Liquid chromatography–mass spectrometry coupled with multivariate analysis for the characterization and discrimination of extractable and nonextractable polyphenols and glucosinolates from red cabbage and Brussels sprout waste streams

• Red cabbage and Brussels sprouts waste contain high total polyphenol content.
• Nonextractable fraction contains more total phenolics than the extractable fraction.
• Polyphenols and glucosinolates from Brassica waste were identified using LC-MS.
• PCA and OPLS–DA were used to distinguish EP from NEP fractions.

Nonextractable polyphenol (NEP) fractions are usually ignored because conventional extraction methods do not release them from the plant matrix. In this study, we optimized the conditions for sonicated alkaline hydrolysis to the residues left after conventional polyphenol extraction of Brussels sprouts top (80 °C, 4 M NaOH, 30 min) and stalks (60 °C, 4 M NaOH, 30 min), and red cabbage waste streams (80 °C, 4 M NaOH, 45 min) to release and characterize the NEP fraction. The NEP fractions of Brussels sprouts top (4.8 ± 1.2 mg gallic acid equivalents [GAE]/g dry waste) and stalks (3.3 ± 0.2 mg GAE/g dry waste), and red cabbage (11.5 mg GAE/g dry waste) waste have significantly higher total polyphenol contents compared to their respective extractable polyphenol (EP) fractions (1.5 ± 0.0, 2.0 ± 0.0 and 3.7 ± 0.0 mg GAE/g dry waste, respectively). An LC-MS method combined with principal components analysis (PCA) and orthogonal partial least squares–discriminant analysis (OPLS–DA) was used to tentatively identify and discriminate the polyphenol and glucosinolate composition of the EP and NEP fractions. Results revealed that phenolic profiles of the EP and NEP fractions are different and some compounds are only found in either fraction in all of the plant matrices. This suggests the need to account both fractions when analyzing the polyphenol and glucosinolate profiles of plant matrices to attain a global view of their composition. This is the first report on the discrimination of the phenolic and glucosinolate profiles of the EP and NEP fractions using metabolomics techniques.