The titratable acidity was measured in accordance with Chinese
national standard 12293 (1990), using sodium hydroxide 0.1 M,
with end point pH 8.1. The titratable acidity was expressed in
terms of g malic acid per l. The pH value was measured by a pH
meter (FE20 series, Mettler-Toledo Instruments Co., Shanghai, China).
All analyses were performed in triplicate