High-Performance Liquid Chromatography Determination of Red Wine

Article pubs.acs.org/JAFC

High-Performance Liquid Chromatography Determination of Red Wine Tannin Stickiness Matthew R. Revelette, Jennifer A. Barak, and James A. Kennedy* Department of Viticulture and Enology, California State University, 2360 East Barstow Avenue, MS VR89, Fresno, California 93740-8003, United States ABSTRACT: Red wine astringency is generally considered to be the sensory result of salivary protein precipitation following tannin−salivary protein interaction and/or tannin adhering to the oral mucosa. Astringency in red wine is often described using qualitative terms, such as hard and soft. Differences in qualitative description are thought to be due in part to the tannin structure. Tannin chemistry contributions to qualitative description have been shown to correlate with the enthalpy of interaction between tannin and a hydrophobic surface. On the basis of these findings, a method was developed that enabled the routine determination of the thermodynamics of the tannin interaction with a hydrophobic surface (polystyrene divinylbenzene) for tannins in red wine following direct injection. The optimized analytical method monitored elution at four different column temperatures (25−40 °C, in 5 °C increments), had a 20 min run time, and was monitored at 280 nm. The results of this study confirm that the calculated thermodynamics of the interaction are intensive and, therefore, provide specific thermodynamic information. Variation in the enthalpy of interaction between tannin and a hydrophobic surface (tannin stickiness) is a unique, concentration-independent analytical parameter. The method, in addition to providing information on tannin stickiness, provides the tannin concentration. KEYWORDS: proanthocyanidins, tannin, grapes, wine, enthalpy, interaction, astringency, stickiness

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vivo.8,9,16−22 The tannin structure variation and corresponding variation in stickiness upon interaction with salivary protein and oral mucosa have been well-documented. Given this, there is a need to develop methodologies that can be used to routinely measure tannin stickiness. Despite the numerous analytical methods available for measuring tannin in the wine world, most focus is on measuring concentration.23−25 The concept of measuring tannin stickiness in relation to wine astringency quality is new. Stickiness in this case is defined as the observed variation in the enthalpy of interaction between tannin and a hydrophobic surface. Using this definition, the tannin structure variation related to grape and wine production has been shown to affect stickiness and is related to anecdotal perception of red wine astringency quality.9 It is of interest to develop an analytical approach, such that tannin stickiness in red wine can be routinely and objectively measured, so that its utility can be determined. The purpose of this study was to develop a reversed-phase HPLC method to measure red wine tannin stickiness following direct injection.

INTRODUCTION Tannins are polyphenolic molecules that are abundant in plants.1 In red wine, tannins are mostly grape-derived,2,3 are extracted during fermentation/maceration,4,5 and are considered essential because of their astringent qualities. 6,7 Astringency is generally considered to be a result of salivary protein precipitation following salivary protein−tannin interaction and/or tannin adhesion to the oral mucosa.6 In red wine, astringency has a range of qualitative sensory descriptors, including terms that range from soft to hard and from ripe to unripe.6,7 On the basis of isothermal titration calorimetry (ITC), the initial interaction between tannin and poly-L-proline is dominated by hydrophobic interaction and variations in the enthalpy of the interaction are consistent with anecdotal observations related to variation in red wine astringency quality.8 Using these findings as a basis, subsequent research has transferred the instrument of measurement from ITC to reversed-phase high-performance liquid chromatography (HPLC), with consistent findings observed.9 Molecular recognition and interaction have importance in the physical and biological world. The chemistry of protein folding and aggregation, bacterial adhesion and its prevention, and pharmaceutical development often consider molecular interaction.10−13 In polymer science, polymer length and branching and associated effects on molecular interaction are of fundamental importance.14 Molecular interactions described in terms of adhesiveness or stickiness are often used to describe the extent of intermolecular noncovalent interaction.15 In these fields and others, the chemistry that gives rise to molecular stickiness is fundamental to understanding molecular activity. For tannins, variations in the tannin structure have been shown to influence intermolecular interaction in vitro as well as in © 2014 American Chemical Society

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MATERIALS AND METHODS

Chemicals. All solvents used in this study were HPLC-grade. Acetone, glacial acetic acid, acetonitrile, L-(+)-ascorbic acid, hydrochloric acid, methanol, phosphoric acid, and sodium acetate anhydrous were purchased from VWR International (Radnor, PA). Phloroglucinol and (−)-epicatechin were purchased from Sigma-Aldrich (St. Louis, MO). All water was purified using an Ultrapure purification system (Evoqua Corporation, Alpharetta, GA). Received: Revised: Accepted: Published: 6626

April 8, 2014 June 6, 2014 June 24, 2014 June 24, 2014 dx.doi.org/10.1021/jf501666z | J. Agric. Food Chem. 2014, 62, 6626−6631

Journal of Agricultural and Food Chemistry

Article

method was modified from previously published methods,9,28 with modifications designed to reduce run time. To determine tannin stickiness, samples were run at four column temperatures (25−40 °C, in 5 °C increments). A blank sample (water) was run at all temperatures and was subtracted from sample signals to eliminate background absorbance. All temperatures were converted into kelvin to calculate thermodynamic parameters in SI units. Tannin elution was monitored at 280 nm. Calculations. Chromatograms were integrated as described previously, with minor modifications.9 Briefly and following baseline subtraction of a water blank, a baseline at 0 mAU across the sample peak area was performed. Following this, the resulting peak area was split at 16.8 min, corresponding to the elution of tanninP. The tannin eluting prior to 16.8 min was separated from overlapping resolved material by tangent skimming. From integration results, the alternative retention factor was calculated as follows:

Instrumentation. An Agilent 1260 HPLC (Santa Clara, CA) was used for all chromatographic analyses. The HPLC system consisted of a model G1311B pump and degasser, G1329B autosampler, G1315C DAD/UV−vis detector, G1316A column heater, and a system controller. Data were processed using ChemStation for LC 3D systems, version B.04.03. Samples and Sample Preparation. Tannins from grape (preveraison Cabernet Sauvignon berry skin tissue) as well as wine (2010 and 2012 Cabernet Sauvignon) were used for standard additions and thermodynamic standards. Extraction, purification, and separation into different molecular mass fractions were accomplished using a previously described procedure.26 The resultant tannin fractions used in the current study have been previously characterized.9 Whenever powdered tannin standards were dissolved in solution, solutions were subsequently allowed to equilibrate for at least 24 h before use. This allowed for stable thermodynamic measurements to be obtained. Wines used for analysis were sourced from either various wineries within California or a wine merchant in the case of the Bordeaux. Wines were sterile-filtered with 13 mm, 0.45 μm polytetrafluoroethylene (PTFE) filters (Grace Davison Discovery Sciences, Waukegan, IL) before injection. For 2012 and 2013 wines, analyses took place approximately 4 months after pressing. For all other wines, only vintage information was available. Phloroglucinolysis for the Mean Degree of Polymerization (mDP) Determination. Phloroglucinolysis, according to the method by Kennedy and Jones,27 using a modified HPLC method,26 was used to determine mDP of isolated tannins. Serial Dilutions. To determine the impact of tannin concentration on tannin stickiness, high and low average molecular mass wine tannin fractions (mDP of 15.4 and 2.9, respectively), previously shown to vary in their retention thermodynamics (ΔH = −12 088 and −2328 J/mol, respectively9), were dissolved (10 g/L) in an aqueous buffer solution (pH 4.6) containing methanol (15%, v/v), 40 mM sodium acetate, and 20 mM HCl and were serially diluted with this buffer, to 0.31 g/L. Solutions were analyzed using the optimized HPLC method. In addition to serially diluted tannin samples, a wine (2012 California north coast Cabernet Sauvignon press wine) was serially diluted 5 times with the same aqueous buffer solution prior to analysis (experiment conducted 10 months after pressing). Standard Additions. Purified wine tannin fractions varying in enthalpy of interaction were added to a 2012 California north coast Petit Verdot wine to examine their impact on wine stickiness. Lowmolecular-weight (mDP = 2.9) and high-molecular-weight (mDP = 22.7) tannins were added in half, equivalent, and double the experimentally measured tannin concentration of the wine. Gelatin Fining. Fining experiments were carried out using Inocolle Extra N1 gelatin (15 000 MM, Scott Laboratories, Petaluma, CA). Gelatin (20 mg) was rehydrated in water (500 mg/L) according to manufacturer guidelines. A total of 2 mg of wine tannin (mDP = 15.4) was rehydrated in an aqueous solution (pH 4.6, 2395 mg/L) containing methanol (15%, v/v), 40 mM sodium acetate, and 20 mM HCl and allowed to equilibrate for 48 h before analysis. After 48 h, the gelatin solution was serially diluted with water from its initial concentration to a minimum concentration of 7.8 mg/L. Gelatincontaining solutions were then mixed with the tannin solution in a 1:1 ratio in microcentrifuge tubes. After mixing, samples were centrifuged at 735g for 2 min in an Eppendorf 5415 C centrifuge (Hamburg, Germany) and the supernatant was then analyzed. Optimized HPLC Method. The optimized HPLC method for measuring tannin stickiness used a polystyrene divinylbenzene reversed-phase column (PLRP-S, 2.1 × 50 mm, 100 Å, 3 μm, Agilent Technologies, Santa Clara, CA) protected with a guard column (PRP1, 3 × 8 mm, Hamilton Company, Reno, NV). The mobile phases consisted of 1.5% (w/w) 85% H3PO4 in water (153 mM, mobile phase A) and 20% (v/v) mobile phase A in acetonitrile (B) with a flow rate of 0.30 mL/min. The linear gradient was as follows: time in min (% B), 0 (14%), 10.0 (34%), 10.0−13.5 (34%), 15.3 (70%), 15.3−17.0 (70%), and 17.0−20.0 (14%). This

kalt =

tanninT tanninT − tanninP

(1) 9

where, as previously described, tanninT is the total tannin peak area and tanninP is the partial tannin peak area. The alternative retention factor is related to thermodynamics of interaction according to the following equation:

ln kalt = −

ΔH ° ΔS° + + ln Ø RT R

(2)

where ΔH° and ΔS° are the specific enthalpy and entropy of interaction, respectively, R is the gas constant, T is temperature, and Ø is the mobile phase ratio. The specific enthalpy was calculated by plotting the natural logarithm (ln) of kalt versus the reciprocal of the column temperature in kelvin at each of the four temperatures (i.e., van’t Hoff plot in Figure 1). The specific enthalpy of interaction (tannin stickiness) was calculated from the slope of the best fit line (slope equivalent to −ΔH°/R).

Figure 1. van’t Hoff plot of the alternative retention factor (kalt) versus the reciprocal temperature (in K) for the retention of red wine tannins on polystyrene divinylbenzene, following the standard addition of a tannins (mDP = 15.4), with best fit equations shown. The concentration of tannin (on the basis of tanninT) was reported in (−)-epicatechin equivalents using an (−)-epicatechin standard and was determined at 303 K (30 °C). In addition to (−)-epicatechin, a grape skin or wine tannin standard (mDP = 39.0 and 15.4, respectively) served as a stickiness standard. Stickiness standards were prepared by dissolving grape skin or wine tannins in an aqueous solution containing methanol (15%, v/v), 40 mM sodium acetate, and 20 mM HCl. Stickiness standards did not vary by more than 10% across all experiments. Given that the variation in experimental results 6627

dx.doi.org/10.1021/jf501666z | J. Agric. Food Chem. 2014, 62, 6626−6631


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was less than 5% suggests that the stickiness standards changed over time.

tannin stickiness in red wine by reversed-phase HPLC following direct injection. The analytical method that forms the basis of this method has been described previously.9 In that study, a modified analytical method was used to measure the stickiness of various grape and wine tannins. In the present study, alterations to the HPLC method were made to increase throughput and remove potential baseline interference. To increase throughput, the flow rate was increased to 0.3 mL/min and the number of temperatures monitored was reduced to four. The binary mobile phase gradient remained identical to the previous method but with alterations made to compensate for the increased flow rate. With regard to a reduction in the number of temperature points, the elimination of higher temperatures resulted in a linear van′t Hoff plot. Following baseline subtraction, integration, and kalt determination, van’t Hoff plots from data collected at 298−313 K had good correlation (Figure 1; r2 = 0.99). The slope of the line from this relation was used to determine the enthalpy of interaction as described previously.29 The enthalpy of interaction is referred to as tannin stickiness. Tannin Stickiness Is an Intensive Property. Unlike the tannin concentration and wine matrix components that change with dilution, compositional variation should not be affected by dilution. Therefore, tannin stickiness, if related to composition/ conformation, should be an intensive thermodynamic property. To explore this, solutions of tannin (in wine and wine-like systems) were serially diluted. In addition, a wine was subjected to a standard addition of tannins with varying degrees of stickiness. With regard to serial dilutions, the stickiness of serially diluted solutions containing high-molecular-weight (mDP = 15.4) and low-molecular-weight (mDP = 2.9) wine tannin fractions was determined. Tannin concentrations in the six serially diluted solutions varied from 10 to 0.31 g/L. The stickiness for the high-molecular-weight samples was −11 928 ± 257 J/mol (±standard deviation, 2.16% error). Similarly, the stickiness for low-molecular-weight tannin solutions was −2261 ± 93 J/mol (±standard deviation, 4.11% error). A 2012 Cabernet Sauvignon press wine, which was serially diluted 5 times, had a stickiness value of −4905 ± 152 J/mol (±standard deviation, 3.10% error). To provide additional evidence that the value for stickiness was a feature of tannin contained in the sample and not related to the concentration, a 2012 Petit Verdot wine was subjected to the standard addition of two wine tannins varying in stickiness (Figure 2). To the base wine (stickiness = −3894 J/mol) was added two tannins (either −2038 or −7283 J/mol). As expected from the results obtained from serially diluted systems, the experimental results were consistent with calculated results and were a function of the fractional contribution of the base wine tannin and the added tannin. In so far as tannin stickiness is sensorially relevant to wine, these results suggest that the addition of exogenous tannins will modify tannin perception beyond concentration alone. Furthermore, the results from standard addition and serial dilution collectively indicate that the analytical method provided good reproducibility and confirmed that stickiness is an intensive property. Effect of Wine Production Practice: The Case of Gelatin Fining. The results above indicate that stickiness is a property of tannin and is concentration-independent, and thus, stickiness can be distinguished from the concentration. This

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RESULTS AND DISCUSSION The mouthfeel quality of red wine is an important management consideration, and tannins, which contribute astringency, are

Figure 2. Effect of the standard addition of (●) low-mDP and (○) high-mDP tannins to a base wine, with theoretical equations representing the fractional contribution of base wine tannin as well as added tannin shown.

Table 1. Effect of Gelatin Addition on the Tannin Concentration and Stickiness tannin

a

added gelatin (mg/L) 0 3.9 7.8 15.6 31.3 62.5 125.0 250.0

concentration (mg/L)

percentage remaining (%)

stickiness (−ΔH, J/mol)

percentage remaining (%)

1198a 1158 1107 1067 963 807 566 286

100.0 96.7 92.4 89.1 80.4 67.4 47.3 23.9

11927 11517 11054 10894 10104 8336 5417 1997

100.0 96.6 92.7 91.3 84.7 69.9 45.4 16.7

The initial concentration was adjusted 50% to adjust for 1:1 dilution.

central to it. In addition to tannin, the matrix of red wine (e.g., ethanol, residual sugar, and polysaccharides) is known to have an impact on astringency perception. With regard to tannins and in addition to concentration, its composition (e.g., subunit composition, size distribution, pigment incorporation, and oxidation) is considered to affect tannin activity or its stickiness. Stickiness is defined as the observed variation in the enthalpy of interaction between tannin and a hydrophobic surface. In this case, the hydrophobic surface is polystyrene divinylbenzene, and previous studies have found that stickiness variation with the tannin structure is similar to variations observed when using ITC to monitor the tannin interaction with poly-L-proline.8,9 In contrast to analytical methodology available for measuring the tannin concentration, methodology for the measurement of stickiness is lacking. Stickiness measurements based on the thermodynamics of the tannin interaction with a hydrophobic surface suggest that this analytical approach has utility. The current study was initiated to develop a method for measuring 6628



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Table 2. Concentration and Stickiness of Tannins in Bordeaux Varietal Red Wines and Across Multiple Vintages tannina

percentage of the maximum value (%)c

rankingb

vintage

regiond

variety

concentration (mg/L)

stickiness (−ΔH, J/mol)

concentration

stickiness

concentration

stickiness

2013

north coast, CA

2012

central coast, CA

2011 2010

central coast, CA central coast, CA

2009

central coast, CA

Cabernet Franc Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Merlot Petit Verdot Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Merlot Cabernet Sauvignon blend blend Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon Merlot Merlot Merlot Cabernet Sauvignon Cabernet Sauvignon Cabernet Sauvignon blend blend blend blend blend blend blend blend blend blend blend blend

4099 4636 5179 4349 4030 4311 4326 4555 4415 4641 3181 3160 2533 2850 2930 3088 2871 4357 2760 4130 4083 3372 2582 4289 1326 3741 3634 4013 3294 4322 4010 4182 4719 5033 4881 4521 3977 4524 4606 4422 4893 4432 4827 4954

2648 3327 3447 2718 2361 2333 2759 2673 2311 2164 2782 2122 2921 2322 1896 2025 1896 2384 2993 2379 2396 2774 2792 1838 2010 2423 2941 2860 3020 2139 3823 4170 3497 3514 3292 3382 3029 3302 3397 3570 3552 2792 3507 3631

25 9 1 18 27 21 19 11 16 8 35 36 43 40 38 37 39 17 41 24 26 33 42 22 44 31 32 28 34 20 29 23 7 2 5 13 30 12 10 15 4 14 6 3

28 12 9 26 33 34 25 27 36 37 23 39 19 35 43 40 42 31 17 32 30 24 22 44 41 29 18 20 16 38 2 1 8 6 14 11 15 13 10 4 5 21 7 3

79 90 100 84 78 83 84 88 85 90 61 61 49 55 57 60 55 84 53 80 79 65 50 83 26 72 70 77 64 83 77 81 91 97 94 87 77 87 89 85 94 86 93 96

64 80 83 65 57 56 66 64 55 52 67 51 70 56 45 49 45 57 72 57 57 67 67 44 48 58 71 69 72 51 92 100 84 84 79 81 73 79 81 86 85 67 84 87

Bordeaux, France

a

Tannin concentration and stickiness, with stickiness equivalent to the negative enthalpy of interaction. bRanking of the tannin concentration and stickiness, with lower numbers ranked higher in concentration and stickiness. cPercent of the highest tannin concentration and stickiness. dRegion where fruit was produced (CA = California).

method can therefore be used to explore the role of grape and wine production practices on stickiness. Previous studies indicate that tannin size, plant tissue origin (seed versus skin), and wine age will affect stickiness.8,9 The effects of a myriad of additional factors on stickiness are intriguing candidates for study (e.g., fruit maturity, vine water status, wine microoxygenation, and pigment incorporation). To provide an example of utility, the effect of gelatin fining on the wine tannin concentration and stickiness was explored.

The protein interaction with tannin is considered the basis of astringency and is used for the removal of excess tannin in wine. From previous studies on the interaction of tannins with macromolecules,30−34 it would be expected that gelatin should preferentially remove larger molecular mass tannins and with potential effects on composition. It would be predicted therefore that as the concentration of tannin declined with gelatin addition, so too would tannin stickiness. To explore this, solutions of serially diluted gelatin were combined with a prepared wine tannin solution (initial stickiness of −11 927 J/ 6629



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Notes

mol). As expected, the results indicated that, as the concentration of tannin declined, so too did its stickiness (Table 1). Previous research has shown that gelatin additions to wines decrease the tannin concentration,32 but the current result suggests that the strength of the hydrophobic tannin− protein interaction plays an important role in perceived astringency in gelatin-fined wines as well. Analysis of Wines. To provide some insight into stickiness variation, a set of commercial wines was analyzed. The wines presented in Table 2 were Bordeaux varietals and were sourced from commercial vineyards or a wine shop. Wines varied in age and region. The concentration of tannin and stickiness for the wines varied considerably. A linear regression across the set of wines analyzed yielded a line with a positive slope (e.g., an observed increase in stickiness with the concentration) although with a low correlation (r2 = 0.24). It is particularly interesting to note that a range of stickiness values is possible at any given tannin concentration. A ranking of wines in order of increasing concentration and stickiness confirmed that varied concentration−stickiness relationships existed. There was a greater variation in the tannin concentration in this set of wines, where the lowest concentration observed was at 26% of the maximum. This is in contrast to the least sticky wine being 44% of the maximum stickiness. It is evident that a much larger data set needs to be gathered, with associated sensory analysis, to understand the extent to which stickiness varies and is relevant to wine production, management, and appreciation. Implications. Historically, tannin structure−activity relationships in red wine have been limited by our understanding of the tannin structure. Considerable effort has been invested into red wine tannin structure elucidation,35−37 yet paradoxically, our knowledge as a proportion of material presence remains limited. Added to this, understanding the role that various grape and wine production practices have on sensory properties is often thought to be limited by our understanding of how process affects structure. Finally, understanding activity of tannins has been dependent upon sensory studies. In addition to the complexity of using humans as analytical instruments, wine matrix and tannin concentration often confound sensory evaluation.38,39 The development of an objective method for measuring tannin activity provides a new avenue for exploring the effect of various processes on activity without first understanding structure and, therefore, has increased our ability to explore process−activity relationships. In addition, this method provides researchers with a method for measuring the effect of known structure variation on activity without the use of human subjects. To fully understand the new information provided by this analytical method, the relationship between tannin stickiness and sensory perception needs to be investigated. Although data to date are consistent with tannin stickiness being related to anecdotal differences in astringency quality between grape and wine tannins, direct relationships between stickiness and perception need to be explored.

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The authors declare no competing financial interest.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-559-278-7179. Fax: +1-559-278-4795. E-mail: [email protected]. Funding

We thank the American Vineyard Foundation for project funding. 6630



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High-Performance Liquid Chromatography Determination of Red Wine

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