Spectrophotometric Analysis of Phenolic ... - ACS Publications

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Spectrophotometric Analysis of Phenolic Compounds in Grapes and Wines Jose Luis Aleixandre-Tudo,*,†,§ Astrid Buica,§ Helene Nieuwoudt,⊗ Jose Luis Aleixandre,† and Wessel du Toit§ †

Departamento de Tecnologia de Alimentos, Universidad Politecnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain Department of Viticulture and Oenology and ⊗Institute for Wine Biotechnology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa

§

ABSTRACT: Phenolic compounds are of crucial importance for red wine color and mouthfeel attributes. A large number of enzymatic and chemical reactions involving phenolic compounds take place during winemaking and aging. Despite the large number of published analytical methods for phenolic analyses, the values obtained may vary considerably. In addition, the existing scientific knowledge needs to be updated, but also critically evaluated and simplified for newcomers and wine industry partners. The most used and widely cited spectrophotometric methods for grape and wine phenolic analysis were identified through a bibliometric search using the Science Citation Index-Expanded (SCIE) database accessed through the Web of Science (WOS) platform from Thompson Reuters. The selection of spectrophotometry was based on its ease of use as a routine analytical technique. On the basis of the number of citations, as well as the advantages and disadvantages reported, the modified Somers assay appears as a multistep, simple, and robust procedure that provides a good estimation of the state of the anthocyanins equilibria. Precipitation methods for total tannin levels have also been identified as preferred protocols for these types of compounds. Good reported correlations between methods (methylcellulose precipitable vs bovine serum albumin) and between these and perceived red wine astringency, in combination with the adaptation to high-throughput format, make them suitable for routine analysis. The bovine serum albumin tannin assay also allows for the estimation of the anthocyanins content with the measurement of small and large polymeric pigments. Finally, the measurement of wine color using the CIELab space approach is also suggested as the protocol of choice as it provides good insight into the wine’s color properties. KEYWORDS: spectrophotometry, phenolics, anthocyanins, tannins, color and wine industry partners.9 Relevant publications have reviewed the existing knowledge on phenolic compounds chemistry10 and the available analytical methods for their quantification.11 However, these papers mainly focus on advanced analytical techniques used to elucidate structural features, characterize, or individually quantify the phenolic compounds.12−15 As far as we know there is no review in the literature on the spectrophotometric analytical procedures used to quantify phenolic compounds in grapes and wines. Moreover, the bibliometric search included in this study provides good insights into the identification of the most commonly accepted methods for phenolic analysis. Additionally, the values reported in this review provide useful information that might help in the interpretation of the results obtained. The aim of this review is thus to investigate the most widely used methods for wine phenolic analysis. With the objective of reducing the gap between research science and industry applications, only the spectrophotometric analyses of different phenolic compounds as well as wine color are considered here. More advanced and complex techniques such as HPLC have not been included because their use as routine analyses is currently not common. The importance that these analytical

1. INTRODUCTION Wine is mainly composed of water and ethanol (95%). Other components such as glycerol, organic acids, carbohydrates, minerals, volatile compounds, and phenolic compounds, among others, represent 0 yellow, b* < 0 blue), and its derived magnitudes chroma (C*) and tone (H*). It also specifies that the color of a wine can be described by using the attributes chromatism (the intensity of the wine color), luminosity (if the wine is more or less luminous), and tonality (color itself based on its red, yellow, green, or blue components). The method requires the spectral measurements of the wine samples over the visible region of the electromagnetic spectrum. The trichromatic components result from the integration through the visible range of the spectrum. The calculations simulate the measurement that will be performed with a spectrophotometer with illuminant D65 and observer at 10°. Moreover, the main advantage relies on the possibility to calculate the total colorimetric difference between two colors (ΔE*), which makes the method suitable for the direct comparison of wines or among treatments in the case of a more detailed research study. The minimal color difference between two samples detectable by the human eye through a wine tasting glass has been fixed as ΔE* = 2.7.83 Additionally, to make the method more user-friendly, a few studies have reported simplifications of the method. PerezCaballero et al.35 proposed a new approach applying statistical characteristic vector analysis (CVA) that allows for the

approximation is thus unavoidable if the aim is to keep the method suitable for routine applications. The estimation of the total anthocyanin content by bisulfite bleaching can lead to an overestimation, as some of the polymeric pigments are also affected by the SO2 bleaching effect. The differences are bigger in young wines and decreased during aging, as the more complex polymeric pigments are probably more resistant to bleaching by SO2. This reasoning may also apply to the values obtained by the bisulfite bleaching method reported by Ribéreau-Gayon and Stonestreet18 previously discussed in this section. Moreover, in a study reported by Versari et al.,77 the polymeric pigment color estimated using the Somers assay was correlated with the HPLC total polymeric pigments. Although a strong correlation was observed (r2 = 0.992), a positive value of the intercept (0.3 AU) also indicated an overestimation of the Somers polymeric pigment color. The above-mentioned findings were also reported by Harbertson et al.,46 but in this case for small and large polymeric pigments quantified with the Adams− Harbertson assay (also known as BSA tannin assay). With regard to the validation parameters, the average coefficients of variation for all parameters were 1.3 and 2.6% for 10 mL and HTP formats, showing both formats’ almost perfect correlations.27 3.2.5. Copigmentation Assay. Copigmentation is based on weak associations among red-colored compounds (anthocyanins) and other mostly colorless substances including a wide range of phenolic compounds. The effect of these associations is observed by an increase in the absorbance at the λvis‑max (hyperchromic effect) and a displacement of the λvis‑max (bathochromic shift) toward higher wavelengths (bluish coloration).78 The extent of the copigmentation effect depends on many factors including the nature of the anthocyanin and the cofactor, the presence of metals, pH, and ethanol.41,79 The color due to copigmentation is thought to account for up to 50% in young red wines, and a possible contribution of this aggregation toward the appearance of more stable polymeric pigments during aging has also been suggested.40 For additional information on the copigmentation phenomena, refer to published literature.40,80,81 The copigmentation assay developed by Boulton39,40 has also received intense attention, as it is the only anthocyanin method that includes the measurement of the color that corresponds to copigmented anthocyanins. The method relies on the dissociation of the copigmented forms after dilution at constant pH to ensure the accurate direct comparison of the wine color. However, the adjustment of the pH individually for each sample increases the time of analysis and makes it very tedious if a large number of samples need to be analyzed. To avoid the individual adjustment of the pH, the measurement of the fraction of the color due to copigmention, polymerization, and free anthocyanins could be done by using the absorbance values obtained from the modified Somers assay reported by Mercurio et al.27 In this case the pH adjustment is achieved by a simple dilution with a buffer solution at pH 3.4. The final concentrations of SO2 (3.75%) and acetaldehyde (1%) in the samples are higher than those used in Boulton’s assay (0.4 and 0.1% for SO2 and acetaldehyde, respectively). However, the wine is only diluted 10 times in the modified Somers assay, whereas Boulton indicated in his protocol that in some cases dilutions higher than 20 times are required. The effect of wine dilution on the copigmentation measurements as 4015

DOI: 10.1021/acs.jafc.7b01724 J. Agric. Food Chem. 2017, 65, 4009−4026

Review

Journal of Agricultural and Food Chemistry

conditions) might explain why astringency perception softens during wine aging.95 Further information on flavan-3-ols, tannins, and the interactions of tannins with salivary proteins is available elsewhere89,90,96,97 3.4.1. Acid Hydrolysis Assay. A number of different chemical properties have been exploited to quantify the total content of proanthocyanidins or tannins in grape extracts and wines. The quantification of this specific group of phenolics has been challenging researchers over the past years, as these compounds are of very diverse nature, a fact that has a strong impact on the ability of the analytical principles to estimate their concentrations. The first proposed spectrophotometric method, based on acid hydrolysis, was developed by Ribéreau-Gayon and Stonestreet18 and relies on the ability of the proanthcyanidins to be transformed into carbocations, which are later partially converted into anthocyanidins when heated in acid medium (Bate−Smith reaction). Briefly, 1 mL of wine diluted 50 times is added to a test tube with 0.5 mL of distilled water and 3 mL of 12 N HCl (treatment sample). The test tube is then heated to 100 °C in a water bath for 30 min. After the sample has been cooled, 0.5 mL of pure EtOH is added. Additionally, a control samples is also prepared but with no heating. The anthocyanidins are then quantified at 550 nm using the ε of the cyanidin-3-glucoside corrected to give grams per liter.

calculation of the coordinates by the measurement of only four wavelengths (A450 nm, A520 nm, A570 nm, and A630 nm). The authors reported a set of equations that allow for the calculations of the tristimulus coordinates in red, rosé, and white wines. The measurement error using this approach was reported to be lower than 2.7 CIELab units. Besides, Perez-Magariño et al.84 studied different regression methods to establish mathematical models that allow for the calculation of the CIELAB coordinates from the traditional color measurements at 420, 520, and 620 nm. Accurate models were reported for the parameters L*, a*, and C* using only the absorbance at 520 nm as an independent variable. However, to predict the parameter b*, the inclusion in the model of the other two original wavelength measurements (A420 nm and A620 nm) becomes a necessity. 3.4. Measurement of Proanthocyanins or Condensed Tannins. Tannins are complex high molecular weight compounds that may be classified into hydrolyzable and condensed tannins.12 Proanthocyanidins are grape-derived polymeric flavanols (condensed tannins) based on flavan-3-ol subunits linked through the 4- and 8-positions or through the 4- and 6-positions. Hydrolyzable tannins are derivatives of gallic acid85 and are derived mainly from wood products. Five catechins or flavan-3-ols are found in grapes: (+)-catechin, (−)-epicatechin, (+)-gallocatechin, (−)-epigallocatechin, and (−)-epicatechin-3-O-gallate. Flavan-3-ols can be found in every part of the berry, but in higher concentrations in seeds and skins and in lower concentrations in the flesh.86 The diversity of proanthocyandins allows for the existence of a wide range of molecules ranging from dimers to large polymers.87,88 The ability of flavan-3-ols and tannins to elicit astringency and bitterness has been mainly attributed to molecular size, stereochemistry, galloylation, substitutions, and concentrations.89 It is widely accepted that flavan-3-ol monomers as well as low molecular weight tannins are more bitter,9,51 whereas oligomers and polymers are described as more astringent than bitter.90 This may explain why less polymerized seed tannins with higher presence of galloyl substitutions are found to be more astringent and bitter than skin tannins with higher degrees of polymerization.89 It is also generally accepted that the astringency perception softens during the aging of wines.4,91 This fact, initially attributed to a decrease in the total proanthocyanidin content in solution, is nowadays ascribed to a number of different phenomena. Starting with the assumption that the ability of the tannin molecules to elicit astringency increases with tannin size,4 other phenomena thus need to also be playing a role. First of all, cleavage reactions leading to lower size tannins have been proposed by Vidal et al.92 to lead to less astringent tannins. On the other hand, molecule conformational arrangements have also been mentioned as a possible reason. The larger tannin molecules may become too bulky, leading to steric hindrance, and their interactions with salivary protein is not possible.93 It is also well documented that anthocyanins indirectly play a role in wine astringency. Their combination with tannins leads to a reduction of the ability of the new polymeric pigment to interact with salivary proteins, thereby reducing astringency, an aspect related possibly to conformational arrangement situations. This is easily identified when tannin−anthocyanin combinations through ethyl linkages are favored by the addition of small amounts of oxygen, the so-called micro-oxygenation.94 The latter phenomena in combination with the precipitation of tannin material from the wine matrix (due to insolubility

tannins (g/L) = (treated samples − control samples) × 19.33

(14)

This method that appears to be the preferred method, being also reported as the most widely method used in Chile,98 shows a number of limitations. First, it gives only an estimation of the total content as it does not take into account the structure of the present tannin pool or the other possible components that interfere in the reaction.25 Moreover, the tannin concentration in wine is often overestimated, and it is possible to observe an increase over time that does not correspond to an increase in tannin content. Moreover, proanthocyanidin values higher than the total phenolic levels observed for the same sample have also been reported.99 On the other hand, the advantages of this method rely on its ease of implementation and reliability.98 3.4.2. BSA Tannin Assay. The ability of tannins to precipitate through interactions with proteins has also been exploited as quantification principle. The method initially reported by Hagerman and Butler43 and later adapted to wine by Adams and Harbertson44 and further improved by Harbertson et al.45,46 relies on the interactions of tannins with bovine serum albumin, precipitation from the wine matrix, and later redissolution in a buffer solution. The absorbance at 510 nm is recorded after a color reaction with ferric chloride. An improvement on the method was later reported by Jensen et al.100 The authors studied the effect of wine dilution on the reliability of the assay and noted that concentrated and very diluted samples consistently underestimated the tannin content, results that were explained by insufficient precipitation agents and by a precipitation threshold, respectively. It was also proposed that the dilutions need to provide an absorbance tannin response between 0.3 and 0.75 absorbance values under the method conditions. It was later reported that an increase in the sugar levels proportionally decreased the protein precipitation of the tannin material and precipitable polymeric pigment. The increased solubility of the tannin−protein complex in the grape extracts or wine samples was proposed as the mechanism to explain the 4016


Review

Journal of Agricultural and Food Chemistry results observed.101 Despite this, the authors concluded that, at sugar levels normally found in wines, the effect on the assay is negligible. The same authors published an improvement of the assay by the reformulation of the resuspension buffer.102 A urea−triethanolamine (TEA) buffer at pH 7 or 8 instead of the traditional sodium dodecyl sulfate (SDS)−TEA buffer at pH 9.4 provided an increased yield of the quantified tannin. The high background signal and the oxidation of some of the phenolic compounds by the former buffer are thus minimized in the new approach. One milliliter of BSA solution (1 mg BSA/mL dissolved in a 0.2 M acetic acid and 0.17 M NaCl buffer adjusted to pH 4.9) was added to a 0.5 mL of properly diluted wine. The dilution was made with a model wine solution containing a 12% v/v EtOH and 5 g/L tartaric acid adjusted to pH 3. After 15 min, the supernatant obtained after centrifugation is discarded. The pellet is then washed twice with 1 mL of the solution adjusted at pH 4.9. After the addition of 0.25 mL of the same solution, the sample is centrifuged for 1 min. The supernatant is again discarded, and the pellet is redissolved by adding 0.875 mL of a TEA buffer at pH 7 or 8. After measurement of the A510 nmbackground, 0.125 mL of 10 mM ferric chloride in 10 mM aqueous HCl was added to the sample. The A510 nmFeCl was measured after 10 min. The total tannin content is calculated as follows and expressed as milligrams per liter of catechin equivalents:

tannins, IRP, anthocyanins, SPP, and LPP). Additionally, 5.3% CV average values were observed for the conventional format.103 Harbertson et al.104 also investigated the type of tannins that are precipitated by the protein and the ability of various sizes of tannin molecules as well as some mixtures to precipitate BSA. The results showed an increased efficacy of condensed tannin to precipitate protein with catechin-based tannins from trimers to octamers and also the inability of monomers and dimers to precipitate with BSA protein. 3.4.3. MCP Tannin Assay. The use of other polymers to precipitate tannins has also been utilized in the methyl cellulose precipitable tannin assay (MCP). The assay developed and validated by Sarneckis et al.20 relies on polymer−tannin interactions in the presence of ammonium sulfate, leading to an insoluble complex that is separated by centrifugation. As for the BSA tannin assay, a high-throughput format using a 96-well plate reader has also been validated by Mercurio et al.27 Both formats lead to a reduction of sample reagents and volumes, thus making their implementation easier as a routine analytical methodology. However, further research investigating the nature of the tannin material that is precipitated by the polymer needs to be conducted. Briefly a treatment sample is prepared by adding 300 μL of a MCP solution (0.04% w/v; 1500 cP viscosity at 2%) to 25 μL of wine. After 2−3 min, 200 μL of a saturated solution of (NH4)2SO4 and 475 μL of distilled water are added. A control sample is also prepared, but with distilled water (775 μL) instead of MCP solution. After 10 min, the samples are centrifuged for 5 min, and the tannin content is obtained by comparing the A280 nmcontrol and A280 nmtreatment referenced to epicatechin equivalents. This precipitation method appears to be simpler than the BSA tannin assay as only two simple solutions that do not require pH adjustment are needed (MCP and the (NH4)2SO4 solutions). Validation parameters for the different versions (10 mL, 1 mL, and HTP) of the protocol were reported with 5.8 and 3.6% CV for wines and grape extracts, respectively. As shown above, precipitation-based methods exploit the ability of tannin material to interact with proteins or complex polymers that can be precipitated from solution. The methods rely on interactions similar to those involved in the astringency mechanism caused by interactions of tannins with human saliva proteins. It may thus be hypothesized that a positive correlation between the values and the astringency intensity of a sample exists. This was observed by several authors,98,105−107 and it can be accepted that the methods provide an estimation of the astringency intensity. However, astringency also depends on many other factors such as acidity, pH, sugar, and alcohol content, as well as the presence of other wine components.108−110 It has been extensively reported that the MCP assay provides 2−3-fold higher values than the BSA protein assay.22,102,106 Caceres-Mella et al.98 also found higher values, but in this case the differences were closer to 5-fold. As the authors did not give a detailed BSA tannin procedure, this result needs to be considered with caution, because it is not possible to ascertain if the reported modifications of the assay have been taken into account. In addition to this, strong positive correlations were also reported for BSA and MCP precipitation methodologies (r2 = 0.8).106,111 When the factors that could account for these differences were investigated, the pigmented polymers formed during fermentation were mentioned as a possible key factor to explain such a difference in absolute values.106 The two assays (MCP

tannins (mg/L) = A510 nm FeCl − 0.875 × A510 nm background (15)

The authors also reported an extension of the method that allows for the estimation of the total red pigments content as well as the small polymeric pigments (SPP) that do not precipitate with the protein and large polymeric pigments (LPP) that do precipitate with the BSA protein.46 The method uses SO2 as bleaching agent and can be performed together with the tannin assay. Briefly 1 mL of acetic acid/NaCl buffer was added to 0.5 mL of diluted wine, and the A520 nm was recorded (A). Then 80 μL of a 0.36 M K2S2O5 was added to 1 mL of the previous mixture. The A1520 nmK2S2O5 is recorded after 10 min. Finally, to 1 mL of the discarded supernatant obtained after tannin protein precipitation and centrifugation is added 80 μL of a 0.36 M K2S2O5, and the A2520 nmK2S2O5 is measured after 10 min. The anthocyanin parameters are calculated as follows: monomeric anthocyanins = A520 nm − A1520 nm K 2S2O5

(16)

small polymeric pigments = A 2520 nm K 2S2O5

(17)

large polymeric pigments = A1520 nm K 2S2O5 − A 2520 nm K 2S2O (18)

The addition of large and small polymeric pigments (SPP + LPP) was found to strongly correlate with the polymeric pigments measured with the Somers and Evans assay. Moreover, Heredia et al.103 validated a comprehensive highthroughput format assay using a microplate reader and also proposed a new parameter, known as the iron reactive phenolics (IRP). Reproducibility results (10 replicates) were reported elsewhere as specified below.103 Due to the complexity of the tannin mixtures it is not possible to obtain true analytical standards. Classical validation methods are thus difficult to implement, this being why the available validation results focus mainly on repeatability tests. The HTP showed an average CV of 4.1% (including total 4017


Giusti and Wrolstad, 2001

pH differential + indices

Iland et al., 2000; RuizHernández, 2004; Mazza et al., 1999; Cliff et al., 2007

Giusti and Wrolstad, 2001

pH differential

HCl

Ribéreau Gayon and Stonestreet, 1965

bisulfite bleaching

Somers and Evans, 1979; Mercurio et al., 2007

Ribéreau Gayon et al., 1998, 2000, 2006

A280 nm

modified Somers

reference

Singleton and Rossi, 1965

protocol

Folin− Ciocalteu

measurements

4018

colored anthocyanins (total phenolics)

chemical age, chemical age 2, % ionization, total anthoycanins, color density, hue, SO2-resistant pigments, total phenolics

anthocyanins, color density, polymeric color, %polymeric color

anthocyanins

anthocyanins

total phenolics

total phenolics

samples

10

10 (×4)

10 (×3)

10 (×2)

10 (×2)

10

10

reagents

HCl

ethanol, tartaric acid, sodium metabisulfite, acetaldehyde, HCl

potassium chloride, sodium acetate, potassium metabisulfite, HCl (pH adjustment)

potassium chloride, sodium acetate, HCl (pH adjustment)

ethanol, SO3HNa, HCl

Folin−Ciocalteu reagent, sodium carbonate

solutions

sodium metabisulfite acetaldehyde HCl

buffer pH 3.4

potassium chloride sodium acetate bisulfite solution

potassium chloride sodium acetate

SO3HNa HCl

ethanol-HCl

sodium carbonate

components

2 (30 + 30) 2 (30 + 30)

yes (180)

yes (180)

2 (35 + 30) 2 (35 + 30)

3 (35 + 30 + 30) 2 (35 + 30)

yes (180)

yes (180)

2 (35 + 30) 2 (35 + 30)

yes (180)

pH

a

2 (35 + 30)

3 (30 + 30 + 30) 2 (35 + 30) 2 (30 + 30)

2 (35 + 30)

a

Table 2. Estimated Time Required for the Measurement of the Levels of Phenolic Compounds and Phenolic Parameters in the Identified Protocols

3 (290)

3 (×4) (1160)

3 (×3) (870)

3 (×2) (580)

5 (×2) (780)

3 (290)

4 (390)

pipetting

a

290

1620

1425

1070

1000

290

405

time of analysis (s)

5

87

54

33

37

5

37

total time (min)a

Journal of Agricultural and Food Chemistry Review


4019

CIElab coordinates (L*, a*, b*, C*, H*)

color density, hue (A420/A520)

tannins

total tannin, total red pigments, polymeric pigments (small + large)

total tannins

proanthocyanidins

measurements

10

10

10 (×2)

10 (×2)

10

10 (×2)

samples

ammonuim sulfate, MCP

acetic acid, sodium chloride, potasium bitartrate, ethanol, triethanolamine, sodium docedyl sulfate, ferric chloride, BSA protein, potassium metabisulfite

glacial acetic acid, sodium chloride, potassium bitartrate, ethanol triethanolamine, sodium dodecyl sulfate, ferric chloride, BSA protein

HCl + ethanol

reagents

MCP ammonium sulfate

FeCl3 BSA potassium metabisulfite

buffer C

buffer B

buffer A

FeCl3 BSA

buffer C

buffer B

buffer A

solutions

+ 30) + 30) + 30)

+ 30 +

+ 30 +

+ 30 +

+ 30) + 30)

+ 30 +

+ 30 +

+ 30 +

2 (35 + 30) 2 (35 + 30)

3 (35 30) 3 (35 30) 3 (35 30) 2 (35 2 (35 2 (35

3 (35 30) 3 (35 30) 3 (35 30) 2 (35 2 (35

components

a

yes (180)

yes (180)

yes (180)

yes (180)

yes (180)

yes (180)

pH

a

a

1 (120)

1 (120)

5 (390) 4 (340)

7 (490) 3 (290)

6 (440)

5 (×2) (780)

pipetting

In parentheses is the required time in seconds to pipet, prepare the solution, or adjust the pH. Total time includes the waiting, incubating, and time required for centrifugation.

CIE and OIV

CIELab

a

Glories, 194

color density

Harbertson et al., 2003

BSA + anthocyanins

Sarneckis et al., 2006

Harbertson et al., 2002

BSA

MCP

Ribéreau Gayon and Stonestreet, 1966

reference

acid hydrolysis

protocol

Table 2. continued

120

120

860

1800

1425

780

time of analysis (s)

2

2

32

75

55

43

total time (min)a

Journal of Agricultural and Food Chemistry Review


Review

Journal of Agricultural and Food Chemistry and BSA) differ in two regards: first, in the tannin precipitation step and, second, in the detection step. With regard to the precipitation step, the authors concluded that MCP is able to complex and precipitate all tannins and pigmented polymers, whereas BSA does not. It seems that the portion of the tannin not removed may be representative of SPP,46 which do not precipitate with BSA, although it remains unclear if the ability to precipitate these compounds is related only to size or if other properties have also an effect on tannin precipitation. Several authors have tested the MCP and BSA methods to confirm that the two assays were capable of precipitating and detecting the same amount of tannin material.106,112 To achieve this, the MCP−tannin complexes were redissolved and analyzed for iron-reactive tannins following the final detection step of the BSA tannin assay. The results showed that although the two assays show different abilities of the precipitants to bind with tannins in their precipitation step, the levels of the precipitated tannins were almost the same. Some other possible factors such as the interaction between the different chemicals in the matrices of the assays, which may modify spectral features of the phenolic compounds, or the influence of the chemicals used in both methods, have also been tested. The reported results showed slightly or no effect of these factors on the absorbance values.27,112 On the other hand, a modification of the BSA tannin assay by using a subtractive procedure, avoiding the ferric chloride step, has also been explored by Boulet et al.107 In this case the calculation was done as the absorbance difference between the control samples and the precipitated treatment samples at 280 nm. However, the main drawback of this proposed methodology relates to interferences caused by the BSA protein’s ability to also absorb UV light at 280 nm, which might introduce an analytical error. Besides, this condition does not apply to the MCP tannin assay as the methyl cellulose polymer does not absorb in the UV region and the measurement can thus be performed at a common maximum absorption peak at 280 nm.20 Further investigations are thus needed, as a full validation of the method reported by Boulet et al.,107 which may include direct comparisons with the original BSA assay and also with other precipitation-based methods, is currently not available in the literature. These results led us to consider that the primary differences between MCP and BSA tannin assays are caused by the different detection methods used (measurement at 280 nm versus colorimetric reaction) rather than the amount of tannins precipitated. Phenolic compounds show absorbance features in the UV region. These compounds together with the abovementioned polymeric pigments could partially account for these differences but again could not account for the 2−3-fold difference observed. The above reasoning led us to focus on the BSA tannin assay where two factors could account for the differences in absolute values. The first one relates to the ability of the ferric chloride to bind and react colorimetrically with tannins, and the second possible factor refers to the tannins solution absorbance maxima, which could not be at 510 nm and therefore an underestimation of tannin concentration would result. Because the values obtained from both methods cannot be directly compared, further work in this area is required with the aim to fully understand the observed differences in tannin ranges. In a study reported by Caceres-Mella et al.98 it was observed that neither precipitation method (BSA protein and MCP polymer) was able to quantify the proanthocyanidin content of rosé and white wines. It seems that the colorimetric methods

(acid hydrolysis and vanillin) are more sensitive to low concentrations of these compounds. Moreover, the acid hydrolysis method gave the highest results, which were up to 20 times higher than those reported for the BSA tannin assay. The results were on average only 2 times higher than those found for the vanillin assay, which is in accordance with the possible overestimation of tannin content reported for the vanillin assay.25 Weak correlations were also reported between the precipitation-based methods and the colorimetric assays, which may be due to different principles used for the estimation of the total tannin content in the samples.98 3.4.4. Other Methods for Tannin Analysis. A modification of the acid hydrolysis assay also known as the acid−butanol assay has been reported by Porter et al.47 and di Stefano and Cravero.48 The colorimetric method that exploits the reaction of proanthcyanidins with vanillin in acid medium was also reported by Sun et al.49 The method seems to be simple, specific, and sensible; however, the main drawback resides in the lack of reproducibility often reported. The authors made a few suggestions to ensure accuracy and reproducibility of the method, which include separation of proanthcyanidins from catechins, with separate measurements, use of absolute methanol, use of H2SO4 as reagent, elimination of interfering substances and anthocyanins, and use of specific standards. All of these requirements make the method time-consuming and therefore difficult for application as a routine analytical procedure. Despite this, the fractionation of the proanthocyanidin fraction adds valuable information, which may sometimes justify its selection.

4. COMPARISON OF ANALYTICAL PROCEDURES IN TERMS OF TIME OF ANALYSIS To investigate the time required to perform the described methods, a direct comparison between methodologies is reported in Table 2. The average time required to analyze 10 samples is obtained by taking into account the following assumptions: the time of analysis was calculated using a microplate reader spectrophotometer and a set of calibrated pipettes. The volumes were scaled down to minimize reagent and chemical usage. The analyses were all performed in test tubes or in 2 mL microcentrifuge tubes to avoid the use of volumetric flasks such as in the Folin−Ciocalteu assay. The tests were performed by an analyst with more than 5 years of experience in wine spectrophotometric analyses. To calculate the expected time of analysis, different volumes were pipetted into test tubes in triplicate. The average times of pipetting 200, 1000, and 5000 μL into test tubes from 10 samples stored in 15 mL screw-capped containers were 116 ± 0.6, 121 ± 3, and 131 ± 2.1 s, respectively. However, the times were decreased to 46 ± 1, 46 ± 2.1, and 55 ± 2.7 s for the same volumes as indicated above when a single solution was used (i.e., pipetting was done from a single solution to each test tube). To simplify the calculations, average times of 120 s when pipetting from centrifuge tubes and 50 s when pipetting from a solution were used to calculate the predicted time of analysis. The average time of weighing a reagent, adjusting a volumetric flask/cylinder, and adjusting the pH of a solution was also recorded (34 ± 10, 27 ± 1.5, and 177 ± 14 s, respectively). Again, to facilitate the calculation, average times of 35, 30, and 180 s were considered for the mentioned operations. The calculation of the pipetting time was estimated considering the addition of the samples into the test tubes, the incorporation of the specific solutions, and the pipetting of the samples into the 4020


Review


Table 3. Levels of Phenolic Compounds Reported in the Investigated Research Strategy (TITLE = (Wine* AND (Anthocyan* OR Phenol* OR (Tannin* OR Proanthocyan*) OR Color*)) protocol

N

average

low to mina

median to low

median

median to high

high to max

anthocyanins

bisulfite bleaching (g/L) Somers assay (g/L) pH differential (g/L) HCl (g/L)

499 160 223 193

0.37 0.38 0.32 0.3

0.02−0.18 0.01−0.14 0.01−0.08 0.04−0.15

0.18−0.31 0.14−0.23 0.08−0.24 0.15−0.28

0.31 0.23 0.24 0.28

0.31−0.47 0.23−0.58 0.24−0.48 0.3−0.43

0.47−2.24 0.58−1.24 0.48−1.19 0.43−1.14

tannins

acid hydrolysis (g/L) BSA assay (g/L) MCP assay (g/L)

521 202 218

2.29 0.42 1.34

0.44−1.63 0.00−0.2 0.06−0.62

1.63−2.1 0.2−0.34 0.62−1.34

2.1 0.34 1.34

2.1−2.78 0.34−0.58 1.34−1.85

2.78−9.69 0.58−1.9 1.85−3.53

total phenolics

TPI TPI (g/L) TPI white wines Folin−Ciocalteu (g/L) Folin−Ciocalteu index Folin white wines (g/L)

631 279 48 1120 113 267

48.3 1.82 0.19 1.89 50.5 0.33

0.75−36.9 0.12−1.04 0.11−0.13 0.00−1.31 13.26−36.8 0.04−0.2

36.90−46.9 1.04−1.59 0.13−0.17 1.31−1.88 36.8−50 0.2−0.26

46.9 1.59 0.17 1.88 50 0.26

46.9−58.35 1.59−2.23 0.17−0.21 1.88−2.45 50−66.13 0.26−0.34

57.35−95.7 2.23−5.74 0.21−0.56 2.45−5.9 66.13−85.8 0.34−1.88

color

color density

1403

11.2

0.04−7.3

7.3−10.6

10.6

10.6−13.7

13.7−37.2

a

Four equal-sized groups were made from the ordered values. For each of the reported methods the values can now be considered as minimum to low, low to median, median to high, and high to maximum.

based methods and the astringency perception makes the MCP tannins assay methodology most suitable for routine analysis. Finally, the analysis of the CIElab coordinates may also be implemented in combination with the traditional approach for the estimation of the wine color. However, the interpretation of the CIELab coordinates requires an understanding of the principle behind the approach, making its interpretation challenging, which might discourage winemakers or scientists from selecting this methodology. On the contrary, the color density is presented as an index that can be easily understood, and the method becomes therefore suitable for routine analysis in a commercial cellar.

microplate. Table 2 shows the number of parameters that can be obtained for each methodology, the reagents needed, and the number of solutions that need to be prepared, as well as if the solution needs to be pH adjusted or not. The number of samples was fixed to 10 samples to make the procedures comparable. On the basis of the described approach, the analysis of the TPI appears as the fastest method as only a dilution step is needed to obtain the total phenolic content of a sample (Table 2). With regard to the analysis of anthocyanins, the method that requires minimal time of analysis corresponds to the HCl assay, which involves only a dilution step. This method also offers the opportunity to calculate the total phenolic content of a sample by recording the absorbance value at 280 nm times the corresponding dilution factor. The bisulfite bleaching and the pH differential method were found to require almost the same predicted analytical time. The pH differential method has the advantage that the anthocyanins content is calculated on the basis of the absorption maxima of the anthocyanins for that specific sample, which might be different from the general 520 nm commonly used. On the other hand, the investigation of the required dilution in the case of having a diverse sample set with various levels of anthocyanins will increase the estimated time of analysis. Within the anthocyanins methods, the modified Somers assay estimated time appears to be longer than any other methodology. However, the large number of parameters, as well as the information extracted from this procedure, makes the method suitable and recommended for routine analysis. The BSA tannin assay appears to be the longest procedure for the total tannins analysis. Moreover, if the analysis of the anthocyanin fraction is also performed, an increase in the time of analysis should be considered. Despite this, the information obtained from this procedure in terms of the polymeric fraction, with the estimation of the small and large polymeric pigments, makes this method also interesting for certain applications. On the other hand, within the tannin analysis methods, the MCP tannin assay appeared as the method that requiref the shortest time of analysis. The fact that strong positive correlations have been reported for the precipitation-

5. LEVELS OF PHENOLIC COMPOUNDS FOUND IN THE LITERATURE Table 3 shows the number of values (N), average, minimum, median, maximum, and quartile values reported for the spectrophotometric methods identified in the records obtained from the search strategy. The data were divided into four equalsized groups compiled from the ordered reported values (quartiles) with the aim of classifying the concentrations into four different categories. For each of the reported methods the values can now be considered as minimum to low, low to median, median to high, and high to maximum. Only data reported in tabular format were included to avoid the inclusion of false values extracted from graphs. Additionally, only data reported for wine samples were included. Despite the suitability of the mentioned methodologies for the analysis of grape extracts, the fact that different extraction conditions (i.e., solvent, concentration, time, etc.) are often used to extract the phenolic compounds from the solid parts of the grape berries, prevented reporting data from grape samples. As can be observed in Table 3, the anthocyanin methods showed values in the same order of magnitude. The highest reported anthocyanin concentration (2.24 g/L) was observed for a young wine made with a Brazilian hybrid named ‘BRS violeta’ using the bisulfite bleaching method.113 With regard to tannins, the three reported approaches do not show the heterogeneity observed for the anthocyanins with the acid hydrolysis method 4021


Review


development of powerful mathematical approaches has been one of the major advances for the establishment of chemometrics and, together with important improvements in the available equipment, contributes to this application being highly successful. However, a small but important number of limitations still hinder the selection of this technique as the preferred analytical method for industry partners. The distrust generated by the lack of an overall understanding of the rationale behind the quantification principle and issues associated with instrument availability and suitability of applications are usually limiting factors. Additionally, a general model that may be used worldwide is currently not available. A limited number of attempts to test a model’s accuracy when samples from other countries are included135 seems to indicate that model accuracy is increased by increasing sample specificity; that is, a model built for a specific wine region should provide more accurate predictions than that created for the entire country. This situation requires further investigation and will significantly help in the process of making this technique the preferred analytical methodology. As spectrophotometric approaches have been found to be reliable and time effective and allow for the measurement of an increasing number of properties and analytes, a steady increase in the application of these techniques as the analytical option of choice is predicted for the future.

showing values much higher on average than the other two methodologies, as previously discussed (see previous section). The highest value reported corresponds to a 1-year-old wine made with the cultivar ‘Primitivo’ and subjected to délestage.114 With the BSA tannin assay, the highest tannin concentration was observed by Harbertson et al.115 in a Cabernet Sauvignon wine from Washington state. McRae et al.95 reported the highest MCP tannin level, which corresponded to a 1988 reserve Cabernet Sauvignon wine produced in Australia (3.53 g/L). In terms of total phenolic levels, both methods showed similar average levels. The highest concentration for the TPI was reported (5.37 g/L in gallic acid equivalents) for a 1-yearold wine made in the Fialhoza region in Portugal,116 whereas the highest Folin−Ciocalteu reported level (5.9 g/L) was found in the Brazilian wine region of the Sao Francisco valley in a Shiraz wine from 2005.117 Finally, the highest reported color intensity (37.2) was found in a 2005 Graciano wine from the la Rioja region in Spain.118

6. MEASUREMENT OF PHENOLIC COMPOUNDS USING SPECTROSCOPY WITH CHEMOMETRICS Different spectroscopy domains, including visible, near-infrared (NIR), and mid infrared (MIR), in combination with statistical multivariate data analysis have been applied to quantify seed, grape, and wine phenolics.119−125 The measurement of wine color components has also been investigated using a wide range of different techniques.126−128 Vis and NIR spectroscopy have also been used to quantify the phenolic content using portable devices129,130 and fiber optic probes.131 The monitoring of phenolic compounds during the red winemaking process was also evaluated by combining Fourier transform-mid infrared spectroscopy with chemometrics.132 Moreover, NIR hyperspectral imaging has been successfully applied to accurately predict the anthocyanin content in wine grapes during the ripening process.133,134 A number of calibrations have also been reported for the estimation of grape and wine phenolics using ultraviolet (UV) spectroscopy, which included sometimes the visible region. Three important wine-producing countries have reported methods for the analysis of wine phenolic composition. Dambergs et al.135 and Skogerson et al.136 in Australia, Aleixandre-Tudó et al.111 in South Africa, and Beaver and Harbertson137 in the United States reported models for the accurate prediction of some wine phenolic parameters, including total phenolics, anthocyanin, and tannin contents. Moreover, the measurement of the phenolic composition during the winemaking process by UV−vis spectroscopy combined with chemometrics was also reported.136 Additionally, efforts to commercialize the prediction models have also been conducted (e.g., the AWRI WineCloud or the WineXRay company). Accurate models to predict the general phenolic composition demonstrated that a close monitoring of the winemaking process is thus possible. The strength of the combination between spectroscopy and chemometrics results in a substantial reduction of the analysis time, as well as in the ability to measure several analytes simultaneously. This makes the technique suitable for both research and industry applications.6 Spectroscopy has been described as a nondestructive and reliable tool for the measurement of grape and wine phenolics and as a suitable technique for online monitoring during the winemaking process.138 Ease of use and the requirement of minimal or no sample preparation have also been reported.139,140 The

7. CONCLUDING REMARKS A number of analytical procedures have been identified on the basis of a bibliometric search study. A critical and intensive discussion of the most commonly reported methodologies is shown in this paper, on the basis of the citation records obtained. Referencing the analytical methods using the original reference (avoiding self-citation or low-frequency-cited references) may help researchers and analysts in the identification and selection of the methods. Moreover, it was confirmed that with minimal equipment, valuable chemical and physical information can be extracted from the spectrophotometric measurement of grape extracts and wines. A UV−vis spectrophotometer preferably with scan capacity (as the measurement of multiple wavelengths is sometimes necessary) is essential. A limited number of chemicals and reagents and general laboratory material are the only necessary requirements. With regard to personnel, it is important to note that a certain level of training is required to ensure accuracy. Brooks et al.141 showed the difficulty of a group of industry practitioners to competently execute well-established analyses such as the BSA tannins assay. The modification of the traditional Somers assay was shown to be a high-throughput, simple, reliable, and multistep protocol for the measurement of the anthocyanin equilibria. Additionally, information is also provided on the color components and total phenolic content. By making only four simple dilutions, a considerable number of phenolic indicators are obtained. The protocol also avoids the time-consuming pH adjustment step as this is adjusted by a dilution with a buffer solution. The measurements obtained after exposing the samples to buffer, SO2, and acetaldehyde dilutions may be used for the calculation of the color due to the copigmented anthocyanins, thus further increasing the value of the methodology. The modified Somers assay is thus proposed here as the method of choice because maximum information is obtained with minimum effort. 4022



■

Both protein and polymer tannin precipitation-based methods have been defined as simple and robust procedures and have also been adapted to high-throughput formats. Additionally, strong correlations between the tannin levels and the astringency intensities are also of relevant additional value. The MCP tannin assay is a subtractive method that also requires minimal effort. The total phenolic content might also be calculated from the control samples after its correction with the corresponding dilution factor. On the other hand, the BSA tannin assay, although a longer procedure, can be combined with SO2 additions to obtain additional anthocyanin-related parameters. This allows for the calculation of the total levels of anthocyanins as well as for the small (SPP) and large polymeric pigment (LPP) fractions. Additionally, the total iron reactive phenolics measurement was recently also included in the protocol. Moreover, and despite the fact that a full validation has not been yet reported in the literature, it is also worth mentioning that the same parameters (SPP and LPP) could possibly be calculated from the MCP assay. The physical evaluation of the wine color by the CIELab color space approach also appears to be a valid alternative. The use of some of the proposed simplified methods or the measurement of the visible spectral range in either absorbance or transmittance modes using a conventional spectrophotometer provides an important insight into wine color characteristics. The methodology also allows for a direct comparison of the color of samples. A color difference of 2.7 CIELab units has been established as the limit detectable by the human eye. Monitoring of the evolution of phenolic compounds during the winemaking process is becoming essential. The possibility of online and in-line measurements during wine fermentation and aging is thus a necessity. Spectroscopy combined with strong chemometrics appears to provide a suitable solution for process control and monitoring. The number of different spectroscopic techniques, instruments, and applications is currently experiencing a steady increase. However, the technology needs now to be transferred to the wine industry, and the benefits of these techniques need to be explained to winemakers. The estimation of the phenolic levels with the methods reviewed in this paper will thus in future be obtained through robust prediction models, rather than with the reference methods, but the interpretation and understanding of the obtained results will remain the same. Moreover, the inclusion of the phenolic analysis results in databases created for individual cellars, wine regions, or even countries, will help in the interpretation and understanding of the wine phenolic data. The comparison of the phenolic levels between cultivars, vintages, or regions will provide additional valuable information that could be effectively used by winemakers or scientists.

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Review

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Funding

We gratefully acknowledge the Vali+d program of the Conselleria de Educacio Cultura i Esport (Generalitat Valenciana) for financial support during the postdoctoral research collaboration between Stellenbosch University and Polytechnic University of Valencia. Notes

The authors declare no competing financial interest. 4023


Review

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