Effects of Caffeic Acid and Chlorogenic Acid Addition on the Chemical Constituents of Lychee Wine Fermented with Saccharomyces cerevisiae DV10

Abstract

This study evaluated the effects of caffeic acid and chlorogenic acid on the chemical constituents of lychee wine fermented with Saccharomyces cerevisiae DV10 when added at 200 mg/L and 300 mg/L before fermentation. Results showed that the caffeic acid and chlorogenic acid addition had no effect on the ability of alcoholic fermentation of S. cerevisiae. The addition of both acids decreased the utilization of amino nitrogen sources and produced less α-ketoglutaric, succinic, and acetic acid. The addition of 200 mg/L of caffeic acid induced a higher product of typical aroma components of the lychee wine, including trans-rose-oxide, precursors of 1-octane-3-ol, octanoic acid, and isoamyl acetate, and produced more esters, such as ethyl caprylate, ethyl caprate, ethyl hexanoate, isoamyl acetate, citronellyl acetate, ethyl-9-decenoate, geranyl acetate, and phenethyl acetate, compared with the chlorogenic acid addition. These findings indicate that caffeic acid addition could enhance the flavorful character and improve the quality of lychee wine.

1. Introduction

Lychee (Litchi chinensis Sonn.), belonging to the Sapindaceae family, is a kind of fruit growing in subtropical to tropical regions [1]. This fruit is widely loved by consumers because of its delicious and juicy taste. However, lychee fruit is difficult to preserve and easily rots and oxidizes. In order to extend their shelf life, lychees have been processed into drinks, dried fruits, and fruit wine [2]. Fermentation, as one of the processing methods for lychee, endows it with rich fruit and flower fragrance [3] and can change the color, taste, flavor, and chemical complexity [4].

As the primary flavor substance of fermented lychee wine, volatile acid is mainly composed of acetic acid [5], and its content determines the quality of lychee wine. A high content of volatile acid in lychee wine leads to its poor quality [6]. In recent years, reducing volatile acid content in lychee wine has been the focus of research. Currently, the research on lychee wine deacidification mainly focuses on the control of brewing process conditions, physical and chemical deacidification, and biological deacidification.

Phenolic acid is a non-flavonoid substance divided into hydroxybenzoic acid and hydroxycinnamic acid according to its structure. Most of them exist in plants in the form of esterification/combination with sugar, organic acid, and alcohol, and a few exist in the form of a free state. Hydroxycinnamic acids can retain more volatile substances in wine than hydroxybenzoic acids [7]. Chlorogenic acid and caffeic acid are important phenolic antioxidants that have attracted increasing attention [8]. Caffeic acid would be co-pigmentation with anthocyanins in wine, thus reducing the color value of wine but promoting the stability of wine color and total phenol concentration [9]. Studies have shown that chlorogenic acid can promote or inhibit aroma substances in fruit wine, depending on its concentration [10].

To the best of our knowledge, few studies have focused on the influence of polyphenols on the chemical constituents of lychee wine. Thus, this study aimed to evaluate the effect of caffeic acid and chlorogenic acid addition on the volatile acid, primary aroma, and amino acid in lychee wine. The result of this study could provide a suitable phenolic acid with which to improve the character of fermented lychee wine.

2. Materials and Methods

2.1. Materials and Reagents

Lychees (L. chinensis Sonn. var. Feizi Xiao), with a total soluble solid of 15.1–18.6% °Brix, were purchased from the Haikou lychee Planting Base in Hainan.

S. cerevisiae DV10 was purchased from Lallemand Inc. (Montreal, QC, Canada); Pectinase was purchased from Kangxi Food & Beverage Co., Ltd. (Shanghai, China) (Origin: France); Caffeic acid and chlorogenic acid were of food grade and purchased from Baiwei Biotechnology Co., Ltd. (Baoding, China); 2-octanol (≥99%, internal standard) was purchased from Aladdin (Chongqing, China); Tartaric acid, α-ketoglutaric (KG) acid, citric acid, acetic acid, pyruvic acid, succinic acid, and L-malic acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Haikou, China); Gallic acid, ferulic acid, caffeic acid, chlorogenic acid, (−)-epicatechin, and (+)-catechin were of chromatographic grade and purchased from were purchased from Sigma-Aldrich (Shanghai, China).

2.2. Experimental Methods

2.2.1. Preparation of Lychee Wine

After the lychees were peeled and cored, lychee juice was obtained by manual pressing. Sodium hydrogen sulfite (300 mg/L) was added to the juice to reduce the microbial load without affecting the activity of fermentative yeasts and preventing oxidation reactions. Then, 80 mg/L of pectinases were added to favor juice clarification at 45 °C for 3 h. Commercial sucrose was added until reaching the concentration of 20 °Brix. In accordance with the single factor and response surface optimization results, 200 mg/L of caffeic acid and 300 mg/L of chlorogenic acid were added; phenolic acids were not added to the control treatment [11]. Finally, 0.6 g/L of S. cerevisiae DV10 was added to start the fermentation at 20 °C for 6 days, and then clear wine samples were obtained by filtering with 1 g/L bentonite for later use. All the samples were stored at 20 °C until chemical and sensory analysis.

2.2.2. Determination of Physical–Chemical Indexes

Total acidity, volatile acid, total sugar, free sulfur dioxide, total sulfur dioxide, alcoholicity, and dry extract in lychee wine were measured using the methods of (GB/T 15037-2006).

2.2.3. Determination of Aroma Compounds of Lychee Wine

Headspace solid-phase microextraction (HS-SPME) coupled with GC-MS (PerkinElmer Clarus 690-SQ8T, Agilent, Santa Clara, CA, USA) was used to analyze the aroma compounds in lychee wine on the basis of the methods of Tang [12] and Villière [13] with slight modifications.

Extraction of aroma compounds: The aroma compounds in lychee wine were extracted by HS-SPME. The wine sample (5 mL) was transferred into a 15 mL headspace bottle with 1.25 g of NaCl and 10 μL of a standard internal substance (2-octanol). The sample was balanced at 45 °C for 10 min and extracted in a water bath for 40 min with continuous heating and agitation at the same temperature. After extraction, the fiber was inserted into the gas chromatograph for analysis.

GC-MS conditions: A DB-WAX chromatographic column was used to separate the aroma substances in lychee wine. The desorped fiber extraction head was inserted into the injection port of the gas chromatograph at 230 °C for 3 min. The initial temperature was 40 °C, which was maintained for 2 min. Then, it was raised to 210 °C at 6 °C/min and held for 0 min. Finally, the temperature was raised to 230 °C at 8 °C/min and maintained for 15 min. Helium was the carrier gas with 1 mL/min flow rate. The ion source was an EI source with an electron energy of 70 eV.

Analysis of aroma components: The aroma components in lychee wine were qualitatively analyzed via the NIST 14 standard spectrum library. The standard internal method was used for the semiquantitative analysis of aroma components. The formula C = N × Cs was used to calculate the content of volatile compounds, where N is the peak area ratio of each aroma substance/standard internal substance, and Cs is the standard internal concentration [14]. Odor activity values (OAVs) indicate the ratio of concentration to the threshold value. To describe the OAVs, refer to [15].

2.2.4. Determination of Organic Acid Content

The organic acid content determination was based on the method of Fu [16] with slight modification. Briefly, when lychee wine fermentation was finished, the supernatant, which was centrifuged at 12,000 r/min for 10 min, was directly filtered through a 0.45 µm membrane for HPLC analysis.

HPLC conditions: Zorbax SB-Aq (250 mm × 4.6 mm, 5 μm) was used, and 0.01 mol/L of potassium dihydrogen phosphate (pH 2.28, potassium dihydrogen phosphate: methanol) served as the mobile phase. The column flow rate was 1 mL/min, and the column temperature was 35 °C. The wavelength was 210 nm for ultraviolet detection. The injection amount was 15 μL. The standard external method was used for quantitative analysis.

2.2.5. Determination of Polyphenol Content

Sample pretreatment: Polyphenol extraction was carried out using the method of Gao [17] and Tzanova [18] with slight adjustments. A wine sample (10 mL) was extracted 3 times with 20 mL of ethyl acetate, which was then separated with a separating funnel. Then, ethyl acetate was evaporated to dryness at 30 °C by using a rotary evaporator. The residue was dissolved in 10 mL of chromatographic methanol and stored at −30 °C for later use. Before determination, the sample was tested by 0.22 μm microporous membrane filtration.

HPLC conditions: The methods of Aznar [19] and Yang [20] were used to determine polyphenols in the wine samples. An Agilent 1260 series HPLC instrument equipped with a diode array detector was used for analysis. The chromatographic column Zorbax SB-C18 (250 mm × 4.6 mm, 5 μm) was adopted, acetonitrile served as organic mobile phase B, and 0.1% formic acid aqueous solution acted as mobile phase A. The flow rate was set to 1 mL/min, the wavelength was 280 nm, and the injection volume was 10 μL. Quantitative determination was performed using a standard external method.

2.2.6. Determination of Amino Acid

Sample pretreatment was conducted in accordance with the method of Bao [21]. Acetonitrile was added to 10 mL of lychee wine sample, mixed well, stood at −20 °C for 30 min, and centrifugated at 12,000 rpm for 10 min. Then, the supernatant was taken for standby. Hydrochloric acid was added to the supernatant, mixed well, extracted at room temperature for 1 h, and centrifuged at 12,000 rpm for 10 min. Afterward, the supernatant was diluted appropriately. The diluted sample (10 μL) was added to a 70 μL AccQ • Tag Ultra borate buffer containing 20 μL of AccQ • Tag reagent, heated at 55 °C for 10 min, and cooled for use.

LC-MS conditions: The methods of Kovács [22] and Glauser [23] were used through UPLC-MS (Thermo Company, Waltham, MA, USA) to determine the amino acids in lychee wine. UPLC combined with ESI was carried out to analyze a Waters BEH C18 (50 × 2.1 mm, 1.7 μm) chromatographic column. Mobile phase: Phase A was ultrapure water (containing 0.1% formic acid), and phase B was acetonitrile (containing 0.1% formic acid). The injection volume was 1 μL, the ion spray voltage was +3000 V, and the temperature was 350 °C. For positive ion scanning, the scanning range was 150–700 m/z. Quantitative analysis was mainly based on the standard curve of the standard sample, while qualitative analysis was based on the precise molecular weight of the compound determined by high-resolution mass spectrometry and secondary mass spectrometry fragments.

2.2.7. Sensory Analysis

The sensory analysis was carried out using the method of [24]. Ten researchers (7 women and 3 men) with professional food knowledge were selected to conduct sensory evaluation and analysis on lychee wine samples. The evaluation was conducted in the tasting room at 20 °C and 65% moisture. Each lychee wine sample (20.0 mL) was randomly numbered in order and poured into glass bottles, and the containers were placed randomly and then presented to the panelists in that order. The chosen sensory attributes were color, floral, fruity, sour, astringent, bouquet, and clarity. Quantification was structured by 10 points, in which 10 represents the strongest feeling, and 0 represents no feeling.

2.2.8. Data Analysis

All the data in the table were collated using Excel 2021 (Microsoft Corporation, Redmond, WA, USA); the significance of data was performed using one-way analysis of variance (ANOVA), followed by Duncan’s multiple range test, using the SPSS 23.0 (SPSS Inc., Chicago, IL, USA), with a 95% confidence level, meaning that differences were considered as statistically significant when p < 0.05. Principal component analysis (PCA) was conducted using Origin 2018 (OriginLab Corporation, Northampton, MA, USA). All samples were tested in triplicate, and the results were expressed as mean ± SD.

3. Results and Discussion

3.1. Physical-Chemical Characterization of Lychee Wine

The physical-chemical parameters of lychee wines are shown in

Table 1. Physical-chemical parameters of lychee wine fermented with S. cerevisiae DV10, with and without caffeic acid and chlorogenic acid addition.

	Control	With 200 mg/L of Caffeic Acid	With 300 mg/L of Chlorogenic Acid
Alcohol (%vol)	11.09 ± 0.89 a	11.32 ± 0.05 a	11.16 ± 0.09 a
Total acidity (g/L)	9.28 ± 0.1 a	8.96 ± 0.1 b	9.04 ± 0.1 b
Volatile acid (g/L)	0.56 ± 0.01 a	0.43 ± 0.01 c	0.52 ± 0.01 b
Total SO2 (mg/L)	166.83 ± 0.60 a	155.73 ± 1.69 a	147.2 ± 1.81 a
Free SO2 (mg/L)	43.22 ± 0.67 a	32.91 ± 1.48 b	29.14 ± 1.62 b
Total sugar (g/L)	5.33 ± 0.07 a	4.51 ± 0.24 c	5.00 ± 0.12 b
Dry extract (g/L)	22.45 ± 1.05 a	24.25 ± 0.55 a	24.4 ± 1.66 a

Note: Different lowercase letters in the same line represent significant differences (p < 0.05).

. The results for alcohol, total acidity, volatile acid, total SO₂, free SO₂, total sugar, and dry extract were within limits established by Chinese legislation. No significant difference was found in the effect of caffeic acid and chlorogenic acid addition on alcohol, total SO₂, and dry extract content (p > 0.05). However, there were significant differences in total acidity, volatile acid, free SO₂, and total sugar compared with the control (p < 0.05). High levels of volatile acetic acid have a negative effect on the quality of lychee wine. When its content is up to 1.2 g/L, the lychee wine is considered unmarketable due to the undesirable acidic taste and unpleasant vinegar aroma. In this study, the content of acetic acid was 0.43–0.56 g/L, and the content of acetic acid added with caffeic acid and chlorogenic acid decreased by 23.2% and 7.1%, respectively, compared with the control. The decreased free SO₂ might be caused by the combination of SO₂ and phenolic acid. Low concentrations of this compound may not be effective in the preservation of fermented lychee wine, reducing its shelf life and causing undesirable effects, such as oxidation, browning, and increased levels of acetaldehyde [25].

3.2. Effect of Caffeic Acid and Chlorogenic Acid on Nonvolatile Acid and Mono-Phenols in Lychee Wine

Malic acid and tartaric acid are the main acids in lychee wine, accounting for more than 80% of the total organic acids. After chlorogenic acid was added, tartaric acid and succinic acid slightly decreased (p > 0.05); α-KG was significantly reduced; malic acid, trace content of pyruvic acid, and citric acid significantly increased (p < 0.05). Pyruvic acid is a key substance in sugar metabolism. Compared with that in the phenol acid treatment group, the pyruvic acid content in the control group was lower (

Table 2. Nonvolatile organic acids (g/L) and mono-phenols (mg/L) in lychee wine fermented with S. cerevisiae DV10, with and without caffeic acid and chlorogenic acid addition.

	Control	With 200 mg/L of Caffeic Acid	With 300 mg/L of Chlorogenic Acid
Organic acids
Tartaric acid	0.84 ± 0.23 a	1.03 ± 0.15 a	0.72 ± 0.01 a
Malic acid	2.48 ± 0.05 b	2.29 ± 0.084 b	3.53 ± 0.64 a
α-KG acid	0.36 ± 0.005 a	0.08 ± 0.004 c	0.10 ± 0.02 b
Pyruvic acid	0.04 ± 0.003 c	0.05 ± 0.001 b	0.06 ± 0.002 a
Citric acid	0.07 ± 0.005 b	0.07 ± 0.001 b	0.60 ± 0.03 a
Succinic acid	0.33 ± 0.02 a	0.20 ± 0.13 b	0.23 ± 0.03 ab
Mono-phenols
Ferulic acid	147.68 ± 10.43 a	95.02 ± 3.33 b	69.31 ± 7.31 c
(+)—catechin	132.71 ± 4.24 a	91.78 ± 1.57 b	76.79 ± 5.35 c
Gallic acid	93.51 ± 5.64 c	129.82 ± 5.42 a	113.75 ± 8.41 b
Caffeic acid	41.93 ± 1.72 b	31.59 ± 0.99 c	111.67 ± 4.51 a
Chlorogenic acid	-	104.35 ± 3.16 a	89.61 ± 5.12 b
(−)—epicatechin	-	57.59 ± 5.86 a	62.55 ± 0.75 a
Total	415.83 ± 22.03 b	510.15 ± 20.33 a	523.68 ± 31.45 a

Note: -: None detected; Different lowercase letters in the same line represent significant differences (p < 0. 05).

), which might be because pyruvic acid was catabolized to produce acetaldehyde. The content of acetic acid in the control group was highest (Table 1). According to previous studies, when the concentration of acetic acid increases, α-KG will also increase because acetic acid can form acetyl coenzyme A, thus entering the TCA cycle. Meanwhile, α-KG is converted into citric acid and malic acid [26]. As an organic acid formed in alcohol fermentation, succinic acid is regarded as a by-product of nitrogen metabolism by yeast cells during fermentation. Succinic acid reduction positively affects wine sensory evaluation by reducing the undesirable salty and bitter taste [27]. For the fermentation with caffeic acid addition, only tartaric acid and malic acid showed the opposite trend compared with that with chlorogenic acid addition. The changing mechanism of this phenomenon needs further study.

From Table 2, six kinds of polyphenols were detected in lychee wine treated with phenolic acid, and four kinds were detected in the control group. Among the phenolic compounds identified, ferulic acid presented the highest concentration in the control treatment, while in the treatments with the addition of phenolic acids, gallic acid presented the highest concentration. After phenolic acid was added, the content of ferulic acid and catechin in lychee wine decreased significantly. There was a clear antioxidant protective effect of caffeic acid that decreased significantly both in the control treatment and in the treatment with caffeic acid, but the latter allowed the preservation of gallic acid and chlorogenic acid. It is possible that the oxidation of caffeic acid contributes to the oxidation of catechins and epicatechin [28,29,30]. The epicatechin compensated for the deficiency of astringency in lychee wine [31]. Moreover, higher content of caffeic acid was detected in the wine samples added with chlorogenic acid, respectively. This was mainly because chlorogenic acid is a kind of caffeic acid ester-linked to quinic acid. When it meets water, it will hydrolyze to produce quinic acid and caffeic acid via a reversible reaction [32].

3.3. Effect of Caffeic Acid and Chlorogenic Acid on the Aroma of Lychee Wine

As shown in

Table 3. Aroma compounds (mg/L) of lychee wine fermented with S. cerevisiae DV10, with and without caffeic acid and chlorogenic acid addition.

Compounds	CAS	Control	With 200 mg/L of Caffeic Acid	With 300 mg/L of Chlorogenic Acid
Esters
Isoamyl acetate	123-92-2	103.85 ± 5.46 b	139.07 ± 1.17 a	110.32 ± 2.43 b
Ethyl butyrate	105-54-4	7.45 ± 0.83 b	8.32 ± 1.04 a	6.28 ± 0.31 b
Isobutyl acetate	110-19-0	7.33 ± 4.39 a	1.99 ± 3.45 a	-
Ethyl Hexanoate	123-66-0	108.81 ± 2.66 b	154.94 ± 11.5 a	157.42 ± 34.46 a
3-Cyclohexen-1-ol, acetate	10437-78-2	19 ± 1.08 b	26.14 ± 0.58 a	19.39 ± 0.4 b
Ethyl caprylate	106-32-1	223.17 ± 23.65 b	388.83 ± 6.69 a	312.53 ± 78.55 ab
Ethyl caprate	110-38-3	96.66 ± 6.49 b	183.13 ± 2.15 a	103.12 ± 17.68 b
Citronellyl acetate	150-84-5	16.99 ± 1.72 b	42.59 ± 1.73 a	16.05 ± 3.53 b
Ethyl benzoate	93-89-0	4.29 ± 0.3 a	4.26 ± 0.74 a	-
Ethyl 9-decenoate	67233-91-4	23.82 ± 1.99 b	48.97 ± 3.42 a	17.42 ± 4.12 b
Acetic acid lavandulyl ester	25905-14-0	5.97 ± 0.66 b	10.1 ± 3.92 a	3.08 ± 2.69 b
Geranyl acetate	105-87-3	14.93 ± 1.29 b	35.79 ± 3.71 a	11.35 ± 2.39 b
Phenethyl acetate	103-45-7	32.88 ± 1.18 b	39.59 ± 0.89 a	20.01 ± 3.2 c
Ethyl undecanoate	627-90-7	2.47 ± 2.16 b	6.64 ± 1.11 a	-
Pentanoic acid, 5-hydroxy-, 2,4-di-t-butylphenyl esters	166273-38-7	1.11 ± 0.97 a	1.73 ± 1.61 a	-
Hexyl acetate	142-92-7	-	0.47 ± 0.82	-
Butyl acetate	123-86-4	-	3.39 ± 3.02	-
Total		668.73 ± 54.81 b	1095.95 ± 47.56 a	776.89 ± 51.00 b
Alcohols
Ethanol	64-17-5	8.54 ± 5.7 a	7.49 ± 3.21 a	-
1-Propanol	71-23-8	6.7 ± 0.48 b	7.79 ± 1.26 a	4.95 ± 1.15 b
2-Methyl-1-propanol	78-83-1	98.66 ± 3.29 c	125.63 ± 3.3 a	111.29 ± 6.07 b
3-Methyl-1-butanol	123-51-3	851.79 ± 2.82 c	949.62 ± 12.26 a	928.13 ± 4.74 b
3-Methylthiopropanol	505-10-2	4.54 ± 0.36 a	5.42 ± 1.23 a	5.2 ± 0.97 a
7-Methyl-3-methylene-6-octen-1-ol	13066-51-8	2.72 ± 0.05 a	1.6 ± 1.45 a	1.68 ± 1.51 a
Nerol	106-25-2	13.3 ± 0.13 a	13.31 ± 2.59 a	12.31 ± 2.53 a
Iso-Geraniol	5944-20-7	11.54 ± 0.43 a	11.38 ± 2.19 a	8.92 ± 2.14 a
Phenylethyl Alcohol	1960/12/8	219.79 ± 9.53 c	255.28 ± 13.24 b	309.74 ± 18.11 c
Cyclopropaneethanol, 2-methylene-	120477-28-3	41.26 ± 2.15 a	47.34 ± 5.21 a	43.43 ± 7.01 a
1-Octanol	111-87-5	-	7.48 ± 1.17 a	1.81 ± 3.14 b
2-Nonanol	628-99-9	-	-	1.46 ± 1.34
1-Heptanol	111-70-6	-	5.34 ± 1.47 a	5.15 ± 0.52 a
2,3-Butanediol, [S-(R*,R*)]-	19132-06-0	12.22 ± 2.02 a	12.8 ± 3.25 a	15.16 ± 3.73 a
2-Decanol	1120-06-5	2.27 ± 0.19 a	3.15 ± 0.63 a	-
1-Octen-3-ol	3391-86-4	4.96 ± 0.5 a	4.82 ± 0.8 a	5.09 ± 1 a
5-methyl-1-Hexanol	627-98-5	1.24 ± 0.07 a	0.78 ± 1.36 a	-
Total		1279.53 ± 27.73 b	1459.23 ± 54.61 a	1454.32 ± 53.95 a
Acids
Hexanoic acid	142-62-1	-	-	10.4 ± 9.16
Oxalic acid	144-62-7	-	4.83 ± 8.37	-
Acetic acid	64-19-7	25.09 ± 0.52 a	17.69 ± 1.91 b	19.87 ± 1.37 b
L-Lactic acid	79-33-4	0.34 ± 0.58 a	0.73 ± 0.64 a	-
Propanoic acid, 2-methyl-	79-31-2	2.51 ± 2.18 a	4.68 ± 0.89 a	4.76 ± 0.82 a
Octanoic acid	124-07-2	90.81 ± 4.28 b	106.78 ± 2.03 a	61.92 ± 3.29 c
n-Decanoic acid	334-48-5	26.75 ± 0.35 b	32.63 ± 3.44 a	14.48 ± 3.61 c
Total		145.5 ± 7.91 b	167.34 ± 17.29 a	111.43 ± 9.08 c
Terpenes
trans-Rose oxide	876-18-6	53.83 ± 3.92 c	74.55 ± 2.2 a	66.81 ± 3.73 b
6-Octen-1-ol, 3,7-dimethyl-, (R)-	1117-61-9	-	143.18 ± 4.85 a	137.13 ± 10.6 a
Citronellol	106-22-9	89.34 ± 7.38	-	-
L-à-Terpineol	10482-56-1	6.17 ± 0.24 a	6.28 ± 1.16 a	5.75 ± 0.99 a
1,6-Octadien-3-ol, 3,7-dimethyl-	78-70-6	31.44 ± 1.62 b	36.05 ± 0.77 a	36.56 ± 2.22 a
Geraniol	106-24-1	39.04 ± 0.38 b	48.07 ± 2.63 a	30.21 ± 3.46 c
Total		219.82 ± 13.54 c	308.13 ± 11.60 a	276.46 ± 21.00 b
Aldehyde ketones
2-Nonanone	821-55-6	5.38 ± 0.06 b	8.74 ± 2.72 a	4.67 ± 0.77 b
Nonanal	124-19-6	10.47 ± 3.32 a	8.09 ± 0.78 a	6.92 ± 0.78 a
Decanal	112-31-2	4.35 ± 3.77 a	8.83 ± 5.43 a	-
Total		20.2 ± 7.15 a	25.66 ± 8.93 a	11.59 ± 1.55 b

Note: -: None detected; CAS: Chemical Abstracts Service. Different lowercase letters in the same line represent significant differences (p < 0.05).

, 50 aroma substances, including 17 esters, 17 alcohols, seven acids, six terpenes, and three aldehydes and ketones, were detected in this study. Higher alcohols and ester compounds were the main volatile components of lychee wine, in concurrence with the results of a previous lychee wine study by [33]. Some polyphenols can interact with aroma compounds in wine, affecting the solubility and volatility and then changing the perception degree [34]. For instance, Pozo-Bayón [35] found that phenolic compounds can interact with different types of aroma molecules to change their volatility and aroma release. Aronson [36] determined that naringin and gallic acid, two polyphenols, could inhibit the diffusion of aroma substances in an aqueous solution. By adding a certain amount of catechin (>2 g/L) to a water-alcohol solution with a volume fraction of 10%, the distribution coefficient of ethyl caproate in the gas phase of the system can be significantly reduced [37]. Similar results have been reported in beverage storage experiments with catechins and other substances [38]. At the same time, studies suggest that the degree of action of phenols and aroma compounds may be related to the chemical structure and properties of the reactive molecules [39].

Most of the esters have a pronounced floral flavor, contributing to the aroma of lychee wine. Compared with the control, the contents of esters in the wine samples with added caffeic acid and chlorogenic acid were increased by 63.9% and 16.2%, respectively. Among them, ethyl caprylate, ethyl hexanoate, isoamyl acetate, and ethyl caprate were the main flavor substances of lychee wine. Compared with added chlorogenic acid, caffeic acid played a better role in promoting the increase of esters in the wine samples. The influence of phenolic compounds on the volatility of esters mainly depends on the hydrophobic interaction between polyphenols and volatile substances [40]. Moreover, because different phenol structures have specific differences, the effect depends on the type and number of substituents on the benzene ring: the more the number of hydroxyl groups, the stronger the inhibition effect [41].

Higher alcohols are an essential component of aroma. They are produced in two ways. First, alcohols are formed through the amino transfer or deamination of corresponding amino acids through the Ehrlich pathway, and the ketoacidosis generated by this pathway is decarboxylated to aldehydes and finally reduced to higher alcohols. Second, alcohols are formed from acid ketones [42]. Compared with the control sample, the alcohols added with caffeic acid and chlorogenic acid increased by 14.0% and 13.7%, respectively. In addition, the contents of β-phenyl ethanol, isobutanol, and isoamyl alcohol were significantly increased, among which isoamyl alcohol accounted for 66.57%, 65.08%, and 63.82% of the total alcohol contents in the control group, caffeic acid group, and chlorogenic acid addition group, respectively. Similar results were reported by Chen [43], who noted that grape seed tannin addition could significantly increase the concentration of β-phenyl ethanol, isobutanol, and isoamyl alcohol.

Acids are the main flavor substances in lychee wine, which can help improve its flavor when the content is lower but will produce an unpleasant feeling when the content is too high. From Table 3, the acid content in caffeic acid treatment increased by 15.0%, in contrast to the 23.4% decrease in wine samples added with chlorogenic acid. In particular, the content of octanoic acid in the wine sample added with caffeic acid increased by 17.6%, whereas the content of octanoic acid in the wine sample added with chlorogenic acid decreased by 31.8%. Compared with the control, the content of acetic acid in caffeic acid and chlorogenic acid decreased significantly, which would improve the flavor of phenolic acid-treated samples. Many studies have shown that phenolic substances are the main factors affecting wine’s color, flavor, and astringency [44]. When caffeic acid and rosmarinic acid were added before fermentation, Li [45] found that the caffeic acid treatment increased the volatile organic acids in dry red wine, such as acetic acid, butyric acid, caprylic acid, caproic acid, and heptanoic acid. However, rosmarinic acid showed the opposite results. This finding implies that some phenolic acids have inhibitory effects on the formation of volatile acids, but the mechanism of their effects still needs further study.

Terpenes and aromatic aldehydes are essential substances that affect wine aroma and flavor compounds, and the glycosylated form of terpenes is usually more common than free terpenes [46]. Compared with those in the control, the terpenes in the wine samples added with caffeic acid and chlorogenic acid increased by 40.2% and 20.8%, respectively, indicating that they had a strong inhibitory effect on the hydrolysis of terpene glycosides and the volatilization of free terpenes; meanwhile, the influence of phenolic acids on terpene compounds was mainly due to the spatial structure, the hydrogen bonds between them, and the interaction between dispersed molecules [7].

In addition, the aldehyde and ketone content in caffeic acid treatment increased by 27.0% compared with that in the control. By contrast, the aldehyde and ketone content in chlorogenic acid-added wine samples decreased by 42.6%.

The substances with OAV > 1 were selected as the critical aroma substances (

Table 4. OAVs of selected key aroma compounds (mg/L) of lychee wine fermented with S. cerevisiae DV10, with and without caffeic acid and chlorogenic acid addition.

Compounds	Control	With 200 mg/L of Caffeic Acid	With 300 mg/L of Chlorogenic Acid	OT	TD
Isoamyl acetate	3.462 ± 0.182 b	4.636 ± 0.039 a	3.677 ± 0.081 b	0.03	Fruity, banana flavors
Ethyl Hexanoate	0.015 ± 0 b	0.021 ± 0.002 a	0.021 ± 0.005 a	7.5	Pineapple, varnish, balsam
Ethyl caprylate	0.372 ± 0.039 b	0.648 ± 0.011 a	0.521 ± 0.131 ab	0.6	Pineapple, pear, flowers
Ethyl caprate	0.483 ± 0.032 b	0.916 ± 0.011 a	0.516 ± 0.088 b	0.2	Aromas of fruit and brandy
Phenethyl acetate	0.132 ± 0.005 b	0.158 ± 0.004 a	0.08 ± 0.013 c	0.25	Floral, fruity, cocoa and whisky notes
Ethyl butyrate	0.372 ± 0.041 ab	0.416 ± 0.052 a	0.209 ± 0.182 b	0.02	Banana, strawberry flavor
1-Propanol, 2-methyl-	0.002 ± 0 c	0.003 ± 0 a	0.003 ± 0 b	40
1-Butanol, 3-methyl-	0.028 ± 0 c	0.032 ± 0 a	0.031 ± 0 b	30	Bitter almond taste, astringent taste
Phenylethyl Alcohol	0.2 ± 0.009 c	0.232 ± 0.012 b	0.282 ± 0.016 a	1.1	Rose, rose fragrance
2-Nonanol	-	-	0.002 ± 0.002	0.6	Fruit, rose fragrance
Octanoic acid	0.091 ± 0.004 b	0.107 ± 0.002 a	0.062 ± 0.003 c	1	Cheese flavor, fruity
Hexanoic acid	-	-	0.003 ± 0.003	3	Barbecue flavor, cheese
trans-Rose oxide	269.149 ± 19.614 c	372.734 ± 10.986 a	334.043 ± 18.658 b	0.0002	Intense aromas of roses
1,6-Octadien-3-ol, 3,7-dimethyl-	2.096 ± 0.108 b	2.404 ± 0.051 a	2.437 ± 0.148 a	0.015	Rose and citrus notes
Citronellol	2.234 ± 1.934	-	-	0.04	Cloves, rose fragrance
6-Octen-1-ol, 3,7-dimethyl-, (R)-	-	3.58 ± 0.121 a	3.428 ± 0.265 a	0.04	Cloves, rose fragrance
Geraniol	0.3 ± 0.003 b	0.37 ± 0.02 a	0.232 ± 0.027 c	0.13	Lemon flavor, peach

Note: -: None detected; OAVs (mean ± SD) of the same compounds followed by different letters are significantly different in the row (p < 0.05). Abbreviations: OT, odor threshold from the literature, mg/L; TD, taste description.

), most of which were esters. The content of trans rose ether in terpenes was well preserved, which contributed significantly to improving the complexity of aroma in wine samples. The PCA of its crucial aroma substances showed that the first and second principal components accounted for 52.8% and 32.6%, respectively. The total variance contribution rate was 85.4%, indicating the dispersion and correlation of most of the data. The volatile aroma components in the three samples exhibited a specific correlation (Figure 1a). The control group was in the negative part of PC1 and PC2 because of high citronellol, which has a pleasant floral scent and was detected only in the control group. The caffeic acid was in the positive part of PC1 mainly because of high propanoic acid, 2-methyl-, isoamyl acetate, and ethyl caprate contents, and the chlorogenic acid group was positively correlated with PC2 because of high phenylethyl alcohol and hexanoic acid. Figure 1b shows that the addition of caffeic acid and chlorogenic acid could promote the formation of esters in lychee wine, and the former had a better effect. However, in terms of promoting effects on alcohols, there was no difference between the two.

3.4. Effect of Caffeic Acid and Chlorogenic Acid on Amino Acids in Lychee Wine

Phenolic compounds can be converted into quinones through enzymatic or non-enzymatic conversion, thus playing a role in the formation and degradation of amino acids, further affecting food flavor [47].

Table 5. Amino acids in lychee wine fermented with S. cerevisiae DV10, with and without caffeic acid and chlorogenic acid addition (μg/mL).

	CAS	Control	With 200 mg/L of Caffeic Acid	With 300 mg/L of Chlorogenic Acid
γ-Aminobutyric acid	1956-12-2	316.95 ± 9.49 c	376.59 ± 5.33 b	646.15 ± 20.11 a
Pro	147-85-3	240.13 ± 16.35 c	282.91 ± 6.35 b	374.12 ± 11.38 a
Trp	73-22-3	0.02 ± 0.01 b	0.04 ± 0.01 b	0.12 ± 0.01 a
Gln	56-85-9	16.46 ± 1.4 b	18.68 ± 1.01 b	22.55 ± 1.71 a
Asn	70-47-3	7.47 ± 0.45 c	8.6 ± 0.17 b	9.44 ± 0.18 a
Ala	56-41-7	13.88 ± 1 c	18.73 ± 0.47 a	17.01 ± 0.8 b
Gly	56-40-6	12.8 ± 0.75 a	14.37 ± 1.14 a	14.37 ± 0.6 a
Thr	72-19-5	1.38 ± 0.33 a	1.74 ± 0.06 a	1.78 ± 0.18 a
Cys	56-89-3	-	-	-
Ser	56-45-1	2.53 ± 0.47 b	3.25 ± 0.27 a	3.41 ± 0.2 a
Asp	56-84-8	5.4 ± 0.76 b	6.7 ± 0.41 ab	7.48 ± 1.09 a
Glu	56-86-0	4.17 ± 0.55 b	6.44 ± 0.52 a	6.51 ± 0.28 a
Arg	74-79-3	3.2 ± 0.1 b	3.74 ± 0.07 a	3.18 ± 0.07 b
His	71-00-1	0.14 ± 0.01 b	0.31 ± 0.01 a	0.15 ± 0.01 b
Lys	56-87-1	1.06 ± 0.05 c	1.93 ± 0.1 a	1.73 ± 0.09 b
Met	63-68-3	0.61 ± 0.17 a	0.76 ± 0.1 a	0.67 ± 0.12 a
Val	72-18-4	1.15 ± 0.19 a	1.49 ± 0.38 a	1.25 ± 0.06 a
Ile	73-32-5	0.43 ± 0.12 b	0.83 ± 0.09 a	0.67 ± 0.07 a
Leu	61-90-5	2.02 ± 0.53 b	3.63 ± 0.21 a	3.42 ± 0.44 a
Phe	63-91-2	1.81 ± 0.18 ab	2.17 ± 0.19 a	1.67 ± 0.2 b
Tyr	60-18-4	1.26 ± 0.15 a	1.44 ± 0.1 a	1.23 ± 0.18 a

Note: -: None detected; Different lowercase letters in the same line represent significant differences (p < 0.05).

presents that 19 kinds of amino acids were detected in the wine samples. The total amino acids in the control group, caffeic acid group, and chlorogenic acid group were 624.97, 746.24, and 1125.54 μg/mL, respectively. γ-Aminobutyric acid and proline were the main amino acids in lychee wine, accounting for 50.18% and 50.02%, 57.41% and 37.36%, and 37.02% and 33.83% of the total amino acids in the control group, caffeic acid group, and chlorogenic acid group, respectively. The contents of γ-aminobutyric acid and proline were significantly increased after caffeic acid and chlorogenic acid addition. γ-Aminobutyric acid is a four−carbon, nonprotein amino acid with the functions of lowering blood pressure, regulating blood sugar, helping sleep and soothing nerves, and adjusting immunity; its formation is mainly related to lactic acid bacteria metabolism during fermentation [48].

The formation content of proline was found to be positively correlated with the caffeic acid and chlorogenic acid addition, increasing from 240 mg/mL (control fermentation) to 282 and 374 mg/mL in the fermentation added with 200 mg/L of caffeic acid and 300 mg/L of chlorogenic acid, respectively (Table 5). Hence, the phenolic acid addition had a more significant influence on the formation of proline than it did on other amino acids. The only reason for its increase could be that by the action of the medium, the yeast synthesizes it in a higher concentration releasing it to the wine product of autolysis. Table 5 also shows that other amino acids increased to varying degrees with the addition of phenolic acids, which indicated that the addition of phenolic acids decreased the consumption of amino nitrogen sources.

3.5. Sensory Evaluation

The sensory evaluation of the tested samples is shown in Figure 2. Compared with the control, after caffeic acid and chlorogenic acid were added to lychee juice before fermentation, the release of volatile aroma substances was inhibited, thus enhancing the flower and fruit fragrance. The main reason for this phenomenon was the addition of low concentrations of phenolic acids that can improve the volatility and perceived intensity of flower and fruit fragrances [49,50].

However, adding caffeic acid increased the acid taste compared with the control group, while the added chlorogenic acid made the acid taste in the lychee wine sample decrease. As for clarity and color, caffeic acid scored lower, which was mainly related to the physical properties of caffeic acid. The strong astringency after caffeic acid treatment was related to the high content of total mono-phenols and isoamyl alcohol in the lychee wine sample.

4. Conclusions

In this study, both caffeic acid and chlorogenic acid additions affected the by-product formation of the sugar metabolism of the lychee wine by reducing the consumption of the preferential amino nitrogen source. Consequently, more amino acids, especially γ-aminobutyric acid, were maintained, whereas minimal acetic acid, α-KG, and succinic acid were produced in the finished lychee wine. Both the addition of caffeic acid and chlorogenic acid increased the total mono-phenols content, decreased the production of (+)-catechin, and reduced the bitter taste of lychee wine. In the primary volatiles of lychee wine, the addition of caffeic acid was better able to enhance the floral and fruity aroma and more able to prompt S. cerevisiae to produce more esters, such as ethyl caprylate, ethyl caprate, ethyl hexanoate, isoamyl acetate, citronellyl acetate, ethyl 9-decenoate, geranyl acetate, and phenethyl acetate in the finished lychee wine. The lychee wine with caffeic acid added had the highest total score. Based on these results, caffeic acid addition for the winemaking practice could be an effective way to form and retain the aroma-active compounds in lychee wine fermented with S. cerevisiae DV10.