Effect of Micro Fluid Ization of Heat-treated Milk on Rheology and Sensory Properties of Reduced Fat...

8/10/2019 Effect of Micro Fluid Ization of Heat-treated Milk on Rheology and Sensory Properties of Reduced Fat Yoghurt

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Effect of micro uidization of heat-treated milk on rheologyand sensory properties of reduced fat yoghurt

Chr. Ian E. Ciron a , b, Vivian L. Gee a, Alan L. Kelly b, Mark A.E. Auty a , *a Food Chemistry and Technology Department, Teagasc Food Research Centre, Moorepark, Fermoy, Co. Cork, Irelandb Department of Food and Nutritional Sciences, University College Cork, Ireland

a r t i c l e i n f o

Article history:Received 19 November 2010Accepted 17 February 2011

Keywords:Reduced-fat yoghurtHomogenizationMicro uidizationRheologySensory analysisPrincipal component analysis

a b s t r a c t

The effects of micro uidization at 150 MPa (MFz) and conventional homogenization at 20/5 MPa (CH) of heat-treated milk on the rheology and sensory properties of non- (0.1%) and low- (1.5%) fat stirredyoghurts were compared. Homogenization conditions clearly affected the sensory properties of reduced-fat yoghurts, but the effect was highly dependent on fat content. MFz of heat-treated milk yieldedproducts with very different sensory pro les from the conventional yoghurts. For non-fat yoghurts, MFzof heat-treated milk enhanced the perception of buttermilk and soft cheese avours, and natural yoghurtaroma and avour, but also increased the intensity of undesirable mouthfeel characteristics such aschalkiness, mouth-dryness and astringency. For low-fat yoghurts, MFz signi cantly improved creaminessand desirable texture characteristics such as smoothness, cohesiveness, thickness, and oral and spoonviscosity. These differences in sensory pro les, especially textural properties, were partially related torheological properties, particularly ow behaviour. MFz of heat-treated milk resulted in non- and low-fatyoghurts with higher yield stress, more pronounced hysteresis effect and higher viscosity than those of CH yoghurts of similar fat contents. These ndings suggest that micro uidization may have applicationsfor production of high-quality yoghurt with reduced-fat content.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

In the dairy industry, consistent production of yoghurt withdesirable texture is achieved by heat treatment and homogeniza-tion of the milk base, increasing the milk solids/protein content,and use of commercial starter cultures. The addition of stabilizers,such as gelatine, modi ed starches and polysaccharides is alsoa common practice in the manufacture of yoghurt. Milk-derivedingredients ( Janhoj, Petersen, Frost, & Ipsen, 2006; Johansen,Laugesen, Janhoj, Ipsen, & Frost, 2008 ) and exopolysaccharide-producing bacterial cultures ( Folkenberg, Dejmek, Skriver,Guldager, & Ipsen, 2006 ) have been investigated to assess theirpotential for manufacture of reduced-fat yoghurts (i.e., at least 25%less fat than the full-fat counterpart) with desirable texture prop-erties. Milk proteins have been modi ed to serve as protein-basedfat replacers by mimicking the functionality of fat in structureformation and imparting attractive sensory properties to yoghurt(Seydim, Sarikus, & Okur, 2005 ). Recent studies have examineda range of new technologies, including high-pressure processing

(Penna, Gurram, & Barbosa-Canovas, 2006 ), thermosonication(Riener, Noci, Cronin, Morgan, & Lyng, 2009 ), high-pressurehomogenization ( Lanciotti, Vannini, Pittia, & Guerzoni, 2004; Serra,Trujillo, Quevedo, Guamis, & Ferragut, 2007 ) and micro uidization(Ciron, Gee, Kelly, & Auty, 2010 ), to determine their potential asalternative processes for producing good quality reduced-fatyoghurts.

Few studies have investigated the potential of micro uidizationto improve the texture and stability of yoghurt. Partial replacementof milk solids with micro uidized starch was shown to enhanceviscosity and reduce syneresis in yoghurt ( Augustin, Sanguansri, &Htoon, 2008 ). Cobos, Horne, and Muir (1995) studied the impact of using micro uidization as a homogenization technique on therheological properties of acid gels. Recently, micro uidization wasutilized for production of stirred yoghurts and shown to affect thetexture, water retention and physical properties of the resultantyoghurt. High-pressure homogenization using a Micro uidizerreduced the particle size in heat-treated non- and low-fat milksamples to sizes smaller than those normally occurring in milkprocessed in a conventional valve homogenizer, and resulted inyoghurts with different gel particle size and microstructure ( Cironet al., 2010 ). Such differences in particle size and structure wouldbe expected to in uence rheological behaviour, which could in turn

* Corresponding author. Tel.: 353 25 42442; fax: 353 25 42340.E-mail address: [email protected] (M.A.E. Auty).

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Food Hydrocolloids

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0268-005X/$ e see front matter 2011 Elsevier Ltd. All rights reserved.

doi: 10.1016/j.foodhyd.2011.02.012

Food Hydrocolloids 25 (2011) 1470 e 1476


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impart changes in sensory properties. Thus, the effects of high-pressure micro uidization and conventional homogenization of heat-treated milk on sensory and rheological properties of non-and low-fat yoghurts were compared in the present study. Thiswork also provides insights into the relationship between sensoryperception of texture and rheological properties of yoghurt madewith micro uidized milk, which has not been reported to date.

2. Materials and methods

2.1. Materials

Medium-heat skim milk powder (36.16% protein, 51.98%carbohydrates, 0.77% fat, 7.93% ash, 3.16% moisture) was obtainedfrom Kerry Food Ingredients (Listowel, Co. Kerry, Ireland), andextra-white anhydrous milk fat (99.9%, w/w, fat) was supplied byCorman, S. A. (Go, Belgium). Granulated white sugar (99.91%, w/w,sucrose) purchased from the local supermarket was used toenhance the avour of yoghurt. Yoghurt culture (FD-DVS YFC-471Yo-Flex ) consisting of a mixed strain of Streptococcus thermophilusand Lactobacillus delbrueckii subsp. bulgaricus was provided as a gift

by Chr. Hansen, Cork, Ireland.

2.2. Production of yoghurt samples

Non- and low-fat yoghurts (0.1, 1.5% fat) were produced fromrecombined milk samples according to the procedure described byCiron et al. (2010) . Brie y, the milk samples were heated (95 C,2 min), then either homogenized using a two stage (20/5 MPa)conventional homogenizer or micro uidized at 150 MPa. Cooledstirred yoghurts (20 C) were apportioned into sterile propyleneconical pots with snap-on caps (Plastiques Gosselin, France); 125 ginto 200-mL pots for rheological measurements and w 800 g into1-L pots for sensory evaluation. All sample treatments wereproduced in duplicate, stored in a walk-in chiller ( w 5 C), and

analyzed after 7 1 days of production.

2.3. Rheological analysis

The rheological properties of stirred yoghurts were character-ized in duplicate at 5 C using an AR 2000ex rheometer (TAInstruments UK Ltd., U.K.), tted with a standard-sized DINgeometry (conical concentric cylinders with 15 mm inner statorradius,14 mm outer rotor radius, 42 mm cylinder immersed height,and 5920 mm gap). Prior to the measurements of viscoelasticproperties or ow behaviour, approximately 17 g of yoghurt samplewas allowed to rebody in the rheometer cup for 30 min at 5 Cwhile the inner concentric cylinder was immersed.

Low-amplitude oscillatory measurements were made as follows

to determine the viscoelastic properties: frequency sweeps(0.1 e 100 rad s 1, in log progression with 10 points per decade)were performed at constant strain of 0.5%, which was within thelinear viscoelastic region as determined in preliminary experi-ments; after this strain sweeps (0.1 e 100%) were performed ata xed angular frequency (1 rad s 1).

Flow behaviours was determined on a new set of samples of yoghurt by shear-rate sweeps (0.1 e 100 s 1, in log progression) at anincreasing shear rate (upward ow), followed by a decreasing shearrate (downward ow) at constant angular frequency (1 rad s 1) andstrain (0.5%) for 10 min. The ow curves were tted with a Her-schel e Bulkley model using a Rheology Advantage Data Analysissoftware (TA Instruments UK Ltd., U.K.). The yield stress ( s 0),consistency coef cient ( k) and ow behaviour rate index ( n) were

calculated using the Herschele

Bulkley model:

s s 0 k _g n (1)

2.4. Sensory analysis

Descriptive sensory analysis was conducted to identify andquantify the perceived attributes in stirred yoghurts. The sensorypro les of the yoghurts were determined by a trained sensory

panel comprised of eight assessors, who were selected based onprevious experience in evaluating products, taste sensitivity, andability to detect sensory differences. A sensory vocabulary of 32attributes describing the appearance, aroma, avour and texture of stirred yoghurt was developed by panel consensus using referencesamples. Creaminess was evaluated by the assessors using theirown de nition. The trained panel evaluated the samples in tripli-cate over three sessions in separate booths in a sensory room. Thesamples were rated for each attribute on a 10-mm line scale(0 none to 10 very high) anchored by appropriate referencestandards for each sensory attribute. The samples ( w 100 g) werekept under refrigeration ( w 5 C) for an hour prior to serving, andpresented to the assessors in random and balanced order in whiteplastic cups coded with three-digit random numbers. Sparkling

water was provided for cleansing the palate in between samples.

2.5. Data analysis

The rheological and sensory data were subjected to analysis of variance (ANOVA) using the general linear model (GLM) to deter-mine signi cant treatment and interaction effects at a 5% level of signi cance. The results were reported as mean values for eachparameter, and Tukey s test was performed for multiple compari-sons of the treatments. Principal component analysis (PCA) wasalso performed separately on rheological and sensory data, and PCAplots were generated. Minitab 15 (Minitab Ltd., U.K.) software wasused for all statistical analyses.

3. Results and discussion

3.1. Effect of micro uidization of heat-treated milk on rheologicalbehaviour of reduced-fat yoghurts

3.1.1. Viscoelastic propertiesAll reduced-fat yoghurts in the study exhibited viscoelastic

behaviour, characterized by frequency and strain dependency,irrespective of fat content and homogenization condition applied tothe heat-treated milk. Micro uidization at 150 MPa (MFz) andconventional homogenization at 20/5 MPa (CH) had similar effectson the viscoelastic properties of non- and low-fat stirred yoghurts.The yoghurts produced from micro uidized milk and convention-ally homogenized milk had almost identical values of elastic

modulus ( G0

) and viscous modulus ( G00

) for both non- and low-fatsamples, as shown in frequency- ( Fig. 1A) and strain-sweep curves(Fig. 2), and Table 1 ( p > 0.05). Their phase angle ( d) values werealso comparable ( p > 0.05, Table 1 ) from very low to highfrequencies ( Fig. 1B). The strain-sweep pro les ( Fig. 2) demon-strated similar linear viscoelastic (LVE) ranges and G0e G00cross-overpoints ( G0 G00), indicating the strain sensitivity and transitionpoint from elastic to viscous behaviour were not affected byhomogenization condition.

Despite the non-signi cance of the effect of homogenizationcondition on the viscoelastic properties, MFz of heat-treated milkresulted in non-fat yoghurt (0% MFz) with marginally lower G0 andG00 values than those of yoghurt produced from milk homogenizedusing the conventional method (0% CH). This is shown in both

frequency- ( Fig.1 A) and strain- ( Fig. 2A) sweep curves. 0% MFz had

Chr.I.E. Ciron et al. / Food Hydrocolloids 25 (2011) 1470 e 1476 1471


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also slightly lower values of yield stress ( s y), and stress (LVE- s ) andstrain (LVE- g) at the limit of LVE compared with 0% CH ( Table 1 ),further indicating a slightly weaker structure. These results were inagreement with the observations on back-extrusion tests usinga texture analyzer in our previous study ( Ciron et al., 2010 ). In fact,MFz of heat-treated milk had detrimental effects on texture and

water retention of non-fat stirred yoghurts. The slightly weakerstructure of 0% MFz compared to conventional yoghurt could beattributed to the differences in microstructures, as discussed in ourprevious report ( Ciron et al., 2010 ). The more heterogeneousmicrostructure of 0% MFz compared to 0% CH, consisting of largeprotein aggregates with less interconnections between each other,

was suggested to be responsible for the low rmness.Figs. 1 and 2B show the effect of homogenization condition on

viscoelastic properties of low-fat yoghurts. Similar G0 and G00 valuesin relation to frequency ( Fig. 1A) and strain ( Fig. 2B) was found forlow-fat yoghurts from micro uidized milk (1.5% MFz) andconventionally homogenized milk (1.5% CH). This indicates thathomogenization condition had no de nite effect on rmness of low-fat yoghurt, although MFz yielded smaller fat globules than CH(Ciron et al., 2010 ) and increased the amount of interacting parti-cles, comprised of milk proteins and fat ( Sharma & Dalgleish, 1993 ).

A

1

10

100

1000

0010111.0

Strain (%)

G ' / G " ( P a )

B

1

10

100

1000

0010111.0

Strain (%)

G ' / G " ( P a )

Fig. 2. Elastic modulus, G0 (solid symbols) and viscous modulus, G00 (hollow symbols) as a function of strain for A) non-fat (0%) and B) low-fat (1.5%) stirred yoghurts made with

conventionally homogenized (CH) or micro uidized (MFz) milk: 0% CH (6

,:

); 0% MFz (,

,-

); 1.5% CH (B

,C

); 1.5% MFz (>

,A

).

10

100

1000

0010111.0

Angular frequency (rad s -1)

G ' / G " ( P a )

0

5

10

15

20

0010111.0

Angular frequency (rad s -1 )

(

A

B

Fig.1. Frequency curves of non-fat (0%) and low-fat (1.5%) stirred yoghurts made withconventionally homogenized (CH) or micro uidized (MFz) milk. A) Elastic modulus, G0

(solid symbols) and viscous modulus, G00 (hollow symbols), and B) phase angle,d (mathematical symbols) as a function of frequency: 0% CH ( 6 , : , d ); 0% MFz(, , - , ); 1.5% CH (B , C , ); 1.5% MFz (> , A , ).

Table 1Rheological behaviour properties of reduced-fat stirred yoghurts as affected by fatcontent (0.1%,1.5% fat) and homogenization condition a (CH, MFz).b

Parameters Non-fat (0.1%) Low-fat (1.5%)

CH MFz CH MFz

Frequency sweeps c

G0 (Pa) 77.97 a 68.46 a 125.45 b 121.85 bG00 (Pa) 19.38 a 16.80 a 31.04 b 28.32 bd ( ) 13.96 a 13.80 a 13.92 a 13.09 a

Strain sweeps d

LVE-s (Pa) 14.00 a 13.38 a 13.59 a 13.00 aLVE-g (%) 0.6398 ab 0.5190 a 0.6373 ab 0.8022 bs y (Pa) 6.14 a 4.94 a 10.17 b 11.52 bg y (%) 42.30 a 53.66 a 45.06 a 37.15 a

Shear-rate sweeps e

s o (Pa) 1.244 a 3.778 b 4.728 b 13.932 dk (Pas n) 5.597 a 10.673 b 13.308 b 25.795 cn 0.3224 c 0.2428 b 0.2446 b 0.2061 ah50 (Pa s) 0.3702 a 0.4724 b 0.5303 b 0.8405 cHL area(Pa s 1)

1018 a 1800 b 2474 c 3888 d

a Homogenization condition: CH conventional valve homogenization (20/5 MPa); MFz micro uidization (150 MPa).

b

Mean values ( n

2) that have different letters across each row signi cantlydiffer ( p 0.05) using GLM-ANOVA and Tukey s test.c Frequency sweep parameters were reported at 1 rad s 1.d Strain-sweep parameters: stress (LVE- s ) and strain (LVE- g) at the limit of LVE,

and yield stress ( s y; ) and yield strain ( gy) at cross-over of G0 and G00.e Shear-rate sweep parameters: h 50 apparent viscosity at 50 s 1; HL hyste-

resis loop area; and Herschel e Bulkley model parameters, where s o yield stress,k consistency coef cient, and n rate index.



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This result is supported by earlier ndings on the effect of MFz ontexture properties as ascertained by back-extrusion test ( Cironet al., 2010 ) and corroborates with that of Cobos et al. (1995) ,demonstrating similar effects of micro uidization and conven-tional homogenization on viscoelastic properties of acid milk gels.To fully understand the mechanism behind the ndings, furtherstudies are required.

3.1.2. Flow behaviour The ow behaviour was also determined since viscosity is an

important quality parameter that in uences the sensory propertiesof yoghurt. Rheometric viscosity has been reported to have a strongpositive correlation with thickness ( Skriver, Holstborg, & Qvist,1999 ). The experimental yoghurts were highly thixotropic, andbehaved as pseudoplastic materials ( Delorenzi, Pricl, & Torriano,1995 ) with a yield point and hysteresis loop ( Fig. 3).

Homogenization condition clearly affected the ow behavioursof non- and low-fat yoghurts. A noticeable increase in viscosity wasobserved for non-fat yoghurt when micro uidized milk was usedfor production, as illustrated by higher consistency coef cient ( k),lower ow rate index ( n), and higher apparent viscosity at 50 s 1

(h50 ) for 0% MFz than for 0% CH (Table 1 and Fig. 3). Moreover, 0%MFz had signi cantly higher yield stress ( s 0) and hysteresis looparea (HL) than 0% CH ( p < 0.05, Table 1 ).

More pronounced changes in ow behaviours were observed inlow-fat yoghurt, compared to non-fat yoghurt; the ow pro le of 1.5% MFz was very different from that of 1.5% CH, showing highershear stress and greater apparent viscosity as the shear rateincreased ( Fig. 3). Higher yield stress and a larger hysteresis loop in1.5% MFz than in 1.5% CH were evident in the ow curves ( Fig. 3),1.5% MFz exhibited a very prominent yielding point as well. Thehigher yield stress ( p 0.05) of 1.5% MFz as compared to 1.5% CH(Fig. 3 and Table 1 ) implies that a greater shear stress was requiredfor ow to commence and thus it is more resistant to shearing. Thisindicates that MFz of low-fat milk produced a yoghurt with a moreconsolidated network compared to the standard process, probably

due to more interactions as consequences of greater size reductionof fat globules ( Ciron et al., 2010 ) and casein micelles ( Pouliot,Britten, & Latreille, 1990 ). The more pronounced hysteresis effect( p 0.05) of MFz of heat-treated milk compared to CH in low-fatyoghurt, as shown by a larger hysteresis loop ( Fig. 3 and Table 1 ),indicates that 1.5% MFz has less ability than 1.5% CH to fully recoverits structure after shear-induced breakdown. The Herschel e Bulkleymodel tted very well to theupward ow curves (0.990 r 0.999)because the shear-thinning ow behaviour of the low-fat yoghurts

had an inherent yield point. The ow model parameters of theHerschel e Bulkley function are presented in Table 1 , and signi -cantly ( p 0.05) higher valuesof s 0, k and h50 , and lower values of nwere obtained for 1.5% MFz in comparison with 1.5% CH. This indi-cates higher viscosity, higher yield stress and more shear-thinningbehaviour of low-fat yoghurt produced from milk homogenized byMFz rather than that made using conventional method.

The positive effects of MFz on ow behaviour of low-fatyoghurts in the present study are in contrast with our earlier

ndings on the viscosity of low-fat yoghurt measured using back-extrusion, wherein the two homogenization conditions resulted inyoghurts with similar viscosity index and consistency ( Ciron et al.,2010 ). A possible explanation for the inconsistency would berelated to the differences in principles and mechanisms of the twomethodologies for assessing the viscosity of yoghurt. Back-extru-sion tests use pseudo-compression (compression and extrusion)while rheometric viscosity is based on shearing of the sample. Theviscosity index and consistency determined by the back-extrusiontest would be more related to gel rmness ( G0) and sensory rm-ness of the yoghurt, while the rheometric viscosity wouldbe a goodindicator of sensory viscosity.

The increase in viscosity of low-fat yoghurt through MFz of heat-treated milk could be attributed to modi cation in micro-structure and particle size (and composition) of gel dispersions. Arecent confocal microscopy study on low-fat yoghurts demon-strated that MFz created fat globules with a more active role instructure formation; micro uidized fat globules were greatlyreduced in size, and incorporated and intimately bound to theproteins in a more highly consolidated gel network, whileconventionally homogenized fat globules appeared to be moreloosely entrapped within the protein networks ( Ciron et al., 2010 ).This increased incorporation of smaller fat globules into the proteingel networks could explain the enhancement in viscosity of low-fatyoghurts by micro uidization.

3.2. Effect of micro uidization of heat-treated milk on sensory

properties of reduced-fat yoghurts

Descriptive sensory analysis was performed by a trained panelto determine the sensory pro les of reduced-fat yoghurts based onestablished descriptors. The list of descriptors consisted of fourappearance, four aroma, nine avour and 15 mouthfeel attributes,together with their corresponding de nitions ( Table 2 ). The meanratings for creaminess and 32 sensory attributes developed by thetrained panel of eight members arepresented in Table 3 . Allsensoryproperties were clearly affected by fat content and homogenizationcondition. Interactions between fat content and homogenizationcondition were signi cant ( p 0.01) for surface water, smoothness,cream aroma, natural yoghurt aroma, soft cheese aroma and

avour, buttermilk avour, astringency, and all mouthfeel attri-

butes, except for oral smoothness and fattiness. The rest of thesensory properties were affected ( p 0.01) by homogenizationcondition, irrespective of fat content.

A multivariate representation was plotted using PCA to havea better understanding of the sensory pro les of the treatmentsamples. PCA of the sensory data ( Fig. 4) showed that the rst twoPCs explained 85.7% of the total variation. PC1 (49.4%), whichsegregated the yoghurts based on homogenization condition(Fig. 4B), was positively correlated with natural yoghurt aroma and

avour, sourness, astringency, shininess, oral smoothness, sticki-ness, cohesiveness, mouth-coating, mouth-drying and chalkiness,and negatively correlated with bitterness ( Fig. 4A). A sensorydifferentiation based on fat content ( Fig. 4B) was evident along PC2(36.3%), which was described by soft cheese aroma, buttermilk

aroma and surface water on the positive side, and featheriness,

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

Shear rate (s-1 )

S h e a r s t r e s s

( P a

)

Fig. 3. Flow behaviour pro les of non-fat (0%) and low-fat (1.5%) stirred yoghurtsmade with conventionally homogenized (CH) or micro uidized (MFz) milk: 0% CH

(:

); 0% MFz (,

); 1.5% CH (C

); 1.5% MFz (

).



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velvetiness, rmness, meltdown rate, thickness, cohesiveness andcreaminess on its negative side ( Fig. 4A).

A distinct segregation of the four yoghurt types in terms of theirsensory properties was shown in the PCAplots ( Fig. 4A and B). Non-

fat yoghurts were positioned on the top half of the sensory space,

and were further segmented as follows with regards to homogeni-zation condition. 0% CH (inthe third quadrant) was characterized byhigh intensities of bitterness, saltiness, soft cheese aroma, butter-milk aroma andcurdiness,and high amountof surfacewater. 0% MFz(in the fourth quadrant) was perceived as astringent, chalky andmouth-drying, but with high soft cheese and buttermilk avours,

natural yoghurt aroma and avour, and mouth-coating. Conversely,low-fat yoghurts were situated in the lower portion of the plot.1.5%CH (in the second quadrant) had the highest score for fattiness, butthe lowest intensities for shininess, chalkiness, mouth-coating,mouth-drying, sourness, and natural aroma and avour. 1.5% MFz(onthe rstquadrant) hadthe highest values for smoothness (spoonand oral), stickiness, cohesiveness, viscosity (spoon and oral),thickness, rmness, velvetiness, featheriness, meltdownrate, cream

avour and aroma, and creaminess. Hence, reduced-fat stirredyoghurts with different sensory pro les can be produced bymanipulating the fat content and homogenization condition.

Combining the results of GLM-ANOVA ( Table 3 ) and PCA (Fig. 4)indicated that MFz of heat-treated milk had a marked effect( p 0.01) on the sensory properties of reduced-fat yoghurts.

Regardless of fat content, MFz enhanced shininess, cream

avour,

Table 2Sensory attributes for stirred yoghurts, as de ned by the trained panel.

Attributes Abbreviation De nition

AppearanceShininess A-Shiny Appears br ight and glossySurface water Surface water Amount of water present on the

surface of the sampleSmoothness A-Smooth Looks smooth and free of irregularities

Spoon viscosity A-Viscous Thickness of the sample ranging fromthick to watery

AromaCream aroma Ar-Cream Aroma of fresh creamButtermilk aroma Ar-Buttermilk Aroma of buttermilkNatural yoghurtaroma

Ar-Naturalyoghurt

Aroma of natural yoghurt

Soft whitecheese aroma

Ar-Soft cheese Aroma of soft white cheese

Taste/ avourSweetness Sweet Taste of sucrose, other sugars and

arti cial sweetenersSourness Sour Taste associat ed with certain acids

such as citric acidSaltiness Salty Taste of sodium chlorideBit terness Bit ter Taste associated with quinine

and caffeineCream avour F-Cream Aromatics/taste of fresh creamButtermilk avour F-Buttermilk Aromatics/taste of buttermilkNatural yoghurt

avourF-Naturalyoghurt

Aromatics/taste of natural yoghurt

Soft cheese avour F-Soft cheese Aromatics/taste of soft white cheeseAstringency Astringent Dry, puckering feeling in the mouth

caused by tannins

Texture (mouthfeel)Oral smoothness M-Smooth Perceived smoothness in the mouth

from smooth to roughOral Viscosity M-Viscous High resistance to ow in the mouthChalkiness M-Chalky A chalky, cloying powdery sensation

in the mouthGrittiness M-Gritty Amount of sandy particles present in

the sampleFeatheriness M-Feathery A light sensation created by a sample

that contains trapped air, reminiscentof whipped productsFat tiness M-Fat ty Perceived amount of fat /grease

in the sampleMeltdown rate M-Meltdown Rate of the created sensation of a

sample melting in the mouthFirmness M-Firm Solid, compact sensation; holds its shapeVelvetiness M-Velvety A silky, velvety sensation that slides

on the surface of the tongue and theroof and sides of the mouth

Curdiness M-Curdy Amount of lumps present in the sampleStickiness M-Sticky Degree to which the sample sticks

or adheres to the teeth and palateThickness M-Thick Perceived thickness of the sample

in the mouthCohesiveness M-Cohesive Degree of holding together rather

than spreading across the tongue andsurfaces of the mouth

Mouth-dryness M-dry Perception of dryness in the mouth; amouth-drying sample is saliva absorbing

Mouth-coating M-coat Sensation of a coating layer left in themouth after swallowing the sample

Creaminess Creamy Overall intensity of the perceivedcreaminess based on each assessor sown concept (could include appearance,

avour and texture)

Table 3Descriptive sensory ratings for reduced-fat stirred yoghurts. a

Sensory attributes Non-fat (0.1%) Low-fat (1.5%) p-Value b

CH MFz CH MFz Fat HCc Fat HC

AppearanceA-Shiny 5.4 a 7.3 b 5.5 a 7.6 c 0.004 < 0.001 NSSurface water 1.0 b 0.8 b 0.8 b 0.3 a < 0.001 < 0.001 < 0.001A-Smooth 6.8 a 8.0 c 7.6 b 8.4 d < 0.001 < 0.001 0.007A-Viscous 6.3 a 6.3 a 6.9 b 7.2 c < 0.001 0.003 NS

AromaAr-Cream 0.6 a 0.5 a 1.0 c 0.8 b < 0.001 < 0.001 0.036Ar-Buttermilk 1.1 b 1.1 b 1.1 b 0.9 a 0.001 0.001 NSAr-Natural yoghurt 4.4 a 6.6 c 4.2 a 6.0 b < 0.001 < 0.001 0.002Ar-Soft cheese 1.0 c 0 .8 b 0 .9 bc 0.5 a < 0.001 < 0.001 < 0.001

Taste/ avourSweet 1.4 b 1.1 a 1.4 a 1.3 b NS < 0.001 NSSour 1.7 a 2.7 b 1.7 a 2.7 b NS < 0.001 NSSalty 1.0 b 0.8 b 0.8 b 0.6 a 0.002 < 0.001 NSBitter 1.1 b 0.8 a 1.0 b 0.8 a NS < 0.001 NSF-Cream 0.6 b 0.5 a 1.1 c 0.8 b < 0.001 < 0.001 NSF-Butt ermilk 1.4 a 1.7 b 1.2 a 1.3 a < 0.001 < 0.001 < 0.001F-Natural yoghurt 5.0 b 6.7 d 4.9 a 6.3 c < 0.001 < 0.001 NSF-Soft cheese 1.0 a 2.4 b 1.0 a 1.1 a < 0.001 < 0.001 < 0.001Astringent 1.3 a 2.5 c 1.1 a 1.6 b < 0.001 < 0.001 < 0.001

Texture (mouthfeel)M-Smooth 7.2 a 8.0 c 7.5 b 8.4 d < 0.001 < 0.001 NSM-Viscous 6.1 a 6.4 a 7.0 b 7.3 c < 0.001 0.002 0.027M-Chalky 0.8 a 3.8 c 0.7 a 2.8 b < 0.001 < 0.001 < 0.001M-Gritty 0.5 a 0.7 b 0.6 b 0.6 b NS < 0.001 0.001M-Feathery 5.7 b 3.7 a 6.2 c 5.4 b < 0.001 < 0.001 < 0.001M-Fatty 1.4 ab 1 .1 a 1.4 b 1.2 ab NS 0.008 NSM-Meltdown 5.9 b 4.8 a 5.8 b 6.0 b < 0.001 < 0.001 < 0.001M-Firm 5.5 b 5.0 a 6.1 c 6.3 c < 0.001 < 0.001 < 0.001M-Velvety 5.7 b 5.3 a 6.6 c 6.6 c < 0.001 < 0.001 < 0.001M-Curdy 1.1 b 1.0 b 1.1 b 0.7 a < 0.001 < 0.001 0.005M-Sticky 4.1 b 5.0 c 3.9 a 5.4 d NS < 0.001 < 0.001M-Thick 4.5 a 4.9 a 5.8 b 6.1 c < 0.001 NS 0.013M-Cohesive 2.3 a 4.1 b 2.1 a 5.7 c < 0.001 < 0.001 < 0.001M-Dry 1.9 a 3.7 c 1 .9 a 3.1 b < 0.001 < 0.001 < 0.001M-Coat 1.9 a 3.6 c 1.8 a 3.2 b < 0.001 < 0.001 < 0.001

Creaminess 5.3 a 5.7 a 6.2 b 7.0 c < 0.001 < 0.001 < 0.001a Mean ratings (replicates, n 2; assessors, p 8; trials, t 3) with different

letters across rows differ signi cantly at p < 0.05 using GLM-ANOVA and Tukey stest.

b NS denotes non-signi cance at p < 0.05.c HC homogenization condition, either conventional homogenization at 20/

5 MPa (CH) or micro uidization at 150 MPa (MFz).



6/7

natural yoghurt avourandnon-oral smoothness, while reducing theperception of sweetness, saltiness, bitterness and fattiness. It is alsonoteworthy that MFz improved the oral smoothness in both types of yoghurt; the oral smoothness of 0% MFz was even higher than that of 1.5% CH. In agreement with an earlier sensory study on low-fatyoghurt ( Janhoj et al., 2006 ), fat content increased oral smoothness,which was further enhanced by MFz of heat-treated milk.

Theeffect ofMFz onmostof thesensory properties depended onfatcontent, especially for mouthfeel attributes ( Table 3 , Fig. 4). In non-fatyoghurt, MFz caused a signi cant reduction in meltdown rate, feath-eriness, rmness and velvetiness, when compared with the controlsample, but in parallel the perception of mouth-coating characterincreased. MFzseemed to have the potential to increasethe intensities

of soft cheese avour, buttermilk avour, and natural yoghurt aromaand avour of non-fat yoghurt. For low-fat yoghurt, MFz was favour-able in terms of enhancing creaminess and some of the fat-associatedtexture attributes, such as non-oral smoothness, viscosity (spoon andoral) and thickness. MFz was also suitable for developing a moremouth-coating mouthfeel and a shiny appearance in low-fat yoghurt,although it reduced the positive effect of the presence of 1.5% fat oncream aroma, featheriness, rmness and velvetiness compared to CH.Furthermore, marked improvements in stickiness and cohesivenesswere achieved by using MFz compared with CH, while reducing thedegree of syneresis (surfacewater)and amount of lumps(curdiness)inlow-fat yoghurts. Hence, there is a synergistic effect of high-pressuremicro uidization and fat content on creaminess and associatedtexture attributes of yoghurt, which has not been previously reported

and will be the subject of further investigation.

As expected, the presence of fat enhanced desirable textureproperties in reduced-fat yoghurts, including smoothness, viscosity,featheriness, rmness, velvetiness, thicknessand creaminess, whilereducing the amount of surface water. The texture-enhancingcapability of fat in yoghurt ( Cobos et al., 1995; Keogh & O Kennedy,1998; Lucey, Munro, & Singh,1998; Patrignani et al., 2007 ) is relatedto the ability of homogenized fat globules to participate in the gelnetwork formation ( Aguilera & Kessler, 1988; Sodini, Remeuf,Haddad, & Corrieu, 2004 ) and consequently strengthen theyoghurt gel structure ( Lucey et al., 1998 ).

Further improvements in creaminess, smoothness, viscosity andthickness of low-fat yoghurt achieved by MFz of heat-treated milkcould be explained by increased interactions between fat globulesand milk proteins due to the changes in particle size and micro-structure. Reduction of fat globules by MFz to size similar to that of casein micelles increased the effective surface area for milkproteins (casein and/or whey proteins) to adsorb on the new fatglobule membrane. Furthermore, the milk proteins became morereactive due to thermal denaturation of whey proteins ( Lucey et al.,1998 ) and micro uidization-induced disruption of casein micelles(Dalgleish, Tosh, & West, 1996; Sharma & Dalgleish, 1993 ). More-over, fat globules that could actively interact with other particleswere created by micro uidization due to the modi cation of fatglobular membranes, which are constituted of semi-intact caseinmicelles or micellar fragments ( Dalgleish et al., 1996; Sharma &Dalgleish, 1993 ). This allowed the casein-coated fat globules tointeract further with casein micelles, micellar fragments, or casein-denatured whey protein complexes, forming dense three-dimen-sional networks of milk proteins and fat as shown by confocalmicroscopy ( Ciron et al., 2010 ). Increased non-oral and oralsmoothness could also be related to the uniform distribution fatglobules in the network structure of low-fat yoghurt besides their very small small size ( w 220 nm) and the lubricating nature of fat.

Although differing sensory pro les of reduced-fat yoghurtscould be attributed largely to the changes in size, microstructureand interactions of proteins and fat globules, some of the texture

attributes could be partially related to ow behaviour. The increasein intensities of spoon and oral viscosity, and thickness of yoghurtsdue to MFz of milk is in agreement with the results of instrumentalviscosity. There were also strong positive correlations for spoonviscosity ( r 0.947; p < 0.001), oral viscosity ( r 0.889; p < 0.001)and thickness ( r 0.867; p < 0.001) with apparent viscosity at50 s 1 (h50 ). A good correlation between oral perception andrheometric viscosity at similar shear rate was reported in an earlierstudy of Skriver et al. (1999) . The increase in number of interactingparticles and fat e protein interactions is the likely reason for theenhancement of the viscosity of low-fat yoghurt.

It should also be noted that the sensory attributes mainly relatedto the fat content ( Fig. 4) were highly correlated with creaminess,which was thus further examined. Not surprisingly, most of these

were texture attributes comprised of oral and visual descriptors,but some avour and aroma attributes were also important forcreaminess. Good correlations (0.76 r 0.95; p 0.05) of creaminess with these sensory attributes were obtained for stirredreduced-fat yoghurts ( Table 4 ). Spoon and oral viscosity, velveti-ness and thickness of yoghurt contributed positively to creaminess,whereas the perception of creaminess was impaired by the amountof visible surface water present. These ndings reinforce theconcept of creaminess as a multidimensional descriptor involvingappearance, avour and texture attributes in food ( Janhoj et al.,2006; Johansen et al., 2008 ). Soft cheese aroma and buttermilkaroma were negatively correlated with creaminess because theseattributes were associated with expelled whey (surface water), asindicated by a strong correlation of cheese aroma ( r 0.960;

p 0.001) and buttermilk aroma ( r 0.979; p

Effect of Micro Fluid Ization of Heat-treated Milk on Rheology and Sensory Properties of Reduced Fat...

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