchased from a local retailer, and was

used without further purification. Modified starch (PurityGum BE) was obtained from the National Starch and Chemi-cal Co. (Bridgewater, N.J., U.S.A.). Premium spray-dried gumarabic was obtained from Importers Service Corp. (JerseyCity, N.J., U.S.A.). WPI (Alacen 895, protein 93.5%) was ob-tained from New Zealand Milk Products (Santa Rosa, Calif.,U.S.A.). Calcium chloride and sodium azide were purchasedfrom Sigma Chemical Co. (St. Louis, Mo., U.S.A.). Distilled,deionized water was used in the preparation of all solutions.

Preparation of emulsionsAn aqueous emulsifier solution was prepared by dispersing20 wt% gum arabic or 14 wt% modified starch or 0.7 wt% WPIwith 0.02 wt% sodium azide in distilled water (for antimicrobi-al agent in this study only, not for food additive) and stirringfor at least 6 h to ensure complete dissolution. A 14 wt% soy-bean oil-in-water emulsion was prepared by weighing 70 gsoybean oil and 430 g emulsifier solution into a 1000-cm3 plas-tic beaker and blending with a high-speed homogenizer for 1min (Bio Homogenizer; Biospec Products Inc., Bartlesville,Okla., U.S.A.). The size of the emulsion droplets was then re-duced further using a high-pressure valve homogenizer (Ran-nie model 8.30R; Wilmington, Mass., U.S.A.). A series of 0.01wt% emulsions with different CaCl 2 concentrations was pre-

pared by diluting the concentrated emulsion with distilled wa-ter containing different concentrations of calcium chloride.Then the diluted emulsions were adjusted to different pH val-ues by adding a small amount of dilute NaOH or HCl solution.

Particle size determination by light scatteringThe particle size distribution of the emulsions was mea-

sured using a laser light scattering instrument (Horiba modelLA-900; Irvine, Calif., U.S.A.). This instrument measures theangular dependence of the intensity of light scattered from adilute emulsion. It then finds the particle size distribution thatgives the best fit between the experimental measurements andpredictions made using light scattering theory. A refractive in-dex ratio of 1.08 was used by the instrument to calculate the

particle size distributions. Measurements are either reported

as the full particle size distribution or as the surface-volumemean radius: r32 = niri

3/niri2, where ni is the number of

droplets of radius ri. To prevent multiple scattering effects, the

concentrated emulsions were diluted with distilled water priorto analysis so that the droplet concentration was less thanabout 0.02 wt%. The dilute emulsions were placed directly intothe measurement cell of the instrument and stirred slowlyduring the measurement. Each sample was analyzed 3 timesand the data are presented as the average. The initial dropletsize distributions of the emulsions stabilized by the 3 types ofbiopolymer emulsifiers are shown in Figure 1.

-Potential measurementsOil-in-water emulsions (0.01 wt%) were injected directly

into the measurement chamber of a particle electrophoresisinstrument capable of measuring the -potential of emulsion

droplets (Zetamaster; model ZEM5003; Malvern Instruments,Worcester, U.K.). The -potential measurements are report-ed as the averages of 3 separate injections, with 3 readingsmade per injection.

Results and Discussion

Influence of pH and CaCl2

on droplet aggregationThe pH and CaCl2 concentration dependence of the mean

Figure 2Mean droplet size (d32) of emulsions stabilized by

WPI at different pH and calcium chloride concentration.

Figure 3Droplet size distribution of emulsions stabilizedby whey protein without calcium chloride at selected pH

values.

Figure 1Droplet size distribution of emulsions measuredby light scattering.

Comparison of emulsifier types . . .


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ldroplet size of emulsions stabilized with gum arabic, modifiedstarch, or whey protein were required. These measurementswere carried out 24 h after CaCl

2was added to the emulsions

and they had been stored at 30 C. There was no significantdifference in mean droplet size of emulsions stabilized withgum arabic or modified starch in pH range from 3 to 9 andCaCl2 concentration 0 to 25 mM (data not shown). Both mod-ified starch and gum arabic have been reported to be mildlyanionic in aqueous solutions (Ray and others 1983; Tse 1990;Tan 1997). The carboxyl (-COO) ions are at the periphery ofthe molecule and are very active in creating an anionic envi-ronment (Tan 1997; Thevenet 1988). However, addition of cal-cium chloride at different pH values did not alter their prop-erties. This finding suggests that these emulsions werepredominantly stabilized by steric interactions, as changes inelectrostatic interactions did not have a significant impact ondroplet aggregation. These results also agree with the -po-tential of the droplets that will be discussed later.

In contrast, the mean droplet sizes of emulsions stabilizedby whey protein differed appreciably from that of the initialemulsion, depending on pH and calcium chloride concentra-tion (Figure 2). In the absence of calcium chloride, there wasa significant increase in the mean particle size around theisoelectric point (4 pH 6) of the whey proteins, indicat-ing that appreciable droplet aggregation occurred (Figure 3).

The addition of calcium chloride to the emulsions signifi-cantly altered the extent of droplet aggregation. When thecalcium chloride concentration was increased from 0 to 3mM, the pH at which the droplets aggregated shifted to ahigher value (Figure 2). As the concentration of calciumchloride increased, the range of pH values above the isoelec-tric point over which the emulsions became unstable grewwider. At high calcium chloride concentration (above 20mM), the emulsions were unstable to aggregation at all pHvalues above the isoelectric point, but at pH 3 CaCl 2 did notinfluence the stability of emulsions stabilized with whey pro-tein. This result indicated the possibility of using whey pro-teins as emulsifiers for beverage emulsions because theseproducts have a low pH.

Influence of pH and CaCl2

on -potential

The role of electrostatic interactions in stabilizing emul-sions was examined by measuring the electrical charge of thedroplets. The dependence of droplet -potential on pH andcalcium chloride concentration is shown in Figure 4 to 7. Inthe absence of calcium chloride, the -potential of gum ara-bic and modified starch remained negative at all pH values(Figure 4), possibly because of the negatively charged (-COO) groups on the acidic polysaccharide (Tan 1997; Garti1999). The modified starch has no cationic groups and there-

Figure 4-Potential of emulsion droplets stabilized by gumarabic, modified starch or whey protein in the absence ofcalcium chloride at different pH.

Figure 5-Potential of emulsion droplets stabilized by gumarabic at different pH and calcium chloride concentration.

Figure 6-Potential of emulsion droplets stabilized bymodified starch at different pH and calcium chloride con-

centration.

Figure 7-Potential of emulsion droplets stabilized by WPIat different pH and calcium chloride concentration.



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fore did not become positively charged at any pH. The gumarabic has some cationic groups in the protein fraction ofthe molecule and, therefore, would be expected to become

positively charged at sufficiently low pH. Nevertheless, theisoelectric point of gum arabic has been reported to be sig-nificantly below the pH studied in this work (Jayme and oth-ers 1999).

Our results also showed that calcium chloride had a smalleffect on the -potential of emulsions stabilized with gum ar-abic or modified starch (Figure 5 and 6). In both gum arabicand modified starch at the lowest pH, in the absence of calci-um chloride, the droplets had a relatively low negative -po-tential (Figure 4). An increase in pH in the absence of calci-um ions caused an increase in the magnitude of negativecharge on the droplets, which was probably due to deproto-nation of some of the protonated carboxyl groups (COOH

COO + H+) or protonation of some of the amino groups

(NH2 + H+

NH3+

).When the calcium concentration was increased from 0 to3 mM, there was a dramatic decrease in the negative value ofthe -potential on gum arabic-stabilized droplets (Figure 5and 6), which could have occurred because of 2 differentphenomena: electrostatic screening and ion binding (Hunter1986). At higher CaCl2 concentration, the zeta potential ofthe gum arabic-stabilized emulsions became slightly lessnegative, whereas there was little change in the zeta potential

of the modified starch-stabilized emulsions. This suggeststhat there may have been some binding of Ca 2+ ions to nega-tive groups on the biopolymer surfaces. The - potential on

the emulsion droplets would contribute to electrostatic re-pulsion, but the magnitude would not be large enough on itsown to stabilize emulsions. These results suggest that stericrepulsion is a more significant stabilizing force in gum arabicor modified starch-stabilized emulsions than electrostaticrepulsion.

Calcium chloride concentration and pH significantly in-fluenced the x-potential of droplets stabilized by whey pro-tein (Figure 4 and 7). At the lowest pH, in the absence of cal-cium chloride, the droplets had a relatively high positivex-potential because the pH was below the isoelectric point ofthe protein (Figure 4). Under these conditions the aminogroups are positively charged (-NH3

+), whereas the carboxylgroups are neutral (-COOH). When the pH was increased,

the magnitude of the positive charge on the droplets de-creased, partly because carboxyl groups became negativelycharged (-COO) and partly because some of the aminogroups become neutral (-NH2). Eventually the x-potential ofthe droplets became zero, which indicates that the numberof positively charged groups balanced the number of nega-tively charged groups. A further increase in pH caused thedroplets to gain a net negative charge, which increased as thenumber of negatively charged groups increased and posi-

Figure 8Mean droplet size of emulsions stabilized by gum arabic, modified starch or WPI with different temperatureat pH 3 (a) with 0 mM CaCl

22222(b) with 25 mM CaCl(b) with 25 mM CaCl(b) with 25 mM CaCl(b) with 25 mM CaCl(b) with 25 mM CaCl

22222.....

fig A fig B


Figure 9Mean droplet size of emulsions stabilized by gum arabic, modified starch or WPI with different temperature

at pH 7 (a) with 0 mM CaCl 22222 (b) with 25 mM CaCl(b) with 25 mM CaCl(b) with 25 mM CaCl(b) with 25 mM CaCl(b) with 25 mM CaCl22222.....

fig Bfig A


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tively charged groups decreased. In the absence of CaCl2, theisoelectric point of the emulsions droplets occurred at pH4.7 (Figure 4), which is close to the mean isoelectric point ofwhey proteins reported in the literature (Swaisgood 1996). Inthe presence of CaCl2, the isoelectric point of the emulsionsdroplets increased to pH 5.5, and was relatively independentof CaCl2 from 3 to 25 mM (Figure 7). This suggested thatCa2+ ions bound to negatively charged groups (-COO) on

the proteins and increased the net positive charge, so that ahigher pH had to be reached before the droplet charge wasneutralized.

An increase in calcium chloride concentration altered the-potential of the whey protein-stabilized droplets (Figure7). At low pH, calcium chloride caused a decrease in the pos-itive potential on the droplets, whereas at high pH it caused adecrease in the negative potential. These results suggest thatat low pH there may have been some binding of Cl ions tothe NH3+ groups of the proteins, causing a decrease in posi-tive charge. In contrast, at high pH there may have beensome binding of Ca2+ ions to the COO groups of the pro-teins, causing a decrease in negative charge. Alternatively, thereduction in charge magnitude may be due to electrostatic

screening by the ions.

Influence of heating and ionic strength on emulsionstability

Particle size distributions of emulsions at pH 3 and pH 7containing either 0 or 25 mM CaCl2 were measured aftersamples had been heated from 30 C to 90 C for 20 min thenstored overnight at 30 C (Figure 8 and 9). Particle size distri-butions of emulsions stabilized with gum arabic or modifiedstarch remained fairly constant over the temperature range,pH values, and CaCl2 concentrations studied. Nevertheless,the change in particle size distribution of the emulsions sta-bilized with WPI depended on pH, CaCl2,and temperature.Particle size distributions of the WPI-stabilized emulsions at

pH 3 (with 0 mM or 25 mM CaCl2) remained fairly constantover the temperature range studied (Figure 8a and 9a),whereas those at pH 7 WPI showed considerable increase inparticle size (Figure 9b).

At pH values notably lower than the isoelectric point, theprotein molecules had a positive net charge that was suffi-cient to prevent droplets aggregating through electrostaticrepulsive forces (Dematriadesand others 1997a, b). At thispH, the counter ions were monovalent Cl ions rather thandivalent Ca2+ ions, and therefore electrostatic screening andcharge neutralization were less effective than at pH valuesabove the isoelectric point. WPI-stabilized emulsions at pH 3were therefore stable to heating in the absence or presenceof CaCl2.

A temperature-dependence was found for the WPI-stabi-lized emulsions at pH 7. In the absence of CaCl2, the particlesize of emulsions changed little between 30 C and 90 C (Fig-ure 9a) except for the emulsion heated to 70 C. A maximumin droplet flocculation has been observed at this tempera-ture in previous studies of the influence of heating on thestability of WPI-stabilized emulsions. The higher degree offlocculation of emulsion droplets above 65 C was most likelycaused by heat-induced unfolding of protein molecules ad-sorbed to the oil-water interface (Hunt and Dalgleish 1995;Dalgleish 1996; Monahan and others 1996). Differential scan-ning calorimetry has shown that lactoglobulin and -lac-talbumin adsorbed to the surface of oil droplets unfoldedwhen the emulsions were heated > 65 C (Dalgleish 1996).

When the molecules unfolded, they exposed reactive amino

acid residues leading to enhanced protein-protein interac-tions via hydrophobic interactions and thio-disulfide inter-changes (Dickinson and Matsamura 1991; McClements andothers 1993; Monahan and others 1993, 1996).

The addition of 25 mM CaCl2 had a strong destabilizinginfluence on the emulsions, and it was strongly temperature-dependent (Figure 9b). Below 70 C, the emulsions were floc-culated because of electrostatic screening and charge neu-

tralization by the divalent ions. Above 70 C, there was anadditional amount of flocculation because of the increase insurface hydrophobicity of the emulsion droplet associatedwith protein unfolding.

Our results clearly showed that the stability of WPI-stabi-lized emulsions were much more sensitive to environmentalconditions (pH, ionic strength, and thermal history) thangum arabic or modified starch-stabilized emulsions. In orderto replace gum arabic or modified starch in beverage emul-sions, WPI could only be used when the pH of the systemwas relatively far from the isoelectric point of the protein,such as < pH 4 or > pH 6 in the absence of minerals. Bever-age emulsions fortified with minerals could only be pro-duced using WPI as emulsifier by adjusting the pH to acid

conditions to avoid flocculation problems.

Conclusions

THISSTUDYSHOWSTHATTHEREARESIGNIFICANTDIFFERENC-es in the properties of model beverage emulsions stabi-lized by gum arabic, modified starch, and WPI. There was noeffect of pH, calcium chloride concentration, or tempera-ture on emulsions stabilized by gum arabic or modifiedstarch. In contrast, droplet aggregation of whey protein-sta-bilized emulsions was strongly dependent on pH, calciumchloride concentration, and temperature. Emulsion dropletsstabilized by whey protein were highly unstable to aggrega-tion near the isoelectric point of the proteins because of therelatively low electrostatic repulsion between the droplets. At

pH values below the isoelectric point, the emulsion stabilitywas relatively insensitive to calcium chloride concentration,but at the pH values above the isoelectric point, calciumchloride promoted droplet flocculation. Heating emulsionsstabilized by whey proteins above 70 C promoted instabilityto flocculation at pH 7, but it had little effect at pH 3. Our re-sults have important consequences for the application ofthese emulsifiers in beverage emulsions. To produce anemulsion stabilized by whey protein that is stable to floccula-tion, it is important to ensure that the pH is sufficiently farfrom the isoelectric point of the protein and that the calciumconcentration is less than that required to promote dropletflocculation. These criteria may be met in many beverageemulsions that have acidic pH.

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MS 20001101 Submitted 11/1/2000, Accepted 4/3/2001

This report is based on work supported by Dairy Management Incorporated and the U.S.Dept. of Agriculture under Hatch Grant 745.

Authors are with the Biopolymers and Colloids Research Laboratory, Dept.of Food Science, Univ. of Mass. Address inquiries to author McClements (E-mail: [email protected]).

emulsifiers used in food and drink

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