2.1. Chemicals and sample preparation

Natural capsaicin was purchased from Sigma Aldrich, and 1% (1g/100g) stock solution was prepared by dissolving 0.1 g into 10 g ethanol (food-grade, 96%). One litre of the flavoured solution was made by adding 100 μL of 3-methylbutanal (food-grade, ≥ 97%, Sigma Aldrich) into pure water. Half of this solution (500 mL) was used as the control sample (CTR), and another 500 mL was made into capsaicin sample (CAP) by adding 250 μL of 1% capsaicin stock solution (final capsaicin concentration = 5 mg/L). The aqueous CTR and CAP solution were prepared one day before the experiment and stored in the Durian bottles with screw caps in the fridge (5 °C). The CTR and CAP samples were labelled with respective three-digit code. During the day of the experiment, these bottles were left at the room temperature 1 h before the static headspace analysis by APCI-MS, and then these samples were served at the room temperature to the participants in randomised orders.