*2.5. Data Analysis*

Each test was conducted in triplicate, and the results are shown as mean ± standard deviation (SD). SPSS 22.0 (IBM, Somers, NY, USA) and Excel 2007 (Redmond, WA, USA) were used to analyze the statistical differences. One-way analysis of variance (ANOVA) and the *post hoc* Tukey test were conducted to compare the differences among the means in more than two samples. The Pearson test was used for the correlation analysis. Statistical significance was defined as *p* < 0.05.

#### **3. Results and Discussion**

#### *3.1. Antioxidant Activities of Fruit Vinegars*

The antioxidant activities of phenolic compounds and other phytochemicals in natural foods are often multifunctional and more than one method is required to measure the antioxidant capacities of fruit vinegars because methods measure different aspects of antioxidant capacities [18]. In this study, FRAP and TEAC assays were both used to evaluate the antioxidant activities of fruit vinegars. The FRAP assay measures the ability to reduce a ferric tripyridyltriazine complex to the ferrous complex [19], while the TEAC method determines the ability to scavenge ABTS•<sup>+</sup> free radicals [20].

The FRAP values of the 23 fruit vinegars ranged from 0.15 to 23.52 μmol Fe(II)/mL, while the TEAC values ranged from 0.03 to 7.30 μmol Trolox/mL (Table 1). These results were consistent with a previous study in which the FRAP value of a wine vinegar was 9.50 mmol Fe(II)/L, while the TEAC value was 3.12 mmol Trolox/L [5]. The five highest FRAP values, in decreasing order, were found in balsamic vinegar of Modena (Galletti) (23.52 μmol Fe(II)/mL), Aceto Balsamico di Modena (Monari Federzoni) (13.39 μmol Fe(II)/mL), red wine vinegar (Galletti) (8.04 μmol Fe(II)/mL), red wine vinegar (Kühne) (5.35 μmol Fe(II)/mL), and apple vinegar beverage (Long He Kuan) (3.99 μmol Fe(II)/mL). Similarly, results for the fruit vinegars with the highest TEAC values were generally consistent with the results for the FRAP values (Table 1).

#### *3.2. TPC and TFC Values*

The TPC values of fruit vinegars ranged from 29.64 to 3216.60 mg GAE/L (Table 1). The fruit vinegars with the highest TPC values were balsamic vinegar of Modena (Galletti) (3216.60 mg GAE/L), followed by Aceto Balsamico di Modena (Monari Federzoni) (1901.92 mg GAE/L), red wine vinegar (Galletti) (993.51 mg GAE/L), red wine vinegar (Kühne) (654.95 mg GAE/L), and apple vinegar (Heng Shun) (495.52 mg GAE/L). The TPC values of the tested fruit vinegars were in accordance with the findings of Ren et al. [13], in which the TPC values of fruit vinegars ranged from 274.08 to 754.50 mg GAE/L, but only three fruit vinegars were tested in that study. On the other hand, the TFC values ranged from 2.22 to 753.19 mg QE/L (Table 1). The fruit vinegars with the highest TFC values were Aceto Balsamico di Modena (Monari Federzoni) (753.19 mg QE/L), followed by balsamic vinegar of Modena (Galletti) (699.67 mg QE/L), red wine vinegar (Kühne) (51.47 mg QE/L), red wine vinegar (Galletti) (50.34 mg QE/L), and apple vinegar (Heng Shun) (31.39 mg QE/L).



Taking the FRAP, TEAC, TPC, and TFC values together, balsamic vinegar of Modena (Galletti), Aceto Balsamico di Modena (Monari Federzoni), red wine vinegar (Kühne), and red wine vinegar (Galletti) showed the highest antioxidant capacities and phenolic contents among the 23 fruit vinegars tested. In general, fruit vinegars made from red grapes, especially balsamic vinegar (balsamic vinegar of Modena (Galletti) and Aceto Balsamico di Modena (Monari Federzoni)), possessed stronger antioxidant activities. This finding is consistent with a previous study in which balsamic vinegars made from red grapes displayed significantly higher TPC values, radical scavenging, and oxidant reducing activities compared to fruit vinegars made from red grapes, white grapes, and apples [21]. In addition, antioxidant activities were found to be higher in red grape balsamic vinegars than in red wine vinegars, probably due to the phenolic contents in different fruit vinegars being affected by the raw materials, such as red grapes, white grapes and apples, and manufacturing processes [22,23].

#### *3.3. Correlations Among FRAP, TEAC, TPC, and TFC Values*

The FRAP values of the 23 fruit vinegars were highly correlated with the TEAC values ( *R*<sup>2</sup> = 0.989) (Table 2), indicating that the components responsible for reducing oxidants were consistent with those scavenging free radicals in fruit vinegars. In addition, a moderate correlation ( *R*<sup>2</sup> = 0.832) was found between the TPC and TFC values (Table 2), indicating that flavonoids were not the only phenolic compounds in fruit vinegars. In addition, the FRAP and TEAC values both showed high positive correlations with TPC values ( *R*<sup>2</sup> = 0.990 and 0.971, respectively) (Table 2), suggesting that phenolic components contribute to both the oxidant-reducing and radical scavenging activities of fruit vinegars. In a previous study conducted by Dávalos et al. [12], the antioxidant activities and TPC values of wine vinegars exerted a positive correlation (*p* < 0.01), consistent with our finding. On the other hand, the FRAP and TEAC values both showed moderate correlations with TFC values ( *R*<sup>2</sup> = 0.804 and 0.767, respectively) as shown in Table 2. Although the four values were correlated with each other, further studies are still needed to evaluate the specific compounds that contribute to each value in fruit vinegars, as most of the methods were based on the same reaction mechanism.


**Table 2.** Correlation analysis among FRAP, TEAC, TPC and TFC values.

\* indicates statistical significance at *p* < 0.01.

#### *3.4. Polyphenols and Organic Acids in Fruit Vinegars*

Some studies have suggested that fruit vinegars possess the ability to improve oxidative stress-related disorders, such as obesity [8], liver damage [9], and diabetes [24]. Our results indicated that polyphenols in fruit vinegars were the major ingredients contributing to the antioxidant activities. We therefore further investigated the main phenolic compounds in fruit vinegars (Table 3). Retention time and UV spectra were used to recognize the phenolic compounds, and the peak areas were used to quantify these phenolic compounds (Figure 1). It was found that gallic acid, protocatechuic acid, chlorogenic acid, caffeic acid, and *p*-coumaric acid were the most widely detected phenolic compounds in 23 fruit vinegars. In addition, the highest concentrations of gallic acid, protocatechuic acid, chlorogenic acid, caffeic acid, and *p*-coumaric acid were found in Balsamic vinegar of Modena (Galletti) (12.56 μg/mL), balsamic vinegar of Modena (Galletti) (3.29 μg/mL), apple vinegar (Zi Lin) (10.91 μg/mL), balsamic vinegar of Modena (Galletti) (3.58 μg/mL), and balsamic vinegar of Modena (Galletti) (1.97 μg/mL), respectively. The data we obtained were in accordance with the data in Phenol-Explorer (a database on polyphenol content in foods), in which the contents of gallic acid, protocatechuic acid caffeic acid, and *p*-coumaric acid in vinegars were 2.59 ±, 0.81, 0.28, 0.29 mg/100 mL, respectively [25]. Balsamic vinegar of Modena (both Galletti and Monari Federzoni) contained high gallic acid and *p*-coumaric acid contents which might be responsible for the strong antioxidant capacities and high phenolic contents and needs further study. Many polyphenols, such as gallic acid, protocatechuic acid, chlorogenic acid, caffeic acid, and *p*-coumaric acid found in fruit vinegars, have been reported to suppress oxidative stress-related damages [26–29].

**Figure 1.** The chromatograms of phenolic compound standards (**a**) and apple vinegar (Zi Lin) (**b**) under 254 nm. Peak identification, retention time and maximum absorption: (1) gallic acid, 10.543 min, 271.3 nm; (2) protocatechuic acid, 15.723 min, 259.4 nm; (3) gallo catechin, 20.821 min, 270.1 nm; (4) chlorogenic acid, 23.661 min, 326.0 nm; (5) cyanidin-3-glucoside, 24.145 min, 279.6 nm; (6) caffeic acid, 24.987 min, 323.6 nm; (7) epicatechin, 25.580 min, 278.4 nm; (8) catechin gallate, 28.334 min, 277.2 nm; (9) *p*-coumaric acid, 29.454 min, 309.3 nm; (10) ferulaic acid, 31.018 min, 323.6 nm; (11) melatonin, 32.325 min, 221.7 nm; (12) 2-hydroxycinnamic acid, 35.562 min, 276.0 nm; (13) rutin, 37.296 min, 255.9 nm; (14) resveratrol, 37.908 min, 304.6 nm; (15) daidzein, 48.543 min, 248.8 nm; (16) equol, 52.706 min, 280.8 nm; (17) quercetin, 53.591 min, 254.7 nm; (18) genistein, 58.002 min, 259.4 nm.

Organic acids are another important component of fruit vinegars. The main organic acids in fruit vinegars and their contents are shown in Table 4. Retention times were used to identify organic acids, and peak areas were used to quantify the contents (Figure 2). In the 23 fruit vinegars tested, tartaric acid, malic acid, lactic acid, citric acid, and succinic acid were the most widely detected organic acids, with the highest content found in white wine vinegar (Kühne) (1566.48 μg/mL), apple vinegar (Guang Wei Yuan) (7691.98 μg/mL), apple vinegar (Guang Wei Yuan) (2541.64 μg/mL), apple vinegar (Guang Wei Yuan) (6485.24 μg/mL), and apple vinegar (Cu Bo Shi) (1775.77 μg/mL), respectively.

**Figure 2.** The chromatograms of the organic acid standards (**a**) and apple vinegar (Cu Bo Shi) (**b**) under 210 nm. Peak identification and retention time: (1) oxalic acid, 3.268 min; (2) tartaric acid, 3.677 min; (3) malic acid, 4.565 min; (4) ascorbic acid, 5.150 min; (5) lactic acid, 5.429 min; (6) citric acid, 7.795 min; (7) succinic acid, 8.869 min.

Fruit vinegars are popular all over the world due to their good flavor and health benefits. Phenolic compounds and organic acids are the main components that contribute to the sensory qualities and health benefits of fruit vinegars. Some research has indicated that different grape varieties possess different phenolic contents and composition [30], and similarly apple varieties [31], depending on factors like cultivars, growing environments, and ripeness stage [17]. Factors, such as yeas<sup>t</sup> strains [32], acetic acid bacteria, and production technology [33] within the fermentation processes, could also affect the phenolic profiles of fruit vinegars [4,34]. In addition, phenolic compounds have been widely explored for the abilities in preventing chronic diseases, including anti-cancer, anti-obesity, anti-aging and anti-diabetes activities [35]. According to our results, a usual serving of fruit vinegar (10 mL) contains approximately 0.30–32.67 mg GAE of polyphenols, and the FRAP and TEAC values of 10 mL fruit vinegar are approximately 1.50–235.20 μmol Fe(II) and 0.30–73.00 μmol Trolox, respectively. As the phenolic contents and composition in fruit vinegars varied, further studies are needed to explore the bioavailability and health benefits of fruit vinegars in vivo.

Organic acids in fruit vinegars are produced through hydrolysis, biochemical metabolism and microbial actions in the fermentation process. In fruit vinegars, the contents and types of organic acids affect their sensory qualities and also their health functions. Our results indicated that fruit vinegars possessed abundant organic acids with different and complex compositions. Among all the tested organic acids, tartaric acid, malic acid, lactic acid, citric acid, and succinic acid were most widely distributed in the 23 fruit vinegars tested, and the results were consistent with another study [36]. Organic acids exert some health benefits such as antimicrobial activities [15], controlling blood glucose levels, and regulating lipid abnormalities [16]. Fruit vinegars were found to contain more complex compositions of organic acids than cereal vinegars [13], indicating fruit vinegars possess a richer taste and fruit flavor compared with conventional cereal vinegars.




**Table 4.** Main organic acids and their contents in 23 fruit vinegars.

#### *Antioxidants* **2019**, *8*, 78
