Elsevier

Food Chemistry

Volume 170, 1 March 2015, Pages 37-46
Food Chemistry

Variation of mineral composition in different parts of taro (Colocasia esculenta) corms

https://doi.org/10.1016/j.foodchem.2014.08.025Get rights and content

Highlights

  • Contents of eleven crucial minerals were studied in four main parts of taro corms.

  • The upper part contained high levels of P, Mg, Zn, Fe, Mn, Cu and Cd.

  • The central part was characterised by high contents of K, P, Mg, Zn, Fe, Cu and Cd.

  • Ca was concentrated in the lower and the marginal parts.

  • The differences amongst genotypes (cultivars) were significant.

Abstract

Taro (Colocasia esculenta) is an important root crop in the humid tropics and a valuable source of essential mineral nutrients. In the presented study, we compared the mineral compositions of four main parts of taro corm: the upper, marginal, central and lower (basal) parts. The freeze-dried taro samples were analysed for eleven minerals (K, P, Mg, Ca, Zn, Fe, Mn, Cu, Cd, Pb and Cr). The upper part, which plays a critical role in vegetative propagation based on headsets, contained high levels of P, Mg, Zn, Fe, Mn, Cu and Cd. The central part, which is essential for human nutrition, was characterised by higher concentrations of K, P, Mg, Zn, Fe, Cu and Cd. Ca was concentrated in the lower and marginal parts. The effect of the genotype was significant for more than half of the analysed minerals (i.e., Mg, Ca, Zn, Fe, Mn).

Introduction

Taro (Colocasia esculenta (L.) Schott) is an important food crop of the humid tropical rain forest areas of the world, growing best where annual rainfall is well distributed, with 2500 mm average annual precipitation or more (Weightman, 1989). The term “taro” is frequently used for four aroid species: Alocasia macrorrhiza (L.) G. Don (giant taro), C. esculenta (taro, true or ordinary taro), Cyrtosperma merkusii (Hassk.) Schott (giant swamp taro) and Xanthosoma sagittifolium (L.) Schott (cocoyam, tania, taro Fiji). For agriculture and human consumption, the more important aroid species is C. esculenta. This species is polymorphic and involves two botanical varieties: C. esculenta var. esculenta or dasheen (characterised by a large main or central corm and several smaller side cormels) and C. esculenta var. antiquorum or eddoe (characterised by a relatively small central corm and well-developed side cormels) (Ivancic & Lebot, 2000).

Taro corm represents a large underground stem that stores starch and other nutrients. Corms can vary in size, shape and colour, depending on the genetic structure, age, and interactions between the genotype and the environment. Taro cultivars are frequently divided in two groups: cultivars adapted to upland conditions and cultivars adapted to permanent irrigation (paddy cultivars). Corms of typical upland varieties are usually round or slightly elongated, whereas extremely elongated corms are more characteristic for paddy genotypes (Lebot, 2009).

The corm consists of three main parts: the skin, the cortex and the core (the central part). The skin is covered by a thick periderm that consists mainly of phellem (cork cells) that are flattened radially and are arranged in compact radial rows. Three regions of phellem can be distinguished: an outer region of 2–3 cell layers, which are brown in colour; a middle region of 5–10 cell layers; and an inner region of 10–15 cell layers. The phellogen is one to two cells in width, and no clear demarcation is evident between the phelloderm and the underlying outer cortex. The cortex consists of parenchymatic tissue characterised by intercellular spaces. The cells of the core are larger, with thinner cell walls than those of the cortex and smaller intercellular spaces (Harris, Ferguson, Robertson, Mckenzie, & White, 1992).

Essential minerals, as inorganic substances, are present in all body fluids and tissues and play important roles in metabolic and physicochemical processes like maintenance of pH and osmotic pressure, muscle contraction, transport of gases. These minerals are important components of enzymes and hormones, crucial for bones’ formation and the synthesis of vitamins (Biziuk & Kuczynska, 2007). Humans require sufficient intakes of many mineral elements which are, on the basis of their requirements, usually divided into macro- (g or mg day1) and microelements (few mg or μg day−1) (Barroso et al., 2009). The availabilities of minerals from agricultural products to humans are affected by the presence of promoter substances and anti-nutrients that can reduce the nutrient utilisation. Mineral malnutrition affects over two-thirds of the world’s population and is considered as one of the main global challenges. Micronutrient deficiencies are a major public health problem in many developing countries. Iron, zinc and magnesium deficiencies may cause health problems in pregnant women and infants. The excessive intakes of some minerals, however, can upset homeostatic balance and cause toxic side effects (Rivera et al., 2003, Soetan et al., 2010). Food is also the main source of consumers’ exposures to some toxic elements like lead, cadmium, arsenic, and mercury. Cadmium and lead affect several tissues and organs including the kidneys, lung, heart and brain, and cause a variety of diseases (Goyer & Clarsksom, 2001). Infants and children up to the age seven are particularly threatened since Pb and Cd intestinal absorption at these ages is significantly higher than in adults (González-Muñoz, Peña, & Meseguer, 2008). The levels of minerals and their accumulation in plants depend on numerous factors, like the type and chemical composition of the soil, soil fertility, the root-soil interface, the absorption mechanism and translocation in the plant (Welch & Graham, 2004).

Studies of taro nutritional composition suggest that it contains a range of important macronutrients and micronutrients. Some studies of the mineral compositions of taro corms suggest that potassium is the more abundant mineral. Other abundant minerals include magnesium, phosphorus, and calcium (Bradbury and Holloway, 1988, Huang et al., 2007, Lewu et al., 2010a, Mwenye et al., 2011). Data from the literature reveal that appreciable amounts of zinc are also present. From a nutritional standpoint, taro is rather low in iron and manganese (Lewu et al., 2010a, Mwenye et al., 2011). The nutritional composition of taro corms can vary widely and depends on the genotype, the growing conditions, and the interaction between the genotype and the environment (Mwenye et al., 2011). Another factor is the age of a plant (Wills, Lim, Greenfield, & Bayliss-Smith, 1983).

Although the literature contains numerous reports on the mineral contents in taro corms, little research attention has been devoted to the mineral distribution within different parts of corms. However, the mineral distribution does not appear to be uniform; calcium oxalate, for example, is more concentrated in the distal part (Bradbury & Holloway, 1988). To our knowledge, only one study in which corms were divided into separate sections has been reported, and the distribution of the studied minerals was not uniform (Sefa-Dedeh & Agyir-Sackey, 2004). This previous study (Sefa-Dedeh & Agyir-Sackey, 2004), however, did not include the marginal part, which is usually removed by peeling. Peeling, especially deep peeling, can significantly influence the concentrations of minerals accumulated in the upper, lower and marginal parts of corm. The material used in the previous study (Sefa-Dedeh & Agyir-Sackey, 2004) did not originate from uniform growing conditions; it was harvested at different local farms, and the authors did not take into account accumulation amongst different varieties.

Several questions related to the quality of the different parts of the corm deserve more data. The pattern of taro partitioning during its growth and development is somewhat different from other root crops. Unlike other species that present phasic partitioning, taro partitioning occurs when the storage organ growth begins and continues throughout the vegetative period. Taro exhibits continuous partitioning with an almost linear increase in fresh and dry weights. Cassava, sweet potato and yams develop through phasic partitioning. The continuous partitioning of aroids appears to be similar to sugar beet. The taro corm includes tissues developed over two consecutive seasons: (1) the tissue from the previous season (the corm base) and (2) the tissue from the current season (the rest of the corm). It is thought that the distributions of different minerals within corm differ from mineral to mineral. A comprehensive analysis could reveal the distribution of essential and potentially toxic elements inside the corm flesh (i.e., inside tissues which are used in human nutrition), which is currently little understood. Hence, it would be possible to predict sections with higher and lower concentrations. Data about chemical composition of the marginal part is also needed because of the adjustment when peeling. If some essential minerals dominate in the marginal part, peeling should be limited to a very thin layer, however, it should be deeper if there are harmful or undesired substances in this part. The central part will always be the more important part but, in order to increase corm yield, it is necessary to reduce waste due to removal of the marginal part. Data about chemical composition of four main parts of corms could also be useful for corm processing, especially for the taro chips industry which is becoming very popular. If there are significant differences amongst the basal and the upper part, horizontal slicing should be avoided, or the part with undesired chemical composition should be removed. The youngest tissue is in the upper part (close to the shoot). This part is characterised by low eating quality (watery tissue, low dry matter content) and farmers could increase the thickness of the slice attached to the head-set in order to improve the qualities of the propagules used for establishing the new crop. Data related to mineral composition could also aid in design programs for micronutrient biofortification through breeding.

Section snippets

Plant material and sample preparation

Our study involved eight accessions from various Vanuatu Islands that were maintained in the Vanuatu National Taro Germplasm Collection at the Vanuatu Agricultural Research and Training Centre (VARTC) on the Island of Espiritu Santo (15° 23′ S and 166° 51′ E, c. 80 m a.s.l.). The studied accessions corresponded to the following cultivar names: (1) VU 105 (‘Peta ni Bankis’ from Banks Islands), (2) Vu 360 (‘Maewo’ from the island of Maewo), (3) Vu 372 (‘Noholihopoe’ from Espiritu Santo), (4) Vu

Moisture content

The averages and variations in moisture contents of the studied cultivars are presented in Table 1. The cultivar Vu 360 had the lowest mean moisture content (64.2%), whereas the highest (79.5%) was determined for Vu 1654. The average moisture content was different in all corm sections. The cultivars Vu 105, Vu 360, Vu 372, Vu 384, Vu 1765 and Vu 1822 exhibited the highest values in the central and upper parts, Vu 468 in the central part and Vu 1654 in the lower part (Table 1). The differences,

Conclusion

The contents of eleven minerals were analysed for four main parts of taro corm: the upper, marginal, central and lower (basal) parts. The concentrations of most minerals (P, Mg, Fe, Cu, and Zn) were higher in the upper and the central parts. K was particularly accumulative in the central part, whereas its contents in other parts were lower but not significantly different. No significant differences were observed amongst the Zn concentrations in the upper, central, and marginal parts, whereas

Acknowledgment

This research was financially supported by the Food Security Thematic Programme of the EU (Grant No. DCI-FOOD/2010/230-267), the International Network for Edible Aroids (INEA, www.EdibleAroids.org), and the Faculty of Agriculture and Life Sciences of the University of Maribor, Slovenia.

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