Extraction of phenolic compounds from bilberry by-products left after pulsed electric field assisted juice production

By-products (press cake) resulting from PEF assisted juice production might still be exploited for recovery of valuable compounds. Therefore, the aim of this work was to investigate the effect of PEF pre-treatment of bilberry fruits on the recovery of phenolic compounds from the press cake left after the juice pressing. Mechanical pressing was used for juice extraction. The experiments were carried out at constant pressure (1.32 bar) using a lab-designed pressing chamber. PEF pre-treatments of constant electric field strength (3kV/cm) and varying specific energy (1-10 kJ/kg) or varying electric field strength (1-5 kV/kcm) and constant specific energy (10 kJ/kg) were applied to fresh bilberries prior to their pressing. Then the press cake left after the juice production was subjected to further extraction with acidified aqueous ethanol for 24 hours at ambient temperature. The total phenolic content of the extracts was determined using the Folin-Ciocalteu method [1]. The antioxidant activity of the extracts was tested using the FRAP assay [2]. Significantly higher amounts of total phenolics were extracted from the press cake of PEF-treated bilberries. The content of phenolic compounds extracted from the press cake of untreated (control) berries was 942 mg/100 g. The application of a PEF pre-treatment at 1, 3 and 5 kV/cm and constant specific energy yielded an increase in total phenolic content by 36%, 63% and 89%, respectively. When PEF pretreatment of constant electric field strength but varying energy input was applied to berries the total phenolic content in the press cake extracts was 58%, 56% and 63% higher for 1, 5 and 10 kJ/kg energy input, respectively. The PEF pre-treatment also resulted in significantly higher ferric reducing antioxidant power of the press cake extracts. In comparison to the control extract, PEF pre-treatments of berries at 1, 3 and 5 kV/cm increased the FRAP values of their press-cake extracts by 44%, 71% and 80%, respectively. Similarly, FRAP of press cake extracts obtained from PEF-treated bilberries at fixed electric field but varying energy input was from 40% (1 kJ/kg) to 71% (10 kJ/kg) higher. The results obtained from this study demonstrate that PEF pre-treatment of bilberries at higher intensity, both in terms of higher electric field strength or higher specific energy at fixed electric field strength resulted in better extractability of phenolic antioxidants from bilberry press cake left after the juice pressing. However, the increase in electric field strength had stronger impact on the extraction efficiency than the increase in delivered specific energy.