3.12. Organic Acid Analysis

Onion vinegar fermentation was evaluated by high performance liquid chromatography (HPLC) (Young Lin Instrument Co., Ltd., YL9100 HPLC system, Anyang, Korea) after filtration of the samples through a 0.45-μm membrane (Millipore). Lactic acid, acetic acid, succinic acid, malic acid, and citric acid dissolved in 0.1% (w/v) perchloric acid solution were used as reference materials. The retention time and peak area of these standards were used to calculate the organic acid content of the fermentation broth. HPLC conditions were modified from and supplemented by the method of Huang et al. [44] and a Shodex RSpak KC-811 (I.D. 8.0 × 3.00 mm) column (Showa Denko K.K., Tokyo, Japan) was used. The oven temperature, mobile phase, flow rate, injection volume, and detector conditions were 4 °C, 0.1% perchloric acid, 1.0 mL/min, 10 μL and UV 210 nm, respectively.