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.