Metabolomic and biochemical insights into bioactive compounds and antioxidant properties of black oilseed testa and peeled seeds

Abstract

Black oilseed crops are rich in diverse phenolic compounds and have excellent antioxidant activities, as reported in traditional Chinese medicine. Testa (seed coat) and peeled seeds (cotyledon, embryo, and other structures) are the seed's crucial components, contributing to the variation in phytonutrient, phenol content, bioactive component, and protective and pharmacological effects. However, comprehensive and comparative information on total phenol, flavonoid, antioxidant, and metabolic profiles in black seed testa and peeled sesame, soybean, peanut, and rapeseed seeds is rare. Here, we investigated the metabolic profiles, phenolic contents, and antioxidant activities of four black oilseed crop testas and peeled seeds. This study revealed that testa has higher total phenol, flavonoid, and antioxidant activities than peeled seeds. A total of 1847 metabolites were identified across all samples and categorized into 17 major classes: flavonoids (20.02%), phenolic acids (15.15%), lipids (11.47%), amino acids and derivatives (9.36%), alkaloids (7.47%), organic acids (5.79%), terpenoids (5.68%), lignans (5.57%), saccharides (4.27%), and nucleotides and derivatives (4.17%) among the top ten. Primary class metabolites such as amino acids, saccharides, and vitamins were higher in the peeled seeds than in the testa, signifying the role of energy reservoirs and nutritive potential. However, flavonoids, phenolic acids, coumarins, chromones, lignans, terpenoids, tannins, organic acids, and lipids were abundant in the testa. Interestingly, the diversity and content of secondary metabolites were more abundant in the testa than in the peeled seeds of each crop, explaining their potential for phenol content, bioactivity, antioxidant activity, and pharmacological potential. The bioactivity of peeled seeds and testas may be associated with the phytochemical composition and content of flavonoids, phenolic acids, terpenoids, alkaloids, lipids, terpenoids, lignans, amino acids, and saccharides. Therefore, according to our results, peeled seeds offer higher nutritional value, and the testa has medicinal and protective properties. This study provides insights into the variations in phytochemical composition, phenolic content, and antioxidant activity of testa and peeled black sesame, soybean, peanut, and rapeseed seeds for further application of oilseeds in food products and to maximize nutritional benefits.

Keywords: Antioxidant activity; Bioactive compounds; Food chemistry; Oil crops; Peeled seeds; Phenol content; Testa.

Graphical abstract

Fig. 1

Morphological appearances of peeled seeds and testa of the samples. The sample labels are described in Table S1.

Fig. 2

Total phenol content (TPC), total flavonoid contents (TFC), antioxidant activity (free radical scavenging activity (DPPH assay) and ferric reducing antioxidant potential (FRAP assay) in the testa and peeled seeds of black sesame, soybean, peanut and rapeseed crops. (A) TPC and (B) TFC, (C) DPPH and (D) FRAP.

Fig. 3

Classification and variation of metabolites. (A) Classification of the 1847 identified metabolites in testa and peeled part samples of oilseeds. (B) Upset plot showing the number of metabolites identified in each crop seed components (CSM, TSM, CSB, TSB, CPN, TPN, CRS, and TRS) and shared metabolites. (C) Venn diagram showing the numbers of common and specific metabolites accumulated in the testa and peeled parts of oilseeds, (D) Classification of shared metabolites among testa and peeled parts of oilseeds, (E) Venn diagram showing the numbers of shared and unique metabolites accumulated in the CSM. vs.TSM, CSB. vs.TSB, CPN. vs.TPN, and CRS. vs.TRS. (F) Relative content of major classes of metabolites in the testa and peeled seeds of SM, SB, PN, and RS. C_: peeled seeds and T_: testa of sesame, soybean, peanut, and rapeseed. AA's: amino acids, and OA's: organic acids. The sample labels are described in Table S1.

Fig. 4

Multivariate analysis showing: A) principal component analysis (PCA), B) hierarchical clustering analysis (HCA), (C–F), OPLS-DA scores plot in a pairwise comparison between the testa and peeled seeds of each crop.

Fig. 5

Differentially accumulated metabolites and KEEG enrichment in the pairwise comparison between testa and peeled black sesame, soybean, peanut, and rapeseed seeds. (A&B) volcano plots showing the number of up and down-regulated metabolites, (C&D) classification of up and down-regulated metabolites in each pairwise comparison, (E) Venn diagram showing the number of key DAMs in each pairwise comparison, (F) Classification of 193 overlapping DAMs and (G&H) the KEGG enrichment result showing significant pathways altered in the pairwise comparison. TSM: sesame testa, CSM: sesame peeled seeds, TSB: soybean testa, CSB: soybean peeled seeds, TPN, peanut testa; CPN, peanut peeled seeds; TRS, rapeseed testa; CRS, rapeseed peeled seeds.

Fig. 6

Major bioactive metabolites' contents include (A) fatty acids and B) lignans.

Fig. 7

Major bioactive metabolites' contents include flavonoids, organic acids, and amino acids. (A) Anthocyanins, flavanols, and flavones; (B) flavanones, flavonols, and isoflavones; (C) amino acids, and organic acids. The colors indicate the degree of concentration or content; pink shows higher, green-lower, and grey-medium.

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