Valorization of Tunisian secondary date varieties (Phoenix dactylifera L.) by hydrothermal treatments: New fiber concentrates with antioxidant properties

•Secondary date varieties were hydrothermally treated for their valorization.•Fiber concentrates (FC) were obtained from the solid fractions after treatments.•FC composition was more influenced by treatment conditions than by date variety.•FC had high antioxidant activity (up to 580 mmolTE/Kg), similar to red grape pomace.•Its chocolate/coffee flavor makes FC especially suitable for using in bakery/dairy.

Two hydrothermal treatments were assayed on secondary date varieties from the Tunisian coastal oasis in order to obtain valuable solid extracts, rich in dietary fiber and antioxidants. The steam explosion treatment (SET) is based on high temperatures and pressures, with an explosive decompression. The steam treatment (ST), without rapid decompression, has lower pressures and temperatures, but longer treatments than SET. The recovery of the fiber concentrates (FCs) was very similar for the different assayed conditions. Only its granulometry showed differences, increasing the fraction with size lower than 4 mm in more intense treatments. Treatment conditions had higher influence than date variety in the FCs chemical composition. It was approximately 0.1-2% soluble sugar, 0.7-2% uronic acids, 4-8% phenols, 5-11% fat, 5-22% non-cellulosic sugars, 9-14% protein, 12-20% cellulose, and 44-71% lignin, depending on treatment conditions. They had also very high antiradical activity (230-580 mmols Trolox/Kg FC). The use of ST reactor is highly recommended because it could be easily scaled up to the industry. The FC obtained by hydrothermal treatments from date fruits could be considered as a very interesting ingredient for healthier food, especially for bakery or dairy products due to its pleasant chocolate/coffee flavor.