Abstract: The objective of this work was to analyze the changes in metabolites during the natural fermentation of apple into an enzyme drink. The pH value, total titrable acid, organic acids (lactic acid, acetic acid, and malic acid), sugars (sucrose, glucose, and fructose), alcohol, total phenolic content and 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging ability were investigated. Results showed that pH value decreased from 5.05 to 3.88 while total titratable acidity and lactic acid increased gradually during the fermentation process. Acetic acid was accumulated from 0.018% to 0.23% during the process; malic acid increased from 0.67 g/L to 2.45 g/L during the first 14 days and then decreased slightly. Glucose and fructose increased until reaching their maximum of 130.6 and 177.2 g/L on the 21th day and then decreased, while sucrose kept decreasing to almost 0 g/L during the whole fermentation process. Alcohol was produced from the 14th to 21th day, reaching 6.6% on the 28th day. Both total phenolic content and DPPH radical scavenging ability increased continuously to their maximum values (259.4 mg/L, 90.9 %) on the 21st day followed by a decrease.

Key words: fermented apple drink rich in enzyme, natural fermentation process, metabolites, DPPH radical scavenging ability