Lyoalfa

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.

Raw onions (Allium cepa L. var. cepa, ‘Recas’) and apples (Mallus pumila Mill., ‘Golden delicious’) were purchased from a local supermarket (Madrid, Spain). The onions were hand-peeled and cut into 10 mm pieces. The apples were washed, divided into quarters without core, cut into 2 cm pieces with skin and quickly packed. Both cut onion and apple (approx. 120 g) were packaged in very low gas permeability bags (BB3255, Cryovac, Barcelona, Spain) and treated by high pressure (400 MPa/25 °C/5 min and 400 MPa/35 °C/5 min, respectively) (High Pressure Iso-Lab System, model FPG7100:9/2C, Stansted Fluid Power Ltd., Essex, UK). After the high pressure treatment, the onion and apple were frozen with liquid nitrogen, freeze-dried in a lyophilizer (model Lyoalfa, Telstar, S.A., Barcelona, Spain) and pulverized with an ultra-centrifugal mill ZM 200 (Retsch GmbH, Haan, Germany), obtaining a fine powder (final size particle ≤250 μm), and stored at −20 ± 0.5 °C until use. Table 1 shows nutritional composition, phytochemical compounds and antioxidant activity of the onion and the apple powder. Analyses were carried out using the methods described by González-Peña et al. (2013) [32 (link)], Colina-Coca et al. (2013) [33 (link)] and Colina-Coca et al. (2014) [34 (link)].