Lipids II: Crystallisation

Learning Objectives

• Describe fat crystal formation.
• Describe triacylglycerol conformation and crystal polymorphism.
• Discuss factors that influence the melting point of fat crystals.

Molecular Organisation of TAGs

• Crystals are often formed during food processing and storage.
• Crystallisation influences food properties, mouth feel, and physical stability.
• A fat is always a mixture of triglyceride crystals and oil.
• This has consequences for quality and functional properties.
• Eating properties:
  • Fracture properties and stickiness (observed when small fat crystals do not melt in the mouth).
  • Cooling sensation: Melting in the mouth is endothermic.
• Physical stability:
  • Formation and sedimentation of crystals in oil
  • Phase separation
  • Bloom on chocolate
  • Gloss of chocolate and margarine.

Crystalline vs. Amorphous Solids

• Solids are either crystalline or amorphous.
• Crystalline solids:
  • Particles arranged in a regular lattice structure.
  • Generally categorized as ionic, metallic, covalent network, or molecular, depending on the bonding in the lattice.
• Amorphous solids:
  • Non-crystalline solids do not have a regular structure.
  • Have a more random arrangement of particles.
• Crystal:
  • A solid in which the atoms, molecules, or ions are arranged in a repeating pattern that extends in all three dimensions.
  • The repeating pattern is termed unit cell.
  • The existence of the crystal lattice implies a degree of symmetry in the arrangement of the lattice.
  • Symmetry about a point (centre of symmetry), line (axis of symmetry), plane (plane of symmetry).
  • Most of the crystals are symmetric although deviations may occur.

Unit Cells and Lattice Systems

• Unit cells are classified into one of the seven lattice systems in terms of the rotational symmetry elements they possess.
• Not very common in food.

Crystallisation Process

• Crystallisation process:
  • Nucleation
  • Crystal growth
  • Recrystallisation
  • Achievement of appropriate conditions

Nucleation

• Conditions:
  • Temperature (supercooling).
  • Concentration (supersaturation).
  • Surface tension.
• Generation of sufficient thermodynamic force.
• Crystallisation occurs from:
  • Solution (i.e., presence of water, sugars, salts).
  • Melt (pure compounds, i.e. only TAGs).
• Final temperature and rate of cooling play key role for crystallisation and properties of TAGs.
• Many minute solid bodies called “embryos”, “nuclei”, or “seeds” must exist in the solution before crystals can develop.
• Nuclei act as centres of crystallisation.
• Nucleation: the molecules of the substance must resist their tendency to redissolve and become oriented into a fixed lattice.
• The critical radius $(r_c)$ represents the minimum size of a stable nucleus.
• Nuclei smaller than $(r_c)$ will dissolve.
• Nuclei larger than $(r_c)$ will continue to grow.

Crystal Growth and Recrystallisation

• The crystal will continue to grow until the solution is not anymore supersaturated, or all material has been converted to crystal.
• Slow cooling rate generate few, big crystals with few impurities.
• Fast cooling rate generate high number of small crystals that grow very fast and contain imperfections.
• Slow cooling rate: crystallisation at higher temperatures.
• Fast cooling rate: crystallisation at lower temperatures.

Types of Recrystallisation

• Once the crystal has formed, further reorganisation takes place.
• Recrystallisation is the change of crystal size, number, shape, or orientation.
• Occurs rapidly at storage temperatures close to melting point or due to temperature fluctuations.
• Influenced by composition of the product and initial crystal size distribution.
• Recrystallisation type:
  • Isomass rounding: Changes in shape.
  • Ostwald ripening: Large crystals grow, small disappear.
  • Accretion: Crystals that are close together grow as one (stick together). Important for foods.
  • Melt-Refreeze: Due to temperature fluctuations during storage. Important for foods.
  • Polymorphic transitions: Changes in polymorphism during storage.

TAG Conformations and Chain Stacking

• The shape of TAGs in solid phase is compared to a “tuning fork” or a “chair”.
• TAGs stack in a repetitive sequence in a unit cell.
• A double chain length structure is formed when the chemical properties of the three fatty acids are the same or very similar.
• A triple chain length structure is formed when the chemical properties of one or two of the three fatty acids are different from each other.

Polymorphism

• The occurrence of several different crystal forms from the same compound is called polymorphism.
• Polymorphic forms are crystals of the same chemical composition that differ in crystal structure but yield identical liquid phases upon melting.
• The most common polymorphic forms of TAGs are alpha $(α)$, beta prime $(β')$, and beta $(β)$.
• They differ in their melting points and crystallographic properties.

Characteristics of TAG Polymorphs

• Crystal structure:
  • Alpha $(α)$: Hexagonal, Tuning fork.
  • Beta prime $(β')$: Orthorhombic, Tuning fork.
  • Beta $(β)$: Triclinic, Chair form.
• Acyl groups orientation:
  • Alpha $(α)$: Acyl groups oriented at 68-70 ° from plane of the glyceryl group. Vertical chain orientation. Randomly ordered. Most loosely packed.
  • Beta prime $(β')$: Acyl groups are tilted about 59 ° from the plane of the glyceryl groups. Tilted chain orientation. In-between. More closely packed.
  • Beta $(β)$: Acyl groups are tilted at 90° to the plane of the glyceryl group. Tilted chain orientation. Highly ordered. Most closely packed.

Crystal Morphology

• Alpha $(α)$: Platelet, 5 μm
• Beta prime $(β')$: Fine needle, 25-50 μm
• Beta $(β)$: Long needle, 1 μm

Thermal Properties

• Alpha $(α)$: Lowest melting point, Unstable.
• Beta prime $(β')$: Intermediate melting point, Unstable.
• Beta $(β)$: Highest melting point, Most stable form.

Colour

• Alpha $(α)$: Translucent
• Beta prime $(β')$: In-between
• Beta $(β)$: Opaque

Polymorphic Transitions

• In many cases, alpha $(α)$-crystals form first upon cooling of a liquid fat because nucleation is easiest in the $(α)$-form.
• In fats of relatively homogeneous composition, the $(α)$-form is short-lived; it may even within a minute be transformed into $(β')$.
• The $(β')$-form generally has a longer lifetime.
• The transitions $(α)$ to $(β')$ to $(β)$ must go along with a change in crystal composition, since compound $(β')$ crystals can host fewer different molecules than alpha $(α)$, and compound $(β)$ crystals hardly exist.
• This also implies that the polymorphic transitions are not true solid-state transitions: they have to proceed via the liquid state.
• Properties of fats may change during storage and influence quality.

Structural Hierarchy of Fats

• Bulk fats have different levels of structure depending on the length-scale of observation (how deep we zoom in the structure).
• Molecularly, TAGs associate to form a lamella and stacking of multiple lamellas form crystalline domains, which in turn they form nanoplatelets.
• The nanoplatelets assemble into spherulites that, with further aggregation, finally form the bulk fat.
• Important technological properties of fats such as spreading, melting, or sensory properties depend on a combination of properties at the different structural levels.
• However, structures with sizes between 1 and 200 μm and the interactions between each other are primarily responsible for the technological properties of fats.

Melting Point and Water Solubility

• The greater the chain length, the higher the melting point and the lower the solubility of the fatty acids.

Melting Point and Double Bonds

• Increase in double bonds results in decrease in melting point of fatty acids.

Melting Range

• Melting range is influenced by:
  1. The number of carbon atoms. The higher it is, the higher are $(T<em>m)$ and $(ΔH</em>f)$.
  2. The presence and the number of double bonds.
  3. Double bond configuration. A trans double bond allows the formation of an almost straight chain and hence causes a smaller difference in melting properties than a single cis bond.
  4. Position of the double bonds in the chain. The most important effect is that two conjugated cis double bonds (- CH = CH – CH = CH -) can give a more nearly straight chain than two non-conjugated ones (- CH = CH - CH2 – CH = CH -).
  5. Branching of the chain tends to cause an appreciably lower melting point.
• Natural fats are always mixtures of a number of different triglycerides and therefore have a melting range.
• This means that they melt $(T_m)$ over a range of temperatures rather than at a specific temperature.
• There is a broad range of melting depending of TAG composition.

Solid Fat Content (SFC)

• SFC is a function of temperature, temperature history, time and composition.
• Food fats are rarely 100% solid at processing temperatures.
• Solid Fat Content is the ratio of solid to liquid fat at a given temperature.
• Important for the functional properties of fats.
• Appearance and stability of salad dressings stored at refrigeration temperatures.
• Spreading of margarines and butters.
• Mouth melting profile of chocolate.
• Texture of baked products.
• Ease of processing.

Chocolate Tempering

• Tempering is a time-temperature process to ensure the formation of chocolate in the right crystal form.
• Tempering is important for demolding, snap, gloss, resistance to bloom and mouth-melting characteristics of chocolates.
• Tempering melts all the undesirable polymorphic forms and keeps only $(β_V)$ crystals that have the optimum melting characteristics.
• Cocoa butter has several crystal polymorphs.
• Good chocolate can only be made from stable crystals.

Crystal Names and Polymorphic Forms

• Higher melting point = More dense = More desirable.

Bloom

• White “moldy” appearance at the surface.
• Major reason for product failure.
• NOT a health hazard.
• Often caused by large fat crystals growing from the surface and scattering light.
• The movement of fat from the center to the surface dissolves some cocoa butter and carries it to the surface.
• Cocoa butter recrystallizes at the surface.
• How to prevent it:
  1. Proper tempering.
  2. Cool storage.
  3. Add butter-fat.
  4. Addition of emulsifiers (e.g. sorbitan monostearate).

Shortenings

• $(β')$-crystals: large amount of small air bubbles.
• $(β)$-crystals: small amount of large air bubbles.
• $(β')$ crystal shortening helps the incorporation of an abundant quantity of small air bubbles in batters for good volume, texture and tenderness of baked goods.
• Shortening is a fat that is used in bakery products.
• The polymorphic structure of the shortening is responsible for the final quality of the product.
• Small crystal sizes allow for more gas retention in dough.
• Melting point of crystals is critical for “mouth-melting” effect in bakery products.

Margarines

• Water-in-oil (w/o) emulsions in which water is dispersed as droplets in the continuous fat phase. Remember: mayonnaise is an oil-in-water emulsion (o/w).
• Lipid phase is a blend consisting of different fats and oils.
• The ratio and type of fats and oils in the fat blend are decisive in achieving margarines with the desired characteristics (e.g., spreading).
• Emulsifiers (lecithin, mono-, di-glycerides), flavours, colours, and antioxidants are also added.
• Many small $(β')$ polymorphs are preferred.
• $(β)$ polymorphs are large crystals and at a high solids content, result in a brittle, hard margarine, while at a low solids content, the product becomes oily.
• Spreadability and mouth melting are two textural properties of margarines that depend on the amount of solid fat and its crystalline structure.
• Good spreadability requires that the product retains its plasticity from refrigerator to room (4–22 oC).
• Desirable mouth melting, however, requires rapid melting at mouth temperature (35–37 oC) for prompt flavor release.
• In-depth understanding of crystal structure is required.

Chemical Manipulation of TAG Composition

• Hydrogenation: Addition of hydrogen to double bonds.
  • Conversion of liquid oils to semisolid fats (margarine, shortenings).
  • Change the crystallisation behaviour and improve the oxidative stability of oil.
  • It is carried out with $(H_2)$ and $(Ni)$ as catalyst.
  • Formation of trans- fatty acids.
• Interesterification: Randomisation (“shuffling”) of fatty acids in triacylglycerols.
  • Complex chemistry that can be carried out with catalysts or enzymes.
  • Change physical properties such as melting point and crystallisation without changing the fatty acid composition.
  • Manufacturing of shortenings, margarines, confectionery oils.
  • A TAG with two linolenic acid residues and a saturated TAG undergo interesterification resulting into two molecules containing one unsaturated residue each.