Characterization of Extruded Blends of Corn and Beans (Phaseolus Vulgaris) Cultivars: Peruano and Black-Querétaro under Different Extrusion Conditions

Abstract

The effect of extrusion conditions in blends of corn and beans (Phaseolus vulgaris) of cultivars Peruano and black-Querétaro were investigated in this study, as an alternative to obtain snack foods. The type of cultivar and beans percentage, and also the extrusion conditions (moisture and temperature) influenced the physicochemical (color and breaking strength) and the functional (water absorption index, water solubility index, and oil absorption capacity) properties of the extrudates. The microstructures showed the presence of cavities and starch granules gelatinized (melted) and plasticized; while, the x-ray powder diffraction patterns revealed the presence of monohydrate glucose due to starch dextrinization. The results demonstrate that extrudates with good properties can be obtained from blends of corn and beans, under selected extrusion conditions, depending on the bean cultivar.

Keywords:

• Peruano bean
• Black-Querétaro bean
• Corn flour
• Extrusion

INTRODUCTION

Food extrusion is widely used as a process for the manufacturing of various products such as expanded snacks, pasta, breakfast cereals, baby foods, pre-gelatinized and modified starches, among others.^[1] This well-known versatile and low-cost method is one of the most preferred to obtain snack foods based mainly on potatoes, corn, or rice, and lately on legumes like lentils. Snack foods are very popular among consumers, in particular potato chips; however, many of these products are high in calories and low in proteins, vitamins, and other nutrients.^[2]

Grain cereals and legumes such as corn and beans (Phaseolus vulgaris) constitute an important part in the diet of many individuals. In some developing countries, beans are considered the only source of protein rich in lysine,^[3] while corn protein provides an important amount of sulfur amino acids. From a nutritional point of view, the combination of cereals and legumes could be an alternative to develop snack foods with increased nutritional compounds for promoting healthy benefits.

Worldwide, there are many bean (P. vulgaris) cultivars such as kidney, black, pinto, navy, Peruano, among others.^[3] Besides the contribution of proteins (27.32 and 21.6% for Peruano and black beans, respectively),^[4] the consumption of beans has other positive effects on human health, such as the protection against diseases associated with oxidative stress like cancer^[5] because of their content of phenolic compounds,^[5–8] that enhance the nutritional value of these legumes.

Recently, the extrusion of legumes have been explored to improve the nutritional quality of extruded food products; some researches focus on the characterization of extruded soybeans, lentils, chickpeas, black beans or pinto beans.^[9–20] Nevertheless, only lentils have been combined with corn, to obtain expanded corn-lentil snacks, with the disadvantage of having high values of breaking stress.^[20] Other disadvantage of lentils consists in its cost, which is higher than that of chickpeas or soybeans, not being fully accepted due to their strong flavor.

The combination of corn and legumes, besides the nutritional aspects, represents an advantage during extrusion, because corn, considered as one of the most suitable cereals for extrusion due to its high starch content, provides good expansion properties required for snacks.^[9] For example, some studies conducting the extrusion of black beans flour (P. vulgaris) have demonstrated an increase in the density of the extrudates at the tested conditions (no corn added), hence the importance of corn.^[20,21]

To characterize extruded foods, functional properties such as water absorption capacity or water solubility index (WSI) are often measured;^[22–24] however, color and texture, which are determinant parameters for consumer’s acceptability of food products, are scarcely studied. The acceptance of food products based on their color is relative, meaning that in some countries consumers might prefer snacks foods of light color tones like potato and corn chips; while in developing countries such as Mexico, yellow (Peruano or Bayo beans) or black beans are equally accepted depending on the region.

Therefore, the combination of corn and legumes to obtain extruded snack foods, from a nutritional viewpoint, is being explored to obtain products of high nutritional value, while taking benefit of the expansion properties of corn. The objective of the present work was to evaluate the physicochemical and morphological properties of extrudates, obtained from blends combining corn, Peruano, and black-Quéretaro flours. Also, X-ray diffraction (XRD) patterns of the extrudates were determined, to understand the changes that these food materials undergo during extrusion.

MATERIALS

Peruano and black-Querétaro beans (P. vulgaris) cultivars used in this work were provided by Verde-Valle Company (Mexican beans processors, Jalisco, Mexico). Corn flour was obtained from Harinera de Maíz de Jalisco, S.A. de C.V. (Jalisco, Mexico).

METHODS

Bean Flour Preparation

Both varieties of beans were milled into flours using a Thomas Wiley^® Mini-mill (Swedesboro, NJ, USA). The particle size of each type of flour was measured by sieve analysis using a set of standard sieves with manual shaking during 8 min (Tyler Standard). After sieving, beans flours were collected from the mesh size #100, which gave the highest percentage of retention with 63.49 and 55.28% for Peruano and black-Querétaro flours, respectively. Flours were packed in polyethylene bags and stored at room temperature until use. Before extrusion, blends with corn and beans flours were adjusted to the selected moisture contents using distilled water, and samples were mixed for 12 min, using a laboratory blender. All samples were stored in polyethylene bags for 4 h at 4°C until reaching the equilibrium; prior to extrusion, samples were tempered at room temperature for 30 min.

Extrusion Cooking

The experiments were performed using a pilot plant scale single-screw extruder, manufactured in Mexico (Jalisco, Mexico), with a barrel of 60 cm length and 14.7 cm in diameter. The speed screw was 200 rpm, the die diameter was 2.99 mm, and the feed rate was 2 Kg/h. The extruder barrel was heated by electrical heaters thermostatically-controlled, reaching temperatures up to 400°C; each section of the barrel was heated with an individual heater. The extrusion temperatures in the heating zones (barrel, screw, and die) were 60/70/110°C and 60/70/135°C, varying only the die temperature. Steady-state conditions for extrusion were reached after 25 min. During extrusion, samples coming out of the die were cut using a sharp knife, and dried for 18 h in an oven drier at 60°C. The extrudates were sealed in polyethylene bags and stored at room temperature, until further analysis.

Experimental Design

For each bean cultivar, a multifactorial experimental design of 2 × 2 × 2 (bean flour percentage, moisture content, die temperature) with three replications was selected for this study. The effects of extrusion were tested on blends prepared with corn and beans (25 and 50% beans flours) with two levels of moisture (23 and 25%) and die temperatures (110 and 135°C), for a total of eight treatment combinations (Table 1). The selection of these conditions was based on preliminary runs and from published data reporting that die temperatures >140°C damage the final extruded beans product.^[14,17]

TABLE 1 Selected treatments for the extrusion of blends of corn combined with Peruano or black-Querétaro flours

Treatment	Peruano or b-Q flour (%)	Moisture (%)	Die temperature (°C)
1	25	23	110
2	25	23	135
3	25	25	110
4	25	25	135
5	50	23	110
6	50	23	135
7	50	25	110
8	50	25	135

b-Q: black-Querétaro.

Physicochemical Measurements

Color

Extrudates were ground and the color of the samples was analyzed with the CIELAB color scale by measuring lightness (L*) and the chromaticity coordinates a* (green-red) and b* (blue-yellow), using a Spectrocolorimeter Hunter-Lab^® “tristimulus” (UltraScan XE, Reston, VA, USA) in the reflection mode. Three color readings were taken in different zones of the samples for each treatment combination. The values of each color parameter were expressed as the average of the readings.

Breaking strength (BS)

The BS of cylindrical extrudates 10-cm long was measured using a TA-XT2 texture analyzer (Stable Micro Systems, Godalming, Surrey, UK). An aluminum cylinder probe TA-54 of 0.5 cm in diameter and 3.5 cm long was selected for this assay. The extrudates were compressed at a constant speed of 2 mm/s until failure, against the flat plate fixed on the loading frame, at a distance of 3 mm. The assay was repeated six times for each treatment. The BS was recorded as the maximum peak force and data was reported in grams-force (gf).

Water absorption index (WAI)

All extrudates were milled using a coffee mill (KSM 2, Naucalpan de Juarez, Estado de Mexico, Mexico). Briefly, 2.5 g of the milled sample was placed in a 50 mL graduated centrifuge tube, with 30 mL of water at room temperature; then, the sample was stirred for 30 min and centrifuged at 12,000 × g for 15 min. The supernatant was removed by evaporation using a heating plate at 40°C. The resulting gel was weighted and the WAI was calculated as follows: WAI = Mg/Ms, where Mg (g) is the weight of the remaining gel and Ms (g) is the weight of the initial sample.

Water solubility index (WSI)

The WSI was determined from the supernatant in the step above, by drying the sample at 70°C until constant weight was reached. WSI was calculated as follows: WSI = (Mds/Ms) × 100, where Mds (g) is the weight of the dried solids and Ms (g) is the weight of the initial sample. Both WAI and WSI were determined using the method proposed by Anderson et al.^[25] and modified by Gujska and Khan.^[15]

Oil absorption capacity (OAC)

The OAC was measured according to a modified method used by Li and Lee.^[26] Briefly, 0.5 g of the milled sample was placed in a 50 mL glass centrifuge tube and 10 mL of corn oil were added. The tube was stirred for 3 min using a vortex mixer and subsequently left for 30 min and centrifuged at 3000 × g for 25 min. The oil was removed carefully with a pipette, and the tube was inverted for 30 min to drain the oil before weighing the residue. The OAC was calculated as: OAC = Mos/Ms, where Mos (g) is the weight of corn oil bound to the weight of the sample and Ms (g) is the weight of the initial sample.

Morphology and XRD Patterns

Scanning electronic microscopy (SEM)

The SEM (SEM JEOL^® 5400 LV, JEOL, Tokyo, Japan) was selected to observe changes in the microstructure of the extrudates. The samples, with a thickness of 1 cm, were cut into pieces of 1 cm × 2 cm; then, each sample was fixed on a copper sample holder. Previously, the samples were coated with gold at 5 kV, 69 mA for 3 min, using an Ernest F. Fullam Sputter coater. The inner and outer surfaces of each sample were observed using secondary electron image (20 kV).

X-ray powder diffraction (XRD) diffraction

To determine the XRD, each sample was ground in an agate mortar and pestle. Approximately 0.1 g of the powdered sample was placed on the surface of a glass and then placed in a Rigaku Miniflex apparatus (CuKα radiation, The Woodlands, Texas, USA). The diffraction angle (2θ) was scanned from 5 to 60°, with a scan speed of 2°C/min, and sampling of 0.020°.

Statistical Analysis

Experimental data were statistically examined using the analysis of variance (ANOVA). Duncan’s Multiple Range Test was used to determine significant differences among means with 0.05 significance level, using the Statgraphics^® Plus software. Unless indicated, all measurements were conducted in duplicate.

RESULTS AND DISCUSSION

The extrudates obtained from the combination with corn and Peruano or black-Querétaro flours had moisture contents between 9.60–10.50%. The expansion index (EI), obtained by dividing the extrudate diameter by the extruder die diameter, of Peruano and black-Querétaro extrudates was lower (2.97–3.89 and 3.11–4.48, respectively) than that reported for potato flour (3.55–4.59) or for blends (3.25–5.24) of 50% potato starch, 35% quality protein maize, and 15% soybean meal.^[27,28] The low EI was attributed to content of bean flour, which contains high amounts of protein. As reported, for soy protein concentrate, high levels (greater than 25%) reduce the radial expansion of the extrudates.^[27,29] In this study, the high percentages of bean flour, from a nutritional point of view, contribute to increasing the protein content in the extruded products; however, lower amounts of bean flour, may be more beneficial to achieve better expansion, in particular for Peruano flour that has a higher protein content than black-Querétaro beans.^[4]

Physicochemical Measurements

Color

Figure 1 shows the color parameters L*, a*, and b* for the extruded blends as a function of the percentage of beans flours, extrusion temperature, and moisture content. In general, for blends of Peruano flour, L* was higher in samples containing 25% flour than those with 50% flour; such increase in L* was not present drastically because of the color of corn flour, which had values (data not show) in a range of that for extruded blends of Peruano flour. Peruano and corn flours possess a natural yellow color, being slightly lighter for corn; thus, their lightness tends to the region of white (L* = 0, yields black; L* = 100, diffuse white). This parameter is important because consumers are more familiar with snacks of light color, such as corn or potato chips. The color coordinates a* and b* also observed an increase with the increase in the percentage of Peruano flour, with exception of the treatment combination 50% flour with 23% moisture at 135°C, where a* and b* decreased probably due to a faster release of water at the highest temperature tested. It is known that under extrusion, the changes in color can be attributed to the development of chemical reactions like browning, and that the extending of such reactions depends on the residence time and the temperatures in the extruder. In addition, the temperatures of extrusion could promote the oxidation of the pigments present in beans and cereals, affecting the color parameters by forming brown pigments. Similarly, L* values of extruded blends of black-Querétaro flour decreased with the increase in the percentage of the flour; because of the dark color of this bean variety, L* values were in a range below that for Peruano flour. On the other hand, a* and b* values did not show a clear tendency, increasing or decreasing, depending on the extrusion conditions. Statistically for extruded blends of Peruano flour, the ANOVA analysis showed that the main effects were significant (p < 0.05) such as the percentage of bean flour (A), temperature (B), and moisture content (C) with the exception of B for the parameter b*; besides, the statistical interactions AC and BC (influencing L*, a*, and b*) and ABC (influencing a*) were also significant (p < 0.05; Table 2). In extruded blends containing black-Querétaro flour, all the main effects A, B, and C and their interactions AB, AC, BC, and ABC were significant (p < 0.05), with the exception of B for the parameter L* and AB for the parameter b*. Some studies report the changes in color of extruded legumes; in one of this studies, likewise to the present findings, L* values decreased in blends of fenugreek, rice, and chickpea, whereas the coordinates a* and b* were increased.^[23]

TABLE 2 Significance of the main effects and their interactions for the physicochemical and functional measurements of extruded blends of corn and Peruano flours

Factors and interactions	L*	a*	b*	BS	WAI	WSI	OAC
A	+	+	+	+	NS	+	+
B	+	+	+	+	+	+	+
C	+	+	NS	+	+	NS	NS
AB	NS	NS	NS	+	+	NS	+
AC	+	+	+	+	NS	+	+
BC	+	+	+	+	+	NS	+
ABC	NS	+	NS	+	NS	+	+

A: percentage of Peruano flour; B: feed moisture; C: extrusion temperature;

BS: breaking strength; WAI: water absorption index; WSI: water solubility index; OAC: oil absorption capacity;

[+] indicate significant and [NS] no significant difference (p < 0.05), according to Duncan’s multiple range test.

FIGURE 1 Color parameters A: L*; B: a*; and C: b* of extruded blends of corn with Peruano and corn with black-Querétaro (b-Q) flours, under different treatment conditions; Error bars indicate the standard deviation of three replicates.

BS

BS is a measure of the texture of food products that, along with color, is considered an important characteristic of snacks foods for consumer’s acceptance. The BS of the extrudates with 25 or 50% Peruano flour, decreased or increased depending on the extrusion conditions, with the highest values found in treatments 6 and 8, containing 50% flour, at a temperature of 135°C. On the contrary, the BS of the extrudates with 25% black-Querétaro flour increased (treatments 1, 2, and 3), as well as those containing 50% flour (treatments 6, 7, and 8). In general, the BS values for those blends with 50% Peruano flour were higher than those obtained for black-Querétaro flour (Fig. 2); such higher values were attributed to the protein content of Peruano beans (27.32%), which is higher than that of black-Querétaro beans (21.60%).^[4] Therefore, high amounts of flour influence the texture of the extrudates, which became harder. Few authors report the combination of legumes and cereals, mentioning that the percentage of legumes in the blend cause an increase in the hardness of the material; while others state that moisture and extrusion temperatures influence the hardness.^[19,23,30] In addition, the BS has been related to the degree of starch gelatinization and degradation, depending on factors, such as temperature and feed moisture. The ANOVA analysis of BS for the extrudates of both beans varieties (Tables 2 and 3), showed that the main effects (A, B, and C) and all the interactions were significant (p < 0.05).

TABLE 3 Significance of the main effects and their interactions for the physicochemical and functional measurements of extruded blends of corn and black-Querétaro flours

Factors and interactions	L*	a*	b*	BS	WAI	WSI	OAC
A	+	+	+	+	+	+	NS
B	NS	+	+	+	+	+	+
C	+	+	+	+	+	NS	+
AB	+	+	NS	+	+	NS	+
AC	+	+	+	+	+	+	+
BC	+	+	+	+	+	NS	NS
ABC	+	+	+	+	+	+	NS

A: percentage of black-Querétaro flour; B: feed moisture; C: extrusion temperature;

BS: breaking strength; WAI: water absorption index; WSI: water solubility index; OAC: oil absorption capacity;

[+] indicate significant and [NS] no significant difference (p < 0.05), according to Duncan’s Multiple Range Test.

FIGURE 2 Breaking strength of extruded blends of corn with Peruano and corn with black-Querétaro (b-Querétaro) flours under different treatment conditions; error bars indicate the standard deviation of three replicates.

WAI

Table 4 shows the values of WAI, WSI, and OAC obtained for the blends containing either Peruano or black-Querétaro flours. In treatments with 25 and 50% Peruano flour, WAI values were fairly close, obtaining the highest values when the highest temperature was used (treatments 2 and 6); moreover, the ANOVA analysis (Table 2) showed that the main effects B and C were significant (p < 0.05), as well as the interactions AB and BC. For legumes like lentils, moisture content had similar effects on WAI.^[13] It has been cited that an increase of temperature causes an increase in WAI,^[14,17] and consequently, that the influence of temperature is related to the starch gelatinization.^[30] On the contrary, the blends of black-Querétaro flour gave different values of WAI depending on the treatment applied, but similarly to the blends containing Peruano flour, the highest values were obtained for treatments 2 and 6. In this case, the ANOVA analysis (Table 3) showed that all the main effects and their interactions were significant (p < 0.05).

TABLE 4 Physicochemical and functional measurements of extruded blends of corn/Peruano and corn/black-Querétaro flours^a

	Corn/Peruano extrudates			Corn/black-Querétaro extrudates
	WAI^b	WSI^c	OAC^d	WAI	WSI	OAC
Treatment	(g/g)	(g/100 g)	(g oil/g)	(g/g)	(g/100 g)	(g oil/g)
1	4.33 ± 0.46	31.48 ± 0.35	1.29 ± 0.03	3.97 ± 0.18	30.47 ± 1.79	1.05 ± 0.02
2	5.59 ± 0.13	31.77 ± 0.69	0.81 ± 0.03	6.52 ± 0.55	32.53 ± 1.56	0.79 ± 0.06
3	4.12 ± 0.08	33.04 ± 0.00	1.37 ± 0.02	4.18 ± 0.02	32.61 ± 0.15	1.52 ± 0.08
4	4.81 ± 0.67	31.36 ± 0.72	1.30 ± 0.03	5.03 ± 0.07	34.48 ± 2.03	1.12 ± 0.27
5	4.09 ± 0.08	32.18 ± 0.84	1.24 ± 0.17	4.17 ± 0.01	34.25 ± 1.74	1.15 ± 0.01
6	5.26 ± 0.00	33.32 ± 0.24	1.17 ± 0.06	5.57 ± 0.36	31.32 ± 1.57	0.94 ± 0.04
7	4.45 ± 0.10	35.83 ± 0.84	0.99 ± 0.06	3.90 ± 0.12	36.85 ± 0.81	1.10 ± 0.01
8	4.71 ± 0.18	32.39 ± 0.75	1.27 ± 0.01	4.90 ± 0.05	31.59 ± 0.25	0.85 ± 0.05

^aEach value is expressed as means ± SD, with n = 3;

^bWater absorption index;

^cWater solubility index;

^dOil absorption capacity.

WSI

In relation with WSI, the values were slightly higher for treatment 7 in blends with both Peruano and black-Querétaro flours, but within a close range. It is noticeable that the increase in the percentage of Peruano flour from 25 to 50% slightly increased the WSI values of the extrudates. The blends of black-Querétaro, gave different values of WSI depending on the treatment applied. Similar results to those reported here, were found in extruded blends combining rice, fenugreek, and chickpea;^[23] nonetheless, there is a controversy in the behavior of this functional property, because extruded corn-starch isolated (from pinto and navy beans) combined with protein had low values of WSI, attributed to the addition of proteins.^[15] The decrease of WSI in products with high starch and protein content may be also attributed to starch dextrinization and protein denaturation, causing a release of hydrophilic groups of proteins bound to the starch molecules, and as a consequence, lowering the WSI of the extrudates.^[15] In this study, the increase in WSI was attributed to the fact that an increase in the percentage of beans also increases the protein content of the extrudates.^[8] Statistically, the main effects A and B and the interactions AC and ABC were significant (p < 0.05) for both blends containing Peruano and black-Querétaro flours (Tables 2 and 3).

OAC

The OAC of the extruded blends was also dependent on the type of beans flour, having different values as a function of the extrusion conditions tested. The highest values were found in treatment 3, in blends of both Peruano and black-Querétaro flours. In this study, the results were similar to those reported for extruded starch isolated from fenugreek, chickpea, and beans,^[14,17,23] and were below those reported for lima beans, cowpeas, and soybean flours.^[31,32] The OAC is associated to the physical trapping of oil due to the exposition of the hydrophobic part of the proteins, which have a predominant association with water instead of oil; also, it is considered an indirect measure of proteins denaturation.^[33] The protein content, the form of the non-polar arrangement of the amino acids, as well as the type of bounding between the oil and the starch and proteins are determinant factors to increase or decrease the OAC of the extrudates.^[13,15] For the blends of Peruano flour, the main effects and their interactions were significant (p < 0.05) with the exception of C, which statistically did not influence the OAC; whereas the blends with black-Querétaro flour were significant (p < 0.05) for B and C, and the interactions BC and ABC (Tables 2 and 3).

Morphology

Figure 3 shows the micrographs of the external and internal surfaces of the extruded blends based on corn with Peruano and black-Querétato flours. The blends containing 25% Peruano flour, 23% moisture and 110°C showed irregular structures with some cavities of different size and number, with the internal surface exhibiting a higher number of cavities (Fig. 4 a–e, 4a–i,). The cavities observed were attributed to the evaporation of water during extrusion. Also, for the treatment mentioned, the increase in temperature from 110 to 135°C resulted in fewer cavities in both external and internal surfaces, observing melted starch granules (Fig. 3b–e, 3b–i). The blends of 50% Peruano with 25% moisture at 110°C, and 50% Peruano with 25% moisture at 135°C exhibited a more uniform structure with the presence of cavities; however, an increase in the temperature causes the collapse of the cavities, giving a more plasticized material. Besides, the structure of the extrudates having 50% Peruano flour was more homogenous, with less presence of cavities in comparison to the above treatment (Fig. 3 c–e, 3c–i, 3d–e, 3d–i). The blends of 50% black-Querétaro with 25% moisture at 110°C, and 50% black-Querétaro with 25% moisture at 135°C, showed more uniform surfaces typical of melted starch, that plasticized at the temperature of 135°C (Fig. 3 e–e, 3e–i, 3f–e, 3f–i). In general, these results were similar to those reported by researchers, reporting that the types of structures obtained correspond to gelatinized starch granules, which are no longer visible, because starch begins to gelatinize or dextrinize when is exposed to high processing temperatures.^[30,34–36]

FIGURE 3 Micrographs of SEM for external (e) and internal (i) surfaces of extruded blends A: 25% Peruano, 23% moisture, 110°C; B: 25% Peruano, 23% moisture, 135°C; C: 50% Peruano, 25% moisture, 110ºC; D: 50% Peruano, 25% moisture, 135ºC; E: 50% black-Querétaro, 25% moisture, 110ºC; F: 50% black-Querétaro, 25% moisture, 135ºC.

FIGURE 4 XRD patterns of corn, Peruano, and black-Querétaro (b-Q) flours before extrusion (a.u.: arbitrary unit, +: starch).

XRD Patterns

Figure 4 shows XRD patterns of raw: corn, Peruano, and black-Querétaro flours. The presence of a high line represents a great amount of amorphous materials, with less amount of crystalline material, which is common of raw materials. The presence of amorphous substances caused a slight intensity in the diffraction peaks formed at 2θ = 15.3, 17.1, 18.2 and 23.5°. According to the Joint Committee for Powder Diffraction Files (JCPDF, 1997) database, these peaks corresponded to starch (#00-039-1911); therefore, both raw corn and beans flours contain amorphous starch referred as the XRD of amorphous starch type A.^[37,38] Crystals from different types of starch can be classified based on their diffractograms as A, B, or C. In particular, starches type A, found in raw flours, are more rapidly hydrolyzed than starches type B.^[39]

The extruded blends of Peruano and black-Querétaro flours, showed a loss of the pattern of the native raw materials, with the formation of new crystallinity peaks due to the effects of extrusion. The XRD patterns of extruded corn and the blends showed a higher base line, indicating that these samples contain more amounts of amorphous rather than crystalline material. The XRD peaks were placed at 2θ = 12.8, 14.7, 19.8, 22.9, 28.6, 35.2, 37, and 41.2° and, according to the JCPDF database, these peaks correspond to monohydrate glucose C₆H₁₂O₆• H₂O (#00-002-0224; Fig. 5).

FIGURE 5 XRD patterns of extruded blends of corn combined with Peruano (P) or black-Querétaro (b-Q) flours, with 23% moisture at the temperatures of A: 110°C; and B: 135°C (a.u.: arbitrary units; *glucose monohydrate).

It is worth noticing that the XRD patterns showed a reduction of the peaks when the percentage of Peruano flour was increased to 50%, indicating the presence of a great amount of amorphous substance. Furthermore, the increase of temperature at 135°C also produced a large amount of amorphous material. The XRD patterns of the extrudates obtained at the highest temperature showed well-defined peaks, because an increase in temperature caused that more water were evaporated from the extrudates, and that the glucose were hydrated faster. During extrusion some phenomena can take place: (1) corn flour contain a great amount of starch composed of many glucose units; therefore, the heat and pressure generated inside the extruder can cause the breakdown of glycosidic bonds of the starch releasing glucose,^[39] (2) after extrusion, the cooling down of the extrudates causes the hydration of glucose, due to the water molecules contained in the extrudates. Hydration is a common property of sugars because of the presence of hydroxyl groups (-OH) that can easily form hydrogen bonds with water; so that, when the extrudates cool down, the hydrated glucose begins to crystallize.

CONCLUSIONS

In this work, the effect of bean flour percentage, temperature, and feed moisture, on the characteristics of extrudates obtained from two beans cultivars was assessed. All factors under study had influence on the independent variables, mainly on the color parameters and BS. There was a dominance of the color of each cultivar in the final extruded products, particularly, at highest flour content. On the other hand, the BS of the extrudates was also dependent on the flour content; at high bean flour contents, the expansion index was reduced, particularly for Peruano extrudates. Other properties such as WAI, WSI, or OAC exhibited slight changes, depending on the treatment applied. The microstructures of the extrudates showed the presence of gelatinized and melted starch, whereas the X-ray diffraction patterns showed the presence of monohydrated glucose; such changes were dependent on the extrusion conditions. Therefore, to obtain extrudates of good properties, the authors recommend using the highest temperature applied, at either 23 or 25% moisture. Further, low percentages of bean flour are recommended, to improve the expansion and texture properties of Peruano extrudates, and also to reduce the color dominance of black-Querétaro beans. Finally, the results achieved in this work demonstrate the feasibility to obtain products with improved nutritional quality, when combining cereals and legumes, processed under extrusion.

ACKNOWLEDGMENTS

The authors would like to thank Dr. Jesus Nungaray for his contribution and to Verde-Valle Company for material donation to conduct this research. A.H.M.P. wants to thank CONACyT scholarship for her Doctoral degree.