Food Structure
Volume 12 Number 4 Article 3
1993
Hydrocolloids as Food Emulsifiers and Stabilizers
Nissim Garti
Dov Reichman
Follow this and additional works at: https://digitalcommons.usu.edu/foodmicrostructure
Part of the Food Science Commons
Recommended Citation Garti, Nissim and Reichman, Dov (1993) "Hydrocolloids as Food Emulsifiers and Stabilizers," Food Structure: Vol. 12 : No. 4 , Article 3. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol12/iss4/3
This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Food Structure by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Food Structure, Vol. 12, 1993 (Pages 411-426) 1046-705X/93$5.00+ .00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA
HYDROCOLWIDS AS FOOD EMULSIFIERS AND STABILIZERS
Nissim Garti and Dov Reichman
Casali Institute of Applied Chemistry, School of Applied Science and Technology The Hebrew University of Jerusalem, Jerusalem, Israel 91904
Abstract Introduction
Hydrocolloids are water-soluble biopolymers con Water soluble high molecular weight polysaccha sisting of high molecular weight polysaccharides known rides have been known for generations as food stabi as viscosity builders, gelification agents and stabilizers lizers; the main application being modification of the of food systems. rheological properties of aqueous systems, viscosity Several hydrocolloids such as gum arabic (acacia), builders, gelification agents and stabilizers (Whistler, tragacanth, xanthan and certain modified gums have 1973; Glicksman, 1969; BeMiller, 1988; Dickinson, been mentioned as food additives having special func 1988). tions such as: "retardation of precipitation of dispersed The primary beneficial effect was related to the abil solid particles and coalescence of oil droplets". The role ity of the hydrocolloid to interact with water and change of these gums as emulsifiers remained somewhat ob its rheological properties. However, the functionality of scure. The present review is an attempt to bring the rel certain hydrocolloids was correlated also to phenomena evant studies together and to throw some light on the such as: "retardation of precipitation of dispersed solid functionalities of the gums as surface active agents and particles", "decreased creaming rates of oil droplets and food emulsifiers. In addition, some recent results foams", "prevention of aggregation of dispersed parti obtained in our laboratory are discussed. cles", "prevention of syneresis of gelled systems contain Gum arabic is compared to colloidal microcrystal ing oils", and "retardation of coalescence of oil droplets" line-cellulose (M CC) and galactomannans (recent results) (Dickinson, 1988). Those functionalities are difficult to in view of their abihty to reduce surface tensions, inter explain in terms of viscosity and water-gum interactions. facial tensions and to stabilize oil in water emulsions via Adsorption onto, or interaction with the oil phase had to the 'steric' and 'mechanical' stabilization mechani sms. be considered in oil-in-water emulsions. Adsorption of It is demonstrated that while gum arabic adsorbs solid particles had to be adapted for solid dispersions of strongly and effectively onto the oil droplets via its solid particles in aqueous systems (fadros and Vincent, proteinaceous moieties, guar gum and locust bean gum 1983). (LBG) adsorb weakly and for the most part only Most of the water soluble polysaccharides are "precipitate" on the oil surface, and form birefringent known to be high molecular weight polymers with a lim layers of the polymer oriented with its hydrophobic ited number of repeating monomers composing the poly mannose backbone facing the oil. The stabilization with mer, low hydrophobicity and limited flexibility (derived MCC is claimed to be achieved via adsorption of solid from the rotational restrictions of the functional groups), particles on the oil droplets (mechanical stabilization). and interfacial density packing limitations. This is con trary to proteins that are composed of both hydrophilic Key Words: Hydrocolloids, food emulsifiers, food sta and hydrophobic moieties, which display a high degree bilizers, food emulsions, gums, galactomannans, gum of flexibility and hydrophobicity (Figure I; Yalpani, arabic. 1988). It is therefore believed that gums will adsorb (onto solid or liquid surfaces) very slowly, weakly and with very limited surface load if at all (Stainsby, 1988). Nevertheless, certain polysaccharides, i.e., gum Paper received June 22, 1993 arabic (Banerji, 1952; Silber and Mizrahi, 1975), Manuscript received November 26, 1993 xanthan (Prud'homme and Long, 1983), and tragacanth Direct inquiries to N. Garti (Dea and Madden, 1986; Bergenstabl et al., 1986; Telephone number: 972 2 633779 Yokoyama er al., 1988) have been noted to display spe FAX number: 972 2 520262 cific surface activities and stabilize dispersed particles of oil droplets in aqueous systems.
411 N. Garti and D. Reichman
HYDROCOLLOID • s '- a HrfffH .;- 0 ~H,OH--0~ - - 0 H 0 OH ~ 1J1 , G(d I /kT
PEPTIDE
Ill Figure 1. Comparison of polysaccharide and peptide chain conformation. Chain conformations are deter mined by tbe dihedral angle Y, and ¢ between adjacent peptides and sugar rings, or planar peptide groups. - s Broken lines indicate "virtual bonds" representing individual residues (Yalpani, 1988). Figure 2. Interaction free energy G(d), in units of kT, Dickinson and Stainsby (1988) distinguish between as a function of surface-to-surface separation d for pairs emulsifiers and stabilizers in food systems and stress the of polymer-coated particles: (I) thin polymer layer; (II) difference between the two using an interesting defini thick polymer layer (good solvent); and (III) thick poly tion: "An emulsifier is a single chemical or mixture of mer layer (poor solvent); ---- shows Van der Waals components having the capacity for promoting emulsion interaction (Dickinson, 1988). formation and short term stabilization by interfacial ac tion. A stabilizer is a single chemical component o r b mixture of components which can offer long tenn stabili ty on an emulsion, possibly by a mechanism involving adsorption but not necessarily so." This definition will '_{L~ ·~~;;;;; be referred to as a guideline for determining efficacy of , ) ;;; )))J)j)J))}) ;;;;;;; ;; ;;;;;;; an emulsifier or stabilizer later in this paper. In considering stability witb respect to coalescence d and flocculation, it is important to distinguish between adsorbing polymers and non-adsorbing polymers . Covering particle surfaces with a thick protective layer of adsorbing polymer inhibits coalescence. The coated particles strongly repel one another at close range through a combination of polymeric elastic and osmotic effects, and at this point, tbe colloid is said to be - d - sterically stabilized. This type of stabilization is most effectively achieved using copolymers having a combina tion of anchoring groups and dangling chains. The an choring groups should have a strong affinity for tbe sur face and a low affinity for the continuous phase, and the opposite is true for tbe dangling chains. In addition, tbere should be enough adsorbing copolymer to cover tbe particle surface quite completely, and tbe layer thick Figure 3. Sketches of interfacial configurations of ness should be sufficient for two particles to be prevent chains witb adsorbing ( • l and non-adsorbing (0) ed from approaching a pair separation of which tbe in groups: (a, top left) copolymer; (b, top right) homo terparticle (Van der Waals) attraction is of an apprecia polymer; (c, bottom left) bridging; (d, bottom right) ble magnitude in relation to tbe thermal energy, kT. A depletion (Dickinson, 1988).
412 Hydrocolloids as Food Emulsifiers and Stabilizers high molecul ar mass favors thick film formation and ,------thus good steric stabilization (Figure 2). A homopoly mer composed of many identical ad sorbing segments is not a good steric stabilizer, since it produces too thin a layer, leading to free energy interaction with an attrac tive minimum, whose depth may be several kT (Figure 3). Where surface coverage is incomplete, a single polymer chain may become attached to more than one particle, as illustrated in Figure 3. Such bridging fl oc culation is most likely when the polymer concentration is approximately half of that required for saturation 0 coverage (Dickinson, 1988). As a result, it seems clear that proteins known to 24------.-----.------.------1 have significant hydrophobicity and flexibility, can readi 0 10 15 20 ly adsorb onto particles or oil droplets, and therefore, Gum arabic concentration (% w/w) will be considered as emul sifiers with short term stabili ty action on dispersed particles (termed "steric stabili Figure 4. The average droplet diameter of emulsions zation mechanism" by Dickinson and Stainsby, 1988). (20% w/w orange oil) as a function of gum arabic con Hydrocolloids, on the other hand, will be considered as centration. Measurements were performed on diluted stabilizers with limited adsorption ability and long term dispersions (1/1000) with distilled water using a ZM stability (termed "depletion stabilization mechanism" by model Coulter counter equipped with a 50 I'm orifice Dickinson and Stainsby, 1988). electrode. Recent studies have concentrated on the role of gum arabic as an emulsifier, mainly in order to understand the nature of the strong adsorption to the oil droplets in emulsions, as opacity builders for citrus beverages cloudy emulsions for beverages (Randall et al., 1988; (Williams et at., 1990). Gum arabic is added at high Williams et at. , 1990). levels (up to 20 %) to an aqueous sugar solution and The present paper is an attempt to criticall y review emulsi fi ed with an oil phase (Figure 4) consisting of the literature with regard to the use of hydrocolloids as orange oil , and possibly wei ghing agents (i.e., bromi food emulsifiers, to analyze their activity at the interface nated vegetable oil - BVO, sucrose esters, sucrose diace and to evaluate their ability to stabilize emulsions. In tate bexaisobutyrate or ester gums) . Stable oil-in-water add ition , some recent results from o ur laboratory on the emulsions with high opacity and low creaming are use o f gal actomannans as macromolecular surfaclants for obtained. Gum arabic is known also to form simple possible food applicati ons are presented. coacervates together with other polymeric materials and to encapsulate various oils within the coacervated Hydrocolloids as Emulsifiers particles (Glicksman, 1982). The hydrocoll oids were classified according to their The controlled reaction at pH 8 in the presence of activity at the interface. Gum arabic is probably the salts between oppositely charged gum arabic and gelatin most studied hydrocoll oid that proved significant surface is a good example of this, yielding coacervate for an activity. Emulsions, as an example of biopolymers and application such as soluble coffee (Glicksman, 1982). galactomannans, are hydrocolloids with significant rigid Gum arabic (Figure 5), was found to consist of ga ity and water-solubility. Micro crystalline cellulose la.ctopyraoose backbone chain linked IJ-(1-Q) to various (MCC) is an example of a hydrocolloid with no solubili branches containing arabinofuranose, arabinopyranose, ty in water that adsorbs mechanically at the interface. rbamnopyranose, glucuronic acid and 4-0-methyl glu curonic acid (Street and Anderson, 1983; Churms et al., Gum Arabic 1983). Gum arabic is always "contaminated" with pro It is well documented that gum arabic, a natural teinaceous matter. polysaccharide , has excellent emulsification properties The role of the proteinaceous components in gum for oil-in-water emul sions (Dea and Madden, 1986; arabic was subject to several recent studies. Anderson Bergenstahl et at., 1986; Yokoyama et at., 1988; (1986) reported that the small amount of proteinaceous Dickinson and Stainsby, 1988; Randall et at., 1988). It material is an integral part of the gum structure. Vande is widely used in food , cosmetics and pharmaceutical ap velde and Fenyo (1985) and Connolly et at. (1988) re plications. An excell ent example of its use is in cloudy lated the significance of this fraction to the molecular
413 N. Garti and D. Reichman
characteristics and properties of the gum. They showed D·GlcpA that the gum consisted of two distinct fractions namely 1 a high molecular mass arabino-galacto-protein complex ! (AGP; representing about 30% of the total) and a lower molecular mass fraction. R->3)-D·Galp It was demonstrated that the emulsification capabili I ty of heat treated gum arabic was reduced dramatically ! due to the denaturation effect of the "active" protein (Figures 4 and 6). A similar effect was recorded at a ->3)-D-Galp(I->3)-D-Galp(I->3)-D-Galp(l-> low pH. In a more recent work, Randall eta/. (1989) 6 6 showed that the gum consisted of three distinct fractions: i i a high molecular mass arabino-galacto-protein complex (AGP) glycoprotein (G 1), and lower molecular mass R->3)·0-Ga lp R->3)-D-Galp fraction which was protein deficient (AG). The AGP 6 6 i i fraction was shown to be degraded by proteolytic en I zymes with the molecular mass decreasing to a value similar to that of the AG fraction (Randall eta/., 1989). R->3)-D-Galp R->3)-D-Galp 6 6 It was suggested that the AGP fraction consists of about i i five carbohydrate blocks of molecular mass 2. 8 x lOS linked together by a polypeptide chain, which may con as as It R->4)·D·GlcpA R->4)-D-GlcpA tain many 1600 amino acid residues. is likely that the polypeptide chain is located at the periphery of D-GlcpA = D-glucopyranosiduron..ic ncid the molecule, thus facilitating its adsorption onto hydro D-Galp = D·galactopyranose phobic substances. The AG fraction is not degraded by L-Rhap = L-r hamnopyrnnose proteolytic enzyme and may simply be a fragment of L-Arap = L-arabinopyranose AGP fraction. At relatively low concentrations, gum L-Araf = L=arab in ofuranose arabic yields solutions, which are essentially Newtonian R = 1.- Rhap (1->, 1.-Araf (1->, D-Galp (1->3)- 1. -Araf in behavior and have very low viscosities compared to (1->, or 1-Arap (1->3)-1.-Araf (I-> other polysaccharides of similar molecular mass. This behavior is similar to globular proteins. The intrinsic Figure S. Schematic structure of the polysaccharide viscosities of gum arabic are similar to those of t1-lacto portion of gum arabic. globulin. The intrinsic viscosities of gum arabic solu tions reach their maximum at pH 5-5.5. Randall et al. (1988) and Williams et al. (1990) have recently review ed the structure of the various fractions in gum arabic and concluded that the high molecular mass AGP frac Q; tion is responsible for the gum's emulsifying ability. a; The complex structure of AGP, where hydrophilic car E "' bohydrate blocks are linked to the maio polypeptide 0 chain, enables strong adsorption at orw interfaces. a; Only a small amount of the gum (high molecular weight) 0. 0 does actually adsorb onto the oil droplets (good corre 0 lation in permeation chromatogram of a solution of gum arabic monitored before and after preparing an orange oil emulsion). Consequently, relatively high concentra oL-----~------~------~----~ tions of gum arabic are required to produce stable emul Q sions of relatively small droplet size. At a lower gum Heating Time (hours ) arabic concentration, there is insufficient surface active material to fully coat all the droplets. Randall et al. Figure 6. Emulsification capability of heat pre-treated (1988) and Williams et al. (1990) have also demon gum arabic solutions as monitored by the average strated that the two glycoprotein components of the gum droplet size diameter in orange oil (20% w/w ) (AGP and G I) are denatured on heating, resulting in a emulsions using gum arabic (19% w/w) heated for loss of emulsification capability and a reduction in various times at l00°C. solution viscosity.
414 Hydrocolloids as Food Emulsifiers and Stabilizers
Table 1. Surface pressure ..- and surface viscosity ~ of HYDROPH IUC POLYS ACCHARIDE adsorbed fi lms of gum arabic after 24 hours at the n hexadecane-water interface (0.005 M, 25•q.
..-lmNm-1 1J i mNm-1s weight percent - 10-3 10-3 w-2 10-3 10-3 pH"" 7.0 2.3 7.0 7.0 2.3 WATER
(I) A. criopoda 19 16 220 200 (5 .27% N) cJJJ OIL J (IT) Commercial 4 II 10 HYDROPHOBIC AMINO ACIDS sample (0 36% N) Figure 7. Schematic illustration of the arabino-galacto (lll) A. ampliceps < gum with the highest nitrogen content (5.27% N) result 40 ed in emul sions with the highest surface pressure ( 71') 0 and surface viscosities ('1) at any of the pH levels
415 N. Garti and D. Reichman
140 x- 20% Emul. (j) o- 20% Emul. 1.5% CL - 611 Q_ + u 120 • - 20% Emul. + 1.0% RC - 591 o- 30% Emul. 100 .A - 30% Emul. + 1.5% CL - 611 >- b. - 30% Emul. + 1.0% RC - 591 .-~ 80 (j) 0 60 u (j) 40 > 20 • 0 I 0 0 200 300 400 500 600 700 Shear Rate (s-1)
Figure 9. The effect of MCC on the viscosity of a basic emulsion. CL-611: type !-commerci al MCC; RC-591: type II commercial MCC (McGinley and Tuason, 1990). has excellent emulsifying properties due to the presence Cellulose, in the chemi cal sense, is a polysaccharide of of fatty acids linked to the amino-sugar backbone, giving sufficient chain length to be insoluble in water or dilute it its amphiphilic character. It combines the hydro acids. It consists of anhydroglucose units linked togeth phobicity of a protein with the hydrophilicity of poly er through the I and 4 carbon atoms with a beta-gluco saccharides (Zosim et al. , 1987) (Figure 8) . sidic linkage. Fibrous cellulose is composed of millions An additional class of hydrocolloids includes those of microfibrils. The individual mi crofibril is composed compounds known to have limited surface acti vity even of two regions, the paracrystalline region, which is an though the crude products are contaminated with amorphous and fl exible mass of cellulose chains, and the proteinaceous materi al. In this category, we could crystalline region, which is composed of tight bundles of include materials such as xanthan, tragacanth and cellulose chains in a rigid linear arrangement (McGinley galactomannans. Some of these polysaccharides have and Tuason, 1990). Microcrystalline cellulose is a puri been claimed to have surface acti vity and promo te fi ed, naturally occurring cellulose, produced by convert fo rmation of stable emulsions (Dickinson, !988). ing fibrous cellulose to crystalline cellulose or a redis The main questions to be answered were: (!) What persible gel. is the role of the proteinaceous matter? (2) How surface Microcrystalline cellulose forms a colloidal disper active are these in comparison to gum arabic? (3) D o sion of solid particles in water if properly sheared. The they really adsorb onto the surface of solid or liquid par cellulose crystallites are claimed to set-up a network ticles? and (4) To what extent does the viscosity (or the with the majority of the particles being less than 0.21'm. depletion action) play a role in the stabilizing properties? Only at this sire range can the insoluble cellulose struc In other wo rds , are these material s stabili zers or emulsi tural network provide the colloidal MCC grades with fi ers? their functional properties. The strong affinity of cellu Very little systematic work has been carried out in lose for both the oil and the water results in precipitation attempt to solve these questions. The information avail and some orientation of the solid particles at the oil-in able in literature is limited at best. water interface (McGinley et al. , 1984). Several meth ods have been used to detect and evaluate the structures Colloidal microcrystalline cellulose (MCC) formed on the interface. A mechanical stabililAtion Effective stabililAtion against coalescence of emulsi mechanism was proposed since the solid colloidal parti fied oil gl obules can be obtained by using certain finely cles, precipitate on the interface and prevent collisions divided powders as emulsifiers (McGinley et al., ! 984; by acting as a physical barrier (McGinley et al., 1984). OlJl and Frank, !989; McGinley and Tuason, 1990). In addition, it was proposed that the colloidal network of
416 Hydrocolloids as Food Emulsifiers and Stabilizers
Guar Gum: I galactose unit for 2 mannose residues. 8000 ~------~
0 - 12% 0il Locust Bean Gum: I galactose unit for 4 mannose ~ 6000 .a - 12% MCC I Guar residues. u C li1 0 H
_;; 4 000
Figure 10. Rheological properties MCC-guar and oil. Figure 11. Schematic illustration of the structure of guar gum and locust bean gum (LBG). the free MCC thickens the water phase between the oil globules preventing their close approach and subsequent MCC and guar gum) results in powdered particles that coalescence. Therefore, the MCC provides long term are spherical, shear resistant and water-insoluble (Figure stability (McGinley and Tuason, 1990). 10) (McGinley and Tuason, 1990). The inherent properties of colloidal MCC products The insoluble, spherical MCC/guar gum particulates dispersed in water have been used to simulate fat in can be agglomerated with a water-soluble hydrocolloid duced rheology in various food applications including and subsequently dried to readily recover water-dispersa ice-cream, salad dressings, sauces, and gravies. The ble spheroidal particles capable of forming a stable aque consistency of oil-in-water emulsions can vary from a ous gel. A potential use for this dried, reconstitutible, thin fluid material at low oil levels to a thick viscous spherical form of colloidal microcrystalline cellulose paste at very high oil levels. Studies have shown that product is in the area of non-nutritive fat substitute for there is a change in the rheological properties of the aqueous food systems. The non-nutritive aspects and emul sions from simple Newtonian behavior to non water-insoluble form of colloidal MCC are key to the Newtonian behavior above an oil concentration of 50 development of a successful product for this kind of use. wt % (McGinley and Tuason, 1990). A patent pertaining to a composi tion and production By incorporating colloidal MCC into the system, the method for a dried, spherical form of a colloidal mi cro level of oil in the emulsion can be effectively reduced crystalline cellulose as a non-nutritive and/or low calorie while still retaining their lubrication and flavor carrying fat substitute in aqueous food systems has been issued properties, as well as rheological properties (McGinley (Durand et al. , 1970). and Tuason, 1990). It has been demonstrated that basic emulsions containing 30% soy oil have similar rheologi Galactomannans cal properties and stability characteristic as 20% soy Galactomannans are the most common plant food re bean oil emulsions containing I% colloidal RC-591 or serve polysaccharides belonging to the legume family 1.5% CL-611 (Figure 9). [RC-591 and CL-611 are two consisting of /1-(1-4-)linked 0-mannan backbone, stabi types of commercial MCC as described in McGinley and lized in water, by substitution of: a-(1-6-)linked D-ga Tuason (1990)). lactose stubs for certain of the mannose residues (Figure The properties of the soy bean oil are in fact im IL) (Seaman, 1980). proved using lower levels of oil in conjunction with Great diversity in the chemical structure of the ga MCC. The emulsions acquire a yield value which lac tomannans derived from different plant sources, was makes the system stable against creaming. It is clear detected (Winter et al., 1984). This diversity includes therefore, that microcrystalline cellulose has considera wide variation in the degree of galactose substitution (ga ble potential for use as a fat substitute in the formulation lactose/mannose ratio of I :5) and significant differences of aqueous-based di etary food formulation. in the distribution of galactose substituents along the It was also shown that in the wet state, guar gum mannose chain (Winter et al. , 1984). physically interacts with colloidal crystalline cellulose Galactomannans exist as fluctuating random coil resulting in the fl occulation of the cellulose. Further chains with non-specific viscosity behavior in aqueous drying of the "hydrocolloid alloy" (particles made from solutions (Winter et al., 1984).
417 N. Garti and D. Reichman
Table 2. Interfacial tension of oil/water as measured with and without the addition of LBG (25°C, 720 min.).
weight% '1 (dyne/em)
Tetradecane 0 44 0.7 25
Paraffinic oil 0 36 0.7 22
Toluene 0 36 0.7 17 Castor oil 0 16 0.7 12 Soya oil 0 30 0.7 16.5 Figure U . Schematic illustration of an oil droplet emul sified witb lecithin and polysaccharide (polysaccharide Table 3. Composition of guar gums at different grades serving as viscosity builder). of purification.
The main characteristics of galactomannans are re Commercial Purified Bi-purified lated to their thickening properties, self-gelling prop (native) erties and contribution to gel formation of other hydro colloids. Differences in structure significantly affe<:t H20 8.49 12.7 13.1 those functionalities. Much bas been reported on the Galactomannan 92.2 98.6 99.0 structural aspects of tbe gums, their rheological and in Ash 0.71 0.25 0.22 teractional behavior in aqueous systems, and their contri Protein 5.95 1.1 0.8 bution to the emulsification of oil droplets in the pres ence of monomeric emulsifiers (Figure 12), but very lit Fiber 1.55 0. 19 trace tle bas been done with respect to their interfacial Cellulose 1.02 0.19 trnce characteristics. Fat 0.92 trace trace GaJactomannans, as well as other biopolymers, have Man./Gal. 3.91 4.58 7.70 been evaluated as flocculants in aqueous systems. Fr()m preliminary work (Bergenstabl et al. , 1986; Lips et al., Young and Torres (1989) studied the surface prop 1991) on competitive flocculation efficiency on latex erties of xanthan gum by drop weight and Wilhelmy particles, it was concluded tbat guar (and similarly also plate methods, and found tbat the surface tension proper xantban gum) adsorption is weak and increases witb con ties of aqueous xanthan solutions are affected by its centration without going through a maximum (minimum molecular conformation in the solution. Xanthan in the flocculation) in comparison to gelatin, casein or gum random coil conformation was more surface-active than arabic tbat adsorb very strongly and exhibit a clear xanthan in the helix conformation. Surface tension maximum at low protein concentration (Figure 13). decreased and adsorption rate increased significantly by However, latex surfaces are quite different as com increasing xanthan concentration and by adding 0.1 N pared to oil/water interface in true food emulsions. The NaCl. Surface excess values, calculated by the Gibbs ability of a number of industrial gums to act as steric equation, suggested that surface tension was affected by stabHizers of oil/water emulsions, in the presence of dif the proportion of molecular segments adsorbed at tbe ferent low molecular weight emulsifiers, has been stud interface. ied (Bergenstabl, 1988). Jt was found tbat several gums LBG and guar have been recently studied in our lab show a pronounced effect on stability already at the ppm oratory in view of their surface and interfacial properties level of concentration. To explain these results, (Reichman and Garti, 1991). Emulsification capabilities Bergenstabl (1988) proposes tbat tbe gums adsorb onto and stability of oil-in-water emulsions have been deter tbe surfactant layer, forming a combined structure of a mined and examined in order to answer the question of primary surfactant layer covered by an adsorbed poly tbe adsorption to tbe oil interfaces (Reichman and Garti, mer layer. 1990; Reichman, 1992).
418 Hydrocolloids as Food Emulsifiers and Stabilizers
-J -2 -1
log [po lymer addition mg/m2]
Figure 13. Comparative tlocculation efficiency of food biopolymers (latex diameter 88 nm, particle density 4.5 x 1011 cm·J, sodium chloride concentration 6.7 x 10·2 mol dm-3, 28 °C, pH 5.9). A: dextran 7500; 0: gelatin; 0: caseinate; +:gum arabic; e : xanthan; . , guar (Lips eta/., 1991). increasing the gum concentration. The lowest surface tensions measured for LBG and guar gum (Reichman and Garti, 1990) were 50 and 55 dynes/em respectively (Figure 14) in comparison to 42.3 and 43 .0 dynes/em for xaothao and tragacanth gums. The gums do not ex hibit clear critical micellar concentration (CMC) at 1 60 · LBG :3,s- '0"6 concentrations up to 0. 7 wt %. This surface activity seems to be relatively pronounced in view of the chemi - ~ ~ ~- t--0 cal structure and expected hydrophilicily. ~ 50 ...... ___ Interfacial tensions (fable 2) were significantly Guor "u lower in comparison to other bydrocolloids or well ~ 40 known emulsifying proteins. "' It was demonstrated (Reichman, 1992) that at least most of the proteinaceous components are not integral component of the polysaccharide and can be almost en 3g o~,----.--. ·~~"'· 'o ~.~ ------. ·~ tirely removed by repeated crystallization (fable 3). Log C (gr/dl) The level of protein contaminating the guar was reduced Figure 14. Surface tension of aqueous solutions of from 5.95 wt% in the crude commercial guar to 0.8 commercial (native) guar gum and LBG. A - for LBG wt% in our bipurified fraction. However, it should be and • - for guar gum. noted that by our techniques, we could not further re move the protein from the guar and some of the gum can still be "bound" to the protein. Wilhelmy plate determinations showed that both Several guar (or LBG) fractions containing increas LBG and guar gum reduce surface tension of water at ing levels of protein have been isolated. The fraction low concentrations (up to 0.5 wt %) and that the surface with the lower nitrogen content (bipurified, BP, 0.8 % tension of the gum solutions is time dependent. The protein) had the best surface properties among all frac gums are sluggish and slow in their migration to the tions and the fraction with the highest nitrogen levels interface. Equilibrium surface tensions decrease and (precipitate, PR, 10.6 wt% protein) showed the lowest adsorption rates increase sigoificaolly, as expected, by
419 N. Garti and D. Reichman
~ w 20 ::;: :J --' 0 15 - Guar >
IO
LBG ---~---
20 ~ 0 •• . ~------~------r-----~ 0.0 0.2 0.4 0.6 0 .8 Droplet Size {!Lm) 30 Gum Concentrations (wt%) B Figure 15. Interfacial tensions of tetradecane-water in the presence of LBG and guar gum.
!U ::;: :J --' 0 > §" "" I S 0 > ""
~ 10 \0
Droplet Size {!Lm)
Figure 16. Droplet size distribution of 5 wt% tetra £0 40 decane in water emulsion prepared with 0.5 wt% gu.ar gum: A: after preparation; 0: after one week, • : after Droplet Size {!Lm) one month. Figure 17. Droplet size di stribution of 5 wt% n-tetra decane-in-water emulsions prepared with 0. 7 wt%: (A) surface activity. The native guar (5.95 wt % protein) crude guar gum solution, and (B) beat pretreated gum bad excellent surface behavior almost similar to the solution (8 hours at 90' C) after: 0 : I day; e : 7 days. bipurified fraction. Figure 15 shows the interfacial tensions of LBG and guar gum upon concentration. did not damage the gum's capabilities to stabilize the Relatively large oil droplets (10-QO I'm) have been emulsion. This indicates that either the proteinaceous formed when gum solution was emulsified with 5 wt % portion of the gum has practically no surface activity or vegetable oil or tetradecane in the presence of 0.5 wt% that it was not affected by the drastic pretreatment gum (Figure 16; (Reichman, 1992). conditions. We tend to believe that the residual protein Pretreatment of the gum solution, prior to its use, does not significantly contribute to the polysaccharide by beat (8 hours at 90'C) or change in pH (pH 3, 7, 10) adsorption on the interface (Figure 17).
420 Hydrocolloids as Food Emulsifiers and Stabilizers
responsible for its surface activity. Increasing the gum concentration and lowering the oil content facilitated formation of stable emulsions with low coalescence rates. Full coverage of the oil phase Ql 20 was obtained at hydrocolloid to oil ratios [r = (wt% ~ hydrocolloid I wt% oil) x 100) of 18 and up, leading to 0 15 > stable and non-flocculated emulsions (Figure 19). "a" 10 The adsorption capability of the gum was limited only to certain oils and in many cases strong flocculation and creaming were detected.
0 Surprisingly, a birefringency effect was observed on 0 "0 emulsion droplets prepared from the gum solutions. The Droplet Size (J 421 N. Garti and D. Reichman Figure 20. Polarized light photomicrographs of oil/ water emulsions stabilized by galactomannans: (A) 0.9 wt % LBG, 5 wt% n-tetradecane; (B) 0.9 wt % guar, 5 wt % toluene; (C) 0.9 wt % guar, 5 wt% soya oil. Photo width (for each micrograph) = 50 Jtm. 10 ~-- 1 9- - ~ - - .[ 8 ---- 7 -- --~~----- : E~-===--=~ ~ ]~ --::::::::=-===------; l_//~ : ------=- · ·-=----= 0 • -~ --·- ,--.-. -- . -- . -~~·-.- 0 0.1 oz OJ o.• o.s o6 O"l os o9 1 To tal Gum Con tent (gr/dl) Figure 21. Surface load of guar gum (• ) and LBG (•) in 5 wt % tetradecane in water emulsions prepared with 0.1 to 0.9 gr/dl gum in the aqueous phase. 422 Hydrocolloids as Food Emulsifiers and Stabilizers Conclusions At present, enzymatic treatment of the guar gum is being done in our laboratory in order to achieve an effi It was demonstrated that guar and gum arabic stabi cient "degalactosidation" of the mannose backbone and lize oil-in-water emulsions. However, it seems that the to clarify the role of the backbone and the side chain two types of gums behave in different modes at oil-water groups in the emulsification behavior of the gum. Com interfaces. The gum arabic behaves as a typical surface petitive adsorption with proteins, small molecule surfac active protein and anchors strongly to the oil phase via tants, and other hydrocolloids is also being carried out. its proteinaceous part of the molecule. When the pro Extensive work is being done at present in order to teinaceous part is removed, denatured, or deactivated , better characterize the oil-water interface in the presence the gum tends to lose its surface activity and emulsifica of the hydrocolloid using more advanced methods such tion capability. Only a small part of the gum mixture is as cryo-scaooing electron microscopy, small angle X-ray responsible for its activity and therefore, large excesses scattering (SAXS), and small angle neutron scattering of gum are required to obtain stability. (SANS). It appears that the spheroidal shape of the gum ara Colloidal microcrystalline cellulose stabilizes oil-in bic together with the proteinaceous portion attached to water emulsions by a mode known as "mechanical stabi the polysaccharides facilitate alignment and close-pack lization • of solid powdered coll oidal particles precipi ing of the gum at the oil-water interface. Spatial separa tating on the oil droplets. tion of the relatively hydrophobic portions of the gum molecule, (i.e., methyl groups of rhamnose in the termi Referenre; nal position) from the hydrophilic hydroxyl and ionic re gions appears to have a significant effect on its surface Anderson DMW (1978). Chemotaxonomic aspects activity (Randall eta/. , 1988; Anderson, 1978). Per of the chemistry of Acacia gum exudates. Kew Bull. haps the minor polyphenolic component of the gum is 32, 529-536. also effective in the same sense, though there are indica Anderson DMW (1986). Nitrogen conversion fac tions that other hydrophobic associations may also be tors for the proteinaceous content of gums permitted as responsible (Ettling and Adams, 1968). food additives. Food Additives and Contaminants 3, Gum tragacanth (not discussed in detail in this re 231-234. view) is an effective emulsifier at as low a concentration Banerji SN (1952). Surface-tension measurement of as 0 .25 wt%, with significant hydrophobicity being some colloidal solutions. I. Gum arabic, gum tragacanth, attributed to the terminal deoxyhexosyl groups of and dextrin solutions. J. Indian Chern. Soc. 29, 270- tragacanthic acid, whi le the spheroidal arabino-galactan 274. component acts like the polysaccharide component of BeMiller JN (1988). Some challenges for gums and gum arabic (Stephen, 1990). gum research. In: Gums and Stabilisers for the Food On the other hand, galactomaooans, being very Industry, Vol. 4. Phillips GO, Wedlock DJ, Williams hydrophilic polymers with rigid structures, migrate to PA (eds.). lRL Press, Oxford, UK. 3-14. the oil interface slowly and only after applying high Bergenstahl B (1988). Gums as stabilizers for emul shears. The oil helps to "precipitate" the hydrophobic sifier covered emulsion droplets. In: Gums and Stabilis portions of the polymer that tend to adsorb on the oil. ers for the Food Industry, Vol. 4. Phillips GO, Wedlock It adsorbs very weakly, but forms ordered and oriented OJ, Williams PA (eds.). IRL Press, Oxford. 363-369. thick films (possibly gelled films) that reveal high Bergenstahl B, Fogler S, Stenius P (1986). The in birefringency. The emulsification ability is weak and fluence of gums on the stability of dispersions. In: Gums high gum to oil ratios are required to retard coalescence and Stabilisers for the Food Industry. Vol. 3. Phillips and reduce flocculation of the oil droplets. GO, Wedlock OJ, Williams PA (eds.). IRL Press, The addition of small molecule surfactants, dilution, Oxford. 285-293. along with heat application, etc. will desorb the gum Churms SC, Merrifield EH, Stephen AM (1983). from the interface and will speed coalescence and rup Some new aspects of the molecular structure of Acacia ture of the emulsion (Garti and Reichman, 1994). senegal gum (gum arabic). Carbohydr. Res. 123, 267- Much work is required in order to better illustrate 279. the ability of galactomannans to adsorb the oil droplets Connolly S, Fenyo JC, Vandevelde MC (1988). Ef and to stabilize the oil/water emulsion via a steric stabi fect of a proteinase on the macromolecular distribution li zation mechanism. It will be even more difficult to ex of Acacia senegal gum. Carbohydr. Polym. 8, 23-32. plain the nature of this adsorption. Is the polysaccharide Dea ICM, Madden JK (1986). Acetylated pectic adsorbed in a molecular form or as a solid colloidal polysaccharides of sugar beet. Food Hydrocolloids 1, particle, much like MCC? 71-88. 423 N. Garti and D. Reichman Dickinson E (1988). The role of hydrocolloids in microcrystalline cellulose. In: Gums and Stabilisers for stabilising particulate dispersions and emulsions. In: the Food Industry, Vol. 5. Phillips GO, Wedlock DJ, Gums and Stabilisers for the Food Industry, Vol. 4. Williams PA (eds.), IRL Press, Oxford. 405-414. Phillips GO, Wedlock DJ, Williams PA (eds.). IRL Oza KP, Frank SG (1989). Drug release from emul Press, Oxford. 249-263. sions stabilized by colloidal microcrystalline cellulose. J. Dickinson E, Stainsby G (1988). Emulsion stability. Dispersion Sci . Techno!. 10, 187-210. In: Advances in Food Emulsions and Foams. Dickinson Prud'homme RK, Long RE (1983). Surface tensions E, Stainsby G (eds.). Elsevier. 1-44. of concentrated xanthan and polyacrylamide solutions Diclcinson E, Murray BS, Stainsby G, Anderson with added surfactants. J. Colloid Interface Sci. 93, 274- DMW (1988a). Surface activity and emulsifier behaviour 276. of some Acacia gums. Food Hydrocolloids 2, 477-490. Randall RC, Phillips GO, WilliamsPA (1988). The Dickinson E, Stainsby G, Murray BS (1988b). Pro role of the proteinaceous component on the emulsifying tein ~dsorption at air-water and oil-water interfaces. In: properties of gum arabic. Food Hydrocolloids 2, 131- Advances in Food Emulsions and Foams. Dickinson E, 140. Stainsby G (eds.). Elsevier. 123-162. Randall RC, Phillips GO, Williams PA (1989). Dickinson E, Elverson DJ, Murray BS (1989). On Fractionization and characterization of gum from Acacia the film-forming and emulsion-stabilizing properties of senegal. Food Hydrocolloids 3, 65-75. gum arabic: dilution and flocculation aspects. Food Reichman D, GartiN (1990). Interactions of galac Hydrocolloids 3, 101-114. tomannans with ethoxylated sorbitan esters- surface ten Dickinson E, Galazka VB, Anderson DMW (1990). sion and viscosity effects. In: Gums and Stabilisers for Emulsifying behaviour of Acacia gums. In: Gums and the Food Industry, Vol. 5. Phillips GO, Wedlock DJ, Stabilisers for the Food Industry, Vol. 5. Phillips GO, Williams PA (eds.). IRL Press, Oxford. 441-446. Wedlock DJ, Williams PA (eds.). IRL Press, Oxford. Reichman D, Garti N (1991). Galactomannans as 41-44. emulsifiers. In: Food Polymers, Gels and Colloids. Dickinson E, Galazka VB, Anderson DMW (1991). Dickinson E (ed.). Royal Soc. Chern. Cambridge, UK. Effect of molecular weight on the emulsifying behavior 549-556. of gum arabic. In: Food Polymers, Gels and Colloids. Reichman D (1992). Characterization of hydrocol Dickinson E (ed.). The Royal Soc. Chern. , Cambridge, loids, surface activity and interactions with monomeric UK. 490-493. emulsifiers at the interface. Ph.D. Thesis. The Hebrew Duraod HW, Fleck EG Jr., Raynor GE Jr. (1970). University of Jerusalem, Israel. Dispersing agent based on IJ-1 , 4-glucans. U.S. Patent Seaman JK (1980). Guar gum. In: Handbook of 3,539,365 . Water-Soluble Gums Resins. Davidson RL (ed.). Ettling BV, Adams MF (1968). Gel filtration of McGraw Hill, New York. 6-19. arabioogalactan from western larch. Tappi, 51, 116-118. Silber D, Mizrahi S (1975). Coalescence of orange Garti N, Reichman D (1994). Surface properties and oil droplets at oil-gum solution interface and some rela emulsification activity of galactomannans. Food tions to emulsion stability. J. Food Sci. 40, 1174-1178. Hydrocolloids (in press). Stainsby G (1988). Foaming and emulsification. In: Glicksman M (1969). Terminology and classification Functional Properties of Food Macromolecules. Mitchell of gums. In: Gum Technology in the Food Industry. JR, Ledward DA (eds.). Elsevier. 315-353. Academic Press, New York. 1-14. Stephen AM (1990). Structure and properties of Glicksman M (1982). Functional properties of hy exudate gums. In: Gums and Stabilisers for the Food drocolloids. ln: Food Hydrocolloids, Vol. I. Glicksman Industry, Vol. 5. Phillips GO, Wedlock DJ, Williams M (ed.). CRC Press, Boca Raton, FL. 47-99. PA (eds.). IRL Press, Oxford. 3-16. Lips A, Campbell U, Pelan EG (1991). Aggregation Street CA, Anderson DMW (1983). Refinement of mechanisms in food colloids and the role of biopoly structures previous! y proposed for gum arabic and other mers. In: Food Polymers, Gels and Colloids. Dickinson Acacia gum exudates. Talanta 30, 887-893. E (ed.). Royal Soc. Chern., Cambridge, UK. 1-21. Tadros ThF, Vincent B (1983). Emulsion stability. McGinley EJ, Thomas WR, Champion S, Phillips In: Encyclopedia of Emulsion Technology, Vol. 1, Basic GO, Williams PA (1984). The use of microcrystalline Theory. Becher P (ed.). Marcel Dekker, New York. cellulose in oil in water emulsions. In: Gums and 129-285. Stabilisers for the Food Industry, Vol. 2. Phillips GO, Vandevelde MC, Fen yo JC (1985). Macromolecular Wedlock DJ, Williams PA (eds.). Pergamon Press, distribution of Acacia senegal gum (gum arabic) by size Oxford, UK. 241-249. exclusion chromatography. Carbohydr. Polym. 5, 251- McGinley EJ, Tuason DC Jr. (1990). Application of 273 . 424 Hydrocolloids as Food Emulsifiers and Stabilizers Whistler RL (1973). Factors influencing gum costs They do not correlate the viscosity reduction to the de and applicatims. In: Industrial Gums, 2nd edition. naturation of the AGP or G 1 fractions. It is difficult, in Whistler RL (ed.). Academic Press, New York, 5-18. our opinion, to see why the polysaccharide fraction AG, Williams PA, Phillips GO, Randall RC (1990). that is left in the solution, will have lower viscosities Structure-functton relationships of gum arabic. In: Gums than the protein-polysaccharide complex, but it is an ex and Stabilisers for the Food Industry, Vol. 5. Phillips perimental fact (Williams et a/., 1990). The giant dif GO, Wedlock DJ, Williams PA (eds.). IRL Press, ference in the MW of the two fractions may be the rea Oxford. 25-36. son for this. Figure 7 is a schematic illustration of the Winter WT, Chien YY, Bouckris H (1984). Struc adsorption of AGP fraction of the oil-water interface. tural aspects of food galactomannans. In: Gum and Sta The sketch was drawn by Randall eta/. (1989) to illus bilisers for the Food Industry, Vol. 2. Phillips GO, trate the role of the protein in the arabino-galacto-protein Wedlock DJ, Williams PA (eds.), Pergamon, Oxford. complex. We agree with the comment that probably 535-539. most of the protein adsorbs on the oil/water interface Yalpani .M (1988). Polysaccharides. Synthesis, with its hydrophobic side groups in the oil phase and the Modifications and Structure/Property Relations. hydrophilic groups facing the water, and the polysaccha Elsevier. 8-49. ride is most probably mostly in the water. However, Yokoyama A, Srinivasan KR, Fogler HS (1988). those are pure speculations, without any good evidence. Stabilization mechanism of colloidal suspensions by gum The authors' intention in this schematic illustration was tragacanth: The influence of pH on stability. J. Colloid to demonstrate that the protein, rather than the poly Interface Sci. 126, 141-149. saccharides, adsorb on the interface. Young SL, Torres JA (1989). Xantban: Effect of E. Dickinson: The substantial reduction in emulsifying molecular conformation on surface tension properties. capacity caused by pre-heating of gum arabic is attrib Food Hydrocolloids 3, 365-377. uted to "denaturation" of the glycoproteinic components Zosim Z, Gmnick DL, Rosenberg E (1987). Effect of the hydrocolloid. This is certainly a possibility, al of protein content on the surface activity and viscosity of though I am sure you will be aware that beat-induced emulsan . Colloid Polym. Sci. 265, 442447. unfolding of globular proteins can often lead to im Dismssion with Reviewers proved emulsifying properties so long as there is no pro tein coagulation. For gum arabic, is there any evidence H.A.C.M. Hendrickx: Can the authors explain why of loss of solubility of change in molecular weight on the denaturation d the protein in gum arabic fractions heating? We have found that a sample of gum arabic AGP and G 1 lead; to a reduction in solution viscosity? that bad been partially degraded by irradiation gave Authors: Willians et a/. (1990) studied the structure poorer emulsion stability than the native gum; this was function relationslip of gum arabic and have shown that attributed by us to poorer steric stabilization caused by gum arabic can re separated into three principal frac the lower average molecular weight of the gum as indi tions: an arabino-galactan {AG); an arabino-galacto cated by intrinsic viscometry. In principle, extended protein complex (AGP); and glycoprotein (G1). Gum beating of gum arabic could lead to slow degradation of arabic solutions ~t low concentrations are essentially the hydrocolloid into smaller fragments, although I sus Newtonian in beh:tviour and have very low viscosities pect that aggregation arising as a result of protein de compared to othet polysaccharides of similar molecular naturation and coagulation is probably more likely, mass. No data 01 the differences in the viscosities of especially in the early stages of the heat treatment. AGP and AG (or J l) are available, but we can assume Authors: Pre-heated gum arabic solutions showed low that since AG ap[OarS to be the hydrocolloid (polysac er emulsification capability in comparison to untreated charide) fraction d' the AGP, its viscosity will be much solutions. The authors (Dickinson, 1988; Randall eta/., higher (similar to guar gum or LBG) than the viscosity 1989) attributed it to the denaturation phenomena. We of AGP. Gum anbic has been shown to be heat sensi are aware that beat-induced unfolding of globular pro tive and some prec.pitation will occur on prolonged beat teins can lead to improved emulsifying properties. We ing. The precipillte bas been shown to be essentially do not know at present what was the heat treatment ef the proteinaceous naterial and the gel permeation chro fect on the proteinaceous fraction contaminating the matography indiettes that both AGP and G 1 fractions galactomannans. All we claim is that the heat treatment are removed. As a consequence of the loss of the high did not damage the gum capabilities (unlike the effect it molecular mass PGP fraction, the solution viscosities had on gum arabic). Close reexamination of our results decrease considenbly. Williams et a/. (1990) explain can indicate even high improvement in the droplet size that the reduction in the gum viscosity is due to the distribution at time zero and after one week. However, removal of the ACP and G I fractions from the solution. the pH change did not reflect in its activity. 425 N. Garti and D. Reichman E. Dickinson: You seem to be suggesting that the sur L.K Jackson: What evidence do you have that gum face activity of guar gum and locust bean gum is greater arabic is spherical? at the oil -water interface than at the air-water interface. Authors: We do not have any direct evidence for such If F igure 14 refers to equilibrium (steady-stale) tensions, claim. However, Randall eta/. (1988, 1989) studied its then the speed of migration to the interface is irrelevant. rheological properties and concluded that its viscosity in I am skeptical as to whether really pure xanthan is as water at low concentrations is similar toP-lactoglobulin, surface-active at the air-water interface as you indicate; which is a globular protein. There are more similarities many commercial samples of xanthan contain cell wall of the gum arabic behaviour that resemble the globular fragments, which can be proteinaceous and hence sur proteins. face-active. You also say that tensions for locust bean gum at oil-water interfaces are lower than for wen B. Bergenstahl: Figure 20 shows emulsions with 5% known emulsifying proteins. If you are referring here dispersed phase. However, it appears that the number to milk proteins, I find this difficult to accept. of droplets is very small (when compared to the concen tration) in Figures 20A and 20B. Please comment. Authors: Figure 14 refers to equilibrium surface ten sions of guar gum and LBG. We are aware that "xan Authors: The concentration of the emulsion in Figure than contains cell wall fragments and can be pro 20 is indeed 5% dispersed phase. We believe that the photograph was taken from an area "on the slide" that teinaceous and hence surface-active·; the xanthan gum was diluted for convenience reasons. results are not from our experimental work. We used guar that was twice purified. LBG reduces interfacial B. Bergenstahl: An adsorbed polymer layer is usually tensions quite remarkably. However, we admit that not observed as a birefringent layer around the droplets. some food including milk proteins are more efficient The observation in Figure 20 has to be interpreted as a than LBG in reducing interfacial tensions. precipitated and/or oriented gel phase around the drop E. Dickinson: Why does heat treatment of guar gum lets. 1f we assume that the birefringent layer is an lead to small er emulsion droplets than with native oriented gel phase, a similar birefringent layer should be hydrocolloid (Figure 17)? observed around the air-water interface on the edge of the microscopic slide. Was this observed? Authors: We do not have a good ex planation for these ftndings. We repeated the experiments several times and Authors: We repeated the experiment as per your sug gestions. We did not see birefringency around the air the resul ts were reproducible. Possible explanations are related to: (I) the role of the protein fraction in the gum, water interface on the edge of the microscopic slide. or (2) possible gum reorganization in the solution lead However, we are no t ruling out the interpretation sug gesting that the gum forms a precipitated and/or oriented ing to some orientation in the random coils. gel phase around th e droplets. As a matter of fact, we E. Dickinson: The birefringence of the adsorbed gu.ar bel ieve that this might indeed be the case. We assume gum layer is said to be lost on dilution. Is the emulsion , that the birefringency is seen only when "salting-out" therefore, destabilized on dilution? conditions are dominating the system (i. e., in the pres Authors: Yes, the emulsion loses both the bire ence of oil droplets). fringency and its stability upon dilution. We attribute it ul sions produced in the experi to a gum desorption from the oil interface. The gum is B. Bergenstahl: The em weakly adsorbed /precipitated and dilution causes "salting ments are usually fairly coarse, with average sizes in" effect and redisolution of the gum in the water. around 10 I'm. In emulsions with large droplets, the Brownian flocculation will be very slow and gravity in L.K. Jackson: Would you please give us more infor duced flocculation (Reddy, Melik and Fogler, J. Colloid mation on the structure of Emulsan? Interface Sci. 82, 116-27, 1981) will be the main floc Authors: Emulsan is an extracellular polysaccharide culation mechanism. Would it be possible that the stabi produced by the oil degrading bacterium Acinetobacter li zation of the emulsion is due mainly to the fo rmation calcoaceiicus RAG-1. It is anionic biopolymer with an of a loose gel [Walstra in "Food Emulsions" , Dickinson amphipbilic character arising from the presence of fatty (ed.), 1986, p. 242-59)? What type of effects were acids linked to the aminosugar backbone. observed on the rate of creaming? Authors: The possibility that the stabilization of the L.K. Jackson: What evidence do you have that MCC emulsion may be induced by the formation of loose gel guar is spherical? was considered. Most emulsions are free-flowing and Authors: The MCC-guar "alloy" was claimed to be non-viscous. Therefore, since guar gum does not form spherical by Oza and Frank (1989), McGinley and gels, we excluded this possibility. Tuason (1990), others. We do not have any good evidence for this. 426