Action of Emulsifers in Promoting Fat DestabilizationDuring the Manufacture of Ice Cream

H. D. GOFF' and W. K. JORDANDepartment of Food Science

Cornell UniversityIthaca, NY 14853

ABSTRACT

Several emulsifiers have been ex­amined in ice cream processing to de­termine their relative emulsion destabi­lizing power. The hydrophilic lipophilicbalance value of the emulsifier did notaccount for all of the differences in de­stabilization; however, destabilizing powercorresponded with the resulting interfa­cial tension between the serum and lipidphases of the mix. Fat destabilizationresults from the combination of icecrystallization and shear forces during icecream manufacture. Neither shear nor icecrystallization alone were sufficient to

cause the magnitude of destabilizationencountered in a typical barrel freezer. Ithas also been shown that polyoxyethylenesorbitan monooleate, the most powerfuldestabilizing agent, reduced the amountof protein adsorbed to the fat globulesurface. Thus, it is suggested that, basedon their ability to lower the interfacialtension, emulsifiers control the adsorp­tion of protein to the fat globule surface.The fat globules thus become moresusceptible to coalescence induced by theshear forces of agitation and ice crystal­lization during ice cream manufacture.

INTRODUCTION

Destabilization of the fat emulsion duringthe whipping and concomitant freezing of icecream is responsible in part for building intothe frozen product an internal structure offer­ing the beneficial properties of dryness uponextrusion during the manufacturing stages (aidsin packaging and novelty molding, for ex-

Received October 8,1987.Accepted August 1, 1988.'Department of Food Science, University of

Guelph, Guelph, Ontario, Canada N1G 2Wl.

ample), a smooth-eating texture in the frozendessert, and resistance to meltdown or goodstand-up properties (necessary for soft serveoperations) (2, 4). The action of emulsifiers inpromoting this fat destabilization and themechanisms of the destabilization phenomenonoccurring during the freezing process have notbeen fully explained and are the focus of thisresearch.

In most dairy emulsions, the aqueouscontinuous phase consists of a sugar and saltsolution and a colloidal casein suspension. Thediscrete, globular phase consists largely of thepartially crystalline lipid components, includinga complex mixture of triglycerides, digly­cerides, and monoglycerides (31). The interfacehas both hydrophilic and hydrophobic com­ponents and consists of any amphiphilic mole­cules present. These include some of the milkproteins, the phospholipids or lipoproteinparticles, and any added surfactant (31). Theamphiphilic molecules have the ability to

lower the interfacial tension between the twophases (1,14).

In an ice cream system, classical theories ofemulsion stability being enhanced by thesurfactant may not sufficiently explain theircomplex action (18, 20). Kloser and Keeney(19) presented evidence that the desired smoothtexture and dryness upon extrusion of icecream was a result of an increase in nonglobularor free fat known as fat destabilization and wasenhanced by some of the emulsifiers in com­mon use. The mechanism whereby good stand­up qualities are obtained in the extruded icecream is not unlike that occurring in whippedcream, a product that depends upon partialchurning of the fat for its foam stability. In icecream dryness studies, all those factors pro­moting dryness also favored partial destabiliza­tion of the fat emulsion (18).

Kloser and Keeney (19) reported thatmonoglycerides of short-chain fatty acids werepowerful destabilizers, whereas monoglycerides

1989 J Dairy Sci 72: 18-29 18

FAT DESTABILIZATION IN ICE CREAM 19

of longer chain fany acids were not so ef­fective. Also, glycerol monooleate (hydrophiliclipophilic balance (HLB) = 2.8) was a moreeffective destabilizer than glycerol mono­stearate (GMS) (HLB = 3.8). They also foundthat Spans (HLB '= 4.3 to 8.6) did not giveextensive destabilization, whereas Tweens (HLB= 15) were very effective. The HLB numberrepresents the portion of the emulsifier mole­cule soluble in the aqueous phase on a scale of20 (1, 14).

Govin and Leeder (16) and Lin and Leeder(21) reported a direct relationship betweendeemulsification and HLB of emulsifiers in theHLB range from 4 to 16. They speculated thatemulsifiers of low HLB become firmly as­sociated with the fat globule, minimizingdeemulsification upon agitation due to theretention of the fat globule identify. However,emulsifiers of higher HLB are only looselyanchored to the fat globule and thus are sweptoff during agitation, removing the protection ofthe globule to stresses and thus promotingconsequent coalescence. Berger and White(7) interpreted Leeder's results as being causedby concentration of Tween at the interface dueto a preferential adsorption of Tween.

Oortwijn et al. (23, 24, 25) and Walstra andOortwijn (30) studied the amount of milkprotein adsorbed to the surface area of fat in anemulsion under various conditions. Theyreported that added chemical surfactantsreduced the surface excess, a function definedas the amount of protein adsorbed per surfacearea of fat (mg/m 2

). This was also found byBarfod and Krog (3) and Goff et al. (15).

In examining the state of the fat in icecream, Berger et al. (5) and Berger and White(6,7) reported that high melting triglycerides inthe form of concentric crystalline layers arepresent on the innermost side of the membrane.A highly concentrated solution of fat crystals inliquid fat exists within these crystalline layers.The solid:liquid fat ratio is temperature de­pendent. Temperature changes may be re­sponsible for fat globule distortion and con­traction during fat crystallization. This distor­tion causes weaknesses in the high meltingtriglyceride shell, which can then fracture understress, releasing liquid oil from the center of theglobule which in turn can form a cementinglayer around other fat globules. The clusters

thus formed actually hold the ice cream serumin their interstices, resulting in the observeddryness. These fat globule chains may alsoenvelope the air cells, thus improving overrun.

Thomas (28) stated that ideal destabilizationof fat should form networks of fat globules,which could strengthen the lamellae of air cellsand prevent serum drainage. However, ifexcessive fat destabilization occurred too earlyin the freezing process, before a certain viscosi­ty is reached, air cells could be ruptured,making it difficult to develop the desiredoverrun. Churning could lead to the develop­ment of a buttery, coarse texture, and a wateryserum could be evident upon melting. Theconcomitant formation of ice crystals also helpsto create a barrier to excessive fat chuning bymechanical obstruction (11, 12).

It is generally accepted that air is necessaryfor rapid flocculation and coalescence of fatglobules (31). When air is incorporated into anemulsion as in flotation churning, the largedifferences in interfacial tension between theair-serum interface and the fat-serum interfacemay cause some of the protein layer of thefat-serum interface to adsorb to the newlycreated air bubble. This desorption of mem­brane then causes a spreading of the liquid fatonto the surface of the air bubble. Because thesystem is dynamic, air bubbles collapse undershear forces, and the resulting layer of liquid fatcoalesces into larger droplets or acts as acementing layer in holding floccules or clumpstogether. As whipping proceeds, the constantformation and collapse of air bubbles and thecontinuous desorption of protein and spread­ing of fat at the air serum interface eventuallyleads to a complete churning of the emulsion(22, 26, 31). Fat destabilization in ice creamhas been attributed to a similar process (2, 4).Liquid fat, either escaping from rupturedhigh melting glyceride (HMG) shells or expelledfrom the partially crystalline globules, acts as acementing agent in tying together clumps orfloes which form during the whipping processin ice cream. If allowed to proceed too far,this creation and destruction of air cells canlead to excessive destabilization and formationof visible butter granules in ice cream.

A partially crystalline fat is necessary forclumping to occur (22, 31). van Boekel andWalstra (29) found emulsion stability of a

Journal of Dairy Science Vol. 72, No.1, 1989

20 GOFF AND JORDAN

paraffin oil in water emulsion to be reduced bysix orders of magnitude when crystals werepresent in the dispersed phase. This has beenattributed (10, 29) to the protrusion of crystalsinto the aqueous phase causing a surfacedistortion of the globule. The crystal pro­trusions can then pierce the film between twoglobules upon close approach. As the crystalsare preferentially wetted by the lipid phase,clumping is inevitable. This phenomena mayaccount for partial clumping of globules undera shear force.

Thus, emulsifier action may be related to

HLB numbers, interfacial tension values be­tween the two phases in the presence of am­phiphilic molecules, and to their effect on theprotein load at the fat surface. The fat de­stabilization mechanism may be related to shearforces or flotatation churning in the freezer andto temperature changes affecting the fat and icecrystallization processes. Hence, the objectivesof this research were to correlate the desta­bilizing ability of the emulsifier to its HLBnumber, since this concept has been exploredbut with contradictory results; to correlatethe destabilizing power of the emulsifier withthe interfacial tension between the serum andlipid phases of the emulsion in the presence ofthe emulsifier; to examine the role of the shearforces during whipping of ice cream, inde­pendent of ice crystallization, in promotingfat destabilization; to examine the role of icecrystallization, independent of shear forces, inpromoting fat destabilization; and to examinethe influence of ice crystallization temper­ature on the magnitude of fat destabilization soas to elucidate any independent temperatureeffects on fat crystallization or protein ad­sorption.

MATERIALS AND METHODS

Destabilization Experiments

The destabilizing power of the following sixemulsifiers were compared: glycerol mono­stearate (GMS, Dimodan PVK), glycerol mono­oleate (GMO, Dimodan, LSQK, GrinstedProducts Inc., Industrial Airport, KA), sorbitanmonostearate (Span 60), sorbitan monooleate(Span 80), polyoxyethylene sorbitan mono­stearate (Tween 60), and polyoxyethylenesorbitan monooleate (Tween 80, Polysorbate80, ICI Americas, Wilmington, DE). Standardice cream mixes containing each of the emul-

Journal of Dairy Science VoL 72, No.1, 1989

sifiers were prepared and frozen. Four rep­licates of each mix for each emulsifier wereperformed. The basic formulation for all mixeswas 10% milk fat, 11 % milk SNF, 10% sucrose,5% corn syrup solids (Dri-Sweet 36, HubingerCo., Keokuk, IA), .1% stabilizer (Dariloid 100,Kelco Div. of Merck, San Diego, CA, a proprie­tary mixture of guar gum, xanthan gum, andlocust bean gum), and .08% emulsifier. Thesources of milk fat and serum solids for allmixes prepared were fresh cream, 40% milk fat;fresh skim milk, .04% milk fat, 9% milk solids;and low heat spray process skim milk powder,97% solids (Land O'Lakes).

All mixes were prepared by incorporatingthe appropriate premixed dry ingredients intothe fluid milk and cream. Mixes were batchpasteurized at 72°C for 30 min in a waterjacketed, air agitated vat. Following pasteuriza­tion, all mixes were homogenized in a MantinGaulin Type E two stage homogenizer (APVGaulin, Everett, MA) at 17.2 MPa, 3.4 MPaon the second stage. Mixes were cooled in anice bath to approximately 4°C and aged 24hours prior to freezing. Mix compositionwas confirmed by checking milk fat (Bab­cock) and total solids (oven drying) afterprocessing. A twofold vanilla-vanillin extract(Virginia Dare Extract Co., Brooklyn, NY) wasincorporated at a rate of 3 mllkg into all mixesafter aging. Mixes were frozen in a Taylorbatch freezer (Taylor Freezers, Rockton, IL).Aliquots of aged mix (2 L) were whippedand frozen to _5°C and held at this tem­perature for the remainder of a total of 15min in the freezer. It took approximately 6 minfor this temperature to be reached, at whichtime the refrigeration was turned off andwhipping was continued. The temperature wasthen maintained at -5 ± 1°C by togglingthe refrigeration as necessary.

Aliquots (40 ml) of mix were removed fromthe barrel of the batch freezer every 2.5 minstarting at time zero and were analyzed for fatdestabilization by specrroturbidity (17). Sam­ples were thawed, gently mixed, a sample wasremoved with a 5 ml pipet, and 3 g wereweighed into a 50-ml Erlenmeyer flask, 27 mldistilled water at room temperature wereadded, 1 ml of the 1: 10 dilution was placed in a50-ml volumetric flask and diluted to the markwith distilled water, and a sample of the 1: 500dilution was placed in a spectrophotometer

FAT DESTABILIZATION IN ICE CREAM 21

tube. The tube was centrifuged for 5 min at1000 rpm (speed 6 on an IEC Clinical Cen­trifuge, Fisher Scientific, Fair Lawn, NJ),allowed to stand for 10 min, and absorbancewas measured at 540 nm on a Bausch andLomb Spectronic 21 Spectrophotometer (FisherScientific). Percent of fat destabilized was cal­culated as: [(Aunfrozen mix - Asample)jAun­frozen mixl x 100%. Distilled water was usedas a blank to zero the instrument. The percentof fat destabilized was then plotted against timein the barrel of the freezer.

The ability of Tween 80 to destabilize theemulsion during freezing when added post­homogenization was determined. Tween 80,.08%, was added to a standard mix containingno emulsifier after aging and 30 min prior tofreezing by sprinkling a 2:1 dilution (45°Cdistilled water) of emulsifier into the gentlyagitated mix at 4°C. Fat destabilization byspectroturbidity was determined as above andcompared to mixes with no emulsifier and withTween 80 added prehomogenization as usual.Four replicates of each mix were prepared andanalyzed.

The influence of shear, independent of icecrystallization, was determined in the batchfreezer by introducing mixes emulsified witheither Span 60, Span 80, Tween 60, or Tween80 to the barrel, whipping for 15 min as before,but in this case, the refrigeration was used onlyto maintain mix temperature at 4°C. Thus, nofreezing occurred. Aliquots were removed andanalyzed for fat destabilization. The experi­ment was replicated four times. Destabilizationcurves from these four trials were compared to

the destabilization seen previously when thesame four mixes were frozen. The influence ofthe degree of ice crystallization was also de­termined in a continuous freezer (CherryBurrell Vogt Model 303, 240 gallh maximumcapacity). A mix of standard composition, 80kg, emulsified with .08% Tween 80, was pre­pared. The mix was added to the flavor tank.The pumps were started and speed, mix backpressure, and overrun vacuum were set and heldconstant. Samples were taken every 15 s afterequilibrium conditions were reached. After the1st min of sample collection, the refrigerationwas turned on and samples were collected untilthe temperature had stabilized at _4°C. Tem­perature was continuously monitored duringthis time. The sample collection period lasted 6

min, and the experiment was replicated threetimes. Fat destabilization was analyzed in all ofthe samples and a plot of percent fat desta­bilized versus temperature of extrusion wasprepared.

The importance to the fat destabilizationprocess of changes brought about by thetemperature drop from 0 to _4°C other thanice crystallization, such as the solid to liquid fatratio, emulsifier crystallization, or proteindesorption, was examined. Mixes were madewith 15% glucose to depress the freezing pointsuch that the decrease in the freezer from4 to _4°C would not cause ice crystallization.Mixes were prepared as described above exceptthat the sweetener content was replaced by15% D-glucose anhydrous. Emulsifiers usedwere Span 60 and Tween 80. Freezing was per­formed in the batch freezer by dropping thetemperature to _4°C, holding for 15 min, andthen lowering the temperature to _8°C andholding for an additional 15 min in the freezer.Samples were collected every 2.5 min andanalyzed for fat destabilization by spectro­turbidity. The experiment was replicatedfour times. The freezing points of the glucosemix and of a control mix sweetened with 10%sucrose and 5% corn syrup solids were deter­mined using an Advanced Milk Cryoscopeat three dilution levels, and the calculatedfreezing curves were then prepared for com­parison (8, 9, 27).

Aged ice cream mixes emulsified with GMS,a commercial blend of monoglycerides anddiglycerides (40% alpha mono, IV<5, German­town Manufacturing Co., Broomall, PA), orTween 80 were placed in a -20°C freezer, 24 h,for quiescent freezing. Following thawing, fatdestabilization by spectroturbidity was an­alyzed and compared with unfrozen mixsamples. This experiment was replicated fourtimes.

Interfacial Tension ElCperiments

The interfacial tension between anhydrousbutter oil (Mid American Dairymen, Inc.,Springfield, MO) and 11% milk SNF in thepresence of the following emulsifiers wasmeasured with a Fisher Surface Tensiometer,Model 20, using a duNuoy ring (1): .08% ofGMS, GMO, Span 60, Span 80, Tween 60, orTween 80 dispersed in their appropriate soluble

Journal of Dairy Science Vol. 72, No.1, 1989

22 GOFF AND JORDAN

I~ GMSI..... GMO100

"0CI> 80~:0ttl1;; 60Q)

0iii 40u.CQ)

20~Q)

D-

O0

Figure 1. A comparison of glycerol monostearate(GMS) and glycerol monooleate (GMO) for theresulting fat destabilization when ice cream mixesemulsified with each were concomitantly whippedand frozen in the barrel of a batch freezer over thecourse of 15 min. Error bars represent 95% con­fidence limits of each value after four trials. Fatdestabilization was determined by spectroturbi­dimetry.

5 10 15Time in Freezer (min)

phase (i.e., monoglycerides and Spans in oil andTweens in milk SNF solution). It was necessaryto limit the concentration of the emulsifier atthe interface, and thus, the emulsifiers wereplaced in their appropriate soluble phase toprevent the formation of an emulsifier layer.The aqueous solution (50 ml) at 70°C wasplaced in a 100-ml jacketed beaker, heated with74°C water, which was circulated with aparistaltic pump, and the platinum iridium ringwas placed in the solution and assembled to theTensiometer. Anhydrous butter oil at 70°C wascarefully layered on the top of the solution andthe interface was allowed to age for 10 minprior to measurement. At that time, the ringwas drawn from one phase into the other andthe interfacial tension (dynes/em) was de­termined. Four replicates of each were per­formed. Analysis of variance and least sig­nificant difference tests were performed on thedata.

RESULTS AND DISCUSSION

Destabilizing Power of the Surfactants

Three pairs of emulsifiers, two monogly­cerides, two sorbitan esters, and two poly­oxyethylene sorbitan esters, were chosen toexamine the relationship between fat desta­bilization and properties of the emulsifier suchas HLB, unsaturation, and polyoxyethylenegroups (Table 1). The monoglycerides hadsimilar HLB, as noted from Table 1. Stearicacid is a 18 carbon saturated fatty acid whereasoleic acid is an 18 carbon unsaturated fatty

TABLE 1. The three pairs of emulsifiers used in thisstudy to examine the importance of hydrophiliclipophilic balance (HLB) or degree of unsaturationas a predictor of fat destabilization when ice creammixes emulsified with these surfactants were frozen.

Commonname Chemical name HLB

GMS Glycerol monostearate 3.8GMO Glycerol monooleate 2.8Span 60 Sorbitan monostearate 4.7Span 80 Sorbitan monooleate 4.3Tween 60 Polyoxyethylene sorbitan 14.9

monostearateTween 80 Polyoxyethylene sorbitan 15.0

monooleate

acid. It can be seen from Figure 1 that glycerolmonooleate produced significantly more fatdestabilization over the course of the 15 min inthe barrel of the batch freezer than did glycerolmonostearate. Span 60, sorbitan monostearate,produced significantly more fat destabilizationthan did Span 80, sorbitan monooleate (Figure2). Similarly, Tween 60, polyoxyethylenesorbitan monostearate, produced significantlymore fat destabilization than did Tween 80,polyoxyethylene sorbitan monooleate (Figure2). It was also observed that Tween 60 pro­duced significantly more fat destabilizationthan did Span 60. Likewise, the unsaturatedpolyoxyethylene emulsifier, Tween 80, pro­duced significantly more fat destabilizationthan did Span 80 (Figure 2).

From these comparisons, it can be seen thatthe degree of fat destabilization was increasedwith the unsaturated emulsifiers and wasincreased with the polyoxyethylene derivatives.The HLB number did not account for the dif­ferences between the three pairs of surfactants(i.e., monoglycerides, Spans, Tweens), althoughthe solubility of the emulsifier is probably oneof the factors involved in interfacial action andfat destabilization. These results are in agree­ment with the early observations of Kloser and

Journal of Dairy Science Vol. 72, No. I, 1989

FAT DESTABILIZATION IN ICE CREAM 23

... Span 60100 -- Span 80 100

"lJell

80 80~:0tIS7ii 60 60ellCliii 40 40LL

'Eell~ 20 20ella.

0 00 5 10 15 0 5 10 15

4- Span 60 ... Span 80100 -- Tween 60 100 -- Tween 80

"i~ 80 80:0IIIiii 60 60ellCliii

40 40LL

'Eell~ 20 20ella.

0 00 5 10 15 0 S 10 lS

TIme in Freezer (min) Time in Freezer (min)

Figure 2. Fat destabilization of the various sorbitan esters as a function of time in the barrel of the batchfreezer. The upper left plot compares ice cream mixes emulsified with sorbitan monostearate (Span 60) andsorbitan monooleate (Span 80). The upper right plot compares the polyoxyethylene sorbitan esters, Tween 60and Tween 80. The lower left plot compares the saturated pairs, Span 60 and Tween 60. The lower right com­pares the unsaturated pairs, Span 80 and Tween 80. Fat destabilization was determined by spectroturbidimetry.Error bars represent 95% confidence limits on four replicates.

Keeney (19), and with Berger (4), as discussedin the introduction; however, the direct in­creasing relationship between HLB and fatdestabilization as reponed by Govin and Leeder(16) and Lin and Leeder (21) may be due to anincreasing concentration of Tween at the fatsurface in their experiments.

Posthomogenization Add itionof Emulsifier

Tween 80 (Polysorbate 80) is widely used inthe ice cream industry to aid in drying a pro­duct that is normally quite wet or sloppy on

extrusion, for example, a strawberry ice cream.In this regard, it is often added just prior tofreezing. The ability of Tween 80 to destabilizethe emulsion if added posthomogenizationneeds to be accounted for in developing anexplanation of emulsifier action and fat de­stabilization. Therefore, the resulting fatdestabilization when Tween 80 was added priorto homogenization and prior to freezing wasexamined. Tween 80 has the ability to promotefat destabilization when added just prior tofreezing (Figure 3); however, the extent of fatdestabilized is not as great as when the sur­factant is present at homogenization.

Journal of Dairy Science Vol. 72, No.1, 1989

24 GOFF AND JORDAN

Figure 3. Fat destabilization resulting from theaddition of Tween 80 to the ice cream mix eitherprior to homogenization (pre homo) or immediatelyprior to freezing (post homo) as determined byspectroturbidity, plotted as a function of time in thebarrel of the batch freezer.

Time in Freezer (min)

TABLE 2. Interfacial tension values (dynes/em)between 11% milk solids-nor-fat solutions and an­hydrous butter oil in the presence of various emul­sifiers dispersed in their appropriate soluble phase(.08%) at 70°C as measured by a duNuoy ring.'

Interfacialtension

(dynes/em)

6.16S.S2a

S.09b

S.64a

S.02b

2.422.24

by the same letter are not

Emulsifier

No emulsifierGMSGMOSpan 60Span 80Tween 60Tween 80

a,bYalues followedsignificant (P<.OS).

1 n = 4.

15105

...... Pre homo.

....... Post homo.-- No emulsifier

100

"0 80CD~:0Cll 60"liiCD01il 40LL

CCDe 20CDa-

00

Interfacial Tension as a Predictor

The interfacial tension between anhydrousmilk fat and a milk SNF solution in the pre­sence of the surfactants was measured. Adifficulty encountered was the solubility ofeach emulsifier in the two phases. In caseswhere the emulsifier was more oil soluble anddispersed in the aqueous phase, it formed alayer at the interface after the lO-min equilibra­tion time. It was thus decided to disperse theemulsifier in the phase in which it was mostsoluble so as to maintain a relative concen­tration at the interface. The results are given inTable 2. The interfacial tension associatedwith the unsaturated emulsifiers was signifi­cantly lower than with the saturated emulsifierswhen the monoglyceride, Span, and Tweenpairs are compared. Also, the Tweens producedthe lowest values, whereas the Spans andmonoglycerides produced significantly highervalues when oleate pairs are compared with thestearates. Thus, an inverse relationship existsbetween the interfacial tension at the twophases and the fat destabilization that resultswhen ice cream mixes with these surfactants arefrozen. Tween 80 produced the highest degreeof fat destabilization and the lowest interfacialtension, various other surfactants fell in be­tween, and a mix with no emulsifier produced

the highest interfacial tension and least fatdestabilization.

An assumption made is that the interfaciallayer in this system is representative of theinterface in an emulsion. This serves as a goodapproximation for the examination of relativetrends and methods for the measurement ofinterfacial tension at the interface in an emul­sion are extremely complex. Also, although theemulsifier would be added to the aqueous phaseduring ice cream manufacture, regardless of itsrelative solubility in each phase, it was thoughtthat concentration at the interface in thisexperiment due to creaming would not berepresentative of surface concentration of thesurfactant in an emulsion.

Influence of Shear on Fat Destabilization

If flotation churning was principally re­sponsible for the destabilization of the fatemulsion during ice cream manufacture, or ifthe membrane was ruptured and swept off, thusexposing the fat and making subsequent co­alescence possible due to mechanical action,then it was hypothesized that fat destabil­ization would occur when mix was subjected tothe shearing action of the dashers and theblades independent of a temperature drop orice crystallization in the mix. This hypothesiswas tested by introducing ice cream mixes to

Journal of Dairy Science Yol. 72, No.1, 1989

FAT DESTABILIZATION IN ICE CREAM 25

the batch freezer, starting the whipping process,but using refrigeration only to maintain the mixtemperature of 4

0C and not to induce freezing.

Destabilization curves for each emulsifierobtained when the mixes were whipped in theabsence of freezing are compared with thedestabilization curves obtained when the icecream mixes were conventionally whipped andfrozen, as presented earlier (Figure 4). It isevident that a limited amount of fat desta­bilization is occurring from the mechanicalaction, that the same relative trends were seenin terms of the destabilizing power of each ofthe four emulsifiers, and that a significantreduction in the extent of fat destabilized wasrealized by removing the presence of thetemperature drop and ice crystallization. Theimportance of ice crystallization can also be

noted in Figure 2 and 3 at the 5 to 7-minrange. When the temperature of _5°C wasobtained, fat destabilization increased rapidly.

The influence of ice crystallization on thefat destabilization process was also examined inthe continuous freezer. The results are pre­sented in Figure 5. As the temperature de­creased through the freezing range from 0 to

_4°C, fat destabilization increased from 10 to60%. The mechanical action of the freezer fromthe pump, dasher, and blades was held constantacross this range. It is evident, therefore, thatsome fat destabilization occurred as a result ofthis mechanical action. However, the magnitudeof fat destabilization normally associated withoptimal ice cream structure and texture, aroundthe 60% range, was not developed until thetemperature had fallen well below the freezing

100--- Span 60 -4C-- Span 60 +4C 100

--- Tween 60 -4C-- Tween 60 +4C

155 10Time in Freezer (min)

60

40

80

20

100

155 10Time in Freezer (min)

--- Span 80 -4C-- Span 80 +4C

o....e:::::::i:-_J-.._"""---_L.-_"""------J

o

40

60

80

80

20

100

Figure 4. The influence of ice crystallization on the fat destabilization process in the batch freezer. Thefour pairs of emulsifiers are compared at _4°C, as in Figure Z, and at 4°C, in which case no freezing was in­volved. Error bars represent 95% confidence limits on four replicates.

Journal of Dairy Science Vol. 72, No.1, 1989

GOFF AND JORDAN

Absorbance

Not frozen Frozen

.79a .79a

.78a .78aGlycerol monostearate40% mono/60% diglyceride

IV<2.STween 80

Emulsifier in Mix (.08%)

TABLE 3. The effect of quiescent freezing of icecream mixes on fat destabilization as measured byspectroturibidity (absorbance at 540 nm).'

a,bValues followed by the same letter are notsignificant (P<.05).

'n = 4.

- 1.0 - 2.0 • 3.0 - 4.0

Temperature (CC)

Figure 5. The influence of ice crystallization onthe fat destabilization process in the continuousfreezer as demonstrated by a reduction in the drawtemperature.

26

100

"0Q) 80N

==:0~ 60Q)

0iii 40u..EQ)

20eQ)

a.00

range. Fat destabilization was still developing asthe temperature reached steady state at _4°C.This confirms the results of the experiment runin the batch freezer. It also confirms reportsthat freezing ice cream at temperatures belowthe normal fteezing range, as is the case in somecommercial freezers that operate at -6 to

-SoC, also enhances fat destabilization (4, 13).The presence of ice crystals and the in­

creased viscosity would contribute to the shearforce exerted on the mix. However, the tem­perature drop would also potentially accountfor other changes in the chemistry of the mix,including a concentration of mineral salts andions influencing electrostatic barriers of repul­sion between the fat globules as water wasfrozen out of solution, a dehydration of mem­brane proteins causing disruption of the mem­brane, and a shift in the solid:liquid fat ratio asthe temperature drop induced further fatcrystallization. Thus, it was evident that eitherthe ice crystallization process or the tem­perature drop and its associated effects were ofprime importance to the destabilization pheno­mena. To elucidate fully the interaction be­tween shear and ice crystallization, it wasdecided to freeze quiescently a number of icecream mixes prepared with various emulsifiersand to examine the extent of fat destabilizedupon thawing: in essence, to examine theinfluence of ice crystallization independent ofany shearing action. No fat destabilizationoccurred as a result of this quiescent freezing

(Table 3). Mean absorbance values prior to andsubsequent to quiescent freezing did notchange, resulting in 0% destabilized fat ac­cording to the spectroturbidity method used inthis project. Dynamic conditions were neces­sary to develop any partial coalescence in themix.

Influence of Crystallization Temperature

The temperature drop from 0 to _4°C is notonly the range of ice crystallization but alsowhen other phenomena, such as a shift in thesolid to liquid fat ratio, occur (22, 30). Thismay partially account for fat destabilization.To test this hypothesis, a mix was made inwhich sugar content was totally replaced withglucose to lower the initial freezing point to_4°C. The freezing point depression of an icecream mix is a function of the number ofmolecules in solution. Because glucose has halfthe molecular weight of sucrose and is also lessthan that of 36 dextrose equivalent (DE)corn syrup solids, the number of molecules inthe 15% glucose mix was at least double that ofthe conventional mix, which was sweetenedwith a mixture of 10% sucrose and 5% 36DEcorn syrup solids. Hence, the initial freezingpoint was reduced. The freezing curves of theglucose mix and the standard ice cream mix areplotted in Figure 6. The initial freezing pointsare on the y intercept. The standard mix beganfreezing at _zoe and the freezing point of the

. 4° 4°C .glucose mIX was - C. At - ,approxI-mately 40% of the water in the conventionalmix was frozen. To freeze approximately 40%

Journal of Dairy Science Vol. 72, No.1, 1989

FAT DESTABILIZATION IN ICE CREAM 27

CONCLUSIONS

ice crystallization was induced by dropping thetemperature in the freezer from -4 to -SoCat the 15 to 20-min interval, the rate of fatdestabilization increased dramatically to ap­proximately the same levels seen at _4°C in theconventional mix (Figure 4). The temperatureis plotted on the right axis of Figure 7.

Changes in temperature from 0 to -4 to

-SoC do affect the solid:liquid fat ratio in milkfat (31). Temperature changes in this range nodoubt have other subtle effects on the chemis­try of these emulsions_ However, these tempera­ture changes had little effect on the extent offat destabilized in the ice cream mix as opposedto the dramatic impacts of the ice crystal­lization process. If the temperature drop from 0to _4°C had a major impact on fat desta­bilization through fat crystallization, forexample, then the glucose mix would haveexhibited more destabilized fat at _4°C than itdid. The ice crystallization process in theglucose mix from -4 to -SoC was similar to

the freezing process in the conventional mixfrom 0 to _4°C. It also had very similar effectsin terms of fat destabilization. Thus, it be­comes clear that the overriding factor involvedin the partial coalescence of the fat emulsion isice crystallization in the dynamic conditions ofthe barrel freezer.

Sucrose/ CSSGlucose

0

G -10-£~::> -20~.,0-E -30{E.

-400 20 40 60 80 100

Percent Water Frozen

of the water in the glucose mix, a temperatureof -SoC was necessary.

Glucose mixes emulsified with each ofsorbitan monostearate (Span 60) and poly­oxyethylene sorbitan monooleate (Tween SO)were then placed in the batch freezer and thetemperature was reduced to -4°C and held for15 min as before. The results are shown inFigure 7. In this case, where no ice crystal­lization occurred, the fat destabilization,shown on the left axis, was greatly reduced andequalled the destabilization seen previouslywhen the mix was held at 4°C. However, when

Figure 6. The freezing curves of the standard icecream mix and of the ice cream mix sweetened withglucose showing the initial freezing point (at 0%water frozen) and the percent of water frozen as afunction of temperature.

Time in Freezer (min)

An inverse relationship was found to existbetween the destabilizing power of the emul­sifier and the resulting interfacial tensionbetween the serum and lipid phases of the mixin the presence of that emulsifier. Emulsifieraction may thus be summarized as follows.Added emulsifiers reduce the interfacial tensionbetween the serum and lipid (fat globule)phases of the mix. It is thus more favorable forthe emulsifiers, rather than caseins, to adsorb tothe fat surface at homogenization, as this leadsto a lowering of the net free energy of thesystem. It has been reported elsewhere thatthese small molecule surfactants can reduce theamount of casein adsorbed onto the surface ofthe fat globule (3, 15, 23). This reduction inthe amount of casein, however, produces anemulsion that is less stable to the shear forcesapplied during ice cream freezing. As a result,the fat emulsion is destabilized to a greaterextent in the presence of the emulsifiers.

10

86

-l4

.,3

2u<D

0 ~c

-2 iil0-

-4 g-6

-8

-1025 305 10 15 20

100

90uQ> 80~:0 70.!'J., 60Q>a

50~u. 40CQ) 30eQ) 200-

lD

00

.... SPAN 60

.. TWEEN 80

... TEMP

Figure 7. The extent of fat destabilized duringthe freezing time in the batch freezer (left axis) andthe plot of temperature during that same time (rightaxis) for the mixes sweetened with glucose and emul­sified with either Span 60 or Tween 80.

Journal of Dairy Science Vol. 72, No.1, 1989

28 GOFF AND JORDAN

Destabilization leads to the production of asmoother ice cream with good melt resistancedue to the enhanced structure of the foamcaused by the fat network.

The addition of Tween 80 posthomogeniza­tion also led to an increase in the percent of fatdestabilized. It can be hypothesized that theproteins adsorbed to the fat globule, par­ticularly the casein micelles, which remainlargely intact and exhibit only limited spread­ing, do not form a cohesive network but can bedisplaced even after adsorption by small mole­cule surfactants, which lower the surfaceexcess.

It appears that ice crystallization in thedynamic conditions of the barrel freezer isnecessary for fat destabilization to occur. Theconcepts of flotation churning through theincorporation of air, or shear sensitivity in­volving the rupturing or stripping of the fatglobule membrane, are not sufficient to explainthe magnitude of fat destabilization thathas been demonstrated in the presence of icecrystallization. These effects might be ac­counted for by the physical presence of the icecrystals contributing to the shear forces on theglobule or to increases in viscosity that occur asice crystallizes.

ACKNOWLEDGMENTS

Gratitude is expressed to the Wisconsin MilkMarketing Board for their financial support ofthis project and to the Cornell Dairy Plant foruse of supplies and equipment.

REFERENCES

1 Adamson, A. W. 1982. Physical chemistry ofsurfaces. 4th ed. John Wiley and Sons, New York,NY.

2 Arbuckle, W. S. 1986. Ice cream. 4th ed. AVI Pub!.Co., Westport, CT.

3 Harfod, N. M., and N. Krog. 1987. Destabilizationand fat crystallization of whipp able emulsions(toppings) studied by pulsed NMR J. Am. OilChern. Soc. 64(1): 112.

4 Berger, K. G. 1976. Ice cream Page 141 in Foodemulsions. S. Friberg, ed. Marcel Dekker Inc., NewYork, NY.

5 Berger, K. G., B. K. Bullimore, G. W. White, and W.B. Wright. 1972. The structure of ice cream. DairyInd. 37:419,493.

6 Berger, K. G., and G. W. White. 1971. An electronmicroscopical investigation of fat destabilization inice cream. J. Food Techno!. 6: 285.

Journal of Dairy Science Vol. 72, No. I, 1989

7 Berger, K. G., and G. W. White. 1976. The fatglobule membrane in ice cream. Dairy Ind. Int.41:199.

8 Bradley, R, and K. Smith. 1983. Finding thefreezing point of frozen desserts. Dairy Rec.84(6):114.

9 Bradley, R. 1984. Plotting freezing curves forfrozen desserts. Dairy Rec. 85(7):86.

10 Darling, D. F. 1982. Recent advances in thedestabilization of dairy emulsions. J. Dairy Res.49:695.

11 Durham, K. 1963. Physical chemistry of ice cream.Chern. Ind. 8:681.

12 Durham, K., T. G. Jones, D. J. Shaw, and D. A.Walker. 1962. Factors affecting the structure of icecream. 1st Int. Congr. Food Sci. Techno!., Vo\.1:769.

13 Frazeur, D. R, and R. B. Harrington. 1968. Lowtemperature and conventionally frozen ice cream.1. The effect of storage conditions and heat shockson body and texture. Food Techno!. 22:910. 2.Interrelationships associated with selected factorsaffecting body and texture. Food Techno!. 22:912.

14 Friberg, S. 1976. Emulsion stability. Ch. 1 in Foodemulsions, S. Friberg, ed. Marcel Dekker, NewYork, NY.

15 Goff, H. D., M. Liboff, W. K. Jordan, and J. E.Kinsella. 1987. The effects of polysorbate 80 onthe fat emulsion in ice cream mix: evidence fromtransmission electron microscopy studies. FoodMicrostruct. 6: 193.

16 Govin, R., and J. G. Leeder. 1971. Action ofemulsifiers in ice cream utilizing the HLB concept.J. Food Sci. 36: 718.

17 Keeney, P. G., and D. V. Josephson. 1958. Ameasure of fat stability in ice cream. Ice CreamTrade J. 54(5): 32.

18 Keeney, P. G., and D. V. Josephson. 1962. Newerconcepts on the mode of action of emulsifiers.16th Int. Dairy Congr. Sect. VI, 53.

19 Kloser, J. J., and p. G. Keeney. 1959. A study ofsome variables that affect fat stability and drynessin ice cream. Ice Cream Trade J. 55(5):26.

20 Knightly, W. H. 1959. The role of the liquidemulsifier in relation to recent research on icecream emulsification. Ice Cream Trade J. 55(6):24.

21 Lin, P. M., and J. G. Leeder. 1974. Mechanism ofemulsifier action in an ice cream system. J. FoodSci. 39: 108.

22 Ml!lder, H., and P. Walstra. 1974. The milk fatglobule. Commonw. Agric. Bur. Farnham Royal,Bucks., Eng\.

23 Oortwijn, H., and P. Walstra. 1979. The mem­branes of recombined fat globules. 2. Composition.Neth. Milk Dairy J. 33:134.

24 Oortwijn, H., and P. Walstra. 1982. The mem­branes of recombined fat globules. 4. Effects onproperties of the recombined milks. Neth. MilkDairy J. 36:279.

25 Oortwijn, H., P. Walstra, and H. Mulder. 1977. Themembranes of recombined fat globules. 1. Electronmicroscopy. Neth. Milk Dairy]. 31:134.

26 Schmidt, D. G., and A.C.M. vanHooydonk. 1980.

FAT DESTABILIZATION IN ICE CREAM 29

A scanning electron microscopal investigation ofthe whipping of cream. Scanning Electron Microsc.3:653.

27 Smith, D. E., Bakshi, A. S., and C. J. Lomauro.1984. Changes in freezing point and rheologicalproperties of ice cream mix as a function ofsweetener system and whey substitution. Milch­wissenschaft 39(8):455.

28 Thomas, E. L. 1981. Structure and properties of

ice cream emulsions. Food Techno!. 35(1):41.29 van Boekel, M.A.J.S., and P. Walstra. 1981. Sta­

bility of oil-in-water emulsions with crystals in thedisperse phase. Colloids and Surfaces 3: 109.

30 Walstra, P., and H. Oortwijn. 1982. The mem­branes of recombined fat globules. 3. Mode offormation. Neth. Milk Dairy J. 36:103.

31 Walstra, P., and R. Jenness. 1984. Dairy chemistryand physics. John Wiley and Sons, New York, NY.

Journal of Dairy Science Vol. 72, No.1, 1989