Abstract
Hazelnut (HS) and walnut (WS) shells, an abundant by-product of the processing industries of these edible nuts, are traditionally considered as a low-value waste. However, they are a source of valuable compounds with an interesting chemical profile for the chemical and pharmaceutical sectors. In this study, the lipophilic and hydrophilic extracts present in HS and WS were quantified and identified, then the polar fractions were chromatographically separated, and their antioxidant capacity was studied. The experimental work includes the isolation of crude lipophilic and hydrophilic extracts by an accelerated extraction process, chromatographic analysis (gas chromatography-flame ionization (GC-FID), GC-mass spectroscopy (GC-MS), high-performance size-exclusion chromatography (HPSEC), thin-layer chromatography (TLC)), and quantification of the components. In addition, a thorough compositional characterization of the subgroups obtained by flash chromatography and their antioxidant capacity was carried out. The gravimetric concentrations showed different lipophilic/hydrophilic ratios (0.70 for HS and 0.23 for WS), indicating a higher proportion of polar compounds in WS than in HS. Moreover, the lipophilic extracts were principally composed of short-chain fatty acids (stearic, palmitic, and oleic acid), triglycerides, and sterols. The polar fractions were screened by thin-layer chromatography and then separated by flash chromatography, obtaining fractions free of fatty acids and sugar derivatives (97:3 in HS and 95:5 in WS), and mixtures richer in phenolic compounds and flavonoids such as guaiacyl derivatives, quercetin, pinobanksin, and catechin. The most polar fractions presented a higher antioxidant capacity than that of the crude extracts.
Highlights
Nowadays, there is an increasing demand for petrochemical-free and safe products
This gap between gravimetric and gas chromatography (GC) quantification could be attributed to the complex mixtures of compounds with low and high molar mass that are present in the extracts and to polymerization reactions during the extraction process [26]
GC facilitates the separation of components with a low molar mass, but high molar mass components are not readable [27]
Summary
There is an increasing demand for petrochemical-free and safe products. The potential of the agroforestry residues in terms of the diversity of natural compounds that can be obtained is very broad and involves markets such as the pharmaceutical, cosmetic, food, and additives, among others [1,2]. The agroforestry sector has a great opportunity to increase the overall mobilization rate due to the higher added value of the output products. In this context, the obtaining of a range of chemical compounds from sustainable sources but focused on the valorization of industrially generated by-products represents an interesting research target with environmental and economic benefits, promoting the development of the circular bioeconomy [3]. Hazelnuts are well-known nuts with an average annual production of about 55,8500 tons (with shell), out of which 75% is produced in Turkey, followed by 15% in Italy [6]. Walnut trees are globally cultivated for their edible kernels, which are enclosed in a hard shell and according to FAO (2018), the annual production is approximately 3.66 million tons (with shell), with China being the principal producer (around 30%), followed by Iran, USA, and Turkey [6]
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