84607_03.pdf

Polymorphism

in Fats and Oils

Kiyotaka Sato and Satoru Ueno

Graduate School of Biosphere Science, Hiroshima University

Higashi-Hiroshima, Japan

1. INTRODUCTION

Triacylglycerols (TAGs) are the major components of fats and oils and biologically

important organic molecules along with proteins and carbohydrates. In industrial

applications, TAGs are the main components in cream, margarine, and confection-

ery fats in foods and as matrix materials in pharmaceuticals and cosmetics. The

physical behavior of TAGs inﬂuences the physical properties of fat-based products,

such as appearance, texture, plasticity, morphology, and rheology. Most fat-based

products are multicomponent TAG mixtures, containing different kinds of fatty

acid moieties. Their complex physical properties are ascribed to polymorphism of

individual TAG components and their mixing behavior. Therefore, research into

the physical properties of the fat-based products usually starts with an understand-

ing of individual TAG molecules and subsequently moves on to an understanding of

the mixed systems, while combining this microscopic information with the macro-

scopic properties of texture, crystal morphology, and rheology. The macroscopic

properties of fats and oils will be discussed in other chapters of this volume.

Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.

Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.

POLYMORPHISM IN FATS AND OILS

This chapter describes the polymorphism of the principal TAGs with saturated and

unsaturated fatty acid moieties and their binary mixtures.

2. BASIC CONCEPTS OF POLYMORPHISM OF FATS

TAGs are three-fold esters of glycerol and fatty acids, having the general formula

shown in Figure 1. There is a number of fatty acid moiety, as indicated in Figure 1.

According to Figure 1, TAGs can be divided into two classes depending on the fatty

acid composition. TAGs having only one type of fatty acid are called monoacid

TAGs, and those having two and three types of fatty acids are, respectively, called

diacid and triacid TAGs, and both are categorized as mixed-acid TAGs. Almost all

natural fats and oils are mixed-acid TAGs. In addition, the diacid TAGs can be

divided into two types: symmetric and asymmetric TAGs. In the asymmetric diacid

TAGs, chiral properties are revealed: For example, sn-R 1 R 1 R 2 and sn-R 2 R 1 R 1 are

stereochemically different from each other, in which sn means a stereospeciﬁc

number. The same chiral properties occur in the triacid TAGs. It is noteworthy

that polymorphism of the symmetric TAGs is largely different from that of the

asymmetric TAGs.

The physical properties of TAGs are determined by the types of fatty acids that

compose them; for example, the number of saturated and unsaturated chains, cis-

and trans-double bonds, short and long chains, chains with even and odd numbers

of carbon atoms, and esteriﬁed positions of fatty acids with glycerol carbon atoms.

Fats are modiﬁed by hydrogenation, interesteriﬁcation, and fractionation to produce

desirable physical properties for fat-based products.

2.1. Polymorphism of Triacylglycerols

Multiple melting points of fats had already been discovered in the nineteenth cen-

tury. Clarkson and Malkin showed that this melting behavior resulted from the

polymorphism of TAGs (1). In the crystalline state, TAG molecules adopt the ideal

sn -1

sn -2

sn -3

CH 2 O

R 1

OCOR 2

OCOR 3

Mono-acid TAG (R 1 = R 2 = R 3 )

* saturated: length, even-odd

* unsaturated: length, even-odd, number-position-conformation

of double bond

Mixed-acid TAG (R 1 ≠ R 2 ≠ R 3 )

* different length

* saturated and unsaturated

* different sn -positioned

CH 2

Figure 1. A triacylglycerol molecule (R: fatty acid moiety, sn : stereospeciﬁc number).

BASIC CONCEPTS OF POLYMORPHISM OF FATS

conformation and arrangement in relation to their neighbors to optimize intramole-

cular and intermolecular interactions and accomplish an efﬁcient close-packing. On

the basis of the structural studies by Larsson (2), the three fundamental polymorphs

are called a, b 0 , and b. The signiﬁcance of the deﬁnition of polymorphism of the

TAGs lies in uniﬁcation of otherwise confused nomenclature of the polymorphic

forms of the fats differently named by researchers, such as sub-a form, vitreous

phase, and so on. In addition, the polymorphic nomenclature makes it convenient

to characterize the crystalline properties of fats employed in many applications. For

example, the structure and texture of ice cream is caused by a network of partially

coalesced a-form crystals and ice crystals that surround air bubbles to form discon-

tinuous foams (3). The small needle-like b 0 crystals impart good plasticity that is

desirable in products such as margarine, shortening, and baking fats (4). Cocoa

butter replacers (CBR) and cocoa butter substitutes (CBS) can crystallize without

tempering into their stable b 0 polymorph upon simple cooling. Tempering is

required for b form, which is used for chocolate, cocoa butter, and cocoa butter

equivalents (CBE) (5).

One may characterize the polymorphic forms of TAGs by thermal stability,

subcell packing, and chain-length structure as described below.

2.1.1. Thermal Stability Among the three main polymorphic forms of TAGs and

their mixtures, generally, b is the most stable, b 0 is less stable, and a is the least

stable form (6, 7). A diagram of the Gibbs free energy (G ¼ H-TS, in which H, S,

and T are enthalpy, entropy, and temperature) versus T for TAG polymorphs is

shown in Figure 2. The G-T relationship determines the transformation pathways

among the polymorphs and liquid (8). The polymorphism of TAGs is monotropic,

and the G values are largest for a, intermediate for b 0 , and smallest for b in the solid

liquid

β′

Temperature

Figure 2. A schematic diagram of Gibbs energy (G) and temperature of three polymorphs of a

triacylglycerol.

POLYMORPHISM IN FATS AND OILS

phase domain at low temperature. Each polymorph has its own melting temperature

(T m ) that is deﬁned as the temperature where the G value of crystal becomes lower

than that of liquid. These thermodynamic conditions inﬂuence the kinetic aspects of

crystallization and transformation of TAGs.

The three basic polymorphic forms shown in Figure 2, which may apply to the

saturated monoacid TAGs, are largely modiﬁed when the shape of a TAG molecule

becomes more heterogeneous. For example, TAGs containing unsaturated fatty acid

moieties or saturated diacid moieties exhibit two b 0 or b forms. In other cases,

b does not occur and b 0 becomes most stable with the highest T m instead. These

properties will be discussed in Section 3.

A primary concern is polymorphic crystallization in which the Ostwald step rule

is very useful (9). This rule predicts that phase changes occur step by step by way of

successively more stable phases. For the relative rate of nucleation of polymorphic

crystals shown in Figure 2, it follows that nucleation of the metastable forms such

as a and b 0 occurs ﬁrst before the most stable b form, when nucleation occurs under

a large supercooling or high supersaturation. When the amount of supercooling or

supersaturation is decreased, the law is broken and the most stable form tends to

nucleate at a relatively slow rate.

Because of its monotropic nature, the polymorphic transformation occurs irre-

versibly from the least stable a form to the most stable b form. The rate of trans-

formation is both time- and temperature-dependent. There are two modes of

polymorphic transformation processes: solid-solid and melt-mediated transforma-

tions. Solid-solid transformations occur below the melting points of all the poly-

morphs involved. In contrast, melt-mediated crystallization occurs when the

temperature is above the melting points of the less stable forms. Melt-mediated

crystallization involves the following processes:

1. Melting of the less stable form

2. Nucleation and growth of the more stable forms

3. Mass transfer in the liquid formed by melting of the less stable form

It has been observed in some TAGs that the rate of melt-mediated crystallization

is often much higher than that of solid-solid transformation (10–14).

2.1.2. Subcell Structure Subcell structure deﬁnes a lateral packing mode of

the hydrocarbon chains (2, 15, 16). Three typical subcell structures are shown in

Figure 3. The a, b 0 , and b forms have hexagonal (H), orthorhombic perpendicular

ð O ? Þ , and triclinic parallel (T // ) subcell structures, respectively (2).

In the hexagonal subcell structure, the two-dimensional lattice is hexagonal and

gives rise to a 0.41-nm wide-angle X-ray diffraction (XRD) pattern. The chain

packing is loose, and the speciﬁc chain-chain interactions are lost because of the

ability of the carbon atoms to rotate several degrees and form disordered conforma-

tions of hydrocarbon chains. The two-dimensional lattice of an orthorhombic

perpendicular ð O ? Þ subcell structure is rectangular, and this represents a tightly

BASIC CONCEPTS OF POLYMORPHISM OF FATS

Figure 3. Typical subcell structures of TAG polymorphs. The a , b 0 , and b forms have hexagonal

(H), orthorhombic perpendicular ðO ? Þ, and triclinic parallel(T // ), respectively.

packed lattice with speciﬁc chain-chain interactions. The subcell parameters of O ?

are typically shown in two wide-angle XRD patterns at 0.37 nm and 0.41 nm. Tricli-

nic parallel subcell structure (T // ) has an oblique two-dimensional lattice and repre-

sents tightly packed chains, in which there are speciﬁc chain-chain interactions.

This subcell structure of T // is characterized by a strong wide-angle XRD pattern

at 0.46 nm and week patterns at 0.39 nm and 0.38 nm. The values given for these

wide-angle XRD patterns of the three polymorphs are typical for the saturated

monoacid TAGs; they vary when the fatty acid moieties change from saturated to

unsaturated acids.

2.1.3. Chain Length Structure The TAG crystals form chain-length structures,

in which a repetitive sequence of the hydrocarbon chains is involved in a unit lamel-

lar along the c-axis (Figure 4) (17). One unit layer made up of one hydrocarbon

chain is called a leaﬂet. Several types of chain-length structures can form as shown

in Figure 4. The TAGs with the same or very similar fatty acids might form a

double chain-length structure. A triple chain-length structure is formed when the

chemical natures of one or two of the fatty acids are much different from the others.

A quarto-chain-length structure consists of two double chain-length structures,

which are combined end-to-end. A hexa-chain-length structure consists of two

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