2. Materials and methods
2.1. Plant materials and cooking methods
Purple-fleshed potatoes (Heimeiren) (length 6–8 cm, diameter 3–4 cm, weight 100–120 g) and soybean oil were obtained from a local supermarket (Hangzhou, China). For boiling, steaming, baking and microwaving, whole tubers (unpeeled) were cooked, and a stainless-steel probe was inserted in the tubers to evaluate the cooking time (Table 1). For normal frying and air-frying, potatoes (unpeeled) were washed and cut into strips (8 × 7 × 60 mm) manually, whereas for stir-frying, the strips measured 2 × 3 × 60 mm. All of the strips were soaked in running water for 1 min to eliminate occluded starch, and stains were removed using tissue paper; the detailed frying conditions were also listed in Table 1. After cooking, the raw and cooked samples were freeze-dried (−50 °C, pressure lower than 0.1 mBar) (FreeZone 6, Labconco, USA), ground using a commercial grinder, sealed with aluminum foil and stored at −80 °C for further analysis