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3 Food Emulsifiers
Emulsifier Types
Actual commercial food emulsifiers are seldom exactly like the organic chemical structures that are discussed in this section. Rather, they are mixtures of similar compounds derived from natural raw materials. The hydrophobic fatty acid (or fatty alcohol) chain reflects the nature of the hydrogenated fat or oil used during manufacture. For example, glycerol monostearate (GMS) made from hydrogenated tallow has a saturated fatty acid composition of about 3% C14, 28% C16, 68% C18, and 1% C20, reflecting the chain length distribution in the source fat. If it is made from hydrogenated soybean oil, the chain length distribution is somewhat different (more C18 and less C16). In addition, the ratio of 1-monoglyceride to 2-monoglyceride varies depending on the temperature during manufacture. Many emulsifiers are the result of rather complex condensation and polymerization reactions. Sorbitan monostearate is made by heating sorbitol and stearic acid together. Sorbitol cyclizes (dehydrates) to a mixture of sorbitans and isosorbides, which in turn is esterified to various extents by the stearic acid. By strict control of reaction conditions, the composition of the final product can be kept in a relatively narrow (and consistent) range, but it is still a mixture. One should keep in mind, therefore, that the chemical structures shown here represent the major components in the commercial material and that related molecular species are also present.

In This Chapter:
Emulsifier Types
Monoglycerides Monoglyceride Derivatives Sorbitan Derivatives Polyhydric Emulsifiers Anionic Emulsifiers Lecithin

Hydrophilic/Lipophilic Balance
Basic Principle of the Concept Experimental Determination of HLB

Foaming Agents Emulsifying Agents


Roughly 22 million kg (50 million lb) of monoglyceride are used annually in the United States in yeast-raised goods to retard staling. At least an equal amount finds its way into cakes, icings, and other applications. The third major use is in the manufacture of margarine. Overall, monoglycerides make up the single most important group for food uses, representing about 75% of total emulsifier production. The use of monoglycerides in baking began during the 1930s, when super-glycerinated shortening became commercially available. Glycerin was added to ordinary shortening along with a small amount of alkaline catalyst. The mixture was heated, causing some interesterification of triglyceride with the glycerin, and the catalyst was removed by neutralization and washing with water. The resulting emulsified shortening contained about 3% monoglyceride and was

Sorbitol Sugar alcohol produced by reduction of glucose, consisting of a chain of six carbons with a hydroxyl group on each carbon. Sorbitan Cyclic structure formed by linking the hydroxyls at the 1 and 4 positions of sorbitol through an ether linkage. Isosorbide Bicyclic structure formed from sorbitol, involving an ether linkage between hydroxyls at the 1 and 4 positions and hydroxyls at the 3 and 6 positions.



H-C-O-C-(CH 2 ) 16 CH 3 H-C-OH H-C-OH H 1-Monostearin

Fig. 3-1. Glycerol 1-monostearate, also referred to as monostearin or GMS.

Staling Phenomenon that occurs in baked products during storage. Stale product has a firmer crumb structure than fresh product; the crumb has a dry, harsh texture; and the flavor impact is significantly reduced. -Monoglyceride Monoglyceride in which the fatty acid is esterified to the 1 position of glycerol. Esterification at the 2 position results in a -monoglyceride. Iodine value A measurement of the number of double bonds in a fat or oil. A higher value means more double bonds.

widely used for making cakes, particularly those containing high levels of sugar. The effectiveness of monoglyceride in retarding staling (crumb firming) in bread became known at about the same time, and bread bakers sought a more concentrated source of monoglyceride. This need was met by suppliers of plastic monoglyceride, which is made by increasing the ratio of glycerin to fat to achieve a final concentration of 4060% monoglyceride. Most of the remainder is diglyceride. When industrial-scale molecular distillation processes were developed, it was logical to subject the plastic monoglyceride to this step, producing distilled monoglyceride containing a minimum of 90% (typically, about 95%) monoglyceride (the rest of the mixture is composed of diglyceride and small amounts of fatty acids and glycerol). The next stage was to make a lamellar mesophase product from this distilled monoglyceride, adding some anionic surfactant (usually sodium stearoyl lactylate [SSL]) to stabilize the hydrated monoglyceride, which contains roughly 25% monoglyceride, 3% SSL, and 72% water. More recently, manufacturers have developed a powdered distilled monoglyceride, in which the composition of the original feedstock fat is balanced between saturated and unsaturated fatty acids. The resulting powder is hydrated fairly rapidly during the process of dough mixing and is functional in complexing with gelatinized starch. Today, bakers use all three monoglyceride types (plastic, hydrated, and powdered distilled) with about equally good results. The monoglyceride structure shown in Figure 3-1 is 1-monostearin, also called -monostearin. If the fatty acid is esterified at the middle hydroxyl, the compound is 2-monostearin, or -monostearin. Both isomers are equally effective at retarding bread staling. In technical specifications, manufacturers usually give the monoglyceride content of their product as the percentage of -monoglyceride. The routine analytical method for monoglyceride detects only the 1-isomer; quantitation of the 2-isomer requires the use of gas chromatography. The total monoglyceride content of a product is about 10% higher than the reported -monoglyceride content. In a practical sense, however, when various products are compared for functionality and cost effectiveness, the -monoglyceride content is a useful number, since for most products it equals about 91% of the total monoglyceride present. The fatty acid composition of a monoglyceride reflects the makeup of the triglyceride fat from which it is made. Commercial GMS may contain as little as 65% stearate if it is made from fully hydrogenated lard or as much as 87% stearate if it is made from fully hydrogenated soybean oil. The other major saturated fatty acid is palmitic acid, and because complete hydrogenation (to an iodine value of 0) is not practical, a small percentage of unsaturated (oleic and/or elaidic) acid is also usually present. A typical commercial GMS has an iodine value of about 5. Iodine values for powdered distilled monoglycerides range from 19 to 36 and for plastic monoglycerides, typically from 65 to 75. The unsaturated fatty acids are a mixture of oleic and linoleic acids and their trans isomers. The phase diagram of a highly unsaturated



monoglyceride is quite different from that of a saturated monoglyceride (Fig. 2-4), but the higher melting point of the partially hydrogenated monoglyceride (both saturated and unsaturated) makes its phase behavior much more like that of GMS than that of the sunflower monoglyceride. To the extent that mesophase behavior governs monoglyceride functionality, the plastic monoglycerides are quite adequate.

Dough strengthener Material added to bread dough to increase the ability of the gluten to retain gas during proofing and baking. -Tending emulsifier Emulsifier that forms a solid film at the oil-water interface under proper conditions of temperature (low) and concentration (high).

The derivatives of monoglyceride shown in Figures 3-2 and 3-3 are of two types: 1) dough strengtheners (succinyl monoglyceride [SMG], ethoxylated monoglyceride [EMG], and diacetyl tartrate ester of monoglyceride [DATEM]) and 2) -tending emulsifiers (acetylated monoglyceride [AcMG], lactylated monoglyceride [LacMG], and

Ethoxylated monoglyceride (EMG)

H-C-O-C-(CH 2 ) 16 CH 3 H-C-OH H-C-O(CH 2 CH 2 O) n H H H O

Succinyl monoglyceride (SMG)


Diacetyl tartrate ester of monoglyceride (DATEM)

Fig. 3-2. Emulsifiers used as dough strengtheners. In the DATEM structure, MG = monoglyceride.



propylene glycol monoester [PGME]). In each case, the structure shown is the main, H-C-O-C-(CH 2 ) 16 CH 3 Acetylated but not the only, component monoglyceride of the mixture. The primary hyH-C-OH (AcMG) droxyl (3-hydroxyl) of monoglyceride is chemically more H-C-O-C-CH 3 reactive than the secondary (2 position) hydroxyl. Thus, acetyH O lation gives mainly 3-acetyl H O 1-monoglyceride, but there is H-C-O-C-(CH ) CH also a certain amount of 2 16 3 Lactylated 2-acetyl 1-monoglyceride plus monoglyceride H-C-OH OH 1-acetyl 2-monoglyceride. In (LacMG) addition, any diglyceride present can also be acetylated. H-C-O-C-CH-CH 3 H SMG is made by reacting O succinic anhydride with monoH O H-C-O-C-(CH 2 ) 16 CH 3 glyceride. The reaction is fairly straightforward. The various H-C-OH positional isomers mentioned for acetylated monoglyceride Propylene glycol can also be found in succinyH-C-H monoester (PGME) lated monoglyceride. DATEM is produced in two H steps. First, tartaric acid (a byFig. 3-3. -Tending emulsifiers. These emulsifiers form solid films at the oilproduct of wine making) is water interface under conditions of low temperature and high concentration. reacted with acetic anhydride, acetylating the two hydroxyl groups and converting the tartaric acid to an anhydride. This is then reacted with monoglyceride. The major product is the structure shown in Figure 3-2, although numerous other closely related compounds are present. The four main reaction products are mono-(diacetyl tartaric acid) ester of monoglyceride, di-(diacetyl tartaric acid) Primary hydroxyl Hydroxyl ester of monoglyceride, mono-(diacetyl tartaric acid) ester of diglycgroup on the terminal carbon atom of a compound. That careride, and mono-(diacetyl tartaric acid) monoacetyl ester of monobon atom is connected to only glyceride. The ratio of these four components in a given DATEM one other carbon. preparation depends mainly on the content of diacetyl tartaric anhydride and the type of monoglyceride used in the reaction. The funcSecondary hydroxyl tionality of the emulsifier depends to a large extent on its composiHydroxyl group on a carbon tion, so it is necessary to use a rather stringent raw material atom that is attached to two specification in order to ensure that performance remains consistent other carbons. from lot to lot. Important indicator properties are saponification value Saponification value Weight and acid value. in milligrams of the potassium The structure of EMG is even more random. Monoglyceride is hydroxide required to saponify treated with ethylene oxide gas under pressure in the presence of al1 g of a lipid. It characterizes a kaline catalyst and at elevated temperatures. Ethylene oxide is polylipid by quantifying the propormerized via a series of ether linkages and also forms ether bonds with tion of ester groups relative to the free hydroxyl groups on monoglyceride. The average chain the total molecular weight.



length is about 20 units (n = 20 in Fig. 3-2). Both the number 2 () and number 3 () positions of the monoglyceride may be derivatized, although because of its greater chemical reactivity, the primary hydroxyl (the position) is more likely to be derivatized than the secondary hydroxyl (the position). The exact distribution of polymer chain lengths and distribution between and positions are functions of reaction conditions, e.g., catalyst type and concentration, gas pressure, temperature, agitation, and length of reaction time. Of course, any diglyceride present may also be ethoxylated. The second group of monoglyceride derivatives, the -tending emulsifiers, are used mainly in cake production. These emulsifiers are dissolved in the shortening phase of the cake formulation, and they contribute to the emulsification of the shortening in the water phase and promote incorporation of air into the fat phase. The particular property of these emulsifiers that makes them valuable in liquid shortening cakes is that they form a solid film at the oil-water interface. This stabilizes the emulsion; but more importantly, it prevents the lipid phase from destabilizing the protein-stabilized foam during cake batter mixing (air incorporation). The production of AcMG and PGME is straightforward organic chemistry. Treatment of monoglyceride with acetic anhydride results in the acetylated product, with the various kinds of isomers listed above. PGME can be made either by direct esterification of propylene glycol with fatty acids or by interesterification of fat (triglycerides) with propylene glycol. The direct esterification product typically contains about 5560% monoester, and the remainder is diester. While a product containing >90% monoester is made by molecular distillation, the extra cost of this process is not warranted for most commercial (cake mix production) uses. The interesterified product is more complex, containing not only mono- and diesters of propylene glycol, but also about 1015% monoglyceride and a small amount of diglyceride. As with DATEM, because such a wide range of product compositions is possible from different manufacturing processes, it is advisable to have stringent raw material specifications for this ingredient. Lactic acid esters of monoglyceride are usually made by reacting lactic acid with a distilled monoglyceride. The complication here is that lactic acid contains a hydroxyl group, and the fatty acid moiety may migrate. For example, if lactic acid is heated with 1-monostearin, the main initial product is 3-lactoyl-1-stearoyl glycerol. However, during the reaction, some portion of the stearic acid may migrate to the lactyl hydroxyl, resulting in glyceryl 3-(stearoyl)-lactylate. In addition, lactic acid can polymerize (form lactoyl lactic esters), and lactoyl dimers and trimers may also be present. Thus, the reaction product mixture from heating lactic acid with a monoglyceride is a complex mixture containing as many as 10 identifiable molecular species. Production parameters must be tightly controlled to obtain a product with consistent functionalities.

Acid value Weight in milligrams of potassium hydroxide required to neutralize the titratable groups in 1 g of lipid. It characterizes a lipid by quantifying the proportion of titratable acidic groups.



Hydrophilic/lipophilic balance (HLB) Ratio of an emulsifiers hydrophilic and lipophilic tendencies.

When the sugar alcohol sorbitol is heated with stearic acid in the presence of a catalyst, two reactions occur: sorbitol cyclizes to form the five-membered sorbitan ring and the remaining primary hydroxyl group is esterified by the acid. The resulting sorbitan monostearate (Fig. 3-4) is oil soluble and has a rather low hydrophilic/ lipophilic balance (HLB) value (HLB is discussed later in this chapter). It is an approved additive for food use in many countries. Other important esters are sorbitan monooleate and sorbitan tristearate. A petition has been filed for affirmation as generally recognized as safe (GRAS) status for sorbitan tristearate, but until it is acted upon by the U.S. Food and Drug Administration (FDA), sorbitan tristearate is being sold as a self-affirmed GRAS crystal modifier. Any of the three esters may be reacted with ethylene oxide to obtain polyoxyethylene derivatives (Fig. 3-5). The monostearate derivative is known as Polysorbate 60, the tristearate is Polysorbate 65, and the monooleate is Polysorbate 80. The discussion above of EMG regarding the length and location of the polyoxyethylene chains applies to these compounds. The average number of oxyethylene monomers is 20 (n = 20), and in the case of the monoesters, chains may be located on more than one hydroxyl group of the sorbitan ring (with the triester, of course, only one hydroxyl group is available for derivatization). Sorbitan monostearate is a good emulsifier for use in making icings with superior aeration, gloss, and stability characteristics. It is also used as part of the emulsifier system in whipped toppings and coffee whiteners. The polyoxyethylene derivatives have found more acceptance; the monostearate Polysorbate 60 is the most widely used of the group. At a level of 0.25% (flour basis), the ability of Polysorbate 60 to strengthen dough against mechanical shock is greater than that of


H-C-O-C-(CH 2 ) 16 CH 3 OH-C-H H C C H H H H C C H 3 C(CH 2 ) 16 C-O O O-C-(CH 2 ) 16 CH 3 O O

H-C-O-C-(CH 2 ) 16 CH 3 OH-C-H O C H C H H H H C C HO OH
Sorbitan monostearate

Sorbitan tristearate

Fig. 3-4. Sorbitan esters. Note the mono- and tristearate esters of the sorbitan ring.



SMG and about equal to that of EMG and SSL. Polysorbate 60 has also been used in fluid oil cake shortening systems, generally in combination with GMS and propylene glycol monostearate.

Polyglycerol esters. Polyglycerol esters (Fig. 3-6) have a variety of applications as emulsifiers in the food industry. The polyglycerol portion is synthesized by heating glycerol in the presence of an alkaline catalyst, and ether linkages are formed between the primary hydroxyls of glycerol. In the structure depicted in Figure 3-6, n may take any value; but for food emulsifiers, the most common are n = 3 (triglycerol), n = 6 (hexaglycerol), n = 8 (octaglycerol), and n = 10 (decaglycerol). (In all cases, n is an average value for the molecules present in the commercial preparation.) The polyglycerol backbone is then esterified to varying extents, either by direct reaction with a fatty acid or by interesterification with a triglyceride fat. Again, the number of

H-C-O-C-(CH 2 ) 16 CH 3 H(OCH 2 CH 2 ) n -OCH H C C H H H H C C HO OH O

Polyoxyethylene (20) sorbitan monostearate (Polysorbate 60)

H-C-O-C-(CH 2 ) 16 CH 3 H(OCH 2 CH 2 ) n -OCH O C H C H H H H C C H 3 C(CH 2 ) 16 C-O O

Polyoxyethylene (20) sorbitan tristearate (Polysorbate 65)

O-C-(CH 2 ) 16 CH 3 O

Fig. 3-5. Polysorbate emulsifiers. Reacting the sorbitan ester with ethylene oxide forms the hydrophilic polyoxyethylene side chain.



OH H-C-H OH-C-H H-C-H O H-C-H OH-C-H H-C-H O H-C-H OH-C-H O H-C-O-C-(CH 2 ) 16 CH 3 H


acid groups esterified to a polyglycerol molecule varies around some central value, so an octaglycerol octaoleate really should be understood as an (approximately octa)-glycerol (approximately octa)-oleate ester. By good control of feedstocks and reaction conditions, manufacturers manage to keep the properties of their various products relatively constant from batch to batch. The HLB of these esters depends on the length of the polyglycerol chain (i.e., the number of hydrophilic hydroxyl groups present) and the degree of esterification. For example, decaglycerol monostearate has an HLB of 14.5, while triglycerol tristearate has an HLB of 3.6. Intermediate species have intermediate HLB values, and any desired value may be obtained by appropriate blending, as described below. The wide range of possible compositions and HLB values makes these materials versatile emulsifiers for food applications. Sucrose esters. Sucrose has eight free hydroxyl groups, which are potential sites for esterification to fatty acids. Compounds containing six or more fatty acids per sucrose molecule are marketed as noncaloric fat substitutes under the name Olestra; this material acts like a triglyceride fat and has no surfactant properties. Compounds containing one to three fatty acid esters (Fig. 3-7), on the other hand, do act as emulsifiers and are approved for food use in that capacity. They are manufactured by the following steps: 1. 2. 3. An emulsion is made of fatty acid methyl ester in a concentrated aqueous sucrose solution. The water is removed under vacuum at elevated temperature. Alkaline catalyst is added, and the temperature of the dispersion is raised slowly to 150C under vacuum, distilling off methanol formed during transesterification. The reaction mixture is cooled and purified.

Polyglycerol monostearate

Fig. 3-6. Basic structure of the polyglycerol esters. The free hydroxyls on the polyglycerol can be esterified to fatty acids to varying extents, resulting in emulsifiers with a wide range of hydrophilic/ lipophilic balance values.


H-C-O-C-(CH 2 ) 16 CH 3 H H OH HO H OH OH H O H H HOH 2 C O H O HO H O CH 2 O-C-(CH 2 ) 16 CH 3

Sucrose diester
Fig. 3-7. One possible positional isomer of a sucrose diester.



TABLE 3-1. Ester Distribution in Sucrose Ester Emulsifiers Percentage of Ester Type Designation F-160 F-140 F-110 F-90 F-70 F-50

Monoester 71 61 50 46 42 33

Diester 24 30 36 39 42 49

Triester 5 8 12 13 14 16

Tetraester 0 1 2 2 2 2

HLBa 15 13 11 9.5 8 6

Hydrophilic/lipophilic balance.

The degree of esterification is controlled by the reaction conditions, especially the ratio of sucrose to methyl ester, and the final product is a mixture of esters (Table 3-1). The HLB value of a particular product is lower (more lipophilic) as the degree of esterification increases, as would be expected.

In addition to SMG and DATEM, some other anionic surfactants (Fig. 3-8) have been tried as dough strengtheners. SSL is currently the one most widely used in the United States. Sodium stearyl fumarate did not find acceptance, and sodium lauryl sulfate (SLS) is used mainly as a whipping agent with egg whites. Lactic acid, with both a carboxylic acid and a hydroxyl function on the same molecule, readily forms an ester with itself. In commercial concentrated solutions, almost all the acid is present in this polylactylic form, and to get free lactic acid, it must be diluted with water and refluxed for a period of time. When stearic acid is heated with polylactic acid under the proper reaction conditions and then neutralized with sodium hydroxide, a product with the structure

O CH 3 (CH 2 ) 16 C-O O
Sodium stearoyl lactylate (SSL)

H 3 C-C-C-O - Na + H O O

CH 3 (CH 2 ) 17 -O-C-C=C-C-O - Na + H H

Sodium stearyl fumarate

O CH 3 (CH 2 ) 11 -O-S-O - Na + O
Sodium dodecyl sulfate (SDS)

Fig. 3-8. Three anionic emulsifiers. SSL and the fumarate are dough conditioners, and SDS is used to solubilize proteins.



shown in Figure 3-8, is obtained. The monomer lactylic acid shown represents the predominant product; the dilactylic dimer is also present as well as lactylic trimers and tetramers. As with all compounds based on commercial stearic acid derived from hydrogenated fats, some percentage of the fatty acid is palmitic and small amounts of myristic and arachidic acids are also present. SSL is readily water soluble, but the calcium salt is practically insoluble. In this respect, it mimics a soap; e.g., sodium stearate is water soluble, but calcium stearate is oil soluble. Either form may be used, depending on the details of the intended application, but as a dough strengthener, the sodium salt is more common. As a stabilizer for hydrated monoglyceride, the sodium form is used because ionization in the water layer is necessary. Stearyl fumarate is a half ester of fumaric acid with stearyl alcohol (octadecanol). Although stearyl fumarate might be expected to have dough-strengthening properties similar to those of SSL, this was not found to be so in practice, and the product was not a commercial success. Stearyl fumarate is still approved by the FDA for use in bread. The third structure shown in Figure 3-8 is sodium dodecyl sulfate (SDS), often used by research workers for solubilizing proteins. It is a sulfate ester of the C12 alcohol dodecanol. The relationship between the pure compound (SDS) and the commercial surfactant (SLS) is as follows. Reduction of coconut oil yields a mixture called lauryl alcohol (from lauric acid, the predominant fatty acid in coconut oil). Sulfation and neutralization result in SLS. The alcohol portion of SLS is a mixture of chain lengths; the approximate composition of the common commercial form is 8% C8, 7% C10, 48% C12, 20% C14, 10% C16, and small amounts of longer chains. The most common food use of SLS is as a whipping aid. The compound is added to liquid egg whites at a maximum concentration of 0.0125% or to egg white solids at a level of 0.1%. It promotes the unfolding of egg albumin at the air-water interface and stabilization of the foam.
Egg albumin Soluble protein found in egg white. Crude gum Material removed during the degumming phase of vegetable oil refining. Water is added to the crude oil, and the polar components (such as phospholipids) become hydrated and associated with the aqueous phase, which is then separated by centrifugation. Phospholipid Diglyceride esterified at the 3 position to phosphoric acid, which in turn is often esterified to another group.

The lecithin generally used by food processors is a by-product of the processing of crude soybean oil; it is the gum that is removed during the degumming step of oil refining. The crude gum is treated and purified to give the various commercial lecithin products that are available today. Crude soybean oil contains about 2% lecithin. Crude corn and cottonseed oils contain about 1% lecithin, but because smaller amounts of these oils are processed in the United States (compared with soy oil), the amount of gum obtained is usually too small for economical processing for human food uses. Instead, it is added back to animal feed formulations as a valuable source of energy. Egg yolk contains about 20% phospholipid, which accounts for its excellent emulsifying functionality, for example, in mayonnaise. However, isolated lecithin from egg yolk is too expensive to be used for food manufacture. The crude gum is dehydrated (to remove water used during degumming), and then insoluble fines are removed by filtration. The crude



material is brown to dark brown (depending on the amount of heat applied during processing) and contains some pigments extracted from the original soybean. It is bleached to attain a more acceptable light brown color. Treatment with up to 1.5% hydrogen peroxide results in a product known as single-bleached lecithin, and addition of benzoyl peroxide up to 0.5% yields double-bleached lecithin. Reaction with hydrogen peroxide at even higher levels plus lactic acid hydroxylates unsaturated fatty acid side chains at the double bond (e.g., yielding dihydroxystearic acid from oleic acid). The hydroxylated lecithin that is formed is more dispersible in cold water than the other types and is more effective as an emulsifier for oil/water (O/W) emulsions. Phospholipids are insoluble in acetone, and the phospholipid content of lecithin is specified as acetone insolubles (AI). The standard commercial lecithin has a minimum AI content of 65%. Crude bleached lecithin is quite viscous. The addition of vegetable oil fluidizes lecithin, and commercial fluid lecithin products are standardized to have a viscosity of 7,50010,000 cP at 25C. A fully deoiled lecithin, a granular, free-flowing product with a typical AI content of 9598%, is also produced. The structures of the main surface-active components of lecithin are shown in Figure 3-9. The phosphatidyl group is a phosphate ester

Acetone insolubles Specification of the amount of phospholipids in gums, based on the fact that the other constituents normally present are soluble in acetone.

-CH 2 CH 2 N + H 3 O CH 3 (CH 2 ) 16 C-O-CH 2 O CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 C-O-CH O H 2 C-O-P-OO

Phosphatidyl ethanolamine

-CH 2 CH 2 N + (CH 3 ) 3



Fig. 3-9. Structure of the major components of soy lecithin. Serine is another possible substituent, but phosphatidylserine is found mainly in animal phospholipids, such as that from egg yolk. Removing one fatty acid from the phosphatidyl moiety results in a lysophospholipid.



of diglyceride. The fatty acid composition of the diglyceride is similar to that of the basic oil, so a number of different fatty acids are found, not just the stearic and oleic acids depicted. The three species are found in approximately equal amounts. Phosphatidylethanolamine (PE) and phosphatidylcholine (PC) are amphoteric surfactants, while phosphatidylinositol (PI) is anionic. Other surface-active materials are also found at somewhat lower concentrations. These include phosphatidic acid (the phosphatidyl moiety plus a hydrogen atom), lysophosphatides (the above species but with one fatty acid removed), and glycolipids (a sugar residue, either galactose or digalactose, attached to the free hydroxyl of a diglyceride). The HLB values of the three types vary: PC has a high HLB, PE an intermediate value, and PI a low value. The HLB of the natural blend is 910, and emulsifier mixtures with values in this range tend to form either O/W or water/oil (W/O) emulsions, although neither type is highly stable. However, emulsifiers with intermediate HLB values are excellent wetting agents, and this is a major application for lecithin. The emulsifying properties of lecithin can be improved by ethanol fractionation. PC is soluble in ethanol, PI is rather insoluble, and PE is partially soluble. Adding deoiled lecithin to ethanol results in a soluble and an insoluble fraction. The phosphatide composition of the ethanol-soluble fraction is 60% PC, 30% PE, and 2% PI plus glycolipids, and that of the ethanol-insoluble fraction is 4% PC, 29% PE, and 55% PI plus glycolipids. The remaining small percentage of both fractions includes oil, free fatty acids, and lysophosphatides. The soluble fraction is effective in promoting and stabilizing O/W emulsions, while the insoluble portion promotes and stabilizes W/O emulsions. Chromatography of the ethanol-soluble fraction can be performed to obtain a material containing 90% PC that is used to make an egg yolk replacer. The emulsifying ability of egg yolk results in part from its high HLB (70% PC and 15% PE in the total yolk phospholipid) plus the protein. The fractionation process is currently used by several European companies to produce industrial food-grade emulsifiers with special functionalities.

Hydrophilic/Lipophilic Balance
As discussed in Chapter 1, emulsifiers consist of a hydrophilic portion (consisting of a wide variety of structures) and a lipophilic portion (usually a fatty acid or occasionally a fatty alcohol). The balance between these two governs the functionality of the emulsifier at interfaces and hence its utility in foods. This balance, called the hydrophilic/lipophilic balance (HLB), attaches a number to emulsifiers that guides the food technologist in choosing one for a particular application. The initial proposal was to calculate HLB as

Glycolipid Diglyceride connected to a sugar moiety (usually galactose or galactosylgalactose) at the 3 position and common in cereal and legume seeds.



HLB = L/T 20 in which L is the molecular weight of the hydrophilic part of the molecule and T is the total molecular weight. Thus, a pure hydrocarbon would have an HLB of 0, while a pure hydrophile (e.g., sugar) would have an HLB of 20. Another method of calculating HLB is to add up contributions by various functional groups. The functional groups and their associated group numbers are listed in Table 3-2. The group values for the hydrophilic and lipophilic functions in the emulsifier are summed and inserted into the equation: HLB = (hydrophilic values) (lipophilic values) + 7 The agreement between calculated and experimentally determined HLB values is generally within a few tenths. Considering that neither method is highly precise, one cannot expect much better, and the calculated value gives a good indication of HLB for a new emulsifier under investigation. HLB values provide a guide to the functionality of the emulsifier system. The following guidelines are based on experience: HLB of 36: a good water-in-oil emulsifier; HLB of 79: a good wetting agent; and HLB of 1018: a good oil-in-water emulsifier.

TABLE 3-2. Hydrophilic/Lipophilic Balance Functional Group Numbers Group Hydrophilic -SO4Na -COOK -COONa Sulfonate -N(CH3)3 Ester (sorbitan ring) Ester (others) -COOH -OH (sorbitan ring) -OH (other) -(CH2-CH2-O)Lipophilic -CH-CH2-CH3 =CHGroup Number 38.7 21.1 19.1 ~11 9.4 6.8 2.4 1.9 0.5 1.9 0.33



While theoretical calculations are fine, emulsifiers are used in food systems that, for the most part, are anything but theoretically simple. For practical purposes, determination of optimum HLB for a particular system is best done experimentally. This approach starts with the concept that the HLB of a blend of emulsifiers is the algebraic sum of their contributions. For example, a blend of sorbitan monostearate (HLB = 4.7) and Polysorbate 60 (HLB = 14.9) is used to determine the optimum HLB for a salad dressing formulation. A 50/50 mixture of the two has an HLB of 9.8 (0.5 4.7 + 0.5 14.9), a 25/75 blend has an HLB of 12.35 (0.25 4.7 + 0.75 14.9), etc. While this is a straightline relationship, it should be noted that in practice the prediction does not hold precisely true. The first step is to prepare a series of emulsifier blends at intervals of 0.5 HLB units. The range should be selected on the basis of the desired product characteristics (W/O or O/W emulsion) and according to the guidelines above. An excess of emulsifier (approximately 10% of the weight of the oil phase) is used to ensure emulsion formation. The emulsifier is dissolved in the oil, the aqueous components are added,



Response surface methodology Method of experimental design to determine the optimum values for two or more variables in a product by using a limited number of experiments and making interpolations based on the experimental data.

and emulsions are made for each trial by using a standardized agitation technique. The emulsions are allowed to stand and are then assessed for stability, e.g., by measuring the thickness of the oil (or water) layer formed at various times. (If all the emulsions are too stable to break down in a reasonable length of time, the experiment should be repeated with less emulsifier.) The result is an approximate optimum HLB range for the system. The next step is to choose the best chemical type of emulsifiers to use. Experience shows that a blend of two emulsifiers (one lipophilic and one hydrophilic) generally produces the most stable emulsions. Numerous such pairs are available, e.g., sorbitan monostearate plus a polysorbate, monoglyceride plus a fatty acid salt (sodium or potassium), triglycerol tristearate plus decaglycerol monostearate, and sucrose esters (e.g., F-160 plus F-50). In addition, the nature of the fatty acid chain (e.g., chain length and degree of unsaturation) can sometimes make a difference that is readily seen by comparing some of the entries in Table 3-3. The melting point can be a factor in some processing systems; solid monostearin may be difficult to add, whereas liquid monoolein (with the same HLB) may be more convenient to use. If the system is acidic (e.g., a salad dressing), acid-stable emulsiTABLE 3-3. HLBa Values of Food Emulsifiers fiers such as the sorbitans or polyglycerols may be the best choices. If the pH is in the neutral range, the Emulsifier Experimental HLB monoglyceride plus fatty acid salt pair may be the Sodium lauryl sulfate 40 most effective. Sodium stearoyl lactylate 22 The various combinations are then subjected to Potassium oleate 20 emulsifying trials as described above, but blends that Sucrose monoester 20 are in the middle of the apparent optimum HLB range are used. These trials should include relatively small Sodium oleate 18 HLB variations (i.e., optimum and 0.5 HLB units) and Polysorbate 60 15 limited variation in concentrations. The goal is to Polysorbate 80 15 choose the emulsifier system that provides the best Decaglycerol monooleate 14 stability at the lowest usage level. Decaglycerol monostearate 13 After the type of emulsifier is selected, the system can Ethoxylated monoglyceride 13 be fine-tuned by running a series of trials at intervals of Decaglycerol dioleate 12 0.1 HLB units and at various concentrations to find the Polysorbate 65 11 minimum amount that yields the desired emulsion staHexaglycerol dioleate 9 bility. At this stage of testing, using response surface Decaglycerol hexaoleate 7 methodology maximizes information gained while miniTriglycerol monostearate 7 mizing the amount of work needed. Glycerol monolaurate 7 Two final points must be emphasized. First, HLB is an empirical system for characterizing emulsifiers, and Sorbitan monostearate 5.9 the values assigned to emulsifiers (as in Table 3-3) are Sucrose triester 5 necessarily somewhat imprecise. Thus, if a particular Propylene glycol monolaurate 4.5 emulsifier combination provides the best results at an Propylene glycol monostearate 3.4 HLB of 11.4, a different combination might function Glycerol monostearate 3.8 best at an HLB of 11.7. The effects of any substitutions Sorbitan tristearate 2.1 must be evaluated, although the use of the HLB system a Hydrophilic/lipophilic balance. greatly decreases the amount of work needed. Second,



emulsifiers interact with other food ingredients, and if formula changes are made (additions or subtractions), the assessment of optimum HLB must be repeated. This is especially true if the changes involve proteins or gums.

Proteins are surface-active molecules. They unfold (i.e., denature, or lose their native three-dimensional structure) at water-air or wateroil interfaces, adsorbing at the interface and stabilizing the foam or emulsion. Their contribution to foam stability is briefly discussed in Chapter 1, and the properties and forces contributing to unfolding and adsorption are discussed here.

The best-known example of proteins as foam stabilizers is the whipping of egg white to make meringue. Ovalbumin (the main protein in egg white) readily denatures and spreads at the air-water interface. When whipping is continued to the dry-peak stage, all the water is immobilized in the interstitial spaces and the meringue does not drain, even after an extended period of time. Air bubble size has been reduced (and numbers increased), generating enough interfacial area to accommodate all the protein in the unfolded state. Many other soluble proteins can also be whipped, but the resulting foam is less stable. Drainage occurs, and after a period of time, the foam begins to break down. The difference may be attributed to reversibility of the unfolding process. For ovalbumin, the process is essentially irreversible, and it stabilizes the interface, even though system energy is high. Soluble soy protein unfolds, but with time (and under the energetic influence to lower free energy by diminishing interfacial area), the adsorbed protein is slowly forced back into the aqueous phase, causing the foam to collapse. Acetylation of soy protein (converting basically charged lysine amino groups into uncharged acetamido groups) provides a soluble protein in which the resolubilizing tendency is greatly reduced, and stable, dry-peak foams can be made from such a protein. In cake batters, the soluble flour proteins (and, of course, any added egg whites) act as foaming agents. In cakes made with oil, proteins are the main agents of air incorporation. The destabilization of foams in these cakes must be prevented by using an -tending emulsifier that encapsulates the oil (see Chapter 4). In cakes made with fractionated and reconstituted flours, omission of the soluble protein fraction results in failures (1).

Proteins at interfaces. A protein is a polypeptide (i.e., a chain of amino acids joined by amide linkages) in which some of the amino acid side

Polypeptide A polymer consisting of amino acids connected by an amide bond, involving the carboxylic acid and amino groups.



chains are hydrophilic and some are hydrophobic. The chain is coiled upon itself in such a way that most of the hydrophobic side chains are in the interior of the protein (and not in contact with water) and the hydrophilic chains are on the exterior surface where they can interact with water (Fig. 3-10, left). A protein has been characterized as an oil drop surrounded by a hydrated Fig. 3-10. Left, protein dissolved in water, folded into its native configuration. Right, shell. This conformation protein unfolded at an interface. The hydrophobic parts of the chain (light gray secis not rigid, and under the tions) penetrate the oil side of the interface while the hydrophilic parts (black secright conditions it can untions) remain in the water. fold. If an interface is present, the hydrophobic parts of the chain will penetrate the air (or oil) side of the interface while the hydrophilic parts remain in the water (Fig. 3-10, right). The protein acts as an amphiphile, and the surface (or interfacial) tension is lowered. Both soluble and insoluble proteins can emulsify oil. Myosin, a low molecular weight protein related to hemoglobin, emulsifies fat during the grinding of meat to make sausage. Other proteins such as isolated soy protein are also often added to the mix, emulsifying fat (and increasing its contribution to sausage net weight) as well as holding water during cooking (thereby increasing yield). The lipoproteins in egg yolk are the main emulsifying agents in mayonnaise (see Chapter 6). Lipoproteins from other sources (e.g., peanut flour) can also emulsify oil, and a type of mayonnaise has been made with such preparations. The emulsification capabilities of egg yolk lipoproteins are enhanced by the lower pH that results from the vinegar and/or lemon juice used in such dressings. A major difference between proteins and ordinary emulsifiers (e.g., a sodium soap) relates to size. The molecular weight of milk proteins is 20,000300,000, while that of sodium oleate is 307. Thus, soap diffuses to the interfacial region much more quickly than a protein. When the protein reaches the interface, the unfolding of the chains and partitioning of the segments between the two phases also take time. If oil is added to a dilute soap solution and to a dilute protein solution, the former system reaches its final interfacial tension value within a matter of seconds, while the latter interface may take up to an hour to reach final equilibrium. Temperature, pH, and different Complexation Combination types and concentrations of salts also greatly affect the time required of two different molecular for the protein to complete its adsorption at the interface. species.



Protein-emulsifier interactions. Protein and emulsifier molecules can interact in two ways: complexation, which is usually observed when both types of molecules are in solution, and competition, which represents the interaction at interfaces. Since a protein is partially hydrophobic in nature, it is not surprising that it can interact with the hydrophobic portion of an emulsifier. The binding of amphiphiles such as SDS or fatty acid to blood serum albumin has been studied in great detail, in part because the protein is readily available in a pure state, allowing easier interpretation of experimental data. A more practical example is the complexation of a dough strengthener such as SSL to gluten (see Chapter 4). At low levels of interaction, up to about 10 moles of emulsifier per mole of protein, there is no alteration in protein configuration, but as the binding ratio increases, the protein begins to lose its native configuration. As the amount of complexed emulsifier increases, the amount of the hydrophobic component of the emulsifier becomes significant compared with the amount of hydrophobic region in the protein, and the influence of amino acid side chains in maintaining the native protein configuration (through hydrophobic bonding) is lessened. The influence of the ionic charge of the emulsifier hydrophilic group is much more disruptive. As the binding ratio approaches 100, the negative charges contributed by the emulsifier (assuming it is SDS or a soap) overcome the charged amino acid side chains, and the protein-emulsifier complex takes on a fairly high charge density. The ionic interactions (e.g., carboxylate-amino group pairs) that contribute to the stability of the native protein configuration are disrupted, and the protein unfolds. The unfolded complex has been called the necklace on a string model (Fig. 3-11). Micelles of the emulsifier incorporate hydrophobic regions of the protein.

Box 3-1. SDS-PAGE

The ability of 1% SDS in buffer to convert most proteins to rod-shaped molecules with lengths proportional to their molecular weights has been used to estimate protein size by polyacrylamide gel electrophoresis (PAGE). The gel is a spongy structure containing buffer. A sample of SDS-denatured protein is placed at one end of the gel slab, and an electrical potential is applied. The complex migrates toward the anode (positively charged pole). The rate at which the complex makes its way through the gel is proportional to its length small rods travel faster than large ones. After an appropriate time, the gel slab is removed from the electrophoretic apparatus, and a protein stain is applied so that the extent of migration can be measured. This is compared with the migration rates of a standard mixture of proteins of known molecular weight (run in the gel at the same time), and the molecular weight of the unknown protein can be estimated.



Since these micelles are all negatively charged, they are mutually repulsive, and the necklace is rod shaped. The binding described occurs only above the critical micelle concentration (CMC) of the emulsifier (see Chapter 2). The binding mode is termed cooperative; i.e., the initial binding (and unfolding of a small region of the protein) enhances further binding and unfolding. Nonionic emulsifiers complex with proteins, although the effect is small, since in general the emulsifiers are relatively insoluble in water. The interaction of a moderately soluble emulsifier, octyl glucoside, with many proteins has been studied. Cooperative binding of more than 100 molecules of the emulsifier per molecule of protein was observed. However, there was no measurable disruption of protein configuration, which highlights the importance of the charge disruption to the unfolding described above. As mentioned above, proteins can unfold and adsorb at interfaces (either air-water or oil-water). If another emulsifier is also present in the system, it too adsorbs at the interface. Several reactions are possible. Competitive adsorption: The two surface-active molecules compete for the available interfacial area. Displacement: The more surface-active compound displaces the less active material from the interface. Enhancement: A complex of the small molecule with the protein increases the interfacial action of the protein. Reinforcement: Interaction at the interface results in more efficient packing of the elements involved (the hydrophobic regions of the protein and the hydrophilic ends of the emulsifier), increasing the total interfacial concentration and decreasing interfacial tension even further. Numerous permutations and combinations of these possibilities have been observed, mainly at air-water interfaces but also occasionally at oil-water interfaces. The proteins studied have ranged from purified (e.g., ovalbumin and lactoglobulin) to complex (e.g., gluten and meat extracts), and all types of emulsifiers have

Fig. 3-11. The necklace on a string model for interaction of a surfactant with an unfolded protein molecule. The surfactant (black circles plus hydrophobic chains) forms micelles around the hydrophobic segments of the protein (light gray sections), stretching out the hydrophilic (black) parts of the protein chain.



been examined, although most reports involve either nonionic or anionic food emulsifiers. A typical example of each case follows. At low concentration, proteins tend to be more surface active than emulsifiers. The addition of a mixture of protein (e.g., -casein) plus a surfactant such as SDS (at a concentration below its CMC) results in a surface tension that is about the same as that of protein alone. Raising the surfactant concentration to near its CMC allows it to become the dominant surface-active molecule, and the surface tension then becomes the same as that of SDS alone. In a drained protein foam, the unfolded protein stabilizes the film between air bubbles and prevents foam breakdown. If a small amount of a nonionic emulsifier (e.g., monocaproin) is present in the liquid, the foam is rather unstable and tends to collapse quickly after it drains. The small molecules lower surface tension (so the foam can be formed) but are not able to prevent the formation of holes in the film that lead to collapse. If the water-soluble emulsifier binds to the protein, it may actually enhance the unfolding and surface-adsorption phenomenon. This is the case with SLS and egg white protein. The surfactant, present at low level, presumably destabilizes the protein conformation, leading to quicker adsorption at the air-water interface and better whippability of the egg white. Surface tension decreases as SDS concentration increases up to the CMC of SDS and then remains constant (Fig. 1-3). In the presence of ovalbumin, for instance, the surface tension is less than that of SDS alone up to the CMC, but at concentrations above the CMC, surface tension continues to decrease. This must result from additional adsorption of the protein-SDS complex at the surface. Most studies of surfactants include measurements of interfacial tension (see Chapter 1). However, adsorbed proteins usually show a large increase in interfacial viscosity, presumably because they are macromolecules. Often, this is the property that makes the protein functional in the food system of interest. A viscous film at the oilwater interfaces in an emulsion, for instance, greatly increases the resistance to coalescence of oil droplets. Certain surfactants may displace (or disrupt) the protein film, lowering interfacial viscosity while at the same time producing the same or lower interfacial tension. An emulsion made with protein alone is more stable (i.e., less coalescence is observed) than an emulsion made with the protein plus a small amount of the displacing surfactant. These considerations play a role in many types of food systems: bakery batters, meat (sausage) emulsions, and egg yolk-stabilized dressings and sauces. The influence of protein at interfaces, however, is most pervasive in dairy products (see Chapter 5).
Interfacial viscosity Resistance to flow in the two dimensions of the interface.



Emulsifiers are additives to traditional foods, and as such their use is regulated by most governments. In the United States, they are approved by the FDA in one of three categories (approved additives, GRAS materials, or affirmed as GRAS compounds) and listed in the Code of Food Regulations, Title 21. In Canada, emulsifiers that may be used are listed in Table IV, Division 16, of the Canadian Food and Drug Regulations. In Europe, the European Union issues European Parliament and Council Directives that list food additives authorized for use in human foodstuffs. Most of the emulsifiers commonly used in foods are listed in Table 3-4. This table should be used only as a guide to the regulations. Many emulsifiers are approved only for certain kinds of food products, and often the allowable amount is limited. Before a large-scale product-development project is begun, the actual regulations concerning the emulsifiers to be used should be consulted to make sure that the final product meets all regulatory guidelines.

1. Howard, N. 1972. Bakers Dig. 46(5):28-30, 32, 34, 36-37, 64.

Supplemental Reading
Friberg, S. E., and Larsson, K., Eds. 1997. Food Emulsions, 3rd ed. Marcel Dekker, New York.



TABLE 3-4. Regulatory Status of Emulsifiers Emulsifier Mono- and diglycerides (GRAS)d Succinyl monoglyceride Acetylated monoglyceride Lactylated monoglyceride Monoglyceride citrate Monoglyceride phosphate (GRAS) Diacetyl tartrate ester of monoglyceride (GRAS) Stearyl monoglyceride citrate Polyoxyethylene monoglyceride Polyoxyethylene (8) stearate Propylene glycol monoester Lactylated propylene glycol monoester Sodium, potassium salts of fatty acids Sorbitan monostearate Sorbitan tristearate Polysorbate 60 Polysorbate 65 Polysorbate 80 Calcium stearoyl lactylate Sodium stearoyl lactylate Stearoyl lactylic acid Stearyl tartrate Stearoyl propylene glycol hydrogen succinate (succistearin) Sodium stearyl fumarate Sodium lauryl sulfate Dioctyl sodium sulfosuccinate Polyglycerol fatty acid esters Sucrose fatty acid esters Sucrose glycerides Lecithin (GRAS) Hydroxylated lecithin Oxystearin Triethyl citrate (GRAS)
a b c d e

United Statesa 182.4505 172.830 172.828 172.852 172.832 182.4521 182.4101 172.755 172.834

Canadab M.4, M.5 e A.2 L.1

European Unionc E 471

E 472a E 472b E 472c

A.94, C.7 A.3 S.19

E 472e E 472f

172.854 172.850 172.863 172.842

P.5 P.14

E 477

172.836 172.838 172.840 172.844 172.846 172.848

S.18 S.18B P.3 P.4 P.2

S.15A L.1A

E 470 E 491 E 492 E 435 E 436 E 433 E 482 E 481

E 483

172.765 172.826 172.822 172.810 172.854 172.859

P.1A S.20

184.1400 172.814 172.818 182.1911

L.2 H.1

E 475 E 473 E 474 E 322 E 322

Code of Food Regulations, Title 21. Canadian Food and Drug Regulations, Table IV, Division 16. European Parliament and Council Directive 95/2/EC, 20 February 1995. Generally recognized as safe. Not listed by this jurisdiction.