Trends in Food Science & Technology 19 (2008) 176e187Review* Corresponding author.

0924-2244/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.tifs.2007.11.012Aerated food gels:

fabrication and

potential applications

R.N. Zuniga and J.M. Aguilera*Chemical & Bioprocess Engineering Department,

Pontificia Universidad Catolica de Chile, Avda. Vicuna

Mackenna 4860, Macul, Santiago, Chile (Tel.: D56

562 3544254; fax: D56 562 6865808; e-mail:

jmaguile@ing.puc.cl)Aerated gels contain both bubbles and entrapped water, thus

offering ample versatility in product development. Dispersed

air (or other gases) provides an additional phase within the

gel that may accommodate new textural and functional de-

mands. Many food polymers form gels and their target proper-

ties may be enhanced by combining materials (mixed polymer

gels) or introducing a finely dispersed fat phase (emulsion

gels). Traditional methods to generate bubbles in foods as

well as non-conventional technologies (membrane processes,

microfluidics, etc.) are revised and their potential applications

in producing aerated gels are discussed. Aerated gels may find

applications in reducing the caloric density of foods and in-

ducing satiety, as carriers of flavors and nutrients, and in novel

gastronomic structures.

IntroductionFood gels are soft solids containing considerable

amounts of an aqueous phase (i.e., >80%) that have receivedmuch attention lately among food scientists (Aguilera &Rademacher, 2004; Hermansson, 2007; Nishinari, Zhang,& Ikeda, 2000; Renard, van de Velde, & Visschers, 2006;Totosaus, Montejano, Salazar, & Guerrero, 2002). The pres-ence of gel-like structures is ubiquitous among most high-moisture processed foods: jellies, yogurt, processed meats,etc. Air is also a component of several food products usuallypresent as a dispersed phase of bubbles or pores within a ma-trix. Air bubbles are abundant structural elements in solidfood foams, for example, bread, cakes, aerated chocolatebars and meringue, in semi-solid foams such as whippedcream or mousses and in beverages like milkshakes(Aguilera, 2005; Campbell & Mougeot, 1999). Even naturalproducts such as fruits and vegetables contain large amountsof water immobilized within cells and some of them, mostnotably apples, contain an appreciable quantity of air (e.g.,w25%) occluded in pores (Lazarides, Fito, Chiralt, Gekas,& Lenart, 1999). The importance of bubbles in food process-ing has been confirmed by a well attended second conferenceof Bubbles in Foods that took place in Windermere (UK)in September 2006.

Emerging evidence that obesity is increasing worldwideat an alarming rate has caught the attention of nutritionistsand food technologists (WHO, 2000). Entrapping abundantamounts of water and/or air in gel matrices may be one al-ternative to design products that promote satiety with re-duced caloric density (Aguilera, 2005). The food industryhas been using proteins and polysaccharides for many yearsas structuring agents to immobilize large quantities of waterin the form of gels (Aguilera, 1992; Harris, 1990; Imeson,1992) and aeration has become one of the fastest growingunit operations in the past decade (Campbell & Mougeot,1999). Thus, the presence of bubbles in gel-based foodproducts may result in unique properties conferred by theadditional gaseous phase and the increased internal surfacearea (Nussinovitch, Velez-Silvestre, & Peleg, 1992). Thispaper reviews initially, traditional and non-conventionalmethods to incorporate bubbles in liquids, since this is a pre-requisite to make aerated gels. Next, some of the materialsand processes used to produce functional gel matrices arerevised. Finally, potential applications of aerated or gasifiedfood gels are presented and discussed.

Dispersing gases in foodsAerated foods

Introducing a gas phase into a food matrix not onlyaffects its texture and firmness making the product lighter,but also changes the appearance, color and mouth-feel(Campbell & Mougeot, 1999). Although the final continu-ous matrix of an aerated food may be liquid (e.g., beerfoam), viscoelastic (e.g., marshmallows) or solid (e.g., me-ringue) bubbles are initially dispersed in the bulk of a liquidsolution or a viscous dispersion. Because aerated liquids arethermodynamically unstable, bubbles must be stabilized attheir aireliquid interface usually by surface active agentssuch as proteins and emulsifiers or solid particles, like fatcrystals. Furthermore, drainage and bubble coalescence is

mailto:jmaguile@ing.puc.cl

177R.N. Zuniga, J.M. Aguilera / Trends in Food Science & Technology 19 (2008) 176e187retarded by increasing the viscosity of the liquid in lamellaebetween the bubbles. If bubbles become physically entrap-ped in a gel network the food will be stable (Boom, 2007).

The type of gas influences structure and stability of aer-ated products. Foams containing N2 or air bubbles willcoarsen slower than those consisting of CO2 bubbles, sincediffusion is largely determined by the solubility of the gas(Weaire & Hutzler, 1999). In aerated chocolate, the type ofgas can be related to the internal structure, as shown recentlyby Haedelt, Beckett, and Niranjan (2007). Gases highlysoluble in chocolate (CO2 and N2O) yielded macro-aeratedstructures while those exhibiting low solubility (N2 andAr) formed structures containing microbubbles.

An aerated structure may facilitate mastication, enzymeaccessibility to substrates and enhance flavor delivery. It isessential for the manufacturer to be able to control the sizeand size distribution of air bubbles as well as the spatial dis-persion of the gaseous phase in order to control the qualityof the product (Lau & Dickinson, 2005).

Bubbles are desirable elements in gastronomy creations.Mousses and souffles, recognized as emblematic forms inculinary art, are classic examples in which the incorpora-tion and retention of bubbles is a critical factor in the suc-cess of the dish. Besides, modern innovative cooks exploitbubbles in their creations, as is the case of champagnegrapes from Homaru Cantu and the famous airs andfoams of Ferran Adria (also known as the chef who in-vented air).

Conventional methods to incorporate bubbles to foodsConventional methods to incorporate bubbles into a liq-

uid or viscoelastic medium include (Campbell & Mougeot,1999; Weaire & Hutzler, 1999): (i) nucleation of gas bub-bles in a liquid which is supersaturated; (ii) shaking or beat-ing the liquid; (iii) generating a gas by fermentation orchemical reaction; (iv) blowing gas through a thin nozzleor single orifice; and (v) by sparging or blowing gasthrough a porous plate.

Mechanical agitation, extensively used to produce liquidfood foams, is inefficient in the use of energy, most of it go-ing into agitation of the bulk liquid and not to form interfa-cial area. High-shear stresses are necessary to deform anddisrupt large bubbles of air incorporated in the whippingprocess (van Aken, 2001). Thus, energy input may shearsensitive ingredients such as proteins resulting in loss offunctional properties (Charcosset, Limayem, & Fessi,2004). The phenomenon called overbeating reportedfor long whipping times, results in excessive protein coag-ulation at the airewater interface, leading to aggregationand low interfacial activity decreasing the final overrun(Lau & Dickinson, 2004, 2005). The problem is one of lo-calizing the energy in the formed elements (bubbles) ratherthan dissipating it in the bulk of the mixture.

There are several reasons why it would be desirable todecrease the size of bubbles in foods and control their shapeand size distribution, among them, to modify texture, toextend the range of physical properties, to increase the in-terfacial area (at constant gas volume), to decrease synere-sis and to disperse air (and possibly aromas) imperceptibly,thus, reducing the caloric density. Air cells smaller than80 mm would be largely unnoticeable to the eye. Bubbleswithin a food increase crack-stopping (effectively enlargingthe radius of curvature of the crack tip), ductility and frac-ture toughness (Buckman & Viney, 2002; Vincent, 1998).Control of bubble size and size distribution is a major chal-lenge not yet achieved in industrial food processing. In con-ventional foaming methods, there is little control over theformation of individual bubbles, therefore they are not uni-formly distributed and the product presents a wide distribu-tion of bubble sizes (including some large size outliers).Decreasing the size of bubbles and avoiding polydispersitywould minimize the effect of buoyancy and the verticalstratification of bubbles, thus giving a better appearance.Monodispersed bubbles can greatly reduce Ostwald ripen-ing by reducing the effective Laplace pressure differencedue to their similar size. From a scientific viewpoint mono-dispersed bubbles are useful in fundamental studies becausethe interpretation of results is much simpler than for poly-dispersed populations (Yasuno et al., 2004). Alternatively,bottom-up approaches can be used to incorporate gasesat the level of individual bubbles. Bulk (conventional)and localized novel techniques to incorporate bubbles intoliquid or viscoelastic media are described schematicallyin Fig. 1.Novel methods to incorporate bubbles to foodsDispersing gases with membranes

Membranes have been used to incorporate air bubblesinto surfactant and protein solutions (Bals & Kulozik,2003a, 2003b; Kukizaki & Goto, 2006, 2007). Membraneprocesses use an applied pressure to force the gaseousphase through the membrane into the continuous liquidphase (Fig. 1a) (Charcosset et al., 2004; Vladisavljevic &Williams, 2005). Although many types of membranes(e.g., ceramic, metallic and polymeric) could be used todisperse gases, Shirasu Porous Glass (SPG) membranesare preferred because of their homogeneity of pore struc-ture and high-mechanical strength (Kukizaki & Goto,2006, 2007). Surface properties of membranes (hydro-philic/hydrophobic surface) influence particle characteris-tics for liquid/liquid dispersions. It is essential that themembrane surface is wetted by the continuous phase(Boom, 2007). However, this has been not studied yet forgas/liquid dispersions.

Droplet or bubble formation using orifices involves oneof the following two mechanisms: shear-induced detach-ment from the surface of the membrane by flow of the con-tinuous phase (Charcosset et al., 2004; Kukizaki & Goto,2006; Vladisavljevic & Williams, 2005) and spontaneousformation by interfacial tension, without continuous-phaseflow (Boom, 2007; Yasuno et al., 2002).

Bulk

Conventional

Bulk

Novel

Mechanical agitationFermentation

Chemical reactionOrifice

(d) Ultrasound

(c) Electrochemical

+

sonotrode

Localized

(a) Membrane

(b) Microdevices

gas

liquid gas liquid

Fig. 1. Conventional and novel mechanisms to incorporate bubbles in food (not at scale).

178 R.N. Zuniga, J.M. Aguilera / Trends in Food Science & Technology 19 (2008) 176e187In membrane processes the energy-intensive, mechani-cally induced shearing is not required to create small bub-bles and foaming is possible with an energy input orders ofmagnitude smaller than in conventional methods (Boom,2007). Hence, materials which are sensitive to localizedshear stresses such as proteins, are not affected to a largeextent (Bals & Kulozik, 2003a, 2003b; Charcosset et al.,2004). However, if shear-induced protein denaturation orliberation of substances is required to improve foaming,shearing or heating may be done before membraneprocessing.

In membrane processing, bubble diameter increases withboth gas flow rate and pore size (Bals & Kulozik, 2003a).The porosity of the membrane surface is an important pa-rameter because it determines the distance between two ad-jacent pores. This distance is critical to ensure that twoadjacent bubbles do not come sufficiently close to coalesce(Charcosset et al., 2004). In SPG processing, mean diame-ters of microbubbles and polydispersity are larger for pro-tein than for surfactant solutions (Table 1), probably dueto the different adsorption kinetics (Kukizaki & Goto,2007).

The main limitation of the membrane process is a lowdispersed phase flux through the membrane associatedwith monodispersed bubbles. However, membrane systemsare particularly suitable for large scale production becausethey are easy to scale-up, by adding more membranes toa module (Charcosset et al., 2004).

Microengineered devicesThe application of micromachining techniques in nano-

technology has prompted the use of microfluidic devices(MFDs). MFDs are process elements that deal with smallamounts of fluids (106e109 L) that flow in channelswhere at least one characteristic dimension on the orderof 10e100 mm (Garstecki, Ganan-Calvo, & Whitesides,2005). They have the advantage of precise fabrication andreplication, portability, small volume and short reactiontimes (Sugiura, Nakajima, Iwamoto, & Seki, 2001). Thehydrodynamics on the micron scale of MFDs involves sig-nificant effects of interfacial tension, rheology and laminarflow, thus allowing a better analytical and reactive perfor-mance (Kobayashi, Nakajima, Chun, Kikuchi, & Fujita,2002). The physics, fabrication methods and applicationsof MFDs in food engineering has been recently reviewedby Skurtys and Aguilera (2008).

For the purpose of this presentation, MFDs will be sep-arated into microchannel (MC) arrays and flow-focusing(FF) devices. The operational principle of MC devices isto drive the disperse-phase fluids to pass through a micro-channel structure which has a designed geometry, similarto membrane processes (Hsiung, Chen, & Lee, 2006).MC is a promising technology to enhance the quality ofbiphasic food products, such as emulsions and foams,since these devices can generate uniform droplets or bub-bles in liquids of micro and nanosizes (Kobayashi et al.,2002; Kobayashi, Nakajima, & Mukataka, 2003; Kobaya-shi, Uemura, & Nakajima, 2007; Sugiura et al., 2001). Ya-suno et al. (2004) studied monodispersed microbubbleformation when the continuous phase was an aqueous solu-tion containing surfactants or proteins. The average bubblesize increased with increasing continuous-phase viscosityand it was larger for protein than for surfactant solutions(Table 1).

Active devices utilizing FF techniques can generatedroplets or bubbles with uniform and tunable sizes. Ina typical microfluidic FF device (Fig. 1b), two immisciblephases flow through separate channels (one central for the

Table 1. Diameters of air bubbles produced by membranes and microengineered devices

Methods Process conditions Continuous phase Bubble diameter (mm) References

Membranes Membrane pores, 7e140 nm Whey protein isolatesolution, 10% wt

180e370 Bals & Kulozik, 2003aContinuous-phase flow, 6.2 L/hShear-induced detachment

Membrane pores, 43e85 nm Sodium dodecyl sulfate(SDS), 0.3% wt

0.36e0.72 Kukizaki & Goto, 2006Transmembrane/bubble pointpressure ratio, 1.1e2.0Shear-induced detachment

Membrane pores, 3.07 mm SDS, 0.3% wt 27.8 Kukizaki & Goto, 2007Transmembrane/bubble pointpressure ratio, 1.1

Tween 20, 1.0% wt 31.3

Spontaneous formation byinterfacial tension

Sodium caseinatesolution, 1.0% wt

39.5

Bovine serum albumin(BSA), 1.0% wt

41.1

Microchannel arrays Microchannel dimension(16 mm width 4 mm depth)

SDS, 0.3% wt 33.6 Yasuno et al., 2004

Spontaneous formation byinterfacial tension

Tween 20, 1.0% wt 39.1Sodium caseinatesolution, 1.0% wt

48.9

BSA solution, 1.0% wt 51.1

Flow-focusing devices Orifice diameter, 100e210 mm Water/ethanol and water/glycerol solutions

5e120 Ganan-Calvo & Gordillo, 2001Liquid flow rate, 24e310 mL/sGas flow rate, 0.2e40 mL/s

Exit channel with square section(100 mm width 500 mm length 30 mm height)

Glycerin solution,50% volume

w46 Gordillo et al., 2004

Liquid flow rate, 20 mL/hGas flow rate, 46 mL/h

Orifice diameter, 250e500 mm Gelatin solution, 1e5% wt 70e200 Skurtys et al., 2007Liquid flow rate, 60e310 mL/sGas flow rate, 4e10 mL/s

179R.N. Zuniga, J.M. Aguilera / Trends in Food Science & Technology 19 (2008) 176e187dispersed phase and one or more outer channels for thecontinuous phase) that meet at the junction of these chan-nels upstream of a small orifice. The outer fluid exertspressure and viscous stresses that force the inner fluidinto a narrow thread, which breaks inside or downstreamof the orifice at regular time intervals (Anna, Bontoux,& Stone, 2003; Garstecki et al., 2005; Skurtys & Aguilera,2008). The diameters of droplets or bubbles generated bythese devices are comparable or smaller than the orificewidth (Anna et al., 2003; Gordillo, Cheng, Ganan-Calvo,Marquez, & Weitz, 2004). A major advantage is that bub-ble formation is not controlled by surface tension as inmembrane processing and in MC devices (Garsteckiet al., 2005; Gordillo et al., 2004). Bubbles with uniformand tunable size as well as sophisticated foam architec-tures may be generated by controlling the flow rate ratioof the continuous and dispersed phases, as shown inFig. 2 (Ganan-Calvo & Gordillo, 2001; Garstecki et al.,2004; Hsiung et al., 2006; Skurtys, Bouchon, & Aguilera,2007). Mean diameters of microbubbles produced by FFdevices are shown in Table 1.FF devices presents two main disadvantages for real in-dustrial applications in which mass production of bubbles isrequired: the gas fraction is limited to very small values andthe scale-up of these devices is a hard task due to the three-dimensional centering of the injection needles with the exitorifices (Gordillo et al., 2004).

Electrochemical reactionsGas bubbles generated by electrochemical reactions

have attracted research interest in many disciplines. Sinceelectrochemical reactions occur primarily in aqueous solu-tions, gases like hydrogen and oxygen are usually generatedin the form of bubbles at the surface of electrodes (Fig. 1c)(Wedin, Davoust, Cartellier, & Byrne, 2003; Zhang et al.,2006). Gases detach from these surfaces as soon as thebuoyancy force overcomes the surface tension effects hold-ing the bubble in place. Electrolysis can generate smallbubbles in the order of micro or nanometers (Lee, Sutomo,Liu, & Loth, 2005; Wedin et al., 2003). Zhang et al. (2006)using tapping mode AFM demonstrated that the formationof bubbles at the surface of electrodes is a sequential

Fig. 2. Different foam architectures generated in a 2.75 mm internaldiameter glass tube for a gelatin solution (3% wt) and selected valuesof gas fraction (U) [U gas flow rate/(gas flow rate liquid flow rate)].(A) U 60%; (B) U 96%; (C) U 97%. Courtesy Dr. Olivier Skurtys.

180 R.N. Zuniga, J.M. Aguilera / Trends in Food Science & Technology 19 (2008) 176e187process of formation, growth and coalescence of nanobub-bles, followed by the release in the form of microbubbles.

Micro-fabrication of electrolytic bubble generatorscould allow direct control of bubble size and size disper-sion, release location and release frequency (Lee, Sutomo,et al., 2005). Tuning the applied voltage or the reactiontime, the formation of nanobubbles and the release of mi-crobubbles can be controlled (Zhang et al., 2006). In addi-tion, some studies on electrochemically generated bubbleshave shown a decrease in bubble size and a narrower bub-ble size distribution with greater applied voltages and theconcomitant increase in gas production (Lee, Sutomo,et al., 2005; Shin & Yiacoumi, 1997; Zhang et al., 2006).Monodisperse bubbles with diameters within the range of100 50 mm were formed by an electrochemical bubblegenerator designed by Wedin et al. (2003). Lee, Sutomo,et al. (2005) were able to produce microbubbles (100 mmin diameter and 90% of the bubbles within 20 mm of themean diameter) by varying the area and spacing betweenelectrodes.

Although electrolytic bubble generators may prove prac-tical at laboratory scale, they may not be viable at produc-tion levels due to high cost, low energy efficiency, etc.Further development is needed before such devices becomepractical for large scale applications requiring formation ofa cloud of bubbles (Lee, Sutomo, et al., 2005).UltrasoundAnother way of creating microbubbles is acoustic cavi-

tation (Fig. 1d) that implies the formation, growth, and im-plosive collapse of microscopic bubbles in liquids subjectedto high-intensity ultrasound (generally between 20 and100 kHz) (Suslick et al., 1999). Depending on the liquidand conditions used, these bubbles may be on the orderof nanometers or micrometers in diameter (Cho, Kim,Chun, & Kim, 2005; Lim & Barigou, 2005; Louisnardet al., 2001). Bubbles emerge from gas nuclei formed inthe bulk of the liquid or in the vessel walls (Louisnardet al., 2001) as well as from ions present in the solution(Cho et al., 2005; Lee, Kentish, & Ashokkumar, 2005). Sur-factants play an important role both in nucleation (by low-ering the surface tension of the solution) and stability of theformed bubbles (Cho et al., 2005; Lee, Kentish, et al.,2005).

Sonication of aqueous solutions of different surfactantsproduced stable nanosize bubbles (

181R.N. Zuniga, J.M. Aguilera / Trends in Food Science & Technology 19 (2008) 176e187(aqueous solution) that shows mechanical rigidity duringthe observation time (Aguilera, 1992). Generally, gels con-tain two dispersed components (or phases): a continuousnetwork imparting the solid character to the gel throughits long-range continuity of structure at the molecular orsub-micron (particulate gels) levels, and a liquid phasethat provides the liquid-like properties (Clark, 1992;Nishinari et al., 2000). In a sense, gels are a form of solidwater at ambient temperature.

The ability of food polymers to produce gels depends onthe formation of junction zones between polymer mole-cules or aggregates that restricts the expansion of the net-work. Thus, continuous network of interconnectedmaterial spanning the whole volume becomes swollenwith a high proportion of liquid (Aguilera & Rademacher,2004; Hermansson, 2007). In effect, polymer moleculesare aggregated into one immense molecule with a three-dimensional structure that entraps the liquid. Structurebuild-up occurs gradually in situ from the polymer solutionto the gel state depending on externally controllable condi-tions such as gelling time, temperature, pressure or solutioncomposition (pH, ionic strength, low molecular weight sol-utes, presence of specific ions, etc.). It is this network struc-ture and its relation with the liquid phase that gives gelstheir characteristic rheological and textural properties. Inthis regard, gels cover a wide spectrum of textural proper-ties as is shown in Fig. 3. These properties, in turn, will de-termine a wide spectrum of sensory and physicochemicalproperties of food gels, for example, appearance, tasteand stability that make them valuable as food matrices(Aguilera, 1992; Clark, 1992; Renard et al., 2006). Differ-ent mechanisms of gel structure formation and examples ofgelling polymers are presented in Table 2.

Mixed and filled gel matricesThe versatility to build functional gels matrices is

greatly extended when mixed and filled gels matricesType of fracture

Gel stren

gth

Whey protein isolateSoy protein isolateGellan

Gelatin Seaweed polysaccharidesStrong

WeakBrittle Ductil

Fig. 3. Schematic classification of food gels based on their strength andfracture properties.are considered (Fig. 4). Gel properties can be dramaticallychanged by addition of a second gelling agent (Aguilera,1992; Aguilera & Rademacher, 2004; Hermansson, 2007;Morris, 1990). Often a blend of gelling polymers providesimproved properties than those of a single component geland sometimes synergistic effects may be found (Table3). A few examples involving milk gels follow. Aguileraand Baffico (1997) reported that addition of small amountsof native cassava starch to a whey protein solution resultedin stronger gels after thermal gelation. In contrast, supple-menting whey protein solutions with skim milk powderproduced weaker mixed gels. Turgeon and Beaulieu(2001) found that addition of 0.1% of k-carrageenan (andadded calcium) increased the hardness and deformation atfracture, and reduced the syneresis of whey protein gels.It is well known that pre-heating of milk induces whey pro-tein aggregation on the surface of casein micelles, increas-ing gel firmness and serum holding capacity in yoghurt(Kulozik, Tolkach, & Hinrichs, 2003).

The incorporation of a finely dispersed lipid phase intothe polymer solution further expands the versatility of theformed gel matrices. The extra lipid phase may serve ascarrier for fat-soluble nutrients and flavors. Moreover,membrane-coated fat globules can interact with thepolymer matrix providing improved strength (Aguilera &Kessler, 1989). Advances in the study of fat-filled gels,including aerated emulsion gels, are reviewed by Dickinson(2006). Aerated gels filled with internally produced CO2gas bubbles were prepared by Nussinovitch et al. (1992).Although mechanical properties of these gels were differentfrom pure gels no control over the bubble size and gas frac-tion could be achieved.

Innovative applications of aerated gelsNew dietetic foods e gels and nutrition

There is increased evidence that the quantity, composi-tion and microstructure of the food ingested affects health(Norton, Moore, & Fryer, 2007; Parada & Aguilera,2007). One approach to development of dietetic foods isbased on proper selection of the food components and theirrelative amounts, for example: (i) use of non-caloric ingre-dients; (ii) immobilizing high quantities of water; and (iii)incorporating air as small dispersed bubbles. Examples oflow calorie analogs occluding large amounts of water inmicrogels are some ultralight margarines (de Deckere &Verschuren, 2000).

The physical form of the food may profoundly alter thesensation of fullness and satiety. Addition of a bulkingagent such as guar gum has been proposed to controlbody weight by increasing the feeling of fullness, thus re-ducing the energy intake (French & Read, 1994; Pasman,Saris, Wauters, & Westerterp-Platenga, 1997). The task isnot easy as it has been shown that, in liquid dietetic foods,calorie content has an additive effect in increasing the senseof satiety (French & Read, 1994; Marciani, Gowland,Spiller, et al., 2001). In solid foods, the microstructure

Table 2. Physical and chemical mechanisms to produce biopolymer gelation

Types Source Example Basis of mechanism

Heat setting Protein Globular proteins Unfolding or dissociation of native protein by heat exposing reactive sites. Unfolded moleculesassociate and aggregate leading to the formation of the junction zones of the gel.

Myosin The mechanism is predominantly tailetail association via non-covalent bonding. Domains of themolecule undergo a conformational change that exposes previously hidden amino acid side-chainsto the solvent, resulting in increased proteineprotein interactions allowing the formation ofjunction zones.

Polysaccharide Starch During gelatinization of starch, amylose and amylopectin are released and become solubilized.Upon cooling, the free amylose (single helices) become ordered into microcrystalline regionssurrounding swollen granules, forming the junction zones. Starch gels are composites of amylosematrices filled with swollen granules.

Cold setting Protein Gelatine Junction zones created by partial reformation of the triple helices found in collagen during cooling.Polysaccharide Agar When hot solutions are cooled below 40 C bundles of double or single helices are formed. These

helices are precursors of superjunctions of helices and of the junction zones.

High pressure Protein Globular proteins Pressures above 200 MPa induce conformational changes leading to denaturation, aggregation andformation of the junction zones. Particulate gels formed are stabilized mainly by hydrogen bonds,although disulphide bonds may be present at higher pressures.

Ionic Polysaccharide Gellan gum Formation of double helices followed by ion-induced association of the double helices to form thejunction zones.

Carrageenans Helical chains become associated of into double helices. Because of the anionic nature of themolecule, carrageenans require counter-ions to form the junction zones.

Alginate, low methoxil pectin Junction zones are regions of polyguluronic (alginate) or polygalacturonic acid (pectin) linked tosimilar regions in another polymer chain by calcium ions (so-called eggbox arrangement).

High methoxil pectin The junction zones are aggregates of chains of various sizes promoted by hydrogen bonding andhydrophobic interaction.

Cold gelation Protein Globular proteins Soluble protein aggregates are prepared by heat treatment at a pH well above the isoelectric pointand in the absence of salts. After cooling, formation of junction zones is induced by lowering thepH or adding salts.

Acid Protein Globular proteins, myosin Lowering the pH, changes the net charge of proteins and allows denaturation to form clusters oraggregates. These clusters may be considered as the junction zones of the gel.

Casein Upon acidification, colloidal calcium phosphate in the micelles is solubilized and aggregation ofthe partly disintegrated micelles occurs due to charge neutralization. At temperatures above 10 Cjunction zones and network structure is formed.

Enzymatic Protein Globular proteins Formation of junction zones is based on the introduction of covalent cross-links between proteinsby the action of the enzyme transglutaminase.

Casein Enzymatic hydrolysis of k-casein by the proteolytic enzyme chymosin releases the so-calledcaseinomacropeptide (CMP) and causes the casein micelles to destabilize and aggregate to formthe junction zones and lead to a particle gel (curd).

18

2R

.N.

Zuniga,

J.M.

Agu

ilera/

Tren

ds

inFo

od

Science

&Tech

nolo

gy19

(2008)

176

e187

Fig. 4. Nomenclature of gels based on the type and number of phases forming their structure.

183R.N. Zuniga, J.M. Aguilera / Trends in Food Science & Technology 19 (2008) 176e187has a major influence on the sensation of satiety by slowingdown the rate of breakdown in the gastrointestinal tract.Several human studies have indicated that meals containingsolids typically have a greater effect on satiety than liquidsmeals of equivalent volume and energy content (Hoadet al., 2004). It is believed that a slower breakdown in theTable 3. Examples of the effect of adding a second polymer on selected p

System Property

Skim milk powder, 1e9% w/w FirmnessWhey protein concentrate, 1e9% w/wTotal solids, 10% w/w; pH, 4.2e4.5

Whey protein isolate, 75e90% w/w of TS Elastic modulusCassava starch, 10e25% w/w of TSTotal solids, 10e15% w/w

Dried egg white, 6e9% w/w Hardnessa-casein, 1e4% w/wTotal solids, 10% w/w; pH, 7.0

Gellan gum, 0.2e0.8% w/v HardnessGelatin, 0.2e0.8% w/vTotal solids, 1% w/v

Water holding capacity

Soy protein isolate, 1e17% w/w Shear modulusWhey protein isolate, 1e17% w/wTotal solids, 18% w/w

Syneresis

k-carrageenan, 0.2e0.5% w/w Youngs modulusLocust beam gum, 0.1e0.4% w/wTotal solids, 0.6% w/w

Force at fractureStrain at fracture

a The single gel is named first in table.b Ratio property of mixed gel/property of single gel, at the same total sstomach can lead to a more lasting sensation of satiety(Norton, Frith, & Ablett, 2006). In a study with human vol-unteers and monitored by MRI hard agar gel beadspresented a delayed exit from the stomach while softergel beads were rapidly broken down and emptied at thesame rate as a liquid model meal (Marciani, Gowland,roperties of mixed gelsa

Ratiob References

1.7e1.9 Aguilera & Kessler, 1989

1.1e1.3 Aguilera & Baffico, 1997

1.5e2.2 Matsudomi, Kanda, & Moriwak, 2003

0.34e0.77 Lee et al., 2003

0.84e1.59

1.43e2.29 Comfort & Howell, 2002

0.25e0.86

0.54e0.94 Koliandris, Lee, Ferry, Hill, & Mitchell, 2008

1.33e1.861.12e1.39

olids content.

184 R.N. Zuniga, J.M. Aguilera / Trends in Food Science & Technology 19 (2008) 176e187Fillery-Travis, et al., 2001). These results confirm thatstructures slowly digested in the stomach increase satietyby a yet unknown mechanism thus providing a higher senseof fullness. Enhancing satiation may restrict the daily foodintake and the desire of overeating, therefore, contributingto control of body weight (Hoad et al., 2004). In this sense,strong aerated gels could increase the initial sense of full-ness but the effect of an aerated structure on the rate ofbreakdown in the gastrointestinal tract needs to be deter-mined. Designed aerated gels with tailored texture, low ca-loric density and flavor properties, may help in developingnew dietetic foods for the treatment of obesity.

According to Wansink (2007), over 85% of the popula-tion with weight problems has consumed an average excessof 25 cal per day over a prolonged period of time. He sug-gests that adding small amounts of water or air to a recipewhile conserving the portion size and taste may help in pre-serving the perceived value of the food while decreasing thecalorie consumption. He concludes that a mere 10% reduc-tion in daily calorie consumption would slow or even re-verse the weight gain of these people in the long term. Infact, it has been demonstrated that within limits peopleeat by volume, thus, incorporating air would reduce the cal-orie density and make them feel just as full as with the nor-mal food (Osterholt, Roe, & Rolls, 2007). A strategy infood design may then be to maintain the taste perceptionof demanded energy-dense foods while imperceptibly add-ing air as small bubbles and/or immobilizing water in thefood matrix as microgels, thus lowering their caloric con-tent per portion.

Aerated aromatic gelsAroma is a key factor in the acceptance of foods by con-

sumers. Taylor (2002) has pointed out that the challenge forfood flavorists is to produce the same flavor perceptionfrom different structures. The transfer of aroma compoundswithin the food and their release in the mouth is influencedby the concentration and chemical nature of the aromacompounds as well as the composition and structure ofthe food (Buettner & Schieberle, 2000; Seuvre, Philippe,Rochard, & Voilley, 2006; Taylor, 2002). Mastication in-creases the surface area of the food exposed to the air inthe oral cavity facilitating the release of volatiles (vanRuth & Roozen, 2000). Flavor release in the mouth isa complex subject and many factors come into play (e.g.,mouth volume, saliva composition, saliva volume, etc.)that alter the rate and profile of volatiles reaching theodor receptors (van Ruth & Roozen, 2000).

Food microstructure affects aroma release but results arenot always clear since flavor molecules may interact di-rectly with macromolecules. Gelatinized starch granuleshaving lost most of granule integrity exhibited a lower vol-atile retention than native starch granules (Lopes da Silva,Castro, & Delgadillo, 2002). Reducing droplet size inemulsions enhanced the release of non-polar aromas (i.e.,linalool) but had no effect on polar aromatic molecules(Miettinen, Tuorila, Pironen, Vehkalahti, & Hyvonen,2002). Emulsions encapsulated in microstructured gelssuppressed the release of lipophilic aromas in a low fatproduct (Malone, Appelqvist, & Norton, 2003) and thetype of gelling biopolymer affected the release of aromafrom these gelled emulsions during mastication (Malone& Appelqvist, 2003). It can be surmised that aromas con-tained in bubbles of aerated gels will be rapidly releasedto the oral cavity during the mastication process. Thus,the inclusion of aromas in aerated gel matrices would ex-pand the spectrum of release mechanisms providing newopportunities in product design and gastronomic creations.

Carriers of nutraceuticals and otherbioactive molecules

Nutraceuticals are defined as foods that contain a compo-nent which gives a specific physiological benefit beyond in-herent general nutrition, thus preventing or alleviatingspecific diseases (Ramaa, Shirode, Mundada, & Kadam,2006). The effectiveness of nutraceuticals depends on pre-serving the bioavailability of the active component duringtransit through the gut (Chen, Remondetto, & Subirade,2006).

Microgels made from food hydrocolloids or proteinshave been proposed as generally recognized as safe(GRAS) carriers for oral administration with the extra ad-vantage that these nutraceutical microgels may help instabilizing the texture of the delivery system (Chen et al.,2006). Moreover, the presence of acidic (carboxylic) or ba-sic (ammonium) groups in proteins chains, which either ac-cept or release protons in response to changes in pH, wouldallow modulating the release of bioactive molecules frommicrogels (Qui & Park, 2001). Particulate gels made ofb-lactoglobulin have been shown to release more ironthan filamentous gels in the presence of pepsin at pH 1.2or pancreatin at pH 7.5, suggesting that enzymatic proteol-ysis depends on the microstructure of these gels (Remon-detto, Beyssac, & Subirade, 2004).

If the trapped bioactive component happens to be an en-zyme then an open porous microstructure would providea larger contact area for interaction with the infiltrating sub-strate (Szymanska, Bryjac, Mrowiec-Bia1on, & Jarzebski,2007; Yiu, Wright, & Botting, 2001). If a digestive enzymeneeds to degrade the gel matrix then a large porosity andhigh-pore interconnectivity would increase the rate of nu-traceutical release (Neves, Kouyumdzhiev, & Reis, 2005;Wachiralarpphaithoon, Iwasaki, & Akiyoshi, 2007). Hence,tailoring the porous structure of an aerated gel would givemore opportunities for protection or release of a physiolog-ically bioactive component (or a nutrient) in the gut.

Control of syneresis and water releaseSome gels experience syneresis or a slow, time-

dependent de-swelling resulting in an exudation of liquid(Lucey, 2002). From a thermodynamic point of view, phys-ical gels are metastable because a significant amount of

185R.N. Zuniga, J.M. Aguilera / Trends in Food Science & Technology 19 (2008) 176e187bonds stabilizing the network have a reversible or temporalcharacter. The permanent structural reorganization withinthe material driven by a reduction in the free energy ofthe system affects water hold-up (Renard et al., 2006). Wa-ter in gels can also be lost as a result of temperature fluctu-ations. The loss of liquid may result in shrinking of gels,changes in texture and reduced quality. In aerated gels,part of the syneresis could be trapped inside multiple gasbubbles and would not appear in the surface of the productduring storage.

Water release from the gel matrix during oral processingmay be desirable in some foods, such as processed meatproducts where it contributes to the perception of juiciness,but in puddings and dairy products the fast release of wateris regarded as a defect. The expulsion of liquid is generallyrelated to large deformations and fracture during oralprocessing (van der Berg, van Vliet, van der Linden, vanBoekel, & van de Velde, 2007). The presence of bubblesis likely to change the fracture pattern of gels in the oralcavity and the release of water.

Innovative high-moisture matricesSeveral semi-solid foods are combinations of high-

moisture gel matrices and dispersed microstructural ele-ments such as fibers, fat globules or air bubbles (e.g.,frankfurters, soft cheeses and mousses). Synthetic caviaris a good example of a high-moisture structured gel ma-trix. It has a gelled center of gelatin and two protein coat-ings: an inner coating consisting essentially of the reactionproducts of gelatin and vegetable tannins and an outercoating formed by the reaction product of polyvalent metalsalts and an acidic polysaccharide (Nesmeyanov et al.,1980).

Fresh fruits are hierarchical superstructures of cells hav-ing fairly thin cell walls that break in a brittle manner giv-ing rise to desirable sensorial features such as crispness,juiciness and enjoyable aromas (Kader, 2002; Vincent,1991). Apple cells (100 mm in diameter) have air spaces be-tween them that have been used to infiltrate fruit pieceswith various solutions (Lazarides et al., 1999). Some ad-vantages of gel analogs of fruit pieces would be uniformityin size and shape, decreased sugar content, improved flavorand color stability during storage. However, a major prob-lem in simulated fruits is related to texture. The crispy/crunchy texture of fresh fruits (related to turgor pressure)should be mimicked by correctly designing the fractureproperties of gels.

Foams and gels have gained notoriety among modernchefs for their light and exquisite textures. Aerated gelsare gastronomic concepts in between the creations of twoof the most reputed chefs in the world: the airs of theSpanish Ferran Adria and the jellies of the British Hes-ton Blumenthal. They and other top chefs are exploiting theproperties of food materials in well equipped laboratories,a trend that has been called molecular gastronomy (This,2006) but is actually a form of gastronomic engineering.ConclusionsGels are ubiquitous structures in high-moisture foods.

The ample availability of edible gelling materials and theimproved properties of mixed systems and emulsion gelsprovide a wealth of opportunities for the design of simpleor complex structures with tailored properties. Introducinga gaseous phase in the form of bubbles or open pores mayincrease the spectrum of possibilities for texture formationand perception, flavor encapsulation and/or release, deliv-ery of bioactive molecules, control of satiety and creationof gastronomic structures. New technologies that allowcontrol of the elements in the dispersed phase at the micronscale are actively explored to make better aeratedstructures.

AcknowledgmentsWork on aerated gels is financed by the National Com-

mittee for Science and Technology (Chile) FONDECYTproject 1060713.

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Aerated food gels: fabrication and potential applicationsIntroductionDispersing gases in foodsAerated foodsConventional methods to incorporate bubbles to foodsNovel methods to incorporate bubbles to foodsDispersing gases with membranesMicroengineered devicesElectrochemical reactionsUltrasound

Gels and gel structure formationFood gelsMixed and filled gel matrices

Innovative applications of aerated gelsNew dietetic foods - gels and nutritionAerated aromatic gelsCarriers of nutraceuticals and other bioactive moleculesControl of syneresis and water releaseInnovative high-moisture matrices

ConclusionsAcknowledgmentsReferences