Spray drying bioactive orange-peel extracts produced by Soxhlet extraction: Use of WPI, antioxidant activity and moisture sorption isotherms

• 7.5% (w/w) WPI could enhance the yield of spray drying orange peel extracts significantly.
• The best operating conditions for scale-up were suggested based on small-scale trials.
• Moisture sorption isotherms for spray-dried pure orange peel powders were determined.
• The net isosteric heat of sorption was calculated from the Clausius–Clapeyron equation.

Spray drying of orange peel extracts was performed using a laboratory-scale spray dryer in order to assess the best operating conditions to be used in a subsequent scale-up process. Addition of whey protein isolate (WPI) to the liquid feed enhanced the yield of the process significantly from 30 ± 10% to 63 ± 3%. The moisture sorption isotherms of the powders were also determined at 30 °C, 40 °C and 50 °C and fitted by the Guggenheim-Andersen-de Boer (GAB) equation. The average net isosteric heat of sorption for the powders was determined using the Clausius–Clapeyron equation to be 70.5 kJ/kg. The total phenolic content (TPC) and the free radical scavenging capacity (SC50) of the powders were retained by 95 ± 5% and 95 ± 15%, respectively, during small-scale spray drying at an inlet air temperature of 150 °C and a corresponding outlet temperature of 70 ± 3 °C. Therefore, the best operating conditions for scale-up were suggested to be an inlet air temperature of 150 °C, a maximum allowable outlet temperature of 80 °C and a maximum allowable relative humidity for the exhaust air of 15%, while using a liquid feed containing 7.5% (w/w), dry basis WPI.