Investigating Phenolic Content in Walnut Milk Residue with UPLC-ESI-MS/MS

Scientists from the University of Chile (Santiago), the University of Almería (Spain), and University of Alberta (Edmonton) characterized the soluble (free, esterified, and etherified) phenolics and insoluble-bound phenolic hydrolysates (IBPH) fractions of walnut milk residues (WMR). The study, which was published in Plants, assessed the WMR for its phenolic content and antioxidant and cytotoxic potential through analysis utilizing ultra-performance liquid chromatography–electrospray tandem mass spectrometry (UPLC-ESI-MS/MS) (1).

Found in different forms depending on their association with the food matrix, phenolic compounds are free, esterified, and etherified soluble phenolics as well as insoluble-bound phenolics (2,3) Most of the reported studies conducted previously only consider the phenolic analysis and antioxidant activity of the soluble phenolic fraction, generally evaluated after obtaining a crude extract, omitting the contribution of each of the phenolic fractions, including free, esterified, etherified soluble, and insoluble-bound phenolics (4,5). Free soluble phenolics are found within the vacuoles of the vegetal cell; insoluble-bound phenolics are bound to carbohydrates and proteins of the plant matrix cell wall by covalent bonds, which can be released by alkaline hydrolysis (6).

An edible nut of high nutritional value, walnut (Juglans regia L.), is mainly composed of unsaturated fatty acids, proteins, dietary fiber, minerals, and B-complex vitamins, in addition to vitamin E (tocopherols) and phytosterols (7-10). Walnuts also contain significant amounts of compounds of a phenolic nature which have shown potential beneficial effects on human health against chronic non-communicable diseases mediated by oxidative stress (including some types of cancer, type II diabetes, and cardiovascular diseases) and neurodegenerative diseases due to their antioxidant and anti-inflammatory capacity in in vitro and in vivo studies (11-16).

The study of this byproduct, according to the authors, is important in the context of waste reduction, optimization of natural resources, and the potential economic and health benefits of the bioactive compounds present in this residue (1). Phenolic acids and flavonoids were identified and quantified by the researchers through UPLC-ESI-MS/MS, with gallic acid, mainly in the free form (3061.0 µg 100 g⁻¹), being the most representative, followed by biochanin A, which reportedly was identified for the first time in a walnut product and mostly present in the fraction released from the esterified form (593.75 µg 100 g⁻¹). The authors of the paper state that 44% w/w of the phenolics present in the walnut were not extracted during the production of walnut milk and remained in the respective residue. Therefore, the authors found that WMR could be considered as a potential source of natural antioxidants contributing to the nutraceutical and food industry in the framework of sustainable processes (1)

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References

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