The generation of surface interactions is closely con-nected to the presence of surface-active material which
is naturally present in food and feed systems, as well as surface-active technical additives. Various national authorities (for instance, the USA Food and Drug Administration) regulate the use of additives in food systems. For feed systems, there are similar restrictions regulating their use. The chemical origin could belong to three classes of compounds, i.e. lipids, proteins and polysaccharides.
4.1 Lipids
The functional properties of polar lipids are determined by their solution properties. The amphiphilic character of such properties can be described as follows (Table 2.8):
1. By the solubilities, ranging from oil-soluble to water-soluble. This was first expressed by the Bancroft rule, which states that water-soluble emulsifiers favour emulsions with an aqueous continuity, while oil-soluble emulsifiers favour emulsions with an oil continuity.
2. By the hydrophilic-lipophilic balance (HLB) sys-tem, according to Griffin (15). The HLB ratio is expressed as a number based on the emulsifying properties of the emulsifier. The HLB can also be estimated from the chemical structure according to the molecular group contributions, as repeated by Davies (16). HLB numbers are closely related to the functional properties of the emulsifiers.
3. The phase-inversion temperature, according to Shin-oda and Saito (17). The effective HLB value is strongly temperature-dependent (the emulsifier becomes less hydrophilic with increasing tempera-ture) when ethoxylated surfactants are used. In an emulsion system, this can be followed by the phase-inversion temperature, which corresponds to the tem-perature at which the effective HLB is about 6. In food and feed applications, this is fairly rarely used as purely ethoxylated surfactants are seldom used in such systems.
Table 2.8. Amphiphilic properties of polar lipids Character
Hydrophilic Balanced Lipophilic
Solubility
Water-soluble Dispersible in water
and oil Oil-soluble
Self-assembling aggregates Micelles Bilayer structures Reversed aggregates
HLBa
Above 12 8-6 Below 6
Use
Detergents, wetting Water-continuous
emulsions, foam Oil-continuous
systems
a Hydrophilic -lipophilic balance.
4. By the formation of self-assembling structures in the presence of surplus water. The type of structures formed range from micellar structures (hydrophilic emulsifiers) to reversed structures (reversed micelles, L2 phases or reversed hexagonal phases) for the more lipophilic emulsifiers. Balanced emulsifiers (HLB around 7) form lamellar liquid crystals. A dispersed lamellar phase appears as liposomal dispersions.
Krog has reviewed the liquid crystalline phases formed by common food emulsifiers (18). Friberg and Wilton, plus other workers, have suggested that the presence of lamellar liquid crystalline phases is a strong indication of a good emulsifier (in simple systems) (19). There is also a strong relation between the presence of swelling self-assembling structures and the ability to generate repulsive interactions (5) on one hand and between non-micellar phases and dense adsorbed layers on the other. The critical packing parameter (CPP) is a generalization of the self-assembling properties of surfactants, describing the properties as a geometrical balance between the area needed for the polar group relative to the area needed for the hydrophobic group (20).
The melting properties are of crucial importance to the technical functionality of emulsifiers, in addition to their amphiphilic properties. Most food and feed emulsi-fiers are based on natural fat sources, thus giving differ-ent melting properties. The consequences of the melting properties can be expressed as the Krafft temperature (i.e. the temperature at which the solubility is above the critical micelle concentration) or as the transition temperature (chain melting temperature, i.e. the melt-ing temperature of the fatty acids in a semicrystaline bilayer). The transition temperature in an emulsifier water system forming a lamellar liquid crystalline phase
represents the transformation between a swelling lamel-lar phase (above the transition temperature) and an a-gel phase (below the transition temperature). The fully crys-talline form of the emulsifier is the /?-form. The melting temperature of the P -phase is higher than the transi-tion temperature. In order to form a liquid crystalline phase or a hydrated gel phase (a -phase) from a solid (/3-crystalline) emulsifier, the latter needs to be mixed above the melting temperature of the p-phase. Technical functionality (such as foaming and emulsifying action) is only obtained when the emulsifier is present in the liquid crystalline form or in the a-gel form. The high-melting monoglycerides used in the baking industry are usually distributed as a stable a-gel in order to overcome the high melting point of the pure emulsifier. The stability of the a-gel is achieved by mixing complex mixtures of different fatty acids, sodium soap and different modified monoglycerides into the gels.
The activation temperatures of the emulsifiers are strongly dependent on the fat base (Table 2.9). High-melting fat bases (fully hardened Ci8-dominated fats) create high-melting emulsifiers with Krafft or transition temperatures in the range 40-60°C. Precipitating emul-sifiers may contribute to fat crystallization, while solid emulsifiers may have a textural functionality, but for most applications such high-melting points are unsuit-able. Intermediate-melting fat bases (Ci4-Ci8 fats with some unsaturation) give emulsifiers with Krafft or tran-sition temperatures between 30 and 500C. These emulsi-fiers could be used to create stable a -gels and do usually display well performing properties in baking applica-tions. Low-melting fats (highly unsaturated fats) give Krafft points below 0°C. Typical of such systems are soybean phospholipids which have the native fatty acid composition. These are present in the liquid crystalline state under all conditions.
Table 2.9. Technical fat bases and emulsifier properties Fat
Lard Tallow
Hardened fish oil Coconut butter Sunflower Soybean Rapeseed oil Hardened rapeseed oil
Typical fatty acids (% composition) 18:1 (35), 16:0 (25), 18:0 (20) 18:1 (50), 16:0(25), 18:0(15) 16:0 (30), 18:0 (30), 20:0 (20),
22:0 (12)
12:0 (44), 14:0 (18), 16:0 (8) 18:2(65), 18:1 (25), 16:0(6) 18:2 (50), 18:1 (30), 16:0 (10),
18:3 (6)
18:1 (50), 18:2(20), 18:3 (10) 18:0 (82), 16:0 (4), 20:0 (3),
22:0 (2)
MPa (0C) 30-35 30-35 50-55 25 - 1 5 - 1 5 - 1 5 50-55
a Melting point of the corresponding triglyceride.
^A measure of the degree of unsaturation; 86 is equal to a monounsaturated fat (triolein).
Iodine value^
50 50 0 10 125 130 100 0
Ricinic (castor) oil is a special oil which is of significant use in the emulsifier industry. This consists of 85% of esters of cw-12-hydroxy-9-octadecenic acid.
The free hydroxy group is used as a starting point for the formation of chemical derivatives such as esters or ethers, for instance, in polyethoxylated or polyglycerol surfactants.
There is a range of different polar groups available for food and feed applications, thus giving a range of different properties. Some examples are given in Table 2.10, while more details are available in the text by Hasenhuettl and Hartel (21).
4.2 Proteins
The consequences of the presence of proteins are nearly always important when we want to understand the colloidal functionalities of food and feed systems as such species are almost always present and are always surface-active in nature. There are a few key para-meters, or rather descriptors, that are helpful when we need to understand protein functionalities in technical systems.
Solubility. The surface-active functionality is closely related to the solubility. A well-soluble protein forming a true solution adsorbs to surfaces and thus creates steric repulsions. The precipitation of a protein from a solution may lead to linkages between dispersed particles, hence resulting in gelation of the system.
Totally insoluble proteins, perhaps resulting from harsh processing conditions, act as insoluble particles, maybe with swelling and waterholding properties.
Denaturation temperature. Denaturation is an irre-versible reduction of the protein solubility at a certain critical temperature.
Isoelectric point. The isoelectric point is the pH when a protein displays a zero net charge. The solubility of proteins is reduced in solutions at a pH close to the isoelectric point and enhanced at pH values above and below the isoelectric point. The pH of the solution relative to the isoelectric point also highly influences its aggregation properties, denaturation temperature and solubility.
Molecular parameters. The molecular parameters include a range of various parameters on a molecular
level, which may be more or less difficult to mea-sure and sometimes even to quantify. They include the molecular weight, for most proteins very well defined, but technically most important for proteins like gela-tine where it may vary and be less defined, and the molecular structure, which might be globular (whey pro-teins), random coil (sodium caseinate) or helical (gela-tine). The molecular flexibility is a more descriptive but less well defined parameter, describing the willingness or the resistance of the molecule towards conforma-tional changes. A further parameter, the protein surface hydrophobicity, is a chemical descriptor based on the binding of a probe to the molecular surface.
Functionality tests. A wide range of functionality tests are available for use in connection with certain protein products and their applications. These include emulsification index according to Kinsella (a measure of the amount of oil that can be emulsified the using the tested protein preparation), the Bloom strength (a gel strength test used to characterize gelatine) and Heini numbers (a measure of egg white quality). A common feature of most of these tests are that they are very sensitive towards the protein solubility, which may influence the results obtained when applied to technical samples.
The characteristic properties of a range of important technical proteins are summarized in Table 2.11.
4.3 Polysaccharides
Most polysaccharides are not very surface active. For instance, starch and dextrane display very weak surface activities in various tests (for instance, in the classi-cal "Gold number" tests). Xanthane has been shown to create depletion flocculation in several types of experi-ment, as shown by an unpredicted fast creaming. How-ever, other polysaccharides, for instance, gum arabic and modified cellulose, do display surface activity. The latter is defined here as the ability to adsorb to sur-faces. The ability to reduce the interfacial tension is associated with the ability to adsorb, as the surface tension represents the strength of the molecule-surface interactions. However, for large molecules, where the mixing entropy is only a minor contribution to the free energy, adsorption can also be achieved also when the adsorption energy is small. The surface activity of gum arabic is explained by the proteinaceous components associated with the molecule (22). When these parts are eliminated, the surface activity is lost. With mod-ified cellulose, the surface activity is more related to
Table 2.10. Surface-active lipids (soaps, lecithins, monoglycenides, monoglycenide derivatives, etc.) used in food and feed applications (emulsifiers)
Lipid
Soaps
Lecithin
PC-enhanced lecithin
Hydrolysed lecithin
Distilled mono-glycerides (MGs)
Monoglycerides/
diglycerides (DGs)
Modified monoglycerides
Examples and USFDA and EEC E numbers Sodium and potassium soaps of
common fatty acids ( - )
( - )
Natural mixture of phosphatidylcholine (pc), phosphatidylethanolamine and other phospholipids (184.1400)
(E 322)
PC concentration increased through a selective extraction of non-PC components of the lecithin
(184.1400) (E 322)
Lysophospholipids (184.1400) (E 322)
About 90% MG, with fatty acid composition depending on fat base; typically lard, tallow, vegetables (182.4505) (E 471)
Typically 40% MG and 60%DG
(182.4505) (E 471)
Lactylated monoglycerides (172.852)
(E 472)
Acetylated monoglycerides (172.828)
(E 472)
Ethoxylated monoglycerides (172.834)
(7)
Diacetyltartaric acid esters of monoglycerides
(182.4101) (E 472)
Properties
Strongly hydrophilic (HLB above 20) at pH values above the pKa of the fatty acid (about 6). Forms micelles with water. Strong soapy taste
The standard lecithin (mainly of soybean origin) is a hydrophobic mixture dominated by the properties of phosphatidylethanolamine (effective HLB about 4).
Forms a reversed hexagonal phase with water
More hydrophilic than the native mixture. When the PC concentration is high enough (about 60-80%), lamellar liquid crystalline phases might be formed
More hydrophilic than the standard lecithin. Water dispersible, and may form lamellar liquid crystals (depending on the quality) Slightly on the more lipophilic
side (HLB about 5). Forms lamellar liquid crystalline phases with water
More lipophilic than distilled monoglyceride (HLB less than 5). Forms an L2 phase with water
Uses
Limited use due to the poor taste and high pH. Is included as a hydrophilizing additive in commercial monoglyceride gels used in the baking industry Used over a wide range of the
industry. Typically used in margarines and spreads as a hydrophobic emulsifier and in chocolates as a viscosity regulator. Used as a wetting additive for powders, and in the feed industry to improve fat digestion
Used in applications where more hydrophilic properties are required. The product has less taste and a purer character than the original material, which might be beneficial in some applications. However, the increased cost level limits its use
Used in water-continuous applications such as mayonnaise and dressings
Used over a wide range of the industry, e.g. in the margarine industry as a lipophilic emulsifier, in the baking industry as an additive retarding the staling of bread, and in whipped toppings
Used as an emulsion destabilizer in the ice-cream industry
Baked products, whipped toppings
Baked goods
Frozen desserts, cakes Baked goods, dairy-type
emulsion replacers
(continued overleaf)
Table 2.10. (continued) Lipid
Polyglycerol esters
Propyleneglycol esters Sorbitane esters
Polysorbates
Sucrose esters
Calcium and sodium stearoyl lactylates Ethoxylated ricinic oil
Examples and US FDA and EEC E numbers Citric acid esters of
monoglycerides (172.832) (E 472) (172.854) (E 475)
(17.854) (E 477)
Sorbitane stearate (solid) and sorbitane oleate (liquid) (172.842)
(E 491)
Polysorbate 80 (oleate-liquid) (172.480)
(E 433) (172.859) (E 173) (172.844) (172.846) (E 482) (E 481) ( - ) ( - )
Properties
A hydrophobic emulsifier (HLB typically less than 4)
A lipophilic emulsifier (HLB about 4). Forms an L2 phase with water
A hydrophilic emulsifier (HLB typically about 12-16).
Forms micellar solutions An intermediate emulsifier
(HLB estimated as about 8)
Emulsifiers with somewhat intermediate hydrophilicity (HLB about 6-8).
Dispersible in water
Uses
Emulsion products
Limited use due to restrictive legislation. Some use in the chocolate industry in combination with lecithin as a viscosity regulator Cake mixes
Emulsions
Frozen desserts, dressings, etc.
Limited use due to restrictive legislation and high cost level
Bread, coffee whiteners
Only permitted in the feed industry. Used as an emulsifier in self-emulsifying feed mixtures
the hydrophobicity of parts of the molecule itself. Mod-ified cellulose also gives a reduction in the interfacial tension.
A second interfacially active component may also induce the surface activity of weakly surface-active pro-teins due to strong intermolecular interactions. Electro-static interactions from anionic bile salts enhance the adsorption of cationic chitosan on emulsion droplets emulsified by using a mixture of phospholipids, choles-terol and such bile salts. Carrageenan interacts strongly with milk proteins, which is of importance in relation to the association to emulsions and to its application in stabilizing neutral dairy products.