al 14

tios ob, M

CIDC

ta 19

La P

2; ac

ent d

at, b

ays.

iffere

compared to control. An untrained sensory panel analyzed consumer acceptability. Even though bers modied certain rheological

Yogurt is one of the dairy products, which should

space. Yogurt is an important nutritional source(Tamine & Robinson, 1985).

vegetables or cereals, include polysaccharides (cellulose,

texture characteristics of yogurts due to the additionof gelatine (Fiszman, Lluch, & Salvador, 1999), pectin

ARTICLE IN PRESSFibers also have benecial effects for human health,with a recommended daily intake of 0.025

(Ramaswamy & Basak, 1992), k-carragenean (Xu,Stanley, Goff, Davison, & Le Marguer, 1992) oat, rice,soy and maize bers (Fern!andez-Garc!a & McGregor,1997). Inulin, in addition to its effects on promotinghealthy bacteria growth in the colon, also enhances

*Corresponding author. Fax: +54-221-4254853.

E-mail address: esparzamarina@hotmail.com (M.D. Staffolo).0958-6946/$ - see

doi:10.1016/j.idacontinue to increase in sales due to diversication in therange of yogurt-like products, including reduced fatcontent yogurts, probiotic yogurts, yogurt shakes,drinkable yogurts, yogurt mousse, yogurt ice-cream,etc. (Fiszman & Salvador, 1999). The key to marketgrowth is a continuos evaluation and modication of theproduct to match consumer expectations. For a longtime, yogurt by itself has been recognized as a healthyfood, due to the benecial action of its viable bacteriathat compete with pathogenic bacteria for nutrients and

hemicelluloses and pectins) and lignins. Both solubleand insoluble bers can be found; however, higheramounts of insoluble bers are used for food fortifyingpurposes (Vicario Romero & Troncoso Gonz!alez, 1997).Fiber may interact with other components of the foodduring processing. These interactions may lead tochanges in bioavailability of nutrients, texture or avorof the product (Fern!andez-Garc!a & McGregor, 1997).Since consumer concerns are related to both nutri-

tional and sensory aspects, several authors studiedKeywords: Yogurt; Dietary ber; Sensory characteristics

1. Introduction

A nutraceutical food may provide expanded utilitybeyond its nutritional benet. These benets can be bothphysical and mental and are commonly attributed to theactive components of the food.

0.030 kg day1 adult1 (Labell, 1990). Certain patholo-gies such as coronary decease, hypertension, diabetes,hypercholesterolemia and gastrointestinal disorders maydisappear, decrease or be prevented with ber consump-tion (Saura Calixto & Larrauri, 1996).Fibers, mainly provided by the cell wall of fruits,characteristics of the plain yogurt, the panelists awarded the supplemented yogurts scores indicating acceptability.

r 2003 Elsevier Ltd. All rights reserved.International Dairy Journ

Inuence of dietary ber addipropertie

M. Dello Staffoloa,*, N. Bertolaa,

aCentro de Investigaci !on y Desarrollo en Criotecnolog!a de Alimentos (

47y 116 La PlabFac. Ingenier!a, Universidad Nacional de

Received 22 August 200

Abstract

The objective of this work was to study the effects of differ

fortied with these bers. Commercial bers from apple, whe

performed by dynamic, shear and compressionextrusion ass

instrumental analysis. Syneresis and pH did not show any dfront matter r 2003 Elsevier Ltd. All rights reserved.

iryj.2003.08.004(2004) 263268

n on sensory and rheologicalf yogurt

. Martinoa,b, y A. Bevilacquaa,b

A), CONICET, Fac. Cs Exactas, Universidad Nacional de La Plata,

00, Argentina

lata, 47 y 116 La Plata 1900, Argentina

cepted 12 August 2003

ietary bers on sensory and rheological properties of yogurts

amboo or inulin were used. Rheological characterization was

Storage time and type of ber were signicant factors for

nce, while only apple ber yogurt showed color differences

2.2. Transient shear stress measurements

ARTICLE IN PRESSal DaRheological behavior was analyzed with a rotationalviscometer Haake Rotovisco RV2 (Karlsruhe, Ger-many) with a thermostatic system. All data wereobtained with a sensor MV IP (medium viscosity rotor)of concentric cylinders with proled surfaces. Thissensor system is appropriate for substances, which tendto slip on cup and rotor surfaces. Rheological propertieswere measured at 20C. Transient shear stress (s vs.calcium absorption and immune functions. Theseattributes magnify the healthy appeal of high-calcium-containing yogurt products.The objective of this study was to evaluate properties

of a yogurt fortied with the addition of dietary berswith an attempt to maintain, at the same time, sensorycharacteristics similar to those of commercial yogurtswithout bers.

2. Materials and methods

2.1. Preparation of yogurts

Three batches of yogurt were prepared in a 250mLglass container using reconstituted whole milk powder(15% w/w), 5% sucrose and 1.3% of each dietary berper glass containers. Each batch of yogurt was analyzedseparately. The amount of ber was selected followingUS regulations for ber-fortied products (Fern!andez-Garc!a & McGregor, 1997). Incorporated bers wereinulin (Frutat-inulin, Imperial Sensus, The Nether-lands), bamboo (Qualicel, CFF, Germany), wheat(Wheatcel, CFF, Germany) and apple (Vitacel, JRS,Germany). Fiber particle sizes as provided by thesuppliers were: 80% of particle o85 mm for inulin,80% of particle o32 mm for bamboo, 80% of particleo32 mm for wheat and 90% of particle o30 mm forapple. While inulin is a soluble ber, the other ones arepartially insoluble. Control (without ber) and ber-fortied mixes were homogenized and heated to 85Cfor 30min, cooled to ambient temperature and inocu-lated with 0.03% starter culture. The starter was a 1:1mixture of Streptococcus thermophilus (Cp2, CIDCAcollection 321) and Lactobacillus delbrueckii subsp.bulgaricus (Lbp, CIDCA collection 332) (Moreira,Abraham, & De Antoni, 2000). Samples were incubatedat 43C in a water bath to pH 4.44.6 (Fiszman et al.,1999; Teggatz & Morris, 1990); gently mixed for 20 s toresuspend the bers, and stored at 4C after completionof the fermentation process (van Marle, van den Ende,de Kruif, & Mellema, 1999). Rheological behavior,color, pH, syneresis and water activity were tested at 1,7, 14 and 21 days of storage.

M.D. Staffolo et al. / Internation264time) curves were obtained at constant shear rates Dranged from 32 to 512 s1. The equilibrium value ofshear stress sN was measured at 180 s at each shearrate to estimate the parameters of the power law model(Navarro, Martino, & Zaritzky, 1997). Apparentviscosities were evaluated at D=32 s1. Each run wasperformed in triplicate with different samples.

2.3. Dynamic oscillatory measurements

Viscoelastic properties of yogurts were also monitoredby dynamic oscillatory assays in a Haake RV20rheometer (Karlsruhe, Germany) with a plateplatesensor (27.84mm diameter and 1mm gap) at 20C.Strain sweeps (050%) were performed to determine thelinear viscoelastic range at 6.28 rad s1. Maximum strainamplitude within this range was 6% for all samples.Frequency sweeps from 0.63 to 13.57 rad s1 at aconstant strain of 5% were carried out to obtain G0;G00 and tan d: The whole experiment was repeated threetimes and three samples at each storage time weremeasured four times for both control and fortiedyogurts.

2.4. Compressionextrusion tests

Compressionextrusion assays were performed in anInstron Universal Testing Machine 1011 (Instron Corp.,Canton, MA, USA) equipped with a 50N-load cell andoperating at 1 cmmin1 head speed. The extrusion celland the compression plunger were 5 and 3.5 cm indiameter, respectively. Compression forcetime curveallowed evaluating maximum force, determined as theforce at which the slope changed. The curve plateaurepresenting the force necessary to continue with theextrusion process was also analyzed (Bourne, 1982). Allmeasurements were performed in triplicate.

2.5. Physico-chemical determinations

Color of yogurts was determined in a tristimuluscolorimeter (Minolta, CR 300, Osaka, Japan); the L, aand b parameters of the Hunter scale were analyzed.Sample pH was measured with a pHmeter Hach, modelEC-30 (Loveland, Colorado, USA) and water activitywas also determined (Aqualab Serie 3TE, Washington,USA). To determine syneresis, 100mL yogurt samplewere placed in a graduated cylinder and stored at 4Cfor 21 days. At different storage times, the volume ofwhey was measured. The syneresis index (%) ismeasured as mL of whey per 100mL of initial sample.All measurements were performed in triplicate.

2.6. Sensory evaluation

A triangle test to evaluate sample differences was

iry Journal 14 (2004) 263268performed. Three coded samples were presented to an

with the non-linear function of the software utilized.

law tting to experimental data of control and wheatyogurt equilibrium shear stress with storage time.Apparent viscosities Zapp calculated at 32 s

1 atinitial time and at 21 days of storage are shown in Table1. ANOVA indicated that type of ber and storage timewere signicant factors for Zapp: Fisher test for mean

ARTICLE IN PRESS

Inulin 17.77a 0.15a 0.94a 0.84c

Apple 30.26b 0.10b 1.31b 1.04d

Wheat 14.80a 0.19a 0.94a 0.77c

None 19.54a 0.14a 1.01a 0.81c

Means with different superscript letters differ signicantly (Po0:05;vertical comparison).

eApparent viscosity at 32 s1 just prepared.fApparent viscosity at 32 s1 at 21 days of storage.

20

25

30

35

40

45

50

55

0 200 400 600

Shear rate (s-1)

0 200 400 600

Shear rate (s-1)

Equi

libriu

m s

hear

stre

ss (P

a)

20

25

30

35

40

45

50

55

Equi

libriu

m s

hear

stre

ss (P

a)

(a)

(b) Fig. 1. Rheological behavior of yogurts at 20C. Equilibrium shear

stress vs. shear rate of yogurts: (a) yogurt with wheat ber; (b) control

yogurt.~, ( ) 1 day;, (- - - -) 7 day; m, () 14 day, , (- - -)21 days storage. Lines correspond to power law model tting. Bars

indicate mean standard deviations of corresponding curves.

al Da3. Results and discussion

3.1. Rheological analysis

Transient shear runs showed a peak corresponding toa typical viscoelastic system. For these type of systems,the power law model (Eq. (1)) can be applied at longshear times using the equilibrium shear stress sNaccording to Navarro et al. (1997) and van Marle et al.(1999)

sN mDn; 1

where sN is the equilibrium shear stress at each shearrate D; m is the consistency index and n is the power lawfactor.The model satisfactory tted the experimental data

for each type of yogurt, obtaining a minimum correla-tion coefcient R2 of 0.987. Table 1 shows m and n forcontrol and fortied yogurts at initial storage time;values of n indicated a pseudoplastic behavior. At initialstorage time, m and n of yogurt fortied with apple berwas signicantly different Po0:05 from those of theother yogurts. Storage time was a signicant factorPo0:05 for both consistency index and the power lawfactor. Consistency indexes decreased with storage time;values are within literature data (Ramaswamy & Basak,untrained panel of 25 members. Each member wasasked to indicate the odd sample. The statistical resultwas obtained from a table with the number of correctidentications corresponding to the number of judge-ments at different signicance levels (ASTM, 1968;Urena, DArrigo, & Giron, 1999). Three independentassays were performed: (a) control yogurt vs. yogurtwith inulin, (b) control yogurt vs. yogurt with wheatber and (c) control yogurt vs. yogurt with bamboober. Another sensory analysis approach was used todetermine consumer acceptability and color; texture,avor and aroma were analyzed. The hedonic scalevaried from 5 (like very much) to 1 (dislike). Yogurtswith apple ber were not included in sensory testsbecause of the obvious color difference. A lightbrownish color characterized apple ber-fortied yo-gurts. In both sensory analysis sessions, 25 untrainedmembers tested the samples.

2.7. Statistical analysis

For statistical analysis, ANOVA was performed onthe three batches and the corresponding replicates, usinga statistical software (Systat Inc., Evenston, IL, USA,5.0, copyright, 19901992). Fisher test was used formean comparison. Model parameters were calculated

M.D. Staffolo et al. / Internation1991; Keogh & OKennedy, 1998). Fig. 1 shows powerTable 1

Consistency index m; power law factor n and apparent viscosityZapp for control yogurt and yogurts fortied with four ber sources

Fiber source m n Zappe Zapp

f

(Pa s1) (Pa s1) (Pa s1)

Bamboo 15.20a 0.18a 0.94a 0.70c

iry Journal 14 (2004) 263268 265comparison indicated that the Zapp of yogurt fortied

where a; b; c and d are parameters that characterize therheological behavior.The model satisfactorily tted experimental data,

obtaining a minimum R2 of 0.937. Our experimentalvalues of b parameter, which varied between 0.110 and0.390, were between the values of concentrate solutions(0.840) and gels (0.037) as presented by Steffe (1996).Table 2 shows a and c calculated at each storage time forall the treatments, control and ber-fortied yogurts.Values of a were between typical gels (5626 Pa sb, Steffe,1996) and concentrated solutions (16.260 Pa sb, Steffe,1996), although the present data shown in Table 2 arecloser to concentrated solution values. Values of cparameter were similar to those of concentrated solu-tions (Steffe, 1996). The d values varied insignicantlybetween 0.037 and 0.276, with most values being

ARTICLE IN PRESSal Dairy Journal 14 (2004) 263268w (rad. s-1)0.1 1 10 100

G' (P

a), G

'' (Pa

)

0.1

1

10

100

Tan

delta

0.1

1

10

100

(a)

M.D. Staffolo et al. / Internation266with apple ber was signicantly different Po0:05compared to the other yogurts.Fig. 2 shows typical frequency sweeps for yogurt with

wheat ber and control yogurt at initial time and at 21days of storage at 4C. In general, curves of G0; G00 andtan d varied with frequency w at initial time showingconcentrate solution characteristics. At longer storagetimes, yogurts with wheat, bamboo and inulin exhibitedcharacteristics closer to a gel, where G0; G00 and tan dshowed a slight variation with w: The following modelwas applied to the dynamic curves of all yogurt samples(Steffe, 1996):

G0 awb; 2

G00 cwd ; 3

around 0.1. Keogh et al. (1998) working with stirredyogurt supplemented with milk fat, proteins andhydrocolloids determined power law dynamic viscosity.Our c values correspond to their calculated dynamicconsistency index of the dynamic viscosity model anddata were consistent.Time and type of ber were signicant Po0:05

variables for the ANOVA analysis; parameters a and cincreased with time (Table 2). In agreement with Keoghet al. (1998) we also found a different trend of

w (rad. s-1)0.1 1 10 100

G'(P

a), G

''(Pa

)

0.1

1

10

100Ta

n de

lta

0.1

1

10

100

(b)Fig. 2. Effect of wheat ber and storage time on frequency sweeps of

G0; G00 and tan delta d of yogurts at 20C: (a) yogurt with wheat ber;(b) control yogurt. G0; G00;m tan d at initial storage time,J G0;&G00; n tan d at 21 days of storage at 4C. Bars indicate mean standarddeviations of corresponding curves.

Table 2Effect of storage time on dynamic parameters a and c for control

yogurts and yogurts fortied with four ber sources

Fiber source Time a sd C sd(days) (Pa sb) (Pa sd)

Bamboo 1 26.29 1.70 24.31 1.18

7 34.58 1.80 26.05 1.21

14 59.14 1.13 36.22 1.11

21 70.01 1.83 36.22 1.26

Inulin 1 21.04 0.78 14.32 0.52

7 25.40 1.12 27.09 1.18

14 29.35 1.15 31.57 1.01

21 54.37 1.70 43.10 1.52

Apple 1 12.02 0.47 12.86 0.35

7 18.58 1.47 25.17 1.21

14 33.61 1.66 30.91 1.72

21 36.71 1.81 35.68 1.41

Wheat 1 18.92 0.53 14.96 0.41

7 17.40 0.85 23.18 0.96

14 28.98 0.94 35.56 1.23

21 49.88 1.31 42.86 1.91

None 1 14.43 0.61 12.16 0.48

7 15.08 0.98 20.42 0.61

14 22.76 1.06 26.53 1.09

21 33.69 1.64 29.05 1.59

Parameters were estimated from the following equations: G0 awb andG00 cwd :

sd: Standard deviation.

consistency index m and c with storage time inrotational and dynamic assays behavior.Forcedistance curves corresponding to compression

extrusion tests are characterized by an increase up to avalue (maximum compression force) that gives the forcenecessary to begin the process of extrusion; afterwardsthe plateau indicates the force needed to continueextrusion. Maximum compression forces of controland fortied yogurts are compared in Fig. 3. Statisticalanalysis of control and fortied yogurts indicated thattime and type of ber were signicant Po0:05 factorsfor maximum compression force. Yogurt fortied withapple ber showed the lowest maximum compressionvalues. This behavior could be attributed to theformation of ber aggregates when added to the milk

that lead to higher probability to interfere with yogurtstructure.In general, the slope of forcedistance curve during

the process of extrusion is approximately horizontal, butsometimes it may steadily increase or decrease, indicat-ing different behavior patterns (Bourne, 1982). Theslight increase in the plateau of forcetime curve (notshown) of our yogurts indicated that during extrusion,shear and adhesion forces act simultaneously.

3.2. Physico-chemical characterization

Color parameters did not show signicant differencesP > 0:05 with time. However, apple ber gave adistinctive brownish color and lower lightness Lvalues, that differed from the other yogurt samples.Values of aw 0:98 remained constant with time for

all yogurts, and pH decreased from initial values to 4.2.In both cases no signicant differences P > 0:05between different bers were found. None of the yogurtsshowed syneresis during storage time.

ARTICLE IN PRESS

0.1

0.2

0.3

0.4

0 5 10 15 20 25

Time (days)

M. C

. F

orc

e (

N)

Fig. 3. Mean maximum compression force (M. C. force) for the

different yogurts at 20C as a function of time:~ control; bamboo; inulin; apple; m wheat. Bars indicate standard deviations.

a-color

controlbamboowheat

0.20.30.40.50.60.7

tive

frequ

ency

d for

M.D. Staffolo et al. / International Dairy Journal 14 (2004) 263268 2671 2 3 4 5category

1 2 3 4 5category

inulin

controlbamboowheatinulin

00.1re

la

00.10.20.30.40.50.60.7

rela

tive

frequ

ency

c-aroma

Fig. 4. Results of sensory analysis using a hedonic scale for control antotal number of panelists (25). Category: 1dislike; 2do not like; 3neit3.3. Sensory analysis

Fig. 4 shows acceptability categories of sensoryproperties as a function of frequency, which correspondsto the number of panelists that chose a category over thetotal number of panelists (25). Hedonic tests showedthat almost all assayed yogurts have color and aromascores above category 3 (neither like nor dislike),considered in the present work as the minimumacceptable value. More than 50% of the panelists

1 2 3 4 5category

1 2 3 4 5category

controlbamboowheatinulin

controlbamboowheatinulin

b-texture

00.10.20.30.40.50.60.70.8

rela

tive

frequ

ency

0

0.1

0.2

0.3

0.4

0.5

0.6

rela

tive

frequ

ency

d-flavor

tied yogurts. Frequency: number of panelists that choose a category/her like nor dislike; 4like; 5like very much.

graded texture of all yogurts with category 4 (like), withwheat ber containing yogurts showing the highest

to consumers preference for rmer yogurts.

consumer preferences. The addition of 1.3% dietary

Universidad Nacional de La Plata is gratefully acknowl-edged.

Journal of Food Science, 57, 96102.

ARTICLE IN PRESSM.D. Staffolo et al. / International Dairy Journal 14 (2004) 263268268ber to supplement yogurts appear to be a promisingavenue for increased ber intake, with high consumeracceptability. Both ber and yogurt itself are wellknown for their benecial health effects, and togetherthey may constitute a functional food with commercialapplications.

Acknowledgements

The Financial support by CONICET (ConsejoNacional de Investigaciones Cient!cas y T!ecnicas),Agencia Nacional de Promoci !on Cient!ca y Tecnol-!ogicas, Proyecto BID1201/OC-AR PICT09-04579 andFor the triangle test with 25 panelists, a minimum of17 correct identications was required for 0.01 level ofsignicance (Urena et al., 1999). All assays showedlower correct identications than 17, indicating that nosignicant differences P > 0:01 between control andfortied yogurts were detected.

4. Conclusions

Yogurt fortied with wheat, bamboo, inulin andapple bers did not show syneresis even after 21 daystorage time at 4C. Also aw; pH, and color parameterswere stable with storage time. Instrumental rheologicalparameters (apparent viscosity, maximum compressionforce and dynamic oscillatory parameters) indicated thatber type and storage time were signicant factors.However, sensory analysis did not detect any differencebetween wheat, bamboo and inulin bers and controlyogurt. The untrained panelists found the fortiedyogurts acceptable, giving high scores for color, avorand texture.The highest differences between control and fortied

yogurts in rheological and sensory characteristics werefound with apple ber. The brownish color associatedwith this ber would make it necessary to add avorcomponents to modify yogurt formulation to matchfrequency at this category. Yogurt with inulin got thehighest scores for avor characteristics.The higher maximum compression force for wheat

and bamboo ber yogurts (Fig. 3) can be related to theirhighest sensory scores for texture (Fig. 4). The highmaximum force could be explained by the insolublecharacteristics of these bers and in turn can be relatedReferences

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testing methods, special technical publication, Vol. 434. Philadel-

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Influence of dietary fiber addition on sensory and rheological properties of yogurtIntroductionMaterials and methodsPreparation of yogurtsTransient shear stress measurementsDynamic oscillatory measurementsCompression-extrusion testsPhysico-chemical determinationsSensory evaluationStatistical analysis

Results and discussionRheological analysisPhysico-chemical characterizationSensory analysis

ConclusionsAcknowledgementsReferences