Designing Multiscale Structures for Desired Properties of Ice Cream

James F. Crilly, Andrew B. Russell, Andrew R. Cox, and Deryck J. Cebula*

UnileVer R&D Colworth Science Park Sharnbrook, Bedford, MK44 1LQ, United Kingdom

Ice cream is a complex multiphase structure consisting of ice, air, and fat as dispersed phases at a range ofdifferent length-scales, all embedded in a continuous phase consisting of unfrozen sugar solution known asthe matrix or serum. The entire structure is the result of both the ingredients and all the processes used in icecream manufacture including emulsification, freezing, and aeration. It is thermodynamically unstable anddelivered quality can only be assured at low and stable temperatures. Physicochemical processes during storagecan lead to loss of quality by coarsening of the ice particles, disproportionation of the air, and the loss ofwater from the matrix. Product design for specific sensory, stability, shape, and increasingly, nutritionalproperties, is a challenging task and must take account of all these aspects of the structure. Almost all propertiesare sensitive to the size, density, and morphology of the dispersed phases as droplets, cells, crystals, or evenmicelles. Finer structures, in general, result in more desirable organoleptic properties such as creaminess andsmoothness but the interfacial dynamics are more rapid, leading to less stability. Even small changes in therelative densities of the dispersed phases such as in the case of low-fat or fat-free products can dramaticallychange key properties such as taste perception, mouth-feel, and rate of melt. Conventional formulation andprocessing techniques complemented by the use of specific additives such as emulsifiers and stabilizers enablesome control, albeit limited, over the interfacial dynamics and stability. New ingredients and new technologies(such as low temperature extrusion) have been developed to enable higher levels of control on the interfacialbehavior either through direct molecular intervention on an interface or new structuring processes whereininterfaces are created in a new or different way. Examples of new ways of influencing the ice, fat, and airinterfaces will be discussed such as “ice structuring protein” and hydrophobins. Challenges that remain highlightthe need for new types of molecular and microstructural interventions to achieve the next levels in designcapability for the ice creams of tomorrow.

1. Introduction

There is a widely held impression that the design anddevelopment of ice cream involves only issues relating to recipeand chefmanship borne out of kitchen skills. Although chef-manship is important, ice cream per se is a deeply technologicalproduct in all of its development, manufacture, distribution. andselling. Whereas product delivery to the consumer is aimed toachieve high appeal through various sensorial attributes, it isdeep inside the structure where the science and technology isembedded. Designing ice cream to meet specific technicalperformance targets is a major challenge.1 This paper aims toshow that, with deeper scientific understanding and some veryexciting new technologies both in use and under development,some of the major barriers to better ice cream in terms of quality,stability, nutrition, and innovation can be overcome.

2. Design Challenge

Ice cream is a product that truly operates on a range of spatialscales. On a macroscale, the sensory properties of the textureare perceived; these are determined by the microscopic detailsof the structure. In turn, microstructure is determined by complexmolecular interactions. The main aim of ice cream manufacturersis to generate the correct microstructure in the ice cream toachieve the desired organoleptic characteristics such that theproduct can breakdown and melt away in the mouth, thusdelivering the consumers’ preferences for taste. In addition, thestructure needs to be sufficiently robust to withstand transporta-tion and storage, so there is quite a balancing act to perform to

reconcile these simultaneous and often conflicting requirements.Therefore, in achieving the optimum microstructure, there aretrade-offs between the formulation (levels and types of ingre-dients and actives such as process aids and stabilizers) and theprocessing regimes (heat transfer rate, temperature of freezing,etc). A general description of the science of ice cream is givenby Clarke.2 Increasingly, as consumers demand healthierproducts, nutritional aspects of formulation become vastly moreimportant. Whereas, for example, reduction of both saturatedfat and sugar are desirable, they may not be immediatelypossible because these are crucial components for both theprocess conditions and the microstructure per se.

A typical microstructure is one that consists of ice crystalsand air bubbles in the size range 20 µm to about 100 µm, andfat droplets in the size range of 1 µm to 0.1 mm (Figure 1).These fine entities are embedded throughout a viscous solutionof sugars, polysaccharides, and milk proteins known as the“matrix”. At another order of magnitude lower in scale, it ispossible to identify the location of the fat. Fat droplets of size<1 µm can be seen that exist as clusters located on the surfaceof the air cells as well as distributed throughout the matrix. Notshown here, milk protein is also partially located on the airinterface and together fat and protein both help to stabilize theair. Fat has an incredibly important role in the microstructurethat relates directly to the sensory properties like mouth-feel,creaminess, and flavor delivery but it is also critical to thestability properties of products such as rate of melt. It can beappreciated that reducing the fat by 50%, or more, to enablehealthier products may not only compromise the sensory qualitybut may put the stability, specifically of the air phase, atconsiderable risk.

* To whom correspondence should be addressed. E-mail:deryck.cebula@unilever.com. Tel.: 44-1234-222748. Fax: 44-1234-222007.

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10.1021/ie701773z CCC: $40.75 2008 American Chemical SocietyPublished on Web 08/06/2008

As previously stated ice cream is thermodynamically unstableand even under ideal storage conditions the structure, specificallythe ice and the air phases, will coarsen over time, resulting inloss of quality and loss of stability. This situation is exacerbatedby upward temperature fluctuations and by pressure changeswhich affect the air phase. In addition, stability becomes a realproblem when distributing products across different altitudeswhen the ice cream expands in response to lower pressures thenshrinks to much lower volume as normal pressure is restored.Low-fat or reduced-calorie products are particularly susceptibleto variations in ambient conditions.

To fulfill the requirements of the complete design challengewithout unacceptable trade-offs of quality and stability, bettercontrol is needed of the starting microstructure and of theunstable dispersed phases: ice and air. These require solutionsto overcome some major technical problems. The remainder ofthe paper will address new technical developments in processand ingredient technology that can help with the design chal-lenge.

3. Technical Developments

3.1. Microstructure Control by Process. One of the mostdirect approaches to better control microstructure is via process-ing. Low-temperature extrusion is a new technology involvinga single screw extruder3 that replaces conventional hardeningand takes fresh ice cream straight from the manufacturing freezer(at about -5 °C) and reduces the temperature rapidly below-10 °C. The process continues to work the product to capturea finer microstructure. The technology consists of an extruderclosely coupled with a conventional scraped surface freezer andoperated at high torque to slowly churn and extrude the icecream at much lower draw temperatures. These lower drawtemperatures are possible because the mechanical energydissipation experienced by the product in the screw extruder ismuch lower than in the conventional scraped surface freezer.4

This energy dissipation becomes more important when the icecream viscosity increases rapidly with declining temperature.Low-temperature extrusion of ice cream has also been achievedusing a twin screw extruder5,6 with draw temperatures as lowas -18 °C reported.

Low-temperature extrusion leads to a much finer microstruc-ture. This is a direct consequence of the lower draw temperature,which slows the coarsening of the newly formed air and icephases, which begins immediately upon creation of the ice creamstructure. Detailed microstructural analysis of low-temperatureextruded ice cream shows that both the air cells and the icecrystals are much smaller, but the presence of smaller air cellsis the dominant factor for enhanced sensory properties, specif-ically creaminess. One additional benefit of low-temperatureextrusion is that because of the optimized microstructure, it ispossible to reduce fat level substantially, in fact by ca. 50%,while delivering the same level of creaminess and bettermeltdown properties. This has been a major step forward inour drive to improve the nutritional profile of ice cream whileretaining high quality and stability.

A number of new higher-quality low-fat, or “light”, productsshowing major improvements have been launched across theworld that exploit the low-temperature extrusion capability andthese have been very much appreciated by the consumers in allmarkets. However, although low-temperature extrusion givesus the benefits of a better initial microstructure, it does not solvethe problem of instability due to temperature abuse on storage.For that, specific technical solutions for the ice phase and forthe air phases per se are needed.

3.2. Controlling Ice Structure. Ice crystals in ice creamcoarsen over time even under isothermal storage conditions,7

and the situation is exacerbated or accelerated by elevatedtemperatures or fluctuations. This is a phenomenon called“recrystallization”. Figure 2 shows the consequence of thisprocesssmany smalls crystals reduce to fewer bigger crystalsand the air bubbles increase in size and may coalesce over time.These effects detract from the creaminess and smoothness,resulting in a product that is cold eating and has poorer resistanceto meltdown (that is to say it melts and loses serum faster).There are two distinct processes involved in recrystallization(Figure 3) more fully described by Sutton et al.8 The first isreferred to as migratory recrystallization and is a manifestationof Ostwald ripening. Here, the tendency to minimize surfacefree energy in ice crystals of different sizes in a multiparticledistribution favors the survival of crystals with a large radius

Figure 1. Ice cream microstructure depicted through scanning electron microscopy. The overall structure is determined largely by freezing and aerationprocess conditions. Complex interactions exist between the structural phases and between fat, protein, and emulsifier. Shown in the inset, air is stabilized bya coating of fat droplets (Pickering stabilization) and because the “matrix” is highly viscous.

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of curvature. Storage of products at low temperatures can retardthis process but cannot arrest it.

The second process is accretive recrystallization and occurswhen two crystals make contact. The negative curvature of thearea of contact drives the development of a neck, leading tothe coalescence of the two crystals. Repeated accretion leadsto agglomerates of crystals of quite irregular shapes althoughthe process of rounding during inevitable temperature fluctua-tions in storage largely offsets irregularity. Using gums and otherthickeners (called stabilizers) retards contact between crystals,thereby reducing accretion. Growth and accretion take place atthe same time. Under highly abusive conditions (sometimes seenin storage or selling cabinets) the ice cream structure cancompletely breakdown. Clearly, with conventional ingredients,the basic laws of thermodynamics prevail and a new approachto ice control is required.

3.2.1. Ice Structuring Proteins. It seemed that nature alreadyhad the solution to ice control. A special type of protein in theblood of Antarctic fish held out the potential for a newmechanism in ice control.9 These proteins bind to ice crystalsand protect the organisms from damage in freezing water.10

Initially referred to as antifreeze proteins (AFP), these have nowbeen identified in more than 200 organisms, all of which survivein cold habitats. These molecules influence the size and structureof the ice crystals and, subsequently, the overall properties ofthe ice phase. The potential for the use of antifreeze proteins infoods has been considered by Griffiths and Ewart11 and, morerecently and particularly for ice cream, by Ragand and Goff.12

These proteins are referred to as “antifreeze” because of theirprotective function in biology, but as an ice cream ingredient,it is their structuring function that is most important; this iswhy they have been renamed ice structuring proteins (ISP) byClarke et al.13

The current body of research literature reports a variety ofmolecules, distinguished by their structure, either helical,globular, or other forms derived from different species. Eachhas been designed by nature most effectively to fulfill its purposeof protecting the organism from ice damage. All ISPs have thecommon property of binding to ice but some lock onto specificcrystal planes giving anisotropic ice structures. The globularstructure (type III) obtained from Ocean Poutsfish from theNorth Atlanticshas the most interesting properties in an icecontrol context. ISPs are associated with three phenomena:thermal hysteresis, recrystallization inhibition, and crystal habitmodification. The latter two properties are of most interest.

First, nearly all ISPs studied can limit the growth of icecrystals. A typical example of difference in size and morphologyis shown in micrographs of ice dispersions with and withoutthe proteins present (Figure 4). They almost completely arrestthe isothermal recrystalliztion process. Second, because of thecrystal face specificity of individual molecules, ISPs can changethe morphology of the growing crystal as shown in Figure 5.These really are the properties being sought and the ISP typeIII is the molecule that has been selected by Unilever to developnew ice control technology.

3.2.2. New Ice Structure. Small crystals with differentmorphologies can result in some new and surprising icestructures in multiple particle dispersions. ISP type III givesvery spicular crystals with high aspect ratios, as much as5-6:1, resulting in high connectivities at ice contents of60-70%. The micrographs in Figure 6 show the differencein ice structures in a water ice with and without ISP. ISPcauses the ice phase, instead of the matrix phase, to becomecontinuous, thereby locking in or encapsulating the matrixphase containing colors and flavors in pockets within the ice.Whereas in an open ice structure, the solution-borne com-ponents such as the color and flavor can leach out, with ISP

Figure 2. Ice cream microstructure shown by scanning electron microscopy. Freshly made ice cream shows a much finer microstructure with smaller aircells and ice crystals, but after storage in which some temperature fluctuations occur, there is a coarsening of all phases. Stability in ice cream can not befully controlled. Both air and ice phases are intrinsically unstable, leading to reduced creaminess/smoothness, colder eating, poorer visual appeal, and rapidmelting.

Figure 3. Ice recrystallization, as observed by low-temperature light microscopy (crystals are approximately 50 µm across), occurs by two mechanisms:migratory and accretive crystallization. These occur simultaneously driven by reduction in curvature of crystal faces and “necking”.

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present, this cannot happen and this behavior has beenexceptionally useful for stabilizing innovative products withhigher levels of solids like fruit purees.

Since 2003, more than 40 products containing ISP havereached the market, some new and some major improvementsof current products, all exploiting the unique properties of this

protein to give new benefits. These illustrate the level of icecontrol for stability with healthier products even under abusiveconditions. We believe this is a breakthrough for better icecream, which consumers continue to demand.14 However,arguably, the biggest challenge remains the effective control ofthe air phase.

Figure 4. Effect of ISP type III is shown using light microscopy. ISP, even at concentrations as low as ∼1 × 10-4mg/mL (∼1 × 10-5 wt %), limits thesize of ice crystals as they grow in sucrose solution.

Figure 5. Light microscopy showing ice crystal habit modification achieved by the ISP adsorbing to specific faces of the crystal and thus altering its shapeas it initially grows.

Figure 6. With planar ice crystal faces comes a greater degree of aggregation. Scanning electron microscopy shows that at high ice content, the ice phasebecomes continuous, increasing the mechanical strength. The hard line in the inset picture shows a continuous line, or path, that can be drawn betweenadjacent crystals. With an ISP modulated structure, no such line can be drawn, indicating that the crystals have become fully networked.

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3.3. Controlling Air: The Next Challenge. The air phasein ice cream is stabilized through a complex combination offat droplets and milk proteins that adsorb onto the surface ofthe air cells during the shearing and aeration processes in thefreezer.15 The semisolid fat droplets under go a process calledpartial coalescence in the high shear freezing process. This leadsto a proportion of the fat particles adsorbing to the air bubbles,which helps stabilize the dispersed air phase by impedingcoalescence of adjacent air cells when in contact. The presenceof clusters of partially coalesced fat particles in the continuousphase also leads to the retention of bulk product shape onmelting and provides creamier in-mouth characteristics. De-emulsifiers (for example, mono/diglycerides) play an importantrole in increasing the level of partially coalesced, or destabilized,fat droplets. They facilitate the process of destabilization throughreducing the stability of the fat particles to shear. This isachieved by displacing some of the protein from the surface ofthe particles during the aging step, prior to freeze-aeration.16,17

This process of controlling the stability of the air phase viaadsorption of fat and protein is very susceptible to small changesin the protein/de-emulsifier ratio, or to changes in the fat content,fat particle size, and also to changes in process conditions.However, although stabilization by fat adsorption to the airbubbles is a key process, long-term stability of the air cells (i.e.,through shelf life of the product) is achieved principally by thepresence of the very viscous matrix phase. At ideal storagetemperatures (-18 °C or below) the matrix is almost solid, yetstill pseudoplastic, and even though the structural componentsof the ice cream are thermodynamically unstable, they areeffectively held in place and kinetically trapped because of thesolid nature of the product. Ultimately, air bubble stabilitythrough these mechanisms can be achieved under ideal storageconditions with constant temperature and pressure. Nevertheless,relatively small changes in these conditions readily leads to airbubble coarsening, product degradation, and loss of quality onconsumption.

3.3.1. HydrophobinssNature′s Solution. With conventionaltechnology, proteins, fat, and de-emulsifiers can help achieve afiner air phase microstructure (post extrusion) of the ice creamfrom the freezer. With temperature cycling (i.e., poor storageor distribution conditions), the coarsening of the air cells isirreversible and will lead eventually to channelling, air loss,and shrinkage of the product.18 Even with the optimum use ofde-emulsifiers and ideal processing, the air stability is finelybalanced and precarious against ambient variations. Thus, abetter and more direct solution is required to make a step changein enhanced air control, an ability that has been long soughtafter by ice cream manufacturers.

In investigations for more powerful emulsifiers, UnileverResearch teams focused on a set of natural proteins referred toas hydrophobins (originally found in arboreal fungi) to helpsolve the problem. As in the case of ISP, it seemed that naturemay have already developed a solution to the problem of airphase control. Currently, two classes of hydrophobins, I and II,distinguished by their specific aqueous solubility, have beenidentified.19 Class II hydrophobins, which readily dissolve inaqueous solutions, currently hold out the best potential as aircontrol agents. The Class II hydrophobin, HFBII, from theTrichoderma reesei fungus exhibits a stable globular amphiphilicstructure, where the majority of the surface is hydrophilic anda smaller area is distinctly hydrophobic.20,21 A distinguishingproperty of this protein is its very high surface activity at air/water interfaces (see Figure 7). Measured surface tensions of

this, and other hydrophobins, make this class of protein one ofthe most surface active known.

However, the most important property of HFBII in relationto air phase stabilization is the ability for it to adsorb to theair/water surface and form exceptionally elastic and cohesivefilms. This is demonstrated by the measurement of surface shearrheology, shown in Figure 8. The surface shear elastic andviscous moduli (G′s and G′′ s, respectively) are measures of themechanical properties of the surface layer.22 The values of G′sfor HFBII are at least an order of magnitude greater than thatof other food proteins. Such mechanical properties are knownto be directly related to the stability of foams.23 Effectively,each air cell has its own elastic membrane, giving it the abilityto respond to and resist pressure and temperature changeswithout significant coalescence. From the preliminary workdone,24 the HFBII layers appear to provide some of theproperties required to stabilize air cells individually so that theyare robust even against temperature cycling and abuse.

4. Conclusions

Designing structures in ice cream to meet all productperformance targets is a very challenging task with manytechnical hurdles. Whereas the aim is to control kinetically

Figure 7. Data for the equilibrium surface tension for hydrophobin at anair/water surface (pH 7). This compares to typical values for skim milkpowder (50 mN m-1) or sodium caseinate (45 mN m-1).

Figure 8. Data for the surface shear rheology comparing hydrophobin andskimmed milk powder (SMP) at an air/water surface using an ARG2rheometer with a De Nouy ring. This shows that surface elasticity ofhydrophobin is at least an order of magnitude greater than for a typicalfood protein.

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stabilized microstructures by processing methods, significantresults can also be achieved by addressing certain molecularfeatures critical for good structure. Air control remains thebiggest technical challenge and the hydrophobins developmentholds out great promise and has real potential for the future.With new developments in science and technology, muchprogress has been made in making better ice cream bothhealthier and better quality to meet modern consumers’ demands.

Literature Cited

(1) van der Graaf, K.; Crilly, J. F. New Horizons for Ice Cream. FoodTechnology International, EFFoST 2004, 13–15.

(2) Clarke, C. J. The Science of Ice Cream; The Royal Society ofChemistry: London, 2004.

(3) Burmester, S.; Russell, A. B.; Cebula, D. J. The Evolution of IceCream Technology. New Food 2005, 2, 42–45.

(4) Russell, A. Process innovation from research and development toproduction in a large companysdevelopment and commercialisation of alow temperature extrusion process. In Case Studies in Food ProductDeVelopment; Earle, M., Earle, R., Eds.; Woodhead Publishing: Cambridge,U.K., 2008; Chapter 11, pp 202-222.

(5) Bolliger, S.; Kornbrust, B.; Goff, H. D. Influence of emulsifiers onice cream produced by conventional freezing and low-temperature extrusionprocessing. Int. Dairy J. 2000, 10 (7), 497–504.

(6) Eisner, M. D.; Wildmoser, H.; Windhab, E. J. Air cell microstruc-turing in a high viscous ice cream matrix. Colloids Surf., A 2005, 263 (1-3), 390–399.

(7) Donhowe, D. P.; Hartel, R. W.; Bradley, R. L. Determination of IceCrystal Size Distributions in Frozen Desserts. J. Dairy Sci. 1991, 74, 3334–3344.

(8) Sutton, R. L.; Lips, A.; Piccirillo, G.; Sztehlo, A. Kinetics of IceRecrystallisation in Aqueous Fructose Solutions. J. Dairy Sci. 1996, 89,741–745.

(9) deVries, A. L.; Wohlschlag, D. E. Freezing Resistance in SomeAntarctic Fishes. Science 1969, 163, 1073–1075.

(10) Harding, M. M.; Ward, L. G.; Haymet, A. D. J. Type I “antifreeze”proteins structure-activity studies and mechanism of ice growth inhibition.Eur. J. Biochem. 1999, 264, 653–665.

(11) Griffiths, M.; Ewart, V. Antifreeze Proteins and Their PotentialUse in Foods. Biotech. AdV. 1995, 13, 375–402.

(12) Regand, A.; Goff, H. D. Ice Recrystallization Inhibition in IceCream as Affected by Ice Structuring Proteins from Winter Wheat Grass.J. Dairy Sci. 2006, 89, 49–57.

(13) Clarke, C. J.; Buckley, S. L.; Lindner, N. M. Ice Structuringproteins: a new name for anti-freeze proteins. Cryo-Lett. 2002, 23, 89–92.

(14) Crilly, J. F. ISP: A Breakthrough for Better Ice Cream. New Food2007, 3, 40–44.

(15) Goff, H. D. Colloid aspects of ice creamsA review. Int. Dairy J.1997, 7, 363–373.

(16) Leser, M. E.; Michel, M. Aerated milk protein emulsions-newmicrostructural aspects. Curr. Opin. Colloid Interface Sci. 1999, 4, 239–244.

(17) Wilde, P.; Mackie, A.; Husband, F.; Gunning, P.; Morris, V.Proteins and emulsifiers at liquid interfaces. AdV. Colloid Interface Sci. 2004,108-109, 63–71.

(18) Chang, Y.; Hartel, R. W. Stability of air cells in ice cream duringhardening and storage. J. Food Eng. 2002, 55, 59–70.

(19) Wessels, J. G. H. Developmental regulation of fungal cell-wallformation. Annu. ReV. Phytopathol. 1994, 32, 413–437.

(20) Hakanpaa, J.; Linder, M.; Popov, A.; Schmidt, A.; Rouvinen, J.Hydrophobin HFBII in detail: ultrahigh-resolution structure at 0.75Å. ActaCrystallogr., Sect. D 2006, D62 (4), 356–367.

(21) Linder, M. B.; Szilvay, G. R.; Nakari-Setala, T.; Penttila, M. E.Hydrophobins: the protein-amphiphiles of filamentous fungi. FEMS Mi-crobiol. ReV. 2006, 29 (5), 877–896.

(22) Bos, M. A.; van Vliet, T. Interfacial rheological properties ofadsorbed protein layers and surfactants: a review. AdV. Colloid InterfaceSci. 2001, 91, 437–471.

(23) Murray, B. S. Stabilisation of bubbles and foams. Curr. Opin.Colloid Interface Sci. 2007, 12, 232–241.

(24) Cox, A. R.; Cagnol, F.; Russell, A. B.; Izzard, M. J. Surfaceproperties of class II hydrophobins from Trichoderma reesei and influenceon bubble stability. Langmuir 2007, 23, 7995–8002.

ReceiVed for reView December 27, 2007ReVised manuscript receiVed July 4, 2008

Accepted July 9, 2008

IE701773Z

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