All specimens were frozen with convective air in a refrigeration device (SU-262, Espec Co., Ltd., Japan) at a controlled temperature (held at 20 °C for 10 min, cooled down to −40 °C at 2.0 K/min, and held at −40 °C for around 1 h). Six cubes of frozen apple were orderly placed on a thin plastic tray (50 × 60 mm side lengths, 20 cm height). The tray was set on the shelf in the freeze-dryer, which was immediately evacuated to carry out freeze-drying. The dryer chamber was covered with a sheet of black curtain in order to avoid unfavorable external radiation. Drying experiments were conducted several times (n = 2–3) for each condition, where the coefficient of variation (CV) was less than 4.0%.