Comparison of the processing and quality of tortillas produced from larger grain borer Prostephanus truncatus (Horn.) resistant and susceptible maize genotypes

Journal of Stored Products Research 55 (2013) 99e105

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Comparison of the processing and quality of tortillas produced from larger grain borer Prostephanus truncatus (Horn.) resistant and susceptible maize genotypes Silverio García-Lara*, Cristina Chuck-Hernández, Sergio O. Serna-Saldivar Centro de Biotecnología-FEMSA, Escuela de Biotecnología y Alimentos, Tecnológico de Monterrey, ITESM-Campus Monterrey, Eugenio Garza Sada 2501, Monterrey, NL C.P. 64849, Mexico

a r t i c l e i n f o

a b s t r a c t

Article history: Accepted 10 September 2013

The aim of this research was to compare the processing and quality of tortillas produced from two kinds of larger grain borer (LGB) Prostephanus truncatus (Horn.) damaged kernels: resistant (IRM) and susceptible (ISM) genotypes. The damaged LGB kernels had significant lower test weight, 1000 kernel weigh and density. The ISM kernels were more negatively affected by insects compared with the IRM counterpart. A significant reduction of 5% in starch was observed in IRM kernels but not in ISM counterparts. Flour acidity and protein increased 8-fold and 5%, respectively parallel to an augmentation of weight grain losses whereas the crude fat content significantly reduced by 29%. Insect damage enhanced the penetration of the hot lime solution into the starchy endosperm. Insect infested kernels which lost 10% and 20% of their weight required 34% and 42% less lime-cooking time compared to sound kernels. The 10% and 20% insect-damaged kernels lost 15 and 23% of their solids during storage and tortilla processing, respectively. Finally, LGB damaged kernels reduced substantially the tortilla quality in terms of color. Ó 2013 Published by Elsevier Ltd.

Keywords: Larger grain borer Insect-resistant maize Lime-cooking Postharvest losses Tortilla

1. Introduction Maize tortillas are considered the most important staple food for the Mexican population. In Mexico, tortillas are consumed by 94% of the population and represent 47% of the average caloric intake and the industrial processing 1% of the gross national product (SAGARPA, 2002). Tortillas and related products, such as tortilla and corn chips, gruels, tamales, among others, are obtained from three different manufacturing processes: traditional, commercial fresh-masa and dry-masa flour. Tortilla production efficiency and yield is affected by dry matter losses (DML) incurred during nixtamalization. The most relevant processing parameters that affect DML are lime-concentration and cooking time, steeping time, degree of agitation, and extent of nixtamal washing (Pflugfelder et al., 1988; Sahai et al., 2000). These losses also are affected by major grain properties. It is well known

Abbreviations: DML, dry matter losses; IRM, insect resistance maize; ISM, insect susceptible maize; GWL, grain weight losses; LGB, larger grain borer; RH, relative humidity. * Corresponding author. Tel.: þ52 81 8358 1400; fax: þ52 81 8328 4262. E-mail addresses: [email protected], [email protected] (S. García-Lara). 0022-474X/$ e see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jspr.2013.09.002

that endosperm texture or hardness, kernel size and ease of pericarp removal are important traits that affect cooking and DML (Serna-Saldivar et al., 1993). In fact, the use of grain lots with high incidence of stress cracks, broken and damaged kernels increase up to 12% the DML (Jackson et al., 1988). A significant proportion of the damage kernel categories are due to insect pests. This is a major constraint in grain lots because their presence is normed by regulatory agencies. The Mexican official normativity for white and yellow maize specifies that no more than 3% and 10% of total damage kernels are allowed in order to categorize premium (Mexico-1) and low quality corns (Mexico-4), respectively (SAGARPA, 2002). Furthermore, the maximum allowable amount of insect fragments used as a sanitary indicator is 1 fragment/g of tortilla (SAGARPA, 2002). There are not enough food quality control measurements for the use of damage kernels in small tortilla factories that manufacture more than 60% of the Mexican domestic production (Mery et al., 2010). Recent investigations have documented that grain damage and postharvest losses of maize due to storage insect pests such as the larger grain borer (LGB), Prostephanus truncatus (Horn.) (Coleoptera: Bostrichidae), are an increasingly important constraint of food security worldwide (Borgemeister et al., 2003; FAO, 2009). Prostephanus truncatus is a woodborer and an invasive post-

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harvest insect native from Mesoamerica that has acquired the status of a serious pest in the Americas, Asia and Africa (Tigar et al., 1994; Kumar, 2002). Studies in Africa and Latin America have shown that subsistence farmers in highlands, tropical and subtropical agro-ecologies experienced 10e45% maize losses and P. truncatus is responsible of 10e80% of damage in storage (Tigar et al., 1994; Bergvinson and García-Lara, 2004). Besides of the relevant dry matter losses, the quality of the damaged kernels for tortilla production is downgraded. Fortunately, several landraces and improved maize varieties available in the market have been characterized as resistant to P. truncatus (Arnason et al., 1994; Kumar, 2002). Kernels of resistant varieties have been characterized for biophysical, biochemical and genetic characteristics (Bergvinson and García-Lara, 2004), and contribute to reduction in losses throughout storage because resistant varieties suffer only 13e50% as much grain weight loss compared to susceptible counterparts (Bergvinson and García-Lara, 2004; García-Lara et al., 2004). Most of maize produced in Mexico is commercialized in formal and informal local markets (Keleman and Hellin, 2009) having unknown levels of grain damage. Thus, the tortilla industry uses local maize which sometimes contains damaged grains due to postharvest pests such as P. truncatus, Sitophilus zeamais, Sitotroga cerealella, among other secondary postharvest pests. Needless to say, the tortilla industry in some instances incorporates damaged kernels into the manufacturing process, however there is not information about the impact in nixtamalization process, cooking time and DML and tortilla quality when a fraction of the kernels used in the manufacturing are infested with P. truncatus. The aim of this research was to compare the processing characteristics and quality of tortillas produced from resistant and susceptible maize genotypes that were purposely damaged with P. truncatus in two different levels. 2. Material and methods 2.1. Maize germplasm Genotypes of maize used in this research were kindly donated by International Maize and Wheat Improvement Center-Global Maize Program. The two genotypes were selected based on previous reports of postharvest insect resistance (García-Lara et al., 2004). The genealogy of these materials included the improved recurrent selection Population-84 C3, Pop84c3, (IRM) and one single cross hybrid CML244X346 (ISM), which served as control. The ISM is the type of grain preferred and commonly used by the tortilla industry in highland zones of Mexico (Miranda et al., 2013). Both maize varieties were grown and harvested during 2009. Crops were managed following the agronomic practices: fertilization with a total of 250 U of N, 60 of P and 30 of K per hectare, pest management and weed control were performed using standard practices in the region. After harvesting and threshing, the shelled corn samples were stored at 4  C for until use. 2.2. Insect pest culture Insect rearing was maintained using the methods previously described by Bergvinson and García-Lara (2011). Briefly, P. truncatus was reared in 0.5 L glass jars with vented lids that were filled with 400 g of equilibrated maize (30 d at 27  1  C, 70  5% of RH) covered with 10 g of maize flour and infested with 250 unsexed adults. Progeny were collected after 6e8 weeks. Adults were obtained by sieving maize containing grain damage in excess of 50%.

2.3. Treatments and evaluation of susceptibility parameters For insect bioassays, three independent treatments were performed under laboratory conditions. The treatments were a) maize kernels infested with P. truncatus until the grain weight losses (GWL) reached 10%, b) 20% GWL and c) control without infestation. All experiments were conducted under controlled conditions in a bioclimatic room (27  1  C, 70  5% of RH and 12:12 L:D). Four replicates for each treatment were performed for both IRM and ISM varieties. Each replicate was conducted on 500 g of maize that was allowed to equilibrate (12% moisture) for 3 weeks prior to infestation with P. truncatus. Moisture kernel content was assayed using a gravimetric method AACC (2000) 44-15A. Each jar was infested with 150 unsexed adults of P. truncatus (0e7 days-old). Susceptibility parameters were determined every 30 days until the GWL reached either 10% or 20% for each genotype. A nest of mesh sieves (#6 and #18, USA, Standard Testing Sieve, VWR, USA) and a collection pan was used to separate damaged kernels, P. truncatus adults, and flour. Grain weight loss, flour production (FP), and number of live P. truncatus adults were recorded according with methods described by Kumar (2002) and modified by Bergvinson and García-Lara (2011). 2.4. Physical properties Kernel test weight was determined using the Winchester Bushel Meter (Seedburo Equip. Co., Chicago IL) according to method 14e40 of the AACC (2000). Thousand-kernel weight by weighing 100 randomly selected kernels and multiplying by 10. Flotation index (FI) was expressed as a percentage of floating kernels on an aqueous solution of sodium nitrate with a specific weight of 1.25 g/cm3 at 35  C. Color parameters L*, a*, b*, Chroma ((a2 þ b2)1/2) and Hue (arctangent (b/a)) for whole kernel samples were obtained with a colorimeter (Minolta CR-300, Osaka, Japan). The pericarp, endosperm, germ and tip cap were manually dissected after soaking 50 g kernels in 100 mL water for 2 min and dried in an oven set at 60  C before weighing. 2.5. Nutrimental and biochemical analyses Moisture content was assayed using a gravimetric method AACC (2000) 44-15A. Total starch was calculated using a commercially kit (Method 76-13, AACC, 2000; Megazyme International, Wicklow, Ireland). Protein (N*6.25) was determined using the micro-Kjeldhal method 46-13 whereas crude fiber, fat, and ash were assayed according to methods 32-10, 30-20 and 08-01, respectively (AACC, 2000). Flour acidity was obtained with the 939.05 Official Fat Acidity Method for Grains (AOAC, 1980). 2.6. Lime-cooking optimization The lime-cooking properties of the two different sound maize genotypes and GWL counterparts were determined according to the nylon bag procedure described by Serna-Saldivar et al. (1993). The procedure consists of lime-cooking kernels contained in perforated nylon bags for three different times (0, 20 and 40 min) followed by 16 h steeping. The lime-cooked corn kernels were washed with tap water and then weighed before and after drying in order to calculate nixtamal moisture and DML. Linear regression equations were calculated to predict optimum cooking and DML. Optimum cooking was considered the time sufficient to increase nixtamal moisture to 48% after 16 h steeping. 2.7. Production of table tortillas One kilogram of each type of maize was lime-cooked at 95e 100  C for the predetermined optimum cooking time with 3 L of

S. García-Lara et al. / Journal of Stored Products Research 55 (2013) 99e105

water containing 10 g of food grade lime. Then, cooked kernels were allowed to steep for 16 h. The wastewater or nejayote was discarded and the resulting nixtamal from each type of maize was washed and manually agitated with one liter of water. The cleaned and washed nixtamals were stone-ground into masa using a commercial mill (Fertitor, Puebla, Mexico) equipped with a pair of carved 23 cm diameter volcanic stones. Water (181 mL/ kg nixtamal) was gradually added during grinding to increase the masa moisture to 56% and to prevent excessive heat generation. The resulting masa was formed and baked into 30  1 g tortillas using commercial equipment located at the commercial tortilla factory “Lopez” (San Nicolás de los Garza, N.L., México). Tortillas were baked on a triple pass gas fired baking oven for 53 s at 330  C.

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genotype achieved at different times the 10 and 20% of GWL. As expected, the IRM required more time to reach the GWL (38% more for 10% and 33% more for 20% GWL) compared with ISM. No significant differences were found for grain damage at 10% of GWL (Table 1). Flour production was different when the degrees of GWL were compared. The observed results in terms of levels of P. truncatus resistance are comparable with previous reports (Bergvinson and García-Lara, 2011) were resistant varieties, such as Pop84c3, were compared to susceptible counterparts, the first having GWL 5 whereas the second 25%. The resistance mechanism associated to Pop84c3 are attributable to two factors (i) physical barrier through a fortified mechanical action of the pericarp cell walls and (ii) antibiosis through the toxic effects of phenolic acid amides and peroxidase activity localized in the aleurone layer (Arnason et al., 1994; García-Lara et al., 2004).

2.8. Tortilla trial and post-production quality 3.2. Physical properties of maize genotypes The baked tortillas were allowed to cool down under a sterile hood to lower their temperature to 25  C sampled for moisture determination and then packaged in sealed plastic bags (SernaSaldivar et al., 1993). Representative samples were taken after 0, 2, 24, 48, and 72 h for further analysis. Tortilla color parameters were determined using a colorimeter (Minolta CR-300, Osaka, Japan) whereas tortilla texture and hardness determined with a Texture analyzer (TAxT2, Model 4465, Stable Micro Systems Ltd., Godalming, England). Ten independent repetitions were performed. The maximum force required to break the tortilla was registered and values were expressed in Newtons (N). 2.9. Statistical analysis The physical, chemical, and tortilla making properties of the different maize genotypes were expressed as means  standard deviations. Linear regression equations based on nixtamal moisture content were calculated in order to predict optimum cooking time (SAS Inst., Cary, NC). All traits were subjected to analysis of variance using the statistical software Statistix v.8 (Analytical Software, Tallahassee, FL) and differences among means were compared by Tukey tests at P < 0.05. 3. Results and discussion 3.1. Treatments and susceptibility parameters As expected, the infestation with P. truncatus caused significant detrimental effects in quality of IRM and ISM kernels (Table 1). Each

Physical differences of IRM and ISM kernel properties have been previously documented by García-Lara et al. (2004). In this study, significant negative effects were observed for physical parameters in maize damaged with P. truncatus being the ISM more affected compared to the IRM counterpart (Table 2). Test weights decreased in average 17 and 25% for 10 and 20% of GWL, respectively. The average kernel weights diminished nearly in the same magnitude compared to test weights, but differences were only significant when compared to control kernels. Flotation indexes of IRM and ISM kernels increased three and two folds at 20% of GWL. Similar decrements in physical traits of maize were recently reported by Chuck-Hernández et al. (2012) who utilized a yellow dent type infested with the postharvest insect pest maize weevil, S. zeamais. As expected, flour acidity also increased significantly according with level of grain damage. For IRM, the acidity values increased 2 and 13-fold for 10 and 20% of GWL, respectively. Likewise, the ISM acidity increased 4 and 6-fold for 10 and 20% of GWL. The ISM white colored kernels had lower acidity values compared with the red colored IRM counterparts. These results indicate that free fatty acids released were due to the metabolic activity of the insects, which is in agreement with other investigations on long term storage of maize (Shobha et al., 2012). In terms of grain anatomical proportions, pericarps of both genotypes did not present significant differences in function of GWL, although the IRM kernels had two fold more pericarp compared to ISM. Interestingly, there was a similar decrease trend in the proportion of both endosperm and germ tissues. The ISM kernels had 12 and 17% more endosperm and germ losses compared with the

Table 1 Insect infestation parameters of insect resistant and susceptible kernels with different levels of grain weight losses produced by Prostephanus truncatus. Maize variety

Pop84 C3

CM244XCML346

a

Ida

IRM

ISM

Treatmentb

Control 10% GWL 20% GWL Control 10% GWL 20% GWL

Insect infestation parametersd Timec

Grain damagee

d

%

e 34 68 e 21 45

e 47.6 65.5 e 46.5 83.8

 2.3 c  3.6 a  1.1 d  3.8 b

Flour production

Adult progeny

e 38.5 72.4 e 41.7 80.7

e 3.8  0.7 d 11.6  0.9 b e 9.1  0.7 c 15.3  1.3 a

No  2.1 c  2.4 b  1.9 c  2.4 a

 2.4 b  0.5 a  0.9 b  0.3 a

IRM, insect resistant maize; ISM, insect susceptible maize. Treatment based on the grain weight loss,(GWL), which is determined by the difference between initial and final sample weight resulting from artificial infestation with Prostephanus truncatus. c Time in days required to reached the set value of grain weight losses calculated according with Bergvinson and García-Lara (2011). d Significant difference (P < 0.001) using a T-test comparison. Mean values  SD (n ¼ 6). e Values with no letter in common within column are significantly different (P < 0.01) by Tukey test. Anova: grain damage (P < 0.0001, F ¼ 301, df ¼ 3); flour production (P < 0.0001, F ¼ 538, df ¼ 3); adult progeny (P < 0.005, F ¼ 24, df ¼ 3). b

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Table 2 Biophysical parameters of insect resistant and susceptible kernels with different levels of grain weight losses produced by Prostephanus truncatus. IDa

Treatmentb

Test weightc

1000 kernel weight

Flotation index

Flour acidity

Anatomical proportions Pericarp

Kg/hL IRM

ISM

Control 10% GWL 20% GWL Control 10% GWL 20% GWL

84.1 70.2 63.9 80.0 67.4 58.7

     

g 0.7 1.3 0.4 0.5 1.2 2.0

a c d b c e

83.5 71.4 67.8 80.2 70.4 63.8

     

0.5 2.0 5.2 0.4 4.1 7.6

a c c b c c

%

mg KOH/100 g

g/100 g

5.0  2.0 a 15.0  2.0 b 15.0  3.0 b 35.0  5.0 c 66.7  4.7 d 86.7  7.5 e

24.8  8.5 c 53.3  11.9 b, c 310.5  8.0 a 24.2  4.7 c 101.8  9.8 b, c 135.1  24.6 b

6.4 6.7 5.6 2.4 3.2 3.0

     

0.9 0.4 0.4 1.1 1.0 0.3

Endosperm

a a a, b b, c b, c b

87.9 81.4 68.9 89.7 78.3 64.2

     

0.6 0.9 0.9 0.5 0.7 0.9

Germ

a a, b c a b, c c

5.6 5.5 4.8 7.7 6.8 5.7

     

0.7 0.5 0.6 0.9 0.7 0.5

c c d a b c

a

IRM, insect resistant maize; ISM, insect susceptible maize. Treatment based on the grain weight loss, GWL, produced under artificial infestation. Values with no letter in common within column are significantly different (P < 0.01) by Tukey test. Anova: test weight (P < 0.0001, F ¼ 330, df ¼ 5); 1000 kernel (P < 0.0001, F ¼ 8, df ¼ 5); flotation (P < 0.0001, F ¼ 194, df ¼ 5); flour acidity (P < 0.001, F ¼ 5, df ¼ 5); pericarp (P<0.05, F ¼ 8.9  104, df ¼ 5); endosperm (P < 0.0001, F ¼ 15, df ¼ 5); germ (P < 0.0001, F ¼ 22, df ¼ 5). b c

IRM counterpart. Considering that P. truncatus is a woodborer and an invasive post-harvest insect pest with reproduction under wide spectrum of conditions and food commodities (Nansen and Meikle, 2002), data reported herein indicates that this particular insect prefers the germ and endosperm as the main food sources. This insect behavior is well known (Nansen and Meikle, 2002) and is the explanation of the origin of dust waste produced during long-term infestation (Kumar, 2002).

3.3. Nutritional properties of maize genotypes Grain moisture content and nutritional parameters and of IRM and ISM kernels with different levels of weight losses are depicted in Table 3. An increase in moisture was detected in all damaged samples, indicating the effect of maize and insect respiration in the kernel environment (Chuck-Hernández et al., 2012). According to Serna-Saldívar (2010), the degree of insect damage and population is mainly affected by grain moisture and storage conditions (temperature, RH) and commonly, insects increased population when moisture exceeds 1.5% above the critical moisture considered 14% in maize and other cereal grains. The moisture increase was higher in the ISM cultivar compared with IRM, indicating the higher availability of free water and respiration level in that particular sample. In terms of chemical composition, both genotypes contained a relatively lower total starch compared to commercial corns used by the industry. Mora-Escobedo et al. (2009) indicated that corn kernels with high starch content are preferred by the tortilla industry. Serna-Saldivar et al. reported an average of 72% of starch for different maize genotypes and 75% for dent maize. The total starch expressed on dry matter basis for IRM and ISM was 63 and 67% respectively, indicating clear differences between the reported values and the genotypes used herein. Regarding the effects of

P. truncatus in the starch fraction, is noteworthy that total starch was almost 5% reduced in the IRM kernels compared to its corresponding sound counterpart (Table 3), a non-observed effect in ISM genotype. The sound IRM, Pop84c3, kernels contained a relatively high protein concentration. At the beginning of the experiment was nearly 1.9% higher compared to the sound ISM kernels. The protein content in IRM was between 2 and 3% points higher than the reported average of 9% for dent maize (Serna-Saldívar, 2010), 10% for Costeño (Mora-Escobedo et al., 2009) and 10.2% and 9.4% for common genotypes from high and lowland agro-ecologies, respectively (Bressani et al., 1990). Surprisingly, the IRM contained elevated levels of protein comparable with quality protein maize, which commonly ranges from 10 to 12% (Serna-Saldivar et al., 2008). The protein fraction increased with a higher insect damage estimated as weight grain losses (Table 3). The concentration increased due to significant losses of starch and fat. On the contrary, the P. truncatus infested kernels contained lower amounts of crude fat likely due to losses of germ tissue. This insect preferably feeds from the germ, the main reservoir of crude fat, instead of the endosperm, where starch and gluten proteins are located. These behaviors also contrast with a recent report of mechanism of resistance in P. truncatus where Mwololo et al. (2012) show a positive correlation between the quantities of protein and fat with P. truncatus resistance.

3.4. Lime-cooking, production of masa and table tortilla The lime-cooking of insect-infested kernels had a profound effect on optimum cooking time. As expected, infested kernels required lower cooking regimes in order to produce nixtamal suitable for stone grinding and tortilla processing. Interestingly,

Table 3 Grain Moisture and nutritional parameters of insect resistant and susceptible kernels with different levels of grain weight losses produced by Prostephanus truncatus. IDa

Treatmentb

Grain moistureb

Protein

Crude fiber

Ash

Crude fat

Total starch

% IRM

ISM

Control 10% GWL 20% GWL Control 10% GWL 20% GWL

12.2 14.6 15.4 12.9 13.5 17.9

     

0.2 0.4 0.2 0.4 0.4 0.6

d c b d c a

12.7 13.0 13.7 10.8 10.9 11.2

     

0.2 0.3 0.3 0.3 0.1 0.4

b a, b a d c, d c

1.3 1.2 1.3 1.5 1.2 1.5

     

0.1 0.1 0.1 0.1 0.1 0.1

b c b a b, c a

2.0 2.5 2.9 2.5 1.7 2.5

     

0.5 0.3 0.3 0.2 0.2 0.2

b, c a, b a a, b c a, b

3.8 3.2 3.0 5.0 4.6 3.0

     

0.3 0.6 0.3 0.8 0.3 0.8

b c c a a c

64.7 64.1 60.4 63.0 67.8 69.7

     

0.1 0.2 0.3 1.1 1.0 1.0

b, c c d a, b, c a, b a

c Values with no letter in common within column are significantly different (P < 0.01) by Tukey test. Anova: grain moisture (P < 0.0001, F ¼ 206, df ¼ 5); protein (P < 0.0001, F ¼ 90, df ¼ 5); ash (P < 0.0001, F ¼ 9, df ¼ 5); crude fiber (P < 0.0001, F ¼ 10, df ¼ 5); crude fat (P < 0.0001, F ¼ 12, df ¼ 5); total starch (P < 0.001, F ¼ 15, df ¼ 5). a IRM, insect resistant maize; ISM, insect susceptible maize. b Treatment based on the grain weight loss, GWL, produced under artificial infestation.

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Table 4 Lime-cooking properties and dry matter losses of insect resistant and susceptible maize under different levels of Prostephanus truncatus grain weight losses. Id

Treatment

Nixtamal moisture Linear regression equationa

IRM

ISM

Control 10% GWL 20% GWL Control 10% GWL 20% GWL

Y Y Y Y Y Y

¼ ¼ ¼ ¼ ¼ ¼

0.12 0.14 0.14 0.13 0.14 0.14

X X X X X X

þ þ þ þ þ þ

47.1 43.6 47.2 44.9 43.7 49.5

Dry matter losses incurred during lime cooking Optimum cooking to achieve 48% moisture (min)

Experimental moistureb (%)

44.3 30.8 5.3 23.1 14.8 2.3c

49.3 50.1 49.2 47.0 49.7 47.6

     

0.1 0.1 0.6 0.7 0.3 0.9

a a a a a a

Linear regression equationd Y Y Y Y Y Y

¼ ¼ ¼ ¼ ¼ ¼

0.13 0.11 0.13 0.14 0.07 0.14

X X X X X X

þ þ þ þ þ þ

1.7 4.0 1.7 2.1 2.6 5.2

Dry matter loss at optimum cooking (%)

Experimental dry matter loss (%)

7.9 7.5 2.5 5.8 5.0 4.8

6.5 6.2 3.1 5.2 4.9 4.1

     

0.1 0.1 0.1 0.1 0.2 0.1

b a e c d d

a

Y ¼ nixtamal moisture and X ¼ cooking time. Values with no letter in common within column are significantly different (P < 0.01) by Tukey test. Anova: experimental moisture (P < 0.0001, F ¼ 13, df ¼ 5); DML (P<0.0001, F ¼ 9*104, df ¼ 5). c Kernels were added to the hot lime solution 2.3 min after discontinuing heat. d Y ¼ dry matter loss and X ¼ cooking time. b

the kernels with the highest damage required negative cooking times because they were introduced into the hot lime solution several minutes after suspending heat (Table 4). Both the insect resistant and susceptible damaged kernels had similar trends in terms of optimum cooking times. Kernels that lost 10% of their weight due to insects required about 30e35% less cooking whereas counterparts which lost 20% weight required less than 10% of the cooking time compared to the sound counterparts. The significantly lower cooking was attributed to the kernel punctures and associated damages that enhanced the penetration of the hot lime solution into the starchy endosperm. In addition, it has been reported that insect-damaged kernels had higher amounts of damaged starch that enhances water uptake by the kernels. Interestingly, damaged kernels lost fewer solids during lime-cooking likely due to their significantly lower cooking regimes. The sound IRM and ISM kernels lost 6.5 and 5.25% solids during lime-cooking, steeping and nixtamal washing operations. These values are higher compared with other studies where loses around 5% or below are reported (Pflugfelder et al., 1988; Serna-Saldivar et al., 1993; GutiérrezUribe et al., 2010). The slightly higher losses compared to the

insect damaged kernels is attributed to the lower cooking schedules. Thus, the accumulated solid losses during faulty storage and lime-cooking exceeded 15 and 23% for kernels that had storage losses of 10% and 20%, respectively. 3.5. Effect of insect damage on grain and tortilla color and tortilla texture The color of IRM and ISM of sound and insect-damaged kernels had red and white pigmentations, respectively (Bergvinson and García-Lara, 2011) and consequently, the L* values of the IRM kernels were lower compared to the ISM counterparts. The ISM infested kernels had negative a* values indicating the loss of red in favor of a green coloration. The negative a* value indicated that insect activity possibly oxidized maize pigments into lighter colorations. Likewise, the b* values were about 5 units lower than control indicating a decrease in yellow color. In all cases they are still greater than the ISM. Changes in color have been previously reported as affected by the level of insect infestation (ChuckHernández et al., 2012). Therefore, it is clear that color changes of maize kernels infested with P. truncatus can directly affect the

Table 5 Effects of Larger Grain Borer Infestation, Prostephanus truncatus (Horn.) on color scores of kernels and texture (rupture force) in tortillas stored for four days. ID

Treatment

Product

La

IRM

Control

Grain Tortilla Tortilla Tortilla Grain Tortilla Tortilla Tortilla Grain Tortilla Tortilla Tortilla Grain Tortilla Tortilla Tortilla Grain Tortilla Tortilla Tortilla Grain Tortilla Tortilla Tortilla

35.7 46.1 45.6 48.2 31.9 45.2 44.5 47.2 37.0 44.7 44.8 44.8 38.0 49.7 50.0 50.8 42.4 46.1 46.6 46.9 44.7 47.0 46.3 48.2

10% GWL

20% GWL

ISM

Control

10% GWL

20% GWL

1 day 2 day 4 day 1 day 2 day 4 day 1 day 2 day 4 day 1 day 2 day 4 day 1 day 2 day 4 day 1 day 2 day 4 day

                       

1.84 f, g 0.30 b, c, d 0.25 b, c, d 0.16 a, b, c 1.6 g 0.4 c, d 0.6 c, d 1.0 a, b, c 1.5 f 0.3 c, d 0.4 c, d 0.7 c, d 1.0 e, f 0.3 a, b 0.2 a, b 0.2 a 0.9 d, e 0.6 b, c, d 0.3 a, b, c, d 0.6 a, b, c 0.9 c, d 0.6 a, b, c 0.5 b, c, d 0.6 a, b, c

a

b

6.7  0.66 a 0.2  0.02 d, e, f, g 0.3  0.02 d, e, f 0.1  0.01 d, e, f, g 4.9  0.56 b 0.4  0.02 d, e, f, g, h 0.4  0.02 d, e, f, g, h 0.5  0.03 e, f, g, h 2.1  0.28 c 0.3  0.01 d, e, f 0.4  0.03 d, e 0.5  0.04 d 0.1  0.01 d, e, f, g 2.1  0.03 i 2.0  0.09 i 1.3  0.07 h, i 0.4  0.08 d, e, f, g, h 0.9  0.06 h 0.7  0.04 gh 0.5  0.04 f, g, h 0.8  0.07 h 0.8  0.03 h 0.9  0.07 h 0.8  0.05 h

25.6 20.6 21.2 19.8 20.0 20.5 20.6 20.5 20.4 18.8 18.8 18.8 12.5 13.4 12.3 10.2 13.9 13.3 12.2 12.1 10.1 11.5 11.3 11.6

Color index, E                        

2.7 0.3 0.4 0.2 1.6 0.2 0.2 0.5 1.5 0.3 0.2 0.3 0.4 0.3 0.2 0.2 0.4 0.3 0.1 0.2 0.6 0.1 0.3 0.2

a b b b b b b b b b b b c c c c c c c c c c c c

44.9 50.5 50.3 52.1 38.3 49.6 49.1 51.5 42.4 48.5 48.6 48.7 40.0 51.5 51.5 51.8 44.6 48.1 48.2 48.4 45.9 48.4 47.6 49.6

                       

0.4 0.3 0.2 0.1 0.9 0.4 0.5 0.8 1.6 0.5 0.4 0.5 1.0 0.3 0.2 0.2 0.9 0.5 0.3 0.6 0.9 0.5 0.4 0.6

f, g, h a, b, c, d a, b, c, d a j a, b, c, d a, b, c, d, e a, b, c hi b, c, d, e a, b, c, d, e a, b, c, d, e ij a, b, c a, b, c a, b g, h c, d, e, f, g c, d, e, f b, c, d, e, f e, f, g, h b, c, d, e, f d, e, f, g a, b, c, d

Rupture force (N) 15.7  0.6 e, f 16.3  0.9 e, f 17.4  0.8 d, e, f 18.5  1.0 d, e 20.1  1.3 c, d 23.9  1.6 b, c 19.9  1.2 d, e 26.4  0.6 b 35.7  2.6 a 10.8  0.6 h 13.6  0.4 f, g 13.8  0.3 f, g 11.6  0.2 h 13.2  0.6 f, g 14.1  1.0 f, g 16.5  0.9 e, f 22.5  1.6 b, c 23.5  1.7 b, c

a Values with no letter in common within column are significantly different (P < 0.01) by Tukey test. Anova: L* (P < 0.0001, F ¼ 7.8, df ¼ 15); a* (P < 0.0001, F ¼ 50, df ¼ 15); b* (P < 0.0001, F ¼ 2.5, df ¼ 15); color index (P < 0.0001, F ¼ 8.8, df ¼ 15), rupture force (P < 0.0001, F ¼ 60, df ¼ 11).

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tortilla pigmentation. In contrast, ISM kernels maintained their L*, a*, b*, and color scores regardless of 10% or 20% of GWL. Processing of IRM and ISM kernels into tortillas significantly increased L* and E values indicating that the pigments mainly associated to the pericarp and outer layers were distributed evenly into the masa after stone grinding. The higher values observed occurred despite the baking of the masa discs into tortillas. In general terms, the storage of tortillas for 4 days did not significantly affect color scores (Table 5). Changes in color of nixtamalized products have been reported before (Sahai et al., 2000). The intrinsic grain properties, lime concentration, masa pH and extent of nixtamal washing greatly affect tortilla color. In terms of chemical compounds, it is known that simple phenolics, anthocyanins and carotenoids affect tortilla color (Serna-Saldívar, 2010). However, this is the first study that relates color changes to the tortilla processing of insect-damaged maize kernels. The potent salivary amylases secreted by insects (Houseman and Thie, 1993; Nansen and Meikle, 2002) hydrolyzes starch into simpler molecules including reducing sugars that upon lime-cooking and tortilla baking generate Maillard or browning reaction compounds. The formation of these compounds is enhanced by temperature. The textural properties of tortillas elaborated from insectdamaged kernels were significantly inferior compared to counterparts produced from sound kernels. Both the IRM and ISM showed similar decrements in this quality attribute. As expected, the fresh tortillas had the lowest firmness values estimated as rupture force (Table 5). The main increment in loss of texture occurred during the first 24 h storage at room temperature and both types of tortillas kept loosing texture during the subsequent days of storage. The IRM fresh and stored tortillas had a significantly higher texture compared to the ISM counterparts. Interestingly, the 20% GWL tortillas had the highest firmness values likely due to its higher amounts of damaged starch which is more prone to retrograde upon cooling and storage. Thus, the processing of insect-damaged kernels produced firmer tortillas and the effect was more negatively affected as the level of infestation increased. Modifications in tortilla texture throughout storage similar to the observed in this study have been reported before (Campas-Baypoli et al., 2002) and are mainly attributed to starch content and retrogradation. Again, this study for the first time related dramatically texture changes with level of insect damaged kernels. Undoubtedly, tortillas prepared with insect-damaged kernels lost texture throughout storage at a faster rate compared to counterparts prepared from sound kernels. The lower levels of crude fat, higher amounts of pericarp rich in fiber and damaged starch contributed to the faster loss of tortilla texture. 4. Conclusions A significant impact in terms of product quality, cooking time and weight losses during the nixtamalization process was observed when P. truncatus susceptible and resistant infested kernels were used for tortilla manufacture. The ISM kernels were more negatively affected compared with the IRM counterpart. The insectaffected kernels produced tortillas with damaged texture properties, rendering a reduced shelf-life. Changes in kernel characteristics, process time as well as product color and texture during storage due to infestation with P. truncatus is useful for tortilla manufacturers and maize producers and distributors. Acknowledgments This research was supported by the Research Chair Funds CAT005 from Tecnologico de Monterrey-Campus Monterrey and

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