A comparative study of the phenolic compounds and in vitro antioxidant capacity of finger millets from different growing regions in Malawi
Graphical abstract
Introduction
Whole-grain foods have gained considerable attention in recent years as their regular consumption has been linked to reduce total mortality, as well as coronary heart disease, ischemic stroke, and type 2 diabetes (Adom and Liu, 2002; Devi et al., 2014). Millets have been considered as first cereals and whole-grain food materials (Lu et al., 2005). Finger millet (Eleusine coracana L.), one of the most important member of millets, is widely cultivated in Africa and Asia for use as a common staple food for populations in these developing countries (Upadhyaya et al., 2007).
Finger millet has gained great importance not only because of its nutritional quality in terms of micro- and macronutrients, but also as a rich source of phytochemicals mainly phenolic compounds which may reduce chronic diseases including diabetes, cancer and cardiovascular diseases (Chandrasekara and Shahidi, 2011a). The polyphenols consist of hydroxybenzoic (protocatechuic, p-hydroxybenzoic, syringic) acids, hydroxycinnamic (caffeic, p-coumaric, ferulic) acids, flavan-3-ols (catechin, epicatechin), flavonoid (catechin, epicatechin, quercetin, apigenin) glycosides, and proanthocyanidins (Devi et al., 2014; Xiang et al., 2019). Phenolic compounds significantly contribute to the overall antioxidant properties of finger millet (Devi et al., 2014; Shahidi and Chandrasekara, 2013).
It has been established that nutrient and phytochemical composition of grains depend both quantitatively and qualitatively on genotype (Badigannavar and Ganapathi, 2018; Chethan and Malleshi, 2007; Sankara Vadivoo et al., 1998; Upadhyaya et al., 2011) and on growing environmental factors (Van Hung, 2016). Contents of phenolic compounds also varied across finger millets from different genotypes and growing regions (Devi et al., 2014; Karki and Kharel, 2012). Finger millet varieties grown in Malawi are a promising source of micronutrients and proteins (Sankara Vadivoo et al., 1998); however, there is limited information on the major polyphenol composition, phenolic contents and corresponding antioxidant capacity of finger millets from different growing regions.
Principal component analysis (PCA), a powerful dimensional reduction technique which aims at explaining most of the variance whilst reducing the data volume to a few uncorrelated components, can be used to distinguish groups of variables. This method works on the basis of transformation of the initial variables into new principal components (PCs), which makes it possible to express maximum total variation in data set in a few PCs and represent the decrease of the variation (Fan and Beta, 2017). The objective of this paper was to evaluate the differences in phenolic composition and antioxidant capacity of finger millets obtained from different growing regions of Malawi.
Section snippets
Finger millet samples
A total of 45 finger millet samples were obtained from seven different districts in central and northern Malawi. Samples were collected from four locations in Dedza (central Malawi), A: Makowe (3), B: Mphathi (8 samples), C: Chuma-Chitsala (3) and D: Khulungira (8), and three locations in Mzimba (northern Malawi), EA: Kabanda (9), EB: Edundu (7), EC: Kabwanda (6). Samples were cleaned and then ground in an electric mill (Micro-Mill, Bel-Art Products Co., Wayne, NJ) so as to pass through a
TPC of finger millet from different growing regions in Malawi
The 45 finger millet samples collected from 39 different villages of seven different growing regions in central and northern Malawi showed an open type consisting of long, thin ribbon-like fingers that open outward (open type), and a fist type of appearance characterized by short fingers that fold inwards (fist type). Most had brown or red seed coats while few had whitish or reddish seed coats (Table 1).
The total phenolic content of finger millets ranged from 148.55 to 589.12 mg/100 g while TPC
Conclusions
Finger millets demonstrated a wide range in TPC, individual phenolic compounds and antioxidant capacity across the 45 samples from central and northern Malawi. Samples from central Malawi (Makowe, Mphathi, Chuma-Chitsala and Khulungira) exhibited higher TPC, ABTS+ radical-scavenging activity, and ORAC values compared to those from northern Malawi (Kabanda, Edundu and Kabwanda) due to the significantly higher (p < 0.05) catechin, epicatechin, and ferulic acid content in the former. Significant
Acknowledgements
This work was partially supported by the Global Affairs Canada through the Malawi Farmer-to-Farmer Agroecological (MAFFA) project led by Western University (Ontario), Canada Research Chairs program and China Scholarship Council (No. 201608410241). Equipment used for analyses of phytochemicals and ORAC assay was generously funded by the Canada Foundation for Innovation (New Opportunities Fund and Leaders Opportunities Fund). The authors are grateful for the assistance in sample collection and
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