Journal of Cereal Science 48 (2008) 420e425www.elsevier.com/locate/jcs

Effect of nixtamalization on morphological andrheological characteristics of maize starch

Guadalupe Mendez-Montealvo a, Francisco J. Garcıa-Suarez a,Octavio Paredes-Lopez b, Luis A. Bello-Perez a,*

a Centro de Desarrollo de Productos Bioticos del IPN, Apartado Postal 24 C.P., 62731 Yautepec, Morelos, Mexicob Centro de Investigacion y Estudios Avanzados del IPN, U. Irapuato, Apartado Postal 629 C.P., 36500 Irapuato, Guanajuato, Mexico

Received 6 December 2006; received in revised form 21 September 2007; accepted 1 October 2007

Abstract

Nixtamalization of maize grain is an ancient process that until now is used for tortilla production. This thermal-alkaline process producesimportant changes in morphology and rheological characteristics of starch that is the major component of maize. The aim of this study wasto evaluate changes in the morphological and rheological properties of starch brought by nixtamalization of maize using image analysis anddynamic rheometry, respectively. Nixtamalized maize starch (NS) presented granule sizes higher than starch isolated from raw maize (S)due to the partial swelling produced in the nixtamalization process. In dynamic tests during the retrogradation kinetics, an inverse effect ofthe temperature was observed in the re-arrangement of starch components. NS was affected due to the thermal-alkaline process presentingan annealing that provoked a reduction in its ability to develop gels. This information is important during the processing of nixtamalized maizeto masa and tortilla production.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Nixtamalization; Maize starch; Image analysis; Viscosity; Dynamic rheology

1. Introduction

The cooking of maize grains in an alkaline solution ofCa(OH)2, known as nixtamalization, is perhaps the most im-portant process for human consumption of this cereal, sincenixtamalized products such as tamales, nachos, pozole andthe most important tortillas, are widely consumed in Mexicoand Guatemala and by Mexican people living in the UnitedStates (Billeb de Sinibaldi and Bressani, 2001; Campas-Bay-poli et al., 1999). Nixtamalization produces changes thatimprove the nutritional quality of maize. Diverse studies hadbeen carried out on nutritional aspects of nixtamalized maize

Abbreviations: Ca(OH)2, calcium hydroxide; G0, storage modulus; G00, loss

modulus; k, consistency index; n, flow behavior index; NS, nixtamalized maize

starch; S, raw maize starch.

* Corresponding author. Tel.: þ52 735 3942020; fax: þ52 735 3941896.

E-mail address: labellop@ipn.mx (L.A. Bello-Perez).

0733-5210/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jcs.2007.10.007

and tortillas, but there are only a few reports on morphologicaland rheological characteristics of starch in nixtamal (Gomezet al., 1991, 1992; Mondragon et al., 2006; Robles et al.,1988). The relatively high temperature during cooking of thegrain (between 85 and 100 �C), and the pH value (z12), facil-itates diverse transformations of the grain components (protein,lipids and its principal component, starch). Among these arethe degradation of pericarp, the loss of soluble proteins (mainlyalbumin and globulin of low molecular weight contained in thegerm), and the partial gelatinization of starch. After cooking,the resulting product (nixtamal) is washed with water to elim-inate the lime, then the nixtamal is ground in stone grind millsto produce the masa (Reguera et al., 2000). During the grind-ing, additional gelatinization of starch is carried out and othertransformations in grain components are produced, since themasa is a mixture consisting of starch polymers (amylose andamylopectin) mixed with partially gelatinized starch and intactgranules, endosperm parts and lipids. All these components

421G. Mendez-Montealvo et al. / Journal of Cereal Science 48 (2008) 420e425

form a heterogeneous and complex matrix within a continuousphase (Gomez et al., 1987). However, it is important to study, inthis first part, only the changes in the starch molecule and itscomponents after the nixtamalization process without grindingof the grain, with the idea to improve the cooking of the grainand to obtain better base-nixtamalized products. Dynamic rhe-ology is a sensible technique; it may be used to study thechanges produced by the processing and storage of food prod-ucts, since the small-amplitude of the equipment does not alterthe arrangement of the macromolecules inside the food matrix(Biliaderis, 1992). Very few studies have been reported usingdynamic rheology, where the effect of the nixtamalization pro-cess on starch structure was evaluated (Mondragon et al.,2006).

This research was carried out to study the changes in themorphological and rheological properties of starch producedby the nixtamalization process.

2. Experimental

2.1. Materials and methods

Maize grains were generated through a breeding programof INIFAP-Mexico (H515) and its agronomic characteristicswere reported previously (Mendez-Montealvo et al., 2005).The grains were ground using a commercial grinder (MapisaInternacional S.A. de C.V., Mexico, D.F.), sieved through 50U.S. mesh and flours stored at room temperature in sealedplastic containers.

2.2. Nixtamalization process

A lot of 5 kg maize was cooked in 15 L of lime solution at1% (grain weight basis). Maize was cooked for 30 min at boil-ing temperature and steeped in the same cooking vessel during16 h. The cooking solution or ‘‘nejayote’’ was discarded andthe resulting nixtamal washed four times with tap water (ap-proximately 60 L) to remove bran and excess lime. Nixtamalwas frozen under liquid nitrogen and ground into flour usinga commercial stone grinder (the nixtamalization process wascarried out in duplicate).

2.3. Starch isolation

Starch was isolated from flours using the modified methodof Gomez et al. (1992). Flours (raw [S] and nixtamalized[NS]) were steeped overnight in ethyl ether. After decantingthe soaked solution, flour was ground with ethanol (96%) ina Waring blender for 1.0 min. The suspensions were passedthrough a series of mesh sieves (40, 100 and 200 U.S.). A res-idue of each sieve was washed with ethanol 96% to recover thestarch. The starch suspension was centrifuged (3000� g for20 min) (refrigerated universal centrifuge Z 300 K, Hemle La-bortechnik GmbH, Wehingen, Germany) and dried at roomtemperature to eliminate residues of ethanol. Starch was storedin a glass container until further analysis.

2.4. Microscopic analysis

Birefringence of individual starch granules was evaluatedwith a polarizing microscope (LEICA, DMLB) with an objec-tive of 10�, equipped with a digital camera (Canon, Power-Shot S40, Japan). Starch was sprinkled in a glass microscopeslide with a drop of distilled water. Starch granules wereselected at random and the presence of Maltese crosses wasobserved.

Granule size distribution was carried out by a polarizedmicroscope (Nikon, model Alpha-Phot II, Japan) with an ob-jective of 10�, equipped with a digital camera (Dage, modelMTI DC-330, Japan) and image analysis (Sigma Scan Pro,version 5.0.0., SPSS, Inc.). A starch suspension (2 mg samplewas weighed on an aluminum pan and 7 mL of deionised waterwas added) was heated at different temperatures (50, 60, 70,80, 90 �C) using a Differential Scanning Calorimeter (DSC,TA Instruments, model 2010, New Castle, USA) with a heatingrate of 2.5 �C/min. The sealed DSC pan was opened withtweezers and samples were placed on a glass microscope slideand a drop of distilled water was added (four samples wereprepared from each pan). The starch granules selected forthe measurement were randomly chosen from each slide. Foreach granule the major axis was determined, selecting at least300 objects.

2.5. Rotational test: flow curves

Starch dispersions with 5% (w/v) of total solids were pre-pared using distilled water. Their flow properties were mea-sured, running rotational tests in a TA Instruments Rheometer,model AR 1000 (New Castle, USA) using a parallel plate system(sandblasted plate) with a diameter of 40 mm, a gap of 1000 mm,and either a heating or cooling rate of 2.5 �C/min. The parallelplates were covered with mineral oil to avoid water evaporationduring the test. The machine was programmed for running timesweeps (first run was a cycle of heating (25e90 �C)ecooking(90 �C, 10 min)ecooling (90 �C to 25 �C)). At that final temper-ature (25 �C), two cycles were made, two cycles upedown from0.06 to 100 s�1 and a third cycle down from 100 to 0.06 s�1. Thepower law equation was applied (the values were obtained intriplicate).

2.6. Dynamic viscoelastic method: retrogradationkinetics (effect of temperature)

Starch dispersions with 5% (w/v) of total solids were pre-pared using distilled water. Their viscoelastic properties weremeasured running oscillatory tests in a TA Instruments Rheom-eter, model AR 1000 (New Castle, USA) using a parallel platessystem (sandblasted plate) with a diameter of 40 mm, a gap of1000 mm, and either a heating or cooling rate of 2.5 �C/min.The parallel plates were covered with mineral oil to avoidwater evaporation during the test. To determine the linear vis-coelastic region (LVR), strain amplitude sweeps were done.Once the LVR was found, the machine was programmed forrunning time sweeps of a heating (25e90 �C)ecooking

15

20

%

a

422 G. Mendez-Montealvo et al. / Journal of Cereal Science 48 (2008) 420e425

(90 �C, 10 min)ecooling (90 �C to different temperatures, 20and 40 �C after maintaining that temperature during 10 h)kinetics. All tests were run at a frequency of 1 Hz and strainvalue of 2.0%. The storage modulus (G0) and loss modulus(G00) were evaluated from each test (the tests were carried outin triplicate).

5

10

3. Results and discussion

3.1. Microscopic analysis

0<10

12 14 16 18 20 22 24 26 28 30 >30

m

0

5

10

15

20

%

<10

12 14 16 18 20 22 24 26 28 30 >30

m

b

Fig. 2. Size distribution of starch granules of: (a) raw [S] and (b) nixtamalized

[NS] maize samples [major axis (mm)] at 25 �C.

The radial order arrangement of starch molecules (amyloseand amylopectin) in the granule has a quasi-crystalline nature,which is evidenced by Maltese crosses (birefringence) in po-larized microscopy (Fig. 1), and the center of the cross isthe hilum. Intact granules (usually after isolation step) exhibita well-defined birefringence pattern with a dark cross as re-ported by Gomez et al. (1991, 1992) and Robles et al.(1988). Birefringence implies only a high degree of molecularorientation within the granule and it does not make referenceto any particular crystalline form.

There were differences in the major axis of S and NS sam-ples (Fig. 2) at 25 �C. Samples showed 10% of granules hav-ing a 6 mm size, and the highest amount ranged between 14and 18 mm (Fig. 2a). In the case of the NS sample, the highestproportion ranged between 18 and 24 mm, indicating that thesegranules were swollen during the nixtamalization process(Fig. 2b). The swelling of the granules was due to waterintake, but there was no loss of birefringence, the internalorganization of starch components is maintained after the ther-mal-alkaline treatment. It is important to mention that starchgranules with small size are in the periphery (Stoddard,1999; Tang et al., 2000) of the endosperm layer and due tothe natural heat transfer by conduction, they are the first thatwere swollen during the heating in the nixtamalization pro-cess. Starches isolated from diverse maize varieties presented

Fig. 1. Polarized-light micrographs of: (a) raw [S] and (b) nixtamalized [NS]

maize starch samples at 25 �C.

mean sizes of major axis between 9.1 and 12.6 mm (Ji et al.,2003), values that were slightly lower than those determinedin this study.

When samples were heated at 70 �C (Fig. 3), the S sampleshowed a size increase due to swelling (compared with thesample at 25 �C). In general, the NS sample showed similargranule size (Fig. 3b) as its counterpart measured at 25 �C(Fig. 2b); it was notable that the alkaline treatment provokedhigher stability in starch granules. At 70 �C, NS sampleshowed birefringence but, in the S sample, only a few starchgranules showed Maltese crosses (images not shown), due tothe majority of the granules losing their internal organizationcaused by starch gelatinization.

3.2. Rotational test: flow curves

Pastes of both starches (NS and S) showed a non-Newto-nian behavior (correction of viscosity was carried out becausedata were obtained with a parallel plate system). The viscositydecreased when shear rate increased (Fig. 4). This behavior isdefined as shear thinning and is produced when the stress dis-organizes the arrangement of the macromolecules inside thematrix. NS samples showed lower viscosity than S starch inthe range of shear rate tested. This pattern is due to the nixtam-alization process; because some granules are in the endosperm

0

2

4

6

8

10

12

%

<10

14 18 22 26 30 34 38

m

m

0

2

4

6

8

10

12

14

16

%

<10

14 18 22 26 30 34 38

b

a

Fig. 3. Size distribution of starch granules of: (a) raw [S] and (b) nixtamalized

[NS] maize samples [major axis (mm)] after heating at 70 �C.

Table 1

Parameters of the Ostwald-de Waele (power law) model for maize starch from

raw [S] and nixtamalized [NS] samples

R n k

[Pa sn]

S 0.997 0.340 1.528

NS 0.990 0.453 0.754

a

1

10

100

1000

5 10 15 20 25 30 35 40Time (s) x 10

3

G’ G

” (P

a)

423G. Mendez-Montealvo et al. / Journal of Cereal Science 48 (2008) 420e425

periphery and they are gelatinized, losing their capacity toproduce more compact pastes with higher viscosity.

The Ostwald-de Waele’s model (power law) was used(Table 1) to fit the experimental data (R> 0.99). The plotshows a slight curvature at low shear rates, which is character-istic of starch suspensions, showing higher viscosities in thatregion (Tecante and Doublier, 1999). Both samples gavea flow behavior index (n) smaller than 1, showing a non-New-tonian pattern and shear thinning, although the NS sample hada value with higher tendency to Newtonian behavior. Consis-tency index (k) values showed an inverse pattern to n, sincethe NS sample had a lower value of k than S. Those values

0.01

0.1

1

10

100

0.01 0.1 1 10 100Shear rate (s

-1)

Visco

sity (P

a.s)

Fig. 4. Flow curve of raw [S -] and nixtamalized [NS :] maize starch

samples.

agree with the fact that partial gelatinization was carried outduring grain nixtamalization.

3.3. Dynamic viscoelastic method: retrogradationkinetics (effect of temperature)

When gelatinization is produced, there is an increase ofdynamic moduli (storage module G0 and loss module G00),which is attributed to: (1) granules swelling and (2) amyloseliberation. When the gelatinized starch suspension is coolingdown, a gel is formed. However, the border between gelationand retrogradation is not defined, since it was reported that ret-rogradation consists of two separable processes: (a) gelation ofamylose molecules lixiviated from starch granules duringgelatinization and (b) recrystallization of amylopectin insidethe starch granules (Biliaderis, 1991).

Differences in the retrogradation phenomenon for S and NSsamples were shown at different temperatures (Fig. 5). For Ssample the reorganization diminished when the temperatureincreased because the G0 value was higher at lower tempera-ture (Fig. 5a). This pattern is indicative that, at high tempera-ture, the mobility of the starch chain increases avoiding the

0.1

1

10

100

5 10 15 20 25 30 35 40Time (s) x 10

3

G’ G

” (P

a)

b

Fig. 5. Kinetics of formation of: (a) raw [S] and (b) nixtamalized [NS] maize

starch gel (5% w/w) at different temperatures: 20 �C (-) and 40 �C (:). Full

symbols G0; empty symbols G00.

424 G. Mendez-Montealvo et al. / Journal of Cereal Science 48 (2008) 420e425

interaction between chains and consequently the reorganiza-tion (Slade and Levine, 1991). For the NS sample, any effectof the temperature in the range of time tested was not shown(Fig. 5b) and moduli values were lower than the S sample,indicating that NS produced a softer gel. These results showedthat the structure of starch granules was modified during thethermal-alkaline process because some linear starch chainswere lixiviated, causing the formation of a softer gel.

When both S and NS samples are compared at the sametemperature (20 �C, Fig. 6a), the NS sample had lower modulivalues than the S sample, due to the partial gelatinization car-ried out during maize nixtamalization, therefore at this tem-perature during the test period, some changes in bothstarches were detected. When the experiment was carriedout at higher temperature (40 �C), lower moduli values(Fig. 6b) were obtained. Higher storage temperature increasesstarch chain mobility, decreasing the chains re-association andthe rigidity of the starch network. The difference of modulivalues between NS and S samples was minor in the experi-ment carried out at higher temperature (40 �C), because starchchain mobility in the NS sample was affected in higher propor-tion than the S sample and a soft network was produced at thistemperature, minimizing the effect of the nixtamalizationwhen the starch gel was stored at higher temperature.

It has been demonstrated by rheological studies that the NSsample has a limited performance to develop gels. This may bedue to its internal organization, presenting higher flexibility inits branching points (amylopectin) or it may also be caused bythe minor elastic behavior that the gels have compared to thoseformed with the S sample; the form and size distribution of

a

0.1

1

10

100

1000

5 10 15 20 25 30 35 40Time (s) x 10

3

G’ G

” (P

a)

0.1

1

10

100

5 10 15 20 25 30 35 40Time (s) x 10

3

G’ G

” (P

a)

b

Fig. 6. Kinetics of formation of raw [S -] and nixtamalized [NS :] maize

starch gels (5% w/w) at: (a) 20 �C and (b) 40 �C. Full symbols G0; empty

symbols G00.

granules, as well as starch components will affect the packingcharacteristics. When granules are in a closed packing, theyhave minor amount of soluble amylose (Eliasson and Gud-mundsson, 1996).

4. Conclusions

Image analysis showed that nixtamalized starch exhibitedprevious swelling, but Maltese crosses were observed, indicat-ing that the starch component organization is maintained afterthe thermal-alkaline process. The nixtamalization did notaffect the flow characteristics of starch dispersions sinceboth starches presented a shear thinning pattern. Gels pro-duced with nixtamalized starch were softer than raw starchdue to the partial gelatinization carried out in the process ofnixtamalization of the maize grain. The morphological andrheological characteristics of starch after the nixtamalizationare important during the processing of nixtamalized maize tomasa and tortilla production.

Acknowledgments

We appreciate the financial support from SIPeIPN, CO-FAAeIPN and EDIeIPN. One of the authors (GMM) alsoacknowledges the scholarship from CONACYT-Mexico. Theauthors are grateful to Dr. Alberto Tecante for all the technicalsupport; Dr. Paul Colonna, Dr. Silvia Bautista, Dr. IsabelleCapron, Jose Luis Trejo, MSc., and Agnes Sabate, MSc., fortheir assistance.

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