Physico-chemical properties and quality assessment of corn flour processed by a continuous ohmic heating system and traditional nixtamalization

Abstract

Traditional nixtamalization is performed to produce corn flour and tortillas. This process has undergone several modifications for industrialization purposes. Extrusion is an industrially used process for nixtamalization and ohmic heating may be considered an alternative. For this study, a continuous ohmic heater was used to produce nixtamalized corn flour and the effects of the process parameters – process temperature, feed moisture, and screw speed – on the physicochemical and textural properties of dough and tortilla were reviewed. Data showed that the process temperature had a greater effect than the feed moisture on the resulting product, while screw speed did not appear to have any significant influence on this. Tortillas showed good quality characteristics because of good water absorption and retention during the process. The products obtained by continuous ohmic heating process show quality characteristics similar to those present in traditional corn flour products.

La nixtamalización tradicional se lleva a cabo para producir harina de maíz y tortillas, esta ha sufrido varias modificaciones para su industrialización. La extrusión es un proceso utilizado industrialmente para la nixtamalización y el calentamiento óhmico puede considerarse como una alternativa. Para este estudio, se utilizó un calentador óhmico continuo para producir harina de maíz nixtamalizado y se evaluaron los efectos de los parámetros de proceso – la humedad, temperatura de alimentación, y velocidad del tornillo – en las propiedades fisicoquímicas y texturales de dough y tortilla. Los datos mostraron que la temperatura del proceso tuvo el mayor efecto en el producto resultante, mientras que la velocidad del tornillo no parece tener ninguna influencia significativa. Las tortillas mostraron buenas características de calidad, teniendo alta absorción de agua y una buena retención del agua durante la cocción de la tortilla. Los productos obtenidos por el proceso de calentamiento óhmico continuo mostraron características de calidad similares a los presentes en los productos obtenidos de la harina de maíz del proceso tradicional.

Keywords:

• corn flour
• ohmic heating
• nixtamalization

Palabras claves:

• harina de maíz
• calentamiento óhmico
• nixtamalización

Introduction

Extrusion is, beyond doubt, one of the most promising alternative processes in the industry of instant corn flours (Bazúa, Guerra, & Sterner, 1979). In this process, there is no nejayote waste, as the whole corn grain is used. Nixtamalization by this process leads to significant changes in rheological, physical, structural, and textural properties of corn flour dough and tortillas, when compared with those obtained when using the traditional nixtamalization process. These changes are due to the interaction between the constituents of the corn grains and the conditions under which the process takes place, such as the temperature generated by a heating jacket, the screw speed and geometry, as well as lime and moisture concentrations. Extrusion presents some disadvantages, i.e., shear stress and temperature, which result in starch dextrinization and a diminished quality of instant flour. Consequently, the industrial use of extrusion for producing instant corn tortillas flour is not widespread (Rodríguez et al., 1996). An alternative technological process that has become increasingly popular in the food industry is ohmic heating. Ohmic heating is an alternative heating process for pumpable foods. It can be used as a continuous in-line heater for cooking and sterilization of viscous liquids and mixtures containing particulate food products. Ohmic heating is a thermal process consisting of the internal generation of heat by the passage of an electric alternating current (AC) through some medium with electrical resistance, such as food. Ohmic heating principles are quite simple: an electric AC is applied to electrodes placed at both ends of the medium. Heating speed is directly proportional to the square of the electric field strength, the electrical conductivity, and the type of material being heated (Salengke & Sastry, 2007). Internal generation of heat thus results in easy temperature control during the process. Gaytán-Martínez et al. (2012) claim that good-quality nixtamalized corn flours were obtained through batch ohmic heating process using a batch cell. The question is whether we get good quality flour using a continuous process for ohmic heating because there is a new variable such as screw speed.

Corn flour technological process

There are several factors to consider in the various technological processes for producing instant corn flour. The traditional nixtamalization process consists of cooking corn grain in limewater, soaking the grains in this solution, and finally grounding them to obtain the dough. The factors that need to be controlled are process temperature, cooking time, lime concentration, and stepping.

The extrusion process in nixtamalization requires feeding the extruder with a mixture of ground corn grain and limewater and the paste goes through a screw conveyor to be cooked; then it is dried and finally milled to obtain the flour. In this case, factors to monitor are the particle size, the amount of water and lime added to the corn grain, the screw speed, process temperature, the out dye, type of dryer, and type of mill.

Another technological process is the ohmic heating process. A screw conveyor coupled to an ohmic heating cell creates a continuous ohmic heater (COH) (Morales-Sánchez, Figueroa, & Gaytán-Martínez, 2010). The nixtamalization process in a COH takes place as follows: the screw conveyor is fed with a mixture of dry ground corn and limewater; the mixture is heated in a continuous heating cell, where it gets cooked with the heat generated by an AC passing through; the cooked paste that comes out of the cell is dried and then it is milled to obtain the flour. Several factors are to be controlled: particle size, amount of water and lime added to the ground corn, process temperature, screw speed, applied voltage or applied electric power, type of dryer, and type of mill.

The research had two objectives. The first objective was to evaluate the effect of feed moisture, process temperature, and screw speed of transport on the quality of corn flour ohmically nixtamalized. The second objective was to compare some physicochemical characteristics of the corn flours obtained by continuous ohmic heating with flour processed utilizing by the traditional method nixtamalization.

Methods and materials

Raw material

Commercial white dent corn was obtained from a local market in Querétaro. Kernel physical properties were as follows: kernel size was 8.64 mm wide, 11.72 mm length, and 4.30 mm thickness; test weight 82 kg/hL; and 23% flotation index, meeting standards for its use in tortilla production (NMX-FF-034/1-SCFI-2002). Food grade calcium hydroxide (El topo, Monterrey, N.L, México) and distilled water were used in all experiments.

Preparation of traditional process corn flour (TPCF)

Corn (1 kg), distilled water (2 L), and calcium hydroxide (10 g) were boiled for 25 min. The cooked corn was steeped for 10 h. The residual liquid was discarded and the cooked corn (nixtamal) was ground in a stone mill and then dehydrated using a flash type dryer (Cinvestav-AV, M2000, Querétaro). The dryer conditions were adjusted to have inlet air at 250°C and to the exhaust air at 90°C to avoid burning the material. Then the material was remilled using a hammer mill (PULVEX 200, Mexico DF, Mexico) equipped with a 0.5-mm screen (Model 200; Pulvex, S.A. de C.V., México DF). The remaining sample was stored in plastic bags at 4°C.

Continuous ohmic heater

The continuous ohmic heater (COH) employed is the equipment patented by Morales-Sánchez et al. (2010). It consists of two parts: a screw conveyor for corn flour coupled to a rectangular ohmic heating cell. The measurements inside the cell are 25 cm long, 5 cm wide, and 2 cm high, resulting in a volumetric capacity of 250 cm³. On both sides of the cell, stainless steel electrodes are connected to an AC voltage. The opening through which the processed material comes out is as wide and tall as the cell itself and has no exit duct, so as not to exert pressure on the material. The COH has a temperature Watlow 981 controller with a K temperature sensor, a Variac 60 Hz variable AC voltage transformer, HP bench multimeters to measure AC voltage and current, and Baldor speed controller to regulate screw speed. Figure 1 shows the continuous ohmic heater employed. Its operation may be described as follows: the temperature controller measures the temperature and increases or reduces the voltage connected to lateral electrodes; voltage is applied to the material that conducts the current and heats it internally. According to feed moisture and screw speed, the controller applies as much power as is needed to reach required temperature, at a maximum power of 1 W/g, as reported by Gaytán-Martínez et al. (2012). The equipment's maximum power is 500 W.

Figure 1. Continuous ohmic heating system.

Figura 1. Sistema de calentamiento óhmico continuo.

Continuous ohmic heating flour (COHF)

One kilogram of raw corn meal ground in a grain mill with a 1.3-mesh (Model 200) was blended with 0.3% of calcium hydroxide (w/w) in a mixer (Kitchen Aid model K45SS; St. Joseph, MI). The water specified for each treatment was added. The mixture was cooked with the COH, which was heated to reach the process temperature for each treatment; maximum applied power was 1 W/g (Gaytán-Martínez et al., 2012). The dough was dehydrated using a flash type dryer (Cinvestav-AV, M2000, Querétaro). The dryer conditions were adjusted to have inlet air at 250°C temperature and the exhaust air at 90°C to avoid burning the material. Then the material was remilled using a hammer mill (PULVEX 200) equipped with a 0.5-mesh (Model 200). The remaining sample was stored in plastic bags at 4°C.

Physical properties of corn flour

Absorption index (WAI) and solubility in water (WSI)

This variable was measured according to the methodology described by Anderson, Conway, Pfeifer, and Groffom (1969). To perform Dunnet's test, water solubility index (WSI) values for COH and traditional process samples were adjusted by adding the solid remains left in the decanted water to the traditional process samples, according to results reported by Campechano-Carrera et al. (2012).

Pasting and rheological properties

A rheometer (Physica Anton Paar model MCR-101, Australia) was used to determine the viscoamilographic curve. The method used was AACC standard program for rapid viscosity analysis with some modification (Approved Method No. 61-02; AACC, 1995); 3 g of each sample was adjusted to obtain 14% moisture. Distilled water was then added to keep the total weight of water and sample constant at 18 g. The rotating paddles were held at 50°C for 2 min to stabilize the temperature and ensure uniform dispersion, then heated to 92°C at a rate of 5.6°C/min, and held constant at that temperature for 5 min. The samples were then cooled to 50°C at 5.6°C/min.

Dough and tortilla preparation and evaluation

Texture of dough and tortillas

The texture of the dough (hardness and adhesiveness) and tortillas (tensile strength and cutting force) was determined using a texture analyser (Texture Technologies Corp., Scarsdale, NY/Stable Micro System, Godalming, Surrey, UK). Hardness and adhesiveness of the dough was determined as reported by Martínez-Bustos, García, Chang, Sánchez-Sinencio, and Figueroa (2000). Preparation of tortillas, yield of tortillas, tensile strength, and cutting force of tortilla were calculated according to the methodology of Mauricio et al. (2004).

Experimental design and statistical analysis

All treatments were performed randomly and the data were analyzed by response surface methodology using MiniTab 14. The significance of the models was tested using variance analysis (F test). The effect of the variables was displayed in surface graphs. A compound central experimental design with 14 treatments and five repetitions in the central point was used for the obtaining corn flour by continuous ohmic heating. The variables evaluated were feed moisture (50–60%), screw speed (10–20 rpm), and process temperature (75–85°C). Dunnet's test was used to compare the quality of nixtamalized corn flour utilizing by ohmic heating continued with the traditional process.

Results and discussion

Rheological and physicochemical characterization of flours

Table 1 shows the analysis of variance for the quality variables of nixtamalized corn flour processed by a continuous ohmic heating system. It can be observed that the moisture feed and temperature process were independent variables with a significant effect on the water absorption index (WAI), WSI, and viscosity, as well as on tortilla yield.

Table 1. Regression coefficients and analysis of the variance and its adjustment to the response variables.
Tabla 1. Coeficiente de regresión y análisis de varianza.

Coefficient	WAI	WSI	Relative viscosity	Adhesiveness force	Hardness	Tensile strength	Cutting force	Tortilla yield
Intercept	−20.83	−38.53	63380.20	1184	938.74	1678	−1934	0.870
Feed moisture (FM)	−0.07^NS	−0.06^NS	−576.75**	−11.88^NS	−7.3^NS	−30.84^NS	−36.17^NS	−0.018^NS
Temperature (T)	0.59*	1.34*	−1116.71***	−19.88^NS	−11.81^NS	−4.93^NS	63.05^NS	0.042*
Screw speed (SS)	0.14^NS	−0.70^NS	−83.99^NS	−7.16^NS	−24.36^NS	−70.24^NS	244.7^NS	−0.065***
FM × T	<0.001	−0.01*	2.02^NS	0.047^NS	−0.03^NS	0.785^NS	3.21^NS	−0.0003*
FM × SS	<0.001^NS	<0.001^NS	−0.15^NS	0.076^NS	−0.15^NS	0.414^NS	−2.916^NS	0.0008***
T × SS	<0.001*	0.01^NS	−0.06^NS	0.011^NS	0.13^NS	0.509^NS	−1.793^NS	0.0001^NS
FM × FM	<0.001^NS	0.01*	3.77**	0.064^NS	0.0^NS	−0.359^NS	−1.646^NS	0.0002**
T × T	0.001***	−0.01^NS	6.06***	0.108^NS	0.09^NS	−0.272^NS	−1.380^NS	−0.0001^NS
SS × SS	0.001^NS	<0.001^NS	2.20^NS	0.082^NS	0.75*	0.209^NS	1.395^NS	0.0003**
Note: *Significant P < 0.05; **Significant P < 0.01; ***Significant P < 0.001. NS, not significant; WAI, water absorption index; WSI, water suability index.
Nota: *Significativo P < 0,05; **Significativo a P < 0,01; ***Significativo a P < 0,001. NS, no significativo; WAI, índice de absorción de agua; WSI, índice de solubilidad en agua.

Figure 2 shows of WAI values. Dunnet's test was used to obtain statistically significant differences between the COHF's and the TPCF's WAI values. The COHF's WAI showed values between 2.48 and 3.15 g gel/g flour, while the TPCF's WAI was 2.23 g gel/g flour. This parameter is one of the most important physicochemical characteristics of nixtamalized corn flour, as it concerns the quality of tortilla texture and effects on its yield (Almeida-Domínguez, Cepeda, & Rooney, 1996). It is observed that WAI increases at high temperatures and high feed moisture (Figure 2). WAI depends on starch potential and its interaction with water; their interaction is not only related to the amount of existing water (Tester & Sommerville, 2000) but also to the characteristics of starch granules (Manelius & Bertoft, 1996). The high value of WAI in COHF may be due to electroporation (electropermeabilization) on the granule's surface, a dynamic phenomenon whose occurrence has been reported in processing food with this technology (Moreno et al., 2012); electroporation can lead to an easy disarray of the granule's surface allowing a greater starch–water interaction, as well as a homogeneous rise in temperature during the process. Water–starch interaction is desirable to obtain better textural characteristics of tortilla as it makes them softer and more flexible.

Figure 2. Physico-chemical properties of ohmic heater corn flour.

Figura 2. Propiedades fisicoquímicas de harinas de maíz óhmicamente calentadas.

The WSI is a feature that indicates the degree of fragmentation while cooking the corn flour, expressed as water-soluble solids (Bressani, Turcios, Reyes, & Mérida, 2001). This characteristic of the flour was not affected by the screw speed (Figure 2), but the temperature and the feed moisture had significant differences (P < 0.05). WSI has been linked to the starch's fractioning degree during the process, especially if shear stress and heat are applied for an extended period of time, as in nixtamalization by extrusion (Bhatnagar & Hanna, 1994; Tang & Ding, 1994). According to the WSI values for COHF, we can say that the damage caused during a continuous ohmic heating process (Figure 2) is not greater than during traditional nixtamalization.

COHF processed at high temperature shows a high value of WSI independently of the feed moisture and screw speed (Figure 2). WSI values for COHF were between 9.43 and 11.55% while those for TPCF were 5.96%. Campechano-Carrera et al. (2012) mentioned that during nixtamalization, there are corn dry matter losses (between 4 and 6%) which are influenced by several key processing parameters such as endosperm hardness, quality grain cooking, the temperature, and steeping time. However, during the ohmic heating, there are no losses because it is a closed system.

Dunnet's test shows significant difference (P ≤ 0.05) between the paste properties for both COHF and TPCF. COHF viscocity values were 391.61–2000 cP and TPCF's were 5063.75 cP. Gaytán-Martínez et al. (2012) reported that pasting properties of ohmically heated flour were different from conventionally heated flour. Ohmically heated flour had lower pasting temperature and viscosity and retrogradation tendency (Figure 2). Different pasting properties between traditional and ohmic heating might be because of different cooking mechanisms. Ohmic heating provides an electrical resistance heating when AC is passed through an electrically conducting food product (Palaniappan & Sastry, 1991). Thus, ohmic heating can simultaneously heat the solid and the liquid phase of a food, whereas conventional heating relies on heating of the liquid phase to transfer heat to the solid phase (Sastry & Palaniappan, 1992). A decrease in pasting temperatures for COHF samples after ohmic heating means less energy was required to cook them. Therefore, ohmic heating COHF starch increase faster than an untreated or conventionally heated sample, probably due to the electroporation effect in the presence of an electric field (Lima & Sastry, 1999). Electroporation made the starch granule more permeable to water during heating, resulting in less viscosity with less cooking stability, rendering it less stable to retrogradation as determined by total setback (Tananuwong & Reid, 2004; Goldstein, Mekondjo-Nantanga, & Seetharaman, 2010), most likely due to greater amylose leaching. It is well known in starch chemistry that amylose leaches from starch granules during heating.

Textural properties of dough

The dough texture test is important in the elaboration of tortillas because it can measure when a dough has the property to be cut or laminated (Ramírez-Wong & Ortega, 1993). Tortillas made with non-cohesive or adhesive dough have poor quality. The texture of the corn dough depends on water absorption and the degree of starch gelatinization (Bedolla & Rooney, 1984), which corresponds with the results found in this research. Figure 3 shows dough yield for COHF and TPCF. Statistical analysis showed no significant difference between the mass yield obtained by ohmic heating and the traditional process. Also it is noteworthy that the adhesion strength of the dough was not significantly affected by the type of process utilized.

Figure 3. Texture of dough of ohmic heater corn flour.

Figura 3. Textura de dough de harinas de maíz óhmicamente calentadas.

The dough adhesiveness for TPCF was 21.70 g-F and was between 16.33 and 30.79 g-F for COHF (Figure 3). The adhesion values did not show statistically significant difference between them. However, there is a trend toward higher values for dough produced at a high temperature and humidity. This behavior is due to a higher degree of starch gelatinization. The adhesion of the dough is a parameter that is related with the functionality to makes tortillas; therefore, high adhesion values means greater “stickiness” or bond strength of the dough, considered undesirable since it impedes the formability of the tortilla (Bello-Pérez et al., 2002).

Hardness values for COHF dough were between 158.87 and 269.98 g-F and for TPCF dough was 175.94 g-F. The dough made from COHF exhibited more hardness than TPCF dough. Dry dough will have high values of hardness and low values of adhesiveness. Quintanar-Guzmán, Jaramillo-Flores, Mora-Escobedo, Chel-Guerrero, and Solorza-Feria (2009) mentioned that with the increase in cooking time, the dough becomes harder, i.e., the mass will become more rigid, possibly due to starch gelatinization. The values of cohesiveness indicate that dough from COHF is appropriate to make tortillas.

Physicochemical, textural, and rheological properties of tortilla

According to statistical analysis, the interaction between feed moisture and screw speed had influence on tortilla moisture (Table 1) (P < 0.05). The tortilla's moisture made with COHF ranged between 43.65 and 46.60%, which is greater than those obtained with TPCF (42.95%) (Figure 4). Although the conditions between the two processes are different because in the traditional process the steeping time is very long and in the ohmic process it is short, the values found for tortilla moisture in the two processes indicate that the water is bonded in a similar way during the baking of tortillas.

Figure 4. Quality assessment of tortilla of ohmic heater corn flour.

Figura 4. Calidad de tortilla de harina de maíz óhmicamente calentada.

Another parameter that was measured for tortilla is the tension strength. The tension strength is an important quality parameter for tortillas because it can measure the flexibility in rolling them (make “tacos”). Figure 4 shows the tension strength for tortillas made from COHF, which had values in the range of 59–221.64 g-F and had value of 160.55 g-F for tortillas made from TPCF. Statistical analysis did not show a significant difference between the two processes.

With respect to the cut strength, there was no significant difference with the types of processes used. The mean value for tortillas made from COHF was between 588.04 and 1156.58 g-F while it was 1782 g-F for tortilla from TPCF. Tortillas from COHF were softer than tortillas from TPCF; this is mainly due to the pericarp gums which are not lost during the ohmic heating process. The pericarp gums help with the cohesion of the tortillas. In the traditional process, the pericarp gums are lost as waste.

Conclusions

It was possible to obtain corn flour with a continuous ohmic heating system. The characteristics of the continuous ohmic heating flour have similar characteristics to those obtained by traditional process but with the advantage of a better tortilla yield and softer texture. The process parameters that have great influence in a continuous ohmic heating system are the feed moisture and the temperature. The screw speed has no influence because there is not any shear stress. This is very important because it is possible to obtain good quality corn flour independently of the rate feeding. So, continuous ohmic heating has a great potential to be used in the nixtamalization industry with the advantage of being an environmentally friendly technology.

Acknowledgements

The authors thank CONACYT and IPN for their financial support through grant SIP-20090967 and SIP 20100209.