Crystallization of Lipids: Fundamentals and Applications in Food, Cosmetics, and Pharmaceuticals

An authoritative reference that contains the most up-to-date information knowledge, approaches, and applications of lipid crystals

Crystallization of Lipids is a comprehensive resource that offers the most current and emerging knowledge, techniques and applications of lipid crystals. With contributions from noted experts in the field, the text covers the basic research of polymorphic structures, molecular interactions, nucleation and crystal growth and crystal network formation of lipid crystals which comprise main functional materials employed in food, cosmetic and pharmaceutical industry. The authors highlight trans-fat alternative and saturated-fat reduction technology to lipid crystallization. These two issues are the most significant challenges in the edible-application technology of lipids, and a key solution is lipid crystallization.

The text focuses on the crystallization processes of lipids under various external influences of thermal fluctuation, ultrasound irradiation, shear, emulsification and additives. Designed to be practical, the book’s information can be applied to realistic applications of lipids to foods, cosmetic and pharmaceuticals. This authoritative and up-to-date guide:

• Highlights cutting-edge research tools designed to help analyse lipid crystallization with the most current and the conventional techniques
• Offers a thorough review of the information, techniques and applications of lipid crystals
• Includes contributions from noted experts in the field of lipid crystals
• Presents cutting-edge information on the topics of trans-fat alterative and saturated-fat reduction technology

Written for research and development technologists as well as academics, this important resource contains research on lipid crystals which comprise the main functional materials employed in food, cosmetic and pharmaceutical industry.

• Language: English
• Publisher: Wiley
• Release date: Jan 24, 2018
• ISBN: 9781118593899

Book preview

1

Introduction: Relationships of Structures, Properties, and Functionality

Kiyotaka Sato

1.1 Introduction

This chapter presents a comprehensive sketch of the lipid species and functionality of lipid crystals present in various end products by outlining different stages of crystallization. In doing so, topics will be highlighted that will be elaborated further in chapters of this book. At the end of this chapter, a particular effort is made to relate trans‐fat alternative and saturated‐fat reduction technology to lipid crystallization because these two issues are the most significant problems in the edible‐application technology of lipids and one of the key solutions is lipid crystallization.

1.2 Lipid Species

Lipids are a class of compounds that contain long‐chain aliphatic hydrocarbons and their derivatives (O’Keefe 2008). There is a wide variety of lipid materials such as hydrocarbons, fatty acids, acylglycerols, sterols and sterol esters, waxes, phospholipids, plasmalogens, sphingolipids, and so on. Typical lipids whose crystallization properties have critical implications in food and other industries include hydrocarbons, fatty acids, alcohols, waxes, and acylglycerols. Because the lipid species of natural lipids of vegetable or animal resources vary from one to another, the understanding of the crystallization, melting, and physical properties must be based on the effects of major and minor lipid components included in every lipid material.

In this section, we take a brief look at the chemical structures of these typical lipid molecules.

1.2.1 Hydrocarbons

Hydrocarbons comprise a group of the simplest lipid molecules and are composed of hydrogen and carbon atoms. A typical molecular shape of hydrocarbons containing all saturated carbon–carbon bonds is expressed as CH3‐(CH2)n‐2‐CH3, in which n is the number of carbon atoms. Hereafter, we use nc as the number of carbon atoms in the all‐hydrocarbon chains. In nature, even‐numbered and odd‐numbered hydrocarbons occur, depending on whether nc is even or odd.

Molecular interactions operating among the hydrocarbon molecules are van der Waals forces, and these comprise the major molecular interactions among lipid molecules when they contain hydrocarbon chains as hydrophobic moieties. When the number of carbon atoms exceeds four, structural isomers occur (e.g., straight chains or branched chains). The straight‐chain hydrocarbons are called n‐alkanes as illustrated for n‐octadecane with nc=18 (Fig. 1.1a).

Image described by caption.

Fig. 1.1 Typical lipid molecules. (a) n‐Octadecane, (b) stearic acid, (c) oleic acid, (d) elaidic acid, and (e) triacylglycerol. In (a)–(d), carbon atoms are shown except for COOH groups for fatty acids. In (e), R is fatty acid moiety.

1.2.2 Fatty Acids

Fatty acids are formed by replacing one end of –CH3 in n‐hydrocarbons with a carboxyl group (–COOH). In contrast, dicarboxilic acids are formed when both end groups of –CH3 in n‐hydrocarbons are replaced with –COOH. There are saturated and unsaturated fatty acids, depending on whether double bonds are included and stereoisomers of cis or trans unsaturated fatty acids occur.

In nature, a wide variety of fatty acids is present, differing in nc, the number of double bonds having cis or trans conformations or the positions of the double bonds at the hydrocarbon chains. Similarly to hydrocarbons, even‐ and odd‐numbered fatty acids occur. The principal fatty acids abundantly occurring in nature are summarized in Table 1.1. Although standard (IUPAC) systematic names are given to fatty acids, the common names and abbreviations presented in the table will be used throughout this book.

Table 1.1 Systematic, common, and shorthand names of principal fatty acids.

As typical fatty acids having nc=18, stearic acid is a saturated fatty acid, oleic acid is a mono‐unsaturated fatty acid having a cis double bond at the 9–10 carbon atoms, and elaidic acid is a mono‐unsaturated fatty acid having a trans double bond at the 9–10 carbon atoms, as seen in Fig. 1.1(b, c, and d). The melting temperatures (Tm) of the three fatty acids in their most stable polymorphic forms are 69° C (stearic acid), 44° C (elaidic acid), and 16.1° C (oleic acid). This typically represents the relationships between Tm and the molecular shapes of the fatty acids in the following aspects.

At a fixed number of nc, Tm decreases with increasing numbers of double bonds, and the conformation of the double bonds changes from trans to cis.

As for saturated fatty acids, Tm increases with increasing nc, although the values of Tm for fatty acids with an odd‐numbered nc is a bit lower than those with an even‐numbered nc–1. For example, Tm of margaric acid nc=17 (palmitic acid, nc=16) is 61° C (63° C). This is ascribed to the instability of molecular packing at the lamellar interfaces, where CH3‐CH3 end groups are stacked against each other, of odd‐numbered fatty acids compared to that of even‐numbered fatty acids.

These relationships apply to other lipids containing fatty acid chains as their hydrophobic moieties.

The –COOH group is hydrophilic (water soluble), and the hydrocarbon chains are hydrophobic (oil soluble). Therefore, the hydrophobicity or hydrophilicity of a fatty acid molecule as a whole depends on nc. Fatty acids with nc ≤6 become water soluble, whereas they are sparingly water soluble when nc exceeds 6. Molecules having a hydrophobic moiety in one part and a hydrophilic moiety in another part are called amphiphilic, as revealed in other lipids: alcohols, mono‐ and di‐acylglycerols, phospholipids, emulsifiers, and so on.

1.2.3 Alcohols and Waxes

Alcohols are formed by replacing one –CH3 end of n‐hydrocarbons with –OH. Similarly to fatty acids, the alcohols become liphophilic as nc increases above 6, and even‐numbered and odd‐numbered alcohols occur.

There are narrow and broad categories of waxes. The former refers to the esters of long‐chain fatty acids and alcohols. The latter represents waxy matter abundantly occurring in nature as epidemic lipids, which include hydrocarbons, ketones, and aldehydes. Here we limit the waxes to the esters of long‐chain fatty acids and alcohols. The nc for constructing naturally occurring waxes vary widely from one wax to another. For example, candellila wax is made of fatty acids with nc = 16–34 and alcohols with nc = 22–34, whereas rice bran wax is made of fatty acids with nc = 16–32 and alcohols with nc = 24–38.

1.2.4 Acylglycerols

Acylglycerols are formed by esterification of the hydroxyls in glycerol molecules (CH2OH‐CHOH‐CH2OH) with fatty acids. Monoacylglycerols (MAGs), diacylglycerols (DAGs), and triacylglycerols (TAGs) are formed when one hydroxyl, two hydroxyls, or three hydroxyls, respectively, are esterified, as summarized in Fig. 1.2.

Image described by caption.

Fig. 1.2 Structure models of acylglycerols. (a) Stereospecific numbering of glycerol, (b) 1‐monoacyl‐sn‐glycerol, (c) 2‐monoacyl‐sn‐glycerol, (d) 3‐monoacyl‐sn‐glycerol, (e) 1,2‐diacyl‐sn‐glycerol, (f) 1, 3‐diacyl‐sn‐glycerol, (g) 2, 3‐diacyl‐sn‐glycerol, and (h) triacylglycerol. C*: chiral carbon; R, a fatty acid moiety; sn: stereospecific number.

TAGs (Fig. 1.1 e) are the principal lipids that construct animal adipose tissues, vegetable and edibles fats, and oils. The term used, fat or oil, depends solely on whether the TAG melts at room temperature (~25° C); at that temperature, fat is in a crystalline state and oil is in a liquid state. MAGs are intermediate products formed during enzymatic decomposition of TAGs during digestion. In addition, MAGs are industrially synthesized and used as emulsifiers because of their strong amphiphilic properties. DAGs are present as relatively minor components in natural oils and fats and are also industrially produced and used as edible fats and oils.

There is no chiral center in a glycerol molecule as seen in Fig. 1.2(a). However, it becomes chiral when for MAGs, a fatty acid is esterified either at the sn‐1 or at the sn‐3 positions (Fig. 1.2b and d), for DAGs, two fatty acids are esterified at the sn‐1 (or sn‐3) and sn‐2 positions (Fig. 1.2e and g) or different fatty acid moieties are esterified at the sn‐1 and sn‐3 positions (Fig. 1.2f), and for TAGs, the three fatty‐acid moieties are all different or different fatty acid moieties are esterified at the sn‐1 and sn‐3 positions (Fig. 1.2g). Instead of a numbering method using the sn‐positions, an alternative description using Greek letters has been employed, as in α‐monoacyl‐sn‐glyverol (1‐monoacyl‐sn‐glycerol), β‐monoacyl‐sn‐glycerol (2‐monoacyl‐sn‐glycerol), α, β‐diacyl‐sn‐glyverol (1, 2‐diacyl‐sn‐glycerol), α, α’‐diacyl‐sn‐glycerol (1, 3‐diacyl‐sn‐glycerol), etc.

Optical isomers can occur for chiral acylglycerols, and the mixing‐phase behavior of the chiral molecules affects the structural and physical properties in natural lipids when racemic mixtures are present.

TAGs can be simply described by using the abbreviated names of the fatty acids listed in Table 1.1. For example, we have tristearoylglycerol (SSS), 1,3‐dipalmitoyl‐2‐stearoyl‐ sn‐glycerol (PSP), and 1,3‐distearoyl‐2‐oleoyl‐ sn‐glycerol (SOS). Chiral TAGs can also be described by using the abbreviated names of the fatty‐acid moieties. For example sn‐POS is 1‐palmitoyl‐2‐oleoyl‐3‐stearoyl‐sn‐glycerol. An equal mixture of both stereoisomers of the chiral TAGs can be described as rac (e.g., rac‐POS), which means that there are equal amounts of sn‐POS and sn‐SOP.

Lipid species can be precisely described by highlighting the atomic‐level crystal structures in Chapter 2.

1.3 Physical States and the Functionality of Lipid Products

The crystallization and functionality of crystallized lipids are complicatedly influenced by the physical states where the lipids are crystallized, as seen in Fig. 1.3. Before going into the details of the crystallization in various physical states, which will be presented in forthcoming chapters, let us briefly view the relationship between the functionality of lipid products and the physical states presented in the figure.

Illustrations of the physical states of lipid products, namely, (top–bottom) liquid, bulk, gel, emulsion, and foam.

Fig. 1.3 Relationships between physical states and functionality of lipid products.

The liquid state simply refers to an oil phase, as represented by frying oil and biofuel, whose functionality is in heat transfer, viscosity, oxidation stability, and so on. The crystallization process in liquid‐state materials may occur as a deterioration of the end product (e.g., the clouding of cooking oils during storage in a refrigerator or precipitation causing an increase in the pouring point for biofuels at chilled temperatures). Therefore, retardation or prohibition of the crystallization of minor‐component lipids becomes critical in these products. Lubricants made of vegetable oils also require similar physical properties for optimum functionality.

The crystalline state in a bulk sample signifies that the major portion of the material is composed of lipid crystals, as typically represented in confectionery fat (chocolate). Fine particles of sugar, cocoa mass, and milk powder are suspended in the continuous phase of cocoa butter crystals, which comprise about 30 wt.% of the total mass of chocolate. Crispy touch, hardness, and sharp melting are typical functionalities of chocolate, which are mostly brought about by the lipid crystals comprising the major matrices of the products. Lipid crystal–based hard lipsticks require the functionalities of hardness, spreadability, gloss, anti‐sweating, and anti‐blooming of the products. Such properties are also determined by the network of lipid crystals, in which pigments, fragrance materials, and biologically active substances (vitamins, hormones, amino acids, etc.) are dispersed.

The gel state is defined as a two‐phase colloidal system consisting of solid components along with water (hydrogels) or oil (oleogels or organogels), in which the solid behavior prevails over the sol state. Oleogels may be defined as lipophilic liquids and solid mixtures in which solid lipid materials (gelators) with lower concentrations can entrap bulk liquid oil by forming a network of gelators in the bulk oil. The gelators can be grouped into two categories: self‐assembly systems and crystal‐particle systems. Water‐barrier films and soft lipsticks are typical products made of oleogels. The morphology, size, density, and crystal networks of lipid crystals are the dominant factors that influence the physical functionalities of the gel state, such as hardness/softness and spreadability.

An emulsion is defined as a two‐phase colloidal system consisting of water and oil along with emulsifiers that reduce the water–oil interfacial energy. There are two types of emulsions, water‐in‐oil (W/O) and oil‐in‐water (O/W). Butter, margarine, and spread (W/O) and whipped (O/W) systems are typical emulsion systems consisting of lipid crystals, in which the physical properties of the emulsion, such as the spreadability, texture, and stability, are influenced by the lipid crystals present in the continuous phase of the W/O emulsion or in the dispersed phase of the O/W emulsion. Both the W/O and O/W emulsions are widely employed in the food, cosmetics, and pharmaceutical industries. In particular, nanometer‐sized lipid droplets are employed as carrier systems for poorly water‐soluble drugs.

Aerated colloidal systems, also known as foams, are widely applied in the cosmetics, food, and porous material production industries. Foams have the significant advantages of shape retention, soft texture, the ability to act as a thermal barrier, and low calorie content. Aqueous foams contain air bubbles in a continuous aqueous phase, like whipped cream and ice cream. Nonaqueous foams are formed by dispersing air bubbles in oil phases and are important for foamed plastics, whipped butter, and confections. In both cases, the dispersibility and stability of air bubbles are major functionalities that are partly governed by the lipid crystals surrounding the air bubbles together with other ingredients such as proteins and starches.

In the lipid crystal–based products displayed in Fig. 1.3, the lipid crystals play critically important roles in revealing the firmness, gloss, melting/crystallization, texture, rheology, and stabilization of water droplets (W/O emulsion) and air cells (foams) by themselves alone or together with emulsifiers, proteins, starch, and so on.

1.4 Formation Processes of Lipid Crystals

The basic principles underlying the formation processes of lipid structures are common to the physical states displayed in Fig. 1.3, including the microscopic and macroscopic features in Fig. 1.4. Polymorphic structures and primary particles of lipid crystals comprise the microscopic features, whereas the formation of flocs and networks of lipid crystals determines the macroscopic features.

Diagram illustrating the flow of external factors affecting formation of lipid crystals, from molecules (polymorph) to primary particles, to flocs, to crystal network, to macroscopic structures.

Fig. 1.4 External factors affecting formation processes of lipid crystals.

The molecular structures of lipids are revealed in polymorphism and primary‐particle formation. Polymorphism remarkably influences the macroscopic properties of fat products. For example, there are three polymorphic forms in TAG crystals, α, β’, and β. In margarines and fat spreads, lipids are first crystallized in the least stable form (α) by rapid cooling of the molten materials. However, the α crystals are very short‐lived and do not exist in the finished products, in which metastable β’ crystals are formed as the most desired polymorphic form. This is because β’ crystals are relatively small and can incorporate a large amount of semi‐solid oil phases and water droplets within the crystal network. Thermodynamic stabilization, however, causes the transformation from the metastable β’ form to the most stable β form during storage or other shelf‐life conditions. The β crystals tend to grow into large needle‐like agglomerates, which results in a sensation of sandiness in the mouth. In contrast, cocoa butter in chocolate should be crystallized in a β polymorph (more correctly, Form V of a β‐type polymorph, see Chapter 3) because of its high density and optimal melting point, resulting in the desired sharp melting of chocolate. As β crystals crystallize too slowly compared with the α and β’ forms, the use of a special processing of crystallization called tempering is necessary for producing cocoa‐butter‐based chocolate.

External factors can produce many of the desired microscopic features of lipid crystals, and knowledge of the relationship between their molecular structures, their particle formation along different dimensions, and their spatial networks under internal and external factors gives us optimal ways of designing materials with the desired functionality. Typical factors that have already been applied, or have high potential to be applied, to the actual industrial processing include the following.

Internal factors

Interesterification (chemical, enzymatic)

Fractionation (dry, solvent, detergent)

Blending

External factors

Intentionally varying the temperature

Applying shear

Applying hydrostatic pressure

Adding foreign materials (additives)

Applying ultrasound waves

Encapsulating of lipids into small droplets (O/W emulsion)

These external factors are thoroughly discussed in this book.

The details of the formation of lipid crystal networks vary from one physical state to another. For example, crystallization in a bulk sample proceeds without the effects of oil–water interfaces, whereas interfacial crystallization in the O/W and W/O emulsion states plays a critical role in creating the lipid crystal network (see Fig. 1.3). The basic streams, however, of the formation of a lipid crystal network can be drawn as in Fig. 1.5, which includes the formation of crystal nuclei (nucleation), the subsequent growth of crystal nuclei (crystal growth), the aggregation of crystal particles, and the formation of a crystal network (network formation). All of these processes should be enabled only when a given set of external conditions (e.g., temperature, pressure, and concentration) provides the driving forces for crystallization as expressed by supercooling or supersaturation. Supercooling (ΔT) is defined as the difference in temperature between the melting point (Tm) and the crystallization temperature (Tc), that is, ΔT = Tm – Tc. Supersaturation (S) is defined as the ratio of the actual solute concentration X in solution to the solubility (Xs) at T = Tc, that is, S = X/Xs. The former refers to crystallization from neat liquid (melt), and the latter to crystallization from solution.

Image described by caption and surrounding text.

Fig. 1.5 A model of formation processes of lipid crystal network. (a) Nucleation, (b) crystal growth, and (c) network formation.

1.5 Polymorphism

Almost all lipids possess two or more different crystal structures under a given set of thermodynamic conditions. This multiplicity of crystalline structures of the same substance is called polymorphism.

The polymorphic behavior of lipid crystals is basically determined by their molecular structure, thermodynamic stability, and phase transformations. The thermodynamic stability of polymorphic forms is illustrated by the relationship of their Gibbs energy values, G = H – TS, where H, S, and T are the enthalpy, entropy, and temperature. Polymorphic forms with greater G values are less stable than those with lower ones, which have higher solubility and lower melting points.

Polymorphic transformations occurring during and after crystallization are also quite important. Two types of transformations can occur from less stable forms to more stable polymorphic forms (e.g., from α or β’ forms to β’ or β forms for TAGs). Solid‐state transformation occurs when the metastable form is stored below its melting temperature in the crystalline state. Another type of polymorphic transformation is melt‐mediated transformation, which occurs as the temperature rises just above the melting point of a metastable form, where melting of the metastable form and successive crystallization of more stable forms occur.

Figure. 1.6 summarizes the elementary processes of the polymorphic crystallization of lipids. We may consider that the nucleation and crystal growth are relatively straightforward in accordance with the theory of nucleation and crystal growth.

Left: Flow from driving force to crystallization and polymorphic transformation. Right: Schematics of monomer, cluster, and nucleus (top) and kink and step in crystal (middle); and graph of Gibbs energy over T.

Fig. 1.6 Elementary processes of crystallization of lipids.

Complicated events, however, must occur during the formation of lipid crystal networks in the actual production stages of the lipid products because the methods of distribution and aggregation of the crystal particles differ greatly from those occurring in the initial stages of nucleation and crystal growth. Network formation may be affected by the following processes.

Nucleation and crystal growth to form primary particles, in which tiny crystals having different sizes and polymorphic forms are present. In addition, the multiple lipid components comprising the lipid products are mixed either in miscible or immiscible phases, depending on the molecular shapes of the lipid components and crystallization.

Recrystallization of primary crystal particles through Ostwald ripening, polymorphic crystallization, and transformation, as well as variations in the mixing behavior and successive crystallization of different lipid materials.

Particle–particle interactions including sintering (Fig. 1.5c) may lead to the formation of crystal networks.

One must recall that lipid materials are produced in factory‐scale machines under external factors, which particularly affect the nucleation and crystal growth. Recrystallization proceeds during the aging period between factory‐scale production and storage in warehouses.

1.6 Aging and Deterioration

The principal flows in the production of lipid materials may be simply drawn as in Fig. 1.7. Raw materials of lipids and water, salts, sugar, protein, starch, and emulsifiers are mixed and then melted or dissolved at elevated temperatures. They are then cooled or evaporated to cause lipid crystallization, which usually is conducted under stirring or shearing conditions for efficiency of heat exchange, emulsification, and aeration. After the dynamic production process has ceased, the lipid materials are kept in storage at optimal temperature ranges over certain periods (days to months) before releasing them into the markets.

Flow diagram of production of lipid materials from mixing raw materials and then melted or dissolved, to cooling, stirring or shearing, to storage of products.

Fig. 1.7 Flows of production of lipid materials.

Throughout the processes shown in Fig. 1.7, including the commodity circulation of final products at the consumer end, the roles of physical and chemical control may be summarized as follows: sustaining the high value of raw materials, stabilizing the final products, and revealing the functionality of the products.

From the viewpoint of stabilization, it is worthy to note that almost all lipid products are actually in thermodynamically metastable conditions when they retain highly functional properties. In contrast, conversions into thermodynamically stable conditions lead to degradation of the functionality, so this stabilization must be prohibited. The conversion proceeds in accordance with thermodynamic laws, so it is impossible to ultimately shut it off; but practical technology is used to retard the conversion as much as possible by proper methods.

For example, chocolate is in a thermodynamically metastable condition because fine particles of ingredients (e.g., cacao mass, sugar, and others) are dispersed in the crystal networks of cocoa butter and other specialty fats (i.e., suspension). The metastability of chocolate is revealed in many aspects. Specifically, the fine particles of ingredients and fat crystals have sizes ranging from submicrons to several tens of microns, and the fats are not simple components but rather are mixtures of different fat components that differ in melting temperatures. Furthermore, the fat crystals in chocolate are in metastable polymorphic forms (e.g., Form V of cocoa butter and β’ of cocoa butter substitute [CBS]). Stabilization may lead to Ostwald ripening, which causes the growth of large particles at the expense of small particles (coarsening) during long‐period storage. Different fat fractions can separate when the mixtures of component fats are eutectic. Furthermore, polymorphic transformations from Form V to Form VI of cocoa butter or β’ to β of CBS may cause fat blooming. These degradations cause inferior mouth feel, loss of gloss, and so on.

Another example is W/O emulsions like margarine and fat spread, which are in metastable states, as water droplets are forced to disperse in the semi‐solid fat phase with the aid of emulsifying reagents by applying a high shear force (emulsification). Also, the fats have multiple components with crystals in metastable β’ polymorph form. When the measures to sustain such metastable condition are not used, various degradations occur (e.g., coalescence of water droplets, separation of oil and water phase in an extreme case, or granulation of fat crystals, which causes a loss of gloss, decrease in spreadability, etc.).

In any case, metastability and stability during production, commodity circulation, and consumption stages vary from one product to another. However, one may consider the stabilization of lipid products in terms of the diagram of free energy and time given in Fig. 1.8.

Graph of free energy over time displaying a descending wave with circles labeled A, B, and C on its troughs. Downward arrow from A to B depicts aging and B to C from deterioration.

Fig. 1.8 Aging and deterioration of lipid products. A and B are metastable states, C is the most stable state.

There are two stages of a metastable state, A and B. A is the stage between factory production and storage, and B is the stage between storage and circulation in the commodity. Both are metastable, but A is less stable than B. Optimal functionality of the products must be revealed in stage B. However, the most stable state is C, where degradation begins to occur because of stabilization.

We may think that aging is the process of transforming from A to B, and that deterioration corresponds to the transformation from B to C. Technological innovation focuses on how to promote the aging and retard the deterioration. Therefore, the physical properties of lipid crystals affecting the microscopic and macroscopic mechanisms of aging and deterioration in lipid products should be clarified.

In this respect, our particular concerns are focused on the kinetic processes involved in forming lipid crystal networks as revealed in the changes from A to B, which follow nucleation and crystal growth under various external factors noted in Fig. 1.5 (a and b).

To summarize, the crystallization of lipids that occurs in the various production stages is quite dynamic (e.g., the time‐size scales differ greatly from one process to another, as shown in Table 1.2). For example, achieving the driving forces for crystallization (e.g. supercooling) may require seconds to minutes, depending on the size of the production system and the rate of cooling. Nucleation begins soon after the driving force is achieved, and the sizes of the crystal nuclei may be on the order of several to several tens of nanometers. Crystals may then grow in several to several tens of minutes. The formation of lipid crystal networks may take longer during aging, typically hours to days, and deterioration such as fat bloom formation in chocolate occurs in weeks to months, depending on the temperature and its fluctuations during the distribution and consumption stages.

Table 1.2 Time‐size scales of crystallization processes.

1.7 Trans‐Fat Alternative and Saturated‐Fat Reduction Technology

Trans‐fats can be defined as fats containing trans‐unsaturated fatty acids, in which the local conformation of hydrocarbon chains at the double bonds is trans‐type, as illustrated for elaidic acid in Fig. 1.1(d). Trans‐fats are rather uncommon in nature except for meat and dairy products from ruminants, but they are produced industrially as partially hydrogenated oils (PHOs) from liquid oils containing cis‐unsaturated fatty‐acid moieties. Trans‐fats have been used in margarine, snack foods, and confections since the 1950s. However, recent nutritional studies have claimed that, depending on the method and quantity of its intake, trans‐fat can be associated with increased risk of coronary heart disease. Therefore, reduction of trans‐fat has been a major concern of national and local governments, consumer organizations, and private enterprises (Kodali 2014; Wang et al. 2016). In addition, the intake of saturated fatty acids (SAFAs) and the reduction of SAFAs have also been a critical nutritional concern, although debates about SAFA‐related issues are still continuing.

We do not intend to discuss the nutritional issues surrounding trans‐fat and SAFAs in this chapter, but we do take a closer look into the necessity to cope with the issues around trans‐fat and SAFAs from the viewpoint of lipid crystallization technology.

There are rational reasons for the long‐time use of trans‐fat: it exhibits such high functionality when employed for edible applications, it is low cost, easily produced, β’‐tending and β’‐stable crystals, fine crystal network formation, high rates of crystallization, sharp melting behavior, and high oxidation stability. Therefore, so‐called "trans‐fat alternative" technology must satisfy the following requirements.

Functionality

Maintaining flakiness, firmness, crispiness, melting, and appearance

Stabilization of end products (e.g., anti‐oxidation)

Availability

Easily available

Smooth processing

Economics

Not highly expensive

Along these lines, typical examples of trans‐fat alternative technology are listed.

Zero‐trans‐fat resources

High‐oleic sunflower, high‐oleic canola, high‐oleic soybean

Semi‐solid fats (palm, palm kernel, coconuts, fully‐hydrogenated fats)

Organogels (low‐molecular/macromolecular organogelators)

Zero‐trans fat resources + processing

Molecular design (esterification)

Zero‐trans fats + processing+ reduced‐SAFA

Blending of high‐oleic soybean/palm/fully‐hydrogenated fats

Algal oil (high oleic >90%)

Fractionation of palm oil, palm kernel oil, coconut oil, high‐oleic high‐stearic sunflower oil, etc.

Esterification (high‐oleic soybean + palm fraction, etc.)

Efficient uses of additives

Emulsifiers

Similarly to trans‐fat alternative technology, SAFA‐reduction technology will have to satisfy the following requirements.

Maintaining functionality with reduced SAFA

Firmness and gloss

Melting and crystallization

Texture, rheology, and spreadability

Stabilization (water droplets, air cells)

Production conditions

Availability

Economics

Minimum changes in processing

Limitations

Hardness of chocolate

Softness of cookies

Along these lines, typical examples of SAFA‐reduction technology are as follows.

Increasing oleic‐acid moiety

High‐oleic sunflower, canola, and soybean

Oleic‐rich TAGs (POO, SOO, OPO, OSO, etc.)

SAFA‐alternative materials

Organogels (see above)

Fat replacers

Increasing starch

Modifying crystallization conditions

(See above.)

To summarize, we are confident that research on lipid crystallization will play a critically significant role in trans‐fat alternative and SAFA‐reduction technologies. Many chapters of this book share the same aims and desires.

References

Kodali, D. R. (ed.) (2014) Trans Fats Replacement Solutions. Urbana, IL, AOCS Press.

O’Keefe, S. F. (2008) Nomenclature and classification of lipids. In: Akoh, C. C., Min, B. D. (eds.) Food lipids: Chemistry, nutrition and biotechnology, 3rd ed. Boca Raton, FL, CRC Press, pp. 3–37

Wang, F. C., Gravelle, A. J., Blake, A. I., & Marangoni, A. G. (2016) Novel trans fat replacement strategies. Current Opinion in Food Science. 7, 27–34.

2

Polymorphism of Lipid Crystals

Kiyotaka Sato

2.1 Introduction

Many lipid crystals possess two or more different structures, categorized as polymorphism and polytypism, under a given set of thermodynamic conditions. The polymorphic behavior of lipid crystals is basically determined by the molecular structure, thermodynamic stability, and phase transformation. This chapter begins with a basic discussion of the thermodynamic stability of polymorphic forms and their transformation behavior. The polymorphic structures and transformation of fatty acids and acylglycerols will then be discussed.

2.2 Thermal Behavior of Polymorphic Transformations

The thermodynamic stability of polymorphic forms is illustrated by the relationship of their Gibbs energy values, G = H – TS, where H, S, and T are enthalpy, entropy, and temperature, respectively. One can usually determine the G‐T relationship by measuring the temperatures and enthalpy of the polymorphic transformation and the temperature variation of the solubility of the polymorphic forms. Polymorphic forms with greater G values are less stable than those with lower values, which have higher solubility values and lower melting points. The general behavior of polymorphic‐phase transformations of lipids can be discussed for two polymorphs, A and B, whose G‐T relationships are depicted in Fig. 2.1.

Graph of thermodynamic stability of 2 polymorphic forms of A and B, displaying enantiotropic (left) and monotropic (right) behavior. Each graph has 3 intersecting lines labeled A, B, and L.

Fig. 2.1 Thermodynamic stability of two polymorphic forms of A and B, showing (a) enantiotropic and (b) monotropic behavior.

Based on Fig. 2.1, possible pathways of crystallization and subsequent transformations during cooling and heating of the melt phase are revealed by the differential scanning calorimetry (DSC) cooling and heating patterns illustrated in Fig. 2.2. For simplicity, crystallization and transformation in the solution phase are not fully discussed except for special cases.

Enantiotropic Polymorphism: Form A (B) is a high‐melting (low‐melting) polymorphs because the G value of A is smaller than that of B around the melting temperatures. The G values, however, are reversed in lower‐temperature ranges, making a cross point at TA‐B.

Case 1: No Solid‐State Transformation between A and B. Form A crystallizes by cooling below Tm(A) and remains kinetically stable in low‐temperature ranges where B is thermodynamically stable, then A melts at Tm(A), which is pattern 1‐1‐1. This apparently means no polymorphism because of steric hindrance prohibiting solid‐state transformation between A and B. However, B can occur in the solution phase because of solution‐mediated transformation.

Case 2 No Solid‐State Transformation from A to B below TA‐B, with Solid‐State Transformation from B to A below TA‐B. Two possibilities arise. First, A crystallizes by cooling below Tm(A) and transforms to B below TA‐B during exothermic heating, B then transforms to A at TA‐B with endothermic heating, and finally A melts at Tm (A), which is pattern 1‐1‐2. Second, A crystallizes by cooling below Tm (A) and transforms to B below TA‐B during exothermic heating. B then does not transform to A at TA‐B but melts at Tm (B), and A crystallizes soon after the melting of B (melt‐mediated transformation). Finally, A melts at Tm (A), which is pattern 1‐1‐3.

Case 3 Solid‐State Transformation between A and B at All Temperature Ranges. Two exothermic peaks (crystallization of A and transformation from A to B) are formed during cooling, solid‐state transformations between B and A occur around TA‐B, and A melts at Tm (A), which are patterns 1‐2 and 1‐2‐1. In this case, hysteresis phenomena may appear as supercooling and superheating for the solid‐state transformations from A to B below and from B to A above TA‐B, respectively, when serious steric hindrance retards the transformation. In this case, the dynamic molecular rearrangement of lipid molecules must be accompanied by the transformations.

Monotropic Polymorphism: Form A (B) is a low‐melting (high‐melting) polymorph because the G value of A is always larger than that of B at all temperature ranges. In this type of polymorphism, the crystallization and transformation behavior largely depend on how crystallization occurs during the cooling process. As a general tendency, the metastable form A crystallizes at a very high rate of cooling, whereas the most stable form B crystallizes at a very low rate of cooling. Cooling at moderate rates causes concurrent crystallization of A and B. Fig. 2.3 illustrates various DSC patterns of this type of polymorphism. Many acylglycerol crystals exhibit monotropic polymorphism, and their crystallization and transformation pathways can be recognized in the patterns in Fig. 2.3.

Case 1: Crystallization of Low‐Melting Form A. The DSC cooling pattern of the crystallization of form A is shown in pattern 2‐1. Two types of transformation may occur with heating, depending on whether or not the solid‐state transformation from A to B occurs. If there is no solid‐state transformation from A to B, A melts at Tm (A) soon after B crystallizes (melt‐mediated transformation) and B melts at Tm (B), which is pattern 2‐1‐1. By contrast, if A transforms to B above Tm (A) with exothermic heating, B melts at Tm (B), which is pattern 2‐1‐2.

Case 2: Concurrent Crystallization of A and B. Two exothermic DSC peaks appear, which is pattern 2‐2, and two types of transformation may occur on heating. A melt‐mediated A–B transformation occurs at Tm (A) and B melts at Tm (B), when there is no solid‐state transformation from A to B, which is pattern 2‐2‐1. If A transforms to B above Tm (A) with exothermic heating, B melts at Tm (B), which is pattern 2‐2‐2.

Case 3: Crystallization of Low‐Melting Form A. A single exothermic peak and a single endothermic peak of crystallization and melting of A appear as seen in patterns 2‐3 and 2‐3‐1. It should be noted here that a single melting peak of B appears when the first‐crystallized A transforms to B by isothermal incubation below Tm (A) without explicit exothermic DSC peaks. In this case, cooling DSC patterns 2‐1 and 2‐2 may result in the heating DSC pattern of 2‐3‐1 when the isothermal stabilization is completed.

Graphical illustration of hypothetical DSC cooling (left) and heating (right) curve patterns of two polymorphs of A and B grown from melt phase, revealing enantiotropic polymophism.

Fig. 2.2 Hypothetical DSC cooling and heating patterns of two polymorphs of A and B grown from melt phase, revealing enantiotropic polymophism.

Graphical illustration of hypothetical DSC cooling (left) and heating (right) curve patterns of two polymorphs of A and B grown from melt phase, revealing monotropic polymophism.

Fig. 2.3 Hypothetical DSC cooling and heating patterns of two polymorphs of A and B grown from melt phase, revealing monotropic polymophism.

The melt‐mediated transformation explained here has technological significance because the tempering process performed during fat crystallization in food engineering corresponds to a melt‐mediated transformation. Furthermore, a solution‐mediated transformation also occurs from A to B below Tm(A)—see Fig. 2.1(b)—even when steric hindrance prohibits a solid‐state transformation and the two forms are present in nearly saturated solution. This transformation has practical significance in the formation of granular crystals in fat crystal networks when minor low‐melting ingredients are melted at elevated temperatures and form an oil matrix in which fat crystals are embedded. Crystallization of a high‐melting (low‐solubility) polymorph can increase at the expense of low‐melting (high‐solubility) polymorph in the oil matrix. The driving forces for this type of solution‐mediated transformation are the differences in the solubilities of the two polymorphs. The solution (oil)‐mediated transformation may act as a partial role of fat‐bloom formation in cocoa butter–based chocolate (Sato & Koyano 2001).

2.3 Molecular Properties

2.3.1 Subcell and Chain‐Length Structures

It is necessary for us to define the technical terms used to characterize the crystal structures that are applied to most lipid crystals.

A unit cell is the smallest group of atoms or molecules comprising a crystal. Repetition of unit cells at regular intervals in three dimensions can construct the crystal lattices of a material. The number of molecules included in a unit cell and the dimension of the unit cell are expressed by Z and lattice parameters (a, b, c, α, β and γ).

An asymmetric unit is a part of a unit cell. We can construct a complete unit cell with an asymmetric unit using the symmetry of the space group.

A crystal system refers to a class of space group that is divided into seven groups: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. Actually, triclinic, monoclinic, orthorhombic, and hexagonal crystal systems often occur in lipid crystals.

Having clarified the crystallographic definitions, let us define fundamental concepts about the molecular properties of lipid crystals by taking as an example the TAG (1, 3‐dipalmitoyl‐2‐oleoyl‐glycerol [POP]) presented in Fig. 2.4. POP is one of the most complicated lipid substances because it is a saturated–unsaturated mixed‐acid TAG. Therefore, using POP as an example of lipids, we may discuss most of the important concepts about the molecular properties of lipid crystals.

Diagram defining molecular structures of triacylglycerol: POP, with lamella, palmitic, and oleic represented by bars as symbols for long‐chain molecules. Subcell, glycerol group, and double bond are labeled.

Fig. 2.4 Definition of molecular structures of a triacylglycerol: POP.

TAG molecules and other lipid crystals are often indicated by bars as symbols representing long‐chain molecules (Fig. 2.4a). Because of the strong interactions of the molecules along lateral directions compared with those along longitudinal directions, TAGs and other lipid crystals construct layered structures. This principle is applied to POP. The term lamella refers to laterally packed POP molecules. We can construct the entire structure of a POP crystal by stacking unit lamellae as depicted in Fig. 2.4(a). The thickness of the lamellae is revealed in X‐ray diffraction (XRD) long‐spacing patterns, which appear in small‐angle diffraction patterns using powder crystal samples.

In the most stable polymorphic form of POP, a lamella is composed of POP molecules whose palmitic acid chains and oleic acid chains are placed in different lamellar planes (Fig. 2.4b). This is because steric hindrance among straight palmitic acid chains and bent oleic acid chains does not allow them to lie within the same lamellar plane. To differentiate each fatty‐acid chain in the lamella, the term leaflet represents a sublayer composed of single fatty‐acid chains (Fig. 2.4c). The lamellar structure of POP depicted in Fig. 2.4(c) is composed of three leaflets. Glycerol groups are placed at the interior parts of the lamellae, and the interfaces among different lamellae are made of methyl end groups. Polar groups are placed in the interior part.

In lipid crystals containing unsaturated fatty‐acid chains, the number, position, and conformation (cis or trans) of the double bonds are the primary factors that influence their physical and chemical properties. In an oleic acid leaflet of POP, a double bond having cis conformation is at the central part of the leaflet. The hydrocarbon chains of the oleic acid leaflet are divided into two segments by introducing the double bond. In the case of a mono‐unsaturated fatty acid like oleic acid, the chain segment between the CH3 group and the double bond is called the ω‐chain, and the other side between the double bond and polar groups (hydroxyl, carboxyl, and glycerol groups) is called the Δ‐chain, as noted in Fig. 2.4(c).

Subcell structures are defined as the cross‐sectional packing mode of the zigzag hydrocarbon chains used to characterize the molecular packing, as illustrated in Fig. 2.4(c) and Fig. 2.5(a). More than nine types of subcell structures have been identified in lipids (Pascher et al. 1992; Dorset 2005), and the seven subcells depicted in Fig. 2.5(a) are often found in lipid crystals.

Schematics of subcell (H, O⊥, and O’⊥) and chain‐length (T//, M//, O//, and O’//) structures, and chain inclination of lipid crystals (single, inter digitated, double, triple, quarto, hexa, and lamellar interface).

Fig. 2.5 (a) Subcell structures, (b) chain‐length structures, and (c) chain inclination of lipid crystals.

Because the subcell structures refer to local arrangements of hydrocarbon chains, the symmetry of the subcell and the unit cell can be different. For example, an orthorhombic perpendicular subcell (O┴) and a triclinic parallel subcell (T//) can be formed in a crystal having a monoclinic crystal system.

All of the hydrocarbon zigzag planes are parallel in T// and are thought to be the densest packing of aliphatic chains. The O┴ subcell consists of zigzag planes that are perpendicular to the planes of its neighbors. Two subcells, O’// and M//, contain parallel‐arranged hydrocarbon zigzag chains. There is also a hexagonal (H) subcell in which the hydrocarbon chains do not assume a specific orientation. Instead, they undergo torsional motion with an aliphatic gauche conformation, making the H subcell less stable than the others.

There is diversified polymorphism in acyglycerol crystals, whose independent polymorphic forms are called α, β’, and β, based on the subcell structures (e.g., α, β’, and β possess H, O┴, and T//, respectively). The subcell structures are revealed in XRD short‐spacing patterns, as indicated for triacylglycerol crystals in Fig. 2.6. In addition to α, β’, and β, sub‐α appeared in acyglycerols when α was cooled, having XRD short‐spacing peaks of 0.37–0.38 nm and 0.41–0.42 nm, as reported for tristearoy‐glycerol (SSS; Akita et al. 2006), and 1, 3‐distearoyl‐2‐oleoyl‐glycerol (SOS) and 1, 3‐dioleoyl‐2‐stearoyl‐glycerol (OSO; Yano et al., 1999). A chain‐length structure comprises a repetitive sequence of the leaflets involved in unit lamella along the long‐chain axis, as seen in Fig. 2.5(b). This property is critically important for TAG crystals because a TAG may contain fatty‐acid moieties whose chemical structures largely differ from each other (e.g., long‐chain and short‐chain or saturated‐chain and unsaturated‐chain).

Top: X‐ray diffraction short spacing patterns of α, β’, and β with peak values indicated. Bottom: Subcell structures of α, β’, and β poylmorphs of triacylglycerol crystals with dimensions indicated.

Fig. 2.6 (a) X‐ray diffraction short spacing patterns and (b) subcell structures of α, β’, and β poylmorphs of triacylglycerol crystals. Unit, nm.

A single chain‐length structure is a lamella composed of one leaflet. A double chain‐length structure is formed when the chemical natures of the acyl chains are the same or very similar. Double means that two leaflets are present within a unit lamella together with glycerol groups. A triple‐chain‐length structure having three leaflets is formed when the chemical nature of one or two of the three hydrocarbon chains largely differ from the others. The POP in Fig. 2.4(c) is a triple chain‐length structure. Higher‐ordered chain‐length structures of quarto‐chain‐length and hexa‐chain‐length can occur. One unique chain‐length structure is called interdigitated, in which methyl end groups and polar groups are alternatively placed in the same lamellar plane side by side. The interdigitated structure is found in the crystals of cis‐unsaturated fatty acid (oleic acid β form) and mixed‐acid TAGs (see below).

The chain inclination is defined as the angle between the long‐chain axis and the lamellar interface (angle of gradient; see Fig. 2.5c). The chain inclination is often different among the polymorphic forms of the lipid as a result of the combined effects of the subcell structures and the methyl end stacking. When single‐crystal structure analysis cannot be done, the degree of the chain inclination can be assessed by dividing the XRD long‐spacing value by the total chain length. In TAG crystals, for example, the TAG molecules are arranged normal to the lamellar interface in α, whereas the angle of gradient becomes small in β’ and β.

2.3.2 Conformation of Hydrocarbon Chains

The molecular packing of the lipids in crystals largely influences their physical properties (e.g., the density of crystals, melting point, and enthalpy and entropy of melting and crystallization, and thereby the rate of nucleation, etc.). The conformation of hydrocarbon chains is defined by a rotation angle (τ) as illustrated in Fig. 2.7. The rotation around the C‐C axis of CH2(1) and CH2(2) noted by a dotted arrow in Fig. 2.7(a) is defined by the angle of rotation (τ) of the C‐C bond between CH2(2) and CH2(B) with respect to the C‐C bond between CH2(A) and CH2(1). The conformations are defined as follows: cis (C) for τ = 0°, ±gauche (G and G’) for = ±60°, ±skew (S and S’) for = ±120° and trans (T) for =180°. The values of τ observed in real lipid crystals differ slightly from the standard values shown because of the bond rotation energy, which varies from one structure to another.

Image described by caption and surrounding text.

Fig. 2.7 (a) Hydrocarbon chains and (b) rotation angle (τ) around C‐C bond between CH2(1) and CH2(2)

In saturated fatty‐acid chains, the conformations of cis and ± skew are too unstable to occur under usual conditions. The conformation of ± gauche (trans) is metastable (stable). In the all trans conformations, the zigzag chain of carbon atoms makes a planar structure. The increase in gauche conformation, however, makes the hydrocarbon chains more flexible and disordered, causing the hydrocarbon chains to be nonplanar structures. Deviations from the planar trans conformation are indicated by defects appearing in the molecular chains. Fig. 2.8 illustrates how hydrocarbon chains are converted from planar to nonplanar conformations by introducing gauche conformations.

Schematics of molecular conformation defects in hydrocarbon chains: all trans (left), end gauche (middle left), kink (middle right), and double‐end gauche (right). Closed circles represent carbon atoms.

Fig. 2.8 Molecular conformation defects in hydrocarbon chains. (a) All trans, (b) end gauche, (c) kink, and (d) double‐end gauche. Closed circles, carbon atoms.

In cis‐unsaturated fatty‐acid chains, there are different conformations of hydrocarbon chains adjacent to the double bond, expressed as skew‐cis‐skew’ (S‐C‐S’), skew‐cis‐skew (S‐C‐S), and trans‐cis‐trans (T‐C‐T; Fig. 2.9). The conformations of S‐C‐S’ and S‐C‐S are considered to be stable. Although these two conformations cause the bent geometry of the hydrocarbon chain, there are clearly different forms in which the ω chain and Δ chains are placed in the same plane in the S‐C‐S' conformation and normal to each other in the S‐C‐S conformation. The two conformations were widely observed in oleic acid, erucic acid, and other mono‐unsaturated fatty acids, whereas the T‐C‐T conformation is unique and was observed in the most stable polymorph of oleic acid (see below).

Schematics of the different conformations of hydrocarbon chains adjacent to the double bond, expressed as skew-cis‐skew’ (S‐C‐S’), skew‐cis‐skew (S‐C‐S), and trans‐cis‐trans (T‐C‐T).

Fig. 2.9 Conformations of hydrocarbon chains adjacent to a cis double bond, viewing normal direction of hydrocarbon chains.

2.3.3 Glycerol Conformations

The conformations of the glycerol groups in the crystals of acyglycerols are important in understanding the molecular arrangements, chain‐packing modes, and lateral interactions of lipids in biomembranes (Pascher 1996). In addition, one may assume that they are strongly related to the occurrence of different subcell structures in the acylglycerol crystals. To date, two types of glycerol conformations have been observed in TAG crystals: the tuning fork and the chair (Fig. 2.10).

Schematic of 2 types of glycerol conformations in TAG crystals: the “tuning fork” and the “chair.” Dots, circles, and zigzag lines represent carbon atom, oxygen atom, and fatty‐acid moieties, respectively.

Fig. 2.10 Conformations of glycerol groups (●, carbon atom; ○, oxygen atom; zigzag line, fatty‐acid moieties).

In the tuning fork conformation, hydrocarbon chains at the sn‐1 and sn‐3 positions connected to a glycerol group are packed in the same direction with respect to the glycerol group, and the hydrocarbon chain at the sn‐2 position is arranged in the opposite direction. The term tuning fork was coined by Jensen and Mabis (1963) when they reported the crystal structure of TAG (the β form of tricapriroyl‐glycerol) for the first time. In the chair conformation, the hydrocarbon chains at the sn‐1 and sn‐2 positions are packed in the same leaflet, and the hydrocarbon chain at the sn‐3 position is arranged in the opposite direction.

The direction of the C1‐C2‐C3 bond of a glycerol group is parallel to the lamellar interface in the tuning fork conformation, whereas it makes a right angle in the chair conformation. This indicates that there is no general rule about how two types of glycerol conformation will occur, though previous studies do indicate that mono‐acid TAGs and symmetric mixed‐acid TAGs exhibit the tuning fork conformation and that asymmetric mixed‐acid TAGs exhibit the chair conformation (see below).

2.3.4 Polytypism

In addition to polymorphism, there is another higher‐order structural variation called polytypism, which is caused by a different stacking sequence of lipid layers. Fig. 2.11 illustrates two polytypes observed in fatty‐acid crystals having the polymorphic structure of a monoclinic system.

Structure models of polytypism, illustrated by parallel lines (left) and zigzag lines (right) for Mon and orth II, respectively. 2 Double-headed arrows of different sizes represent unit repeating lamellaes.

Fig. 2.11 Structure models of polytypism. An arrow means a unit repeating lamellae.

In Mon, the layers having a monoclinic crystal system are simply stacked in such a way that the chain axes of the fatty‐acid molecules in the adjacent layers are arranged in the same direction. By contrast, the chain axes of the fatty‐acid molecules in the adjacent layers are rotated by 180 degrees along the normal to the lamellar interface in Orth II. The name of Orth II was given to differentiate it from the usually employed orthorhombic crystal system. Thus, the nomenclature of Orth II was given to the polytypic structures to distinguish them from polymorphic orthorhombic structures.

Polytypic structures have been widely observed in the crystals of n‐alkanes and saturated and unsaturated fatty acids. Interestingly, Orth II can be more thermodynamically stable than Mon because of the stabilization of the lamellar interface. This property will be elaborated on in the next section.

2.4 Fatty Acids

Here we discuss the polymorphic structures of the principal saturated and unsaturated fatty acids. Fatty acids are the main hydrophobic moieties of lipids present in biotissues and are also employed for multiple purposes in the food, pharmaceutical, and polymer industries (Chow 1992). Saturated fatty acids exhibit a straight chain configuration, allowing for melting points higher than those of unsaturated fatty acids with the same number of carbon atoms. The inclusion of a double bond reduces the melting point in an unsaturated fatty acid, depending on its conformation (cis or trans) and its position in the carbon chains. It also introduces multiplicity in molecular conformations of chain segments separated by a double bond, which results in the complex polymorphic structures of unsaturated fatty acids.

The polymorphic properties of saturated and unsaturated fatty acids must affect the physical properties of lipid materials composed of fatty acids and their esters, but they also affect the physical properties of acylglycerols containing fatty acids as their hydrophobic moieties.

2.4.1 Saturated Fatty Acids

Table 2.1 summarizes the polymorphic forms of saturated fatty acids with nc values of 12 through 18 (Kaneko 2001; Moreno et al. 2007). Triclinic (monoclinic) forms commonly occur in even‐(odd)‐numbered saturated fatty acids. In addition to polymorphism, polytypism was found for the monoclinic B and E forms of stearic acid and arachidic acid (C20; Kaneko 2001; Moreno et al. 2007). In this chapter, we refer to the polytypic structures as B (Mon), B (Orth II), E (Mon), and E (Orth II).

Table 2.1 Polymorphism in major saturated fatty acids.

m, monoclinic; nc, number of hydrocarbon atoms; t, triclinic.

2.4.1.1 Crystal/Molecular Structures

Table 2.2 summarizes the lattice parameters of polymorphic and polytypic forms of selected saturated fatty acids.

Table 2.2 Lattice parameters of polymorphic and polytypic forms of selected saturated and unsaturated fatty acids.

Fig. 2.12 portrays three different types of triclinic crystals of lauric acid and myristic acid. The peculiar properties of the three triclinic structures are revealed in their lamellar interfaces, called segregated and nonsegregated layer structures (Kaneko 2001). In the segregated layer, the (COOH)2 terminals and the CH3 terminals are separated from each other, whereas the (COOH)2 terminals and the CH3 terminals are placed on the same lamellar interface, neighboring each other. The A1 and A‐super forms of lauric acid, both of which have an interdigitated structure, form the nonsegregated layer structures. The subcell structure of the A1 form of lauric acid is T// and the conformation of the hydrocarbon chains are all trans (Fig. 2.12a). It is worth noting that the c‐axis of a unit cell corresponds to a one‐molecule length. The A‐super form of lauric acid also has a T// subcell and an all‐trans conformation (Fig. 2.12b), but it is a modified structure of A1 in which three lauric acid molecules (indicated by arrows in Fig. 2.12b) comprise the a‐axis, as a superlattice. A segregated‐layer triclinic structure is formed in the A2 form of myristic acid (Fig. 2.12c), in which the length of the c‐axis equals that of the dimer of myristic acid. In this case, the chain‐length structure is double. The occurrence of A2 in palmitic and stearic acid and of A‐super in myristic and palmitic acid was confirmed (Moreno et al.