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Effect of microfluidization 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 2010

Accepted 17 February 2011

Keywords:

Reduced-fat yoghurt

Homogenization

Microfluidization

Rheology

Sensory analysis

Principal component analysis

a b s t r a c t

The effects of microfluidization 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 stirred

yoghurts 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 yielded

products with very different sensory profiles from the conventional yoghurts. For non-fat yoghurts, MFz

of heat-treated milk enhanced the perception of buttermilk and soft cheese flavours, and natural yoghurt

aroma and flavour, but also increased the intensity of undesirable mouthfeel characteristics such as

chalkiness, mouth-dryness and astringency. For low-fat yoghurts, MFz significantly improved creaminess

and desirable texture characteristics such as smoothness, cohesiveness, thickness, and oral and spoon

viscosity. These differences in sensory profiles, especially textural properties, were partially related to

rheological properties, particularly flow behaviour. MFz of heat-treated milk resulted in non- and low-fat

yoghurts with higher yield stress, more pronounced hysteresis effect and higher viscosity than those of

CH yoghurts of similar fat contents. These findings suggest that microfluidization may have applications

for 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 with

desirable 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, modified starches and polysaccharides is also

a common practice in the manufacture of yoghurt. Milk-derived

ingredients (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 modified to serve as protein-based

fat replacers by mimicking the functionality of fat in structure

formation and imparting attractive sensory properties to yoghurt

(Seydim, Sarikus, & Okur, 2005). Recent studies have examined

a range of new technologies, including high-pressure processing

(Penna, Gurram, & Barbosa-Canovas, 2006), thermosonication

(Riener, Noci, Cronin, Morgan, & Lyng, 2009), high-pressure

homogenization (Lanciotti, Vannini, Pittia, & Guerzoni, 2004; Serra,

Trujillo, Quevedo, Guamis, & Ferragut, 2007) and microfluidization

(Ciron, Gee, Kelly, & Auty, 2010), to determine their potential as

alternative processes for producing good quality reduced-fat

yoghurts.

Few studies have investigated the potential of microfluidization

to improve the texture and stability of yoghurt. Partial replacement

of milk solids with microfluidized starch was shown to enhance

viscosity and reduce syneresis in yoghurt (Augustin, Sanguansri, &

Htoon, 2008). Cobos, Horne, and Muir (1995) studied the impact ofusing microfluidization as a homogenization technique on the

rheological properties of acid gels. Recently, microfluidization was

utilized for production of stirred yoghurts and shown to affect the

texture, water retention and physical properties of the resultant

yoghurt. High-pressure homogenization using a Microfluidizer

reduced the particle size in heat-treated non- and low-fat milk

samples to sizes smaller than those normally occurring in milk

processed in a conventional valve homogenizer, and resulted in

yoghurts with different gel particle size and microstructure (Ciron

et al., 2010). Such differences in particle size and structure would

be expected to influence rheological behaviour, which could in turn* Corresponding author. Tel.: 353 25 42442; fax: 353 25 42340.

E-mail address: mark.auty@teagasc.ie (M.A.E. Auty).

Contents lists available at ScienceDirect

Food Hydrocolloids

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f o o d h y d

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

doi:10.1016/j.foodhyd.2011.02.012

Food Hydrocolloids 25 (2011) 1470e1476

mailto:mark.auty@teagasc.iehttp://www.sciencedirect.com/science/journal/0268005Xhttp://www.elsevier.com/locate/foodhydhttp://dx.doi.org/10.1016/j.foodhyd.2011.02.012http://dx.doi.org/10.1016/j.foodhyd.2011.02.012http://dx.doi.org/10.1016/j.foodhyd.2011.02.012http://dx.doi.org/10.1016/j.foodhyd.2011.02.012http://dx.doi.org/10.1016/j.foodhyd.2011.02.012http://dx.doi.org/10.1016/j.foodhyd.2011.02.012http://www.elsevier.com/locate/foodhydhttp://www.sciencedirect.com/science/journal/0268005Xmailto:mark.auty@teagasc.ie

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impart changes in sensory properties. Thus, the effects of high-

pressure microfluidization and conventional homogenization of

heat-treated milk on sensory and rheological properties of non-

and low-fat yoghurts were compared in the present study. This

work also provides insights into the relationship between sensory

perception of texture and rheological properties of yoghurt made

with microfluidized 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 obtained

from Kerry Food Ingredients (Listowel, Co. Kerry, Ireland), and

extra-white anhydrous milk fat (99.9%, w/w, fat) was supplied by

Corman, S. A. (Go, Belgium). Granulated white sugar (99.91%, w/w,

sucrose) purchased from the local supermarket was used to

enhance the flavour of yoghurt. Yoghurt culture (FD-DVS YFC-471

Yo-Flex) consisting of a mixed strain ofStreptococcus thermophilus

and 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 from

recombined milk samples according to the procedure described by

Ciron et al. (2010). Briefly, the milk samples were heated (95 C,

2 min), then either homogenized using a two stage (20/5 MPa)

conventional homogenizer or microfluidized at 150 MPa. Cooled

stirred yoghurts (20 C) were apportioned into sterile propylene

conical pots with snap-on caps (Plastiques Gosselin, France); 125 g

into 200-mL pots for rheological measurements and w800 g into

1-L pots for sensory evaluation. All sample treatments were

produced in duplicate, stored in a walk-in chiller (w5 C), and

analyzed after 71 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 (TA

Instruments UK Ltd., U.K.), fitted with a standard-sized DIN

geometry (conical concentric cylinders with 15 mm inner stator

radius,14 mm outer rotor radius, 42 mm cylinder immersed height,

and 5920 mm gap). Prior to the measurements of viscoelastic

properties orflow behaviour, approximately 17 g of yoghurt sample

was allowed to rebody in the rheometer cup for 30 min at 5 C

while the inner concentric cylinder was immersed.

Low-amplitude oscillatory measurements were made as follows

to determine the viscoelastic properties: frequency sweeps(0.1e100 rad s1, in log progression with 10 points per decade)

were performed at constant strain of 0.5%, which was within the

linear viscoelastic region as determined in preliminary experi-

ments; after this strain sweeps (0.1e100%) were performed at

a fixed angular frequency (1 rad s1).

Flow behaviours was determined on a new set of samples of

yoghurt by shear-rate sweeps (0.1e100 s1, in log progression) at an

increasing shear rate (upwardflow), followed by a decreasing shear

rate (downward flow) at constant angular frequency (1 rad s1) and

strain (0.5%) for 10 min. The flow curves were fitted with a Her-

scheleBulkley model using a Rheology Advantage Data Analysis

software (TA Instruments UK Ltd., U.K.). The yield stress (s0),

consistency coefficient (k) and flow behaviour rate index (n) were

calculated using the Herschele

Bulkley model:

s s0 k _gn (1)

2.4. Sensory analysis

Descriptive sensory analysis was conducted to identify and

quantify the perceived attributes in stirred yoghurts. The sensory

profiles 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, and

ability to detect sensory differences. A sensory vocabulary of 32

attributes describing the appearance, aroma, flavour and texture of

stirred yoghurt was developed by panel consensus using reference

samples. Creaminess was evaluated by the assessors using their

own definition. The trained panel evaluated the samples in tripli-

cate over three sessions in separate booths in a sensory room. The

samples were rated for each attribute on a 10-mm line scale

(0none to 10 very high) anchored by appropriate reference

standards for each sensory attribute. The samples (w100 g) were

kept under refrigeration (w5 C) for an hour prior to serving, and

presented to the assessors in random and balanced order in white

plastic 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 significant treatment and interaction effects at a 5% level of

significance. The results were reported as mean values for each

parameter, and Tukeys test was performed for multiple compari-

sons of the treatments. Principal component analysis (PCA) was

also performed separately on rheological and sensory data, and PCA

plots were generated. Minitab 15 (Minitab Ltd., U.K.) software was

used for all statistical analyses.

3. Results and discussion

3.1. Effect of microfluidization of heat-treated milk on rheological

behaviour of reduced-fat yoghurts

3.1.1. Viscoelastic properties

All reduced-fat yoghurts in the study exhibited viscoelastic

behaviour, characterized by frequency and strain dependency,

irrespective of fat content and homogenization condition applied to

the heat-treated milk. Microfluidization at 150 MPa (MFz) and

conventional homogenization at 20/5 MPa (CH) had similar effects

on the viscoelastic properties of non- and low-fat stirred yoghurts.

The yoghurts produced from microfluidized 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 were

also comparable (p> 0.05, Table 1) from very low to high

frequencies (Fig. 1B). The strain-sweep profiles (Fig. 2) demon-

strated similar linear viscoelastic (LVE) ranges and G0eG00 cross-over

points (G0 G00), indicating the strain sensitivity and transition

point from elastic to viscous behaviour were not affected by

homogenization condition.

Despite the non-significance of the effect of homogenization

condition on the viscoelastic properties, MFz of heat-treated milk

resulted in non-fat yoghurt (0% MFz) with marginally lower G0 and

G00 values than those of yoghurt produced from milk homogenized

using the conventional method (0% CH). This is shown in both

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

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also slightly lower values of yield stress (sy), and stress (LVE-s) and

strain (LVE-g) at the limit of LVE compared with 0% CH ( Table 1),

further indicating a slightly weaker structure. These results were in

agreement with the observations on back-extrusion tests using

a 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 weaker

structure of 0% MFz compared to conventional yoghurt could be

attributed to the differences in microstructures, as discussed in our

previous report (Ciron et al., 2010). The more heterogeneous

microstructure of 0% MFz compared to 0% CH, consisting of large

protein aggregates with less interconnections between each other,

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

viscoelastic properties of low-fat yoghurts. Similar G0 and G00 values

in relation to frequency (Fig. 1A) and strain (Fig. 2B) was found for

low-fat yoghurts from microfluidized milk (1.5% MFz) and

conventionally homogenized milk (1.5% CH). This indicates that

homogenization condition had no definite effect on firmness 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"(Pa)

B

1

10

100

1000

0010111.0

Strain (%)

G'/G"(Pa)

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 microfl

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"(Pa)

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 with

conventionally homogenized (CH) or microfluidized (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 1

Rheological behaviour properties of reduced-fat stirred yoghurts as affected by fat

content (0.1%, 1.5% fat) and homogenization conditiona (CH, MFz).b

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

CH MFz CH MFz

Frequency sweepsc

G0 (Pa) 77.97 a 68.46 a 125.45 b 121.85 b

G00 (Pa) 19.38 a 16.80 a 31.04 b 28.32 b

d () 13.96 a 13.80 a 13.92 a 13.09 a

Strain sweepsd

LVE-s (Pa) 14.00 a 13.38 a 13.59 a 13.00 a

LVE-g (%) 0.6398 ab 0.5190 a 0.6373 ab 0.8022 b

sy (Pa) 6.14 a 4.94 a 10.17 b 11.52 b

gy (%) 42.30 a 53.66 a 45.06 a 37.15 a

Shear-rate sweepse

so (Pa) 1.244 a 3.778 b 4.728 b 13.932 d

k (Pasn) 5.597 a 10.673 b 13.308 b 25.795 c

n 0.3224 c 0.2428 b 0.2446 b 0.2061 a

h50 (Pa s) 0.3702 a 0.4724 b 0.5303 b 0.8405 c

HL area

(Pa s1)

1018 a 1800 b 2474 c 3888 d

a Homogenization condition: CH conventional valve homogenization (20/

5 MPa); MFzmicrofluidization (150 MPa).b

Mean values (n

2) that have different letters across each row signifi

cantlydiffer (p 0.05) using GLM-ANOVA and Tukeys test.c Frequency sweep parameters were reported at 1 rad s1.d Strain-sweep parameters: stress (LVE-s) and strain (LVE-g) at the limit of LVE,

and yield stress (sy;) and yield strain (gy) at cross-over ofG0 and G00.

e Shear-rate sweep parameters: h50 apparent viscosity at 50 s1; HL hyste-

resis loop area; and HerscheleBulkley model parameters, where so yield stress,

k consistency coefficient, and n rate index.

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This result is supported by earlier findings on the effect of MFz on

texture properties as ascertained by back-extrusion test (Ciron

et al., 2010) and corroborates with that of Cobos et al. (1995),

demonstrating similar effects of microfluidization and conven-

tional homogenization on viscoelastic properties of acid milk gels.

To fully understand the mechanism behind the findings, further

studies are required.

3.1.2. Flow behaviour

The flow behaviour was also determined since viscosity is an

important quality parameter that influences the sensory properties

of yoghurt. Rheometric viscosity has been reported to have a strong

positive correlation with thickness (Skriver, Holstborg, & Qvist,

1999). The experimental yoghurts were highly thixotropic, and

behaved as pseudoplastic materials (Delorenzi, Pricl, & Torriano,

1995) with a yield point and hysteresis loop (Fig. 3).

Homogenization condition clearly affected the flow behaviours

of non- and low-fat yoghurts. A noticeable increase in viscosity was

observed for non-fat yoghurt when microfluidized milk was used

for production, as illustrated by higher consistency coefficient (k),

lower flow rate index (n), and higher apparent viscosity at 50 s1

(h50) for 0% MFz than for 0% CH (Table 1 and Fig. 3). Moreover, 0%

MFz had significantly higher yield stress (s0) and hysteresis looparea (HL) than 0% CH (p< 0.05, Table 1).

More pronounced changes in flow behaviours were observed in

low-fat yoghurt, compared to non-fat yoghurt; the flow profile of

1.5% MFz was very different from that of 1.5% CH, showing higher

shear stress and greater apparent viscosity as the shear rate

increased (Fig. 3). Higher yield stress and a larger hysteresis loop in

1.5% MFz than in 1.5% CH were evident in the flow curves (Fig. 3),

1.5% MFz exhibited a very prominent yielding point as well. The

higher 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 required

for flow to commence and thus it is more resistant to shearing. This

indicates that MFz of low-fat milk produced a yoghurt with a more

consolidated 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-fat

yoghurt, 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 recover

its structure after shear-induced breakdown. The HerscheleBulkley

model fitted very well to theupwardflow curves (0.990 r 0.999)

because the shear-thinning flow behaviour of the low-fat yoghurts

had an inherent yield point. The flow model parameters of the

HerscheleBulkley function are presented in Table 1, and signifi-

cantly (p 0.05) higher valuesofs0, k and h50, and lower values ofn

were obtained for 1.5% MFz in comparison with 1.5% CH. This indi-

cates higher viscosity, higher yield stress and more shear-thinning

behaviour of low-fat yoghurt produced from milk homogenized by

MFz rather than that made using conventional method.

The positive effects of MFz on flow behaviour of low-fat

yoghurts in the present study are in contrast with our earlier

findings on the viscosity of low-fat yoghurt measured using back-

extrusion, wherein the two homogenization conditions resulted in

yoghurts with similar viscosity index and consistency (Ciron et al.,

2010). A possible explanation for the inconsistency would be

related to the differences in principles and mechanisms of the two

methodologies 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. The

viscosity index and consistency determined by the back-extrusion

test would be more related to gel firmness (G0) and sensory firm-

ness of the yoghurt, while the rheometric viscosity wouldbe a good

indicator of sensory viscosity.

The increase in viscosity of low-fat yoghurt through MFz of

heat-treated milk could be attributed to modification in micro-structure and particle size (and composition) of gel dispersions. A

recent confocal microscopy study on low-fat yoghurts demon-

strated that MFz created fat globules with a more active role in

structure formation; microfluidized fat globules were greatly

reduced in size, and incorporated and intimately bound to the

proteins in a more highly consolidated gel network, while

conventionally homogenized fat globules appeared to be more

loosely entrapped within the protein networks (Ciron et al., 2010).

This increased incorporation of smaller fat globules into the protein

gel networks could explain the enhancement in viscosity of low-fat

yoghurts by microfluidization.

3.2. Effect of microfluidization of heat-treated milk on sensory

properties of reduced-fat yoghurts

Descriptive sensory analysis was performed by a trained panel

to determine the sensory profiles of reduced-fat yoghurts based on

established descriptors. The list of descriptors consisted of four

appearance, four aroma, nine flavour and 15 mouthfeel attributes,

together with their corresponding definitions (Table 2). The mean

ratings for creaminess and 32 sensory attributes developed by the

trained panel of eight members arepresented in Table 3. Allsensory

properties were clearly affected by fat content and homogenization

condition. Interactions between fat content and homogenization

condition were significant (p 0.01) for surface water, smoothness,

cream aroma, natural yoghurt aroma, soft cheese aroma and

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

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

condition, irrespective of fat content.

A multivariate representation was plotted using PCA to have

a better understanding of the sensory profiles of the treatment

samples. PCA of the sensory data (Fig. 4) showed that the first two

PCs explained 85.7% of the total variation. PC1 (49.4%), which

segregated the yoghurts based on homogenization condition

(Fig. 4B), was positively correlated with natural yoghurt aroma andflavour, sourness, astringency, shininess, oral smoothness, sticki-

ness, cohesiveness, mouth-coating, mouth-drying and chalkiness,

and negatively correlated with bitterness (Fig. 4A). A sensory

differentiation 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

)

Shearstress(Pa)

Fig. 3. Flow behaviour profiles of non-fat (0%) and low-fat (1.5%) stirred yoghurts

made with conventionally homogenized (CH) or microfluidized (MFz) milk: 0% CH

(:

); 0% MFz (,

); 1.5% CH (C

); 1.5% MFz (

).

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velvetiness, firmness, meltdown rate, thickness, cohesiveness and

creaminess on its negative side (Fig. 4A).

A distinct segregation of the four yoghurt types in terms of their

sensory 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 by

high 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 and

mouth-drying, but with high soft cheese and buttermilk flavours,

natural yoghurt aroma andflavour, 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, but

the lowest intensities for shininess, chalkiness, mouth-coating,

mouth-drying, sourness, and natural aroma and flavour. 1.5% MFz

(onthefirst quadrant) had the highest values for smoothness (spoon

and oral), stickiness, cohesiveness, viscosity (spoon and oral),

thickness,firmness, velvetiness, featheriness, meltdownrate, cream

flavour and aroma, and creaminess. Hence, reduced-fat stirred

yoghurts with different sensory profiles can be produced by

manipulating 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, creamfl

avour,

Table 2

Sensory attributes for stirred yoghurts, as defined by the trained panel.

Attributes Abbreviation Definition

Appearance

Shi ni ness A-Shi ny Appear s br ight a nd glossy

Surface water Surface water Amount of water present on the

surface of the sample

Smoothness A-Smooth Looks smooth and free of irregularities

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

Aroma

Cream aroma Ar-Cream Aroma of fresh cream

Buttermilk aroma Ar-Buttermilk Aroma of buttermilk

Natural yoghurt

aroma

Ar-Natural

yoghurt

Aroma of natural yoghurt

Soft white

cheese aroma

Ar-Soft cheese Aroma of soft white cheese

Taste/flavour

Sweetness Sweet T aste of sucr ose, oth er sugars a nd

artificial sweeteners

Sourness Sour Taste associat ed with certain acids

such as citric acid

Saltiness Salty Taste of sodium chloride

B ittern ess B itter T aste associ ated wi th quin in e

and caffeine

Cream flavour F-Cream Aromatics/taste of fresh cream

Buttermilk flavour F-Buttermilk Aromatics/taste of buttermilk

Natural yoghurt

flavour

F-Natural

yoghurt

Aromatics/taste of natural yoghurt

Soft cheese flavour F-Soft cheese Aromatics/taste of soft white cheese

Astringency Astringent Dry, puckering feeling in the mouth

caused by tannins

Texture (mouthfeel)

Oral smoothness M-Smooth Perceived smoothness in the mouth

from smooth to rough

Oral Viscosity M-Viscous High resistance to flow in the mouth

Chalkiness M-Chalky A chalky, cloying powdery sensation

in the mouth

Grittiness M-Gritty Amount of sandy particles present in

the sample

Featheriness M-Feathery A light sensation created by a sample

that contains trapped air, reminiscentof whipped products

Fatti ness M -Fatty Per ceived a mount of fat/grease

in the sample

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

sample melting in the mouth

Firmness M-Firm Solid, compact sensation; holds its shape

Velvetiness M-Velvety A silky, velvety sensation that slides

on the surface of the tongue and the

roof and sides of the mouth

Curdiness M-Curdy Amount of lumps present in the sample

Stickiness M-Sticky Degree to which the sample sticks

or adheres to the teeth and palate

Thickness M-Thick Perceived thickness of the sample

in the mouth

Cohesiveness M-Cohesive Degree of holding together rather

than spreading across the tongue and

surfaces of the mouth

Mouth-dryness M-dry Perception of dryness in the mouth; a

mouth-drying sample is saliva absorbing

Mouth-coating M-coat Sensation of a coating layer left in the

mouth after swallowing the sample

C reaminess C reamy Overa ll i nten sity of the p er ceived

creaminess based on each assessors

own concept (could include appearance,

flavour and texture)

Table 3

Descriptive sensory ratings for reduced-fat stirred yoghurts.a

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

CH MFz CH MFz Fat HCc FatHC

Appearance

A-Shiny 5.4 a 7.3 b 5.5 a 7.6 c 0.004

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natural yoghurtflavour and non-oral smoothness, while reducing the

perception of sweetness, saltiness, bitterness and fattiness. It is also

noteworthy 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-fat

yoghurt (Janhoj et al., 2006), fat content increased oral smoothness,

which was further enhanced by MFz of heat-treated milk.

Theeffect of MFz onmostof thesensory properties depended onfat

content, especially for mouthfeel attributes (Table 3, Fig. 4). In non-fat

yoghurt, MFz caused a significant reduction in meltdown rate, feath-

eriness, firmness and velvetiness, when compared with the control

sample, but in parallel the perception of mouth-coating character

increased. MFz seemed to have the potential to increasethe intensities

of soft cheese flavour, buttermilk flavour, and natural yoghurt aromaand flavour of non-fat yoghurt. For low-fat yoghurt, MFz was favour-

able in terms of enhancing creaminess and some of the fat-associated

texture attributes, such as non-oral smoothness, viscosity (spoon and

oral) and thickness. MFz was also suitable for developing a more

mouth-coating mouthfeel and a shiny appearance in low-fat yoghurt,

although it reduced the positive effect of the presence of 1.5% fat on

cream aroma, featheriness,firmness and velvetiness compared to CH.

Furthermore, marked improvements in stickiness and cohesiveness

were achieved by using MFz compared with CH, while reducing the

degree of syneresis (surfacewater)and amount of lumps (curdiness)in

low-fat yoghurts. Hence, there is a synergistic effect of high-pressure

microfluidization and fat content on creaminess and associated

texture 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 texture

properties in reduced-fat yoghurts, including smoothness, viscosity,

featheriness,firmness, velvetiness, thickness and creaminess, while

reducing the amount of surface water. The texture-enhancing

capability of fat in yoghurt (Cobos et al., 1995; Keogh & OKennedy,

1998; Lucey, Munro, & Singh,1998; Patrignani et al., 2007) is related

to the ability of homogenized fat globules to participate in the gel

network formation (Aguilera & Kessler, 1988; Sodini, Remeuf,

Haddad, & Corrieu, 2004) and consequently strengthen the

yoghurt gel structure (Lucey et al., 1998).

Further improvements in creaminess, smoothness, viscosity and

thickness of low-fat yoghurt achieved by MFz of heat-treated milk

could be explained by increased interactions between fat globules

and 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 milk

proteins (casein and/or whey proteins) to adsorb on the new fat

globule membrane. Furthermore, the milk proteins became more

reactive due to thermal denaturation of whey proteins (Lucey et al.,

1998) and microfluidization-induced disruption of casein micelles

(Dalgleish, Tosh, & West, 1996; Sharma & Dalgleish, 1993). More-

over, fat globules that could actively interact with other particles

were created by microfluidization due to the modification of fatglobular membranes, which are constituted of semi-intact casein

micelles or micellar fragments (Dalgleish et al., 1996; Sharma &

Dalgleish, 1993). This allowed the casein-coated fat globules to

interact 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 confocal

microscopy (Ciron et al., 2010). Increased non-oral and oral

smoothness could also be related to the uniform distribution fat

globules in the network structure of low-fat yoghurt besides their

very small small size (w220 nm) and the lubricating nature of fat.

Although differing sensory profiles of reduced-fat yoghurts

could be attributed largely to the changes in size, microstructure

and interactions of proteins and fat globules, some of the texture

attributes could be partially related to flow behaviour. The increasein intensities of spoon and oral viscosity, and thickness of yoghurts

due to MFz of milk is in agreement with the results of instrumental

viscosity. There were also strong positive correlations for spoon

viscosity (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 at

50 s1 (h50). A good correlation between oral perception and

rheometric viscosity at similar shear rate was reported in an earlier

study ofSkriver et al. (1999). The increase in number of interacting

particles and fateprotein interactions is the likely reason for the

enhancement of the viscosity of low-fat yoghurt.

It should also be noted that the sensory attributes mainly related

to 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 flavour and aroma attributes were also important for

creaminess. Good correlations (0.76 r 0.95; p 0.05) of

creaminess with these sensory attributes were obtained for stirred

reduced-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 amount

of visible surface water present. These findings reinforce the

concept of creaminess as a multidimensional descriptor involving

appearance, flavour and texture attributes in food (Janhoj et al.,

2006; Johansen et al., 2008). Soft cheese aroma and buttermilk

aroma were negatively correlated with creaminess because these

attributes were associated with expelled whey (surface water), as

indicated by a strong correlation of cheese aroma (r 0.960;

p 0.001) and buttermilk aroma (r 0.979;p