Composite wheat-plantain starch salted noodles. Preparation, proximal composition and in vitro starch digestibility.

Rodolfo Rendón-Villalobos, Perla Osorio-Díaz, Edith Agama-Acevedo, Juscelino Tovar and Luis A. Bello-Pérez

Rodolfo Rendón-Villalobos. Biologist, Universidad Autónoma del Estado de Morelos, Mexico. M.Sc. in Marine Ecology, Centro de Investigación Científica y de Educación Superior de Ensenada, Mexico. Researcher, Centro de Desarrollo de Productos Bióticos (CEPROBI-IPN), Mexico.

Perla Osorio-Díaz. Nutritionist and M.Sc. in Food Science and Technology, Instituto Politécnico Nacional (IPN), Mexico. Ph.D. in Food Science and Technology, Universidad Autónoma de Querétaro (UAQ), Mexico. Researcher, CEPROBI-IPN, Mexico.

Edith Agama-Acevedo. Biochemical Engineer, Instituto Tecnológico de Acapulco (ITA), Mexico. M.Sc. in Biotic Product Development, CEPROBI-IPN, Mexico. Ph.D. in Food Science and Technology, UAQ, Mexico Researcher, CEPROBI-IPN, Mexico.

Juscelino Tovar. Biologist, Universidad Central de Venezuela (UCV). Ph.D. in Applied Nutrition, University of Lund, Sweden. Professor, UCV, Venezuela.

Luis A. Bello-Pérez. Biochemical Engineer, ITA, Mexico. M.Sc. in Bio-engineering and Ph.D. in Plant Biotechnology, CONVESTAV-IPN. Professor, CEPROBI-IPN, Mexico. Address: CEPROBI-IPN, Apartado 24 C.P., 62731, Yautepec, Morelos, México. e-mail: labellop@ipn.mx

SUMMARY

Salted noodles were prepared with different contents of wheat grits and plantain starch (PS). The blends were hydrated with 2% NaCl (w/v), homogenized, and the resulting doughs were sheeted through a pasta machine, cut into strips ~30cm in length, cooked, and their composition and in vitro starch digestibility was assessed. Moisture (6.43-7.60%) and ash contents (2.08-3.12%) increased by the addition of PS. Fat level decreased from 0.41 to 0.31% as the substitution of wheat grits increased. Results showed a 7.39% lower total starch content in the control sample as compared to the noodle containing 30% PS. A similar pattern was observed for potentially available starch content, but the difference was greater (12.46%). Approximately 50% of total resistant starch (RS) in the noodles was resistant starch associated to fiber, showing that a part of RS is due to the physically inaccessible and retrograded starch fractions. Pure wheat (control) noodles presented a greater final a-amylolysis value, which is suggestive of potentially lower glycemic impact for the plantain/wheat products.

RESUMEN

Se elaboraron tallarines salados con diferentes contenidos de sémola de trigo y almidón de plátano (AP). Las mezclas fueron hidratadas con solución NaCl 2% (p/v), homogenizadas y las masas laminadas en una máquina para elaborar pasta. Se cortaron en tiras de ~30cm de longitud, se cocinaron y se estudió su composición, así como la digestibilidad del almidón in vitro. El contenido de humedad (6,43-7,60%) y cenizas (2,08-3,12%) incrementaron con la adición del AP. El contenido de lípidos disminuyó de 0,41 a 0,31% conforme el nivel de sustitución de la semolina incrementó. El contenido de almidón total fue 7,39% menor en la muestra control comparado con el tallarín con 30% de AP. Un comportamiento similar se encontró en el almidón disponible, pero la diferencia fue mayor (12,46%). Aproximadamente 50% del almidón resistente total (AR) en los tallarines fue almidón resistente asociado a fibra, mostrando que una parte del AR es debido al almidón físicamente inaccesible y al almidón retrogradado. El valor final de la a-amilólisis fue mayor en el control de trigo puro, lo cual sugiere que los tallarines elaborados con la mezcla semolina/AP tienen menor impacto glucémico.

RESUMO

Elaboraram-se talharins salgados com diferentes conteúdos de sêmola de trigo e amido de banana da terra (ABT). As misturas foram hidratadas com solução NaCl 2% (p/v), homogeneizadas e as massas laminadas em uma máquina para elaborar massa. Cortaram-se em tiras de ~30cm de comprimento, foram cozidas e estudada sua composição, assim como a digestibilidade do amido in vitro. O conteúdo de umidade (6,43-7,60%) e cinzas (2,08-3,12%) se incrementou com a adição de ABT. O conteúdo de lipídios diminuiu de 0,41 a 0,31% na medida em que o nível de substituição da semolina se incrementou. O conteúdo de amido total foi 7,39% menor na amostra controle comparado com o talharim com 30% de ABT. Um comportamento similar se encontrou no amido disponível, mas a diferença foi maior (12,46%). Aproximadamente 50% do amido resistente total (AR) nos talharins foi amido resistente associado à fibra, mostrando que uma parte do AR é devido ao amido fisicamente inacessível e ao amido retrogradado. O valor final da a-amilólisis foi maior no controle de trigo puro, o qual sugere que os talharins elaborados com a mistura semolina/ABT têm menor impacto glicêmico.

KEYWORDS / Chemical Composition / Digestibility / Pasta / Plantain / Starch /

Starch, which is the major dietary source of carbohydrates, is also the most abundant storage polysaccharide in plants. It occurs as granules in the chloroplast of green leaves and the amyloplast of seeds, pulses, and tubers. Starch is a major component of wheat grain; it is located in the endosperm and has some unique properties, which determine its functionality in many food applications (Shibanuma et al., 1994).

Unripe banana flour, for instance, contains between 47 and 57% RS, depending on the analytical method employed (Faisant et al., 1995). More recently, an RS level of 17.5% was recorded for the flour of a plantain variety (Juárez-García et al., 2006). With nearly 40% of wheat being processed into noodle products in Asian countries (Baik and Lee, 2003), the consumption of wheat noodles is, globally, second only to bread. The instant noodle market is growing fast in Asian countries, and is gaining popularity in the Western market (Yu and Ngadi, 2004). Depending on the method of dehydration, instant noodles can be divided into fried and non-fried types (Wu et al., 1998). Fried instant noodles are made by a continuous steaming and frying process that gelatinizes starch and quickly dehydrates the noodles. The resulting product has a porous spongy structure and an excellent flavor. Non-fried instant noodles can be dehydrated after expansion from a tight non-expanded structure, or using high-temperature expansion to produce a porous, honeycomb-like structure, e.g., the expanded type (Wu et al., 1998).

Wheat grits is the main ingredient for making Asian noodles (Baik and Lee, 2003). About three parts of flour are usually mixed with one part of salt or alkaline salt solution to first form a crumbly dough and then "white salted noodles". In Japan, instant noodles, which are steamed, deep fat-fried and packed in polyethylene bags, are a popular industrially processed food. Chinese-type alkaline instant noodles, called "instant ramen", are manufactured in the highest quantity. The ingredients of instant noodles are wheat grits, starch, salt, alkali (soda/potash) and water (Noda et al., 2006). Park and Baik (2004) observed the significance of the amylose content of starch in wheat grits with respect to the textural properties of instant noodles. They indicated that there was a positive correlation between the hardness of cooked instant noodles and the amylose content. Variations in suitability as a material for the production of starch-based noodles were found among starch samples prepared from several potato cultivars (Singh et al., 2002). On the other hand, Batey et al. (1997) have confirmed the importance of the starch component, reporting significant correlations between textural properties of alkaline noodles and selected flour pasting characteristics or swelling parameters derived of flour or whole meal. However, information on starch digestibility in white salted noodles is yet rather scarce.

The objectives of this research were to prepare plantain starch-containing noodles and evaluate its proximal composition. The impact of adding different plantain starch levels on the in vitro digestibility of starch in the noodles was also evaluated.

Unripe plantains (Musa paradisiaca L.) were purchased at the local market of Cuautla, Morelos State, Mexico. Starch was isolated by the procedure described by Flores-Gorosquera et al. (2004).

White salted noodles (WSN) were prepared by mixing of wheat grits and plantain starch in proportions of 90:10, 80:20 and 70:30, respectively. Control WSN was prepared with 100% wheat grits. The control sample and blends were mixed with enough 2% NaCl (w/v) to obtain a completely hydrated flour. Mixing was carried out in an N50 mixer (Hobart, North York, Canada) for 5min using low speed (speed position 1). The mixed dough was sheeted through the rolls of a pasta machine with a gap setting of 3.0mm. After the first pass, the noodle sheet was folded and passed twice through the rollers at this same setting. The dough sheet was cut through #12 cutting rolls into strips ~30cm in length and cross-section of 0.3×0.2cm. The resulting noodle strip was placed uniformly into a steam pan and then put into a preheated (100°C) steamer, and cooked for 12min until the noodle strip had a smooth surface and an elastic texture, as it is normally consumed, according to the finger test. All samples were frozen in liquid nitrogen, freeze dried and stored at room temperature in sealed plastic containers.

Total starch content in the noodles was assessed according to Goñi et al. (1997). Samples of 50mg were dispersed in 3ml distillated water and 3ml of 4M KOH. The mixture was intensely stirred with a magnetic bar during 30min at room temperature. After neutralization, the mixture was treated with amyloglucosidase (Boehringer, Mannheim, Germany) in order to release glucose, which was measured colorimetrically using the glucose oxidase peroxidase assay (SERA-PAK^® Plus, Bayer de México). Starch analyses were performed in triplicate.

The content of potentially available starch was assessed following the multienzymatic protocol of Holm et al. (1986). Briefly, the sample was treated with a heat-stable a-amylase (Termamyl^® Novo A/S, Copenhagen) in a boiling water bath for 20min and then with amyloglucosidase (Boehringer, Mannheim, Germany). Released glucose was analyzed using the glucose-oxidase-peroxidase (GOD/POD) system. Resistant starch (RS) was measured by two different protocols: 1) The content of RS associated to fiber (RSAF or RS3) was measured as starch remnants in dietary fiber residues, according to the so called "Lund method" as modified by Saura-Calixto et al. (1993); 2) the method proposed by Goñi et al. (1996) was employed to estimate the total amount of indigestible starch (comprising RS2, RS3 and part of RS1 fractions; Tovar, 2001). In brief, removal of protein with pepsin P-7012 (Sigma Chemical, St. Louis, MO, USA) was followed by incubation with a-amylase A-3176 (Sigma) to hydrolyze digestible starch; after this, the insoluble material was treated with 2M KOH in order to disperse the resistant starch, which was immediately digested with amyloglucosidase A-7255 (Sigma). Finally, released glucose was determined using the glucose oxidase/peroxidase assay (SERA-PAK^® Plus, Bayer de México). The in vitro rate of a-amylolysis was measured according to Holm et al. (1985). The percentage of digested starch was estimated at different incubation times from the maltose produced, assessed with the 3,5-dinitro-salicylic acid (DNS) acid reaction. Each assay was run with 500mg available starch. All in vitro digestibility tests were performed in duplicate on noodles cooked (boiled) as for eating. The a-amylolysis assays were carried out with homogenized cooked noodles.

The results are presented as mean ±SEM. A commercial software program (Sigma Stat ver. 2.03, Jandel Corporation, San Rafael, CA, USA) was used to conduct two-way analysis of variance (ANOVA) for determining significant differences among means. Statistically significant differences (p<0.05) among means were determined using the Tukey multiple comparison procedure.

The addition of plantain starch to wheat noodles increased the product moisture content (Table I). This is likely due to the well known water binding properties of starch. Moisture content of noodles is important during cooking, as the presence of higher water levels in the product may result in complete starch gelatinization, a change that influences the product, resulting in a rubber-like texture. Since plantain starch has a low fat content, a slight reduction of this component was observed in the composite noodles (Table I); furthermore, fat content did not change with the increase in plantain starch level (p<0.05). The wheat grits control noodle exhibited higher protein content than plantain starch-added preparations; this parameter decreased as substitution with plantain starch increased. A dilution effect is likely responsible of this pattern, since plantain starch has low protein content (Aparicio-Saguilán et al., 2005). Protein contents in the noodles are similar to those quoted by Nagao (1996) for alkaline noodle flours (10.5-12.0%). Crosbie et al. (1999) recorded protein contents ranging between 10.0 and 12.2% in 21 experimental noodle samples, while the commercial ramen noodle (used as a reference) contained 11% protein. Additionally, Baik and Lee (2003) and Wang et al. (2004) reported protein values between 10.5-16.4 and 10.1-19.3% for cooked white salted noodles, respectively. The ash content in the plantain starch-containing noodles increased (p<0.05) with the substitution (Table I). This result reflects the relatively high ash content of isolated plantain starch (0.4-0.45%; Aparicio-Saguilán et al., 2005; González-Soto et al., 2007). Plantain fruits are rich in various minerals, such as potassium (Aparicio-Saguilán et al., 2005). Thus, the intake of these composite noodles might contribute to satisfy potassium requirements.

The total starch (TS) content of the noodles increased following the addition of plantain starch (Table II). Thus, a 7.39% TS difference was recorded between the control sample and noodles added with 30% plantain starch.

Similarly, as the plantain starch level in the formulation rose, potentially available starch (AS) in the noodle increased (Table II). It is noteworthy that the AS content in the 30% plantain starch-containing noodle was about 12% higher than in the control sample. This pattern might be associated with a perceived compactness reduction in the composite noodles; addition of plantain starch decreases the pasta protein (gluten) content, which may lead to lower compactness and this, in turn, may result in increased physical starch availability to enzyme action. AS content in two experimental cookie types containing 15-17% plantain starch was 51% (Bello-Pérez et al., 2004). In another study, a bakery product added with 15% modified plantain starch exhibited a 40% AS content (Aparicio-Saguilán et al., 2007).

When total resistant starch (RS) was assessed, the lowest value was recorded in the control noodle (1.87%). Noodles containing plantain starch showed a slight but significant (p<0.05) RS increase (Table II), although values did not differ among the various composite preparations. Native banana starch is among the richest natural sources of RS (Bello-Pérez et al., 2004; Aparicio-Saguilán et al., 2007).

The limited digestibility of this starch is often attributed to its particular granular structure (Englyst et al., 1992; Faisant et al., 1995), corresponding to the so-called type 2 RS (Englyst et al., 1992). However, in heat treated products, like the boiled noodles evaluated in this study, starch suffers gelatinization, a phenomenon that rules out type 2 RS (Tovar, 2001). Since composite noodles exhibited only 2-2.2% RS content (Table II), it can be concluded that gelatinization of plantain starch in the cooked noodles was practically complete. Such a low RS content may also be related to the putative weak interaction (low compactness) between the wheat grits protein and plantain starch, which probably results in pasta where starch is more easily digested than in a compact noodle. It is conceivable that the type of pasta may play a role in the enzymatic hydrolysis behavior of starch in composite noodles; however, further studies are necessary to evaluate this possibility. RS contents were 3.24 and 4.9% in cookies added with native plantain starch (Bello-Pérez et al., 2004) and 8.42% in cookies containing a high RS modified plantain starch (Aparicio-Saguilán et al., 2007). Taking all these results together, it may be suggested that the type of product prepared with wheat/plantain starch blends markedly influences the final RS content.

Approximately 50% of the RS determined in the noodles was resistant starch associated to fiber (RSAF; Table II), showing that a significant part of the indigestible starch in these samples is due to a combination of physically inaccessible and retrograded starch fractions, which is a common characteristic of pasta products (Granfeldt, 1994). The pattern exhibited by RSAF is in agreement with that of RS values, since its values did not change with the plantain starch ratio in the composite noodle. Lower RSAF values (0.2 and 0.4%) were determined in cookies added with native plantain starch.

The course of the in vitro a-amylolysis reaction for the noodles is represented in Figure 1. All noodles studied showed similar behavior, as during the first 15min of reaction the hydrolysis rate was high and thereafter the hydrolysis index did not change. The sample added with 30% plantain starch exhibited the lowest final starch digestion point (~43%) and no statistical difference (p<0.05) was observed between the 10 and 20% plantain starch-containing noodles. The control sample displayed the highest final hydrolysis value (~52%). The amylolysis pattern of the studied noodles resembles that shown by freshly prepared tortillas from commercial dough and commercial dry dough flour, which were shown to reach a maximal hydrolysis level during the first 15min of reaction and remained constant thereafter (Bello-Pérez et al., 2006). However, the hydrolysis values in those tortillas (60-65%) were higher than in the noodles herein reported. Again, the microstructure of this pasta product, related to physical compactness, may be responsible for a decreased accessibility by digestive enzymes. Furthermore, permanence of intact plantain starch granules entrapped within the pasta structure might also be controlling the lower hydrolysis rate. Low in vitro a-amylolysis indices are often related to moderate postprandial glycemic responses in vivo (Holm et al., 1985), which is an important factor in the dietary management of altered metabolic conditions, such as diabetes (Englyst et al., 1992; Granfeldt, 1994).

White salted noodles prepared from wheat grits only and those prepared from wheat grits mixed with different plantain starch proportions exhibited clear differences in proximal composition. Noodles with greater plantain starch levels showed lower in vitro digestibility compared to those with a low content of this starch or with control wheat grits noodles. The results could be applied in further formulation studies to optimize quality attributes of composite noodles and may be useful in the development of new products for groups with special caloric and glycemic requirements.

The authors recognize the financial support from SIP-IPN, COFAA-IPN and EDI-IPN.

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