Comprehensive profiling of phenolic compounds by HPLC-DAD-ESI-QTOF-MS/MS to reveal their location and form of presence in different sorghum grain genotypes

https://doi.org/10.1016/j.foodres.2020.109671Get rights and content

Highlights

  • Free & bound phenolics from the bran & kernel of 5 sorghum genotypes were examined.

  • 110 phenolic compounds were identified or tentatively identified.

  • Brown sorghum had the highest concentration of total phenolic contents.

  • Brown sorghum bran had the most abundant and diverse phenolic compounds.

Abstract

In this study, comprehensive profiling of the phenolic compounds in sorghum grain was achieved by analysing the free and bound extracts of sorghum bran and kernel fractions from five Australian sorghum genotypes (1 white, 2 red, 1 brown and 1 black coloured), using HPLC-DAD-ESI-QTOF-MS/MS. A total of 110 phenolic compounds were annotated, out of which 56 were reported for the first time in sorghum grain. Compounds with matched authentic standards were quantified/semi-quantified. Multiple factor analysis (MFA) was performed and heatmaps generated, which provided direct visualisation of the distribution of individual phenolic compounds/subclasses between the sorghum samples. The results indicated that phenolic compounds were concentrated on the bran, and free and bound extracts had different phenolic composition. The phenolic compound/subclass profile varied greatly among sorghum genotypes. Brown sorghum genotype (IS131C) had the highest concentration of total phenolic contents, and the bran fraction of brown sorghum had the most abundant and diverse phenolic composition among all tested samples. This study provides the most comprehensive phenolic profile of Australian representative sorghum grains up to date.

Introduction

Sorghum is the fifth most-produced cereal crop in the world after maize, wheat, rice, barley (FAO, 2017). Sorghum has attracted increasing attention not only because it can be grown in diverse and harsh environments including at high temperature, high altitudes and drought conditions (Schittenhelm & Schroetter, 2014); but also, importantly, due to its health-promoting bioactive phenolic compounds (de Morais Cardoso, Pinheiro, Martino, & Pinheiro-Sant'Ana, 2017).

The phenolic compounds in sorghum are abundant and diverse, and a wide range of phenolic compounds have been reported, including phenolic acids, flavonoids (such as flavonols, flavones, flavanones and 3-deoxyanthocyanidins) and proanthocyanidins (tannins) (Xiong et al., 2019a, Xiong et al., 2019b). The phenolic profile of sorghum is also unique depending on the genotype, environmental and growth conditions. Some sorghum grain genotypes, especially brown and black coloured grain genotypes, contain a significant level of 3-deoxyanthocyanidins and condensed tannins, which are absent in other major cereal crops (Awika et al., 2004, Wu et al., 2016, Wu et al., 2012). These phenolic compounds have antimicrobial and potent antioxidant activities with huge potential to be exploited in food and therapeutical applications (Xiong et al., 2019b).

Understanding sorghum phenolic profile is the prerequisite for its industrial applications. Most of the published research has focused on the phenolics in sorghum whole grains. Dykes et al. investigated the total phenolic content (TPC) of sorghum whole grains (Dykes, Rooney, Waniska, & Rooney, 2005); and several reports investigated the individual phenolic/flavonoids profile of sorghum whole grains (Dykes et al., 2011, Dykes et al., 2013, Dykes et al., 2009, Kang et al., 2016). A few studies have investigated the free and bound phenolic profile of sorghum whole grains (Wu et al., 2017, Wu et al., 2017). The research on sorghum bran phenolic profile is currently limited. Although sorghum bran is a major industry by-product with great potential commercial value due its rich phenolic contents, previous research has been focused on examining the bioactive effects of the bran phenolic extract rather than profiling its phenolic compounds (Xiong et al., 2019b). Awika et al. examined the TPC of sorghum bran and intact whole grains and found that the phenolic compounds are concentrated in the bran layer, at a level of up to six times higher than that of the whole grain (Awika, McDonough, & Rooney, 2005). However, most of these previous studies on sorghum have only focused on the identification and quantification of a few phenolic compounds/subclasses (using the LC-MS method) or the TPC (using the colourimetric assay methods).

In order to have a deeper understanding of the phenolic profile and its distribution in sorghum, this work investigated the individual phenolic profile, its location (i.e. bran and kernel) and form (i.e. free and bound) in different coloured sorghum grain genotypes (i.e. 1 white, 2 red, 1 brown and 1 black coloured). To our best knowledge, this could be the most comprehensive phenolic profiling of sorghum grain which provides important information into sorghum material selection and processing design for its food and other industrial applications.

Section snippets

Chemicals and reagents

Ethyl acetate, hydrochloric acid, formic acid, methanol and acetonitrile were purchased from Sigma-Aldrich (Castle Hill, NSW, Australia). Standards of apigeninidin chloride, luteolinidin chloride and 7-methoxy-apigeninidin chloride were obtained from ChromaDex (Los Angeles, CA, USA). All other standards were obtained from Sigma-Aldrich (Castle Hill, NSW, Australia).

Samples and preparation

White colour Liberty (W), red colour Mr-Buster (RM), red colour Nuseed Cracka (RC) sorghum grains were obtained from Nuseed

Identification of phenolic compounds

Representative chromatograms of the sorghum phenolic compounds are shown in Fig. 2, and the qualitative analysis and results are summarised in Table 1, in which most of the detected peaks shown in the chromatograms were characterised. The MS chromatograms of samples/standards, and MS/MS spectra of individual phenolics/standards, are also available in Xiong et al. (2020 Co-submission) Fig. S1-S3. In the following sections, to present a clear illustration of the molecular characterisation, the

Conclusion

Sorghum grain is a rich source of diverse health-beneficial phenolic compounds with great potential in the food and pharmaceutical industries. Phenolic characterisation is an essential step for the development selection and utilisation of specific sorghum grain genotypes for these applications. A total of 110 phenolic compounds (38 phenolic acids, 59 flavonoids and 13 other phenolic compounds) belonging to various phenolic subclasses were identified or tentatively identified, 56 of which, to

CRediT authorship contribution statement

Yun Xiong: Conceptualization, Methodology, Validation, Investigation, Software, Data curation, Formal analysis, Visualization, Writing - original draft. Pangzhen Zhang: Supervision, Investigation, Writing - review & editing. Robyn Dorothy Warner: Supervision, Writing - review & editing. Shuibao Shen: Writing - review & editing. Stuart Johnson: Resources, Writing - review & editing. Zhongxiang Fang: Conceptualization, Methodology, Supervision, Validation, Writing - review & editing, Project

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by Taiyuan Brand Will Firm Biotechnology Development Co., Ltd, China (project No. GL 022055 - TA 39201).

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