xRetention time. yMethod of identification: RF, mass spectrum and retention time identical with a reference compound; TI, tentative identification by mass spectrum; SEM, standard error media. a, b, c: Different letters in the same row indicate significant statistical differences (Tukey's test, p < 0.05). zOrigin L: compound derived from lipid oxidation; M: compound derived from Maillard reaction; O: compound derived from other reactions/sources; –: compound derived from unknown