Antioxidant phenolic extracts obtained from secondary Tunisian date varieties (Phoenix dactylifera L.) by hydrothermal treatments

• Thermal treatments help to the valorization of secondary Tunisian date varieties.
• Steam treatments generate a liquid phase with antioxidant activity in vitro.
• The liquid is rich in phenolic acids and sugars.
• The fractionation of the liquid phase increases its antioxidant activity.
• Fractionated phenols have a similar activity in vitro than commercial antioxidants.

Three common non-commercial Tunisian date varieties were treated by two thermal systems, obtaining a liquid fraction which was characterized and its antioxidant capacity determined. The concentration of total phenols in the three varieties (Smeti, Garen Gazel, and Eguwa) was increased by steam explosion treatment up to 5311, 4680, and 3832 mg/kg of fresh dates, and their antioxidant activity up to 62.5, 46.5 and 43.1 mmol Trolox®/kg of fresh date, respectively. Both thermal treatments increased the content of phenolic acids. Additionally, a long scale study was carried out in a pilot plant, with steam treatment at 140 °C and 160 °C for 30 min. The liquid phase was extracted and fractionated chromatographically using adsorbent or ionic resins. The phenolic profiles were determined for each fraction, yielding fractions with interesting antioxidant activities with EC50 values of up to 0.08 mg/L or values of TEAC of 0.67 mmol Trolox®/g of extract.