The onion bags were treated at 400 MPa for 5 min at 25 °C. These treatment conditions were selected due to the good results in terms of health-promoting characteristics demonstrated for this HPP-treated onion by in vitro and in vivo assays carried out with Wistar rats [40 (link),41 (link),70 (link),71 (link)]. The hydrostatic pressure unit was formed by a vessel of 1925 mL capacity and it was able to work to a maximum pressure of 800 MPa and a maximum temperature of 70 °C (High Pressure Iso-Lab System, Model FPG7100:9/2C, Stansted Fluid Power LTD., Essex, UK). Two bags of packaged onions were placed into the vessel unit filled with water as a pressure-transmitting medium. The compression and decompression rates were both 3 MPa/s. Because of adiabatic compression, the maximum temperature in the vessel was 25 °C at 400 MPa. Pressure, time, and temperature were controlled by a computer program, being constantly monitored and recorded during the process.
Onion treated by high-pressure processing (HPP-treated onion) was immediately frozen with liquid nitrogen and stored at −80 °C until lyophilization (100 mTorr, −90 °C) using a lyophilizator model Lyoalfa (Telstar, Barcelona, Spain). Lyophilized samples were pulverized in an ultracentrifugal grinder ZM 200 (Retsch GmbH, Haan, Germany) to obtain a fine powder (particle size ≤ 0.5 mm), and maintained at −20 °C until analyzed.
Onion treated by high-pressure processing (HPP-treated onion) was immediately frozen with liquid nitrogen and stored at −80 °C until lyophilization (100 mTorr, −90 °C) using a lyophilizator model Lyoalfa (Telstar, Barcelona, Spain). Lyophilized samples were pulverized in an ultracentrifugal grinder ZM 200 (Retsch GmbH, Haan, Germany) to obtain a fine powder (particle size ≤ 0.5 mm), and maintained at −20 °C until analyzed.
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