Lipid Technology April 2012, Vol. 24, No. 4 83

Feature

Influence of minor components on fatcrystallization

Geoff Talbot, Kevin Smith, Krish Bhaggan

G.T. is Consultant, The Fat Consultant, Bedford, MK40 1ZL, United Kingdom. E-mail: geoff@thefatconsultant.co.ukK.S. is Consultant, Fat Science Consulting Ltd., Bedford, MK41 8HP, United Kingdom. E-mail: kevin.w.smith@fatscience.co.ukK.B. is R&D Manager, Product and Process Development, Loders Croklaan BV, Wormerveer, The Netherlands.E-mail: krish.bhaggan@croklaan.com

Summary

Crystallization in fats is of fundamental importance in the production and consumption of fats per se and of food and home and personalcare (HPC) products in which fats form a major part. While crystallization of fats as such has been extensively reviewed over the pastdecade there has been less emphasis on the role of minor components. A review by Smith et al. [1] redressed this; this article is based onthat review.

Introduction

In foods, sensory characteristics such as creaminess, smooth-ness, sandiness and

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snap’ are all affected by the way in whichfats in the product crystallize; mouthfeel is defined by the melt-ing profile of the fats. The crystal structure of a product has aneffect on the rate of release of flavors in foods and active ingredi-ents in HPC products. Even in liquid oils the presence of minorcomponents can affect the clarity of these oils. Minor compo-nents already present in the fat or materials deliberately addedto a fat can affect crystallization in three main ways – nuclea-tion, crystal growth and polymorphism. This article looks ateach of these in turn and defines the effects that the most impor-tant minor components and additives have on each of them.

Minor components

Minor components in fats can be either present indigenously orthey can be deliberately added. Deliberate addition is usuallydone to enhance certain attributes such as crystallization (rateand type), surface gloss, rheology, temperature and poly-morphic stability. In some instances, indigenous minor compo-nents are also removed or reduced for the same reasons. Themain indigenously found minor components are free fatty acids(FFA), monoacylglycerols (MAG), diacylglycerols (DAG) and phos-pholipids (mainly lecithin). Conventional refining will removeFFA down to levels below 0.1% and, usually, MAG are alsoremoved. However, DAG are not removed by normal refiningmethods and so they often remain in the fat to affect its crystal-lization. Lecithin is found in many vegetable oils but its maincommercial sources are soyabean oil, sunflower oil and rapeseedoil. Because of its use as an emulsifier or moderator of rheologyit is often also used as an additive as well as simply being indi-genously present in the oil. Other minor components naturallyfound in vegetable oils are phytosterols and waxes. Phytosterolshave been studied as structuring agents for fats but not as crys-tallization modifiers. The main area where waxes have an effectis in bottled liquid vegetable oils such as sunflower oil where, atlow temperatures, they can crystallize and cause cloudiness in

the oil. For that reason they are usually removed by winteriza-tion before bottling.

Minor components deliberately added to fats can be dividedinto (i) components that are naturally found in fats and (ii) com-ponents that aren’t. In the first category are materials such asMAG which, while they are largely removed during refining, arealso added for their emulsification properties. Specific triacyl-glycerols (TAG) also come into this category. In some cases, thismay be an enhancement of a TAG already present; in others acompletely foreign TAG may be added to affect crystallization.An example of the latter is the use of 1,3-dibehenoyl-2-oleoylgly-cerol (BOB) to modify the crystallization of chocolate. Additivesnot naturally found in fats that are used to modify crystalliza-tion are often lipid-based and are often classed as emulsifiers.These are, for example, esters of fatty acids with sugars such assorbitan (e.g. sorbitan tristearate (STS) and sorbitan monostea-rate (SMS)), citric acid esters and propylene glycol esters. A pro-blem with defining the effects of addition of components suchas these is that they are usually used in their commercially avail-able form, i. e. under their trade name. The

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purity’ of these canvary widely. For example, commercially-available STS can be amix of many components with the true tri-ester of stearic acidforming only a small part.

Effects

These minor components have a number of effects on crystalli-zation but we will divide them into three main types:* Crystallization: Nucleation* Crystallization: Crystal Growth* Polymorphism

In terms of these main effects, additives can affect them eitheras promoters or inhibitors. Figure 1 shows these different modesof action. Often, nucleation or growth are promoted or inhibitedby minor components without affecting the final equilibriumsolid fat content, although they can affect the rate at which thatequilibrium is achieved. Exceptions to this are (i) when theminor component affects the equilibrium conditions by eitherbeing incorporated into the solid phase or by stabilizing the

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DOI 10.1002/lite.201200180

84 April 2012, Vol. 24, No. 4 Lipid Technology

liquid phase, or (ii) a kinetically stable situation (as opposed totrue thermodynamic equilibrium) is achieved. This can happenwhen, for example, the influence of the component is depen-dent on heating or cooling rates.

Crystallization: NucleationWhile nucleation and growth are both parts of the whole crystal-lization process they are distinctly separate in terms of minorcomponent effects. Some additives can promote (or inhibit) bothprocesses while others promote one while inhibiting the other.That makes it useful to separate them when discussing minorcomponent effects. Even within the initial stage of nucleationthings are not always clear-cut. Differential scanning calorime-try (DSC), for example, is often used to study crystallization,especially in terms of the effects of minor components on theonset and peak temperatures of minor and major peaks. Inmany cases, though, the minor peak is formed as a result of theminor component crystallizing. Changing the amount of thiscomponent may only have an effect on that particular peak – orit may also influence the main peak as well. In other cases thefirst peak is indicative of an unstable polymorph which thentransforms into a more stable one. Palm oil, for example crystal-lizes first in the a form and then transforms into b9 before grow-ing further in that polymorph. Adding saturated MAG (frompalm oil itself) promotes nucleation while unsaturated MAG(from sunflower oil) does not.

Much research on nucleation is based on studying the initialcrystals or ,

seeds’, often assuming that the components thataffect nucleation will be over-represented in the seeds. This isnot always the case, though. Trisaturated (SSS) TAG and phos-pholipids are over-represented in the initial crystals formedwhen cocoa butter nucleates but adding extra tristearin (StStSt)

only affects the initial DSC crystallization peak and does notaffect the main crystallization step of chocolate. In a similar sys-tem, Cebula and Smith [2] added SSS and DAG to a cocoa butterequivalent and found that higher levels increased the degree ofcrystallization in the early stages and raised the nucleation tem-perature. As is implied in Figure 1, not all minor componentspromote nucleation. Some, such as free fatty acids and phospho-lipids can inhibit it meaning that good refining to remove thesecomponents can result in an oil that nucleates faster.

Nucleation is dependent on the critical crystal size. Below thissize, clusters of molecules redissolve instead of growing. Abovethis size, however, they grow further and nucleation is said tohave occurred. Nucleation is influenced by minor components ina number of ways – by providing (for nucleation promotion) orshielding (for nucleation inhibition) heterogeneous nucleationsites, by stabilizing the developing nuclei or by affecting the driv-ing force for nucleation. One proposed mechanism is that com-plex lipids with surface-active properties combine with high-melting lipids to form the initial crystals. The main TAG compo-nents are then incorporated in order of decreasing melting point.This is where nucleation and crystal growth begin to deviatefrom each other in that this mechanism would imply that thesecomponents have an effect on both. However, DSC shows distinctdifferences between the first and second crystallization stages.The first stage is mainly the crystallization of SSS TAG while thesecond is usually the crystallization of monounsaturated (SUS)TAG, and SSS TAG do not necessarily provide a good base for thegrowth of SUS TAG. Faster nucleation does not always mean fas-ter crystal growth. Fast crystal growth occurs when there is agreater degree of similarity between the structure of the nucleiand that of the main part of the fat in terms of chain length,degree of saturation and type and position of unsaturation.

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Figure 1. Modes of action of additives; reproduced from [1] with a permission of the publisher.

Lipid Technology April 2012, Vol. 24, No. 4 85

Crystallization: Crystal growthDAG, particularly saturated DAG generally has an inhibitingeffect on crystal growth in a wide range of fats (lauric fats, cocoabutter and cocoa butter equivalent component oils). Unsatu-rated DAG can have variable effects, both promoting and inhibit-ing, depending on the substrate and on the level of addition.Phospholipids also generally inhibit crystal growth reducingthe growth rate of the fastest growing crystal faces and produ-cing denser spherulites. There is, for example, a greater reten-tion of wax in sunflower oil in the presence of phospholipidsmaking dewaxing more difficult. Bleaching of coconut oil inwhich the phospholipids are largely removed increases the rateof crystal growth. Free fatty acids generally also reduce the rateof crystal growth although Smith et al [3] found that adding lau-ric acid to trilauroylglycerol increased its growth rate. There isoften an optimal level of minor component in terms of crystalgrowth; fortunately, this is often the amount found naturally ina fat. For example, Tietz and Hartel [4] removed the minor lipidsfrom milk fat and added them back at twice the natural level.When milk fat containing either no or double the level of minorcomponents was then added to cocoa butter at a level of 10%,longer nucleation times, slower crystallization and faster bloomformation resulted compared to adding the original milk fat.

As we said earlier, fast crystal growth occurs when there is agreater degree of similarity between the structure of the nucleiand that of the main part of the fat. This may be because compo-nents such as DAG and phospholipids interfere with the crystalsurface preventing further molecules from joining the crystaland thereby inhibiting crystal growth. For a component or addi-tive to have an influence on crystal growth it should be suffi-ciently similar to the bulk material that is crystallizing to beable to join the crystal matrix. If it is included within the crystalmatrix in this way then the material that is crystallizing willcrystallize around it; if not, then further crystallization will beblocked at that site (Figure 2). In this way, even very smallamounts or additive of minor component can be enough to havea large effect on crystal growth.

PolymorphismThis brings us into the area of more complex crystallization pro-cesses but an area with great relevance for a number of impor-tant food products, notably chocolate and margarines andspreads. In both of these cases it is important to stabilize theproduct in a particular polymorphic form in order to inhibit theformation of fat bloom in chocolate or of sandiness in margar-ines and spreads. Considerable research has been carried out onthe use of additives to achieve these aims.

As far as chocolate is concerned it is the inhibition of the tran-sition from bV to bVI that is important. Monitoring this is compli-cated by the fact that (a) DSC is often used to define polymorphicchanges and this is not an unambiguous method, and (b) fatbloom can be caused by other polymorphic changes such asfrom b9IV to bV in a poorly tempered chocolate. X-ray diffractiongives the only truly unambiguous indication of polymorphicform. Although additives such as DAG (particularly distearoyl-glycerol) have been shown to retard polymorphic changes inCBEs and high melting fractions of milk fat have been shown toincrease the bloom stability of chocolate the greatest amount ofwork has been done studying the use of sorbitan tristearate (STS)as an additive. This is said to retard the transition of bV to bVI butto promote other transitions such as from the b9 forms to bV. Ona temperature program that would fully transform bV to bVI add-

ing 5% STS retained all the cocoa butter in the bV form. Themechanism of this is subject to some debate. One proposal isthat the bV to bVI change is a solid-state transition and that add-ing STS increases the liquid fraction of bV. Garti et al [5] proposedthat such a solid-state transition is inhibited by the rigidity ofthe STS molecule. An alternative mechanism is one in which thebV to bVI change goes via the liquid phase and that STS acts as acrystal poison at the sites where bVI crystals would grow. Indeed,we have shown [6] that the addition of a small amount of hazel-nut oil accelerates the bV to bVI transformation in cocoa butter.

In margarines and spreads the problem is not one of fat bloombut of sandiness being produced in the spread on storage givingthe spread an unacceptably grainy structure. DAG from lardretards the b9 to b transformation in a partially hydrogenatedrapeseed oil based margarine that, otherwise, would haveresulted in sandiness. In the absence of the DAG the b phase wasobserved after 4 weeks. Adding 5% DAG to the margarineextended the b-free shelf life to 44 weeks. In this regard, DAGalso shows a positional effect with the sn-1,2 isomers delayingthe transformation more than the sn-1,3 isomers.

Conclusions

Minor components and additives can have varying effects oncrystal nucleation and growth and subsequent polymorphicchange. Their effects are strongly dependent on the similaritybetween the bulk fat and the minor components. The greaterthe similarity, the stronger is the effect. The degree of undercool-ing also influences their effects – increasing undercoolingreduces the effects of minor components. Their concentrationin the bulk fat is also of importance and the level at which theyhave an influence can vary with the mechanism involved. Theirimportance in affecting the crystallization of the bulk fat, how-ever, should not be underestimated.

References

[1] Smith, K.W. et al., J. Am. Oil Chem. Soc. 2011, 88, 1085–1101.

[2] Cebula, D.J., Smith, K.W., J. Am. Oil Chem. Soc. 1992, 69,992–998.

[3] Smith, P.R. et al., J. Am. Oil Chem. Soc. 1994, 71, 1367–1372.

[4] Tietz, R.A., Hartel, R.W., J. Am. Oil Chem. Soc. 2000, 77,763–771.

[5] Garti, N. et al., J. Am. Oil Chem. Soc. 1986, 63, 230–236.

[6] Smith, K.W. et al., Food Chem. 2007, 102, 656–663.

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Figure 2. Blocking a growth site by an additive; reproduced from[1] with a permission of the publisher.