INDIGESTIBLE STARCH ASSOCIATED TO DIETARY FIBER RESIDUES FROM COOKED LEGUME SEEDS CONSUMED IN VENEZUELA

Carolina Peñalver, Irma M. Herrera and Juscelino Tovar

Carolina Peñalver. Nutritionist. M.Sc in Food Science Technology., Universidad Central de Venezuela (UCV). Professor, School of Nutrition and Dietetics, UCV, Venezuela.

Irma M. Herrera. Biologist and MSc., UCV. Researcher, Instituto Nacional de Nutrición. Venezuela.

Juscelino Tovar. Biologist, UCV. Ph.D., University of Lund, Sweden. Professor, UCV. Address: Instituto de Biología Experimental, UCV. Apartado 47069, Caracas 1041A. Venezuela. email: jtovar@ciens.ucv.ve

SUMMARY

Dietary fiber residues were prepared from the cooked seeds of eleven legume varieties, following the AOAC enzymatic method. Resistant starch associated to fiber, i.e. retrograded resistant starch (RS), was assessed after alkaline dispersion of the indigestible residues. All legumes contained significant amounts of RS, ranging between 3.0% (g per 100g cooked seed on dry matter basis) in red cowpeas and 7.2% in green peas. RS contents in the four common bean (Phaseolus vulgaris) varieties analyzed fell within a narrow interval (5.1 to 6.7%). Chick peas also exhibited substantial indigestible starch levels (6.7%). Employing previously reported dietary fiber values, RS levels recorded for each legume were used to calculate starch-corrected insoluble dietary fiber. Corrected figures were 11 to 38% lower than original values.

ALMIDÓN RESISTENTE ASOCIADO AL RESIDUO FIBROSO DE SEMILLAS DE LEGUMBRES COCIDAS CONSUMIDAS EN VENEZUELA

RESUMEN

El residuo fibroso de las semillas cocidas de once variedades de leguminosas comestibles fue preparado empleando el procedimiento enzimático de la AOAC. Una vez sometidos a un tratamiento de dispersión alcalina, dichos residuos fueron evaluados para determinar su contenido de almidón, que corresponde a fracciones de almidón resistente retrogradado (AR). Todas las leguminosas presentaron cantidades apreciables de AR, cuyos valores variaron entre 3,0% (g por 100g de semilla cocida en base seca), encontrado para el frijol caupí, y 7,2% para las arvejas verdes. El tenor de AR difirió muy poco entre las cuatro variedades de frijol común (Phaseolus vulgaris) analizadas (5,1-6,7%). El garbanzo también presentó valores considerables de AR (6,7%). Los tenores de AR se utilizaron para calcular el contenido de fibra dietética corregido, tomando como base valores de contenido de fibra insoluble descritos previamente para estas leguminosas. Dichos contenidos corregidos resultaron 11 a 38% menores que los valores originales.

AMIDO RESISTENTE ASSOCIADO A RESÍDUO FIBROSO DE SEMENTES DE LEGUMES COZIDAS CONSUMIDAS NA VENEZUELA

RESUMO

O resíduo fibroso das sementes cozidas de onze variedades de leguminosas comestíveis foi preparado empregando o procedimento enzimático da AOAC. Uma vez submetidos a um tratamento de dispersão alcalina, ditos resíduos foram avaliados para determinar seu conteúdo de amido, que corresponde a frações de amido resistente retrogradado (AR). Todas as leguminosas apresentaram quantidades apreciáveis de AR, cujos valores variaram entre 3,0% (g por 100g de semente cozida em base seca), encontrado para o feijão-caupi, e 7,2% para as ervilhas verdes. O teor de AR deferiu muito pouco entre as quatro variedades de feijão comum (Phaseolus vulgaris) analisadas (5,1-6,7%). O Grão-de-Bico também apresentou valores consideráveis de AR (6,7%). Os teores de AR se utilizaram para calcular o conteúdo de fibra dietética corrigido, tomando como base valores de conteúdo de fibra insolúvel descritos previamente para estas leguminosas. Ditos conteúdos corrigidos resultaram de 11% a 38% menores que os valores originais.

KEYWORDS / Beans / Dietary Fiber / Legumes / Resistant Starch / Retrograded Starch /

Received: 01/18/2007. Accepted: 08/08/2007.

Introduction

The definition of fiber has changed since the recognition of an indigestible portion in foods of plant origin. Thus, terms like crude fiber, roughage and, more recently, dietary fiber have appeared in the literature as the analysis of the indigestible portion of foods evolved (Asp, 1996; Saura-Calixto et al., 2000; McCleary, 2003). As a matter of fact, it has been extremely difficult to obtain an undisputed definition for dietary fiber (Cho and Prosky, 1999; Saura-Calixto et al., 2000), although some proposals hold manifest approval (Prosky et al., 1988; McCleary, 2003). One recent issue relates to the inclusion of the indigestible starch fraction of foods as dietary fiber (Champ et al., 2003; Sajilata et al., 2006; Tovar et al., 2006).

Indigestible, or resistant starch (RS) is defined as the sum of starch plus starch degradation products not absorbed in the small intestine of healthy individuals (Asp, 1992). Because of its indigestible character, RS may be regarded as dietary fiber (Champ et al., 2003; McCleary, 2003), although it does not make part of plant cell walls. In addition to a number of physiological motivations, the possible acceptance of RS as fiber is supported by the frequent occurrence of indigestible starch in dietary fiber residues prepared by enzymatic means (Johansson et al., 1984; Saura-Calixto et al., 1993; Tovar et al., 2002; McCleary, 2003). Actually, the first definition of RS was introduced to describe the presence of a-glucans in the dietary fiber fraction of heat-treated starchy foods (Englyst et al., 1982). It is now known that RS associated to dietary fiber residues is only a part of the total indigestible starch occurring in foods (Englyst et al., 1992; Champ et al., 2003) and corresponds to firmly retrograded starch fractions arising upon cooling and storage of gelatinized starch or cooked starch-containing food items (Saura-Calixto et al., 1993; Fredriksson, et al., 2000). Due to its physicochemical characteristics, amylose is the main responsible for the formation of retrograded RS (Siljeström et al., 1989), whereas the role of amylopectin in this process is controversial (Fredriksson et al., 2000).

Even if RS is to be included as part of dietary fiber, it may be still advantageous to know the exact contribution of the former to whole fiber contents (McCleary, 2003), particularly when dealing with foods whose starch constituents have significant retrogradation proclivity, such as legume seeds (Tovar et al., 1990a, b, 2002). In this study, cooked seeds from 11 different legumes consumed in Venezuela were analyzed for starch remnants in dietary fiber residues prepared with the enzymatic protocol of Prosky et al. (1988). Such figures may be employed to calculate starch-corrected dietary fiber values for these edible pulses.

Materials and Methods

The following legume seeds were analyzed: lentils (Lens culinaris L), green and yellow peas (Pisum sativum L), white and red cowpeas (Vigna sinensis L), chickpeas (Cicer arietinum L), pigeon peas (Cajanus cajan L Mills), and black, white, red and kidney beans (Phaseolus vulgaris L). All samples were purchased from a local market (Caracas, Venezuela). Since commercially available dehulled green and yellow peas are often consumed in Venezuela, these coat-free seeds were also evaluated.

Seeds were processed as in a previous study of dietary fiber in Venezuelan pulses (Herrera et al., 1998). In brief, seeds were washed and wetted in water (2h; room temperature; 4:1 water:seed ratio, v/w). The water/seed mixture was then boiled for 2h with liquid reposition as needed. After cooling to room temperature, cooked seeds were drained, dried under forced-air convection (25ºC) and milled (60 mesh). Dehulled peas were boiled for 1h and dried/milled without draining (Herrera et al., 1998).

Insoluble dietary fiber residues were prepared in duplicate for each legume, following the enzymatic method of Prosky et al. (1988), the same protocol employed in the study by Herrera et al. (1998). Fiber residues were analyzed enzymatically for starch remnants, i.e. resistant starch, previous dispersion of the indigestible residue/celite mixture in 2N KOH (Johansson et al., 1984; Tovar et al., 1990a; Sambucetti and Zuleta, 1996). Results from the two residues were averaged and expressed as resistant starch (g) per 100 g cooked seed (dry matter basis).

Results and Discussion

Starch fractions remaining in dietary fiber residues prepared by enzymatic means consist of firmly retrograded a-glucan chains not susceptible to amylolytic attack (Siljeström et al., 1988; Saura-Calixto et al., 1993; Champ et al., 2003). Such an indigestible portion is called type 3 RS, according to the classification proposed by Englyst et al. (1992). As shown in Table I, all of the samples contained appreciable amounts of fiber-associated RS, whose levels varied between 3.0 and 7.2% (dmb). This finding is not surprising since starch in leguminous seeds has been shown to have a marked tendency to retrograde, thus generating indigestible material (Tovar et al., 2002) in proportions that surpass levels commonly found, for instance, in boiled roots and tubers (Blanco-Metzler et al., 2004), bread (Johansson et al., 1984) and other cereal products (Saura-Calixto et al., 1993; Sambucetti and Zuleta, 1996).

Compared to RS3 data reported previously for other pulses, present values are in the same order as those found in red and brown beans (Tovar et al., 1990a, b), cowpeas (Velazco et al., 1997) and black beans (Vargas-Torres et al., 2004). The highest RS level was recorded in green peas (Table I), while the two cowpea cultivars exhibited the lowest concentrations. It is noteworthy that RS contents in the four common bean (Phaseolus vulgaris) varieties analyzed fell within a narrow interval (5.1-6.7%).

In addition to being good sources of protein, starch and dietary fiber, pulses are also regarded as functional foods, whose indigestible components may have a number of health-beneficial effects (Duranti, 2006). Functional features of the indigestible components of foods mainly depend on their fermentability (Nyman, 2003). Different dietary fibers may be fermented to different extents, depending on the botanical source and processing conditions (Asp et al., 1993). Similarly, not all resistant starches are fermented equally; for instance, indigestible starch in common beans is fermented in the rat gut to a larger extent than lentil RS (Tovar et al., 1992). Moreover, fiber and RS may also exhibit different fermentation product patterns (Nyman, 2003). Hence, in spite of the current tendency to integrate all indigestible fractions to the dietary fiber concept (Saura-Calixto et al., 2000; Sajilata et al., 2006) it is yet advisable to know the composition of the indigestible portion in a particular food (McCleary, 2003; Gordon, 2007). Fiber-associated RS contents may be subtracted from insoluble dietary fiber (IDF) in order to calculate the starch-corrected IDF (Tovar et al., 1990a; Sambucetti and Zuleta, 1996). With this aim, data reported by Herrera et al. (1998), who analyzed the IDF content of cooked samples of the legumes studied here was used. Corrected IDF contents (II) ranged between 11.6 and 24.0% (dmb), which are substantially lower than the uncorrected fiber levels (16.4-27%).

The most evident changes recorded after correcting IDF values involved red cowpea, black bean and pigeon pea. The former exhibited 1% (dmb) more IDF than black beans and almost 3% more than pigeon pea, whereas corrected IDF values revealed equivalent contents for black bean and pigeon pea (~20%), which was 4% lower than in red cowpea. Also, despite the similar IDF content in yellow and green peas, their corrected IDF figures were considerably different.

Conclusions

Cooked seeds from eleven legume varieties consumed in Venezuela were evaluated for dietary fiber-associated resistant starch, i.e. type 3 RS. All of the samples contained appreciable amounts of RS, ranging between 3 and 7.2% (dmb). RS contents were subtracted from previously reported insoluble dietary fiber values in order to obtain starch-corrected IDF contents, which may be valuable for food labeling, dietetic and physiological purposes.

Acknowledgements

The authors acknowledge the financial support from the International Foundation for Science (IFS-Grant E-2009/3, Stockholm, Sweden) and LANFOODS (Quito, Ecuador).

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