Characterization of phenolic compounds and antioxidant activity in sorghum grains
Introduction
The current trend in the consumer market reveals a considerable preference for foods that provide added health benefits. Sorghum whole grains have been demonstrated to have substantial health benefits, in part as a result of antioxidant active phenolic compounds present in the pericarp layer (Khan et al., 2015; Khoddami et al., 2015). This free radical scavenging activity has been correlated to health beneficial properties such as anti-microbial (Kil et al., 2009), reduced oxidative stress (Khan et al., 2015), anti-inflammatory (Burdette et al., 2010) and anti-cancer activity (Awika et al., 2009; Wu et al., 2011).
Sorghum grains have pigmented and non-pigmented pericarp including white, black, red and brown coloured pericarp (Dykes et al., 2005). Research has shown that pericarp colour is governed by the expression of B1 and B2 genes and the presence of tannins require both the genes to be expressed. Hence, sorghum can be classified into three groups; Type 1 non-pigmented pericarp with no tannins, Type II pigmented pericarp containing tannins that are extractable by acidified methanol and Type III pigmented pericarp containing tannins that are extractable with neat methanol (Dykes et al., 2005).
Many polyphenolics have been identified in sorghum including a range of hydroxybenzoic acids, hydroxycinnamic acids and flavonoids (Dicko et al., 2002; Nguyen et al., 2015; Wu et al., 2016). Phenolic compound in sorghum are often found in bound form fractions, the composition and concentration of these compounds vary across sorghum genotypes and production environments (Carbonneau et al., 2014b; Hahn et al., 1983; Wu et al., 2016; Wu et al., 2017a; Wu et al., 2017b; Wu et al., 2017c). A derivative of anthocyanin, 3-deoxyanthocyanidins (3DXA) have been identified as a potential bioactive compound that is unique to sorghum (Carbonneau et al., 2014a). In comparison to anthocyanins, 3DXA are more stable at high pH due to a missing hydroxyl group at the C3 position on the C ring (Awika et al., 2004; Yang et al., 2014). According to Awika et al. (2009) this arrangement of 3DXA gives it a higher bioavailability than anthocyanin. Additionally, the high antioxidant activity of 3DXA has been found to correlate specifically to anti-proliferative and anti-cancer properties (Suganyadevi et al., 2013; Yang et al., 2009).
In addition to 3DXA, sorghum has many other polyphenols that have antioxidant potential however, these have not received much attention. Therefore, the aim of this study was to identify and quantify phenolic compounds present in sorghum that have significant antioxidant activity by examining six varieties of pigmented and non-pigmented pericarp sorghum. Characterisation through identification and quantification of phenolic compounds was conducted using UHPLC-online ABTS for antioxidant activity and QTOF LC/MS for polyphenolic identification and quantitation. The purpose of the UHPLC-online ABTS detection system was to produce a comprehensive understanding of the antioxidant potential of the sorghum polyphenolics. This information may assist in selection of sorghum genotypes as functional grains with maximum health benefits.
Section snippets
Standards and reagents
Chemicals were purchased from either Sigma-Aldrich (St Louis, Missouri, USA) or Chem Supply Pty Ltd (Port Adelaide, South Australia, Australia). Acetone, acetic acid, acetonitrile, ethanol, methanol, sodium carbonate, anhydrous sodium acetate, potassium persulfate, hydrochloric acid, sulphuric acid were sourced from Chem Supply Pty Ltd. Folin-Ciocalteu reagent, Trolox (6-Hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), ABTS (2,2′-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid), DPPH
Variation in total phenolic content (TPC)
The six sorghum varieties investigated exhibited significant variation in TPC at p < 0.05 (Table 1). The black pericarp variety Shawaya short black 1 had the highest TPC at 11.50 ± 1.81 mg/g GAE, followed by the brown pericarp IS13116 at 3.58 ± 1.63 mg/g GAE. The red and white pigmented pericarp varieties did not exhibit a significant difference in TPC (Table 1). While the pigmented pericarp varieties of Shawaya short black 1 (black) and IS13116 (brown) exhibited higher phenolic content than
Conclusion
Results from this study demonstrated that pigmentation of sorghum pericarp is quite variable hence, an exact correlation was not observed between pericarp pigmentation and of its phenolic content or antioxidant activity. In addition, substantial variation in phenolic content and antioxidant activity was observed across the six varieties. The black pericarp variety Shawaya short black 1 and the brown pericarp IS11316 were found to be the most active in scavenging for free radicals, this
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
Funding: This work was funded by the Australian Research Council Industrial Transformations Centre for Functional Grains [Identifier Number: IC140100027]. Shiwangni Rao is a recipient of the Australian Research Council Industrial Transformations Centre for Functional Grains scholarship through Charles Sturt University.
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