Guoguang apples (Malus pumila Mill.) were purchased from a local fruit market (Tai’an, China). Commercial wine yeast SIHA Active Yeast 3 (Saccharomyces cerevisiae WET 136) was ordered from Eaton (Langenlonsheim, Germany). Methanol standard (chromatographic grade) was purchased from Shandong Yuwang Pharmaceutical Co., Ltd. (Dezhou, China), and tert-amyl alcohol provided by China National Research Institute of Food and Fermentation Industries (Beijing, China) was used as an internal standard. PME, PL and PG ELISA kits were ordered from Shanghai Hengyuan Biotechnology Co.,Ltd (Shanghai, China). All other reagents were of at least analytical grade.
