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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.ie7/30/2019 1-s2.0-S0268005X11000555-main
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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