LWT - Food Science and Technology 43 (2010) 708–714
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LWT - Food Science and Technology
journal homepage: www.elsevier.com/locate/lwt
Viscoelastic properties of orange fiber enriched yogurt as a function of fiber dose,
size and thermal treatment
E. Sendra a, *, V. Kuri b, J. Fernández-López a, E. Sayas-Barberá a, C. Navarro a, J.A. Pérez-Alvarez a
a
b
Dpto. Tecnologı́a Agroalimentaria, Escuela Politécnica Superior de Orihuela, Universidad Miguel Hernández, Ctra. Beniel km 3.2, 03312 Orihuela (Alicante), Spain
Food and Nutrition, Faculty of Science and Technology, School of Biomedical and Biological Sciences, University of Plymouth. Drake Circus, Devon PL4 8AA, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 December 2008
Received in revised form
1 October 2009
Accepted 11 December 2009
The effect of orange fiber addition on yogurt viscoelastic properties was studied, the following factors
were evaluated: (i) fiber doses (0, 0.2, 0.4, 0.6, 0.8 and 1 g/100 ml) (ii) fiber particle size (0.417–0.7 and
0.701–0.991 mm) (iii) fiber addition prior or after pasteurization. (i) In yogurts with pasteurized fiber G0 ,
G00 and complex viscosity increased with fiber dose, whereas in non-pasteurized fiber yogurts smaller
fiber particles (<0.4 g/100 ml) rheological parameters decreased due to the disruptive effect of the fiber,
and over 0.6 g/100 ml rheological parameters increased. The presence of particles alters yogurt structure
but when the fiber dose is high enough the water absorption compensates the weakening effect of the
fiber. (ii) G0 , G00 and viscosity were higher in yogurts with large particles than in yogurts with fiber of
smaller size. The higher the number of fiber particles, the higher the disrupting effect. (iii) Fiber
pasteurization in the mix enhances its integration in the gel matrix.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Rheology
Citrus fiber
Yogurt
Low fat dairy
1. Introduction
Fermented milk products already have a positive health image
due to the beneficial action of its viable bacteria. Dietary fibers have
beneficial effects for human health. The recommended daily intake
of fiber is about 38 g for men and 25 g for women (Trumbo,
Schlicker, Yates, & Poos, 2002). Dairy products, as yogurt, can
provide major opportunities for the development of fiber enriched
foods. Their acceptability by the consumers is mainly based on
satisfactory textural and sensory attributes.
Yogurt manufacturing process causes irreversible changes in the
properties of milk. Typically, milk is fortified with dairy ingredients
to produce a milk base which is then submitted to a drastic heat
treatment, which results in a high level of thermal denaturation of
the whey proteins and their partial fixation on the casein micelles. As
a consequence, aggregation is promoted, giving stronger gels and
decreasing the extent of acidification required to allow association.
Finally, the lactic acid production during the fermentation step
results in the destabilization of the micellar system and associated
gelation of the proteins. As the isoelectric point of denatured proteins
(pH 5.2), and casein (pH 4.6) are reached, low-energy bonds, mainly
hydrophobic, are progressively established between the proteins
(Lucey & Singh, 1998). Slow acid development favours formation of
* Corresponding author. Tel.: þ34 966749735; fax: þ34 966749677.
E-mail address: [email protected] (E. Sendra).
0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.lwt.2009.12.005
grains in yogurt, a coarse microstructure and low viscosity may be
due to short gelation time with a lower degree of casein aggregation
and a looser network (Sodini, Lucas, Tisier, & Corrieu, 2005). Rheometry is a useful technique for measuring shelf life and texture, as
many consumers evaluate the sensory texture to assess the freshness
and quality of a product. In a previous study at Universidad Miguel
Hernández (Garcı́a-Pérez et al., 2006) the rheology of fiber enriched
yogurts was assessed by means of viscosity, texture and syneresis
determinations. Set and stirred yogurts were manufactured: set
yogurt textural characteristics were evaluated by a penetration test
and stirred yogurts’ characteristics were evaluated by means of back
extrusion tests and viscosity. All the applied methods were
destructive and no information was obtained on the viscoelastic
behaviour of yogurt. Viscoelastic properties are useful in the food
industry for observing the onset of gelation, measuring the extent
and strength of internal structures, such as those present in yogurt.
Clearly, measurements in the linear viscoelastic region involve
probing the structure of the sample in a non-destructive manner. In
the mouth, as in the texture analyzer, this is not the case, and irreversible deformation takes place. However, it is hypothesized that
viscoelastic properties can give an indication of the initial experience
of a consumer (Kealy, 2006).
A controlled stress rheometer allows the shear stress on the
sample to be controlled; and it is possible to gradually increase the
shear stress on the material and measure the deformation. When
a small load is applied, the material stretches and a small deformation
resisted by the internal structure of the sample may be measured. The
E. Sendra et al. / LWT - Food Science and Technology 43 (2010) 708–714
internal structural strength is overcome when the yield stress is
exceeded. As the load is increased beyond the yield stress the material
structure ceases to be stretched, and deforms irreversibly (flows).
Commercial yogurts are typically either set or stirred. In set style
yogurts the milk is fermented in the retail containers, resulting in
a continuous gelled structure in the final product. A stirred product
is the result of fermentation commonly in large tanks, followed by
the disruption of the acid gel by stirring and sieving to give a more
fluid product which is often used as a base for inclusion of fruit
before packaging.
The rheological properties of stirred yogurt have been well
studied; their flow properties are characteristic of a non-Newtonian
and weakly viscoelastic fluid (Lubbers, Decourcelle, Vallet, & Guichard, 2004). The rheological characteristics of yogurt are governed
by milk composition, temperature and time of milk heat pretreatment, type and quantity of starter culture employed to inoculate the
milk, fermentation temperature and storage conditions of the final
product. Several authors have studied the correlation among yogurt
rheology and structure, evaluating the effect of milk heat treatment,
type of starter culture, incubation temperature, storage time, etc.
(Girard & Schaffer-Lequart, 2007; Ozer, Robinson, Grandison, & Bell,
1997; Remeuf, Mohammed, Sodin, & Tissier, 2003; Renan et al.,
2009; Sodini et al., 2005). Scaning electron microscopy (SEM) (Ozer
et al., 1997; Remeuf et al. 2003; Sodini et al., 2005) and Confocal
Laser Scaning Microscopy (CLSM) (Girard & Schaffer-Lequart, 2007;
Guggisberg, Cuthbert-Steven, Piccinali, Bütikofer, & Eberhard, 2009;
Renan et al., 2009) are the most widely used techniques. Results
from rheology studies usually show good correlation with the
observations of the microstructural analysis, although several times
uneven results may be obtained. As an example, Guggisberg et al.
(2009) studied the rheology and microstructure of inulin fortified
low fat yogurts and reported minimal microstructural differences as
observed by CLSM, although inulin addition provided high consistency as assessed by rheological and sensory evaluation. May be
inulin formed a non-CLSM visible network. Oscillatory tests are
widely accepted for the evaluation of rheological characteristics of
yogurt (Ozer et al., 1997; Remeuf et al., 2003; Sodini et al., 2005).
Keogh and O’Kennedy (1998) studied the role of milk fat,
protein, gelatin and hydrocolloids (starch, locust bean gum/xantan
mixture) on the rheology of stirred yogurt, reporting that the
consistency index and syneresis were more frequently influenced
by the composition than the behaviour index (n) and the critical
strain. Hess, Roberts, and Ziegler (1997) concluded that polymer
(starch, pectin and guar gum) association with the casein network
prevents disruption of a portion of the network. Thickeners are
generally incorporated in yogurts as a part of a fruit preparation,
and then a decrease in the viscosity of yogurt with fruit preparation
in comparison to control was observed (Kratz & Dengler, 1995; Celik
& Bakirci, 2003). Conflicting results have been reported by different
authors regarding the effect of milk supplements in yogurt
rheology, probably due to the different methodology and equipment used for the rheological analysis (Sodini et al., 2005).
The addition of novel fibers to milk products such as yogurt is
seldomly reported: oat, rice, soy and maize fibers (FernándezGarcı́a & McGregor, 1997), apple, wheat, bamboo and inulin (Dello
Staffolo, Bertola, Martino, & Bevilaqua, 2004), inulin in yogurt ice
cream (El-Nagar, Clowes, Tudorica, Kuri, & Brennan, 2002),
b-glucan in yogurt (Tudorica, Jones, Kuri, & Brennan, 2004).
Concerning citrus fiber in dairy products, Dervosiglu and Yazici
(2006) reported that citrus fiber as a single stabiliser could not
improve the viscosity, overrun and sensory properties of ice cream but
had a positive effect on the melting resistance. The addition of citrus
fiber had a negative effect on the viscosity values of ice cream mixes.
Dello Staffolo et al. (2004) observed that the type of fiber significantly affected the rheological properties of the yogurts. Apple fiber
709
fortification decreased yogurt compression values, probably due to
the formation of fiber aggregates that interfered with yogurt structure. Wheat and bamboo fiber fortification increased yogurt
compression force and texture sensory scores, Consumer’s preferred
firmer yogurts, probably, resulting from the insoluble nature of these
fibers. Tudorica, Jones, Kuri, and Brennan (2004) observed that,
when added to milk, b-glucan seems to promote shelf association of
caseins. Caseins appear to promote the association of b-glucans as
well, which at high concentrations could result in the formation of
a gel network, probably reinforcing the casein network. The inclusion of b-glucan in milk affected the viscoelastic properties of the
resulting coagulum, at higher levels of glucan, higher values for
G prima and G00 were obtained. Full fat milk curds have significantly
lower G0 than their low fat counterpart samples. Fat in solid state
increases the flexibility of the casein matrix due to the decrease in
tan d and so increases its gel like behaviour.
The addition of inulin at more than 1 g/100 ml increased whey
separation, inulin addition caused a decrease in organoleptic scores
(Guven, Yasar, Karaka, & Hayaloglu, 2005). Pectin hindered the
formation of the casein matrix in rennet-induced gels (Fagan,
O’Donell, Cullen, & Brennan, 2006).
Gelatine interacted with the network of milk proteins as
a connection between the clusters formed and so gelatine was
found suitable to improve the texture in milk products (Fiszman,
Lluch, & Salvador, 1999).
In our previous study on the rheology of orange fiber enriched
yogurt (Garcı́a-Pérez et al., 2006) measured by destructive methods,
it was observed that fiber concentration modified yogurt rheology
but not the fiber particle size. The addition of 1 g/100 ml orange
fiber reduced syneresis and improved the creaminess sensory
scores, together with: increased gel firmness, stickiness and average
force measured by back extrusion. Unexpectedly, lower fiber doses
increased syneresis and decreased textural parameters when
compared to control samples. The present study aimed to complement previous knowledge of the rheology of orange fiber enriched
yogurts. The impact of milk heating on yogurt structure when citrus
fiber is added prior to pasteurization has not been yet evaluated. The
following factors were included on the design: fiber dose, fiber
particle size and, fiber heat treatment.
2. Materials and methods
2.1. Materials
Fiber was obtained from orange fiber by-products by a procedure described by Fernández-López, Fernández-Ginés, AlesónCarbonell, Sendra, Sayas-Barberá, & Pérez-Alvarez (2004). Powders
with two different particle sizes (0.417–0.701 and 0.701–0.991 mm)
were obtained. The citrus fiber product used has a total dietary fiber
content of 53.65 g/100 g (determined by method 985.29, AOAC,
1995), 80.1 g/100 g crude fiber (determined by Weende method
962-09, AOAC, 1995), 4.19 g/100 g ash, 7.34 g/100 g moisture, 11.3
g/100 g water holding capacity (WHC), pH 3.92, 30 CFU/g aerobic
mesophilic bacteria and 25 CFU/ g of moulds and yeasts. For all the
tests the same batch of skim milk powder was used (34 g/100 g
protein, 52 g/100 g lactose, 1 g/100 g fat, 6.8 g/100 g ash, 5.2 g/100 g
moisture; Central Lechera Asturiana, CAPSA, Granda-Siero, Spain).
Skim milk powder was reconstituted with deionised water at 15
g/100 ml total solids. Commercial starter cultures of Streptococcus
thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (EzalÒ
MY900, Rhodia Food-Danisco A/S, Sassenage, France) were used at
the concentrations prescribed by the suppliers, but instead of direct
vat use, a poach for 20 L milk was pre-suspended in 20 mL of sterile
peptone water (1.5 g/ml) from which a corresponding aliquot was
added to the milk and gently shaked.
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E. Sendra et al. / LWT - Food Science and Technology 43 (2010) 708–714
2.2. Experimental design and statistical analysis
2.5. Overall composition and pH
A total of 21 experimental groups were obtained: 5 fiber
concentration (0.2, 0.4, 0.6, 0.8 and 1 g/100 ml), 2 fiber particle sizes
(0.417–0.701 and 0.701–0.991 mm), 2 fibre heating procedures
(pasteurized and non-pasteurized fiber) (5 2 2) plus a control
group without fiber. Overall composition and pH were determined
in duplicate on each group; rheological measurements were made in
triplicate on set and stirred yogurt of each group. Statistical analysis
was run on SPSS 14.0 (Chicago, IL, USA) for Windows. General Lineal
Model procedures and pairwise comparison (Tukey’s test) for means
was used. The whole experiment was run in triplicate.
Protein, fat, ash and water content were determined following
standard methods (AOAC, 1995) on 24 h old yogurt preparations.
The pH was determined with a pH meter GPL 21 Crison (Alella,
Spain).
2.3. Sample preparation
2.3.1. Procedure to prepare yogurt
For treatment samples that required fiber heat treatment,
orange fiber powder was dispensed into 50 mL PyrexÒ flasks
according to the fiber doses corresponding to each treatment. Milk
was poured into the flasks to obtain the final mix. The flasks were
closed and immersed into a water bath for heat treatment at 80 C
for 30 min, followed by immersion in ice-water baths to cool down
to 43 C, at this point the starter culture was added and gently
shaken. For those treatments not requiring fiber heating, the fiber
was added to the pasteurized milk at this point. The inoculated mix
was poured into the cylindrical containers and incubated at 43 C to
reach pH 4.7, and then cooled down to 4 C.
2.3.2. Preparation of cylindrical samples for rheometry
The cylindrical containers in which yogurts’ fermentation took
place were prepared by cutting the narrow end of 60 mL graduated
syringes (150 mm height, 25 mm diameter). After fermentation, gel
cylinders of 1.2 0.2 mm thickness were carefully sliced with
a sharp blade while pushing the plunger very slowly to avoid
breakages. All samples were handled similarly, and no evidence of
early breakages was apparent from rheological measurements.
Stirred yogurt samples were obtained similarly, but fermentation took place in sterile 100 mL plastic cups. After fermentation,
samples were stirred with a spoon 10 times clockwise and 10 anticlockwise. In order to create a creamy texture and remove hydrated
fiber particles, an aliquot of yogurt was sieved by gravity through
a screen (1.65 mm).
2.4. Rheological measurements
An oscillatory test was applied to determine the rheological
behaviour of the set yogurt slices and sieved stirred yogurt samples
prepared as described before, using a rheometer (Rheostress 600,
with Rheowin 3.21 Haake, Karlsruhe, Germany). Measurements
from all samples were within the range of linear viscosity. The
geometry used was plate and plate with serrated platens (35 mm
diameter, 1 mm gap) to prevent slippage, and it was set at 7 C.
Haque et al. (2001) suggested that when the yogurt network is very
weak, sedimentation of casein aggregates occur, and that the
syneresis triggered by this agglomeration leads to the formation of
a depleted layer at the upper surface of the sample. When oscillatory measures are to be taken in a horizontal geometry, this layer
causes slippage of the moving element, thus giving rise to an
apparent reduction in modulus. In the present work, serrated plates
were used to overcome this possibility.
Two types of test were run: (i) stress sweep from 0.015 to 1.5 Pa
at a constant frequency of 1 Hz and (ii) frequency sweep from 0.01
to 10 Hz at a constant stress of 0.4 Pa with three cycles of
measurements in both tests. The measurements were conducted in
triplicate.
3. Results and discussion
The mean fermentation time was 5 h and 30 min. The total solid
content of yogurts ranged from 14.35 to 14.85 g/100 g, the fat
content was less than 0.1 g/100 g, protein from 4.89 to 5.12 g/100 g
and ash from 0.68 to 0.81 g/100 g. The pH values ranged from 4.58
to 4.71. Increasing citrus fiber concentration from 0.2 to 1 g/100 g
did not significantly affect the pH, total solids, and fat, ash and
protein contents of yogurts.
Dello Staffolo et al. (2004) recommended the addition of fruit
fibers to yogurt at levels up to 1.3 g/100 ml, in the present study
doses up to 1 g/100 ml were tested following evidences from
previous studies, indicating that higher doses produced low sensory
scores. Previous studies on fiber addition to yogurts reported the use
of fiber of less than 85 mm, a distinctive feature of this study is that
biggest fiber sizes were used (dry particles were from 417 to 991 mm,
and the size increased due to water absorption). Smaller sizes were
reported to be inappropriate because they were linked to sandiness
perception (Garcı́a-Pérez, 2002). Orange fiber particles have an
uneven composition, they may include both albedo and flavedo.
Additionally, it should be noted that the fibre preparations are
diverse both in composition and structure. Upon microscopical
examination of the bigger particle sizes, it is evident that the
structure of the orange tissues is to some degree intact (results not
shown).
Considering that the structure of the protein network in yogurt
is not well established in the first days of ageing, no rheological
measurement was made right after manufacture (Lubbers et al.,
2004). In young gels (shortly after formation) the elastic or storage
modulus (G0 ) is low and the pH is still at high levels (>5.0), where
the electrostatic attraction between casein particles is not yet as
high as it would be in aged gels (Lucey, 2001). However, the fracture
properties of young gels may be relevant as an indicator of possible
rearrangements.
In the potential interactions between proteins and polysaccharides in aqueous solutions three equilibrium situations could
be possible: miscibility, thermodynamic incompatibility (protein
polysaccharide repulsion) and complex coacervation (attraction of
protein and polysaccharide) (Tudorica et al., 2004). Incompatibility
consequently leads to the separation of the mixture into liquid
phases, with a corresponding increase of their relative concentration in the different phases.
3.1. Rheology of set style yogurt
The data obtained through oscillatory measurements are the
contributions to the internal structure of the sample from
the elastic and viscous portions of flow, G0 and G00 (Pa), respectively,
the complex viscosity h* (Pa s), the tan (d) which is equal to G00 /G0
and the deformation (g). G0 is the energy stored per deformation
cycle during an oscillatory test. It is related to the stiffness of the
network (Lucey, 2001). It was hypothesized that the elastic
contribution to flow, G0 , would relate to the elastic nature,
measured by the texture analyzer, and the complex viscosity to the
cohesiveness (estimation of the amount of deformation before
rupture) measured by the panel and the texture analyzer (Kealy,
2006). G00 is the viscous contribution to flow and tan (d) reflects the
viscoelastic behaviour.
E. Sendra et al. / LWT - Food Science and Technology 43 (2010) 708–714
3.1.1. Results of the stress sweep test
The studied yogurts remained in the linear viscoelastic region
over a short range of applied stress (0.1–0.4 Pa). G0 , G00 , h*, tan (d)
and g at 0.363 Pa were selected to run statistical analysis of the data
(Table 1). The statistical analysis output in the table includes
univariate comparisons (type of yogurt). Steffe (1996) reported
values of 129 Pa, 153 Pa, 20 Pa s and 1.19 for G0 , G00 , h* and tan (d),
respectively, in concentrated solution whereas values of 5187 Pa,
363 Pa, 520 Pa s and 0.0699 were typical for gels. Data from the set
yogurts of the present study would suggest a more concentrated
solution behaviour.
All the studied factors significantly affected rheological properties of yogurt. However, the best indicator of viscoelasticity, tan (d)
was not significantly affected. The values of G0 , G00 and h* increased
with fiber dose in pasteurized fiber yogurts; whereas in nonpasteurized fiber only slight changes, not significant for all groups
were observed. Actually low doses of 0.2 and 0.4 g/100 ml resulted
in lower values for these parameters, coinciding with the observations of Garcı́a-Pérez et al. (2006). Possibly, at fiber dose lower than
0.4 g/100 ml, rheological parameters decreased due to a disruptive
effect of the fiber, and when fiber dose was higher than 0.6 g/100 ml
rheological parameters increased. It is suggested that although the
presence of fiber particles always alters yogurt structure, when
the fiber dose is high enough the water absorption compensates the
weakening effect of the fiber and strengthens gel structure. Yogurt
deformation (g) decreased with increased fiber levels.
Regarding particle fiber size, G0 , G00 and h* increased with fiber
size and deformation was lower for the largest fiber size. For a given
dose, the total number of fiber particles is higher when the small
fiber size is used, and consequently the disrupting effect is higher.
Regarding fiber pasteurization, G0 , G00 and h* were higher in
pasteurized fiber yogurts and deformation tended to decrease
although no significant differences were observed in all cases. It
seems that fiber pasteurization would solubilise some components
of the orange fiber powder which lead to an enhanced yogurt
structure whereas if it is not pasteurized fiber is mainly insoluble
and only reinforces yogurt structure in the measure that it absorbs
711
water. It is suggested that orange fiber will present thermodynamic
incompatibility with milk proteins and only when the fiber is
pasteurized with the milk some solubilization and miscibility of
compounds will occur.
3.1.2. Results of the frequency sweep test
The yogurts showed a predominantly elastic behaviour (G0 > G00 )
over the whole range of frequencies tested (Fig. 1), which corresponds closely to that of a true gel. When analyzing the results from
frequency sweeps, moduli (G0 and G00 ) are a strong function of
frequency in dilute and concentrated solutions, but practically
constant in gels (Steffe, 1996). In the present study moduli
increased with increased frequency. The tangent of the phase shift
of phase angle is also a function of frequency: tan (d) ¼ G00 /G0 . Steffe
(1996) suggested the following numerical ranges for tan (d) of
polymer systems: very high for dilute solutions, 0.2–0.3 for amorphous polymers, low (near 0.01) for glassy crystalline polymers and
gels. The obtained results (tan (d) from 0.250 to 0.330) point to
a concentrated amorphous polymer rather than a gel.
The delta value for a gel is practically constant, indicating
consistent solid like behaviour over the entire frequency range
(Steffe, 1996). In the present study the set yogurt behaves more
than a concentrated solution. Materials usually exhibit more solid
like characteristics at higher frequencies (Steffe, 1996) as in the
present study where a decrease in tan (d) with increased frequency
has been observed. The linear decrease of the complex viscosity
corresponds to a typical shear thinning profile. The obtained set
yogurt seems to behave as a very weak gel, much closer to
a concentrated solution than to a true gel.
3.2. Rheology of stirred yogurt
3.2.1. Results of the stress sweep tests
No significant differences within stirred yogurt samples were
observed for the studied rheological parameters (data not shown).
Values for the following parameters were between: G0 from 64.99
to 115.97 Pa; G00 from 14.00 to 29.82 Pa; h* from 8.43 to 18.53 Pa$s;
Table 1
Rheological parameters of orange fiber enriched set yogurts under oscillatory testing (stress sweep values obtained at 0.363 Pa, 1 Hz, 7 C): G0 , elastic moduli; G00 , viscous
moduli; h*, complex viscosity; tan (d) and g deformation.
G0 (Pa)
Mean
G00 (Pa)
Std
Mean
h* (Pa$s)
Std
tan d (–)
g (–)
Mean
Std
Mean
Std
Mean
Std
9.72a
0.81
0.327
0.012
0.0071a
0.0017
Control
58.03a
Pasteurized fiber
0.2-M
79.51b
0.4-M
121.45c
0.6-M
210.23d
0.8-M
228.40d
1-M
257.70d
0.2-G
68.84b
0.4-G
285.94d
0.6-G
264.46d
0.8-G
212.66d
1-G
333.78e
4.91
18.91a
1.26
8.91
20.79
18.31
17.22
24.60
5.26
13.29
27.06
17.04
3.46
26.28b
39.17c
68.92d
76.35e
84.69e
22.74b
89.25e
79.12e
64.68d
101.04f
3.05
7.25
12.28
8.22
8.44
1.45
4.91
14.86
5.44
2.36
13.31b
20.30c,d
35.17e
38.30e
43.13e
11.56b
47.77e
43.99e
35.41e
55.52f
1.48
3.48
6.35
2.99
2.30
0.87
2.26
9.40
2.83
0.57
0.332
0.323
0.329
0.335
0.329
0.331
0.312
0.303
0.304
0.303
0.001
0.007
0.014
0.010
0.009
0.008
0.005
0.018
0.002
0.004
0.0052b
0.0034c
0.0020d
0.0018d
0.0018d
0.0059b
0.0014e
0.0017e
0.0019d
0.0012e
0.0002
0.0002
0.0001
0.0001
0.0005
0.0013
0.0002
0.0010
0.0005
0.0000
Non-pasteurized
0.2-M
0.4-M
0.6-M
0.8-M
1-M
0.2-G
0.4-G
0.6-G
0.8-G
1-G
4.77
9.39
15.14
10.56
13.93
16.37
5.17
14.15
19.02
17.57
16.36a
15.02a
21.99a
25.68a,b
36.29c
25.34b
20.76a
26.74a
36.91c
22.01a
4.14
3.03
4.85
3.30
2.25
7.00
7.76
9.39
3.58
4.02
8.45a
7.75a
11.33b
13.19b
19.41c,d
20.80c,d
16.52c
21.19d
30.48e
17.15c
2.42
1.58
2.53
1.77
2.19
2.77
1.98
1.82
3.51
7.87
0.327
0.325
0.326
0.328
0.314
0.315
0.325
0.330
0.313
0.333
0.013
0.000
0.002
0.002
0.018
0.023
0.013
0.013
0.019
0.011
0.0085a
0.0090a
0.0063b
0.0053b
0.0036c
0.0014c
0.0017e
0.0013e
0.0010e
0.0016d
0.0021
0.0016
0.0013
0.0006
0.0004
0.0003
0.0003
0.0002
0.0002
0.0001
fiber
50.55a
46.27a
67.68a.b
78.50b
116.21c
81.56b
64.43a.b
82.56b
119.55c
66.79a
Fiber addition levels: 0.2. 0.4. 0.6. 0.8. 1 g/100 ml; fiber size M (0.417–0.701 mm); G (0.701–0.991 mm).
Values with different superscript letters within the same column significantly differ (*P > 0.05).
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E. Sendra et al. / LWT - Food Science and Technology 43 (2010) 708–714
Non Pasteurized
G, G” Pa
Pasteurized
ƒ* Pas
G, G” Pa
10000
1000
ƒ* Pas
1000
1000
100
100
10
10
1000
Medium
particle
size
100
100
10
10
0.01
0.10
1.00
1
10.00
1
0.01
f [Hz]
a
0.10
b
G, G” Pa
ƒ* Pas
1000
1.00
G, G” Pa
1000.0
1
10.00
f [Hz]
ƒ* Pas
1000
10000
100.0
1000
100
Fine
particle
size
10.0
100
100
10
1.0
1
0.01
0.10
1.00
0.1
10.00
10
10
0.01
f [Hz]
c
d
0.10
1.00
1
10.00
f [Hz]
Fig. 1. G0 , G0 0 and complex viscosity obtained from frequency sweep (0.01 to 10 Hz) at a stress of 0.4 Pa and 7 C: set yogurt with (a, c) non pasteurized and (b, d) pasteurized fiber of
different particle sizes; (a, b) medium (0.701 to 0.991 mm) and (c, d) fine (0.417 to 0.701 mm) at levels from 0 to 1 g/100 ml. Green G0 (Pa); Blue: G00 (Pa); Fuchsia: complex viscosity
(Pa’s).- Control; -x- 0.2 g/100 ml fiber; -6- 0.4 g/100 ml fiber; -,- 0.6 g/100 ml; -A- 0.8 g/100 ml; -B- 1 g/100 ml fiber.
tan (d) from 0.290 to 0.335; and deformation (g) from 0.002 to
0.003. A single significant difference was observed, the tan (d) of
yogurts with 1 g/100 ml pasteurized fiber (0.401–0.711 mm) was
significantly higher than that of all the other groups, meaning
a decreased solid behaviour of this type of yogurts. A decrease in
tan d in a viscoelastic material is generally associated with more
pronounced solid like behaviour (Tudorica et al., 2004).
Once lost the original structure of the yogurts, the rheological
properties will depend mainly on the total solids, particularly the
amount and type of protein (Oliveira et al., 2001). Although no
significant differences on composition were detected between
samples, the fact that fiber has an important water holding
capacity could have modified the rheological behaviour of the
yogurt which has been previously described in the case of inulin
addition to yogurt (El-Nagar et al., 2002). A non-significant
tendency to increase G0 , G00 , h* and tan (d) and to decrease
deformation due to fiber presence was observed, but it was not
always dose dependent.
Rheology assessment in particulate foods such as fiber enriched
yogurts is difficult. The selected procedure did not yield the
expected results and so it does not seem suitable for the study of
the rheology of stirred yogurts.
3.2.2. Results of the frequency sweep tests
G0 , G00 increased with frequency increases whereas complex
viscosity dramatically decreased showing a clear shear thinning
profile, tan (d) remained almost constant during the frequency
sweep (Fig. 2). Stirring the yogurts hindered differences among
different samples. When the fiber had been pasteurized differences
between samples with different fiber concentrations almost
disappeared. The fiber with small particles resulted in a weaker gel
structure (lower G0 , G00 and h* decreased).
Although some types of non-pasteurized fiber yogurts with
doses lower than 0.4 showed weaker network structure than the
control, as described by Garcı́a-Pérez et al. (2006), it has to be
pointed out that in the cited work only pasteurized fiber was tested.
E. Sendra et al. / LWT - Food Science and Technology 43 (2010) 708–714
G, G” Pa
Non Pasteurized
ƒ* Pas
1000
G, G” Pa
10000
713
Pasteurized
ƒ* Pas
1000
1000
1000
100
100
Medium
particle
size
100
100
10
10
10
1
0.01
0.10
1.00
1
10.00
10
0.01
f [Hz]
a
0.10
G, G” Pa
ƒ* Pas
G, G” Pa
1000
ƒ* Pas
1000
1000
100
Fine
particle
size
100
100
100
10
10
10
0.01
c
1
10.00
f [Hz]
b
1000
1.00
0.10
1.00
1
10.00
f [Hz]
10
0.01
d
0.10
1.00
1
10.00
f [Hz]
Fig. 2. G0 , G0 0 and complex viscosity obtained from frequency sweep (0.01 to 10 Hz) at a stress of 0.4 Pa and 7 C. Stirred yogurt with (a, c) non pasteurized and (b, d) pasteurized
fiber of different particle sizes; (a, b) medium (0.701 to 0.991 mm) and (c, d) fine (0.417 to 0.701 mm) at levels from 0 to 1 g/100 ml. Green G0 (Pa); Blue: G00 (Pa); Fuchsia: complex
viscosity (Pa’s).- Control; -x- 0.2 g/100 ml fiber; -6- 0.4 g/100 ml fiber; -,- 0.6 g/100 ml; -A- 0.8 g/100 ml; -B- 1 g/100 ml fiber.
The fact that this effect has not been observed in pasteurized fiber
yogurts in the present study may be due to the nature of the
measurements: Garcı́a-Pérez et al. (2006) only used destructive
tests with large deformations instead of the non-destructive
methods with small deformations of the present study (Table 1).
Related to sensory properties, Kealy (2006) observed a good
correlation between the sensory cohesiveness and complex
viscosity measurements. As h* is a function of both viscous and
elastic contributions to flow, they would be expected to correlate
with the human sensory experience. Hardness (the force required
to evenly deform the sample) assessed by a taste panel and
a texture analyzer showed a strong correlation. Probably due to
the difference in techniques (destructive vs. non-destructive),
texture analyzer and rheometer ranked samples in opposite order
in their study. This neatly illustrates the differences between the
extensional properties (which dominate in large deformations in
destructive assays) and elastic modulus (small deformation, nondestructive measurement). The combination of these techniques
could provide practitioners with an effective tool for predicting
the properties of a product.
4. Conclusions
Citrus fiber from orange by-products is a novel ingredient that
can be successfully used in yogurt production. Yogurts behave as
shear thinning fluids and very weak gels. Orange fiber addition
modifies yogurt rheology; when the fiber is with the pasteurized
mix G0 , G00 and complex viscosity increase with fiber dose, whereas
in non-pasteurized fiber yogurts, rheological parameters remain
low at low fiber doses due to the disruptive effect of the fiber, while
at higher fiber dose over 0.6 g/100 ml rheological parameters
increased. Although the presence of fiber particles always alters
yogurt structure, when the fiber dose is high enough the water
absorption may compensate the weakening effect of the fiber and
strengthens the gel structure. G0 , G00 and viscosity are higher in
yogurts with large fiber particle size than in yogurts with smaller
fiber particles. For a given dose, the total number of fiber particles is
higher when fiber size is small, and consequently the disrupting
effect is higher. Fiber pasteurization in the mix enhances its integration in the gel matrix decreasing the differences in rheological
behaviour between yogurts with different fiber levels. The
714
E. Sendra et al. / LWT - Food Science and Technology 43 (2010) 708–714
experiments on the set gel provided more detailed information
than those on the stirred gel.
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