Effects of Calcium Hydroxide and Screw Speed on Physicochemical ...

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Food Engineering and Physical Properties

Effects of Calcium Hydroxide and Screw Speed on Physicochemical Characteristics of Extruded Blue Maize J. ZAZUETA-MORALES, F. MARTINEZ-BUSTOS, N. JACOBO-VALENZUELA, C. ORDORICA-FALOMIR, AND O. PAREDES-LOPEZ

ABSTRACT: The effects of calcium hydroxide and screw speed on expansion, pasting, crystallinity, torque, water absorption, and solubility indices upon extruded blue maize meal products were studied, using a central composite experimental design. All the characteristics measured presented a significant quadratic regression model, indicating relationship between the responses and extrusion conditions. It was found that the expansion, crystallinity, torque, water absorption, and solubility indices decreased and the Rapid Viscosity Analyzer’s (RVA) pasting characteristics increased with the increase of calcium hydroxide concentrations. The Pearson correlation indicated that the responses presented strong correlation (r > 0.7, p < 0.01) between them. Extruded products with physical appearance and expansion similar to the control were found at concentrations lower than 0.1% of calcium hydroxide. Keywords: blue maize, calcium hydroxide, extrusion, physicochemical characteristics, RVA-pasting characteristics


Introduction

A

T LOW COST AND HIGH EFFICIENCY, EXTRUSION PROCESSING IS A

widely used technology to process cereals or starches into food and industrial products, like snack foods, ready-to-eat cereal, and infant formulas. Extrusion variables (such as screw speed, temperature, feed moisture, and the addition of other materials) can alter the physical and chemical properties of starch materials (Owusu-Ansah and others 1983; Chinnaswamy and Hanna 1988; Pan and others 1998). The addition of diverse ingredients also has the objective of improving the nutritional quality of foods and imparting desirable functional properties. In México, races of diverse colored maize are used in the preparation of traditional foods. The natural pigments (anthocyanins) present in pigmented maize currently are considered as nutraceutical compounds and effective food additives (Nakatani and others 1979; Thadens and Verstrynge 1989; Marcus 1992; Lee and others 1997; Salinas and others 1999a; Delgado-Vargas and Paredes-López 2000). It has been suggested that anthocyanins may prevent oxidative damage caused by active oxygen radicals in living organisms. Calcium is a major essential macroelement that can make a positive contribution to the human diet as required for infants and adolescents during fast growing periods. Calcium is the principal constituent for bone and teeth formation. Calcium-modified starch complex formation imparts desirable functional properties, besides increasing the calcium supply through processed food (Joseph 1982; Robles and others 1988). Data on the nature of calcium-starch complexes induced by extrusion cooking are scarce. Gomez and others (1989) studied the structural changes in both corn and sorghum during alkaline processing and found that some of starch granules in the peripheral endosperm were destroyed. In fact, the reduction of relative crystallinity of starch during nixtamalization was 15 to 25%, and the loss of birefringence of starch granules was 4 to 7%. Bryant and Hamaker (1997) studied the effect of lime cooking and con3350

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centrations on corn flour-water suspensions and postulated that at low lime levels, the granules’ matrix is stretched by exchange of protons with Ca++ or CaOH+ ions. They also found that at high concentrations of lime, crosslinking occurs and produces breakdown-resistant granules with high viscosity. Martínez-Bustos and others (1998) studied the effects of added calcium hydroxide concentrations, feed moisture content, and barrel temperature on the properties of corn meal extrudates. These authors found that by using concentrations below 0.25% of calcium hydroxide, water absorption and solubility indices as well as crystallinity of starch granules increased, which further suggested the formation of starch-calcium complexes. Zazueta-Morales and others (2001) reported the effects of calcium hydroxide and screw speed on blue maize meal extrudates’ physicochemical characteristics (bulk density, penetration force, specific mechanical energy, and expansion). They suggested it is possible to elaborate appropriate extruded products from blue maize fortified with calcium in an optimized area between 0.02 to 0.078% of calcium hydroxide, at 117 to 180 rpm screw speed. The present study was designed to investigate the effects of calcium hydroxide and screw speed on some physicochemical characteristics of blue maize extrudate products.

Materials and Methods Materials Harvested from January to February 1998, dented blue maize was obtained from the state of Puebla, México. The calcium hydroxide obtained was reagent grade.

Flour preparation This step’s objective was to reduce the lipid content of germ and pericarp that can affect the expansion property. Blue maize was steeped in distilled water at room temperature (24 to 26 °C) for 2 min, drained, and ground in a disk mill (Nixtamatic Model © 2002 Institute of Food Technologists

Extruded blue maize with lime . . .

Extrusion process Used for extrusion was a single-screw Brabender® laboratory extruder (Model 20DN/8-235-00, C.W., Brabender Instruments Inc., Diusburg, Germany). It was equipped with a 19-mm barrel dia, 20:1 barrel:length-to-diameter ratio, 3:1 extruder screw compression ratio, and a 4.0-mm die-nozzle. The barrel was separated into independent electrically heated zones (feed and compression), and cooled by compressed air which circulates around the barrel to maintain precise temperature control. The die assembly was electrically heated. The temperatures of the feed, compression, and die assembly zones were held at 85, 120, and 150 °C, respectively. Blue maize meal samples were supplemented with calcium hydroxide (0.0 to 0.2% d.b.) in accordance with the experimental design. Initially the calcium hydroxide was manually dispersed into a small portion (about 50 g) of the meal, using a laboratory glass beaker. Later, this portion of flour was added slowly to the rest of flour sample (about 1500 g) using a laboratory size mixer (Kitchen Aid, Model K5SS, Benton Harbor, Mich., U.S.A.), at minimum speed. The obtained meal samples were adjusted at 16% moisture content and force-fed into the extruder at constant rates of 65 g · min –1. The independent variables were: calcium hydroxide concentration (% d.b.) and screw speed (rpm).

Analytical methods The proximate composition was determined using official AOAC (AOAC 1990) procedures: Method 925.09 for moisture, 979.09 for protein, 923.05 for lipids, 923.03 for ash, and 962.09 for fiber.

Expansion index (EI) The expansion index (EI) was calculated for 15 samples by dividing the average diameter of the extruded products by the internal diameter of the extruder die-nozzle orifice (Faubion and Hoseney 1982; Colonna and others 1989; Balandrán-Quintana and others 1998).

Pasting properties of extruded products Pasting characteristics were evaluated using a Rapid Visco Analyzer (RVA-3, Newport Scientific, New South Wales, Australia). Flour sample suspensions were prepared by weighing 4 g milled, dried (50 ± 2 °C, 12 h) extrudates with 7.5 to 8.5% moisture content into an RVA canister and individually adjusting each sample to the total weight of 28 g, using distilled water. Under constant stirring at 75 rpm, the heating profile was held at 50 °C for 2 min, heated to 92 °C at a constant rate of 5.6 °C/min –1, held at 92 °C for 5 min, then cooled to 50 °C at the same rate, and finally held at 50 °C for 1 min. Recorded then were RVA-pasting parameters such as viscosity at 92 °C (V92), minimum viscosity (MinV, or lowest viscosity at the end of heating constant period at 92 °C), and final viscosity (FinV, or maximum viscosity attained during cooling to or holding at 50 °C). From these parameters’ values, the to-

tal setback viscosity (final minus minimum viscosity) (Zeng and others 1997) was calculated.

X-ray diffractometry Ground samples with 7.5 to 8.5% moisture content, which passed through a 1.19-mm mesh sieve, were packed in a glass sample plate (0.5 mm deep), and mounted onto a Rigaku X-Ray Diffractometer (Model Ultima D/Max-2100, Rigaku Denki Co. Ltd, Japan). The scans were made from a Bragg angle of 5° to 30° on a 2␪ scale with a step-size of 0.02, operating at 30 KV and 16 mA with CuKµ radiation wavelength ␭=1.5406 Å. Relative crystallinity was calculated on the basis of Herman’s method, as described by Nara and others (1978) and Gomez and others (1989). The area of the crystalline fraction was divided by the diffraction area of the control sample. The area of the crystalline fractions in the X-ray diffraction pattern of the non-extruded blue maize meal (BMM) was considered as the control sample with 100% crystallinity. Thus, relative crystallinity was calculated as: RC(%) = (CRCP x 100) / CRRC where RC = relative crystallinity (%), CRCP = crystalline region of cereal product, and CRRC = crystalline region of raw cereal (control). Two measurements were taken for each sample.

Torque The torque (N·m), which indicates the resistive load on the motor (Harper 1981), was registered directly from the Brabender® Do-Corder DCE drive unit.

Water absorption (WAI) and water solubility (WSI) indices WAI and WSI were determined in triplicate by procedures described by Anderson and others (1969) and are reported as percentage in this study. The extrudates were milled and passed through a 1.19-mm mesh sieve. A 0.5-g sample was dispersed in 20 mL of distilled water into tared centrifuge tubes and stirred for 30 min, then centrifuged (Sorvall Model RC-2, Norwalk, Conn., U.S.A.) at 15000 × g for 15 min. The supernatant was then decanted for determination of solid content, and the sediment was weighed and expressed as g of solids per g dry sample to give the WSI (%). After decanting the supernatant, the remaining sediments were weighed and expressed as g of water absorbed per g dry sample to give the WAI (%).

Statistical analysis Pearson correlations and comparisons of means (Duncan test) were performed using the Statistica ® Version 5 software package from StatSoft, Inc. (1997). Two factors with 5 levels each were evaluated: calcium hydroxide concentration (%), 0, 0.05, 0.1, 0.15, and 0.2; and screw speed (rpm), 100, 120, 140, 160, and 180. A central composite nonroutable experimental design with ␣ = 2.0 was used. Experimental data were fitted by a 2nd-order model (Myers 1971): Yi = b0 + b1X1 + b2X2 + b11X12 + b22X22 + b12X1X2 where Yi is the response; X1 is the calcium hydroxide concentration; X2 is screw speed, and b0, b1, b2, b11, b22, and b12 are the regression coefficients. The experimental design was composited by 4 factorial points, 4 “star” points, and 5 central points. Studied responses were: EI, pasting properties (V92, MinV, FinV, and setback), relative crystallinity, torque, WSI, and WAI. All treatments were performed randomly and the data were analyzed by response surface methodology (RSM) with SAS ® software (SAS 1992).

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CGP-134M, Koldenz, México). The ground maize was dried in an oven at 80 ± 3 °C for 30 min. The fraction passing through a 1.19mm mesh sieve was separated, and the fraction retained was milled using a Pulvex Model 200 hammer mill from México, equipped with an 0.8-mm sieve. Through this process the lipids originally presenting in the whole grain were reduced by approximately 50%. The proximate composition (% d.b.) of the blue maize meal (BMM) was 8.54 ± 0.01 moisture, 3.62 ± 0.3 protein, 3.73 ± 0.02 lipids, 1.42 ± 0.03 ash, 2.21 ± 0.08 fiber, and 81.03 carbohydrates (by difference).

Extruded blue maize with lime . . . Results and Discussion Expansion index (EI)


The addition of calcium hydroxide in concentrations from 0.0 to 0.2% wt dry matter decreased EI from 3.4 to 1.4. Higher values of EI were found at lower calcium hydroxide concentrations with similar values to the control (0.0% Ca(OH)2) (Table 1). Physical appearance of the extrudates with concentrations of 0.0 to 0.2% of calcium hydroxide and 140 rpm are shown in Figure 1. The products obtained with 0.05% of calcium hydroxide and 120 and 160 rpm screw speeds presented physical appearance similar to the control (data not shown). Under these conditions the values found for EI were larger than 2.1. Concentrations of calcium hydroxide higher than 0.1% probably resulted in the formation of complexes with starch, decreasing EI. Presumably, the complex starch-calcium restricted the extensive flash-off of internal moisture, once the starch emerges from the die. This behavior likely can be explained through the calcium ions electrostatically binding to the OH - group of the starch, thus making the molecule more compact (Camire and Clydesdale 1981). Statistical analysis indicated that only the calcium hydroxide presented significant effect in its lineal and quadratic (p < 0.0001) terms. The screw speed did not have significant (p > 0.11) effect. The characteristics of the starch-based foods are modified with the change of conformational and structural characteristics, due to the addition of complexing minerals (like Ca++) (Nurul-Islam and Mohd.-Azemi 1994). Acidic or alkaline conditions considerably decrease the average molecular weight of the starch. They also reduce the expansion index (Kervinen and others 1984). Other results showed that sodium bicarbonate added to wheat and corn starch before extrusion decreased expansion index (Chinnaswamy and Hanna 1988b; Lai and others 1989). Martínez-Bustos and others (1998) reported that in corn meal extrusion, the best values for samples’ EI (2.87) of samples were found when samples were extruded with the addition of 0.15% calcium hydroxide at 18.0% feed moisture and 130 °C barrel temperature. Pan and others (1998) observed an expansion index loss when expanded products were produced with urea and NaHCO3. This behavior was attributed to partial degradation of starch molecules by the addition of these ingredients. These authors found similar behavior when screw speed was increased to values above 110 rpm.

Figure 1—Physical appearance of blue maize extrudate products as affected by Ca(OH) 2 concentration at 16% moisture content and 140 rpm screw speed of extrusion 3352


Pasting properties Figure 2 (viscosity at 92 °C, V92) and Table 1 (minimum, final, and setback viscosity) show the RVA-pasting characteristics of extruded blue maize products. All these response variables showed significant RSM quadratic regression models (R2 > 0.75, CV < 11.93, p de F < 0.0001, and lack-of-fit > 0.05). In all pasting characteristics, statistical analysis indicated that Ca(OH)2 presented a highly significant effect in the lineal term (p < 0.0001) and in the quadratic term (p < 0.024). On the other hand, screw speed did not present a significant effect (linear p > 0.54, quadratic p > 0.37), except for the quadratic term of setback viscosity which was highly significant (p < 0.0005). Furthermore, in all cases the interaction did not have significant (p = 0.05) effect. An increase in Ca(OH)2 concentration resulted in an increase in RVA-pasting values, in all the evaluated ranges of calcium hydroxide concentrations. The strongest changes in viscosity properties were found in concentrations from 0.1 to 0.2% (Figure 3). As documented by researchers Nurul-Islam and Mohd.-Azemi in 1992 and 1994, calcium interacts with starch resulting in the formation of complexes. The alkaline pH and extrusion conditions induce starch swelling and gelatinization, and may unfold the starch molecules and expose the reactive sites. Under such conditions, it is expected that more calcium would be bound with higher alkalinity of the starch (Rendleman 1978a, 1978b; NurulIslam and Mohd.-Azemi 1992, 1994). Also, the addition of calcium hydroxide in extrusion conditions may facilitate the formation of complexes which influence the starch granules’ structure and properties. Using concentrations of 0.1% calcium hydroxide or higher resulted in products that apparently were not fully gelatinized; at lower concentrations than 0.1% and 0.0%, the products were fully or almost completely gelatinized (Figure 3). The behavior in the RVA of extruded blue maize meal products with and without 0.05% calcium hydroxide, during heating in excess water, indicated a high initial uncooked viscosity and the absence of a gelatinization peak during heating, as well as a continuous decline in viscosity during holding at 92 °C and cooling to 50 °C (Figure 3). The addition of calcium hydroxide in concentrations from 0.1 to 0.2% may have resulted in reinforcing the granules through bonded bridges, which do not rupture on being heated with water or continuous stirring. The starch granules

Figure 2—Effect of Ca(OH)2 concentration and screw speed on the viscosity at 92 °C (V92, RVU) of blue maize extrudate products


Figure 3—RVA profiles of blue maize extrudates products as affected by Ca(OH)2 concentration and screw speed of extrusion

Bryant and Hamaker (1997) reported the effect of lime cooking and concentrations (from 0.0 to 1.0%) on the viscosity of defatted corn flour-water suspensions, using Brabender amylographs. These authors found that the hot-paste peak viscosity was highest at 0.1% lime concentration. Further addition of lime (0.1 to 0.5%) decreased peak viscosity. The behavior found by these authors on hot-paste peak viscosity as well as minimum and final viscosity presented from 0.0 up to 0.1% lime concentration, which was similar to this work’s results.

X-ray diffractometry Figure 4 shows the diffractograms of analyzed samples. The diffractograms of unextruded BMM samples were similar to those reported by Inouchi and others (1984) for normal and waxy corn starches, and also those reported by Martinez-Bustos and others (1998) for corn meal. These structures showed a typical Apattern of cereal starches (Hoseney 1994). Extruded samples with 0.0, 0.1, and 0.2% of Ca(OH)2 showed similar structures between them during X-ray analysis, with 2 main peaks at 2␪ values of 13 and 19.8 Å, corresponding to d-spacing of 6.86 and 4.50 Å, respectively. Also, these 3 diffractograms presented a type of structure different to those of the unextruded BMM and commercial corn starch (CCS) samples; furthermore, it was noted that they differed between them. The diffractograms from extruded BMM with 0.0, 0.1, and 0.2% of Ca(OH)2 were similar to those reported by Mercier and others (1979) as well as Singh and others (1998), in their examination of extruded starch in the pres-

Figure 4—X-ray diffraction pattern: commercial corn starch (CCS), unextruded blue maize meal (BMM), and extruded product at different concentrations of calcium hydroxide Vol. 67, Nr. 9, 2002—JOURNAL OF FOOD SCIENCE

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absorb water and swell, showing a peak during the heating cycle, decreasing the viscosity during holding at 92 °C and increasing it during cooling to 50 °C. The introduction of a very small number of crosslinks between carbohydrate molecules is responsible for the increase in paste consistency. According to Rendleman (1966a) and Oosten (1982), some starch hydroxyl groups in alkaline pH can be ionized, making them sites for interaction with the Ca ++ or CaOH + of dissociated Ca(OH)2. Thus, the calcium hydroxide seems to have a protective effect on BMM extruded starch granules. This has been demonstrated using optical microscopy (data not shown). These microphotographs showed some starch granules that were not completely gelatinized by the effect of calcium hydroxide during the extrusion process. Accordingly, these starch granules could be responsible for the increase in RVA-viscosity, as shown by the extruded samples when the concentration of calcium hydroxide was increased. Some authors (Rendleman 1966a, 1966b, 1966c; Rorabacher and Moss 1970; Angyal 1973; Rendleman 1978a, 1978b) have reported that the formation of mineral binding is dependent on various factors such as ionic radius, ionic strength, coulombic field, electromotive forces of ligands and cations, stearic arrangements of hydroxyl groups, in addition to the capability of replacing the water of hydration or cation-solvent bonds from around the metal ions in aqueous media or in other solvents, as well as pH, temperature, and holding time for interaction. The physical action of crosslinking agents (like formaldehyde) has been likened to that of alkali on starch (Samec 1927). With crosslinked starches, the granule may swell as the hydrogen bonds are weakened. After rupture, however, the chemically bonded crosslinks may provide sufficient granule integrity to keep the swollen granules intact and minimize or prevent viscosity loss. On the other hand, extreme values of pH generally tend to have a negative impact on viscosity by hydrolyzing bonds and disrupting the granules’ molecular integrity.

Extruded blue maize with lime . . . Table 1—Mean values of EI, RVA-pasting properties, and torque of extruded products Treatment 1 2 3 4 5* 6 7 8 9 10 11 12 13

Ca(OH)2 (%)

Screw speed (rpm)

0.05 0.15 0.05 0.15 0 0.20 0.10 0.10 0.10 0.10 0.10 0.10 0.10

120 120 160 160 140 140 100 180 140 140 140 140 140

EI 2.44 1.46 2.80 1.50 3.06 1.48 1.60 1.74 1.41 1.41 1.60 1.75 1.54

(0.04)a (0.01) (0.07) (0.07) (0.11) (0.04) (0.06) (0.04) (0.01) (0.01) (0.07) (0.04) (0.11)

FinV (RVU) 51.5 (0.7) 94.0 (4.2) 58.0 (5.7) 98.0 (1.4) 45.5 (2.1) 109 (1.4) 93.5 (6.4) 91.0 (5.7) 94.5 (9.2) 96.0 (1.4) 94.5 (3.5) 69.0 (2.8) 87.0 (4.2)

MinV (RVU) 45.0 80.0 43.5 84.5 33.5 86.0 77.5 80.5 80.0 82.5 81.5 68.2 75.0

(2.0) (2.0) (1.5) (3.5) (1.5) (1.0) (2.5) (0.5) (5.0) (0.5) (1.5) (4.5) (2.0)

Setback Viscosity (RVU) 9.5 15.0 13.5 16.0 12.5 21.0 17.0 15.5 13.0 14.0 13.5 13.0 12.5

(0.7) (0.0) (0.7) (1.4) (0.7) (0.0) (2.8) (0.7) (0.0) (0.0) (2.1) (1.4) (2.1)

Torque (N.m) 15.5 9.5 14.5 9.5 16.5 9.5 14.0 8.5 9.3 9.2 10.5 10.2 10.8

(0.5) (0.5) (0.5) (0.5) (0.5) (0.5) (0.0) (0.5) (0.8) (0.6) (0.5) (0.3) (0.3)

*Treatment used as control a Value between parentheses is the standard deviation


ence of lipids. Similar structures were reported by Martínez-Bustos and others (1998) in extruded corn meal with added calcium hydroxide. These reported structures are known as Vhydrate (Vh) types. Other authors as well (Colonna and Mercier 1983; Bhatnagar and Hanna 1994) have reported the formation of Vh-type (or a mixture of Vh and E h types) when starches are extruded with fatty acids, monoglycerides, or in samples with their native lipids. Extrusion cooking either partially or completely destroys the organized crystalline structure, depending on the amylase:amylopectin ratio and on extrusion variables such as

Figure 5—Relative crystallinity of commercial corn starch (CCS) and unextruded blue maize meal (BMM) samples, and processed by extrusion at different Ca(OH)2 concentrations and 140 rpm samples. The data are expressed as mean of 3 replicates. Graphic with different letters differ significantly (p = 0.01) (Duncan method). 3354


moisture, shear, temperature, and so forth. At higher temperatures the structure is completely destroyed, leading to an X-ray pattern typical of an amorphous state; in fact, a new type of structure may even be formed (Colonna and others 1989). The diffractograms showed that the effect of the extrusion process induced strongest changes on crystallinity loss than the calcium hydroxide concentration did. The actual crystalline content of the CCS and BMM were 7.29 ± 0.094% and 12.01 ± 0.021%. On the other hand, the crystalline content of extruded BMM at 0.0% Ca(OH) 2 was 2.60 ± 0.076%, and with 0.1 and 0.2% were 1.84 ± 0.009% and 1.91 ± 0.039%, respectively. Gomez and others (1989) found a decrease in starch crystallinity and birefringence in corn cooked with lime. The relative crystallinity of extruded samples is shown in Figure 5. By increasing the calcium hydroxide concentration from 0.0 to 0.1%, the relative crystallinity of extruded samples decreased significantly (Duncan, p = 0.05), then leveled off. The crystallinity of extruded samples can be attributed to the complexation between amylose solubilized with native lipids (3.71%). Increasing the calcium hydroxide concentration increased the pH, facilitating the linkage between calcium ions and amylose, thus reducing the active sites of amylose that may link with native lipids. Also, optical microscopy showed that an increase of calcium hydroxide concentration in extruded products resulted in increasing the number of starch granules not gelatinized (data not shown). This is probably due to the protective effect of calcium hydroxide on the starch granules. Because of this and the fact that the amylose’s reactive sites will be bound to the calcium, there will be a smaller amount of amylose molecules in the medium to react with the sample’s native lipids, therefore decreasing the area of crystallinity in the diffractograms. In the concentrations of calcium hydroxide used, the increase of non-gelatinized starch granules was relatively small, and for this reason only small peaks were detected upon X-ray. However, this behavior was also detected during viscosity analysis, where an increase in viscosity parameters with the increasing of calcium hydroxide was observed. The relative crystallinity of extruded starch increases with the addition of fatty acids or monoglyceride (MG) in concentrations from 0 to 1%, thereafter remaining constant (Singh and others 1998). Martinez-Bustos and others as well (1998) reported an increasing crystallinity in corn meal extrudates when calcium hydroxide concentrations were increased from 0 to 0.15% at 16% moisture content, and at 130 °C or 150 °C barrel temperature. However, when these authors increased the calcium hydroxide

Table 1 shows a reduction of torque when both Ca(OH)2 concentration and screw speed were increased. The torque exhibited a significant RSM quadratic regression model (R2 = 0.83, coefficient of variation, CV = 9.76%, p of F < 0.0001, and lack-of-fit = 0.06), indicating a relationship between this response and extrusion conditions. Both the lineal and quadratic terms presented a highly significant effect in the hydroxide concentration (p < 0.0001) and screw speed (p < 0.02), but not the interaction term (p > 0.44). As the corn meal dough in the extruder presents both thixotropic and pseudoplastic behavior, there is a linear relationship between speed and torque. Also, this behavior probably is due to the barrel-fill length which decreases with increasing screw speed (Harper 1981; Frame 1994). According to Meuser and Wiedmann (1989), both SME and product temperature increased with increasing screw speed. But in this work, the addition of calcium hydroxide and/or increase of screw speed decreased the torque. This behavior was similar to reports by Garber and others (1997) and Chang and others (1969), who used a twin-screw extruder and corn meal. These authors reported that the pseudoplastic behavior of corn meal and a reduction in fill of screw channels caused torque to decrease as screw speed increased. Furthermore, Yeh and Jaw (1999), using a single-screw

extruder and rice flour, reported similar behavior. They postulated that feed rate and screw speed are 2 major variables that affect the barrel fill, operating characteristics, and properties of the extrudates. These authors studied the effects of screw speed and feed rate on torque, SME, and extrudate properties during single-screw extrusion cooking of rice flour with 37% moisture content. They reported that increasing screw speed resulted in reduction of torque, and that torque was affected by the screw channel’s degree of fill in the extruder (that is, the increase in degree of fill resulted in torque’s increase). These authors additionally concluded that torque was more sensitive to change in operating conditions and therefore appeared to be a good control variable. Screw configuration controls the specific mechanical energy input range as well. The decline in mechanical energy input and residence time results in viscosity and product density increases (Meuser and Wiedmann 1989). In this study, it is possible that the decrease of torque upon the increase of Ca(OH)2 resulted in alkaline conditions which unfolded the starch during extrusion cooking. For example, this might have decreased the shear-effort into the extruder and therefore decreased the torque value. Pan and others (1998) reported that during corn starch extrusion, an increase in urea concentration produced a strong energy reduction (as SME). On the other hand, when these authors added sodium bicarbonate, the SME values remained almost constant, with only slight increase at the salt’s higher concentrations (> 5%). Meuser and Wiedmann (1989) reported that product temperature increases with SME as well as conduction of thermal energy through the barrel wall. Higher temperature of the mass leads to a change in its viscosity, which also depends on the degree of starch plastification. Both the energy input and the structural changes to the mass affect the plastification and thus, the torque value. On the other hand, Grossmann and others (1988) studied the extrusion effects on properties of hydratation in manioc starch and indicated that the most important factor which affects the torque was screw speed. Screw speed was identified as having intense effect on how fast the die pressure and motor torque responses reached steady-state (Akdogan and Rumsey 1996). Akdogan (1996) also studied the extrusion conditions at a higher moisture content on pressure, torque, and ener-

Figure 6—Effect of Ca(OH)2 concentration and screw speed on the water absorption index (WAI, %) of blue maize extrudate products

Figure 7—Effect of Ca(OH)2 concentration and screw speed on the water solubility index (WSI, %) of blue maize extrudate products

concentration from 0.25 to 0.35%, the crystallinity was decreased when evaluated under the same conditions. The severity of the extrusion conditions can be related to the starch structure, as revealed by the crystallinity of the extruded samples with 0.0% calcium hydroxide. Some researchers have reported that important granular damage occurs when the specific mechanical energy (SME) input is higher than 500 to 600 0.63, p < 0.01) with all other sections of RVA profile, but negatively with WAI and WSI (r > –0.63, p < 0.01). In studies of corn nixtamalization, Sahai and others (1999) reported a direct correlation between peak viscosity and setback and final viscosity of nixtamalized fresh masa. Our results, however, are in disagreement to those reported by Bryant and Hamaker (1997). They studied the cooking of defatted flour-water suspensions, finding a strongly positive correlation between peak viscosity and water uptake (WRC) when lime concentration was increased up to 0.5%. They reported that under this circumstance, the peak viscosity and WRC decreased. This difference may be due to the varying processes used that can produce different degrees of molecular degradation. Pan and others (1998) indicated that samples with low water absorption display a high degree of molecular degradation. In this work,

it was postulated that the combined effect of extrusion conditions (screw speed, barrel temperature, and so forth) and increasing Ca(OH)2 concentrations induced starch degradation, and thus lower WAI values. Extruded products with higher expansion showed values of texture similar to the control (data not shown). Other important correlations found among all the analyzed responses are shown in Table 2.

Conclusions

T

HE ADDITION OF CALCIUM HYDROXIDE DURING EXTRUSION DE-

creased the dependent variables EI, torque, WAI, WSI, and crystallinity, whereas RVA-pasting characteristics were increased. Screw speed had significant effect only on torque and WAI values. Higher expansion values were found at concentrations lower than 0.1% of calcium hydroxide, with similar values to the control. These products also showed physical appearances similar to the control. The high pH of the alkaline system may promote starch hydroxyl group ionization, thus creating freedom for interaction between Ca++ or CaOH+ and starch molecules. The decreasing of EI with concentrations of calcium hydroxide higher than 0.1% is attributed to the formed starch-calcium complex that restrained the extensive flash-off of internal moisture, once the starch emerges from the die. Most likely, the calcium ions electrostatically bound to the starch’s OH – group participate in making the molecule more compact. The extrudates showed similar behavior in viscosity characteristics, water absorption, and water solubility indices than those of crosslinked starches that could be used in food-based starch products. The crystallinity of extruded samples decreased with the addition of calcium hydroxide, but did not affect the pattern of diffractograms. Blue maize extruded samples with calcium therefore impart some desirable functional properties, besides increasing the calcium supply through diverse prepared foods and anthocyanin content. Although extruded products showed texture values similar to the control, it will be necessary to carry out sensorial tests to determine this product’s consumer acceptability.

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Author Zazueta-Morales is with the Food Science Dept. of the Chemistry Faculty at Univ. A. de Querétaro and CINVESTAV-IPN, Unidad Querétaro, Mexico. Author Martínez-Bustos is with CINVESTAV-IPN-Unidad Querétaro, Libramiento Norponiente No. 2000, Fracc. Real de Juriquilla, Apdo. Postal 1-798, C.P. 76230, Querétaro, Qro., México. Authors JacoboValenzuela and Ordorica-Falomir are with the Univ. A. De Sinaloa, Apdo. Postal 1354, Culiacán, Sin., C.P. 82000, México. Author Paredes-López is with CINVESTAV- IPN- Unidad Irapuato, Mexico. Direct inquiries to author Martinez-Bustos (E-mail: [email protected]).