**1. Introduction**

Traditional vinegar is made from cereals and has been consumed for a long time. Another type of vinegar, fruit vinegar, made from fruit or fruit juices, has become increasingly popular in recent years because consumers are paying more attention to the functional properties of food products.

Oxidative stress is one of the main causes of certain chronic diseases such as liver, neurodegenerative, and cardiovascular diseases [1,2], which can be prevented by antioxidants [3]. Fruits are rich in antioxidants and are widely consumed by humans. Fruit vinegars can retain a number of antioxidants from fruit or fruit juices [4] and possess relatively high antioxidant capacities compared to wine and fruit juices [5]. Furthermore, fruit vinegars can increase antioxidant capacities of diets [6] as the fermentation process can produce functional components such as organic acids [7] which are not, or are only rarely, present in raw fruit materials. Fruit vinegars have also been reported to possess several health benefits, such as suppressing obesity-induced oxidative stress [8], regulating lipid metabolism, and decreasing liver damage [9], which can be at least partly due to the antioxidant activity of fruit vinegars [10]. Hence, it is valuable to determine and compare the antioxidant capacities of different fruit vinegars.

In addition, raw fruit materials are the main sources of phenolic compounds in fruit vinegars [11], and phenolics play key roles in the organoleptic properties and health effects of fruit vinegars. However, scientific research has reported that different fruit vinegars vary in their phenolic composition and contents [12], due to differences among the raw materials and manufacturing processes [7]. Only one study revealed the differences in phenolic profiles among three fruit vinegars [13]. Therefore, the phenolic profile is another important factor in measuring the value of fruit vinegars.

Fermentation is a key process in the production of fruit vinegars, during which most organic acids are produced through chemical and microbial actions [7]. Organic acids can contribute to the organoleptic qualities of fruit vinegars [14]. Furthermore, organic acids demonstrate antimicrobial activities [15] and can control blood glucose levels and regulate lipid abnormalities [16]. The organic acids in fruit vinegars have been found to be different from those in traditional cereal vinegars [13]. Therefore, it is valuable to understand the organic acid profile of fruit vinegars.

This research, therefore, was conducted to determine the antioxidant activities, total phenolic contents (TPC), and total flavonoid contents (TFC) of 23 commonly-consumed fruit vinegars. Moreover, the main phenolic compounds and organic acids were also identified and quantified in the 23 fruit vinegars. This study provides a good reference for the public as to consuming fruit vinegars rich in antioxidant phenolics and organic acids.

#### **2. Materials and Methods**

#### *2.1. Chemicals and Materials*

The chemicals for the determination of ferric-reducing antioxidant power (FRAP), Trolox equivalent antioxidant capacity (TEAC), TPC, TFC and phenol analysis were bought according to the paper [17] we published previously. Eighteen standard phenolic compounds, including gallic acid, protocatechuic acid, gallo catechin, chlorogenic acid, cyanidin-3-glucoside, caffeic acid, epicatechin, catechin gallate, p-coumaric acid, ferulaic acid, melatonin, 2-hydroxycinnamic acid, rutin, resveratrol, daidzein, equol, quercetin, and genistein were purchased from Sigma-Aldrich (St. Louis, MO, USA). Standard organic acids, including ascorbic acid, lactic acid, citric acid, and succinic acid, were obtained from Sigma-Aldrich, and oxalic acid, tartaric acid, and malic acid were bought from National Institutes for Food and Drug Control (Beijing, China). Phosphoric acid and potassium phosphate monobasic used for organic acid analysis were of analytical grade and bought from Damao Chemical Factory (Tianjin, China) and Yongda Chemical Reagent Company (Tianjin, China), respectively. Double-distilled water was used in all the experiments. The 23 fruit vinegars (Table 1) were bought from online shopping platforms and local markets in Guangzhou, China, and were stored at 4 ◦C before use.

#### *2.2. Determination of FRAP, TEAC, TPC, and TFC Values*

The FRAP, TEAC, TPC, and TFC values were evaluated based on the methods published previously [17], and were expressed as μmol Fe(II)/mL, μmol Trolox/mL, mg gallic acid equivalent (mg GAE)/L, and mg of quercetin equivalent (mg QE)/L, respectively.

#### *2.3. Phenolic Composition Analysis*

The phenolic components in 23 fruit vinegars were analyzed by High Performance Liquid Chromatography coupled with Photometric Diode Array detector (HPLC-PDA) (Waters, Milford, MA, USA) based on the literature [17]. Separation was conducted using an Agilent Zorbax Extend-C18 column (250 × 4.6 mm, 5 μm) (CA, USA) at 40 ◦C. Mobile phase A was formic acid solution (0.1%, *v*/*v*), and B was methanol. The procedure of gradient elution was set as: 0 min, 5% (B); 15 min, 20% (B); 20 min, 30% (B); 25 min, 37% (B); 40 min, 40% (B); 60 min, 50% (B); 65 min, 50% (B); 65.1 min, 5% (B); and 70 min, 5% (B). The spectra were scanned between 200 and 600 nm. Peak area was used to quantify phenolic compounds and the results were expressed as μg/mL.

#### *2.4. Organic Acid Analysis*

HPLC-PDA was used to analyze the organic acids in 23 fruit vinegars based on the literature [13] with slight modifications. Separation was conducted using an Agilent TC-C18(2) column (250 × 4.6 mm, 5 μm) at 35 ◦C with a mobile phase of 0.01 mol/L monopotassium phosphate buffer solution (pH = 2.5). The injection volume was 20 μL and the flow rate was 1 mL/min. The spectra were recorded at 210 nm. Peak area was used to quantify organic acids and the results were expressed as μg/mL.
