Physicochemical Characterization of Extruded Blends of Corn Starch–Whey Protein Concentrate– Agave tequilana Fiber

Page 1

ORIGINAL PAPER

Physicochemical Characterization of Extruded Blendsof Corn Starch–Whey Protein Concentrate–Agavetequilana Fiber

Abigail Santillán-Moreno & Fernando Martínez-Bustos &

Eduardo Castaño-Tostado & Silvia L. Amaya-Llano

Received: 4 November 2008 /Accepted: 9 June 2009 /Published online: 26 June 2009# Springer Science + Business Media, LLC 2009

Abstract The objective of this work was to prepareextruded blends of corn starch supplemented with wheyprotein concentrate and Agave tequilana fiber (AF). Theextruded blends were prepared by blending whey proteinconcentrate (WPC 80, 25%) with a mixture of corn starch(60%, 67%, and 74%) and A. tequilana fiber (1%, 8%, and15%) and then adjusting its pH (5 and 8). The extrusionprocess was performed using a laboratory single-screwextruder. The screw compression ratio was 2:1 with a5.0-mm die nozzle. Barrel temperature in the final zone was140°C. Small differences in expansion index and bulkdensity values were found between extruded samples withor without fiber addition, while the samples extruded at pH5 showed the lowest penetration force. Alkaline pH andhigh fiber content resulted in the highest total and insolubledietary fiber. The addition of fiber to the extrudedformulations decreased lightness, greenness (−a), and totalcolor (ΔE). AF incorporation increased water absorptionindex in all the assays, but these values were notsignificantly different. In vitro digestibility values variedbetween 83% and 90%, and the addition of AF in differentlevels did not change these values. The inclusion of AF intoextruded blends of whey protein and corn starch reducedpeak, minimum, and final viscosity but increased the extent

of gelatinization when highest levels of AF were added inthe blends. Extruded samples showed good functionalcharacteristics with improved health benefits (more fiberand protein content) due to whey protein and fiber additionto starch.

Keywords Corn starch .WPC80 . Extrusion .

Physicochemical characteristics

Introduction

Extrusion processing is popular in the food industry andcan efficiently create novel products that might not bepossible with other processing methods (Cisneros andKokini 2002). Food extruders provide the thermomechanicaland mechanical energy (shear) necessary to cause physico-chemical changes of raw materials with an intense mixing toobtain dispersion and homogenization of ingredients (Antonand Luciano 2007). Many researchers have worked onunderstanding the extrusion cooking for process and productdevelopment in food and feed (Chevanan et al. 2008, 2009;Larrea et al. 2008; Nwabueze and Iwe 2008; Shankar et al.2008; Kannadhason et al. 2009). Even though starch is theprimary ingredient in expanded breakfast cereals and snacks,addition of whey protein can enhance their nutritional valuebecause their incorporation in extruded products mayprovide a nutritionally sound and economical approach tofortification (Allen et al. 2007). Whey protein can beextruded into expanded products (Kim and Maga 1987;Sokhey et al. 1996; Onwulata et al. 1998; Walsh andCarpenter 2003; Liu et al. 2006; Allen et al. 2007) as well asfibrous-textured products (Hale et al. 2002; Taylor and Walsh2002; Walsh and Carpenter 2003; Walsh et al. 2008).Undesirable effects of proteins extrusion include reduction

A. Santillán-Moreno : E. Castaño-Tostado :S. L. Amaya-Llano (*)Programa de Posgrado en Alimentos del Centro de la República(PROPAC), Universidad Autónoma de Querétaro,Apdo, 184, Querétaro, Qro 76010, Mexicoe-mail: [email protected]

F. Martínez-BustosCiencia de Materiales, CINVESTAV Querétaro,Apdo, 1-798, Querétaro, Qro 76230, Mexico

Food Bioprocess Technol (2011) 4:797–808DOI 10.1007/s11947-009-0223-x

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of protein quality due to, e.g., the Maillard reaction, decreasein palatability, and loss of heat labile vitamins (Tran et al.2008). The extrusion of mixtures of proteins and starchesincreases sites for cross-linking and affects textural quality(Onwulata et al. 2001b). Addition of whey proteins inamounts higher than 10% tends to reduce expansion, animportant textural parameter (Onwulata and Heymann 1994;Martinez-Serna and Villota 1992; Amaya-Llano et al. 2007).

Health benefits associated with dietary fiber, with arecommended daily intake of 0.025–0.030 kg day−1adult−1

(Labell 1990), have led to its use in virtually all foodproduct categories, including many products, which aremanufactured using extrusion processing. The addition offiber to extruded snacks has been limited to a few fibersources such as wheat and oats (Hsieh et al. 1991), sugarbeet fiber (Hsieh et al. 1989; Ralet et al. 1991), soy fiber(Jin et al. 1995), and cauliflower fiber (Stojceska et al. 2008).Addition of high-fiber, high-protein alternate ingredients tostarch has been shown to significantly affect the texture,expansion, and overall acceptability of extruded snacks (Liuet al. 2000; Veronica et al. 2006). Whey protein and dietaryfiber both tend to have detrimental effects on extrudatecharacteristics, decreasing the expansion ratio and increasinghardness and density (Kim and Maga 1987; Onwulata et al.1998, 2001a). Sugar beet fiber included in significantamounts (>100 g/kg) in products increased structuralstrength but reduced expansion. Increases in mechanicalenergy associated with increasing fiber do not result ingreater expansion or reduced specific bulk density, which aredesirable snack characteristics (Guha et al. 1997), or to otherapplications.

Despite the apparent disadvantages related to theaddition of fiber and other components like whey proteinin extruded products, the nutritional value is increased, andthis can provide other benefits for products such as snackfoods. Blue agave (Agave tequilana Weber), a member ofthe lily family, is grown extensively in the regions east andwest of Guadalajara in Central Mexico. It is the rawmaterial for production of the alcoholic beverage tequila.The agave plant has two main parts: the long spiked leavesfrom which sisal type fibers can be obtained and the “head”or “piña” from which juices are extracted for alcohol(tequila) production. The fiber in the leaves is composed ofagrate bundles of short fibers. The fiber bundles vary inlength with an average length of 40 cm (23±52 cm) and anaverage width of 0.12 mm (0.6±13 mm). The averagelength of the ultimate fiber is 1.6 mm with an average widthof 25μm (Iñiguez-Covarrubias et al. 2001a). The fiber isthick-walled and long (10–12 cm). Some of the bagasse ismixed with clay and used to make bricks. More bagassefinds its way, after mechanical pith separation and sundrying, into mattresses, furniture, and packing materials,but most of it is treated as waste and returned to the fields.

In recent years, the tequila market has grown and gainedinternational recognition. This will mean even morebagasse, increasing disposal problems for the tequilacompanies (Iñiguez-Covarrubias et al. 2001b). Duringextrusion, pulp fibers are shortened; fiber bundles, consistingof elementary fibers, are defibrated, and elementary fibers arefibrillated (Westenbroek 2000). However, a search ofreferences indicates that no research on the incorporation ofA. tequilana fiber into food extruded products has beenpublished.

The aim of this research was to study extruded blends ofcorn starch, whey protein concentrate, and A. tequilanafiber in relation to their physicochemical characteristics.Such information could help food processors predict theperformance of extruded material to be used in newproducts being developed in response to various consumerdemands, including those with nutritional benefits orconvenience foods.

Materials and Methods

Corn starch (CS) and whey protein concentrate 80 (WPC80) were purchased from IMSA, S.A. (Mexico DF, Mexico)and América Alimentos, S.A. de C.V. (Zapopan Jal,Mexico), respectively. A. tequilana Weber bagasse (AF)was donated by La Herradura (Guadalajara Jal, Mexico), aTequila producer.

Chemical Composition

Official methods (AOAC 1999) were used to quantify totalprotein (no 988.05) and lipid (no 920.35) contents. Themoisture content was determined using Approved Method44-19 (AACC 2000).

Preparation of Samples

The CS–WPC–AF blends were prepared by mixing WPC80 (25%) with CS (60%, 67%, and 74%) and AF (15%,8%, and 1%), respectively, and then adjusting the pH (5 and8) by adding NaOH or HCl (concentration of 0.1–1.0 N)according to the levels indicated in Table 1 and accordingto Method 943.02 of AOAC (AOAC 1999).

AF was milled using a hammer mill (model 200, Pulvex,Mexico) with a 250-μm sieve. The total moisture contentsof the blends were adjusted to 30%. Weights of thecomponents to be mixed were calculated according to thefollowing formula:

cCS ¼ rCS �M � 100� 30ð Þ½ �= 100� 100� wCSð Þ½ �

798 Food Bioprocess Technol (2011) 4:797–808

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cWPC80 ¼ rWPC80 �M � 100� 30ð Þ½ �= 100� 100� wWPC80ð Þ½ �

cAF ¼ rAF �M � 100� 30ð Þ½ �= 100� 100� wAFð Þ½ �

wx ¼ M � cCS � cWF � cAF

where cCS, cWPC 80, and cAF were mass of CS, WPC 80,and AF powder, respectively; rCS, rWPC 80, and rAF were apercentage of CS, WPC 80, and AF in the blend, dbrCS þ rWPC80 þ rAF ¼ 100%ð Þ; M was the total mass of theblend; 30, the moisture of the final blend,%, wb; wx was thewater to be added; and wCS, wWPC 80, and wAF werethe moisture of CS, WPC 80, and AF, respectively. Themoisture content of the blends was measured by drying thesamples to a constant weight in a vacuum oven at 105°C.The samples were stored in polyethylene bags at 4°C forsubsequent extrusion processing. The extrusion process wasconducted using a single-screw extruder, designed, andmanufactured by Cinvestav-IPN, Mexico. The screwcompression ratio was 2:1 with a 5.0-mm die orifice. Thebarrel was equipped with electrical cartridge heaters andthree independently controlled heating and cooling zones(cooling jackets and cooled with air). The barrel wasseparated into independent electrically heated zones andcooled by compressed air, which circulates around thebarrel to maintain precise temperature control. The dieassembly was electrically heated. Barrel temperature in thefeed zone (zone 1) was 100°C, transition zone (zone 2) was120°C, and final zone (zone 3) was 140°C. The feed rate(73 g/min) and the screw speed (43 rpm) were constantthroughout the experiment. Extruded blends were dried tothe desired moisture (9.5–10.5%) in a convection oven(Felisa, model FE294AD,Mexico; 40°C) for 18 h. Dependingon the analysis, the final blends were either used whole orwere powdered by a hammer mill (model 200, Pulvex) with a250-μm sieve and packed into polyethylene bags for storageand further analysis.

Expansion Index

Extruded samples (expanded) were cut into 5-cm pieces,and then the expansion index (EI) was determined accordingto the method described by Jin et al. (1994). The EI wascalculated by dividing the average transversal area of theexpanded products by the area of die (5 mm diameter). Theexpansion index (average) for each extruded product wasderived from 20 measurements.

Bulk Density

The bulk density (BD) was determined using the method ofGujska and Khan (1991). BD (grams per cubic centimeter)T

able

1Exp

erim

entalresults

forextrusionandrespon

sevariablesof

extrud

edblends

ofcorn

starch–w

heyproteinconcentrate–

Aga

vetequ

ilana

fiber

Blend

Factors

Responsevariables

Expandedsamples

Milled

extruded

samples

CS(%

)AF(%

)pH

EI

BD

(gcm

−3)

PF(N

)Color

La

bΔE

160

155

1.0±0.02b

0.63

±0.03a

14.7±2.1b

58.44±0.07c

3.71

±0.02a

11.25±0.04a,b

30.79±0.07a

267

85

1.1±0.05a,

b0.65

±0.08a

24.2±4.1b

60.59±0.01b,

c3.59

±0.04a

12.26±0.06a

29.19±0.09a,b

367

88

1.1±0.04a,

b0.75

±0.07a

30.0±3.8b

55.31±4.52c

3.11

±0.04a,b

10.70±0.51a,b

33.47±0.07a

474

15

1.1±0.02a,

b0.71

±0.06a

17.9±3.5b

76.68±0.07a

0.77

±0.02b

11.71±0.04a,b

15.11±0.06c

574

18

1.2±0.04a

0.67

±0.06a

28.4±5.2b

73.47±0.10a,

b0.87

±0.03b

12.13±0.04a,b

17.34±0.06b,c

660

158

1.0±0.03a,

b0.70

±0.04a

33.5±4.1b

50.89±0.13c

3.40

±0.04a

10.01±0.04b

37.53±0.12a

C5

750

51.0±0.08a,

b0.67

±0.06a

75.7±7.9a

85.79±0.06a

−1.04±0.01b,

c12.68±0.14a

11.76±0.03c

C8

750

81.2±0.06a

1.01

±0.02a

79.9±7.3a

85.24±0.07a

−1.61±0.01c

12.42±0.08a

11.57±0.09c

AFcolorvalues:L=48

.58±0.21

,a=4.83

±0.04

,b=11.34±0.07

,ΔE=40

.26±0.14

.Valuesaremeans

±SD.V

alueswith

differentlettersin

thecolumns

aresign

ificantly

different(Tuk

ey'smultip

lecomparisontestat

α=0.05

)

CScorn

starch

contentin

theblend,

AFAga

vetequ

ilana

fibercontentin

theblend,

EIexpansionindex,

BD

bulk

density

(gramspercubiccentim

eter),PFpenetrationforce(new

tons)

Food Bioprocess Technol (2011) 4:797–808 799

Page 4

was calculated by dividing the extrudate piece weight by itsapparent volume. The average diameter and length weremeasured and the apparent volume (V, cubic centimeter)was computed as

V ¼ 1=4 p � d2 � 1� �

where d (centimeters) is the average diameter, andl (centimeter) is the length of the extruded product. BDvalues were derived from 20 analyses of each product.

Penetration Force

Penetration force (PF) trials were performed using a textureanalyzer (TA-XT2, Stable Micro Systems, Texture Tech-nologies Corp., Scarsdale, NY, USA) according to themethod of Fernández-Gutiérrez et al. (2004) with somemodifications. Ten randomly chosen extruded piecesadjusted at 8% moisture content were cut into 2.5-cmlength and were placed in an Ottawa compression cell. Themaximum penetration force (in newtons) necessary topenetrate extruded products was recorded. Each assay wasthe average of ten measurements.

Color

The color was measured in extruded treatments milled to250μm. Color was recorded using a Mini Scan Hunter Labinstrument (CE96, Hunter Associates Laboratory, Reston,VA, USA) following the method of Jin et al. (1994). Thecolor difference for each sample was calculated using theequation

Color ¼ ΔE ¼ ΔL2 þ Δa2 þ Δb2� �1=2

Each assay was the average of 20 samples. The blankvalues were L=92.26, a=−0.81, and b=0.62.

Water Absorption and Water Solubility Indexes

Water absorption (WAI) and water solubility (WSI) indexeswere measured in powdered extruded samples at twotemperatures (30 and 75°C). The sample (2.5 g) wassuspended in 30 ml distilled water at 30 or 75°C for30 min, gently stirred during this period, and thencentrifuged at 3,000×g for 15 min. The supernatant wasdecanted into an evaporating dish of known weight. TheWSI was the weight of dry solids in the supernatantexpressed as a percentage of the original weight of sample.The WAI was the weight of gel obtained after removal ofthe supernatant per unit weigh of original dry solids,according to the method of Anderson et al. (1969). Threerepetitions were made for each analysis.

In Vitro Digestibility

In vitro digestibility of protein was measured according tothe method of Hsu et al. (1977). Fifty milliliters ofpreviously defatted, aqueous extrudate suspension(6.25 mg protein/ml) in glass distilled water was adjustedto pH8.0 with 0.1 N HCl and/or NaOH while stirring in a37°C water bath. A multienzyme system consists of trypsinfrom porcine pancreas (Type IX-S, 15700 BAEE units/mgprotein, product no.T-0303), α-chymotrypsin from bovinepancreas (type II, 66 units/mg solid, product no. C-4129),and peptidase from porcine intestinal mucosa (grade III,102 units/g solid, product no. P-7500). All these enzymeswere purchased from Sigma Chemical Co. (St. Louis, MO,USA). The multienzyme solution (1.446 mg trypsin,2.818 mg α-chymotrypsin, and 0.5098 mg peptidase)/mlwas maintained in an ice bath and adjusted to pH8.0 with0.1 N NaOH. Five milliliters of the multienzyme solutionwas added to the protein suspension, which was beingstirred at 37°C. A rapid decline in pH occurred immedi-ately. This was caused by the freeing of amino acidcarboxyl groups from the protein chain by the proteolyticenzymes. The pH drop was recorded automatically over a10-min period using a recording pH meter. Percentdigestibility (Y) was calculated according to the regressionequation (Y=210.464−18.103X, where X is the pH of thesuspension) of Hsu et al. (1977). The multienzyme solutionwas freshly prepared before each series of tests, and itsactivity was determined using casein (from bovine milk,purchased from Sigma Chemical Co.) of known in vitrodigestibility as reference. Two repetitions were made foreach analysis.

Total Dietary Fiber and Insoluble and SolubleFiber Content

The total dietary fiber (TDF) and insoluble (IDF) andsoluble fiber (SDF) content of extruded samples weredetermined according to AOAC method (991.42 and993.19; AOAC, 1999) in duplicate, described by Proskyet al. (1988). The AOAC method uses a heat-stable α-amylase, amyloglucosidase, and protease treatment. Theinsoluble dietary fiber was determined by weighing 1 g ofsample into 400-ml tall form beakers, and 50 mlof phosphate buffer pH6.0 was added and adjusted topH6.0±0.2 with 0.275 N NaOH, then 100μl heat-stable α-amylase (Termamyl 120 l, 120KNU/g, Novo A/S, Copen-hagen) solution was added, and the beaker was coveredwith aluminum foil and placed in boiling water bath for15 min then was shaken gently at 5-min intervals to reachan internal temperature of 100°C, for 30 min. Then, it wascooled and adjusted to pH7.5±0.1 by adding 10 ml of0.275 N NaOH solution. Immediately after that, 5 mg of

800 Food Bioprocess Technol (2011) 4:797–808

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protease (Alcalase 0.6 l, 0.6AU/g, Novo A/S) solution inphosphate buffer was added, covered with aluminum foil,and incubated at 60°C for 30 min with continuousagitation. Then, it was cooled and added with 10 ml of0.350 M hydrochloric acid solution to adjust pH to 4.0 to4.6 and was then added with 0.3 ml of amyloglucosidase(AMG 200, 200 AGU/ml, Novo A/S) solution, coveredwith aluminum foil, and incubated at 60°C for 30 min withcontinuous agitation. Cooled residue was filtered throughtare crucible and weighted after being dried to 105°C for2 h and cooled again. The residue was analyzed from onesample of the set of duplicates for protein using Kjeldahlanalysis as specified in AOAC, using 6.25 for the proteinfactor. A second sample of the duplicate was incinerated for5 h at 525°C, cooled in desiccator, and weighed for ash.IDF %ð Þ ¼ weight of residue mgð Þ; minus weight ofð½protein; minus weight of ash:� � 100. The soluble dietaryfiber was determined using the filtered solution in IDF,adding three 20-ml portions of 78% ethanol, two 10-mlportions of 95% ethanol, and two 10-ml portions ofacetone. It was then filtered through tare crucible and driedovernight at 105°C in air oven and cooled in desiccators.SDF %ð Þ ¼ weight of residue mgð Þ; minus weight ofð½protein; minus weight of ash:� � 100. The total dietaryfiber values were obtained by the addition between theinsoluble and soluble dietary fiber fractions.

Pasting Properties and Extent of Gelatinization

Pasting properties of extruded blends were measured on aRapid Visco Analyzer (RVA-4; Newport Scientific Pty. Ltd.,Warriewood, Australia). An extruded sample of 4.0 g wasmilled and sifted (250 mmwide mesh), adjusted to 14% (wb),and transferred into a canister, and approximately 24±0.1 mldistilled water was added (corrected to compensate for 14%,wb). The time–temperature sequence was as follows: an initialstage at 50°C for 2 min, followed by heating to 95°C at aconstant heating rate of 5.6°Cmin−1, maintaining temperatureat 95°C for 5 min and then cooling to 50°C at the same rate.Data from the RVA were processed by the software(Thermocline version 1.2, Newport Scientific Inc.). Allprocedures were performed in duplicate.

Gelatinization (GE) was determined by the methoddescribed by Guha et al. (1998). The GE of each extrudedsample was calculated in relation with that of nonextruded.The area under the peak occurring during the heatingphase of the pasting curve exhibited by the extrudedsamples, relative to that of raw samples (as 100%),represented the ungelatinized or uncooked part, whichafter subtraction from 100 yielded the extent of GE. Tomeasure the area under the peak, a base line was drawnconnecting the inflections at the beginning and the end of thepeak. The peak area was then determined using software

Origin Pro 8 (OriginLab Corporation, Northampton, MA,USA).

GE %ð Þ ¼ 1� area under the peak of extruded sample

area under the peak of non extruded sample

� �

� 100

Experimental Design and Data Analysis

Our interest is to compare blends with a fixed 25% ofWPC80 and 75% of a mixture of CS and AF; therefore,blends were prepared by mixing WPC 80 (25%) with CS(60%, 67%, and 74%) and AF (15%, 8%, and 1%),respectively; we were also interested in studying effects ofpH change (5 and 8) on properties of mixtures; eachmixture was replicated once. The experimental runs areas shown in Table 1. Tukey’s multiple comparisons test(α=0.05) was used to establish statistical significance.

Results and Discussion

Chemical Composition

The chemical composition (% wb) of the CS, WPC 80, andA. tequilana fiber was as follows: protein (N×6.25) 0.45,75.18, and 3.59, respectively; lipid content 0.60, 0.56, and0.1, respectively. Moisture content was 10.7 for CS, 7.4 forWPC 80, and 7.8 for A. tequilana fiber.

Physicochemical Characteristics of Extruded Products

Expansion Index, Bulk Density, and Penetration Force

The EI of the extrudates varied between 1.0±0.02 and 1.2±0.06. For the most part, the EI values did not showstatistical differences (Table 1) among the blends. Excep-tions were the extruded with higher AF at pH5 that had theminimum EI, and the assay extruded with 1% AF and pH8 that had the highest. However, all EI and BD values,including controls (without fiber addition), were verysimilar, probably due to operating conditions principallythe die nozzle diameter (5.0 mm) or particle size of feedingmaterials (restriction caused by the presence of fiber). Thedegree of expansion determines the extrudate structure andconsequently its texture (Arhaliass et al. 2009). The highestPF was observed in blends without fiber addition (controls);thus, PF was affected by fiber incorporation, showing asignificant difference when compared to the controls.However, different levels of AF in the blends were notsignificantly different in PF. Probably, the re-arrangement

Food Bioprocess Technol (2011) 4:797–808 801

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of the segments of fiber in the starch matrix is different,modifying the values of EI and BD. Most extruded foodsare actually made out of complex formulations, whichinclude, besides starch, protein, fat, sugar, fibers, and so on.All these ingredients have different effects on extrudateexpansion (Faubion and Hoseney 1982). The resultsobtained for PF showed correlation with extent of gelatini-zation (P=0.031). Our results can be explained by the factthat the additions of AF in the extrudate matrix possiblyweaken the extrudate by destroying the continuity of thealigned mass. Some authors have reported that fiberincreases the hardness of extruded products (Mendonca etal. 2000; Yanniotis et al. 2007; Ainsworth et al. 2007) as aresult of its effect on cell thickness; however, these authorswere using dietary fiber from cereal. These differencescould be explained due to use of different ingredients aswell as operating conditions.

Color

The color values recorded for the extrudates containingdifferent levels of AF are summarized in Table 1. L valueswere higher in blends with low fiber content, including thecontrols. The samples without AF displayed significantlyhigher L values than other samples. L values give a measureof the lightness of the color from 100 for perfect white to 0for black; thus, AF level decreased the lightness of extrudedsamples. Lightness showed strong correlation with extent ofgelatinization (P=0.0086). The redness (+a)/greenness (−a)color of extrudates indicated that samples containing a lowAF level showed a higher greenness color. Redness of theproduct was related to the level of AF with significantdifference between lower levels (including controls) andmedium and higher levels (8% and 15%) of AF; however,statistically, these levels were not different. From the dataobtained, it appears that different levels of AF had no effecton the +b (yellowness) and −b (blueness) color ofextrudates. Lightness, redness (+a)/greenness (−a), andtotal color were influenced by the fiber concentrations,mainly due to the residues of sugar and lignin present in thefiber, resulting in a dark color. For this reason, samples withlow fiber content showed an off-white coloration due toother ingredients in the blends. Also, the various tonalitiesfound in different fiber concentrations could be due toreactions that originated during the extrusion process,therefore causing the pigment degradation. The reactionsthat can affect the color during the extrusion process arediverse and basically include Maillard, caramelization,hydrolysis, and others, as well as a nonenzymatic reaction,such as the degradation of pigments (Camire et al. 1990).Nevertheless, changes in color can also be an indicator ofthe intensity of the process and may be related to chemicalchanges (Berset 1989).

Since color is an important quality factor directly relatedto the acceptability of food products, color measurement isan important physical property to report for extrudateproducts (Patil et al. 2005). In this study, the AF contentin the blend had an influence on the color changes of theextrudates, as did the biochemical changes occurring insidethe barrel during processing.

Water Absorption and Water Solubility Indexes

Extrudates without fiber added showed a lower WAIevaluated at 30°C (WAI30). When AF was added to theblends, WAI30 increased, but the AF levels were notsignificant (Table 2). These findings were in agreementwith a study in which the by-product of brewer’s spentgrain (BSG), containing approximately 70% fiber, wasincorporated into extrudates (Stojceska et al. 2008), wheredietary fiber addition of BSG increased WAI. Hashimotoand Grossmann (2003) observed a similar effect on WAIwhen adding cassava bran to cassava starch processed in asingle-screw extruder, which can be explained by the addedfiber having a high water absorption capacity. At the sametime, starch was reduced. The denaturation increased theaccessibility to the protein's polar amino-acid groups,enhancing their affinity for water. This phenomenonincreases the capacity for water absorption. Common beansdo not only have proteins but also starch, which could begelatinized and their crude fiber swollen, playing a veryimportant role in the water absorption capacity too (Rocha-Guzman et al. 2008). On the other hand, Matthey andHanna (1997) reported that addition of WPC reduced theexpansion and water absorption index under some conditions.They conducted a split-plot experiment to investigate fourlevels of amylose in starch with four levels of WPC in WPC–corn starch blends with a co-rotating intermeshing twin-screwextruder at 220 g/kg moisture content, 140 rpm screw speed,and 140°C barrel temperature. They reported that increasingthe WPC content decreased the WAI of starch with highamylose content, suggesting that WPC decreased starchmolecular degradation. Diverse researchers have reported thatthe behavior of different materials have different responses tothe same processing variables. In agreement with the findingsof this research, Aguilar-Palazuelos et al. (2007) reported thatan increase in fiber content at low barrel temperature resultedin an increase in the WAI of the extruded blends of cornstarch and coconut fiber. However, at high barrel tempera-ture, the effect was contrary; an increase in fiber contentproduced a decrease in WAI. The WAI tends to increase withincreased screw speeds. WAI and WSI are related to thedegree of starch fragmentation (de Mesa et al. 2009). In thisstudy, lowest WAI coincides with lowest GE and showed asignificant relation (P=0.0063). When increasing fibercontent in an extruded product, the effort of cutting in the

802 Food Bioprocess Technol (2011) 4:797–808

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Tab

le2

Fun

ctionalcharacteristicsof

extrud

edblends

ofcorn

starch–w

heyproteinconcentrate–Aga

vetequ

ilana

fiber

Blend

Factors

Respo

nsevariables

CS

(%)

AF

(%)

pHWAI

WSI

Invitro

digestibility

(%)a

TDF(%

)ID

F(%

)SDF

(%)

Pastin

gtemperature

(°C)

RVAviscosity

(cP)

Extentof

gelatin

ization

(%)

30°C

75°C

30°C

75°C

Peak

Minim

umFinal

160

155

6.19

±0.25

a5.73

±0.01

a5.57

±0.23

a,b,

c8.36

±0.85

a85

.75±

0.26

b30

.74±0.60

a30

.37±0.57

a0.37

±0.03

a,b

53.8±1.1c,d

209.9±

2.0e,f

175.5±11.2e

297.4±

12.8

80±3a

267

85

6.21

±0.36

a6.24

±0.73

a5.97

±1.02

a,b,

c11.33±

1.14

a89

.91±

0.51

a22

.56±0.13

d22

.04±0.21

d0.52

±0.08

a54

.8±0.8b

,c

265.55

±7.1d

,e21

5.5±9.8d

417.0±

17.0e

63±2b

367

88

6.85

±0.50

a6.38

±0.33

a4.92

±0.69

c8.79

±0.79

a82

.81±

0.70

c24

.19±0.24

c23

.83±0.22

c0.37

±0.02

a,b

56.9±0.3a,b

338.5±

43.6d

196.2±4.1d

,e

550.6±

9.0d

75±3a

474

15

5.66

±0.67

a,b

6.53

±0.73

a4.85

±0.03

c8.41

±1.28

a88

.51±

0.32

a15

.820

.11±f

15.45±0.14

f0.37

±0.03

a58

.5±0.1a

437.0±

9.1c

290.8±10

.7c

594.5±

50.6c,

d51

±3c

574

18

6.51

±0.59

a7.63

±0.83

a5.23

±0.35

b,c

9.59

±1.64

a84

.62±

0.32

b18

.73±0.11e

18.56±0.08

e0.17

±0.03

b58

.2±0.5a

419.35

±19

.3c

296.6±4.9c

646.2±

15.6c

78±1a

660

158

6.79

±0.20

a5.88

±0.15

a5.39

±0.79

a,b,

c11.08±

1.36

a85

.07±

0.32

b29

.07±0.39

b28

.53±0.33

b0.54

±0.06

a54

.8±0.2b

,c

180.65

±13

.8f

133.7±9.2f

349.3±

16.4

79±3a

C5

750

55.58

±0.68

a,b

6.50

±0.31

a8.56

±0.54

a13

.44±

0.77

a86

.3±

0.13

bN.d.

N.d.

N.d.

52.2±0.6d

1,39

8.5±

15.7b

811.8±7.2b

2,30

4.1±

8.3b

43±1c,d

C8

750

83.50

±0.14

a,b

7.04

±0.12

a8.49

±0.25

a,b

14.25±

0.63

a85

.8±

0.30

bN.d.

N.d.

N.d.

51.8±0.2d

1,74

6.4±

12.0a

1,13

0.7±3.7a

2,89

7.7±

5.9a

35±2d

Valuesaremeans

±SD.Valueswith

differentletters

inthecolumns

aresign

ificantly

different(Tuk

ey'smultip

lecomparisontestat

α=0.05

)

CScorn

starch

contentintheblend,

AFAga

vetequ

ilana

fibercontentintheblend,

WSI

water

solubilityindex,

WAIwater

absorptio

nindex,

TDFtotald

ietary

fiber,ID

Finsolubledietaryfiber,SD

Fsolubledietaryfiber,N.d.no

tdetected

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aCaseinwas

used

asreference(89.2%

invitrodigestibility)

Food Bioprocess Technol (2011) 4:797–808 803

Page 8

interior of the extruder favors the plasticization andhomogenization of the processed materials (Guan et al.2004). On the other hand, WAI at 75°C (WAI75) was notaffected. As we expected, the largest changes took placeduring extrusion process and were registered at 30°C;furthermore, when the temperature of the analysis wasincreased at 75°C, the changes in WAI were minimum.WSI at 30°C (WSI30) was affected by AF levels. Blendswithout AF showed higher WSI30, and AF addition caused adecrease in WSI; however, WSI was not related to the levelof AF. Hashimoto and Grossmann (2003) reported that anincrease in bran content tended to decrease WSI in extrudedcassava bran/cassava starch blends; these authors explain thatstructural modifications involving bran fiber could beresponsible for increasing WAI and decreasing WSI and thatthis modification could have resulted in interactions betweenfiber and starch, reducing solubility. The results obtainedfor WSI showed correlation with extent of gelatinization(P=0.023). WSI at 75°C did not show differences. Therelatively high hydration capacity of the blends probablydepends mainly on the inter–intra-molecular bonds betweenWPC, amylose and amylopectin induced by pH andextrusion conditions, as well as on the changes that the fiberand starch undergo during extrusion.

In Vitro Digestibility

In general, the in vitro digestibility (IVD) values of thesamples with added AF were very similar, with valuesbetween 83% and 90% (Table 2). The IVD value of thesample with medium AF content at pH5 was lower than therest of the samples. The lowering of IVD seems to be aresult of a combination of shearing, heat, pressure, and pHduring extrusion. It has been suggested that low proteindigestibility may result from changes in the proteinsthemselves during cooking (Duodu et al. 2002). Theformation of enzyme-resistant, disulphide-bonded oligomersmay be the cause of the low digestibility. In contrast, thesample with medium AF content at pH8 showed the highestIVD. Amaya-Llano et al. (2007) found that protein contentand pH were the most important variables influencing thedigestibility in vitro, although in general, most of the blendsof whey protein concentrate–corn starch showed improvedvalues. During extrusion, protein increases its susceptibilityto enzymatic hydrolysis and therefore improves digestibilityby denaturation. However, some nonenzymatic browningreactions and the formation of cross-linking reactions candecrease digestibility (Camire et al. 1990; Arêas 1992;Ledward and Tester 1994; Camire 2000, 2001; Moraru andKokini 2003). The extrusion process does not influence theoverall protein content even at severe extrusion conditions(Onwulata et al. 2003). Also, these researchers concludedthat although the amount of denatured protein increased with

increasing barrel temperature, denaturation had a minimaloverall effect on protein digestibility. The values ofdigestibility of extruded whey protein isolate reported bythese researchers varied from 85% to 90%. This is verysimilar to the values found in this research. Probably, thedifferences between the above and the results of this researchwere due to the different barrel temperatures and rawmaterials used.

Total Dietary Fiber and Insoluble and Soluble Fiber Content

The dietary fiber and insoluble and soluble content ofextruded samples are given in Table 2. TDF and IDF wereaffected by pH and low and medium AF content in theblends; however, high AF content at pH5 resulted in moreTDF and IDF content. SDF did not show correlation withAF levels. Englyst et al. (1995) suggested the increase inTDF contents of starchy foods after heat processing is dueto the formation of enzyme-resistant starch during thecooling of the product. Vasanthan et al. (2002) reported thatextrusion cooking increased the TDF of barley floursattributed primarily to a shift from IDF to SDF, as well asthe formation of RS3 and “enzyme-resistant indigestibleglucans” formed by transglycosidation. Several researchers(Aoe et al. 1989; Ralet et al. 1990; Wang and Klopfenstein1993; Gualberto et al. 1997) have shown a significantincrease in SDF content in extruded wheat bran. Somestudies have also reported an increase in the TDF and SDFcontent of potato peels using extrusion processing (Camireand Flint 1991; Camire et al. 1997). Similar changes in thedietary fiber profile of various extruded products have beenreported in numerous studies. SDF increased due to theextrusion processing of wheat flour (Siljestrom et al. 1986;Wang and Klopfenstein 1993), barley (Berglund et al.1994), sugar beet fiber with corn meal (Lue et al. 1991),and soy fiber (Qian and Ding 1996). Larrea et al. (2005)modified the properties of fiber components in orange pulpusing extrusion technology. They reported that extrusionconditions decreased insoluble dietary fiber and increasedsoluble fiber. The increase in SDF was usually at theexpense of IDF due to fragmentation or other types ofthermomechanical decomposition of cellulose and lignin,which are major components of insoluble fiber. The above-mentioned studies have reported conflicting results withregard to TDF, which remained unchanged (Siljestrom et al.1986), decreased (Lue et al. 1991), or in some cases evenincreased (Camire and Flint 1991; Camire et al. 1997).Increase in TDF due to extrusion was attributed to theformation of resistant starch. Decrease in TDF could be dueto the fragmentation of IDF into sugars or lower molecularweight fragments, which are not detected as SDF byenzymatic assay methods. However, contradictory resultshave been reported as well (Asp and Bjorck 1989).

804 Food Bioprocess Technol (2011) 4:797–808

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According to Varo et al. (1983) and Artz et al. (1990),extrusion cooking causes no significant changes in solubleand insoluble fiber; however, reduction in fiber content hasalso been reported (Fornal et al. 1987) because of extrusioncooking. It is important to note that most of the above-mentioned extrusion studies were carried out under con-ditions of relatively low moisture (for example, wateraddition of 6.1% to 17.5% dry basis in the case of Ralet etal. [1990]), high screw speed (up to 450 rpm in the case ofGualberto et al. 1997), and high barrel temperatures (up to160°C in the case of Aoe et al. 1989). The extrusionconditions in the current study were relatively lower (highermoisture and lower screw speed and barrel temperature).This could explain the low SDF values measured.

Pasting Properties and Extent of Gelatinization

The RVA pasting results for the extruded blends with andwithout AF are presented in Fig. 1. Extruded samples with

AF showed significantly lower peak, minimum, and finalviscosity with relation to extruded samples without AF(Table 2). It was attributed to a decrease in the water-swelling starch fraction due to its replacement with fiber.Another possible reason could be the interference of fiberwith the gelatinization or water absorption of starchgranules. GE of extruded blends with and without AF wascalculated (Table 2). Highest GE was obtained in extrudedblends with highest AF (80% GE for 15% AF pH5 and79% GE for 15% AF pH8). The extent of starchgelatinization has been reported primarily as a function ofthe barrel temperature; however, extrusion conditions andmaterial used possibly increased frictional damage. Whenprotein and fiber are present along with starch, theycompete for the limited amount of water available in thefood system. Some fibers and proteins are very strong waterbinders (Wang and Kim 1998). In contrast, lowest GE wasobtained in extruded blends without AF (43% GE in controlat pH5 and 35% GE in control at pH8). Extruded blendswith 1%AF and 8%AF at pH5 showed 51%GE and 63%GE,respectively. It has been shown that the starch pastingcharacteristics change considerably when the compressionratios of the screws, the moisture content of the raw materials,the presence of others ingredients, and operating conditions ofthe extruder are varied (Bhattacharya et al. 1999).

Conclusions

The A. tequilana fiber addition to starch and whey proteinconcentrate greatly affected extruded blends in theirphysicochemical characteristics. Extruded blends showedsmall differences in EI and BD, while the samples extrudedat pH5 showed the lowest penetration force. The penetra-tion force values were strongly influenced by AF addition.The addition of fiber to the extruded formulations decreasedlightness, greenness, and total color. Extruded blends withAF resulted in higher water absorption indexes thancontrols; however, water solubility index diminished forextruded samples with AF. Higher digestibility values werefound in the processed samples with low and medium AF atpH5; however, different levels in AF did not change theIVD values. Results showed that high AF content at pH5resulted in more TDF and IDF content. SDF did not showcorrelation with AF levels. The inclusion of AF into blendsof whey protein and corn starch reduced peak, minimum,and final viscosity but increased the extent of gelatinizationwhen highest levels of AF were added in the blends.

Acknowledgments We would like to thank the CINVESTAV-QROfor the facilities provided for the accomplishment of the present work,CONACYT for the MSc degree scholarship provided to the firstauthor, and FOMIX-HGO for the financial support for this project.

0 200 400 600 800 1000 1200 14000

400

800

1200

1600

2000

2400

2800

Tem

perature (˚C)V

isco

sity

(cP

)

Time (sec)

0% AF pH8 0% AF pH5

60

80

100a

0 200 400 600 800 1000 1200 14000

100

200

300

400

500

600

700b

Tem

perature (˚C)V

isco

sity

(cP

)

Time (sec)

1% AF pH 5 8% AF pH 5 15% AF pH 5 1% AF pH 8 8% AF pH 8 15% AF pH 8

60

80

100

Fig. 1 Effect of Agave tequilana fiber (AF) on RVA profiles (a),extruded blends without AF (b), extruded blends of corn starch–wheyprotein concentrate and Agave tequilana fiber

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