food emulsions food science and technology

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title:Food Emulsions Food Science and Technology (Marcel Dekker, Inc.) ; 81author:Friberg, Stigpublisher:CRC Pressisbn10 | asin:0824799836print isbn13:9780824799830ebook isbn13:9780585138664language:EnglishsubjectEmulsions, Food.publication date:1997lcc:TP156.E6F66 1997ebddc:664/.024subject:Emulsions, Food.

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1

Emulsion Stability

Stig E. Friberg

Clarkson University, Potsdam, New York

The stability of food emulsions is complex because it covers both a number of phenomena and a variety of systems with different content. Among the phenomena the stability is concerned with the destabilization steps including flocculation, coalescence, creaming, and final phase separation with different stabilization mechanisms. The widely varied systems are exemplified by a series of cases in this book.

In the first example, the classical concept of emulsion of two liquid phases is discussed [13] with special attention to the effect of surfactant/polymer combinations. Subsequent examples extend the discussion to emulsions that contain another phase in addition to the two liquid ones, in accordance with the actual definition of an emulsion [4].

The third phase may be a liquid, in which case energy-efficient emulsification [5,6] is possible, or a solid [7,8], or the phase may be a liquid crystal [911], in which case the phase equilibria are essential as well as the formation of liposomes. The liquid crystal offers a potential for substances to be solubilized, especially substances that are only poorly soluble in the two liquids of the emulsion.

Introduction

Food emulsions cover an extremely wide area in practical applications, as is amply demonstrated in this volume. One finds the semisolid varieties such as margarine and butter as well as the liquid ones such as milk, sauces, dressings, and various beverages. In addition, the concept of food emul-

Food_Emulsions__Food_Science_and_Technology__Vol_81_/0824799836/files/page_10.html

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Figure 5

A force between two droplets gives rise to a stabilizing potential

energy between them. (Adapted from Ref. 14.)


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Table 7 Recipe for the Ice Cream Emulsion ModelComponentAmount in % (w/w)

Fat 10.0

Milk powder 11.5

Sugar 14.0

Mono-/diglyceridea 0.3

Stabilizerb0.10.25

aApproximately 40% monoglyceridebOne of the following: sodium alginate, carboxymethylcellulose, carrageenan, xanthan, guar gum, or locoust bean gum.

Figure 22

Flocculation rate of a mono-/diglyceride coated emulsion stabilized by different gums or

casein. The flocculation is induced by the addition of salt.


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Figure 23

Interfacial tension between oil and water when carrageenan, the mono-/diglyceride

mixture, casein, or mono-/diglyceride and casein are present.

enough (d < 1 mm) to allow for turbidimetric flocculation rate measurements [157159].

Primarily, it was assumed that the mono-diglyceride emulsifier was the most surface-active component of the mixture. The emulsion was destabilized by the addition of salt (the stability of this emulsion toward flocculation was initially due to electrostatic charges, probably originating from impurities in the oil and the emulsifier). The adsorption of a polymer generated a steric repulsion which was then observed as a reduced flocculation rate. The influence of different gums (hydrocolloids) and casein on the flocculation rate of this emulsion is shown in Fig. 22.

It is obvious that most gums do adsorb to the mono-diglyceride interface. However, it is also clear that the adsorption is rather weak for most of the polysaccharides. The concentration of the different gums in the actual ice cream (Table 7) is too low to give rise to significant adsorption. However, the caseinate concentration in the ice cream is high - considerably higher than needed for adsorption. Hence, it is clear that the oil-water interface contains casein. However, interfacial tension measurements (Fig. 23) show that both the mono-diglyceride emulsifier and the casein adsorb


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Figure 24

Flocculation rate of an emulsion coated with casein and the mono-/diglyceride emulsifier

when stabilized by different gums. The flocculation was induced by addition of an acid

buffer.

to the interface, and that they form a mixed layer. The last conclusion is based on the fact that the surface tension is lower when both the monoglyceride and the casein are present, compared to when only one of the components is included in the mixture.

To investigate the stability of an emulsion coated with both mono-diglyceride and casein, an acidic buffer was added to induce flocculation. The flocculation rate when different gums were present in the emulsion is shown in Fig. 24. The results show that several anionic gums reduce the flocculation rate, which is an indication of adsorption to the emulsion droplet surface. The adsorption is enhanced considerably by the presence of casein on the surface. The surface activity of the gums is much lower than that of casein (Fig. 23). Hence, the polysaccharide should not be able to displace the casein or the mono-diglyceride at the interface. Instead the observed stabilization is due to the formation of a layered structure. The inner layer is composed of mono-diglyceride and casein, whereas the outer layer is made up by polysaccharides. The complicated structure of this


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Figure 25

Proposed layer structure formed by gums, casein, and the

mono-/diglyceride mixture.

emulsion droplet surface is illustrated in Fig. 25. In the picture of the ice cream emulsion that emerges, the stability is cause mainly by the casein. The emulsifier and the casein are mixed in the first layer, and the only effect of the emulsifier may be to destabilize the emulsion by softening the protein film. Such a destabilization of the protein film is assumed to be beneficial to the taste. Hydrocolloids are added to increase the viscosity, which reduces sandiness. Besides changing the viscosity, some hydrocolloids also adsorb to the emulsion droplets and stabilize them, a stabilization that might be unwanted and negative to the taste.

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127. J. Sjblom, P. Stenius, and I. Danielsson, in Nonionic Surfactants, (M. Schick, ed.), Marcel Dekker, New York, 1987, pp. 364434.

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154. E. Cook and A. P. Lagace, U. S. Patent 4, 533, 254, 1985.

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Page 11

Figure 6

A square interdroplet potential simplifies the evaluation

of its stabilizing effects, where Wo is potential height, a

is radius of droplets, and (y - x)a is potential width.

and height constant but moving it toward increased distances from the droplet makes the factor smaller; the reduction is approximately proportional to the square of the distance. (y - x is constant; (y - x)/xy y-2) Hence, the barrier is considerably less efficient at greater distances from the droplet.

The remaining variable, the height of the barrier, is the important factor, as demonstrated by Table 3.

Table 3 Approximate Half-Lives for Emulsions in TextWo(kT)Half-life

00.6 sec

11 sec

530 sec

101.2 hr

1523 hr

203 yr

503.1013 yr


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3

Molecular Organization in Lipids

Kre Larsson

Camurus Lipid Research, Lund, Sweden

Introduction

Our knowledge of lipid structure is based on studies of simple systems containing only a few components, for example, ternary systems comprised of polar lipids, fats, and water. Most of their behavior in complex systems can be explained by the structures and phase properties exhibited by model systems. The different states of order, their structures, and available experimental methods for structural analysis will be presented here. The chemical as well as physical structures of lipids have recently been reviewed [1].

Lipids are unique substances with regard to the variety of structures that they can form, ranging from micellar solutions, liquid crystals, and plastic crystals, to true crystalline phases. The reason for this is the amphiphilic character of the lipid molecule, which is also responsible for the well-known orientation phenomena of lipids at water/oil or water/air interfaces. Furthermore they exhibit alternative crystal structures (polymorphism), and different crystal forms have different effects on emulsion structure.

Molecular Arrangement In The Solid State

In the crystalline state it is possible to obtain exact and detailed information on the molecular conformation as well as the lateral molecular packing. Knowledge about lipid crystal structures is therefore of fundamental importance for discussions of the structure in disordered states, such as liquid crystalline lipid-water phases. Furthermore, there is a close resemblance between crystal structures and the structures of lipid monolayers at inter-


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faces, for example, on a water surface. A remarkable property of lipid crystalline phases that should be mentioned in this connection is that complex mixtures like natural fats can form perfect solid solution with wide variations in molecular size.

Experimental Technique

When single crystals are available, it is possible to perform a complete structure determination by the x-ray diffraction technique. A useful textbook on the application of x-ray diffraction for studies of lipids is given in Ref. 2. There are, however, two experimental difficulties. It is usually a very difficult problem to obtain single crystals that are perfect enough for an x-ray analysis. Twinning of growth is a frequent complication, and the crystals must be handled with extreme care in order to avoid relative displacements of the bimolecular lipid layers along the end-group contact planes. Another difficulty is that most lipids do not contain heavy atoms, so that standard methods utilizing the Patterson function cannot be applied in analyzing the structure. It has been found, however, that terminating methyl groups often can be replaced by bromine or iodine atoms without changing the structure [3]. Furthermore, the recent development of computer programs for so-called direct methods will probably also make possible structure determinations of complex lipids without substitution.

Even if single crystals are not available, a considerable amount of information on solid-state packing can be obtained by x-ray powder diffraction. This has been demonstrated in studies of the crystallization of fats [4]. It is thus possible to obtain a so-called long spacing from which the main features of the arrangement of the molecules in layers can be obtained, such as the number of molecular layers in the repetitive unit layer and the angle of tilt of the molecules within each layer. In the wide-angle region of the diffraction pattern it is possible to identify dominating so-called short-spacing lines, which in many cases are sufficient for identification of the hydrocarbon chain packing.

X-ray diffraction is by far the most powerful method used to determine the structures in the solid state, even if important complementary information has been obtained by other methods, e.g., by infrared spectroscopy [1].

Molecular Orientation

In this general discussion on structure in the solid state, a simple lipid molecule with a polar head group attached to a hydrocarbon chain will be considered. As will be shown later, the same principles are also valid in complex lipids with alternative possible molecular conformations.


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The characteristic feature of lipid crystal structures is the arrangement of molecules in layers with the polar groups at the surface of the layer. A typical structure, that of n-hexadecanol [5], is shown in Fig. 1. The molecular layers form unit layers of double molecular length, lipid bilayers, in

Figure 1

Molecular packing of

n-hexadecanol viewed along

the two short axes of the unit

cell. (From Ref. 5.)


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order to permit polar interaction (hydrogen bonding in this structure) between the molecular layers. This means that there are only relative weak van der Waals attractive forces between these bilayers separated by the methyl end-group gap. This explains the crystal morphology; the crystals form very thin plates parallel to these planes, with a pronounced tendency for cleavage at the methyl gap interface.

The major types of layer arrangements are illustrated in Fig. 2. The arrangement shown in Fig. 2a, head-to-head, is the same as that in Fig. 1 and by far the most common type of structure in simple lipids. The angle of tilt of the hydrocarbon chains is determined by the size of the polar end group. In n-alcohols with a quite small polar head group there are two vertical polymorphic forms besides the tilted one shown in Fig. 1. In n-fatty acids, for example, with a larger polar end group, all crystal forms are tilted.

In cases with very weak attractive forces between the polar head groups, or when the polar groups are not end groups, a head-to-tail arrangement as shown in Fig. 2b can be adopted. An example is ethyl stearate

Figure 2

Schematic illustration of the major types of

molecular orientations in layers: (a) head-to-head

arrangement, all molecules directed in the same way

within each layers; (b) head-to-tail arrangement, all

molecules directed in the same way within each layer;

(c) head-to-tail arrangement within each layer;

(d) alternating directions of the tilt.


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[6], in which the polar group is too far from the end of the linear molecule to give dimerization. Methyl stearate [7], on the other hand, forms the usual head-to-head structure.

An arrangement head-to-tail within each layer, as shown in Fig. 2c, has been observed in cases with very bulky polar head groups. By means of such a structure, the polar groups obtain twice as large a cross-sectional area as the hydrocarbon chains.

In cases where the molecules are tilted they are usually all parallel. In some structures, however, the angle of tilt alternates between opposite directions in successive layers as indicated in Fig. 2c. Such an arrangement possesses particular advantages in cases of solid-state phase transitions, as translations along the methyl end-group planes can be avoided [8]. Finally, it should be mentioned that structures with two chain directions within each molecular layer have been observed in cases where the forces between the polar head groups are of dominating importance, for example, in amides [9].

Hydrocarbon Chain Packing

The hydrocarbon chains are always in the extended planar zigzag conformation in lipid crystal forms. There are a few alternative close-packing arrangements of such chains, which can be best described by the corresponding subcell (the smallest repetition unit within the layer). All chain packing alternatives that have been observed are summarized in Fig. 3. The triclinic packing, , and the orthorhombic one, 0^, represent the most efficient packing from an energetic point of view, and are observed, e.g., in n-paraffins. All chain planes are parallel in the triclinic packing, whereas every second chain plane is perpendicular to the rest in the orthorhombic packing. The monoclinic packing, , has been observed in 3-thiadodecanoic acid [10], and in racemic 1-monoglycerides [11]. An orthorhombic subcell, O'^, closely related to the common one, O^, was found in a branched fatty acid [12]. In other structures with disturbances in the hydrocarbon chain region, two more orthorhombic chain packings have been observed: and , both with all chain planes parallel [13,14].

An interesting disordered crystal form, usually termed the a form [15], is observed near the melting point of many lipids. Its x-ray single-crystal data are in agreement with a hexagonal lateral symmetry of the hydrocarbon chains with rotational freedom [16]. Electron diffraction data on a thin film of this crystal form have also been reported [17], and beside the hexagonal diffraction pattern a halo was observed with a geometry corresponding to the Fourier transform of a rotating hydrocarbon chain. Due to the rotation of the molecules and its physical properties, this form is


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Figure 3

Hydrocarbon chain packing alternatives as defined by the corresponding

subcell. One zigzag period is seen in the direction of the hydrocarbon

chains with the atomic positions in the chain direction given

by their fractional coordinates. Open circles represent hydrogen atoms

and filled circles represent carbon atoms.


Page 117

in fact a plastic crystal. In some lipids, however, the chains are anchored at the polar groups, e.g., in triglycerides, and it is obvious that only oscillational movements of the chains are possible. Finally it should be mentioned that this hydrocarbon chain arrangement occurs in lipid-water phases and even in emulsions.

Polymorphism

The multiple melting phenomena exhibited by triglycerides was first explained by Malkin [18] as due to the occurrence of an alternative crystal form: polymorphism. One possibility for polymorphism is the arrangement of the different hydrocarbon chain close-packing types described above. The most common reason for polymorphism, however, is the possibility for variations in the angle of tilt of the hydrocarbon chains. The chains are successively displaced one or more whole zigzag periods in relation to adjacent chains so that the angle of tilt toward the end group plane is increased or decreased and the hydrocarbon chain packing and thus the van der Waals interaction in the hydrocarbon chain region will not be changed.

Most lipids exhibit polymorphism. This can be illustrated by fatty acids. An even n-fatty acid in the chain-length range C12 to C18 can be obtained in three different crystal forms: the A, B, and C forms. The crystal forms B and C have the same packing (O^) with different angles of tilt, whereas the A form shows another chain packing ().

Lipid Structures and Phase Properties

The structure shown in Fig. 1 is characteristic for simple lipids with normal and saturated hydrocarbon chains. Branches can be accommodated into the hydrocarbon chain region by relative displacements of adjacent chains or by changes in the tilt direction at the branch position [19]. Examples of the two types of structures, termed t or g forms, are shown in Figs. 4 and 5.

When there are double bonds in the hydrocarbon chain they will effect the chain packing differently according to the bond configuration. In oleic acid, where there is a cis double bond, there is a change in the chain direction at the double bond, whereas the corresponding trans isomer, elaidic acid, shows only one chain direction [14]. The crystal structure of oleic acid is shown in Fig. 6.

The crystal structures of glycerides will be described shortly, in order to illustrate changes in the molecular layer arrangement when going from simple complex lipids.

The crystal structure of a 1-monoglyceride is shown in Fig. 7. Although there is a complicated hydrogen bond system linking the molecules


Page 118

Figure 4

Molecular packing of 9-DL-methyloctacecanoic acid viewed along the

a-axis (From Ref. 19.)

Figure 5

Molecular packing of 14-DL-methyloctadecanoic acid viewed along the

a-axis (From Ref 19.)


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Figure 6

Crystal structure of oleic acid viewed along

the b-axis. (From Ref. 14.)

together within the layers, the structure is of the same simple type as that shown in Fig. 1. The crystal structure of a symmetric diglyceride (with a sulfur atom in the chain tail) is shown in Fig. 8. There are two alternative molecular conformations of diglycerides in the solid state: one with extended molecules forming a unit layer of double chain length, and the other (folded) with the two chain tails in the same chain layer. It can be seen that


Page 12

Theory of Colloidal Stability: The DLVO Theory

The DLVO (Derjaguin-Landau-Verwey-Overbeek) theory [27,28] states that the stability of a colloidal suspension essentially depends on the distance relation of two independent interactions between colloid particles. These interactions, as shown in Fig. 7, are:

1. The van der Waals attraction

2. The electrostatic repulsion between electrical double layers of identical sign

The sum of the two potentials with signs gives the potential W in Eqs. A3 and A4. In spite of the fact that the flocculation rate is influenced by the distance distribution of the potential, Eq. A1 and Table 3 well illustrate that the influence of the barrier height predominates.

Taking this into account, the theory essentially predicts that if the repulsion potential B exceeds the absolute value of the attraction potential

Figure 7

The DLVO theory refers colloidal stability to the distance

dependence of two independent potentials.


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Figure 7

Molecular arrangement in the asymmetric monoglyceride of 11

bromoundecanoic acid as seen along the short unit cell axes.

the structure is of the former type. The asymmetric diglyceride of lauric acid was found to exhibit the other structure alternative [20].

The crystallization behavior of triglycerides can be fairly complex. Generally, triglycerides crystallize in layered structures, with glycerol groups and methylene groups each in one plane and the axes of the hydrocarbon chains straight and parallel to each other within a layer. When the glycerol group is considered a cross-link between hydrocarbon chains, it is clear that it may force spatial proximity on hydrocarbon chains that are not able to cocrystallize, and thus cause more complex structures. Besides the covalent bonds, lateral van der Waals interactions between hydrocarbon chains, van der Waals interactions of methyl end groups, and polar interactions of the ester bonds will control crystallization. The polar interactions are highly significant, as seen in the fact that melting temperature of triglycerides are much higher than the melting temperatures of the corresponding alkanes, and even slightly higher than those of the corresponding fatty acids.

It was recognized early that triglycerides exist in a range of polymorphs. The usual type of triglyceride polymorphy is monotropic, i.e.,


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Figure 8

Molecular arrangement in the asymmetric diglyceride of 3-thiododecanoic

acid as seen along the short unit cell axes.

unstable forms are formed on cooling of melt, but even enantiotropic polymorphs, changing crystal habitus reversibly in solid state, have been found. There is no complete consensus on triglyceride polymorph terminology. Historically, a polymorph is the (unstable) crystal found on cooling of melt and b the stable form. A rational scheme is based on measurable properties of crystal lattice, on the lateral packing of hydrocarbon chains in the subcell, the repeating unit along the axis of the hydrocarbon chain:

a One interchain distance only, single strong x-ray diffraction line at @4.15 , i.e., hexagonal subcell

b' Two diffraction lines, at 3.80 and 4.20 , and infrared (IR) doublet at 720 1/cm, i.e., orthogonal subcell with mutually perpendicular (nonparallel) carbon planes

b Other


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Figure 9

Molecular arrangement in trilaurin as seen along the short unit cell axes.

Multiple crystal forms are distinguished by numerical subscripts; thus b'2 is the second highest melting b' crystal.

At subfreezing temperatures, the a form may further crystallize in several nonhexagonal forms, which are distinct from the b and b' forms. Usually, these low-temperature forms are termed sub-a.

Simple saturated triglycerides crystallize in double layers. The acyl chains in 1 and 2 positions are linearly stretched opposite each other and the 3 chain is hinged on its carboxyl group and parallels the 1 chain. The unit cell contains two triglyceride molecules in tuning fork configuration, with the single 2 chain interleaved between the double chains of its neighbors (Fig. 9).


Page 123

Apart from subcell type, the angle of tilt of the hydrocarbon chain axis to the layer plane distinguishes crystal forms. In the a crystal, the lateral distance between next neighbors allows for rotation of CH2 groups and the chains are perpendicular to the layer plane (Fig. 10). In b and b' crystals, the chains are tilted, down to 50 angle. In a simple triglyceride, the 1 chain will reach further out from the glycerol residue plane than the 1 and 2 chains. If the unit cell a-axis were to be the same as the subunit cell a-axis, the terminal methyl residue of chain 1 would project out of the methyl plane, and there is no way in which large voids in the methyl region could be avoided. Thus the arrangement is not allowed for simple triglycerides. Instead, the neighboring doublet can slide one or two zigzag periods with respect to the subunit cell a-axis, and the distinctive appearance of the methyl terrace appears (Fig. 11). De Jong [21] gives 9 allowed shapes of the methyl terrace for saturated triglycerides with various combinations of chain lengths, assuming arbitrarily a translation of at most one zigzag period per chain. As a consequence of terracing, the angle of tilt in the plane defined by the carbon chain backbone assumes distinct values, compatible with translation of the neighboring chain by one zigzag period. Tilting of the chain can be interpreted as a way of reducing the voids in the methyl group region where the chains do not fit together perfectly. However, the tilt may introduce strain in other parts of the structure, and even the allowed configurations are not energetically equivalent and have therefore varying melting points.

The double-layer structure will tolerate minor deviations in chain length between the fatty acid chains, 24 carbons, but the packing imperfection is reflected in lower melting temperature at constant melting enthalpy per carbon group, i.e., increased entropy of the mixed triglyceride crystal.

Figure 10

Hexagonal and orthorhombic perpendicular packing of chains. Due to

the linkage at the glycerol residue chain planes in the hexagonal form

are expected to be locally parallel but random on larger scale.


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Figure 11

Methyl plane packing of monoacid saturated

triglycerides. (top) Not allowed packing;

(middle) usually encountered form, de Jong

form A; (bottom) allowed packing, de Jong

form E. (After Ref. 21.)


Page 125

If one of the acyl chains differs substantially from the other two, a trilayer structure is formed. The differing chains of two molecules interdigitate and form a separate middle layer (Fig. 12). If necessary, the orientation of the glycerol group is changed, so that, e.g., in C16C16C8 the 1 and 2 chains are next to each other. No clear-cut distinctions exist, however; e.g., saturated C16C16C10 can form either a classical bilayer or a trilayer.

In unsaturated triglycerides, the trans-fatty acids appear to be incorporated into the crystal structure in the same way as the saturated chains (Fig. 12). cis-Fatty acids do not mix in one layer with saturated acyl chains and thus trilayer crystals are formed. There is a change in tilt of the unsatu-

Figure 12

Examples of complex glyceride crystal structures.

(top left panel) Triple layer structure of saturated

triglycerides with one chain much shorter; (top

right panel) double layer structure of triglycerides with

one trans-unsaturated chain; (bottom left panel)

triple layer structure of triglycerides with one

saturated and two unsaturated chains; (bottom right

panel) triple layer structure of mixed crystal of a

saturated triglyceride and an unsaturated triglyceride.


Page 126

rated layer, comparable to the tilt of the oleic acid crystal. In mixed triglycerides, the tilt of the separate saturated layer is the same as the tilt of the carbonyl part of the unsaturated chain (Fig. 12). For symmetry reasons, even six-layered structures may be necessary to describe a unit cell.

On cooling a triglyceride melt, a crystals are generally formed. In contrast to, e.g., alkanes, where the a form can be stable at temperatures close to the melting point, a crystals are never stable in triglycerides and convert to b' forms. In most cases, b' crystals are slowly converted to stable b form; however, in mixed triglycerides, e.g., in C16C18C16, C16C16C8, and in triglyceride mixtures, b' forms seem to be stable, as in odd saturated fatty acids and alkanes.

The kinetics of triglyceride crystallization is unsufficiently understood. In triglyceride melt, lamellar structure exists at least at temperatures close to the melting point, and may persist 40 above. It is known that the crystallization rate of melt is dependent on the temperature to which the melt has been heated; thus the structure of the melt affects crystallization directly, especially when the rate of cooling is high. The a form crystallizes without appreciable undercooling when the temperature is lowered from melt to a temperature below its melting point, so it seems that the structure in solution is related to a. The stable polymorphs can be often obtained directly from melt if the crystallization temperature is sufficiently high, above melting of the other polymorph, but the crystallization rate may be extremely slow (order of days for b form of trimargarine). So far, it has not been possible to separate the kinetics of nucleation and of crystal growth. At a given temperature, the rate of crystallization of b- and b'-tripalmitin from high-temperature melt was found to be 3 to 10 times slower than crystallization from melting a form. The solid-state transformation of a .gif b was found to be much more rapid than b' .gif b. The cross-section area of the b' configuration is smaller than the cross section of b, and thus lateral van der Waals attraction is likely to be higher in the b' form and to have to be balanced by the contribution of the methy1 layer in b. As the lateral attraction is highly cooperative, the activation energy of the transformation to the b form is expected to be high.

No comprehensive theoretical treatment of the behavior of triglyceride mixtures is available. As expected, the melting point in mixtures is smeared out as the high-melting components dissolve in the low-melting ones, but even in the case of simple tripalmitin-triolein mixtures, the agreement with ideal mixing theory is poor. Intersolubility of different triglycerides in solid state is low; even fats as close as C16C16C16 and C16C18C16 show only 5 to 15%. Solid-state crystal transformations are of practical importance in storage hardening of edible fats and blooming of chocolate. A number of empirical additives is used to influence the kinetics of the


Page 127

important b' b transition. The rationale is to increase the disorder of the methyl group region and thus lower the difference in free energy between b and b'. However, the effects on nucleation and activation energy are not immediately obvious.

Phase behavior can be quite complex even in a pure lipid when many polymorphic forms exist. The thermal phase relations for such a lipid, a pure glycosfingolipid, is illustrated in Fig. 13. It is obvious that the phase behavior of a multicomponent system including such lipids can be very complicated. Fortunately there is a strong tendency for a lipid mixture to behave as one component with regard to the phase rule. A complex glyceride mixture constituting a natural fat, for example, often shows the same or a similar phase behavior as a pure triglyceride. This also illustrates the possibility of accommodating various molecular structures in the same crys-

Figure 13

Phase behavior of a cerebroside,

N-octadecanoyl-psychosine [22].

A, B, and B' are crystal forms and

LC a liquid crystal.


Page 128

tal form by disorder in the hydrocarbon chain region. If there are large and systematic variations in the molecular structure, however, molecular compounds may be formed. This is illustrated by a binary fatty acid system shown in Fig. 14.

Liquid Crystal Structures

Phases possessing physical properties characteristic for both the liquid and the solid states have been termed liquid crystals or mesomorphic phases. They have for a long time been divided into three classes: smectic, nematic, and cholesteric [23]. The numerous liquid crystalline phases formed by

Figure 14

Binary phase diagrams as evident from the melting point curves of

octadecanoic acid and of heptadecanoic acid-hexadecanoic acid. (After Ref.

26.)


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lipids and lipid-water mixtures do not, however, fit into this classification. The reason is that each molecule is considered as a rigid body, and the disordered phases differ only by the orientation and repetition properties of these units. The reason for the occurrence of liquid crystalline phases in lipids is the amphiphilic properties of the molecules, and the only order existing in such phases is an orientation of the polar groups into open and closed surfaces with different curvatures. The simplest structure is obtained by heating a crystalline phase, for example, the monoglyceride phase shown in Fig. 7. Due to relatively weak van der Waals forces between the hydrocarbon chains compared to the hydrogen bonds in the polar end group region, the chains melt before the true melting point is obtained. A lamellar structure with semiliquid properties is thus obtained, which differs from the structure shown in Fig. 7 in the hydrocarbon chain region, showing complete disorder. A liquid crystal obtained by heating a pure component in this way is termed thermotropic as opposed to those obtained by addition of a solvent, which are called lyotropic.

A requirement for formation of a lipid-water liquid crystal is thus that the hydrocarbon chains are transformed into a state with disorder such as that in the liquid state. In spite of the high degree of short-range disorder in these structures-liquid hydrocarbon chains and liquid polar regions-there is a perfect long-range repetition in at least one dimension. These phases can therefore be analyzed by x-ray diffraction methods, and the basic structural features were established by Luzzati and co-workers [24] in their classical work on soap-water systems, using this technique. The dominating phases are thus lamellar (long-range order in one dimension), hexagonal (long-range order in two dimensions), or cubic (long-range order in three dimensions). According to the various criteria used for definition of liquid crystals, the cubic phase is not a true liquid crystal. Since there is complete repetition in three dimensions, this phase is in fact a crystal, and due to the occurrence of orientational disorder of the structural units it should be termed a plastic crystal [27]. On the other hand it has a close relation to the lipid-water liquid crystalline phases and it has therefore usually been described as a liquid crystal. Another related phase, which is not strictly a liquid crystal, the gel, will also be treated in connection with the liquid crystal structures.

A complementary description of lipid liquid-crystalline phases is given in Chap. 5 by Nylander and Ericsson.

Experimental Methods for Phase Analysis

X-ray diffraction is the most informative method for identification and characterization of liquid crystalline phases. As samples of the single crystal type are hard to obtain, the x-ray powder technique is usually applied in a


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A by a value W kT at any distance between the particles (Fig. 7), the suspension will be st

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