A comparative study of pulsed electric field, ultrasound, milling and soaking as pre-treatments for assistance in the extraction of polyphenols from willow bark (Salix alba)

White willow (Salix alba) has a long history of use as an herbal remedy for treating common pain and inflammation. Pre-treatment is a crucial step that assists the subsequent extraction process and may affect the extraction efficiency of polyphenols from plant materials. The objective of this work was to study the impact of pre-treatment methods on the hot water extraction of polyphenols from willow bark. Pulsed electric field (PEF) and ultrasound (US) pre-treatments were compared to commonly used pre-treatment methods, milling and soaking. All pre-treatments significantly increased the total phenolic content (TPC) and antioxidant activity (p < 0.05) in willow extracts. At a similar energy level, PEF with 400 pulses yielded a higher TPC (44.33 mg GAE/g d.w.) and antioxidant activity (DPPH of 74.58 mg AAE/g and FRAP of 186.41 mmol TE/g) than US pre-treated samples, but a small increase in ultrasonic energy resulted in the highest TPC (46.20 mg GAE/g d.w.) and antioxidant activity (DPPH of 76.18 mg AAE/g and FRAP of 220.20 mmol TE/g) of all the pre-treatments. Compared with PEF and US, milling (42.69 mg GAE/g d.w.) and soaking (40.29 mg GAE/g d.w.) were less effective in recovering polyphenols but they were comparable with PEF and US when employed at low energy levels. Overall results indicate emerging technologies PEF and US can be alternatives to milling to enhance the extractability of polyphenols and to reduce energy consumption. The obtained willow extracts were a rich source of polyphenols that could be used for food and pharmaceutical applications.