ORIGINAL RESEARCH ARTICLES

 

Phenolic characteristics and antioxidant activity of merlot and cabernet sauvignon wines increase with vineyard altitude in a high-altitude region

 

 

Xiao-duo Jin; X. Wu; X. Liu^*

College of Oenology, Northwest A& F University, Yangling 712100, China

 

 

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ABSTRACT

Altitude, as an important factor in the expression of terroir, may affect wine quality. We evaluated the effect of altitude and its related climatic conditions on the phenolic characteristics and antioxidant activity of red wines made from grapes originating from high-altitude areas. The content of total phenolic compounds, total flavonoids and total anthocyanins increased with altitude in Merlot (ME) and Cabernet Sauvignon (CS) wines. Cabernet Sauvignon wines showed richer tannins with increasing altitude. Merlot and CS wines from higher altitude vineyards, showed a greater antioxidant capacity. Salicylic acid, syringic acid, caffeic acid, (+)-catechin, (-)-epicatechin, and the sum of individual phenolic compounds in the wines increased with altitude based on the results of HPLC. The scores of the sensory evaluation of ME wines increased with higher altitude. The highest score was determined for CS wine originating from 2 608 m. A clear grouping of wines according to grape cultivar and vineyard altitude was observed by principal component analysis. Regression analysis showed that altitude, followed by sunshine hours, made the greatest contribution to differences in the phenolic characteristics and antioxidant activity of red wines at different sites in a high-altitude region.

Key words: Wine, phenolic characteristics, antioxidant activity, terroir, high-altitude region

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INTRODUCTION

Environmental factors such as light and field management practices have a combined effect on grapevine physiology and wine quality (Spayd et al., 2002; Feng et al., 2017). The French term "terroir" is used to define the geographical and environmental origin of grapes grown and harvested during a certain vintage (Deloire et al., 2008). This term includes all regional parameters, such as soil type, climate and orography (Douglas et al., 2001). Terroir is an important factor affecting grape and wine composition (Roullier-Gall et al., 2014).

Altitude is usually considered an indirect control on the environmental conditions affecting plant metabolism (Kumar et al., 2006). Therefore, altitude may have a strong effect on climatic conditions that affect grape development and wine quality (Mateus et al., 2002). In particular, altitude gradient may reflect the integrated variation in temperature, humidity and solar radiation in a mountainous region with high vertical zonality (Zhang et al., 2005).

Phenolic compounds contribute to a wine's organoleptic characteristics, such as mouthfeel and colour, and are associated with anti-cancer and cardiovascular protection and antioxidant activity (Burns et al., 2000; Skerget et al., 2004). The phenolic compound concentrations of grapes are affected by time of harvest and grape ripeness levels, thereby affecting the flavour and quality of the resulting wine. Phenolic compound concentrations are also affected by genotypes, viticultural practices and environmental conditions (Downey et al., 2006). Variation in phenolic characteristics of wine grapes has often been observed at different altitudes and also has been characterised by regional differences (Mateus et al., 2001, 2002; Liang et al., 2014). Higher altitudes synchronise with a lower temperature and higher humidity, which may be beneficial to carotenoid accumulation (Oliveira et al., 2004). An increase in ultraviolet (UV) intensity, with an increase in altitude, promotes anthocyanin biosynthesis in Malbec grape skin via upregulating the genes encoding phenylalanine ammonia lyase (PAL) and chalcone synthase (CHS) (Berli et al., 2015).

The effect of vineyard altitude on wine quality has not been reported extensively (Jiang et al., 2015; Yue et al., 2015). Touriga Nacional wine made from grapes harvested at higher altitudes - ranging from 300 m to 350 m - showed a higher level of anthocyanins (Mateus et al., 2002). The concentrations of 2-methoxy-3-isobutylpyrazine in CS wines were found to be positively correlated with vineyard altitude (Falcäo et al., 2007). Altitude played a more important role than rainfall and growing degree days on the differences in phenolic composition of Malbec wine produced in Mendoza and California at an altitude of 1 103 m and 190 m, respectively (King et al., 2014). However, contrasting results have also shown that total phenolic compounds, flavonoids and flavanols in red wine made from grapes originating from lower altitude vineyards were significantly higher than higher altitude vineyards, although anthocyanin content increased with altitude (Jiang et al., 2011). No relationship was observed among α- and β-ionone, β-damascenone content and vineyard altitude in Brazilian CS wines (Falcäo et al., 2007). Most of the aforementioned vineyards were located at altitudes ranging from 39 m to 1 354 m.

This study aimed to determine the differences in phenolic characteristics and antioxidant activity of red wines produced at different altitudes in the high-altitude region of Hengduan Mountain. The objective of this work was to evaluate the effect of altitude and climatic factors on wine quality.

 

MATERIALS AND METHODS

Chemicals

Methanol, Folin-Ciocalteu reagent, acetonitrile (HPLC grade) and acetic acid were purchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). Gallic acid, rutin, cyanidin-3-O-glucoside, malvidin-3-O-glucoside, (+)-catechin, trolox, 2, 2-diphenyl-1-picrylhydrazyl (DPPH), and all individual phenolic standards for HPLC analysis were purchased from Sigma-Aldrich Company (MO, USA).

Experimental region and grape management

The experimental region is located in Danba County, situated in the north of the Hengduan Mountains, China. The Hengduan Mountain system includes a complex of ridges and river valleys that rise between the western margin of Yunnan Plateau and the eastern margin of Tibet in China and run approximately north to south. This region is famous for vertical zonality, and hence altitude may make a greater contribution to the terroir of wine from this area. Furthermore, the vineyards are mostly located at altitudes from 2 000 m to 3 000 m.

This wine region is located at the low latitude of 30°N to 31°N and is characterised by a cool climate with a mean annual temperature of 13°C, with substantial differences between day and night temperatures (23°C minimum temperature and 37°C maximum temperature) and, with 600 mm of annual rainfall, less rainfall compared to the Shandong wine regions in China. Vineyards are often established on steep slopes and therefore vines are not covered during the winter months. Two red wine grape cultivars of Vitis vinifera L., viz. Cabernet Sauvignon (CS) and Merlot (ME), were selected in 2014. The vineyard sites were spread over three altitudes (2 282 m, 2 435 m and 2 608 m). Cabernet Sauvignon and ME grapevines were planted in a same vineyard at 2 282 m and 2 435 m respectively. One CS vineyard was located at 2 608 m. All grapevines were established in 2006, planted in east-west row orientations and trained to a vertical trellis with Guyot pruning. The vines were planted with a 2.0 m spacing between rows and a 1.0 m spacing between vines. Shoot thinning was applied to adjust the grape yield of each vineyard to a similar level of about 10 tons per hectare.

Climate data

Climate data were collected from the automatic meteorological stations (TRM-ZS3, Yangguang Company Ltd., Jinzhou, China). One station was positioned in the centre of the vineyard at 2 282 m, 2 435 m and 2 608 m. The average annual values of climatic parameters were calculated based on the daily data.

Winemaking process

Grapes from each vineyard were manually harvested at commercial maturity in 2014. The total soluble solid content of grapes from the ME vineyard at 2 282 m, the CS vineyard at 2 282 m, the ME vineyard at 2 435 m, the CS vineyard at 2 435 m and the CS vineyard at 2 600 m was 20.4°Brix, 21.2°Brix, 20.7°Brix, 21.4°Brix and 21.6°Brix, respectively. The grape bunches were destemmed and crushed with an experimental destemmer-crusher and transferred to 50 L stainless steel containers. Sulphur dioxide (60 mg/L) was added to the must. Saccharomyces cerevisiae (RC 212, Lallemand, Danstar Ferment AG, Switzerland) active dry yeast (0.2 g/L) was inoculated to initiate fermentation, which lasted for seven to eight days at 25 to 28°C. Density control and temperature were monitored daily during fermentation. The contents of the tank were homogenised once a day to dissolve the cap into the wine. Wines were transferred to another tank after fermentation for three weeks of cold treatment at 4°C, after which the wines were bottled. Chemical analyses of the wines were carried out two months after bottling. Wines of each sampling vineyard were produced in three replications.

Analyses of general oenological parameters

The general parameters of wines, including alcohol content (AC), titratable acidity (TA), volatile acidity (VA), pH, reducing sugars (RS) and colour intensity (CI) were analysed according to the official methods established by the OIV (2014).

Determination of phenolic content

Total phenolic content (TPC)

The TPC was measured using the Folin-Ciocalteu method (Singleton & Rossi, 1965). In brief, 5.99 mL of distilled water, 0.1 mL of wine sample and 0.2 mL of Folin-Ciocalteu reagent were added to a test tube in sequence. Sodium carbonate solution (2 mL, 10%, w/v) was added to the tube after the contents had been reacting for 5 min. The mixture was placed in a dark room to react at room temperature for 120 min. Absorbance was measured at 765 nm. The results were expressed as gallic acid equivalents (GAE) in mg/L.

Total monomeric anthocyanin content (TMAC)

The TMAC was determined using the pH differential method (Wrolstad, 1976). Wine samples were diluted 20-fold with a buffer at pH 1.0 and pH 4.5, respectively. Absorbance was measured at 510 nm and 700 nm in both the pH 1.0 and pH 4.5 buffers and calculated by the following equation: A = (A₅₁₀- A₇₀₀)_pH1.0 - (A₅₁₀ - A₇₀₀)_pH4.5. The TMAC (expressed as cyanidin-3-O-glucoside equivalent, C3GE) was calculated using the equation: TMA content (mg/L of C3GE) = (A χ MW χ DF χ 1000)/(ε χ 1), where A is the absorbance, MW is the molecular weight of cyanidin-3-O-glucoside (449 g mol/L), DF is the dilution factor, and ε is the molar extinction coefficient of cyanidin-3-O-glucoside (29 600).

Tannin content

Tannin content was determined by protein precipitation using the method of Harbertson et al. (2003). Tannins were precipitated from wines by adding bovine serum albumin solution of 1 g/L, followed by centrifugation, collection and resuspension of the precipitate in an alkaline triethanolamine/ sodium dodecyl sulphate buffer. Background absorbance was determined at 510 nm, and tannin absorbance was measured after adding a FeCl₃ solution (2.7 g/L). The results for quantification were compared to a (+)-catechin standard and reported as (+)-catechin equivalents (CE) in mg/L.

Content of anthocyanin composition

Wine anthocyanin composition was analysed by using the reversed-phase HPLC method as described by Lamuela-Raventós and Waterhouse (1994), with some modifications. Twenty microlitres of wine samples were directly injected after filtration using 0.45 μm organic filters. The chromatograph system (Shimadzu, Japan) was equipped with a LC-20AT pump, a SIL-20A autosampler, a SPD-M20A diode array detector (Shimadzu, Japan), and a CTO-20A column oven. A SEP-PAK C18 (250 χ 4.6 mm i.d., 5 μm) (Waters, USA) was used and the column temperature was 30°C. The detection wavelength was 520 nm. Solvent A was 10% (v/v) formic acid in water and solvent B was acetonitrile:solvent A (80:20, v/v). These solvents were passed through a 0.20 μm filter. The gradient was as follows: 0 to 45 min, 10% to 60% B; 45 to 46 min, 60% to 100%; 46 to 50 min, 100% to 10% B; 50 to 55 min, 10% to 10% B, with a flow rate of 1.0 mL/min. All analyses were performed in three replicates. External calibration was performed using a malvidin-3-O-glucoside standard. All other anthocyanins were quantified using this calibration curve and reported as malvidin-3-O-glucoside equivalents in mg/L.

HPLC analysis of individual phenolic compounds

The HPLC analysis was performed using a LC-20A HPLC system (Shimadzu, Kyoto, Japan) with a Hibar RT LiChrospher RP-C18 column (250 χ 4.0 mm, 5 μm) purchased from Merck, Darmstadt, Germany, based on the method of Cheng (2008). Solvents A and B were composed of 2% (v/v) acetic acid in water and 100% acetonitrile respectively. The following gradient was optimised after preliminary experiments: 0 to 15 min, 3 to 6% B; 15 to 35 min, 6 to 15% B; 35 to 55 min, 15 to 30% B; 55 to 65 min, 30 to 30% B; 65 to 80 min, 30 to 0% B. The column temperature was 30°C, and the flow rate was 1.0 mL/min. The photodiode array detector scanned from 200 nm to 400 nm. The analysis for each sample was performed in triplicate.

The identification of phenolic compounds was based on a comparison of their retention time with those obtained from authentic standards and spiked samples with the standard solutions. Quantification was performed using the external standard method. A six-point calibration curve was used for each standard.

Measurement of antioxidant activity

DPPH free radical elimination activity

The DPPH was determined by the method of Brand-Williams et al. (1995). The wine sample and DPPH methanolic solution were mixed and kept in a dark room for 30 min. The absorbance of the reaction mixture was measured at 517 nm. The results were expressed as trolox equivalents (TE) in μmo1/L.

Cupric-reducing antioxidant capacity (CUPRAC)

The CUPRAC was assayed based on the method of Apak et al. (2004). In brief, wine samples were diluted 10-fold with distilled water. The dilutions were mixed with CuSO₄, neocuproine and distilled water. Absorbance was measured at 450 nm after 30 min. The results were expressed as TE in μmo1/L.

Sensory evaluation

The sensory profiles of the ME and CS wines originating from different altitudes were analysed in the wine sensory laboratory, College of Oenology, Northwest A&F University. The sensory panel was composed of15 trained wine panellists (eight males and seven females, 21 to 45 years of age) who major in oenology. The panellists included graduate students and teachers and were chosen according to their experience in wine sensory analysis training programmes. All had attended a wine-tasting training class once a week for three months.

The evaluation consisted of describing the visual aspect, aroma, taste and harmony, which accounted for 15, 30, 44 and 11 scores respectively (Li, 2006). Wine samples (30 mL) were presented in standard ISO wine glasses (ISO 3591:1977) covered with plastic lids. Wines were coded randomly and were presented to the panel arbitrarily. Wine samples were evaluated in triplicate. Water and unsalted crackers were provided as palate cleansers and all samples were expectorated.

Statistical analysis

All the analyses were performed in triplicate. Data were expressed as the mean ± standard deviation values and analysed by one-way ANOVA with a Tukey post-hoc test at 5% using SPSS 19.0 software (SPSS, Chicago, IL). A two-tailed Pearson's correlation test was conducted to determine the correlations between the phenolic content and antioxidant activity of the wines. Principal component analysis (PCA) was used to explore the contribution of each parameter of phenolic characteristics and antioxidant activity to the clustering among the wines originating from different altitudes. The least absolute shrinkage and selection operator (LASSO) method was performed using Matlab R2010b software (Mathworks, Natick, MA) for regression analysis between environmental factors and phenolic characteristics and the antioxidant activity of the wines.

 

RESULTS AND DISCUSSION

Soil properties and climate parameters of different altitudes and vineyards

The terroir, consisting mainly of climate, soil and geomorphology properties, was one of the factors affecting wine characteristics. The soil properties of the sampling vineyards at different altitudes are presented in Table 1. The same soil texture and similar chemical properties were established in the different soil samples originating from the different vineyards. Table 2 shows the main annual climate parameters in 2014. Mean annual temperature, annual maximum temperature, annual minimum temperature, effective accumulative temperature and active accumulated temperature parameters decreased with increasing altitude. Average temperature differences between daytime and night time increased as altitude increased, up to 13.0°C at 2 608 m. An increase in rainfall and sunshine hours was concomitant with an increase in altitude. The maximum wind speed and minimum relative humidity were determined at 2 435 m.

Oenological parameters

The general composition of five wine samples at three different altitudes are described in Table 3. No significant differences in the alcohol content (AC) of ME and CS wines were detected with increasing altitude. Higher AC was determined for CS wines compared to ME wines at the same altitude. The colour intensity (CI) of the wines increased with an increase in altitude and differences in CI were also measured between ME and CS wines. The CS wines had a higher CI value than the ME wines at 2 282 m and 2 435 m. The CI of the wine was strongly related to the anthocyanin content, which varied by grape cultivar and was strongly affected by climatic conditions (Buzo et al., 2014). The increased radiation, especially UV radiation, usually observed at higher altitudes would benefit the synthesis of anthocyanins in grape skin (Zhang et al., 2012; Berli et al., 2015), which could be ascribed to the higher CI observed in wines at higher altitudes.

Phenolic content and antioxidant capacity of wines at different altitudes

Phenolic compound concentrations are one of the most important factors affecting the wine quality because of their contribution to colour, mouthfeel and the multiple biological effects of antioxidant activity. The phenolic content and antioxidant activity of the wines at different altitudes are presented in Table 4. The total phenolic content (TPC) of ME and CS wines increased significantly from 2 282 m to 2 435 m. The CS wine made from grapes originating at 2 608 m showed the highest TPC value, which was significantly higher than that of the other wine samples.

The total monomeric anthocyanin content (TMAC) of the wine increased markedly with altitude in the ME and CS wines produced in the higher altitude region. This was in agreement with the results for Touriga Francesca wines, in which anthocyanin content increased with increasing altitude from 150 m to 250 m (Mateus et al., 2002), as observed for the CS grapes (Liang et al., 2014). Anthocyanins originally synthesised in the skin of red grapes after véraison are responsible for the colour and stability of wine. Sunlight, especially UV radiation, is one of the major environmental factors generally suggested to affect anthocyanin biosynthesis by prolonging the activity of phenylalanine ammonia lyase (PAL), which is the key enzyme in the flavonoid biosynthesis pathway (Berli et al., 2015). Hence, the exposure of grape bunches to sunlight could increase the accumulation of total anthocyanins in the grape skin (Spayd et al., 2002). The sunshine hours usually increased with altitude, as shown in Table 2. Furthermore, the intensity of UV radiation would increase with increasing altitude in the mountainous region (Zhang et al., 2005). The CS wines showed a higher anthocyanin content than ME wines at 2 282 m and 2 435 m, respectively. The anthocyanins in the wine were extracted from grape berries during fermentation and maceration. Varietal genetic characteristics and the extended harvest time of CS grapes might be beneficial to anthocyanin accumulation and contribute to their high content in wine under the same maceration and fermentation conditions (Jiang & Zhang, 2012). Furthermore, the average temperature difference between daytime and night time was found to increase from 2 282 m to 2 608 m in the present study (Table 2). The temperature difference between daytime and night time has an essential impact on grape sugar accumulation, which could be coupled with the conversion of anthocyanidin into anthocyanins in grapes (Liang et al., 2014). Therefore, grapes at higher altitudes subjected to major differences in temperature between day and night could explain the high anthocyanin content observed in the wines (Yamane et al., 2006).

Tannins in red wine are mostly composed of proanthocyanidins, also known as condensed tannins. Condensed tannins are polymeric compounds made up of flavan-3-ol subunits, which contribute mainly to the bitterness and astringency of wine, and to colour stability in combination with anthocyanins (Vidal et al., 2002). No significant variation in tannin content was observed for the ME wines between 2 282 m and 2 435 m. However, the tannin content increased with altitude for CS wines, up to a maximum of 975 mg/L CE at 2 608 m. Moreover, there were significant differences between ME and CS wines originating from 2 435 m. The tannins in wine are extracted primarily from the seeds and skins during the vinification process and their content in wine depends mainly on the total content in the skins and seeds. Tannin content also varies between grape cultivars (Mateus et al., 2001). Hence, the differences in tannin content between CS and ME wines could be attributed to differences in grape cultivar and altitude.

The chemical diversity of antioxidants makes it difficult to separate and quantify antioxidants in a wine matrix. Therefore, the total level of antioxidant activity is usually measured. A single antioxidant test for the examination of multifunctional antioxidants is insufficient; therefore, more than one method is used to provide adequate information on the antioxidant properties of phenolics (Lutz et al., 2011). To characterise the antioxidant potential of wines made from grapes harvested at different altitudes, two antioxidant assays, viz. DPPH and CUPRAC, were performed. The same variation in altitude was determined for the two antioxidant parameters in this study. The DPPH and CUPRAC values increased significantly with altitude for the ME and CS wines. A maximum value of 5 184 μmo1/L TE for DPPH and 16 886 μmo1/L TE for CUPRAC was determined in the CS wine at 2 608 m. However, there was no clear difference between the antioxidant parameters of ME and CS wines at 2 282 m and 2 435 m.

Phenolics are characterised by polyphenolic structures containing numerous double bonds and hydroxyl groups that can donate electrons through resonance to stabilise free radicals, and thus act as powerful antioxidants to protect plants against oxidative stress (Machlin & Bendich, 1987). Hence, a positive correlation may be found between the phenolic composition and the antioxidant activity of wines. The correlation coefficient between the phenolic content and the antioxidant activity of wines in the high-altitude region is presented in Table 5. The TPC, TMAC and tannin contents observed were positively and significantly correlated with the results of DPPH and CUPRAC. This is in agreement with the findings of previous research (Jiang et al., 2011; Meng et al., 2012).

 

 

Content of wine anthocyanin composition

Five monomeric anthocyanins, namely delphinidin-3-O-monoglucoside (Dp), cyanidin-3-O-monoglucoside (Cy), petunidin-3-O-monoglucoside (Pt), peonidin-3-O-monoglucoside (Pn), and malvidin-3-O-monoglucoside (Mv), were determined (Table 6). Mv was the most abundant in the CS and ME wines. This is in agreement with previous reports by Bouzas-Cid et al. (2016) and González-Neve et al. (2016). Furthermore, significant differences in Mv content were observed among the wines originating from different altitudes and grape cultivars. ME wines originating from 2 435 m showed a higher content of five monomeric anthocyanins compared to the wines originating from 2 282 m. As for the CS wines, the Dp, Cy and Mv content increased significantly with increasing altitudes. There was no notable difference for Pt and Pn content between CS wines originating from 2 282 m and 2 435 m.

Anthocyanins in Vitis vinifera L. cultivars are glycosylated derivatives (He et al., 2012). The monomeric anthocyanins in red wines originate mainly from the grape skins. Environmental factors could affect the biosynthesis and accumulation of each anthocyanin in the skin (Liang et al., 2014). A higher altitude generally corresponds to more intense sunlight, lower temperature, greater temperature difference between day and night, and more extreme environmental conditions (Mateus et al., 2001), all of which are important factors affecting the production of anthocyanins. Furthermore, the effects of altitude on the different monomeric anthocyanins in CS grapes were also different in high-altitude areas (Xing et al., 2015).

Changes in concentration of individual phenolic compounds in wine originating from different altitudes

Individual phenolic compounds in wine were measured (Table 7) to further understand the phenolic characteristics of the ME and CS wines produced at different altitudes. An increase in the sum of individual phenolic compound concentrations was shown with an increase in altitude for ME and CS wines. There also was a clear difference in phenolic compound concentrations between ME and CS wines. CS wines originating from 2 282 m and 2 435 m showed a higher total content of individual phenolic compounds compared to ME wines. The gallic acid and salicylic acid content contributed mainly to the sum of individual non-flavonoid phenolics in the wine from the two cultivars at any altitude. Gallic acid is one of the hydroxybenzoic acids present in wine and usually shows the highest concentration. It not only originates from the grape, but is also formed through the hydrolysis of hydrolysable and condensed tannins, i.e. gallic acid esters of flavan-3-ols (Dewick & Haslam, 1969). However, no significant changes in gallic acid concentration were determined in wine from the different altitudes and grape cultivars in the current study. Salicylic acid in the wine increased markedly with altitude for both the ME and CS wines; CS wines originating from 2 282 m and 2 435 m showed a higher content compared to ME wines. Salicylic acid is a secondary metabolite in plants and is stimulated by certain abiotic stresses such as increased UV radiation (Cohen & Kennedy, 2010). Hence, grapes at higher altitude might synthesise more salicylic acid for protection against UV radiation, which could explain the higher salicylic acid content observed in wine produced from grapes at the higher altitude sites. The syringic acid content in ME and CS wines increased with increasing altitude, up to a maximum of 9.61 mg/L for CS wine originating from 2 608 m. Two hydroxycinnamic acids were detected in the wines. A higher caffeic acid content was found in ME and CS wines with increasing altitude. However, no significant differences in ferulic acid content were found between wines originating from 2 282 m and 2 435 m. The traws-resveratrol content in the CS wines increased with increasing altitude, although there were no differences between ME wines originating from 2 282 m to 2 435 m. This is in agreement with the results from CS wines produced in the Loess Plateau region (Jiang et al., 2015).

The sum of individual flavonoids contributed the most of the total phenolic content at any altitude, ranging from 60.72% to 66.60%. Catechin, which is one of the common subunits of proanthocyanidins, was the predominant phenolic monomer compound in ME and CS wines. It increased significantly with an increase in altitude for the two grape cultivars, up to a concentration of 40.75 mg/L for CS wine at 2 608 m. Epicatechin, the epimer of (+)-catechin, showed the same trend with altitude for both wines. Furthermore, CS wines showed higher (+)-catechin and (-)-epicatechin contents than ME wines from the same altitude. This is in agreement with work by Li et al. (2011), who found that CS wines from different regions in China were higher in (+)-catechin and (-)-epicatechin contents when made from grapes grown at an altitude of 1 214 m compared with those grown at 1 036 m in the western regions. The terroir effect might explain the differences because of the great variation in climate, for example the temperature difference between daytime and night time and the increased annual sunshine between these two regions (Liang et al., 2014). However, the sampled vineyards for the present study were located in the same wine-producing region with similar soil properties. Hence, the flavan-3-ols in grapes, including (+)-catechin and (-)-epicatechin, might be very sensitive to environmental factors, even though the climatic parameters varied little among the three altitudes.

Flavonols are generally considered to act as UV protectants and free-radical scavengers (Downey et al., 2003). Hence, the flavonol content of grapes increases with an increase in sun/light exposure in the fruiting zone (Spayd et al., 2002). Quercetin derivatives such as rutin, including quercetin and kaempferol, comprised most of the flavonol content of ME and CS wines at the different altitudes. The rutin content of ME wines increased significantly from 2 282 m to 2 435 m, but no increase in the same compounds was observed in CS wine. However, the rutin content of CS wines at 2 608 m was notably higher than that at other altitudes. The quercetin content showed no significant increase with altitude in ME wines. The quercetin content in CS wines increased from 20.39 mg/L to a maximum of 21.98 mg/L with an increase in altitude from 2 282 m to 2 608 m. The quercetin content in grape berries usually increases in response to increased exposure to UV radiation in the fruiting zone (Azuma et al., 2012), resulting in high levels of quercetin in wines made from grapes originating from higher altitude vineyards.

Projection of wines on the first two principal components responsible for 70.82% showed a separation mainly according to grape cultivar and vineyard altitude (Fig. 1A). Furthermore, wines from the three altitudes were clearly separated from each other. Cabernet Sauvignon wines showed more separation between the three altitudes than ME wines. Merlot wines at 2 282 m and 2 435 m were located in the positive part of PC2, while CS wines at these two altitudes formed two groups in the negative part of PC2. Cabernet Sauvignon wines at 2 608 m grouped together and were located in the upper positive part of PC1, separate from all the other wines.

The PCA results of the variables used for the characterisation of the wine samples are displayed in the first two principal components and are presented in Fig. 1B. It is notable that component 1 was responsible for 70.82% of the cumulative variance and was mainly associated with the phenolic content and antioxidant capacity and prevailed in the positive part of PC1. Component 2 was responsible for 17.34% of the cumulative variance and was associated with kaempferol and is present in the negative part of PC1. Anthocyanins and tannins are two of the most important flavonoid groups in wine because of their contribution to wine colour, astringency and ageing potential. The TMAC, monomeric anthocyanins and condensed tannin monomers, including (+)-catechin and (-)-epicatechin, were located in the positive part of PC1 and showed a high factor loading with PC1. Hence, wines located in the right half of the diagram were associated with anthocyanins and tannins and those in the left half were related to both varietal character and altitude.

Sensory analysis

The average scores of each wine originating from different altitudes are presented in the global sensory evaluation (Table 8). Higher scores indicated improved attributes of the wine. Cabernet Sauvignon wines originating from 2 282 m had higher scores than ME wines with respect to taste quality and harmony indicators. Cabernet Sauvignon wines originating from 2 435 m showed higher scores for taste intensity compared to ME wines. Furthermore, CS wines produced at 2 608 m were characterised by improved aroma intensity, aroma quality, taste purity, taste intensity, taste quality and harmony compared to the other four wines. The total score for ME wines increased with increasing altitude. Cabernet Sauvignon wine produced at 2 608 m was described as "very good", with a score of 84.1. The four other wine samples belong to the "good" level. This was in agreement with work done by King et al. (2014), who found that Malbec wines made from grapes originating from higher altitudes were considered more complex than those from vineyards at a lower altitude (King et al., 2014). Cabernet Sauvignon wines from higher altitudes have also been found to possess clearer "red fruits" and "jam" aromas (Falcäo et al., 2007).

Regression analysis of terroir parameters, phenolic characteristics and antioxidant activity of red wines

LASSO is a regression analysis method that performs both variable selection and regularisation in order to enhance the prediction accuracy and interpretability of the statistical model it produces (Tibshirani, 1996). LASSO involves penalising the absolute size of the regression coefficients. The larger the penalty applied, the further the estimates are shrunk towards zero, which means the variable would be excluded from the independent variable pool. Moreover, all the independent variables could be analysed simultaneously, which is different from the traditional analysis using the step-by-step method.

Factors such as altitude and climatic conditions can have an impact on the phenolic characteristics and antioxidant activity of red wines. However, the separate contribution of each factor to the phenolic characteristics of wine should be clarified. Hence, 11 independent variables, including altitude and 10 climate parameters, as well as 21 dependent variables, comprising phenolic characteristics and antioxidant properties, were used to model the regression relationship among them by LASSO analyses. Typical trace plot coefficients as fitted by LASSO are presented in Fig. 2.

The regression coefficients between the dependent and independent variables were determined in this study. The number of dependent variables correlating positively with the independent variable is a primary factor while the independent variables are optimised. Moreover, the regression coefficients with a higher cumulative value should be considered simultaneously (Tibshirani, 1996). Six independent variables, namely altitude, annual minimum temperature, relative humidity, rainfall, sunshine hours and wind speed, showed a positive correlation with the dependent variables based on the accumulative value of the coefficients. The highest accumulative value of the coefficients was determined for sunshine hours, which was associated with six dependent variables. Rainfall as an independent variable was correlated with only two dependent variables, although a high cumulative value of the regression coefficient was observed. A more effective regression model was obtained for altitude, because altitude was positively correlated with 10 dependent variables and showed a relatively high cumulative value of coefficients. Hence, altitude might act as a more effective independent variable and play an important role in its effect on phenolic characteristics and the antioxidant activity of red wines in the high-altitude region.

 

CONCLUSIONS

The results of this study suggest that total phenolic, total flavonoid, and total anthocyanin content of ME and CS wines increased with altitude in high-altitude areas. Malvidin-3-O-monoglucoside was the primary monomeric anthocyanins and increased with increasing altitude. CS wines originating from higher altitude showed richer tannins. A greater antioxidant capacity was reported for ME and CS wines made from grapes originating from higher altitude vineyards. Salicylic acid, syringic acid, caffeic acid, (+)-catechin, (-)-epicatechin, and the sum of individual phenolic compounds in wines increased with altitude. The scores of sensory evaluation for ME wines increased with higher altitude. Highest score was determined for CS wine originating from 2608 m. PCA suggested a clear association of wines with grape cultivar and vineyard altitude. It was found that altitude followed by sunshine hours made the greatest contribution to the differences in phenolic characteristics and antioxidant activity of red wines at different sites in a high-altitude region.

 

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Submitted for publication: October 2016
Accepted for publication: March 2017

 

 

Acknowledgements: This study was supported by the National Natural Science Foundation of China (31401819), the China Agriculture Research System for the Grape Industry (CARS-30-zp-09), the Special Fund for Agro-scientific Research in the Public Interest (201203035), and the Key Science and Technology Programme of Shaanxi Province, China (2014K01-08-01)
* Corresponding author: E-mail address: liuxu@nwsuaf.edu.cn [Tel.: +86-29-87092107; Fax: +86-29-87092107]