Effect of processing conditions on the production of nixtamalized corn flours by the traditional method

Abstract

Nowadays, in Mexico and Latin America, calcium content in the diet is a very important issue. This fact is critical since research studies have shown a relationship between consumption of dietary calcium and chronic degenerative diseases. Deficiencies of calcium in Mexican population involve the search for alternative foods that lead to an increase in the daily calcium intake. In this context, the traditional nixtamalization process is a good option for food processing in order to increase calcium content in foodstuffs. This work focused on the development of an effective methodology to establish the relationships among the processing variables in the production of nixtamalized corn flours by the traditional method. A full compound factorial design was used to determine the effect of some independent variables, involved in the nixtamalization, on the response variables such as (a) Ca⁺² content in corn flours, (b) particle size distribution (homogeneity), and (c) average energy consumption during grinding.

Actualmente en México y Latinoamérica el contenido de calcio en la dieta es un tema de gran interés debido a que diversos trabajos de investigación han demostrado que existe una relación entre el bajo consumo de calcio y la presencia de enfermedades crónico degenerativas. Por lo anterior, el proceso tradicional de nixtamalización para la producción de alimentos procesados es una alternativa excelente para incrementar la ingesta de calcio en la población mexicana. Este trabajo presenta el desarrollo de una metodología efectiva para establecer las relaciones entre las variables de proceso involucradas en la elaboración de harinas de maíz nixtamalizado por el método tradicional. Se utilizó un diseño compuesto factorial completo para determinar el efecto que algunas variables independientes involucradas en la nixtamalización tienen sobre variables de respuesta tales como: (a) contenido de Ca²⁺ en harinas de maíz nixtamalizado, (b) homogeneidad en el tamaño de partícula y (c) gasto energético promedio.

Palabras claves:

• contenido de calcio
• harina de maíz
• tiempo de reposo
• distribución de tamaño de partícula

Keywords:

• corn flour
• calcium content
• steeping time
• particle size distribution

Introduction

In recent decades, scientific interest has increased the focus on understanding the physicochemical changes of organic compounds such as biopolymers and food systems in order to evaluate changes during the transformation process to facilitate food manufacturing, transportation, and storage. Furthermore, these compounds are important for human diet. Nixtamalization process is a process required for the production of flour or dough in corn industry. Corn kernel is a basic foodstuff for human diet in Latin America and the southern United States. In these regions, corn is consumed as corn tortillas and snacks. Additionally, raw materials such as flour and starch obtained from maize have crossed Mexico borders with a broad perspective and rapid growth in the European community and the United States.

During the nixtamalization process, the diffusion of water and calcium into the corn kernel is one of the most important processes and produce important physicochemical changes in pericarp, endosperm, and germen, which are reflected in nutritional and sensory food properties (Bressani, Benavides, Acevedo, & Ortíz, 1990). Through an engineering perspective, the instant corn flour production was one of the most important technological advances in Latin America, due to reduced processing time and increased shelf life of the nixtamalized products compared with traditional products (Flores-Farias, Martínez-Bustos, Salinas-Moreno, & Ríos, 2002). Nevertheless, the industrial process does not attain the quality standards of products obtained with a smaller traditional scale. Palacios-Fonseca, Vazquez-Ramos, and Rodríguez (2009) conducted studies on industrial nixtamalized corn flours of the three main commercial producers in Mexico in order to compare them with nixtamalized corn flours obtained by the traditional method. They found that the calcium concentration in industrialized corn flours is lower than that in nixtamalized corn flours prepared by the traditional method.

This is significant as corn tortillas are the main sources of calcium in Mexican population diet, with a daily per capita consumption of 234 g (Rojas-Molina et al., 2007). In addition, Gutierrez et al. (2007) evidenced that nixtamalization process conditions modify calcium concentrations in the end-product. The authors reported that the cooking temperature and steeping time of corn kernels are critical variables for increasing the calcium content in nixtamalized products. On the other hand, Rojas-Molina et al. (2008) established that there is a correlation between the calcium content and protein quality in nixtamalized corn flours prepared with high quality protein maize. Rojas-Molina et al. (2009) analyzed the calcium/phosphorus ratio in nixtamalized corn flours prepared at different process conditions by the traditional method. The results of this study show that the calcium content in commercial nixtamalized corn flour is lower than calcium concentration in corn flours obtained by the traditional method. Additionally, the authors explain that during traditional nixtamalization process, calcium content in nixtamalized corn flours increases at high cooking temperatures (92°C) and long steeping times (7–9 h). The  search for the methodologies to increase calcium content in corn flour has attracted interest from researchers; hence, others scientists consider increasing the amount of calcium in diet by adding other compounds such as nopal flour (Cornejo-Villegas et al., 2010).

According to the aforementioned works, nixtamalization process does not require very long corn steeping times (>9 h) in its cooking liquid, as long times would imply the softening of tissues and loss of material (pericarp, germ, and endosperm). Furthermore, such a loss would modify the mechanical and rheological properties in intermediate products such as dough, impairing handling for tortilla elaboration and therefore increasing production time, affecting the economy of business companies (González, Reguera, Figueroa, & Sánchez-Sinencio, 2004).

The nixtamalization process consists in subjecting the corn kernels to a thermo-alkali treatment by using an oversaturated solution of calcium hydroxide. The treatment is performed in two stages: (1) cooking stage which mainly affects the pericarp of corn kernels. This fact governs the ion calcium diffusion paths into the inner structures of grain, (2) steeping stage where the grains remain in the cooking liquid (Valderrama-Bravo et al., 2010). During these steps, the diffusion of water and calcium takes place simultaneously in the germ and the endosperm, promoting starch gelatinization phenomenon, which is decisive in the textural properties of the nixtamalized products (Fernández-Muñoz, Acosta-Osorio, Zelaya-Angel, & Rodríguez-García, 2011; Pineda-Gómez et al., 2012).

The purpose of this study was to find the optimal conditions to prepare nixtamalized corn flour by using the traditional method in order to obtain a high quality end-product. This criterion comprises the following targets: (1) the highest calcium content, (2) maximum flour yield related to an adequate homogeneity of granular solids or particle size distribution according to NMX-046-S-1980 (both criteria applied to nixtamalized corn flours in Mexico), and (3) maximum savings in energy consumption during grinding of dehydrated granular solids from cooked and washed corn kernels named “nixtamal” during nixtamalization process.

Materials and methods

Materials

Commercial instant corn flour (CICF)

A commercial corn flour sample produced in Mexico and obtained in spring 2012 from a commercial store was use for this study. The corn flour was stored at a temperature of 4°C in closed jars for analysis. Triplicate samples were analyzed and the means with standard deviations of each determination were reported.

Traditional instant corn flour preparation with calcium (TICFC) and without calcium (TICFWC)

The nixtamalized corn flours were prepared as was reported by Gutierrez et al. (2007). In the initial step of this process, each sample was prepared by cooking 2 kg of Pioneer corn kernels in a solution of 4 L of distilled water and 20 g of calcium hydroxide (reagent powder, Fermont, Monterrey, N. L., and Mexico). The corn kernels were added to the container and cooked at 92°C for 25, 45, and 65 min. These cooking times were selected according to the criterion established by Gutierrez et al. (2007) to determine the cooking time in nixtamalized corn kernels. Each sample was prepared separately from the others to constitute an independent event. After cooking, the maize was steeped for 3, 7, and 11 h. The control sample was prepared by cooking the corn kernels without the addition of calcium hydroxide. Also, corn samples were processed at temperatures of 72 and 82°C, with the same cooking and steeping times outlined above. The cooking liquor (nejayote) was drained off and the samples (cooked corn kernels) were washed twice in distilled water (2:1 v/w ratio) by stirring the kernels in the washing water for 1 min. After washing and draining, 2.0 kg of cooked and washed corn kernels (nixtamal) were ground (FUMASA, M100, Queretaro) into corn dough and then dehydrated using a vacuum furnace. The drying conditions were adjusted to 40°C for 8 h.

Calcium content in nixtamalized corn flours

Calcium content in nixtamalized corn flours was determined by the 968.08 method (AOAC, 1998). The calcium ion concentration was measured with a double-beam atomic absorption spectrometer (Analyst 300 Perkin Elmer, Waltham, MA, USA), equipped with a deuterium lamp. The equipment was operated with 0.083 Mpa of dry air and 0.483 Mpa of acetylene, using a 422.7 nm flame, a 10 mA lamp current, and 0.7 nm slit width.

Determination of the power and energy consumption average for grinding

The material (dehydrated granular solids) obtained from experimental samples was triturated by using a hammer mill (PULVEX 200, Mexico, D.F., Mexico). For this purpose, a dispenser was used to feed the feedstock to the mill. The rate used was 32.48 kg/h ± 0.47, which was constant for all samples to produce a muffled grinding. A mesh of 0.8 mm was placed at the output restriction. The energy required for grinding was determined by Equation (1)(1).

(1)

where E is the necessary energy to carry out the grinding expressed in kWh/kg; P_c is the consumed power by the mill during grinding (Kw) and T is the feed rate during grinding (kg/h). The P _o and P _c, values were determined through voltage and current intensity measurements made during the grinding with a hook multimeter. (Soar corporation, model  CT-300, Hanishinagun, Japan).

The amount consumed during milling was quantified in amperes and the power was calculated by Equation (2)(2).

(2)

where P is the power required for grinding (Kw), I is the power current intensity, and V is electric power voltage (V).

Corn flour particle size distribution and yield

Particle size distribution of traditional nixtamalized instant corn flours prepared at 72, 82, and 92°C of cooking temperature, cooked for 25, 45, and 64 min, and steeped for 3, 7, and 11 h were measured by using a RO-TAP equipment with a set of meshes (U.S. standard Rot-tap model KH59986-60, with an horizontal and vertical automatic stirrer, mesh numbers: 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 100, and pan). The mesh size establishes the categories for the particle size. The separating procedure was done according to ASAE Standards (1995), where 100 g of flour was separated during 12 min with rigorous stirring. Then, the fractions retained on each one of the different meshes were collected and weighed. The fractionation process was performed in triplicate. Means with standard deviations were used to the full factorial design. The yield was a response variable and was calculated according to the standard NMX-046-S-1980, which specifies that at least 750 g/kg of commercial flour must sift through 60 USA. Subsequently, corn flour particle size distribution in CICF, TICFC, and TICFWC was carried out. Experimental samples (TICFC and TICFWC) were prepared by using the central point processing conditions obtained from a Box-Wilson central composite design. The traditional flour with and without calcium was prepared at 82ºC of cooking temperature, 45 min of cooking time, and 4 h of steeping time. These parameters represent the best process conditions according to the statistical analysis. The yield for experimental samples and commercial nixtamalized corn flour was calculated as indicated above. The weights of flour retained on the mesh: 70, 80, 100, 120, and plate were added and the percentage that passed through the mesh 60 USA was calculated. Finally, a graph was constructed for the granulometric analysis by plotting the differential data of average diameter (Dpi) versus mass fraction retained (Xi). Dpi (in) was calculated with the summation of the sieve mesh opening that the sample pass through and the sieve mesh opening that the sample was retained divided by two. Xi (g) is the sample weight fraction retained on each sieve divided by total sample weight used for the analysis.

Chemical proximate analysis

Commercial nixtamalized corn flours, traditional nixtamalized corn flours, and instant corn flours processed without calcium hydroxide at 82°C of cooking temperature, 45 min of cooking time and 4 h of steeping time (central point for central composite design) were studied. The crude protein (N-6.25) by the micro-Kjeldahl (Method 46–13, American Association of Cereal Chemists [AACC], 2000), moisture (Method 925.10, AOAC, 2000), total ether extract (Method 30–25, AACC, 2000), ashes (Method 08–01, AACC, 2000), insoluble dietary fiber, soluble dietary fiber, and total dietary fiber (Method 992.16, AOAC, 2000) were measured. All the measurements were carried out three times.

Instant corn flour morphology CICF, TICFC, and TICFWC

The characterization was made by low vacuum scanning electron microscopy (LV-SEM) and energy dispersive spectrometry (EDS). The corn flour morphology of each sample was analyzed in an LV-SEM, JSM 5600LV (JEOL, Tokyo, Japan), with a resolution of 5 nm in low vacuum and fitted with an energy dispersive X-ray spectrometer (Noran instrument, model Voyager 4.2.3, Middleton, WI, USA). Prior to the analysis, the corn flour samples were fixed on an aluminum specimen holder with carbon tape. The analysis was performed using a 20 kV electron acceleration voltage and a pressure of 12–20 Pa in the specimen chamber, thus obtaining images on the fracture surfaces with the backscattering electron signal (Arenas, 1999).

Statistical analysis

A full factorial design (2 ^k ) was used to determine the effect of independent variables A (cooking temperature), B (cooking time), and C (steeping time) and their interactions on calcium content and particle size distribution in nixtamalized corn flours, as well as, power consumption for grinding, in order to find process conditions to obtain nixtamalized corn flours with the highest calcium content, an adequate homogeneity of granular solids (particle size distribution), and maximum savings in energy consumption during grinding in nixtamalization process. Variables A, B, and C are coded variables with three levels for each one as is shown in Table 1.

Table 1 Independent variables (coded and uncoded) and factor levels for full factorial design 2 ^k .Variables independientes (codificadas y no codificadas) y niveles de variación para el diseńo factorial completo 2^k.

Uncoded variables or factors	Coded variables or factors	Factor levels
		–1	0	+1
Cooking temperature (^°C)	A	72	82	92
Cooking time (min)	B	25	45	65
Steeping time (h)	C	3	7	11

The analysis of the data for full factorial design and Pareto charts were based on the analysis of variance (ANOVA) and t-statistic, respectively. Furthermore, Box-Wilson central composite design was conducted to allow for a good estimation of the curvature of the output. All the statistical analysis was performed by using a computational software (Minitab Statistical Software v 16.0. Minitab Inc. Penn. State University). This software estimates the effects of each factor (cooking temperature, cooking time, and steeping time) or individual effect and the influence of the interaction amongst them (interactive effects). These effects, called standardized effects, were represented in Pareto charts. The standardized Pareto charts contain a bar for each effect, sorted from the most significant to the least significant one. The length of each bar is proportional to the standardized effect, which equals the magnitude of the t-statistic that was used to test the statistical significance of that effect. A vertical line was drawn at the location of the 0.05 critical value for Student's t-test. Any bar that extends to the right of that line indicates effects that are statistically significant at the 95% significance level. Box-Wilson central composite design was developed in order to find the optimal conditions to prepare nixtamalized corn flours by using the traditional nixtamalization process.

Results and discussion

Calcium content in nixtamalized corn flours

Figure 1  shows the Pareto charts for the independent variables: cooking temperature (A), cooking time (B), and steeping time (C) and their interactions versus effect estimate. The numerical values on the X-axis are based on the values of the response variable (calcium concentration). The standardized effect has a value of 2.28, with α = 0.05. Variables and interaction values higher than 2.28 are statistically significant as is shown in Table 2. Figure 1a evidences that the effect of cooking temperature (variable A) and steeping time (variable C) in calcium content of samples is greater than the effect of the cooking time (variable B). These results agree with the data reported by Gutiérrez-Cortéz et al. (2010) who explained that the calcium diffusion into the corn grain is higher at 92°C and long steeping times than at 82 and 72°C of cooking temperature and short steeping times. Furthermore, the interaction effect among all independent factors was not significant (p ≥ 0.171) (see Table 2). The mathematical model that predicts this behavior with R ² = 0.97 is shown in Table 3.

Figure 1. Effect of cooking temperature, cooking time, and steeping time on a) the residual calcium concentration, b) yield, and c) energy consumption in nixtamalized corn flour prepared by using the traditional method.Efecto de la temperatura de cocción, el tiempo de cocción y el tiempo de reposo en a) la concentración de calcio residual, b) rendimiento y c) consumo de energía en harina de maíz nixtamalizado elaborada empleando el método tradicional.

Table 2 Analysis of variance (ANOVA) and regression coefficients of response variables for (independent variables) factors.Análisis de varianza (ANOVA) y coeficientes de regresión para las variables de respuesta correspondientes a las variables independientes estudiadas.

	Calcium (mg/g)		Yield (g)		Energy consumption (kWh/kg)
Term	Coefficient	p-value	Coefficient	p-value	Coefficient	p-value
β0	1.576^a	0.000	54.05^a	0.000	1.638^a	0.000
β1	0.165^a	0.000	14.93^a	0.000	−0.233^a	0.000
β2	0.008^ns	0.594	1.835^a	0.000	−0.088^a	0.000
β3	0.078^a	0.000	13.77^a	0.000	−0.175^a	0.000
β11	−0.075^a	0.001	−1.007^a	0.006	0.028^ns	0.123
β22	−0.113^a	0.000	−0.534^ns	0.091	0.090^a	0.000
β33	−0.008^ns	0.594	−0.180^ns	0.540	0.005^ns	0.741
β123	−0.021^ns	0.171	−0.920^ns	0.010	−0.015^ns	0.369
R ^2	97.89		99.84		97.71
R ^2-adj	96.02		99.69		95.67
Note: βo= Regression coefficient; βi = Lineal coefficient; βii = Quadratics coefficient; and βij = Interaction coefficients; ^ns Not significant term (p > 0.05); ^aSignificant term (p < 0.05).

Table 3 Mathematical models for response variables.Modelos matemáticos par a las variables de respuesta.

Response variables	R ^2	Equations
Calcium (g/100g)	97.89	−2.28 + 0.004A + 0.016C – 1.867^−5 A.B – 1.598^−4 A.C
Yield (g)	99.84	−93.905 + 145A – 0.139B + 0.395C + 0.003A.B – 0.001A.B.C
Energy consumption (kWh/kg)	97.71	6.40–0.052A – 0.028B – 3.05C + 0.003A.C

Nixtamalized corn flour yield

Figure 1b shows the independent variables and their interactions versus effect estimate for nixtamalized corn flours yield. The numerical values on the X-axis are based on the values of the response variable (yield). The standardized effect has a value of 2.28, with α = 0.05. Variables and interaction values higher than 2.28 are statistically significant. The cooking time had the least significant effect on yield compared with the other independent factors (p ≥ 0.741). On the contrary, the most influential factor in this response variable and factor interactions was the cooking temperature (p ≥ 0.000) (see Table 2). This is a result of material loss (biopolymers comprised in the corn pericarp), which is more evident at high processing temperatures as was reported by Rojas-Molina et al. (2009). Pericarp removal takes off a significant amount of calcium attached to the corn grain. However, the most fibrous portion of the grain represents higher particle sizes. The mathematical model shown in Table 3 predicts flour yield with R ² = 0.99. It is important to denote that though the long steeping times and high temperatures of processing increase corn flour yield, the calcium content in samples decreases.

Power and energy consumption average for grinding

Figure 1c shows the interactions among the independent factors: cooking time, steeping time, and cooking temperature. Interactions between the three independent factors, as well as, cooking temperature–cooking time, and cooking time–steeping time had no significant effect on the energy consumption in the nixtamalization corn process. During grinding, to obtain flours, the energy consumption is an important factor, since most of the power required for this unit operation is dissipated as heat and noise. Moreover, this operation in food engineering is extremely expensive. According to Table 2, all factors independently and cooking temperature–steeping time interaction affect the energy consumption during grinding stage (p ≥ 0.000). Pericarp losses reduce the corn flour fraction that comprises the highest fiber content (Gutiérrez-Cortéz et al., 2010), so it reduces the energy consumption significantly. Table 3 shows the mathematical model that predicts energy consumption average for grinding with R ² = 0.95. At this point, it is important to note that some industrial nixtamalization processes avoid the steeping step; consequently, excess of fibrous material content remains in the final product and thereby increases the energy costs (Montemayor & Rubio, 1983).

Figure 2  shows the Box-Wilson central composite design, whose corners correspond to lower and upper limits of the independent variables evaluated: cooking temperature (from 72°C to 92°C), cooking time (from 25 to 65 min), and steeping time (from 3 to 11 h). Numbers in central point and cube points correspond to calcium content in g/kg of nixtamalized corn flour. The central point represents experimental conditions where equilibrium in the correlation between response variables is reached, that is to say, the highest calcium concentration in corn flours, higher yield, and lower energy consumption for the nixtamalized corn flour production. This central point also indicates the most suitable process conditions for the processing of the Pionner corn variety by using the traditional nixtamalization method to obtain a high quality end-product.

Figure 2. Predictions of the experimental design for calcium content in nixtamalized corn flour prepared by the traditional method and maximum response at the central point.Predicciones para el modelo experimental correspondiente al contenido de calcio en harina de maíz nixtamalizado empleando el método tradicional y la respuesta máxima en el punto central.

In adittion, this figure demonstrates the behavior of calcium concentration in nixtamalized corn flours due to the interaction of the independent variables with the maximum response evaluated at the central points of the model. The highest calcium content in samples (2.2 g of Ca/kg of nixtamalized corn flour) is achieved at 82°C of temperature, 45 min of cooking time, and 4 h of steeping time. These processing conditions were selected considering a balance between the higher calcium concentrations, adequate particle size distribution (at least 750 g/kg passing through 60 US mesh), and average energy consumption. The cube evidences that minimum calcium content value is 1.2 g of Ca/kg of nixtamalized corn flour; in addition, an increase in temperature and cooking time increase the calcium content in samples. Nevertheless, it does not occur with the steeping time, for the reason that at longer steeping time calcium concentration decreases. This is due to the dry matter loss during steeping time that involves pericarp fractions, germ, and endosperm, which are placed in the nejayote and drags a significant amount of calcium.

Corn flour particle size distribution and homogeneity of CICF, TICFC, and TICFWC

Figure 3  shows a differential curve for particle size distribution analysis of CICF and experimental samples (TICFC and TICFWC), which were prepared by using the central point processing conditions obtained from the central composite design (82°C of cooking temperature, 45 min, and 4 h for cooking and steeping time, respectively). According to this figure, the corn flours added with calcium hydroxide (TICFC) had a particle size distribution similar to that observed in the commercial nixtamalized corn flours (CICF). These flours included granular solid particles, whereas the flour without calcium hydroxide (TICFWC) had large granular solids. The area under the curves is directly proportional to the homogeneity of samples. Similarly, commercial nixtamalized corn flours and corn flours obtained by the traditional method with calcium hydroxide had the same consistency and an adequate size distribution according to the NMX-046-S-1980 values. Instant corn flours prepared without calcium hydroxide addition had a deficient homogeneity and a broad size distribution.

Figure 3. Particle size analysis of instant corn flour CICF, TICFC, and TICFWC.Análisis del tamańo de partículas en harina de maíz instantánea CICF,TICF y TICFWC.

Chemical proximate analysis

Table 4   shows the results of chemical proximate analysis of experimental samples and commercial instant corn flours. According to this table, there are no significant differences (p < 0.05) in moisture, crude protein, and ether extract (fat) content in all samples. Whereas, it is evident that ash content (mainly associated with calcium concentration) in nixtamalized corn flour obtained by the traditional method (TICFC) was significantly higher (p < 0.05) than those detected in commercial nixtamalized corn flours (CICF) and instant corn flours without calcium hydroxide (TICFWC). Crude fiber content in TICFC was significantly lower (p < 0.05) in comparison with TICFWC and CICF. There were no statistical differences in nitrogen-free extract (NFE) content between TICFC and TICFWC. NFE content in CICF was significantly higher (p < 0.05) than the content detected in TICFC and TICFWC. With respect to these results, Rojas-Molina et al. (2009) mentioned that short steeping times used in industrial production processes of commercial nixtamalized corn flours is partly responsible for the differences between the calcium concentrations in TICFC and CICF. Furthermore, the authors mention that there is a correlation between dry matter loss and steeping time. This fact suggests that during industrial nixtamalization process, the fibrous material remains attached to corn grains allowing the reduction of dry matter loss constituted for NFE and other components.

Table 4 Chemical proximate analysis of instant corn flours (g/kg of dry matter).Análisis químico proximal de las harinas instantáneas (g/kg de materia seca).

Samples	Moisture	Crude protein	Ether extract	Crude fiber	Ash	NFE
CICF	95.7 ± 6.4^a	82.1 ± 1.7^a	30.6 ± 1.6^a	41.4 ± 8.2^a	10.2 ± 1.3^a	748.5 ± 4.3^b
TICFC	92.2 ± 1.9^a	86.3 ± 4.3^a	28.7 ± 3.2^a	30.7 ± 2.9^b	18.0 ± 2.4^b	737.0 ± 2.6^a
TICFWC	93.1 ± 2.5^a	86.6 ± 4.2^a	28.4 ± 7.2^a	39.2 ± 3.4^a	11.6 ± 1.1^b	741.5 ± 2.5^a
Notes: CICF = Commercial instant corn flour; TICFC = Traditional instant corn flour with calcium; TICFWC = Traditional instant corn flour without calcium; and NFE = Nitrogen free extract. This value was calculated by the addition of moisture, crude protein, ether extract, crude fiber, ash, and the difference to 1000; Assays were performed in triplicate. Mean ± SD values followed by the same letter in each column are not significantly different (p < 0.05).

Instant corn flour morphology CICF, TICFC, and TICFWC

Figure 4   shows the micrographs of the three instant corn flours. Figure 4a belongs to TICFWC, the arrow points to the protein matrix that covers the packages of starch granules without apparent changes; this fact evidences an order in the packing of starch granules . This figure also supports the absence of major morphological transformations due to thermal treatment, applied during cooking and steeping stages, when water is used without calcium.

Figure 4. Micrographs of the morphology of instant corn flour (a) CICF, (b) TICFC, and (c) TICFWC.Micrografías de la morfología de harina de maíz instantánea (a) CICF, (b) TICFC, and (c) TICFWC.

Figure 4b corresponds to the TICFC and shows starch granules greater than those shown in Figure 4a. The white circle shows that starch granules appear free with different sizes depending on water absorption, that the protein matrix is fragmented and that portions of germ and lipid bodies can be seen between starch granules. These changes are morphological transformations associated with the gelatinization process.

Figure 4c shows the CIFC morphology in this sample, the white circle focuses on starch granules that are free and have a similar size than those observed in Figure 4b (see the micrographs’ scales) and the protein matrix is barely noticeable. Additionally, this sample shows a disorder in the packing of starch granules similar to that shown in Figure 4b. This fact can explain that the particle size distribution in TICFC and TICFWC was comparable in both corn flours.

Conclusion

According to the statistical analysis, the effect of cooking temperature and steeping time on calcium content of nixtamalized corn flours was greater than the effect of the cooking time. In the same way, the cooking time had the least significant effect on yield of experimental samples. On the other hand, all independent variables affect energy consumption during grinding stage to obtain instant nixtamalized corn flours. The optimal conditions to prepare nixtamalized corn flour by the traditional method, in order to obtain a high quality end-product with the highest calcium content, were achieved at 92ºC of cooking temperature, 25 min of cooking time, and 4 h of steeping time. The nixtamalized corn flours obtained by the traditional method and the commercial instant corn flours showed a similar particle size distribution that complies with NMX-046-S-1980 criterion. In contrast, the instant corn flours prepared without calcium did not satisfy this standard. Scanning electron microscopy analysis evidenced that morphological changes in experimental samples and commercial instant corn flours were similar. Nevertheless, calcium content in nixtamalized corn flours prepared by traditional method was higher than in commercial instant corn flours. Furthermore, there is energy saving during grinding stage to obtain instant corn flours by using the traditional method. This fact can be explained due to dry matter losses during steeping stage that diminish fibrous material in corn grains and thus reduce the energy consumption.

Acknowledgements

This work was partially supported by projects PAPIME (No. PE203711), PAPIIT No. (IT231511), and PACIVE: NCONS-15 UNAM, México. We thank Roberto Hernández, Microscopy Laboratory, Instituto de Física, Universidad Nacional Autónoma de México for giving us the technical support and access to microscopy equipment. The authors also thanks Chair: Controlled Release Systems in Process for Vegetable Products Preservation for advices on statistical methods.