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Rheological and thermal properties of masa as related to changes in cornprotein during nixtamalization

Adriana Quintanar Guzmn a,*, Mara Eugenia Jaramillo Flores a, Javier Solorza Feria b,Mara Guadalupe Mndez Montealvo c, Ya-Jane Wang c

aDepartamento de Graduados en Alimentos, Escuela Nacional de Ciencias Biolgicas, Instituto Politcnico Nacional, Carpio y Plan de Ayala, Col. Plutarco E. Calles, CP 11340,Deleg. M. Hidalgo, Mxico D.F., MexicobCentro de Desarrollo de Productos Biticos, Instituto Politcnico Nacional, Km 6 Carretera Yautepec-Jojutla, Calle Ceprobi 8, Col. San Isidro, C.P.62731 Yautepec, Morelos, MexicocDepartment of Food Science, University of Arkansas, 2650 N. Young Ave., Fayetteville, AR 72704, USA

a r t i c l e i n f o

Article history:Received 21 April 2010Received in revised form11 October 2010Accepted 4 November 2010

Keywords:NixtamalProteinecalcium interactionsRheologyProtein cooking

a b s t r a c t

Traditional nixtamalization process produces a masa (dough) with appropriate cohesiveness andadhesiveness. Masa is considered as a network of solubilized starch polymers with dispersed, uncookedand swollen starch granules, cell fragments, proteins and lipids. In this work, the influence of proteins onthe masa viscoelastic behavior was studied in corn kernels under different nixtamalization conditions.Scanning electron microscopy, SDS-PAGE, differential scanning calorimetry and rheological analysis wereused to characterize the corn samples. The micrographs showed that the nixtamalization modified theshape of the starch granules and protein bodies, but no changes in appearance were observed whenprotein was removed. SDS-PAGE showed that corn proteins polymerized during cooking. Lime promotedboth calciumeprotein and proteinecalciumeprotein interactions mainly by calcium bridges, which weredifficult to disrupt and increased the protein thermo-resistance. In the absence of lime, corn proteinspolymerized mainly by disulfide bond cross-linking. Thermal analysis (DSC) indicated that the gelati-nization temperature increased in lime-treated samples compared to control samples. Rheologicalstudies showed that the corn protein exhibited greater influence on gel strength by enhancing the elasticcharacter of the samples (G0). These results suggested that polymerized corn proteins stabilized the gelstructure, which in consequence influenced the viscoelastic behavior of masa.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

Nixtamalization is the process of cookingmaize grains in a limesolution, soaking and washing them, to obtain nixtamal. Thisnixtamal is then stone-ground to obtain nixtamal dough ormasa. Avariety of products (e.g. tortilla and corn chips, tamales, tostadas,tacos, enchiladas, atoles, etc.) are obtained frommasa and tortilla isthemost popular one. Lime cooking alters themicrostructure of theoutermost layers of maize pericarp, which shows a corrugated-likestructure. Surface materials dissolve partially and this facilitatespericarp removal during washing. The aleurone layer remainsattached to the endosperm; it behaves as a semi-permeable enve-lope andmight contribute to reduce protein losses.Most of the germis retained during the nixtamal and tortilla making process. Corn

boiling in lime causes removal of starch granules, so that the soft(inner) endosperm is greatly altered: starch arrangement becomesirregular and some fibrils connect the dispersed starch granules(Paredes-Lpez and Saharpulos, 1982). Important structural alter-ations, caused by heat denaturation of proteins, cross-linkages anddisruption of the tertiary structure of proteins occur. The endospermproteins remain attached to the starch granules; lime cookingchanges the physical appearance of protein bodies.

Alkaline cooking and the steeping step cause water and calciumto be taken up by the grain. The role of lime is important, as itallows faster water absorption and distribution throughout thegrain components and it modifies the outer layers, so that thepericarp fraction becomes gummy and sticky. The presence ofgerm, which is not lost during nixtamalization, gives moremachinability to the masa, with a higher tolerance to mixing andless susceptibility to breakdown. The traditional process results inmasa with desirable properties of cohesiveness and adhesiveness.During steeping, the grains absorb water and are softened due tothe distribution of water (Martnez-Bustos et al., 2001). Lime

* Corresponding author. Tel.: 1 479 5756824.E-mail addresses: [email protected], [email protected]

(A.Q. Guzmn).

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Journal of Cereal Science 53 (2011) 139e147


cooking alters starch crystallinity, and reassociation of starchmolecules during steeping is important to develop the rheologicalproperties of nixtamal dough. Grinding disrupts the grain structure,dispersing cellular components and starch polymers. Masa can beconsidered to be a network of solubilized starch polymers withdispersed, uncooked and swollen starch granules, cell fragments,proteins and lipids (Gmez et al., 1990).

Oates (1990) suggested the possible existence of peptide cross-links within the amylopectin fraction, which are responsible formaintaining the structure of gelatinized starch granules. Han et al.(2002a,b) studied the influence of corn starch granule-boundassociate proteins on pasting (RVA measurements) and viscoelasticproperties (dynamic rheological measurements) of gelatinized cornstarch. They reported that starch granule-bound associate proteinsreduce paste viscosity and the elastic properties of the starch pasteswere reduced by removal of such compounds.

According to Duodu et al. (2002), zein polymerizes duringcooking. Furthermore, it has been proposed (Ezeogu et al., 2005,2008) that prolamin polymers of Mr> 100 kDa are formed inmaize on cooking. More recently, Emmambux and Taylor (2009)studied the properties of heat-treated sorghum and maize mealand their prolamin proteins and reported that the highly poly-merized zein occurs as a result of extensive disulfide bonding ofzein monomers during cooking.

2. Experimental

2.1. Materials

The commercial corn variety named Pioneer 30G54 coming fromValle de Santiago region (harvested in the provinces of Mxico,Guanajuato and Michoacn) was used. The proteolytic enzyme,thermolysin from Bacillus thermoproteolyticus rokko [E.C.3.4.24.27]was sourced from SigmaeAldrich (St. Louis, MO). The alkaline treat-ment was done with commercial lime [calcium hydroxide; purity88 2% (cal pirmide) Grupo Bertrn, Mexico City], commonly usedin the tortilla industry. The rest of the reagentswere analytical grade.

2.2. Corn cooking conditions

Lots of 500 g of corn in 1.5 L of water were nixtamalized, using1% (w/w) of calcium hydroxide (lime)/weight of corn. Different lotsof samples were subjected to a thermalealkaline treatment at 90 Cfor different times, with lime (nixtamalization) making a total offive treatments, with processing times of 20, 30, 40, 90 and150 min. Another series of non-nixtamalized samples (controls)were cooked for the same cooking times. Corn nixtamalizationcooking time varies according to the final products, i.e., standardcooking time to produce tortillas is 20e40 min, prolonged cookingtime (90e150 min) is used to produce corn chips (Gmez et al.,1992). All lots of cooked corn were then left to rest for 14 h,which is the traditional procedure. Then, the samples were washedthoroughly with deionizedwater until the rinsing waters pHwas 7.All washed corn samples were milled in a Cyclone mill (UDYCorporation, Fort Collins, CO.), then dried in an oven at 40 C andsieved through a 150-mm sieve.

2.3. Microscopy of corn kernels

Dried corn samples were sputter-coated with gold using thedeionizer equipment (DESK II, Denton vacuum), and micrographswere obtained at 15 kV and 1000, using a scanning electronmicroscope JEOL model JSM-5900LV. SEM micrographs wereobtained from nixtamalized and control corn kernels cooked at90 C for different cooking times.

2.4. Treated corn kernel protein hydrolysis

Treated corn kernel protein was removed using a proteolyticenzyme before sample preparation, using the method described byMu-Foster and Wasserman (1998). Before its protein hydrolysis,corn kernels were defatted by washing them with hexane at roomtemperature. Remaining materials from corn kernels were recov-ered by centrifugation (7500g for 15 min), and this procedurewasrepeated three times. The remaining solvent was removed bystanding the corn kernels in the fume hood overnight at roomtemperature. Dry and defatted corn kernels (50 g) were mixed with3300 units of thermolysin in the presence of 5 mM calcium chloridein 1 L of aqueous solution. The mixture was kept in a water bath at60 C for 4 h and gently hand-mixed at 30-min intervals. Aftercooling down to room temperature, the mixture was filtered andthe enzyme action was terminated by washing the solids withdeionized water. This procedure was repeated five times, and then,the protein hydrolyzed corn kernels were dried in an oven at 40 Cfor 24 h and sieved through a 150-mm sieve.

2.5. Electrophoresis (SDS-PAGE)

The protein content of dried samples was determined using theCoomassie Plus1 protein assay reagent kit (Pierce Biotechnology)with BSA as standard (Bradford method) (Bio-Rad Corporation,Hercules, CA).

The protein composition of treated corn kernels was analyzedusing SDS-PAGE under reducing (heat and 4% 2-mercaptoethanol[2-ME]) and non-reducing (neither heat nor 2-ME) conditions. SDS-PAGE was done on a vertical gel system Mini-Protean 3 (Minigel,Bio-Rad Corporation, Hercules, CA) using a 15% acrylamide gel witha 4% stacking gel. Samples of freeze-dried flour of nixtamalized andcontrol samples of approximately 0.20 mg protein were added into0.3 mL of buffer (0.01% bromophenol blue, 10% glycerol, 0.625 MTriseHCl, 10% SDS) and reduced with 4% 2-ME to obtain a finalconcentration of 10 mg protein/mL. Under the reducing conditions,samples were placed in a boiling water bath for 5 min, and thencentrifuged (Eppendorf centriguge model 5415D, Hamburg,Germany) at 13,000 rpm (15,700g) for 15 min. For both reducingand non-reducing conditions, 20 mL of supernatant was loaded intosample wells along with 5 mL of standards with molecular massranging from 10 to 250 kDa (BioRad, Richmond, CA) to determinethe molecular masses (Mr) of polypeptide bands of the sample.Electrophoresis was performed at 200 V for approximately 30 minor when the running front reached the end of the gel. Gels werestained with Coomassie Brilliant Blue R-250 staining solution(BioRad, Richmond, CA) and then destainedwith 40%methanol and10% glacial acetic acid. All gel tests were performed in triplicate. Themolecular masses of proteins were estimated from the logelog plotof relative mobility versus molecular mass of protein standards.

2.6. Thermal analysis

The peak transition temperatures (Tp) and enthalpies (DH) of allsamples (before and after protein removal) were determined usinga differential scanning calorimeter (DSC) (Pyris-1, PerkineElmerCorp., Norwalk, CT). The DSC was calibrated with indium, and datawere analyzed using the Pyris software. Approximately 100 mg offreeze-dried sample was mixed with deionized water to obtaina mixture of 50% moisture content and equilibrated at roomtemperature for 1 h, to make sure that hydration had been carriedout (Cameron and Wang, 2006). The moisture content for thethermal analysis was selected according to Sahai et al. (2001), whoreported that the properly processed nixtamal and masa havemoisture contents of 46e51%. Samples of approximately 8 mgof the

A.Q. Guzmn et al. / Journal of Cereal Science 53 (2011) 139e147140


equilibrated mixtures were placed in pre-weighed aluminum pans.The panswere hermetically sealed and scanned from25 to 120 C at10 C/min. All measurements were carried out in triplicate.

2.7. Viscoelastic behavior

The viscoelastic properties of all samples subjected to differentcooking times (before and after protein removal) were studied witha stress controlled AR-2000 rheometer (TA Instruments, NewCastle, DE), used in feedback on its strain controlled mode,assembled with a parallel plate system (crosshatched surface) witha diameter of 60 mm and a sample gap of 1 mm (Quintanar-Guzmn et al., 2010). Approximately 5 g of dried sample wasadded with water to reach 50% moisture content and manuallymixed to obtain a masa (dough). The masa was rested in a plasticcontainer at room temperature for 24 h. Thereafter, a portion (3 g)of masa was compressed between two flexible plastic sheets untilreaching approximately 60 mm in diameter and then loaded ontothe lower plate of the rheometer. The heating program selectedstarted at 25 C (first set of measurements) with a heating rate of7 C/min until reaching a final temperature of 60 C (second set ofmeasurements). The parallel plates were covered with mineral oilto avoid water evaporation during the 2nd test at 60 C. Themeasurement temperatures selected (25 and 60 C) were similar tothose commonly used in the tortilla industry. All measurementswere done in triplicate. To determine the linear viscoelastic region(LVR), strain amplitude sweeps were run from 0.01 to 3% ata frequency of 1 Hz. Once the LVR was determined, the rheometerwas programmed for running frequency sweeps (0.1e100 Hz) ata constant strain value of 0.3% to obtain the storage modulus (G0),the loss modulus (G00), and the complex viscosity (jh*j).

2.8. Statistical analysis

The Minitab statistical software, version-Minitab 15 (MinitabInc., State College PA) was used to analyze the data by applying oneway analysis of variance at a significance level of 5% (P 0.05), andsignificant differences among means were defined by using theStudenteNewmaneKeuls test (Steel and Torrie, 1980).

3. Results and discussion

3.1. Scanning electron microscopy of whole and protein removedcorn kernels

Figs. 1 and 2 show the SEM photographs of corn cooked kernelswith lime (nixtamalized) (Fig. 1) and without lime (control) (Fig. 2)for 30, 40 and 90 min before and after protein removal. In nixta-malized samples (Fig. 1AeC) before protein removal, starch gran-ules were polygonal to round in shape and showed some adheredprotein bodies (arrows).

All samples showed some signs of damage on the surface of thegranules because of heat and alkaline treatment. Serious exocorro-sion of granular surface and even the destruction of such granuleswere observed on the sample extensively cooked (90 min). Afterprotein removal (Fig.1DeF) no protein bodies adhering to the starchgranule were observed.

Control samples before protein removal (Fig. 2AeC) showedstarch granules rounded in shape with some adhered proteinbodies and smoother surface than nixtamalized samples(Fig.1AeC). All of these samples showed less damage on the surfaceof the granules than nixtamalized samples. Starch granules of thesample cooked for 90 min showed the most severe signs of damageand erosion. These results suggest that the combination of heat andlime caused more erosion to the granules than heat solely. After

protein removal, the size, shape and apparent erosion of thegranules seem to be unaltered for nixtamalized (Fig. 1DeF) andcontrol samples (Fig. 2DeF). These results suggest that the presenceof protein was not affecting the granule appearance.

Smoother granules were observed in the control samples(Fig. 1AeC) when compared to the granules from the nixtamalizedkernels (Fig. 2AeC). It has been suggested that starch gelatinizationand consequently, granule swelling is decreased by calciumestarchinteractions (Robles et al., 1988).

3.2. Protein analysis SDS-PAGE

In thiswork, for SDS-PAGEunder reducing conditions, samples ofcontrol corn kernels (Fig. 3A) showed several groups of bands in theregions of 10e25 kDa and 50e75 kDa for all cooking times. Bandintensitywas apparently similar among all cooking times, except forsamples of 90 and 150 min cooking. For samples of nixtamalizedcorn kernels cooked for varying times (Fig. 3B), similar groups ofbands were observed with similar intensities among them. Undernon-reducing conditions, a completely different band pattern wasobserved (Fig. 3C and D) compared to reducing conditions (Fig. 3Aand B). Samples of control corn kernels (Fig. 3C), solely showedbands for bothuncookedand20 minof cooking samples (lanes2and3), which proved that proteins polymerized during cooking (longercooking times: 30e150 min) and were too large to pass through thepolyacryamide gel (15%). The rate of polymerization was greaterafter 30 min of cooking. The polymerizationwas probably the resultof cross-linking of proteins by disulfide bonds. The absence of thesebands in control samples after 30 min (Fig. 3C lanes 4e7) could beexplained because these disulfide bonds were destroyed underreducing conditions by the presence of 2-ME.When disulfide bondswereundisturbed (non-reducing conditions), no bandswere visible.

Under both reducing (Fig. 3B) and non-reducing conditions(Fig. 3D), samples of nixtamalized corn showed several groups ofbands in the region of 10e25 and 50 kDa (reducing conditions) and18e25, 40e50 and >75 kDa (non-reducing conditions) for allcooking times. These results indicate that lime promoted cal-ciumeprotein interactions by calcium bridges, which were difficultto disrupt and increased protein thermo-resistance. Proteinthermo-resistance was also observed from the results of the DSCanalysis of extracted proteins from control and nixtamalizedsamples (Table 1), where an increase of denaturation temperaturesand enthalpies from nixtamalized samples over all cooking timeranges was observed. Denaturation temperatures and enthalpiesfrom control samples were lower than those of the nixtamalizedones. Whether structural changes of proteins and their interactionwith calcium occur due to nixtamalization is not clear withoutfurther experimentation, but so far, it has been shown that there isan effect on their thermal properties.

It has been proposed (Emmambux and Taylor, 2009; Ezeoguet al., 2005, 2008) that zein polymers of Mr> 100 K are formed inmaize on cooking. This was based on the fact that with SDS-PAGE,there was a large increase in zein oligomers and monomers whenflours were cooked in the presence of 2-mercaptoethanol, and anincrease in the number of protein bands when samples werecooked under non-reducing conditions and then, subsequentlytreated with 2-mercaptoethanol.

Trejo-Gonzlez et al. (1982) reported that during the nixtamali-zation process, the content of albumins globulins, prolamin andglutelin fractionsdecreased. Similar resultswereobservedbyOrtegaet al. (1986), Rojas-Molina et al. (2008) and Vivas et al. (1987). Theseresearchers inferred that the reduced protein content after nix-tamalization is mainly due to protein changes in solubility.

Ortega et al. (1986) reported that changes in protein solubilitythat occurred during tortilla making, i.e., the protein components

A.Q. Guzmn et al. / Journal of Cereal Science 53 (2011) 139e147 141


soluble in 0.3 M NaCl (albumins, globulins, amino acids, and smallpeptides), were either lost during the process or converted intoan insoluble form. These researchers concluded that grain germproteins, are probably the main components that became insolubleor lost after thermoalkaline treatment. The zeins also becameinsoluble during the process. The solubility of zeins decreased 58%from the raw corn tomasa and during tortilla making. Hydrophobicinteractions, protein denaturation, and cross-linking of proteinswere probably responsible for changes in the solubility of thesefractions during processing (nixtamalization).

3.3. Thermal analysis

Results from thermal analysis of nixtamalized and controlsamples are shown in Table 1. Nixtamalized and control samples

cooked for 20e150 min, showed an endothermal transition at73e83 C and at 72e80 C, respectively. While samples cooked for90 min from both nixtamalized and control samples, showedendothermal transitions at 64e65 C.

Overall, the gelatinization temperatures and enthalpies werehigher in lime-treated samples compared to control samples. Thiseffect could also be due to interactions of starch chains withcalcium ions, producing structural changes from cross-linkedstarch, through calcium (Bryant and Hamaker, 1997; Gmez et al.,1992) that was disorganized in a broader temperature range.Results of this work are in agreement with those reported by otherresearchers (Gutirrez-Dorado et al., 2008). Retrogradation hasbeen used to describe starch structural changes following gelati-nization, and also as a stage of increased order (degree of crystal-linity) from initially dispersed starchmolecules (Atwell et al., 1988).

Fig. 1. SEM micrographs of corn kernels cooked with lime at 90 C for 30, 40 and 90 min (1000) before (AeC) and after (DeF) protein removal. The arrows show protein bodies.



In this study, samples of kernels treated with lime and control,cooked for 90 and 150 min showed a transition related to retro-gradation at about 65 and 60 C, respectively, which is related toamylopectin recrystallization (Seetharaman et al., 2002).

Yglesias et al. (2005) investigated the thermal properties ofground raw corn, freeze-dried nixtamal, and masa at differentcooking temperatures (86e96 C) and steeping times (3e11 h).These authors concluded that nixtamal DSC enthalpy was affectedmostly by steeping time, while masa enthalpy was affected exclu-sively by cooking temperature. Robles et al. (1988) and Sahai et al.(2001) suggested that endotherm peak temperatures and enthalpyvalueswere greatly influenced by steeping time, due to an annealingphenomenon that takes place close to gelatinization temperatures.

Starch gelatinization during alkaline cooking and steeping isrestricted by insufficient heat and moisture (Gmez et al., 1992)

and by amyloseecalcium interactions (Robles et al., 1988). Masacan be considered as a network of solubilized starch polymerscontaining uncooked, swollen, and dispersed starch granules. Inaddition, retrogradation of gelatinized starch granules takes placevery rapidly upon cooling cooked kernels. Annealing of starchoccurs in nixtamalized corn kernels (Gmez et al., 1990).Annealing takes place during steeping as starch is heated in excesswater, for a certain period of time at subgelatinization tempera-tures and thus, starch undergoes reorganization to a more orderedstructure (Gmez et al., 1992). It has been suggested (Qi et al.,2004; Tester et al., 2001) that annealing of starch causesa higher peak gelatinization temperature. This alteration withinthe starch crystallites of nixtamalized corn kernels, might explainthe increase in their gelatinization temperature over the cookingtime range (Table 1).

Fig. 2. SEM micrographs of control corn kernels cooked at 90 C for 30, 40 and 90 min (1000) before (AeC) and after (DeF) protein removal. The arrows show protein bodies.



3.4. Viscoelastic behavior

In Fig. 4, it is seen that the plots of the storage modulus (G0) andthe loss modulus (G00), are essentially flat over all of the appliedstrain range (0e3%), confirming clearly the linear viscoelasticregion (LVR), within the domain involving strain values less than1.0% deformation, i.e., no breakage point can be observed, whichwould otherwise indicate a sample structure disruption (Ferry,1980). In consequence, 0.3% strain was selected to carry out thefrequency sweeps. In all cases, the G0 (storage modulus) valuespredominated over all of the applied strain range, over those of G00

(loss modulus). The storage and loss moduli (G0 and G00) profiles ofall studied samples are shown in Fig. 5 as functions of frequency,using a deformation value within the linear viscoelastic region(0.3% strain). Samples of control corn kernels after protein removal(Fig. 5A and E) showed higher G0 values than those before proteinremoval at both measurement temperatures (25 and 60 C);a behavior which is typical of gel-like viscoelastic materials. Theseresults suggest that the absence of protein promoted a moreelastic structure due to the absence of proteinestarch interactionsall over the starch granules. Storage and loss moduli (G0 and G00), ofsamples cooked for 30, 40 and 90 min are shown in Fig. 5BeD andFeH. The storage modulus (G0) values of all samples subjected toa thermalealkaline treatment, before protein removal (Fig. 5

squares) were also higher than those of the control samples, forall frequency profiles, at both measurement temperatures (25 and60 C) (Fig. 5BeD and FeH). After protein removal, thermal-ealkaline treated samples (Fig. 5 circles) showed higher G0 valuesthan those of the control samples (Fig. 5 diamonds) over thecooking time range and at both measurement temperatures (25 Cand 60 C), except for samples cooked for 20 and 90 min, whichshowed similar G0 values when measurements were done at 60 C.

This rheological behavior suggests that when protein and limewere present, the networks formed by interactions among lime-polymerized proteins and starch during the heat treatment (cook-ing), were more elastic than those formed by starch-polymerizedproteins without lime. This effect was not observed in uncookedsamples (untreated corn kernels), where the proteins were native.

The loss modulus (G00) values of the thermalealkaline treatedcorn kernels before and after protein removal, were also higher atlonger than at shorter cooking times. This could be due to resultingdifferences in the volume and mobility of the swollen granules.Mondragn et al. (2006) studied the effect of lime on structuralmodifications of starch in nixtamalized maize, using dynamicrheometry. These researchers found that the storage modulus (G0)and the loss modulus (G00) behavior seemed to be lime dependent.This behavior was attributed to changes in the starch structure,due to Caestarch interactions that affected starch swelling and

Table 1Peak temperatures and transition enthalpies of corn samples cooked with lime and control, measured with DSC (mean SD, n 3)d.

Cookingtime (min)

First endothermic phase transition Second endothermic phase transition

Tp (C) DH (J/g, d.b.) Tp (C) DH (J/g, d.b.)

Lime Control Lime Control Lime Control Lime Control

0 e e e e e 71.7 0.3a e 9.4 0.1a20 e e e e 73.1 0.7a 72.3 0.1a 7.4 1.2a 8.5 0.1b30 e e e e 76.9 0.2b 73.5 0.3b 6.9 0.1b 8.6 1.2b40 e e e e 77.8 0.6c 74.1 0.1b 3.6 0.2c 8.6 1.4b90 64.0 0.4a 59.6 0.6a 0.8 0.06a 1.1 0.1a 81.7 0.1d 79.1 1.1c 3.6 0.1c 3.4 1.3c150 64.9 0.5a 60.2 0.6a 0.8 0.08a 0.9 0.1a 82.9 3.1e 80.3 2.9c 2.4 0.1d 3.1 1.4cd Values followed by the same letter in the same column are not significantly different (P> 0.05).

Fig. 3. SDS-PAGE corn kernels. Control: A and C. Cooked with lime: B and D. Mmarker, line 2 uncooked control corn, line 3 20 min, line 4 30 min, line 5 40 min, line6 90 min, and line 7150 min.



solubility. They found that the gel storage modulus (G0) of nixta-malized corn flour cooked for 30 min, exhibited the highest valuesat 0.2% lime treatment. On further lime addition, G0 tended todecrease to the values of the control flour. Also, the G0 values werehigher than those of the G00 values, indicating that, in the maize gel

produced, the elastic character had predominated. Some otherworks have also shown that lime concentration affects the thermaland rheological properties of calcium-treated starch, nixtamal,nixtamalized flour and tortillas (Bryant and Hamaker, 1997; Gmezet al., 1992).

Fig. 4. Strain sweep (frequency 1 Hz) of corn kernels cooked at 90 C for 0, 20, 90 and 150 min: with lime (triangles), with lime protein removed (circles), control (squares) andcontrol protein removed (diamond), uncooked (X) and uncooked after protein removal (*). Measurements were done at 25 C and 60 C. Filled symbols G0; open symbols G00 .



It looks as though the cross-linking of granules caused by thethermalealkaline treatment, favored a decreased capacity ofswelling and mobility, with an increase in the complex viscosityvalues [h*G*(complexmodulus)/w (frequency)] (data not shown).This system showed shear-thinning behavior, i.e., the viscositydecreased as the corresponding applied strain at different times(frequency) increased, due to increasing strain disorganizing themacromolecular arrangement inside the matrix.

These results are consistent with the viscoelastic behaviorpreviously observed during the frequency sweep, a pattern thatmight be attributed to the effect of lime and protein interaction.

The structure formed by the lime-polymerized proteinestarchnetwork was stronger than the one formed solely by polymerized

proteinestarch. The overall rheological results suggest that allsystems involved in this study behaved as gel-like viscoelastic mate-rials with predominance of the elastic character (G0) (Ferry, 1980).

Overall, from this work on maize masa, the results from scan-ning electron microscopy suggest that heat and lime combinedcaused more erosion to the starch granules than heat solely. Afterprotein removal, the size, shape and apparent erosion of thegranules seemed to be unaltered for all samples. Thus, the presenceof protein was probably not affecting the granule appearance.Electrophoresis (SDS-PAGE) indicates that lime promoted cal-ciumeprotein interactions by calcium bridges, increasing proteinthermo-resistance, which was also observed from results of theDSC analysis of extracted proteins from control and nixtamalized

Fig. 5. Frequency sweep (0.3% strain) of corn kernels cooked at 90 C for 0, 30, 40 and 90 min: with lime (triangles), with lime protein removed (circles), control (squares) andcontrol protein removed (diamond), uncooked (X) and uncooked after protein removal (*). Measurements were done at 25 C and 60 C. Filled symbols G0 , open symbols G00 .



samples. Thermal properties from control samples were lower thanthose of the nixtamalized ones. Further research is needed to showwhether structural changes of proteins and its interaction withcalcium occur due to nixtamalization, but so far, it has been seenthat there is an effect on its thermal properties.

All masa systems involved in this study, behaved as gel-likeviscoelastic materials with predominance of the elastic character(G0 >G00), polymerized corn proteins stabilized the gel structure,which in turn influenced the viscoelastic behavior of masa, in bothnixtamalized and control samples. It may well be that the inter-actions in lime-polymerized proteinestarch networkwere strongerthan those in polymerized proteinestarch.

Acknowledgments

Thanks are due to the Instituto Politcnico Nacional in Mexico.Jaramillo-Flores and Solorza-Feria thank to SIP-20100386, EDI/COFAA and SNI.

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