Artículos de investigación

 

Nutritional and antinutritional evaluation of wild yam (Dioscorea spp.)

 

Evaluación del valor nutricional y factores antinutricionales de Dioscorea spp. silvestre

 

P.S. Shajeela¹, V. R. Mohan²*, L. Louis Jesudas³ and P. Tresina Soris²

 

¹ PG & Research Department of Botany, St. John's College, Palayamkottai, Tamil Nadu, India.

² Ethnopharmacology Unit, Research Department of Botany, V.O.Chidambaram College, Tuticorin, Tamil Nadu, India. *Corresponding Author

³ PG & Research Department of Botany, St. Xavier 's College, Palay amkottai, Tamil Nadu, India.

 

Submitted December 16, 2010
Accepted March 22, 2011
Revised received April 12, 2011

 

Abstract

The wild yam tubers consumed by the tribes Kanikkars / Palliyars of South- Eastern slopes of Western Ghats, Tamil Nadu (Dioscorea alata, D. bulbifera var vera, D. esculenta, D. oppositifolia var dukhumensis, D.oppositifolia var. oppositifolia, D. pentaphylla var. pentaphylla, D. spicata, D. tomentosa and D. wallichi) were evaluated for its nutritional quality. From the present investigation, it is observed that most of the wild edible yams were found to be a good source of protein, lipid, crude fibre, starch, vitamins and minerals. Antinutritional substances like total free phenolics, tannins, hydrogen cyanide, total oxalate, amylase and trypsin inhibitor activities were quantified.

Key words: Proximate and mineral composition; vitamins; in vitro digestibility; antinutrients.

 

Resumen

Los tubérculos de variedades silvestres de Dioscorea spp. Son consumidos por las tribus Kanikkars / Palliyars de la región sur y oriental de los Ghats Occidentales de Tamil Nadu. Dioscorea alata, D. bulbifera var vera, D. esculenta, D. oppositifolia var dukhumensis, D.oppositifolia var. oppositifolia, D. pentaphylla var. pentaphylla, D. spicata, D. tomentosa y D. wallichi fueron evaluados en cuanto a su calidad nutricional. Se encontró que fueron una buena fuente de protein, lípidos, fibra cruda, almidón, vitaminas y minerales. Adicionalmente se cuantificó el contenido de fenoles totales libres, taninos, cianuro de hidrógeno y oxalatos totales. Se cuantificó también la actividad de los inhibidores de amilasa y tripsina.

Palabras claves: composición proximal; contenido de minerales; vitaminas; digestibilidad in vitro.

 

INTRODUCTION

With ever-increasing population pressure and fast depletion of natural resources, it has become extremely important to diversify the present-day agricultural in order to meet various human needs (Janardhanan et al, 2003). The world food crisis has been and will continue to be a major obstacle to humanity. The observed interest in search for alternative / additional food and feed ingredients is of paramount importance mainly for two reasons, one is the low production of oil seeds and grains and another is the stiff competition between man and the livestock industry for existing food and feed materials (Siddhuraju et al, 2000). Root and tubers are the most important food crops since time immemorial in the tropics and subtopics (Behera et al., 2009). Roots and tubers refers to any growing plant that stores edible material in subterranean root, corm and tuber. The nutritional value of roots and tubers lies in their potential ability to provide one of the cheapest sources of dietary energy in the form of carbohydrates in developing countries (Ugwu, 2009).

Yams (Dioscorea) belong to Dioscoreaceae family. They are herbaceous plants with twine. Approximately 600 Dioscorea species are eaten in various parts of the world. (Agbor-Egbe and Treche, 1995). Yams, the edible starchy tubers, are of cultural economic and nutritional importance in the tropical and subtropical regions of the world (Coursey, 1967). Yam has been suggested to have nutritional superiority when compared with other topical root crops. They are reported as good sources of essential dietary nutrients (Baquar and Oke, 1976; Bhandari et al, 2003; Shanthakumari et al, 2008; Maneenoon et al, 2008; Arinathan et al, 2009). Earlier reports have also pointed out that a few yam species contain some toxic compounds and can impact serious health complications (Anthony, 2004). Some species of wild yams, particularly wild forms, are toxic and / or unpalatable, taste bitter and cause vomiting and diarrhoea when large amount are ingested without proper processing or if eaten raw (Webster et al, 1984).

These wild yams make a significant contribution in the diets of the tribal people of India. The tubers were found with a high amount protein, a good proportion of essential amino acids and appeared as a fairly good source of many dietary minerals. However, their wider utilization is limited due to the presence of some toxic and antinutritional factors. In India the cooked wild tubers are known to be consumed by the Palliyar and Kanikkar tribes (Arinathan et al, 2007; Shanthakumari et al., 2008) living in South-Eastern slopes of Western Ghats, Tamil Nadu. Information regarding the chemical and nutritional content of wild edible tuber is meager (Babu et al., 1990; Nair and Nair, 1992; Rajyalakshmi and Geervani, 1994; Shanthakumari et al, 2008; Alozie et al, 2009; Arinathan et al, 2009). Studies of nutritional value of wild plant food are of considerable significance since it may help to identify long forgotten food resources. In this context, an attempt was made to understand the chemical composition and antinutritional factors of the under utilized tubers of nine species of Dioscorea to suggest ways and means to remove the antinutritional / toxins and make the edible tubers as the safe food sources for mass consumption.

 

MATERIALS AND METHODS

Nine samples of wild yam tubers {Dioscorea alata, D. bulbifera var vera, D. esculenta, D. oppositifolia var dukhumensis, D.oppositifolia var. oppositifolia, D. pentaphylla var. pentaphylla, D. spicata, D. tomentosa and D. wallichi) grown in sandy loam soil consumed by the tribal Kanikkars / Palliyars were collected using multistage sampling technique in three consecutive rainy seasons during August and January 2009 from the South Eastern Slopes of Western Ghats, Virudhunagar district, Madurai district and Kanyakumari district, Tamil Nadu.

Moisture content was determined by drying the samples in an oven at 80°C for 24hrs and was expressed on a percentage basis. The samples were powdered in Willey mill 60 mesh sizes and stored in screw cap bottles at room temperature for further analysis. Nitrogen content was estimated by the micro-kjeldhal method (Humphries, 1956) and crude protein was calculated (N x 6.25).

The contents of crude lipid, crude fibre and ash were estimated by AOAC (2005) methods. Nitrogen free extract was obtained by difference method by subtracting the sum of the protein, fat, ash and fibre from the total dry matter (Muller and Tobin, 1980). The energy value of the tuber was estimated (KJ) by multiplying the percentages of crude protein, crude lipid and NFE by the factors 16.7, 37.7 and 16.7 respectively (Siddhuraju et al, 1996). From the triple acid digested sample, sodium, potassium, calcium, magnesium, iron, copper, zinc and manganese were analyzed using an atomic absorption spectrophotometer (Perkin Elmer Model 5000) (Issac and Johnson, 1975). Phosphorus was estimated colorimetrically (Dickman and Bray, 1940). The total starch was determined by the titrimetric method of Moorthy and Padmaja (2002). The antinutritional factors, total free phenolics (Sadasivam and Manickam, 1996), tannins (Burns, 1971), hydrogen cyanide (Jackson, 1967), total oxalate (AOAC, 1984), trypsin inhibitor activity (Sasikaran and Padmaja, 2003) and amylase inhibitor activity (Rekha and Padmaja, 2002). In vitro protein digestibility was determined using the multi-enzyme technique (Hsu et al, 1977) and in vitro starch digestibility was assayed by Padmaja et al. (2001).

 

RESULTS AND DISCUSSION

The crude protein (Table 1) content of the various species of Dioscorea tubers investigated in the present study was found to be in agreement with the earlier investigation in the species of Dioscorea tubers (Onyilagha and Lowe, 1985; Rajyalakshmi and Geervani, 1994; Akissoe et al, 2001; Shanthakumari et al, 2008; Alozie et al, 2009; Arinathan et al, 2009). Among the two varieties of D. oppositifolia tubers, the variety dukhumensis contained more crude protein than the variety oppositifolia. This value is found to be consonance with earlier reports (Arinathan et al, 2009). The crude lipid content of D. oppositifolia var dukhumensis was found to be higher when compared to the presently investigated other Dioscorea species. The content of crude lipids in the tubers oí Dioscorea species exhibited more crude lipid content than the earlier reports in the tubers of D. alata (Udensi et al, 2008); D. oppositifolia, D, bulbifera, D. pentaphylla, D. hispida (Rajyalakshmi and Geervani, 1994) and D. rotundata (Akissoe et al, 2001); D. calicola, D. daunea, D.wallichi, D. stemonoides and D. glabra (Maneenoon et al, 2008). The crude fibre content in the presently investigated tubers of D. esculenta, D. oppositifolia var oppositifolia, D. pentaphylla var pentaphylla, D.spicata andD. wallichi were found to be more than that in the earlier reports inD. bulbifera (Pramila et al, 1991); D. oppositifolia, D. pentaphylla (Murugesan and Ananthalakshmi, 1991); D. alata (Udensi et al, 2008); D. bulbifera, D. deltoidea, D. versicolor and D. triphylla (Bhandari et al, 2003). The nitrogen free extractives (NFE) in the tubers of D. alata, D. bulbifera var vera, D. pentaphylla var pentaphylla, D. spicata and D. tomentosa were higher (above 75%).

Table 2

This value was found to be higher than that of the previous studies in the Dioscorea spp (Rajyalakshmi and Geervani, 1994; Akissoe et al, 2001; Pramila et al, 1991). The calorific value of all the investigated Dioscorea spp. was less than that of earlier studies in the tubers oí Dioscorea spp. (Arinathan et al, 2009). Robinson (1987) reported that a diet that meets two-third of the RDA (Recommended Dietary Allowance) values is considered to be adequate for an individual. The tubers of D. oppositifolia var. oppositifolia and D. wallichi were found to contain higher calcium content than that of RDA's of NRC/NAS, (1980) for infants and children. The magnesium content of D. alata was found to be more when compared to that of the other Dioscorea species.All the investigated tubers were found to contain higher magnesium content than that of RDA's of NRC/NAS (1980) for infants and children. The tubers of D. bulbifera var vera, D. esculenta, D. oppositifolia var dukhumensis and D. oppositifolia var oppositifolia were found to contain higher level of potassium when compared with RDA's of infants and children (<1550mg) (NRC/NAS, 1980). The high content of potassium can be utilized beneficially in the diets of people who take diuretics to control hypertension and suffer from excessive excretion of potassium through the body fluid (Siddhuraju et al., 2001). The manganese content of D. bulbifera var vera was found to be high when compared to that of the other investigated Dioscorea species. All the investigated tubers appeared to have a higher level of manganese content compared to ESADDI of infants, adults and children of NRC/NAS (1989).

The amount of starch (Table 3) estimated in the tubers of Dioscorea sp. were higher than that of the earlier reports in the species of Dioscorea (Rajyalakshmi and Geervani, 1994; Arinathan et al., 2009). The niacin content in the tubers of Dioscorea alata, D. bulbifera var vera, D, esculenta, D. oppositifolia var oppositifolia, D. pentaphylla var pentaphylla, D. spicata and D. wallichi were found to be higher than in the tubers of Dioscorea species (Rajyalakshmi and Geervani, 1994; Arinathan et al., 2009). The starch content of D. bulbifera var vera, D. oppositifolia var dukhumensis, D. pentaphylla var pentaphylla and Dioscorea tomentosa was found to be more when compared with the earlier reports of same wild edible yams (Arinathan et al., 2009). This difference may be due to some edaphic factors. Among the investigated tubers D. bulbifera var vera, D. oppositifolia var dukhumensis and D. pentaphylla var pentaphylla registered the highest ascorbic acid content than the earlier studied tubers of D. alata (Udensi et al., 2008).

The antinutritional factors like total free phenolics, tannins, hydrogen cyanide, total oxalate, amylase inhibitor and trypsin inhibitor activities are presented in Table 4. Phenolic compounds inhibit the activity of digestive as well as hydrolytic enzymes such as amylase, trypsin, chymotrypsin and lipase (Salunkhe, 1982). Among the various species of Dioscorea, the tubers of D. bulbifera var vera contained more free phenolics (Table 4). This value was found to be higher than that of the earlier studies in the tubers of Ipomoea batatus (Adelusi and Ogundana, 1987). D. esculenta, D. alata, D. rotundata (Babu et al., 1990; Sundaresan et al., 1990); Manihot esculenta and Ipomoea batatus (Babu et al., 1990). Recently phenolics have been suggested to exhibit health related functional properties such as anticarcinogenic, antiviral, antimicrobial, anti-inflammatory, hypotensive and antioxidant activity (Shetty, 1997). The level of tannins, hydrogen cyanide and total oxalate were found to be lower when compared with the earlier reports of the tubers of Dioscorea alata, D. cayenensis, D. rotundata and D. esculenta (Esuabana, 1982). The tubers of D. oppositifolia var dukhumensis and D. oppositifolia var oppositifolia contained more trypsin inhibitor activity when compared with earlier reports in the tubers of D. dumetorum and D. rotundata (Sasikiran et al., 1999). The phenolics and tannins are water soluble compounds (Uzogara et al., 1990) and as such can be eliminated by soaking followed by cooking (Singh, 1988; Murugesan and Ananthalakshmi, 1991; Kataria et al., 1989; Singh and Singh, 1992; Shanthakumari et al, 2008). A lot of hydrogen cyanide (known to inhibit the respiratory chain at the cytochrome oxidase level) is lost during soaking and cooking (Shanthakumari et al, 2009) so that its content in the tubers posses no danger of toxicity. Boiling for sufficient time makes the tuber soft enough and inactivates all the trypsin inhibitor (Bradbury and Holloway, 1988).

Table 5 shows the data of in vitro protein digestibility and in vitro starch digestibility. In all the presently investigated samples, the in vitro protein digestibility (IVPD) is found to be very low. The in vitro protein digestibility of D. oppositifolia var. oppositifolia and D. spicata was found to be higher when compared with the earlier reports of D. oppositifolia var. dukhumensis, D. pentaphylla var. pentaphylla, D. tomentosa and D. spicata (Mohan and Kalidas, 2010). However, in vitro starch digestibility (IVSD) of the tubers Dioscorea bulbifera var vera, D. pentaphylla var. pentaphylla, D. spicata and D. tomentosa is found to be higher than that of the previous reports of D. oppositifolia, D. bulbifera, D. pentaphylla, D. hispida and the pith of Caryota urens (Rajyalakshmi and Geervani, 1994).

 

CONCLUSIONS

Based on the nutritive evaluation studies on the wild edible yams consumed by the tribals Kanikkars and Palliyars, it can be summarized that most of them were found to be a good source of protein, lipid, crude fibre, starch, vitamins and minerals. All the investigated samples exhibited variations in the levels of total free phenolics, tannins, hydrogen cyanide, total oxalate, amylase and trypsin inhibitors. Except phenolics, tannins, hydrogen cyanide, total oxalate, amylase and trypsin inhibitors, these antinutritional can be inactivated by moist heat treatments. Phenolics, tannins, hydrogen cyanide and total oxalate can be eliminated by soaking followed by cooking before consumption. It is recommended as a means of removing harmful effects of these antinutritional.

 

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