re

o

rcaento

ano

a r t i c l e i n f o

thermally processed foods (Tareke et al., 2002).The effect of water activity (aw) on acrylamide formation has

been widely studied. Depending on the water activity rangestudied, positive or negative correlations between water activityand acrylamide content have been observed. De Vleeschouweret al. (2007) showed very slight promoting effect of water activity

., 2011), revealingng water aon that thgar-rich pr

(Mathlouthi, 2001)where a small variation inmoisture conteresponds to a large variation in aw for water activities low0.9. For water activities higher than 0.9, a steep increase in tresponding moisture content can be observed (De Vleeschouweret al., 2007). In contrast, most foods are a mixture of different com-pounds (for example, carbohydrate, protein, fat, minerals, vitamins,salts and others). Hence, real foods tend to have a sigmoidal mois-ture sorption isotherm due to the presence of solutes with verystrong afnity for water that could maintain a larger amount ofbound moisture at lower humidity than solutes with less afnity.

Corresponding author. Tel.: +52 442 211 9900; fax: +52 442 211 9938.E-mail address: rsalazar@qro.cinvestav.mx (R. Salazar).

Journal of Food Engineering 143 (2014) 17

Contents lists availab

Journal of Food

lset al., 2002). Acrylamide, a neurotoxic compound (Spencer andSchaumburg, 1974) and probable human carcinogen (IARC,1994), has attracted the attention of the scientic community inrecent years due to it has been found in high concentrations in

plantain paste (0.43 6 awP 0.97) (Bassama et ala decrease in acrylamide formation with increasi

The above-mentioned studies have in commmodel showed a sorption isotherm typical of suhttp://dx.doi.org/10.1016/j.jfoodeng.2014.06.0290260-8774/ 2014 Elsevier Ltd. All rights reserved.ctivity.e foodoductsnt cor-er thanhe cor-1. Introduction

The Maillard reaction is the main responsible of the generationof avor and color in thermally processed foods (Ames, 1990).However, this reaction has been also linked to the acrylamideformation, specically with the presence of carbonyl compoundswith groups capable of forming a Schiff Base with the amino acidasparagine (Hidalgo et al., 2009; Mottram et al., 2002; Stadler

(0.34 6 awP 0.92) on acrylamide amount in an asparagineglucose model system. Similar results were reported by DeVleeschouwer et al. (2008) (0.88 6 awP 0.99). The same tendencywas observed in a potato model system (0.11 6 awP 0.97) wherethe acrylamide content was rather dependent upon the moisturecontent than upon the water activity (Mestdagh et al., 2006). How-ever, an opposite trend was found in aqueous glycerol asparaginemodel system (0.33 6 awP 0.71) (Hedegaard et al., 2007) andArticle history:Received 24 March 2014Received in revised form 11 June 2014Accepted 20 June 2014Available online 27 June 2014

Keywords:AcrylamideTortilla chipsMinimum integral entropya b s t r a c t

The objective of this study was to relate the tortilla minimum integral desorption entropy with acrylam-ide content during processing of tortilla chips. Tortilla pieces were stored at 30 C at aw of 0.110.84 for4 days and fried later in soybean oil at 180 C for 25 s. The lowest acrylamide content was observed intortilla chips made of non-stored tortilla (aw = 0.98) as well as in those prepared from tortilla stored inthe minimum integral entropy (aw = 0.53). In addition, the color and texture values were similar in bothcases. These results suggest that the reduction of the acrylamide content during processing of tortillachips and other tortilla based foods thermally processed might be modied by factors such as moisturecontent, aw, and the physical state of water in the tortilla. Thus, the minimum integral entropy showed tobe a reliable indicator to establish the most appropriate moisture conditions to obtain tortilla chips withreduced level of acrylamide when tortilla is dehydrated.

2014 Elsevier Ltd. All rights reserved.d Facultad de Ciencias Agrcolas, Campus El Cerrillo, Universidad Autnoma del Estado de Mxico, Piedras Blancas, Carretera Toluca Ixtlahuaca Km. 15.5,5020 Toluca, Estado de Mxico, MexicoEffect of water activity in tortilla and itscontent after frying

Rosa M. Delgado a, Gabriel Luna-Brcenas b, GernimPatricia Lpez-Pera d, Ricardo Salazar b,a Instituto de la Grasa, Consejo Superior de Investigaciones Cientcas, Avenida Padre GabCentro de Investigacin y de Estudios Avanzados del I.P.N., Unidad Quertaro, Librami76230 Quertaro, Quertaro, Mexicoc Instituto de Ciencias Bsicas, Universidad Veracruzana, Av. Dr. Rafael Snchez Altamir91192 Xalapa, Veracruz, Mexico

journal homepage: www.elationship on the acrylamide

Armbula-Villa b, Ebner Azuara c,

Tejero 4, 41012 Seville, SpainNorponiente # 2000, Fraccionamiento Real de Juriquilla,

s/n, Col. Industrial Animas, Carretera Xalapa-Las Trancas Km. 3.5,

le at ScienceDirect

Engineering

evier .com/ locate / j foodeng

dough with proper consistency to make tortillas. The dough was

Foodshaped into thin disks (11 cm diameter and 1.0 mm thickness)using a commercial tortilla roll machine (Casa Herrera, Mxico,D.F.). The dough shaped into tortillas were cooked on both sidesfor around 1.0 min by using an iron hot plate (270 10 C) namedcomal. The resulting tortillas were cut into circular pieces withan average area of 10 cm2 and its aw and pH value (AACCInternational, 2010) determined. Two set of tortilla pieces wereobtained. The rst set was fried immediately in soybean oil at180 C for 25 s (control). The second set was used in the construc-tion of desorption isotherms. After frying, tortilla chips werecooled on a paper towel to remove supercial oil and the color,fracture force and acrylamide content determined.

2.3. Water desorption isothermsWater activity inuences on the stability and chemical reactionrates in foods (Labuza et al., 1972). Thus the mobility of acrylamideprecursors and as a consequence, the acrylamide formation isaffected by the physical state of the water sorbed during thermalprocessing. In this context, it is expected that the nal acrylamidecontent will be mainly affected by the moisture content (mono-layer value) where strong bonds between the water (adsorbate)and the food (adsorbent) occur.

In an attempt to understand the inuence of water activity ofreal foods on acrylamide content, the level of acrylamide wascompared in tortilla chips prepared from tortilla equilibrated atdifferent water activities, including that corresponding to themonolayer value. The monolayer was calculated from tortilladesorption isotherms by using both, GAB equation and the mini-mum integral entropy criteria.

2. Materials and methods

2.1. Materials

Labeled [2,2,3-2H3]acrylamide was purchased from SigmaAldrich (St. Louis, MO). All other chemicals were analytical gradeand purchased from Sigma (St. Louis, MO) or Merck (Darmstadt,Germany). Soybean oil was obtained from local supermarkets inQuertaro, Mxico.

2.2. Elaboration of nixtamalized corn our and tortilla chips

Nixtamalized corn our was prepared with commercial cornvariety named Pioneer 30P16 and commercial lime (Ca(OH)2) (ElTopo, Monterrey, N.L. Mexico), commonly used in the tortillaindustry. This our was prepared by cooking (8 kg) of whole cornkernels in a solution of 16 l of water with 80 g of Ca(OH)2, corre-sponding to 1.0 g/100 g of lime relative to the corn weight used.The corn was boiled in an aluminum pan for 25 min and steepedfor 16 h at room temperature (22 1 C). The steep liquor wasremoved. The cooked corn was washed with 16 l of water, thenground into corn dough (FUMASA, M100, Quertaro, Mxico),and nally dehydrated using a ash type dryer (Cinvestav-AV,M2000, Quertaro, Mxico). The drying conditions were adjustedto have 250 C inlet air temperature and 90 C to the exhaust airto avoid burning the material. Before storage, the nixtamalizedcorn our was milled using a hammer mill (PULVEX 200, MxicoDF) equipped with a 0.5 mm screen.

For tortilla chips elaboration, nixtamalized corn our (100 g)was rehydrated with enough water (118 mL) to provide fresh

2 R.M. Delgado et al. / Journal ofWater desorption isotherms were determined using a staticequilibrium method. Tortilla samples (4.0 g) (aw = 0.98) wereweighed in triplicate into standard weighing dishes with a circularsection on the bottom. Samples were placed in separate desiccatorscontaining saturated salt slurries in the range of water activityfrom 0.11 to 0.85 using the aw values reported by Labuza et al.(1985). The samples were held at 25, 30, and 35 C until equilib-rium was reached. Values of water activity were generated usingequations reported in the same paper. A small amount of toluenewas placed in each desiccator to prevent the growth of fungi. Theequilibrium condition was attained within 710 days, when thedifferences among two consecutive weights were within 0.001 g.The dry matter content of the tortilla was determined by the dry-ing in a conventional oven at 105 C for 24 h. The GuggenheimAndersonDe Boer (GAB) equation was used in modeling watersorption (Quirijns et al., 2005):

M M0Ckaw1 kaw1 kaw Ckaw 1

where aw is water activity; M is water content of the sample on drybasis;M0 is the monolayer water content; C is the Guggenheim con-stant, given by C = c0exp (hm hn)/RT; where c0 is the equation con-stant; hm is the heat of sorption of the rst layer; hn is the heat ofsorption of the multilayer; R is the gas constant; T is the absolutetemperature; and k is the constant correcting properties of multi-layer molecules with respect to bulk liquid, and given by k = k0exp(h1 hn)/RT; where k0 is the equation constant; h1 is the heat of con-densation of pure water. The parameters values of GAB equation(M0, C and k) were estimated by tting the mathematical modelto the experimental data, using non-linear regression with the Kale-idagraph 4.0 package (Synergy Software, Perkiomen, USA). Good-ness of t was evaluated using the average of the relativepercentage difference between the experimental and predicted val-ues of the moisture content or mean relative deviation modulus (P)dened by the following equation:

P % 100N

XNi1

jMei McijMei

2

where Mei is the moisture content at observation i; Mci is the pre-dicted moisture content at that observations; and N is the numberof observations. It is generally assumed that a good t is obtainedwhen P < 10% (Lomauro et al., 1985).

2.4. Determination of minimum desorption integral entropy

The determination of the integral (enthalpy and entropy) ther-modynamic properties, and the water activity-temperature condi-tions where the tortilla minimum integral entropy occurred,considered as the point of maximum storage stability (monolayervalue), was established as indicated by Pascual-Pineda et al.(2013) and Vigan et al. (2012). These authors have provided athorough description of the procedure followed and equationsused for this purpose. Briey, the integral enthalpy changes(DHint)T (J/mol) at the watertortilla interface and, at differentstages of the adsorption process, were determined using the equa-tion of Othmer (1940).

d ln Pvd ln P0v

HvTHovT

3

where the desorbed substance is water; Pv (Pa) is the vapor pressureof water over the adsorbent; P0v (Pa) is the vapor pressure of purewater at the temperature of sorption; Hv(T) (J/mol) is the integralmolar heat of sorption, and H0v (T) (J/mol) is the heat of condensa-tion of pure water. Since all these terms are temperature-depen-dent, the equation can be integrated:

Engineering 143 (2014) 17lnPv HvTHovT

/

lnP0v A 4

ture force was evaluated in freshly prepared samples. The test

70 eV.

Foodwhere A is the adsorption constant, and / (J/mol) is the pressure ofdiffusion or surface potential. A plot of lnPv versus lnP

ov gives a

straight line if the ratio HvT=H0vT is constant within the rangeof temperatures used.

The molar integral enthalpy (DHint)T can be calculated using Eq.(5), at a constant pressure of diffusion (Nunes and Rotstein, 1991):

DHintT HvTHovT

1

/

HovT 5

/ lapla RTWapWv

Z aw0

Md ln aw 6

where lap (J/mol) is the chemical potential of the pure adsorbent;la (J/mol) is the chemical potential of the adsorbent participatingon the condensed phase; Wap (g/mol) is the molecular weight ofthe adsorbent, andWv (g/mol) is the molecular weight of the water.

By calculating Hv T=H0vT from Eq. (4) and substituting it intoEq. (5) it becomes possible to calculate the integral enthalpy at dif-ferent temperatures, provided that a good means of estimatingH0vT is available, such as that proposed by Wexler (1976):H0vT J=mol K 6:15 104 94:14T 17:74 102T2

2:03 104T3 7Using the values obtained for (DHint)T changes, the molar inte-

gral entropy (DSint)T can be estimated using the followingequation:

DSintT S1 SL DHintT

T R ln aw 8

where S1 S=N1 (J/mol K) is the integral entropy of water desorbedin the foodstuff; S (J/mol K) is the total entropy of water desorbed inthe foodstuff; N1 is the moles of water desorbed in the foodstuff,and SL (J/mol K) is the molar entropy of pure liquid water inequilibrium with vapor.

2.5. Storage of tortilla before frying

Since water retention capacity in tortillas is poor, the moistureloss in tortilla after processing is often caused by starch retrograda-tion. Taking advantage of this phenomenon, tortilla samples wereleft at room temperature (25 C) during 4 h to reduce their aw from0.98 to 0.85 and therefore, reducing the moisture content and pre-venting spoilage reactions during the time that the samplesreached the desired aw. The tortilla pH was monitored in order toconrm that acrylamide levels were not affected by pH changesduring storage. Duplicate samples containing ca. 5 g of tortillawere placed in desiccators containing the same saturated salt slur-ries referred in Section 2.3 at 30 C. Tortilla samples were with-drawn at day 5 to prepare tortilla chips.

2.6. Color determination in tortilla chip samples

Color changes were determined using a colorimeter MiniScanXE, model 45/0-L (Hunter Associates Laboratory, 11491 Sunset HillRd., Reston, VA, U.S.A.). Total color differences (DE) at the differentperiods of time were calculated from the determined CIELAB L* a*

b* values according to Hunter (1973): DE = [(DL*)2 + (Da*)2 +(Db*)2]; where L* = brightness or lightness (100 = perfect white,to 0 = black); a* = greenness/redness [negative (green) to positive(red)]; b* = yellowness/blueness [negative (blue) to positive(yellow)]; DL*, Da*, and Db* = absolute differences of the valuesbetween the reference tile (white porcelain) and sample values;

R.M. Delgado et al. / Journal ofDE = total difference between reference and sample color. Thereference values (calibration) were: L* = 92.22, a* = 0.82 andb* = 0.62.Quantication of acrylamide was carried out by preparing stan-dard curves of this compound. Acrylamide content was directlyproportional to the acrylamide/internal standard area ratio(r = 0.999, p < 0.0001). The coefcients of variation at the differentconcentrations were lower than 10%.

2.9. Statistical analysis

All results were expressed as mean SD values (n = 3). Whensignicant F values were obtained, group differences were evalu-was carried out using a 2.03 mm diameter stainless-steel probeand a platform accessory with a hollow cylindrical base with33.5 and 10 mm external and internal diameters, respectively.The probe traveled at a velocity of 10 mm/s until it cracked thesample (distance 6 mm).

2.8. Acrylamide determination in tortilla chips

Acrylamide was analyzed as the stable 2-bromopropenamidederivative by gas chromatographymass spectrometry (GCMS)using a combination of the methods of Andrawes et al. (1987)and Castle et al. (1991) as described previously (Salazar et al.,2012). Tortilla chips were ground in a mortar lab and powderedsamples (0.8 g) were successively weighed in centrifugal tubes,spiked with 20 lL of internal standard solution (0.5 mg/mL of deu-terium-labeled [2,2,3-2H3]acrylamide in acetonitrile), and stirredwith 8 mL of distilled water and 10 mL of n-hexane at roomtemperature for 5 min. After centrifugation at 2000 g for10 min, organic phases were removed. Co-extractives from super-natants were precipitated with 30 lL of carrez I and 30 lL of carrezII solutions. Later, supernatants were centrifuged at 2000 g for5 min and ltered. These extracts (4 mL) were treated with 0.45 gof potassium bromide, 200 lL of sulfuric acid (10 mL/100 mL),and 300 lL of potassium bromate solution (0.1 mol/L). After 1 hin the dark at 4 C, the bromination reaction was terminated byadding of 1 mol/L sodium thiosulfate until solutions becamecolorless, and solutions were extracted with 5 mL of ethylacetate/hexane (4:1). Organic layers were recovered after centrifu-gation at 2000 g for 10 min, and were dried with sodium sulfateand evaporated to dryness under nitrogen. Each sample wasdissolved in 50 lL of ethyl acetate, treated with 25 lL oftriethylamine, and analyzed by GCMS. The ions monitored forthe identication of the analyte, 2-bromopropenamide, were[C3H4NO]+ = 70, [C3H479BrNO]+ = 149, and [C3H481BrNO]+ = 151,using m/z 149 for quantication. The ions monitored for the iden-tication of the corresponding derivative 2-bromo[2H2]propena-mide were [C22H2H81Br]+ = 110 and [C32H2H281BrNO]+ = 153,using m/z 153 for quantication.

GCMS analyses were conducted with a Perkin Elmer GCClarus 500 coupled with a Perkin Elmer Clarus 560 MSD (MassSelective Detector-Quadrupole type). In most experiments, a30 m 0.32 mm i.d. 0.25 lm Elite-5MS capillary column wasused. Working conditions were as follows: carrier gashelium (1 mL/min at constant ow); injector, 250 C; oventemperature: from 50 (10 min) to 240 C at 5 C/min and then to300 C at 10 C/min; transfer line to MSD, 280 C; ionization EI,2.7. Texture determination in tortilla chips

The fracture force of the tortilla chips was evaluated using theTexture Analyzer TA-XT2 (Texture Technologies Corp., N.Y.). Frac-

Engineering 143 (2014) 17 3ated by the Tukey test. All statistical procedures were carried outusing the JMP 9.0 package (SAS Institute Inc., Cary, NC). The signif-icance level was p < 0.05 unless otherwise indicated.

3. Results and discussion

Fig. 1 shows the desorption isotherms of the tortilla used tomake tortilla chips at 25, 30 and 35 C. Sigmoidal isothermsdescribed as type by Brunauer et al. (1940) were determined. Sincedesorption is an endothermic process, the increment of the tem-perature at constant aw enhances the moisture desorbed fromthe tortilla. Similar desorption isotherms have been reported forstarchy products such as potato (McLaughlin and Magee, 1998),maize (Samapundo et al., 2007) and banana (Yan et al., 2008),chestnut our (Chenlo et al., 2011) and Japanese noodle (Inazuet al., 2001). Table 1 gives the estimated parameters obtained bytting the GAB equation to experimental data. As can be seen fromthe table, the GAB equation describes adequately the experimentaldata over the whole measured range of aw. Thus the values of r2

were very close to unit and the P value was610% under the studiedconditions, which it indicates a good t (Lomauro et al., 1985). Asexpected, maximum water desorption by strongly binding sites(monolayer) predicted for GAB equation decreased with increasingtemperature. The moisture content in the monolayer (Mo) was10.12, 9.46 and 8.96 g water/100 g dry solid (corresponding to awof 0.32, 0.31 and 0.30) for 25, 30 and 35 C, respectively. It isassumed that the value of Mo is a parameter for estimating theamount of water bound to specic polar sites in dehydrated foodand in this water content; a storage product should be stableagainst degradation reactions (Rahman and Labuza, 1999). Theconstant C is a measure of the attraction force between the waterand the sorption sites. The values obtained are higher than thosereported by Palou et al. (1997) in commercial tortilla chips. In thisstudy, the k values increased with increasing temperature. Thevalue of k provides a measure of the interaction of the molecules

the energy value of the molecules in the monolayer and the liquidwater. When k = 1, the properties of the water present in the mul-tilayer are similar to those of the free water (Prez-Alonso et al.,2006).

Variations in the differential and integral entropy with respectto aw of the tortilla used to make tortilla chips are represented inFig. 2. The intersection of the curves is found in the water contentand aw corresponding to the minimum integral entropy. It could beobserved that as tortilla desorbed moisture, their integral entropyfell to a minimum. Integral entropy can be directly related to theorderdisorder of water molecules sorbed on food, and thereforeit is a useful function to study the effect of drying method on thestability of the product. The minimum integral entropy is expectedwhere strong bonds between the adsorbate and the adsorbent are(Nunes and Rotstein, 1991), and hence water molecules are lessavailable to participate in spoilage reactions. This point has beenproposed as the most suitable for storage and it is considered asthe water content corresponding to the monolayer (Domnguezet al., 2007; Hill et al., 1951).

4 R.M. Delgado et al. / Journal of Food Engineering 143 (2014) 17in the multilayers with the adsorbent, and tends to fall between

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1a

w(-)

Moi

sture

con

tent

(g H

2O/1

00 g

d.s.

)

Fig. 1. Moisture desorption isotherms of tortilla used to make tortilla chips at 25(d), 30 (h) and 35 (4) C. The continuous lines are the tted points using the GABmodel.

Table 1Estimated GAB parameters for tortilla desorption isotherms.

25 C 30 C 35 C

Mo (g H2O/100 g d.s.) 10.11 9.46 8.96C 14.67 15.72 14.26

k 0.66 0.67 0.70r2 0.99 0.99 0.99P (%) 0.96 1.36 1.58Although the value of minimum entropy may be unique, thereare food products with zones in which this minimum does not varyappreciably in a dened range of moisture. As it can be observed inFig. 2, the minimum integral entropy at 30 C for tortilla corre-sponds to aw value of 0.53 (13.20 g water/100 g dry solids). Itshould be noted that although the minimum integral entropyoccurs in a given aw, this minimum does not vary appreciably ina dened range of water activities (0.450.62). Differential entropyhad a minimum at 7.3 g water/100 g dry solids (aw = 0.18). Thisparameter does not mean order or disorder of the total system.The differential entropy represents the algebraic sum of the inte-gral entropy at a particular hydration level, plus the change oforder or disorder after new water molecules were desorbed bythe system at the same hydration level. If the values of moisturecontent corresponding to minimum integral entropy and mini-mum differential entropy are different, this particular hydrationlevel at the minimum differential entropy cannot be consideredas the maximum stability point, because not all available activesites have been occupied at that particular water content, andtherefore it is possible to obtain after this point lower differentialchanges that provide a better ordering of the water molecules des-orbed on food (Beristain et al., 2002).

The minimum integral entropy approach has been veried inseveral studies (see for example: Beristain et al., 1994; Bonillaet al., 2010; Carrillo-Navas et al., 2011; Rascn et al., 2011).

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0-60

-40

-20

0

20

40

Entro

py (J

/ mol

K)

aw

(-)Fig. 2. Differential (j) and integral (s) entropy changes as a function of wateractivity for tortilla at 30 C.

On the other hand, frequently the thermodynamic analysis isnot in accordance with the monolayer obtained with the GABmodel (Beristain and Azuara, 1990; Beristain et al., 2002; Nunesand Rotstein, 1991). In this study, the minimum integral entropypoint is higher than the calculated Mo value with GAB equation.Since the BET theory, which assumes that the surface of the adsor-bent is energetically uniform and does not take into account hori-zontal interactions between the molecules in the adsorbed layer(Dollimore et al., 1976), is also present in the GAB equation, theGAB monolayer concept has a limited applicability.

In this context, the Maillard reaction (main pathway acrylamideformation pathway) is a reaction that requires the interaction oftwo reagents. Therefore, those factors which affect the concentra-tion and mobility of the reactants will have a signicant impacton the reaction rate (Bell, 2007). Thus, the plasticizing effect ofwater signicantly inuences on the rate of the Maillard reaction(Bell et al., 1998; White and Bell, 1999). In the minimum integralentropy point, water is less reactive to carry out spoilage reactions

formed at temperatures below 100 C, an increase on acrylamideformation is observed when frying is carried out at low aw wherea rapid increase of the internal temperature of the food occurs(Mestdagh et al., 2006).

Foods are heterogeneous and dynamic mixtures of macromole-cules, solvent and solutes, therefore, the positive or negative corre-lation between water activity and acrylamide content are probablydue to differences in composition and structure of used food sys-tems. According to Viveros-Contreras et al. (2013) and Viganet al. (2012), food with the same chemical composition and differ-ent microstructure could show different moisture sorption behav-ior, and therefore the aw where spoilage reactions occurs, could bemodied. This fact suggests that the microstructure and physicalstructure of the water in the food is able to restrict the mobilityof the precursors responsible for the formation of acrylamide to aspecic humidity level during thermal processing and it explainsthe decrease of the acrylamide content of tortilla elaborated fromtortillas equilibrated at the aw corresponding to minimum integralentropy zone. Thus, the inuence of aw on the acrylamide forma-tion will be different depending on each particular food system.

Texture and color are considered the most important parame-ters of quality and acceptability of fried products. Fig. 4 showsthe effect of tortilla water activity on the parameters of tortillachips above mentioned. The color has been correlated with theacrylamide generation in thermally processed foods (Gkmenand Senyuva, 2006; Lukac et al., 2007; Majcher and Jelen, 2007;Pedreschi et al., 2007). In this study, the appearance of the tortillachips (Fig. 4a) remained constant in the range of aw from 0.43 to0.84 and increased proportionally to aw reduction (0.430.11). It

R.M. Delgado et al. / Journal of Food Engineering 143 (2014) 17 5in the food, so the mobility of acrylamide precursors is reduced,affecting it formation during thermal processing.

Fig. 3 shows acrylamide content in tortilla chips prepared fromtortillas stored at different water activities. All water activitiesinduced the formation of acrylamide in the tortilla chips, but torti-lla chips made of no-stored tortilla (aw = 0.98) always containedless acrylamide (688.55 65 lg/kg) than those made of tortillastored at different water activities (p < 0.05). This suggests a dilu-tion of acrylamide precursors caused by high moisture content.This effect is similar to that reported for the Maillard reaction inwhich a reduction in reaction rate is observed in aw close to 1because of the reagent dilution as well as the inhibitory effect thatwater exerts in this reaction (Ames, 1990; Acevedo et al., 2008).Thus, a reduction in the acrylamide content was observed in theminimum integral entropy point. The acrylamide content of tortillachips prepared from tortilla stored at aw = 0.53 (741.85 88 lg/kg)was similar to those prepared from no-stored tortilla (p < 0.05).This point is characterized by strong waterfood interactions caus-ing a reduction in the diffusion and mobility of the system andaffecting to the development of chemical reactions during fryingsuch as the acrylamide formation. On the other hand, an increaseon acrylamide content on tortilla chips proportional to reductionof aw (0.430.11) on tortilla was observed. This increase may belinked to the concentration of the precursors as well as the evapo-ration of water which takes place during the start of frying. At thisstage, the temperature of the tortilla does not exceed 100 C until acertain amount of water is evaporated. Since acrylamide is not

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

300

600

900

1200

1500

Acr

ylam

ide

(g/

kg)

aw

(-)Fig. 3. Effect of tortilla water activity on acrylamide content in tortilla chips fried at180 C for 25 s.should be emphasized that this change was observed in the wateractivity zone corresponding to the minimum integral entropy. Thefracture force (Fig. 4b) showed similar values over the whole rangeof water activities studied. The obtained fracture force values areclose to those found (8.739.63 N) by Tseng et al. (1996), and

42

48

54

60

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

3

6

9

E (-

)

b

aw

(-)

Frac

ture

forc

e (N

)

aFig. 4. Effect of tortilla water activity on: color (a) and fracture force (b) in tortillachips fried at 180 C for 25 s.

Waals adsorption of gases. J. Am. Chem. Soc. 62 (7), 17231732.

FoodCarrillo-Navas, H., Gonzlez-Rodea, D.A., Cruz-Olivares, J., Barrera-Pichardo, J.F.,Romn-Guerrero, A., Prez-Alonso, C., 2011. Storage stability andphysicochemical properties of passion fruit juice microcapsules by spray-drying. Revista Mexicana de Ingeniera Qumica 10 (3), 421430.higher than those (5.336.17 N) described in commercial tortillachips (Lujan-Acosta and Moreira, 1997).

4. Conclusions

The effect of aw on the acrylamide content of tortilla chips couldbe related to the moisture sorption behavior. The minimum inte-gral entropy in conjunction with the corresponding aw obtainedfrom the desorption isotherm provided a reliable indicator toestablish the most appropriate moisture conditions to obtain torti-lla chips with reduced level of acrylamide when tortilla was dehy-drated. Furthermore, the acrylamide content reduction was similarwhen tortilla chips were prepared from non-stored tortilla with amoisture content corresponding to water activities close to 1. Itshould be noted that a partial tortilla dehydration to a water activ-ity different to the minimum integral entropy results in tortillachips with higher acrylamide content. Although a detailed sensoryevaluation of tortilla chips prepared is needed, preliminary resultsshowed that the products obtained from tortilla stored in the min-imum integral entropy aw did not show any signicant change inthe appearance of the food product. Therefore, the results suggestthat the control of aw, moisture content and the physical state ofwater in the tortillas are factors to reduce the formation of acryl-amide during processing of tortilla chips. This is of the upmostimportance, because it provides the food manufacturer with infor-mation of which aw is more likely to get a low acrylamide contentproduct over a wider range of processing conditions.

Acknowledgments

We are indebted to Juan Veles, Edmundo Gutierrez, CarlosAlberto vila, Araceli Mauricio and Veronica Flores from CINVE-STAV Quertaro for their technical assistance.

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R.M. Delgado et al. / Journal of Food Engineering 143 (2014) 17 7

Effect of water activity in tortilla and its relationship on the acrylamide content after frying1 Introduction2 Materials and methods2.1 Materials2.2 Elaboration of nixtamalized corn flour and tortilla chips2.3 Water desorption isotherms2.4 Determination of minimum desorption integral entropy2.5 Storage of tortilla before frying2.6 Color determination in tortilla chip samples2.7 Texture determination in tortilla chips2.8 Acrylamide determination in tortilla chips2.9 Statistical analysis

3 Results and discussion4 ConclusionsAcknowledgmentsReferences