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entists in understanding and controlling the thermodynamics and (slow) dynamics of structured foods. We

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principles of soft matter physics to food structuring [13]. Their activi- food structuring processes. Also, we hope this review challenges soft

Advances in Colloid and Interface Science 176177 (2012) 1830

Contents lists available at SciVerse ScienceDirect

Advances in Colloid an

.eties receive increasing attention from the eld of soft matter physics,as indicated by a growing number of food related papers in the high-impact journal Soft Matter. To give this growing eld a good exposureto food scientists, and also to expose physicists to the rich complexityof food, a Faraday Discussion will be held at Wageningen University in2012 (www.rsc.org/FD158). With this review paper we aim to providea thorough introduction to concepts from soft matter physics as recent-

matter physicist and colloids scientists to delve into the seemingly com-plexworld of foodmaterials.Wewill show that this complexmaterial isamendable to the tools of their trade.

Soft matter is a complex uid having dispersed, mesoscopic struc-tures, such as gas bubbles, colloidal particles, emulsion droplets, amphi-philes or polymers. The length scale of these mesoscopic structures is inthe order of microns. It happens that this size is similar to the lengthly introduced into the eld of food science. Furknown applications of these concepts in the fopotential future applications.We target this rether food science or soft matter physics, whofrom the other eld. We hope that the food s

E-mail address: ruud.vandersman@wur.nl.

0001-8686/$ see front matter 2012 Elsevier B.V. Alldoi:10.1016/j.cis.2012.04.002proach in understandingarchers are applying the

the thermodynamics and dynamical behavior of complex food mate-rials. Such an understanding will aid them in the design and control ofBecause of the perceived benets of this apthe behavior of complex materials, food reseReferences . . . . . . . . . . . . .

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

to examine the potential of this relatively novel branch of physics [4],and that they discover its ability to provide a better understanding of7. Conclusions . . . . . . . . . .Acknowledgments . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266. Application of soft matter concepts in food structuring . . . . . . .. . .4. Self assembly . . . . . . . . . . . . . . . . . . . . . . . . . .5. Jamming and slow dynamics . . . . . . . . . . . . . . . . . . .Soft matter physicsMicrostructureFree volumeEffective temperature

discuss the use of these concepts in four topics, which will also be addressed in a forthcoming Faraday Dis-cussion on food structuring.

2012 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182. Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193. Concepts for strong driven food materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Soft matter approaches to food structurin

R.G.M. van der SmanAgrotechnology and Food Sciences Group, Wageningen University & Research, The Netherla

a b s t r a c ta r t i c l e i n f o

Available online 20 April 2012

Keywords:

We give an overview of thespective of soft matter physof van der Waals, such as fre

j ourna l homepage: wwwthermore, wewill list theod science literature, andview at scientists from ei-are unaware of the workcientists are encouraged

rights reserved.ny opportunities that arise from approaching food structuring from the per-. This branch of physics employs concepts that build upon the seminal workolume, the mean eld, and effective temperatures. All these concepts aid sci-

d Interface Science

l sev ie r .com/ locate /c isscale that humans can sense with the tongue, and thus often sets thescale for structured foods. Soft matter physicists approach such diversi-ty and complexity by coarse graining, i.e. they ignore chemical details atthe molecular scale up to the level that similarities in the physical be-havior emerge. By doing so, the physicist still keep to the principles ofthermodynamics [4,5]. For a description of these similarities they regu-larly borrow theories and concepts from other branches of physics, suchas the, quite mature, liquid state theory.

At the mesoscale and beyond, the properties of (edible) softmatter are largely independent of the chemical details of its constitu-ents, which is due to the large separation of the length scales of themesoscopic structures and the chemical moieties at the molecularscale. The results of this makes the interactions at the mesoscale to bedominated by entropy rather than by chemical bonds. This also makesthe material much more susceptible to externally applied elds, suchas mechanical stress. This gives the materials their soft appearance.

The mesoscopic structures are best characterized by coarse-grainedproperties such as size, exibility and amphiphility. Using these proper-ties it is shown that all classical soft matter systems are part of one con-tinuum of systems [5], which is illustrated in Fig. 1. This classication isone of the examples of how physicists try to nd universality in softmatter.

A seemingly universal trait of the physicist is to view everything asa hard sphere. Though, it is often regarded as oversimplistic it hasproven to be a valuable instrument in the quest for universality inphysics. An example of the use of the hard sphere concept is shownin Fig. 2, where mesoscale structures are disguised as hard spheres.This illustrates the universality of the jammed state of various kindsof soft matter [6]. In the jammed state the mesoscale structures

19R.G.M. van der Sman / Advances in Colloid and Interface Science 176177 (2012) 1830form a percolating network, which is immobilized. In food science,the jammed states are well studied, but are better known as the glassystate or the gel state.

Our daily food is extremely rich in types of soft matter [1,2], andoften contains mixtures of various mesoscale building blocks, asshown in the CSLM images in Fig. 3. Ice cream is an extreme case,and contains air bubbles, fat crystals, casein micelles, ice crystals, allembedded in a sugar solution. The multitude of mesoscopic objectsavailable for foods, offer them seemingly unlimited possibilities fortheir structuring [3]. The focus of the coming Faraday discussion willbe food structuring, which we will also take as the guiding line inthe discussion of soft matter concepts.

Food structuring processes can often be divided into 3 steps: 1) de-struction of native structures or raw food materials, 2) creation of newfood structures, 3) the arrest of the structured food in a jammed state,as discussed in detail in our review paper in SoftMatter [3]. For the pur-pose of this review, we consider that there are two basic, different waysof organizing food microstructure, via 1) self-assembly of specicallydesigned building blocks, or 2) non-equilibrium structure formationby strong external elds [3].

After bringing food materials into the arrested state, they arethought to be shelf stable. However, frequently they are prone to slowdeterioration of the structure, as evident in the staling of bread crumb

Fig. 1. Various kinds of soft matter can be arranged in a triangle, showing there is a con-tinuum in dispersed phases, which can be characterized by size, exibility, and amphi-phility.

The gure is taken from [5], by courtesy of Springer Verlag.(which is arrested in the gel state), and the growth of ice crystals inice cream during storage. These processes are examples of what softmatter physicists call slow dynamics, for which a universal theory iscurrently developing [7].

Finally, food structure is broken down again when humans con-sume their food. The destruction starts in the mouth [8,9], and is com-pleted in the digestive tract, where it is broken down to molecularbuilding blocks, which can be absorbed by the human intestines[10,11]. Recently, novel food structures have been created, which aredesigned as controlled release vehicles [12], which should be brokendown at certain locations in the human digestive tract. Hence, de-struction of food structure is becoming an important research direc-tion, because of its implications for bioavailability of the functionalingredients present in foods, and also in the more sustainable use ofraw agricultural materials, providing us the building blocks for foodstructure [1315].

Below we will discuss concepts and theories from the eld of softmatter physics, that are relevant to food structuring, of which we es-pecially focus on a) structuring via self-assembly or strong drivingexternal elds, b) (un)jamming of the food structure, and c) slowdynamics near or in the arrested state. The destruction of the foodstructure as in digestion can be viewed as unjamming or controlleddisassembly, and will be discussed in context of jamming, or self as-sembly. An overview of the discussed soft matter concepts for theabove mentioned topics is shown in Table 1. To keep this review con-cise, we have been selective in the number of concepts. The selectionis driven not only by personal experience, but also by whether theconcepts build upon the seminal work of van der Waals. Hence, werefrain from discussing mechanical and rheological properties offood matter. The discussed concepts have found wide application insoft matter physics due to their universal traits, and apply to multipletopics in the eld of food structuring, as indicated in Table 1.

2. Thermodynamics

While physicists have a quite open mind for coarse-graining out(irrelevant) details, they are quite rigorous in the application of ther-modynamics. It must be said that this rigor is often lacking in currentfood science. Van der Waals is one of the early champions for the useof thermodynamics, and particularly the use of the free energy or,equivalently, the equation of state. In his famous equation of state,van der Waals introduced concepts that are still extensively used inthe eld of soft matter, namely: free volume and the mean eld [16].These concepts are derived by splitting the interaction betweenmole-cules into a strong short-range repulsive force, and a weak long-rangeattractive force. The repulsive forces determine the free volume,which is equal to that part of the volume of a system, where new par-ticles can be inserted. Free energy is to be formulated in terms of theeffective volume of the free volume, rather than the total volume. Theeffect of attractive forces between particles can be captured with a(negative) internal pressure, which is added to the actual pressureeld, to make it an effective pressure or mean eld. With these effec-tive volumes and pressures van der Waals reformulated the equationof state of an ideal gas as: peffVeff=nRT.

The free volume of a hard sphere suspension is shown schematical-ly in Fig. 4, where the free energy is purely determined by the free vol-ume (in the absence of attractive forces the mean eld is zero). Thefree volume of a hard sphere suspension can be computed using theWidom insertion method or experimentally via confocal microscopy[18,17]. In Fig. 4 the free volume is indicated by the gray area. Observethat around each particle there is an excluded volume, where onecannot insert new particles which is an effect due to the nite sizeof particles. The equation of state, obtained for a monodisperse sus-pension, agrees with the empirical relation by Carnahan and Starling.Using the free volume concept one can predict the phase behavior of

strong asymmetric bidisperse suspensions, which phase separate via

re, a

20 R.G.M. van der Sman / Advances in Colloid and Interface Science 176177 (2012) 1830so-called depletion interaction [19]. Next to thermodynamic proper-ties, free volume also determines transport properties if the systemis near the jammed state [20], as we will discuss below.

After its introduction by van der Waals, other physicists have gen-eralized the concept of the mean eld. A mean eld theory replacesthe interaction between elements in the system by the interaction ofa single element with an effective eld, which is the sum of the exter-

Fig. 2. Various dispersed phases of soft matter can approximated as a hard spheFigure is taken from [6], by courtesy of RSC.nal eld and the internal eld. Mean eld theories have been devel-oped for ferri-magnetism by Weiss, for alloys by Bragg and Williams,and for superconductors by Ginzburg and Landau. Building on thesetheories for condensed matter, mean eld theories have been devel-oped also for soft matter, for example by Flory and Huggins for poly-mers, and by de Gennes for liquid crystals. A detailed discussion offree energy functionals for various soft matter systems building onmean eld theory can be found in ref. [21].

Building on the van der Waals picture of a liquid, the liquid statetheory has been further extended [22]. Central to the liquid state the-ory is the pair distribution function, describing the structure of theuid. This function is mainly determined by the repulsive part of theparticle interaction. The attractive part of the particle interaction canbe handled as a perturbation of the hard sphere uid. As interactions

Fig. 3. Examples of food structures, from left to right: ice cream, yogurt and cheese. PicturesFigure is taken from [3], by courtesy of RSC.between colloidal particles often show similar characteristics as theinteractions between molecules of simple liquids, liquid state theoryis often applied to these colloidal systems [23]. Via generalization ofthe van der Waals equation of state, several authors derive the freevolume from the pair distribution function [2426].

3. Concepts for strong driven food materials

nd be viewed as a soft glass, having a percolated network in the arrested state.Food structuring often requires processing involving intensiveow, heat and mass transfer [27,28,3]. Hence, in these cases food canbe perceived as strongly driven soft matter. The strong driving isoften combined with phase transitions to induce novel structures.The driving signicantly modies these phase transitions, and can beviewed from the framework of non-equilibrium thermodynamics a eld which is still under active development, even formore commonplace soft matter systems [29]. However, in the eld of soft matternovel concepts have already been deduced, which can be very usefulto food processing.

Studies of driven soft matter may have the strongest link to thetraditional approaches applied to food processing. Food engineers re-ceive a training, which is quite similar as that of chemical engineers.

were obtained by CSLM, with uorescent dyes coloring fats (green) and proteins (red).

Consequently, they tackle problems in food processing via solving thebalance equations for mass, momentum and energy [30]. These equa-tions are of course applicable to strongly driven soft materials, as theconservation of mass, momentum and energy equally holds for them.However, the balance equations can be generalized to have a propercoupling to thermodynamics, so that they have the correct drivingforces for transport and phase transitions. The theory of generalizedbalance equations is described in Chaikin and Lubensky [31]. The gen-eralized balance equations encompass 1) a generalized NavierStokesequation, with an osmotic pressure replacing the conventional hydro-static pressure, describing the evolution of the velocity eld for thecomplete mixture, 2) a generalized convectiondiffusion for the evo-lution of the volume fraction of the dispersed phase with diffusiondriven by a chemical potential, and 3) the energy balance describing

Table 1Soft matter concepts used in the discussion topics.

Topic Driven softmatter

Self assembly Directedassembly

Soft glasses

Free energy x x xGeneralized balance eq. x xPhase/state diagram x x x xMean eld theory x xFree volume x x x xEffective temperature x x

21R.G.M. van der Sman / Advances in Colloid and Interface Science 176177 (2012) 1830the change of enthalpy of the mixture. The osmotic pressure andchemical potential are to be derived from a free energy functional,which describes the thermodynamics of the soft matter system.

Awide variety of driven (soft) condensedmatter systems can be de-scribed with this framework of generalized balance equations, such aspolymer blends [32,33], surfactant stabilized emulsions [3437], alloys[38], granular matter [39], or solutions with liquid crystals [40,41]. Forsystems with immiscible phases, like emulsions, it is necessary thatthe free energy functional is extended with the square gradient term.This is yet another concept that has been introduced rst by van derWaals [4244]. The squared gradient term generates a diffuse interfacebetween immiscible phases, and complex phenomena like dropletbreak-up and moving contact lines then emerge naturally from thegeneralized balance equations, if they are properly coupled to the freeenergy functional [4547]. The framework of generalized balanceFig. 4. Free volume in a suspension of hard spheres. Black indicated the volume occupiedby the hard spheres, white indicates the excluded volume the center of other hard cannotenter, and the gray area represents the free volume.The gure is taken from [17], by courtesy of Nat. Ac. Sci.equations also captures systems undergoing phase transitions via heattransfer, such as solidifying alloys [48] or polymer melts [49], evaporat-ing liquidgas systems [50], and drying polymer gels [51].

The governing equations of driven soft matter systems are oftensolved using mesoscale simulation methods [5255], which are fre-quently part of amultiscale simulation framework [5658]. Inmesoscalesimulations, the dispersed phases of soft matter are fully resolved on thecomputational grid. Often they are represented as hard spheres [59], orrepresented via a continuous phase eld having surface free energymodeled by the square gradient term in the free energy functional [42].

Strongly driven systems appear to defy classical thermodynamics, asshown by the fact that phase transitions are inuenced by strong sheareldswhich is not captured by classical thermodynamics. For severalphenomena, it appears that they t into the framework of classical ther-modynamics via the denition of an effective temperature [60].

The concept of effective temperature has been introduced rst fordriven (uidized, vibrated or sheared) granular matter. It is taken tobe linear with the kinetic energy of the velocity uctuations of particlesinduced by the driving [61]. For sheared granular matter the effectivetemperature is quadratic in the shear rate, Teff _2. The governing equa-tions for granularmatter have a strong resemblance to those of classicalthermodynamics. The only difference is that the classical temperaturereplaced by the effective temperature.

Subsequently, the concept of the effective temperature has beenintroduced for shear-induced melting of (colloidal) crystals [62]. Forsmall departures from equilibrium the effective temperature scalesquadratically with the shear rate [62,63], while at strong drivingthe effective temperature scales linearly with the shear rate, Teff _1[62,64,65]. Shear-induced phase transition can be depicted conve-niently in a non-equilibrium phase diagram, using volume fraction andeffective temperature as state variables. At high volume fractions andhigh effective temperatures the non-equilibrium phase diagram alsoshows a re-entrant solid phase probably induced by shear-thickening.

An effective temperature has also been dened for sheared granularmedia and emulsions [66], which is probably related to free volume[6769]. These systems often show shear-banding, as well as having ayield stress. If sheared, they show coexisting jammed state and owingstates which are governed by both temperature and hydrodynamicuctuations.

Recently, we have shown the applicability of the effective tempera-ture concept to strongly sheared suspensions conned to small channels,or concentrated in a boundary layer in the crossowmicroltration pro-cess [65]. Via this denition of the effective temperature, these stronglysheared suspensions can also be described by the above generalized bal-ance equations.

In pharmacy a frequently used method of manipulation of phasetransitions is ball milling. Recently, it is shown that ball milling is alsoamenable to the concept of effective temperature [70]. In food scienceball milling is also occasionally investigated, where it has shown thechange of phase transitions in starchy powders [71,72].

4. Self assembly

Under certain conditions some mesoscopic structures present insoft matter can self assemble. Requirements for self assembly arethat the structural elements have sufciently weak (non-covalent)attractive interactions, and that the interaction leads to more orderedstate [73]. Often the structural elements have some degree of asym-metry, for example in shape or hydrophilic properties, such as hardrods or amphiphilic molecules, which enables the existence of an or-dered, self-assembled state [74]. Often, self assembly can only occur,if there is a balance between repulsive and attractive forces. Theinteraction has to be weak enough such that (thermal) uctuationsallow the elements to adjust their position and orientation, to attainthe more favorable self-assembled conguration. If the uctuations

are too weak, or the interactions too strong (irreversible), it is more

function. Of course, the concept of caging has strong similarities withthe free volume concept. Via the caging, a jammed system remainsout of equilibrium, and its state cannot be described by a free energyfunctional or phase diagram. The jammed state can often be correlatedto a certain temperature for a given composition. Hence, the jammedstate is often represented together with phase transitions in a statediagram. Liu and Nagel [99] have generalized the jammed state tovarious soft matter systems. They have represented this with thejamming state diagram, which is reproduced in Fig. 5. The jammedstate diagram also shows alternative routes to unjam the soft glasses[100,101]. If the system is arrested via cooling, it can be unjammedvia shear or dilution. Also gels are viewed as jammed systems, andthey can be unjammed via application of shear or pressure [102]. Theunjamming of gels is often thought of as failure or collapse of thestructure.

The primary relaxation mode is related to translation of the dis-persed phase (or molecule/chain in case of hard glasses). The primarymode determines physical properties like viscosity and mobility,which is the inverse of diffusivity. These relaxation modes diverge atthe glass transition temperature Tg, meaning that translation of thedisperse phase is immobilized. Many glassy systems also show sec-ondary or beta relaxation modes. They are relatively faster than theprimary (alpha) relaxation mode, and therefore they can persist attemperatures below the glass transition. In the case of polymers thesecondary relaxations are associated with rotational and vibrational

22 R.G.M. van der Sman / Advances in Colloid and Interface Science 176177 (2012) 1830likely that the system ends up in an arrested state. The structural ele-ments can self assemble in the bulk solution like a crystal, or they canassemble at an interface between immiscible phases. Classical exam-ples of self assembling systems are liquid crystals, surfactants, bersuspensions, and binary colloid suspensions with strong size asym-metry. The constraint of the reversibility of the interactions sets alimit to the size of the structural elements.

Self assembly has many similarities with crystallization, and manytools from (soft) condensedmatter physics can be applied; such as thephase diagram and group theory [73]. Phase diagrams have beenobtained theoretically for liquid crystals, block copolymers and surfac-tant systems using free energy functionals. These functionals arebased on orientational order parameters, which express the topologyof the system rather than the chemical details similar to otherkinds of soft matter [7578].

For binary suspensions containing hard objects, with strong asym-metry in size or shape, the interactions are purely entropic [79]. Con-sequently, the free energy functional can be described in terms of freevolume [19]. These theories, developed for classical self assemblingsystems, show that the geometry of the structural elements is impor-tant. Furthermore, they have provided insight into the universality ofself assembled phases [73], which can be used for more complex selfassembling systems like polysoaps, patchy colloids, synthetic DNAand proteins [8082]. These modern self assembling systems areapproached theoretically using numerical tools developed earlier forthe classical soft matter systems, such as Brownian Dynamics, Dissi-pative Particle Dynamics, Self Consistent Field theory or the relatedDynamic Density Functional theory [8386].

In addition to the above described equilibrium self assembly,ordered states of soft matter can also be obtained via directed (self)assembly, which is facilitated by external templates and elds suchas a shear ow or electric eld [87]. These dissipative structures areformed via a constant supply of energy. Therefore, their thermody-namic description closely resembles that of strongly driven soft mat-ter systems, described above. The manufacturing of colloidal particles,delivery vehicles and emulsion droplets with microuidic devices isalso viewed as an example of directed assembly [88,89].

Directed assembly via templating can be achieved via coacerva-tion. Coacervation is a liquidliquid phase transition in a solution ofpolyelectrolytes, where oppositely charged polyelectrolytes undergoassociative interactions [9092]. An example of a system showingthis kind of behavior is a mixture of proteins and pectins. If the proteinis rst absorbed at the interface of emulsion droplets, they function asa template for pectinwhich binds to the proteins as in a coacervate.Pectin can bind proteins again, and consequently multiple layers canbe deposited. Via this multilayer deposition microspheres for con-trolled release purposes can be created [9395].

For controlled release of functional ingredients in the digestivetract, directed disassembly is often required [9698]. Via (enzymatic)cleaving of proteins, the amphiphilic properties can be altered andthe self assembled state is not favored. The conformation of proteinscan also be changed by altering the pH or ionic strength. The size,shape and surface hydrophobicity of the proteins are changed andcan lead to controlled disassembly.

5. Jamming and slow dynamics

Close to the jammed state many systems of (soft) condensed mat-ter exhibit slow relaxation phenomena with quite universal traits [7].This universal behavior has been called slow dynamics, and the softmatter systems showing this behavior are named soft glassy materials.

The systems are in the jammed state because the thermal uctua-tions are too small to sample the equilibrium state. The dispersed ele-ments of the soft matter cannot escape the cage formed by the othersurrounding dispersed elements. The signature of the occurrence of

caging is the growth of the primary peak in the pair distributionmodes. Key features of the slow dynamics are spatial and dynamicalheterogeneity, and the slow dynamics is driven by stress relaxation.The slowly (primary) relaxing physical properties often show stretched(Kohlrausch Williams Watts) exponential behavior [7].

Only a few universal theories exist for describing the slow dynamicsof (soft) glasses, such as free volume theory [103], Gibbs-diMarzio[104], and Mode Coupling Theory (MCT) [105]. None of them is totallysatisfactory, and they explain only part of the observed phenomena.All these theories are semi-empirical and they often break down attemperatures far above the glass transition. For soft glassy materialsthe Mode Coupling Theory is most often employed. MCT predicts theexistence of primary and secondary relaxation modes, with the formershowing stretched exponential behavior. However, it predicts thedivergence of primary relaxation at a cross-over temperature Tc. It hasbeen found that the cross-over temperature is often different from

max/

T/T

g

Liquid

1

1

1

Jammed

Fig. 5. Jamming state diagram, with axes indicating volume fraction , temperature T,and stress . Systems get jammed if these state variables crosses a critical value, as in-dicated on the axes by the random close packing max, the glass transition temperatureTg, and the yield stress .

The gure is taken from [3], by courtesy of RSC.

using generalized balance equations, as with other driven soft mattersystems. The particle migration is shown to be a kind of diffusive be-havior [124], which takes place at much slower time scales as hydro-dynamics. Hence, shear induced migration might also be termed asslow dynamics. This slow dynamics is also described by an effectivetemperature, which is linear with the local stress. The local stress islinear with the suspension viscosity, for which expressions derivedvia effective medium theory show best correspondence with empiri-cal relations for shear induced diffusivity [65].

23R.G.M. van der Sman / Advances in Colloid and Interface Science 176177 (2012) 1830experimentally observed glass transition, Tc1.2Tg. In the regime ofTcbTbTg MCT often fails to explain experimental relaxation phenom-ena [7], as observed in viscosity and diffusivity for example. Variousextensions of MCT have been proposed to explain the phenomena inthis range [106]. In this regime free volume theory also offers semi-empirical explanations for the observed behavior of viscosity and dif-fusion [103,107].

Slow dynamics is also shown for soft glassy systems under shear,which is described by Soft Glassy Rheology [108]. The shear eld pro-vides hydrodynamic uctuations via which the system can samplestates with a lower free energy. An effective temperature is taken asa measure for these uctuations. The concept of effective temperatureis expected to describe quantitatively the slow dynamics and aging ofsoft glassy materials [7]. However, experimentally it is still difcult todetermine the effective temperature, and often experiments yieldcontradictory results. The effective temperature is related to the localstress [109]. A similar statement is found above for strongly drivensoft matter. The fact, that slow dynamics is dominated by stress relaxa-tion, strengthens the plausibility of the hypothesis that the effectivetemperature is linear with the local stress.

Via combining Soft Glassy Rheology with effective medium theory,it has been shown that the viscosity of soft glassymaterials shows sim-ilar behavior to hard glasses [110]. For glasses, the viscosity diverges atthe glass transition Tg, as follows from the free volume theory [103]. Ef-fective medium theory is a kind of mean eld theory, which states thatphysical properties of a mixture can be obtained from theories consid-ering a single particle immersed in a uid. In the effective medium the-ory the uid obtains properties equal to those of themixture. Often, theproblem has to be solved in a self-consistent way. Effective mediumtheory has been applied successfully to predict the viscosity of hardsphere suspensions, using the Einstein theory of dilute suspensions[111]. The celebratedKriegerDoghourty relation for suspension viscos-ity is derived via effective medium theory, which states that the viscos-ity diverges at random close packing volume fraction [112114]. Bothfree volume theory and the KriegerDoghourty relation for viscosityof hard sphere suspension are also related to free volume [115]. Boththeories state that the viscosity diverges if the free volume reaches acritical value.

Recently, slow dynamics has been observed in livingmatter, namelythe cytoskeleton commonly referred to as active gels [116,117]. Asthese active gels constantly require input of energy (ATP), the distinc-tion between directed self assembling systems, strongly driven systemsand soft glassy systems becomes somewhat blurred [118]. Also, for ac-tive gels, the effective temperature concept has been proposed [119].

Other phenomena illustrate the fuzziness of the boundaries be-tween soft glassy materials and strongly driven soft matter systems.These are for example, viscoelastic phase separation [120] and shearinduced migration [65].

Viscoelastic phase separation occurs in mixtures having strongasymmetry in viscoelastic properties of the mixed components, whichis satised by many soft matter systems. Viscoelastic phase separationleads to the formation of a transient network (gel) of the dispersedphase, under the constraints of sufciently strong attractive interac-tions, and sufcient concentration of the dispersed phase. The transientnetwork exhibits quite slow dynamics. In the case of asymmetric poly-mer blends, viscoelastic phase separation is described by the general-ized balance equations as used for strongly driven soft matter systems[121].

Shear induced migration occurs in pressure driven hard spheresuspensions, conned in narrow ow channels [122]. Hydrodynamicinteractions between particles give rise to an osmotic pressure [123],which is linear with the local shear stress, and which can be viewedas an effective temperature [65]. In narrow ow channels there aregradients in the shear stress, which lead to gradients in the osmoticpressure. These gradients are relaxed via particle migration towards

the center of the ow channel. Shear induced migration is solved6. Application of soft matter concepts in food structuring

In the above topics in food structuring processes, we have discussedrepeatedly various soft matter concepts, which have been found usefulin the theoretical description of relevant physical phenomena. These re-curring concepts we have listed in Table 1. Below, we discuss in how farthese concepts have been used already in the eld of food science. Fur-thermore, we indicate novel problems for which the soft matter con-cepts are expected to have a large added value.

As an indication of the use of soft matter concepts in food science,we have classied the papers, published by food scientists in the SoftMatter journal, in Table 2. This shows that the most investigatedtopic is self-assembly, which is driven by 1) the interest in the correctdelivery of functional ingredients in the digestive tract, and 2) thequest for food structures having less fat, but with similar functionality.Thermodynamics and jammed systems receive moderate interest,while driven systems and directed assembly are the least investigatedtopics. Furthermore, observing the list of principal investigators fol-lowing the soft matter approach in Table 3, one nds that they are notevenly spread over the globe. We nd they are concentrated in a smallnumber of sites, namely Wageningen University and Research, ETH inZurich, Birmingham and Leeds in England, Guelph University, the re-search institutes NIZO and IFR, and the multinationals Unilever andNestle. Most sites are specialized in certain topics, while Wageningenand themultinationals aremore diverse, which is likely due to their size.

Due to the specic drivers for the above mentioned food research,the physics of many food systems under investigation is quite novel tothe main-stream research in Soft Matter physics. Below, we will dis-cuss the typical nature of various food systems, with the objective ofraising the interest of these main-stream physicists in food materials.

Under the topic of self-assembly, food scientists are trying to createmicrospheres that are able to function as controlled delivery vehicles[139]. These microspheres are self-assembled via absorption of food-grade colloids onto the interface of emulsion droplets. Controlled deliv-ery can then happen in the digestive tract, via the disassembly of thecolloid layer on the interface [10]. This can be done via change in pH(changing conformation of proteins), enzyme activity or displacementof colloids by (bile) surfactants. Another major theme in self-assemblyis organogels, whose primary function is to replace gels formed by fatcrystal networks as present in fatty food products like butter, marga-rine and bakery doughs [138].

The topic of thermodynamics is found typically in studies investi-gating biopolymer mixtures, which are also termed water-in-wateremulsions. These systems consist of solutions of proteins with poly-saccharides, that are phase separated [178]. The driver for these stud-ies is again the quest for low-fat food materials, but now having a

Table 2Food studies taking soft matter approach, sorted according to topics.

Topic References

Thermodynamics [94,125129]Self assembly [130,10,131139]Directed (Dis)assembly [140,11,141,142]Driven systems [143,65,144,145]Jammed systems [146151,6,152]

24 R.G.M. van der Sman / Advances in Colloid and Interface Science 176177 (2012) 1830Table 3Principal investigators taking a soft matter approach to Food.

Name Address Topics Ref.

Aken NIZO/Wageningen, Food Disassembly/emulsions [126]Bakalis Birmingham, Food CFD, driven systems [143]creamy texture. The construction of phase/state diagrams has alreadya long-standing tradition in food science, but it is often done usingonly experimental input [192]. The phase/state diagram is acclaimedto be a convenient tool for depicting the processing path during foodprocessing, where phase transitions are used to induce structuring ofthe food material [3]. Recently, we have shown that these phase/

Boom Wageningen, Food Driven systems/directedassembly

[141]

Bot Unilever, Food Organogels/self assembly [153]Brady Cornell MD sugars [154]Callaghan MacDiarmid, Physics NMR/MRI/rheology [155]Cohen-Stuart Wageningen, Food physical chemistry

proteins[126]

de Kruif Utrecht, Physics/NIZOFood

Dairy colloids [156]

Dickinson Leeds, Food SCF/Brownian dynamics/scattering

[94]

Dutcher Guelph, Physics AFM, proteins [132]Ettelaie Leeds, Food Brownian dynamics/SCF [157]Euston Edinburgh, Food MD proteins [158]Fisher ETH, Food Food rheology [159]Foegeding North-Carolina, Food Oral processing [160]Frith Unilever, Food Rheology/soft glasses [161]Fryer Birmingham, Food Driven systems [143]Garti Hebrew Thermodynamics liquid

crystals[162]

Groot Unilever, Food Mesoscale simulation [163]Gunning IFR, Food Disassembly/digestion [10]Hartel Wisconsin Crystallization/thermo [164]Hermansson SIK/Food Biopolymer mixtures/

driven[149]

Horne Hannah Research Inst.,Food

Protein moleculardynamics

[165]

Leermakers Wageningen, Food SCF [85]Leser Nestle, Food Liquid crystals [166]Limbach Nestle, Food Molecular dynamics [146]Livney Technion, Food Self-assembly [167]Marangoni Guelph, Food Organogels/fat crystals [168,133]Mayama Japan Fat crystals [125]McClements UMass Controlled delivery [169]Meinders Wageningen, Food Emulsions/foams [170]Mezzenga ETH, Food SCF simulations [1]Michel Nestle, Food Liquid crystals [166]Mitchell Nottingham, Food Starch [171]Moldenaers Leuven, Physics Rheology/biopolymer

mixtures[172]

Morris IFR, Food Polymers/glasses [10]Nicolai Lemans, Physics Protein gels [173]Normand Firmenich, Food Glassy dynamics [174]Norton Birmingham, Food Oral processing/

destructuring[175]

Opheusden Wageningen, Food Brownian dynamics/gels [176]Parker IFR, Food Polysaccharides [140]Rogers Saskatchewan, Food Organogels/fat crystals [177,133]Rousseau Toronto, Food Fat crystals [147]Stokes Queensland, Food Rheology/soft glasses [6]Stoyanov Unilever, Food Self-assembly/delivery [128]Schurtenberger Friburg, Physics Scattering techniques [178]Terentjev Cambridge, Physics theory, thermodynamics [179]Tromp NIZO, Food Driven biopolymers [180]Tuinier IFF Juelich, Physics Soft matter/scattering [156]Turgeon Lafayette, Food Thermodynamics

biopolymers[90]

Ubbink (formerly) Nestle Glassy systems [2,146]van der Linden Wageningen, Food Protein self-assembly [181]van der Sman Wageningen, Food Mesoscale simulations/

driven systems[3,58]

Velikov Unilever, Food Self-assembly/delivery [139]Vilgis MPI Mainz, Physics Polymers [182]Wilde IFR, Food Digestion [11]state diagrams can be constructed using the thermodynamic theoryof FloryHuggins, complemented with the free volume extension byVrentas and Vrentas [189,188].

Also, the investigation of jammed systems, such as glasses of carbo-hydrates or biopolymers, and gels of proteins, already has considerablehistory in food science [227]. These are subjects addressed by food sci-entists having a background in colloid science. Such systems are alsoregularly addressed by soft matter physicists [99]. Also for a long time,food researchers have been investigating slow dynamics in these sys-tems [236]. However, the theories of soft matter physics are not yetwidely applied but their potential has been appreciated [6]. Foodmaterials are very rich in phenomena having slow dynamics, whichare hardly investigated by soft matter physicists. Examples of theseslow dynamics are blooming of chocolate [125], retrogradation (recrys-tallization) of starch [241], and swelling of starch granules [145]. Fur-thermore, it is expected that many food systems exhibit visco-elasticphase separation (VPS) [120]. This is a phenomenon quite unknownto the food science community, given a few exceptions [1,239]. Insome food systems VPS is conned to the two-dimensions of the inter-face. This happens in self-assembled delivery vehicles, which are disas-sembled via displacement of proteins by surfactants. The displacedprotein network displays similar characteristics to a transient gel net-work, a typical signature of VPS [94].

As stated in Table 2 directed assembly or structuring via strongdriving is still minor subjects of investigation. However, the investigatedsystems are quite non-conventional to the soft matter community. Sev-eral directed (dis)-assembly studies involve enzymes, which assemblevia making crosslinks [242], or disassemble via incisions in biopolymers[11], thereby imparting their stabilizing functionality. Several studiesinvestigate food structuring using the conning walls of microuidicdevices [141]. Connement can also be used for destructuring, as inthe mouth during eating or in microuidic devices, via which raw foodmaterials are separated into various functional ingredients [175].

From Tables 2 and 3 we conclude that food research, taking a softmatter perspective, is gaining momentum. This research addressestopics that are shared with the mainstream research in soft matterphysics, albeit that their main focus is on systems near equilibrium.Systems driven far from equilibrium are still a challenge, but highly rel-evant for food structuring processes. From the literature one also ob-serves that various soft matter concepts, as summarized in Table 1,are used in the analysis of the phenomena occurring in food structuringprocesses. This we have made more explicit in Table 4, where we havelisted in which reference these concepts are used, together with thefood system under investigation and the principal investigator. As al-ready discussed partly above, the use of some concepts has alreadysome history in food science, like the phase/state diagram, slow dy-namics and the generalized balance equations. The concepts of thestate diagram and slow dynamics are in use already for two decades,due to the early recognition that shelf-stable foods are in the glassystate [227]. However, it must be said that the analysis of slow dynamicsis largely empirical. The use of the generalized balance equation is dueto the rooting of food engineering in chemical engineering approaches,where these balance equations are heavily used in the description ofmomentum, heat and mass transfer. To make this a proper soft matterapproach, the driven forces in the description of these transport pro-cesses have to be linked to a proper description of the thermodynamicsof the food material, as we have done in our recent papers [37,211,65].

The thermodynamics of various food systems is investigated theo-retically. However, the community performing this research is quitedifferent from the one investigating transport phenomena food sys-tems, using balance equations of conserved physical quantities. Thiscan be seen from the principal investigators listed in Table 4. It is ob-vious that understanding of food structuring is enhanced if exchangebetween these communities is stimulated. Recognizing that the foodmaterial being structured is an example of strongly driven soft mat-

ter, the use of soft matter concepts like mean eld theory, free volume

thermodynamics and transport coefcients. There are a few examples

Table 4Soft matter concepts applied to food systems.

Topic System PI Ref.

Free energy Calciumproteins Penfold [183]Microchannel emulsication Nakajima [184]Protein solutions McClements [185]

Thermodynamictheory

Casein micelles Kruif [156]Casein micelles andpolysaccharides

Tuinier [186]

Protein/polysaccharidemixtures

Turgeon [90]

Proteins/starch at highpressure

Heeremans [187]

Meat proteins van der Sman [188]Phase/statediagram

Starch van der Sman [189,190]Milk Vuataz [191]Proteins Kokini [192]Baking Guilbert [193]Waterwater emulsion Tuinier/deKruif [194,195,93]Casein/thickener deKruif [196]Microemulsions Garti [197]High pressure freezing LeBail [198]Liquid crystals/lipids Mezzenga [199]Starch-based foods Mitchell [200]Watersugar solution Blond [201]Organogels Bot [202]Surfactant/biopolymer mixture McClements [203]Gelatin Sobral [204]

Free volume Starch lms Benczedi [205]Maltodextrins glasses Ubbink [146]Starch thermodynamics van der Sman [189]Water diffusion Karel/van der

Sman[206,152]

Anti-plasticizing Seow [207]Biopolymer rheology Kokini [208]Rheology sugar/biopolymermixture

Kasapis/Mitchell [209,210]

Generalizedbalance eq.

Sheared suspensions van der Sman [65]Heated meat van der Sman [211]Emulsion van der Sman [212]Intensively heated food Datta [28]Heated chocolate Bakalis/Fryer [143]Rehydrated foods Saguy [213]Hydrogels Mizrahi [214]Osmotic dehydration Fito [215]Frozen foods LeBail [216]Microwave heating Dincov [217]Baking Trystram [218]Baking Andrieu [219]PEF/electroporation tissue Vorobiev [220]

Non-equil. phasediagram

Emulsion/thickener McClements [221]Directed assemblyhigh-protein

van der Goot/Boom

[222]

Gels of protein bers van der Linden [223]Gels at high pressure Dickinson [224]Polysaccharide/carbohydrateglasses

Roos [225,226]

Polysaccharide/carbohydrateglasses

Slade/Levine [227]

Mean eldtheory

Sheared suspensions van der Sman [65]Sheared emulsions Agterof [228]Creaming emulsions Robins [229]Electroporation Vorobiev [220]Ice recrystallization Sutton [230]Thermal conductivity Rotstein [231]

Effectivetemperature

Sheared suspension van der Sman [65]Sheared waterwateremulsions

Frith [161]

Slow dynamics Ice recrystallization Sutton [230]Maltodextrins Hemminga [232]Protein aggregation Parker/Noel [233]Protein gelation/aggregation Nicolai [173]Foam disproportionation Dickinson/

Ettalaie[234]

Sugar glassy dynamics Ludescher [235]Sugar glassy dynamics Noel/Parker [236]Beta-relaxation meat Johari [237]Viscoelastic phase separation Dickinson [238,239]

25R.G.M. van der Sman / Advances in Colloid and Interface Science 176177 (2012) 1830of the use of these concepts within food science, which are listed inTable 4.

Often the complexity of foodmaterials is so rich that the applicationof the concepts of soft matter implies the use of advanced simulationtechniques which are also commonly applied in soft matter physics.The simulation techniques used range from molecular dynamics (MD)at themolecular microscale, Lattice Boltzmann techniques at the meso-scale of the food structure, tomacroscale approaches based on the (gen-eralized) balance equations. These advanced methods often requirequite expert knowledge. Consequently, this implies again a differentcommunity with little exchange with other communities. Again, tostimulate communication we have listed illustrative examples of theuse of such simulation techniques in Table 5. On occasion multiplelengthscales present in structured foods are addressed in multiscaleapproaches [251].

We believe that there is a huge potential for the soft matter ap-proach in various other problems concerning food structuring. To givethe reader an idea of these potential applications, we have listed themin Table 6. Therewehave indicated towhich systems several softmatterconcepts might be applied. The listed references indicate studies wherethese systems have been investigated or reviewed. As one can observe,we foreseemuch potential for concepts from the eld of strongly drivensoft matter. In several food systems shear banding has been observed,but it has not yet been analyzed using theories for these systems, suchas Soft Glassy Rheology (SGR) or Shear Transition Zones. Mesoscalesimulations with phase eld theory can be used to describe foodsbeing structured via crystallization, such as chocolate and ice cream.Protein assembly/aggregation can be described via patchy colloids,which is an enhancement of the often applied hard sphere representa-tion of colloids. Here, the hard sphere is sticky of several patches on thesurface. These augmented hard spheres can easily be included inmeso-scale simulation techniques such as Brownian Dynamics or LatticeBoltzmann. After having applied free volume theory for describingmoisture sorption in polysaccharides, we have recently investigatedthe application of this theory to moisture sorption in proteins [188]and to moisture diffusion in polysaccharides [20]. Free volume theorycan also be applied to mixtures exhibiting depletion interactions. Infood structuring processes the matter is frequently driven by elds,which are not commonly applied in soft matter physics, such as electri-cal elds and high pressure elds. This would make the problems veryattractive for physicists. Theoretical work has yet to be performed, butanalysis by means of qualitative tools as non-equilibrium phase dia-gram has great potential. Of course, these tools also have potential infood structuring processes, where material is driven in a more conven-tional way, namely by shear ow. For shear-driven systems theory ismore developed, where such concepts as effective temperature and ef-fective medium theory can be very helpful.and effective temperature can be advantageous for describing the

Table 4 (continued)

Topic System PI Ref.

Slow dynamics Fat crystallization Starov [240]Starch retrogradation Blanshard [241]Dense emulsions Wilde [229]7. Conclusions

In this review we have shown that the soft matter approach to foodmaterials is denitely gaining ground in food science, and it also receivesrecognition from the eld of soft matter physics. In the different phasesof the food structuring process a multitude of concepts from soft matterphysics can be applied. Surprisingly, many of these universal conceptsbuild upon the original work of van der Waals. Their universality offers

Micro Molecular Carbohydrates [154,243,146]

Surfactants at interfaces [158]

26 R.G.M. van der Sman / Advances in Colloid and Interface Science 176177 (2012) 1830Bulk protein/polysaccharidemixtures

[158]

Mesoscale Brownian dynamics Colloids at interfaces [248]Food gels [176]Colloids displacement atinterface

[249]

Lattice Boltzmann Emulsions [212]Suspensions [250]

Macro Porous media approach Intensively heated foods [28]Multiscale Cell model+macro Expanded snacks [251]dynamics Protein/Polysaccharide mix [149]SCF Casein at interfaces [244]

Liquid crystals [245]Protein/polysaccharide mixat interface

[157]

MonteCarlo Proteins absorption atinterfaces

[246,247]Table 5Advanced simulation techniques used in the Soft Matter approach to Food.

Scale Technique System Ref.good opportunity for improved understanding of the complexity of thefood materials and their structuring processes.

Researchers are performing food research taking a soft matter ap-proach, but they do not form a united community, as they come fromstarkly differing disciplines, like soft matter physics, computationalphysics, food colloidal science and food engineering. Hence, it is time-ly to form such a community, which is stimulated via this review, andthe upcoming Faraday Discussion.

Acknowledgments

Hereby, we acknowledge the fruitful discussions with Job Ubbinkon this review. Furthermore, we appreciate the careful reading of themanuscript by Bill Frith.

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