1.1. Chemicals

Deionised water was purified with a Milli-Q^® water system (Millipore, Bedford, MA, USA). Acetonitrile and methanol were of high-performance liquid chromatography (HPLC) grade and purchased from Sigma Aldrich (Saint Louis, USA), as were the following: (+)-catechin, malvidine-3-O-glucoside, gallic acid, Folin–Ciocalteu’s phenol (2N), sodium metabisulfite, sodium carbonate, phloroglucinol, L-ascorbic acid, hydrochloric acid (37 %), formic acid (98 %), 2,2’azobis(2-methylpropionamidine) dihydrochloride (AAPH, 97 %), disodium phosphate, sodium dihydrophosphate, fluorescein, trolox, polyvinylpolypyrrolidone (PVPP, 110 µm), sodium acetate, syringaldazine, 2,2-diphenyl-1-picrylhydrazyl (DPPH), sodium hydroxide and acetic acid.

1.2. Fruit sampling and Botrytis cinerea inoculation

Mature grapes (Vitis vinifera, cv. Merlot noir) were collected from several Saint-Émilion vineyards of the Union de Producteurs de Saint-Émilion (Saint-Émilion, south-west France) in 2017. The vineyards had been planted on sandy-gravely or chalky-clay soils with a density of 5,500 vines/ha. The vineyards had not been sprayed with anti-Botrytis synthetic fungicides during the season.

Healthy grape bunches (approximately 66 kg) were randomly selected and then inoculated as described hereafter. The inoculum originated from a 7-day-old culture in Petri dishes of the B. cinerea strain 213. This single-spore isolate was selected from a collection belonging to UMR SAVE, Bordeaux, since it is a highly virulent strain belonging to the transposa type (Martinez et al., 2003; Martinez et al., 2005; Martinez et al., 2008). Fruits were processed in 2-kg batches of grape berries. They were inoculated until the complete fruit surface was covered by spraying a mycelio-spore suspension at the rate of 1.4 x 10⁵ propagules/mL, with fungal propagules comprising a third of conidia and two thirds of mycelium fragments about 60-200 µm in length. To promote grey mould development, inoculated grape bunches were incubated in the dark at 13 °C for 16 days in a climatic phytotronic CONVIRON chamber (at 95-100 % relative humidity). Visual sorting of berries was then performed to remove undesirable rotten berries following other fungal development (e.g., Penicillium spp., identifiable via a blue-green colour). Botrytised grapes were used for micro-winemaking.

2.1. Vatting

Typical infected and symptomatic berries (rotten berries often totally covered by Botrytis mycelium and/or sporulation) were added to healthy berries with a proportion of either 20 or 50 % (ratios were based on number of infected berries rather than on their weight), thus ending up with 7-kg batches of grapes. A control with 100 % healthy grapes was also prepared. Every batch was mechanically crushed and destemmed, then collected in a 10-L aluminium tank. A 3 g/hL dose of SO₂ was added to the control tanks only (botrytisation rate 0 %) using a Bisulfite18 solution at 183 ± 3 g/L of SO₂ (Laffort, Bordeaux, France). Commercial tannins from Laffort and IOC were chosen for their various botanical origins and structures: grape seed (Gse), grape skin (Gsk), quebracho (Q), oak (O) and nutgall (N). In the tanks with botrytised grapes, tannins were added at 100 g/hL, or not at all to obtain controls (20 % and 50 % Botrytis controls). The various treatments are presented in Figure 1. There were 13 tanks (1 x 2 with a botrytisation rate of 0 %, 6 x 2 with a botrytisation rate of 20 % and 6 x 2 with a botrytisation rate of 50 %) in duplicate with about 5 L of musts in each.

2.2. Alcoholic fermentation

Saccharomyces cerevisiae yeasts (Zymaflore^®FX10, Laffort) were prepared by rehydration in warm water (37 °C) for 15 minutes. Rehydrated yeasts were applied to each fermentation tank in order to reach concentrations of 20 g/hL. An alcoholic fermentation activator (Thiazote^®, Laffort) was added along with the leaven when the nitrogen concentration was lower than 140 mg/L and during fermentation (when a density of 1.06 kg/L had been reached) to complete nutriments at 200 mg/L. Fermentations were conducted at a constant temperature of 25 °C and temperature and density were monitored daily in each tank. The cap was punched down daily and O₂ (2 to 4 mg/L) was added when the density had reached 1.02. When the density was stable (0.993-0.997) in each batch and concentrations of reducing sugars were lower than 2 g/L, a 2-day post-fermentation maceration was conducted at 30 °C under CO₂.

Figure 1. Experimental process of micro-winemaking from 20 % and 50 % botrytised grapes, with or without tannin addition.

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(Ø = no addition of tannins; Gse = grape seed tannins; Gsk = grape skin tannins; O = oak tannins; Q = quebracho tannins; N = nutgall tannins; AF = alcoholic fermentation; MLF = malolactic fermentation; VAT = vatting; AF1, AF2, AF3 = 1, 2, 3 days after beginning of alcoholic fermentation respectively; MAC = after post-alcoholic fermentation maceration; WINE = 3 months after bottling).

2.3. Malolactic fermentation

Wines were separated from the pomaces by moderate manual pressing and poured into 5-L glass bottles. Malolactic fermentation was conducted by inoculating a commercial lactic acid bacteria, Oenococcus oeni (Lactoenos® SB3 Direct, Laffort), at 1 g/hL. The decrease in malic acid concentrations was monitored with an enzymatic kit (R-Biopharm, Saint Didier au Mont d’Or, France). When they were lower than 0.2 g/L, finished wines were racked in 0.75-L bottles and 2 g/hL of SO₂ using the Laffort Bisulfite18 solution were added. Bottles were stored at 4 °C.

5.1. CIELAB parameters

Lightness (L*), position in a red-green axis (a*) and position in a yellow-blue axis (b*) (CIE, 1986) were measured with a Konica Minolta CM-5 apparatus (Nieuwegein, Netherlands). C*_ab and h_ab were calculated as follows:

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Delta E (ΔE) was also calculated to determine the colour difference between two wines:

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Values of ΔE higher than 3 are detectable with the naked eye (García-Marino et al., 2010).

5.2. Spectrophotometric chromatic parameters

Absorbances at 420, 520 and 620 nm were measured in a 1-mm path length cuvette by a UV-Vis Helios Alpha^TM spectrophotometer (Thermo Fisher Scientific Inc., Waltman, MA, USA). Chromatic parameters (Glories, 1984) were calculated as follows: were calculated as follows:

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6.1. Total polyphenol index (TPI)

Samples were previously diluted with a dilution factor (DF) of 50 (VAT-samples) or 100. The absorbance at 280 nm was measured in a 1-cm path length quartz cuvette and TPI was calculated as (Ribéreau-Gayon et al., 2006):

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6.2. Folin-Ciocalteu index (FCI)

FCI was determined according to Singleton and Rossi (1965) with the following solutions placed in 50-mL volumetric flasks: 2.5 mL of Folin–Ciocalteu solution, 0.5 mL of sample or standard solution, 10 mL of Na₂CO₃ solution and water up to the 50-mL mark. After 30 min, absorbance was measured at 760 nm on a UV-vis spectrophotometer. Gallic acid was used as standard at concentrations ranging from 100 to 800 mg/L in order to express FCI in mg of gallic acid equivalents per litre of must or wine.

6.3. Total anthocyanins (TA)

Total anthocyanins were determined according to Ribéreau-Gayon and Stonestreet (1965). Solution A containing 250 μL of sample, 250 μL of ethanol acidified with 0.1 % HCl and 5 mL of 2 % HCl (v/v) was prepared beforehand. In two tubes, 1 mL of solution A was added to 400 μL of distilled water in tube 1 and 400 μL of 15 % potassium bisulfite in tube 2. After 20 min, the absorbance at 520 nm from both tubes was measured in a 1-cm optical path cuvette. Malvidin-3-O-glucoside was used as standard and TA was expressed in mg/L malvidin-3-O-glucoside equivalent.

7.1. ORAC

All solutions were prepared in 7.4 pH phosphate buffer at 37 °C. In a 96-well black plate, 30 μL of diluted musts or wines (1/2000) or trolox (2.5 to 40 μM), 180 μL of 117 nM fluorescein and 90 μL of 40 mM AAPH solution were added to each well. The fluorescence (excitation = 485 nm and emission = 520 nm) was measured every minute for 90 min at 37 °C. For each kinetic, the area under the curve (AUC) was calculated and then from the standard curve AUC = f (trolox concentration), the antioxidant capacity of each sample was determined as being the concentration mM trolox equivalent (González-Centeno et al., 2012).

7.2. DPPH

According to Milat et al. (2019), 190 μL of 60 μM DPPH in methanol and 10 μL of the diluted sample (1/25 for the wines and 1/10 for the musts) or solutions of trolox (0.1-1 mM) were deposited in a 96-well transparent plate. The absorbance at 515 nm of each well was measured at 25 °C after 30 min. The result was expressed in mM trolox equivalent.

9.1. Anthocyanin analyses

Musts and wines were filtered (0.45 µm) and directly injected for HPLC analyses. HPLC-UV analyses were performed by means of a Thermo Scientific Accela (Thermo Fisher Scientific, Waltham, MA, USA) with an Accela 600 pump module and a UV-Visible diode array detector and Xcalibur Software according to González-Centeno et al. (2017). Separation was performed on a reverse-phase C18 Nucleosil column (250 x 4.6 mm, 5 µm). The injected volume was 20 µL. The mobile phases were water/formic acid (95:5, v/v) (solvent A) and acetonitrile/formic acid (95:5, v/v) (solvent B), at a flow rate of 1 mL/min. Initial solvent B was set at 10 %. The mobile phase gradient was as follows: 35 % B at 25 min, 100 % B at 35 min, 100 % B from 35 to 40 min, 10 % B at 41 min, and then 10 % B for 4 min before the next injection. Eluting peaks were monitored at 520 nm. The mean peaks were identified by comparison with injected external standards and previous results (Chira, 2009). Concentrations were expressed as milligrams of malvidin 3-O-glucoside equivalents per litre.

9.2. Proanthocyanidin monomer and oligomer analyses

Musts and wines were filtered (0.45 µm) and directly injected for HPLC analyses. The equipment used for HPLC analysis consisted of a Thermo-Finnigan UV-Visible detector (UV-vis 200), a Thermo-Finnigan autosampler and a Thermo-Finnigan (San Jose, CA, USA) ternary pump coupled to a Xcalibur data treatment system. Separation was performed on a reverse-phase Lichrosphere 100-RP18 (250 mm x 2 mm, 5 µm; Merck, France) column according to González-Centeno et al. (2012). The mobile phases were water-acidified with formic acid 0.5 % (solvent A) and acetonitrile-acidified with formic acid 0.5 % (solvent B). The gradient at a flow rate of 1 mL/min was 3 % B for 3 min, 3 to 5 % B for 11 min, 5 to 10 % B for 8 min, 10 to 14 % B for 4 min, 14 to 25 % B for 14 min, 25 to 100 % B for 1 min, 100 % B for 7 min, 100 to 3 % B for 2 min, and 3 % B for 5 min. Eluting peaks were monitored by a UV-detector at 280 nm and a fluo-detector (λ_excitation = 280 nm, λ_emission = 320nm). Identification of mean peaks was performed by comparison with injected external standards and previous results (Chira, 2009). Calibration curves were established using the catechin as external standard. The results were expressed as milligrams of catechin equivalents per litre.

9.3. Determination of mean degree of polymerisation (mDP)

Proanthocyanidin mDP was determined for wines by phloroglucinolysis (Drinkine et al., 2007). The oligomeric and polymeric products were depolymerised in the presence of a nucleophilic agent (phloroglucinol) in an acid medium. Briefly, wine (5 mL) was evaporated to dryness using a rotary evaporator and then resuspended in 10 mL of milliQ water. The 10 mL sample (or 5 mL of musts) was passed through a pre-activated C18-SPE column (60 mL) and then washed with 60 mL of milliQ water. The samples were then eluted with 60 mL of methanol and the eluate was dried using a rotary evaporator and resuspended in 2 mL of methanol (extract 1). In an Eppendorf tube, 100 μL of extract 1 and 100 μL of methanol solution containing phloroglucinol at 50 g/L, ascorbic acid at 10 g/L and 0.1 N HCl were added. The tube was placed for 20 min in an oven at 50 °C, after which 1 mL of a 40-mM sodium acetate solution was added. The new extract obtained was filtered onto a 0.45-μm pore diameter syringe filter before injection. Reaction products were analysed by HPLC-MS on an Accela series (Thermo-Scientific, Illkirch-Graffenstaden, France), equipped with a pump module and a UV detector. Separation was performed on a reverse-phase Waters XTerra^®RP C18 (Waters, Milford, MA, USA) (100 mm x 4.6 mm, 3.5 µm) column at room temperature. A binary gradient system was employed using 1 % (v/v) aqueous acetic acid (solvent A) and MeOH (solvent B) at a flow rate of 1 mL / min. The elution conditions were as follows: 5 % B for 25 min; 5-20 % B for 20 min, 20-32 % B for 15 min, and 32-100 % B for 2 min. The column was then washed with 100 % B for 5 min and re-equilibrated with 5 % B for 5 min before the next injection. The mDP, % of prodelphinidin (% P) and % of galloylation (% G) were estimated using the response factors of proanthocyanidin cleavage products at 280 nm and calculated as described by Chira et al. (2012). The mDP was obtained by dividing the sum of all subunits (flavan-3-ol monomers and phloroglucinol adducts, expressed in moles) by the sum of all flavan-3-ol monomers (expressed in moles).

10.1. Triangle tests

Several triangular tests (ISO4120: 2007) were carried out to determine significant differences between

i) the main control wine [0 % Botrytis, Ø tannins] and the other control wines [20 % Botrytis and 50 %, Ø tannins], and

ii) between control wines [20 % and 50 % Botrytis, Ø tannins] and the corresponding wines supplemented with tannins.

During the sessions, the judges were asked to observe, smell and/or taste the samples in the defined order and indicate which one seemed different, even if they were not sure of their answer. The panel was also asked to freely note one or more descriptor(s) in order to differentiate the samples. The descriptors were retained only in the event of a correct answer from the judge.

10.2. Ranking tests

Ranking tests (ISO8587: 2007) were conducted in two sessions of six wines each (for each botrytisation rate). The panel of tasters classified the six tested wines according to the "overall quality" of the product (visual, aromatic and taste qualities), knowing that they were young wines (three-months old) made from Merlot. Ratings were assigned from 1 (the most qualitative) to 6 (the least qualitative). Results were processed using a Friedman test on the sum of the ranks assigned by the judges. The smallest significant difference was then calculated and compared to the absolute value of the difference in the sum of the ranks (for two wines), to determine which products were significantly different from each other.

1.1. Musts

The wines were obtained from healthy grapes (0 % Botrytis) and from a mix of healthy grapes and totally infected grapes to reach a final 20 % or 50 % botrytisation rate. Table 1 presents the oenological and chemical parameters of the control (0 %), 20 % and 50 % botrytised musts and their corresponding 3-month-old wines.

The classic oenological parameters (pH, density, concentrations of reducing sugars and total sugars) varied slightly according to the level of botrytisation of the musts, although significant differences were found. Volatile acidity content increased by 17 % with a botrytisation level (trend) of 0 to 20 %, and significantly by 36 % with a 20 % to 50 % botrytisation level. Beside the possible production of acetic acid by Botrytis, the highest volatile acidity at higher botrytisation levels can also be explained by the action of acetic bacteria, most likely present in damaged grapes (Ribéreau-Gayon et al., 2012).

The greatest differences were observed for phenolic compounds. The total phenolic compounds in botrytised musts decreased significantly to levels approximately two-fold less significant (TPI) or even three-fold less significant (FCI) than in the healthy musts. Anthocyanins were also drastically degraded in both 20 % and 50 % botrytised musts. Their concentration decreased significantly from 108.2 (control must) to less than 10 mg/L malvidin-3-O-glucoside eq. in the botrytised musts (anthocyanins identified and quantified by HPLC in botrytised musts in trace amounts). These results may be due to the presence of laccase with corresponding activities of 25.8 LU and 42.9 LU in 20 % and 50 % botrytised musts respectively (Table 1). Hence, there was sufficient laccase activity in the 20 % botrytised must to degrade all the anthocyanin content in the healthy musts. The major anthocyanin compound, malvidin-3-O-glucoside, present in the healthy must at about 35 mg/L, was thus nearly absent from both botrytised musts. These results are in agreement with those obtained by Ky et al. (2012). Moreover, the results in Table 1 show that the molecular tannins were significantly affected by enzymatic oxidation. Like the anthocyanins, the tannins had probably been oxidised since they are also substrates of laccases (Oliveira et al., 2011).

Furthermore, since the chromatic characteristics and antioxidant capacities of botrytised musts were also markedly affected, the consequences of the degradation of phenolic compounds by laccase were again directly observable (Table 1); for example, the spectrophotometric “hue” parameter (A_420 nm / A_520 nm) was approximately twice as high in botrytised musts as in healthy musts, indicating a major oxidative degradation of botrytised musts.

The presence of laccase produced by B. cinerea thus had a considerable negative impact on the phenolic compounds of the musts as soon as they were in the vat, the degradation having begun in the grape berries (Ky et al., 2012). The oxidation of polyphenols by laccase therefore directly affected the visual characteristics and antioxidant properties of the musts, which is expected to impact the final quality of the wines.

1.2. General description of 3-month-old wines

In contrast to the wines from healthy Merlot grapes, the botrytised control wines obtained from the botrytised musts were damaged, their colour and phenolic compounds having been drastically affected (Table 1). The botrytised wines were less intense (CI); they had a stronger yellow hue (increasing % yellow and decreasing % red) and a brickish colour. These characteristics increased with botrytisation rate. Tannin and anthocyanin contents decreased with botrytisation level.

All wine variables (oenological, chromatic, spectrophotometric, fluorometric and chromatographic parameters of the phenolic compounds and laccase activity) are represented in a PCA plot (Figure 2), with three main axes accounting for over 83 % of the overall variability.

In the PCA plot, the first PCA axis classifies the wines according to their botrytisation rate very clearly (increasing from right to left). However, two wines were differentiated owing to positive axis-2 coordinates; i.e., the wines at 20 % and 50 % botrytisation levels with added nutgall tannins (N). These wines had higher TPI, ORAC and DPPH values than the other wines. The plot of 1 and 3 axes shows differences between wines supplemented with the various tannins, especially within the 20 % botrytisation group. Wines with Gse and Gsk tannins were the furthest from the control wines without added tannins, suggesting that their addition had a positive impact on the final phenolic content (Figure 2B). Wines obtained from 50 % botrytised grapes were more grouped, showing lower variability. As a preliminary result, the added oenological tannins at 100 g/hL may have had less effect on the studied variables in the 50 % botrytised wines than in the 20 % botrytised wines.

Figure 2. Plot of principal component analysis obtained from healthy or botrytised (20 % and 50 %) 3-month-old wines, with or without added tannins.

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A) plot of axes 1 and 2, B) plot of axes 1 and 3. Gse = grape seed tannins, Gsk = grape skin tannins, Q = quebracho tannins, O = oak tannins, N = nutgall tannins. Groups corresponding to the level of botrytisation are highlighted in green (50 %), red (20 %) and black (0 %).

2.1. Laccase activity during micro-winemaking

In musts (VAT) and wines at the end of alcoholic maceration (MAC), laccase activity was lower with the 20 % botrytised treatment than with the 50 % one (Figure 3A). According to both MAC and VAT data (two-way ANOVA with "Botrytis rate” and "Tannin" as main factors), botrytisation level had a highly significant effect (p-value = 5*10^- 8) on laccase activity. Without any added tannin, laccase activity in 50 % botrytised musts (≈43 laccase units) was 1.65-fold higher than in 20 % botrytised musts (≈26 laccase units) (Figure 3A). However, laccase activity in wines after post-alcoholic fermentation maceration (MAC) fell by 28 % and 47 % in the 50 % and 20 % botrytised wines respectively (compared with the musts). With the addition of tannins, a decrease in laccase activity was also observed in the wines at the end of post-fermentation maceration compared to the musts.

In the 20 % botrytised musts and wines (Figure 3A), the two-way ANOVA with "Tannin" and "Time" (VAT, MAC) as main factors showed significant effects of both "Time" (p-value = 5.6*10^- 7) and "Tannin" (p-value = 0.015), and of their interaction (p- value = 0.031). The Gsk, O and N tannins showed a similar inhibitory effect, with a significant decrease in laccase activity between vatting and the end of post-fermentation maceration, reaching 64 % for Gsk, 65 % for O and 70 % for N (compared to the 20 % botrytised control, which showed a 47 % drop in activity). As regards musts and wines made from 50 % botrytised grapes, the two-way ANOVA revealed a "Time" effect (p-value = 6.1*10^- 7) on laccase activity (Figure 3A). The botrytisation level was too high to show any differential effects between the tannins.

In terms of relative laccase activity (Figure 3B) at the 50 % botrytisation rate, values were often greater than or close to 100 %, indicating that added tannins did not have an inhibitory effect on laccase activity. The tested dose of tannins (100 g/hL) was not high enough to inhibit laccase activity. At the 20 % botrytisation rate (Figure 3B), the results of the two-way ANOVA with "Tannin" and "Time" (VAT, J1, J2, J3 and MAC) as main factors indicate a “Tannin” effect (p-value = 0.0016). The mean comparison of tannins showed that the Gse and Gsk tannins were the most efficient, and that their overall relative laccase activities were lower than those of the control, regardless of the alcoholic fermentation step. The addition of Q and N tannins led to relative laccase activity equivalent to that of the control, although it was higher for Q (> 100 %) and lower for N than that of the control at the end of alcoholic maceration. Finally, the relative activity calculated for oak tannins (O) showed greater overall laccase activity than that of the control; it was therefore not efficient.

Figure 3. Laccase activity in botrytised (20 % and 50 %) musts and wines, either with or without added tannins.

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Gse = grape seed tannins, Gsk = grape skin tannins, Q = quebracho tannins, O = oak tannins, N = nutgall tannins.

A - Laccase activity in musts (VAT) and post-alcoholic fermentation maceration wines (MAC) in laccase units. For each botrytisation level, the various letters show significant differences between samples for the two-way ANOVA with "Tannin" and "Time" (VAT, MAC) as main factors.

B - Relative laccase activity (%) in musts (VAT) and wines (AF1, AF2, AF3, and MAC) compared to controls (wines without tannins) at each step of alcoholic fermentation. At the 20 % botrytisation level, the various letters show significant differences between tannins for the two-way ANOVA, with "Tannin" and "Time" (VAT, J1, J2, J3 and MAC) as main factors considering all "Time" data, since the “Time” effect was not significant.

C - Laccase activity in 3-month-old wines after bottling in laccase units.

During vatting, relative laccase activity was in most cases lower than in the control, with the Gse tannin having the lowest relative activity. In contrast, it did not decrease in the “O” oak tannin. These results are in accordance with those of Vignault et al. (2020) related to white musts, whereby grape seed tannins were found to be the most efficient at precipitating laccase proteins and thus inhibiting laccase activity, whereas oak tannins (ellagitannins) did not precipitate laccase proteins. The impact of tannin addition on the laccase activity of musts may therefore be the result of the putative aggregation between tannins and laccase proteins, owing to the colloidal properties of tannins that cause partial laccase precipitation, which makes laccase inactive.

Furthermore, the origins and structures of added oenological tannins could also explain their differential effects on laccase inhibition during winemaking. Besides precipitation, they can also inhibit laccase activity without protein precipitation (Vignault et al., 2020), making the active site of the enzyme inaccessible due to tannin-protein interactions.

2.2. Laccase activity in 3-month-old wines

Laccase activity was observed in all wines obtained from 50 % botrytised grapes (Figure 3C). Only the "Botrytis" effect was significant (p-value = 7.93*10^-7). Free SO₂ added before bottling (20 mg/L) can usually inhibit and completely destroy laccase after a few days, as long as there is free SO₂ remaining. Nevertheless, if SO₂ combines before the complete destruction of the enzyme, laccase activity can persist, since the combined form of SO₂ does not show any antioxidasic properties (Ribéreau-Gayon et al., 2012). Laccase activity in 50 % botrytised wines was between 5 and 11 laccase units (no significant differences between wines: p-value = 0.22), which is enough for a wine to be at significant risk of oxidasic haze. Indeed, the colour of red wines with a residual activity greater than 3 laccase units per millilitre is unstable and evolves into haze (Grassin and Dubourdieu, 1989). Wines obtained from 20 % botrytised grapes had little or no laccase activity, with no significant differences between tannins. However, the trend confirms that Gsk, Gse and N tannins might lead to lower laccase activity in wines compared to the control wine. These results confirmed those observed during winemaking.

3.1. Impact on phenolic compounds and antioxidant capacities

The ANOVA results of the phenolic compounds and antioxidant capacities (Table 2) showed a significant "Botrytis" effect, except for the variable “% P”. Total anthocyanins, malvidin-3-O-glucoside, and % G were significantly influenced by the botrytisation rate only and not by tannin addition.

Figure 4A shows that total anthocyanin (TA) concentrations decreased by approximately 35 and 70 % on average with 20 and 50 % botrytisation respectively. A significant degradation of anthocyanins in wines induced by botrytisation ranging from 5 to 20 % has been demonstrated by Ky et al. (2012).

Significant "Botrytis" and "Tannin" main effects were found for other variables (Table 2): TPI, FCI, catechin content, T, mDP, % purple, and ORAC and DPPH antioxidant capacities. These variables were thus not only affected by the botrytisation rate, but also by the addition of tannins, without having a protective effect on these variables.

Figure 4. Total anthocyanins and epicatechin concentration in botrytised (20 % and 50 %) 3-month-old wines after bottling, either with or without added tannins.

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Gse = grape seed tannins, Gsk = grape skin tannins, Q = quebracho tannins, O = oak tannins, N = nutgall tannins. Different letters show significant differences: A and B for the “Botrytis” effect (Figures A), and a, b, c, d for the “interaction Botrytis*Tannin)” effect (Figure B).

Finally, several variables (i.e., epicatechin, B4 and B3 procyanidin dimers) were significantly affected by the botrytisation rate, the addition of tannins and the interaction of "Botrytis" and "Tannin" factors (Table 2). For example, Figure 4B shows that the level of epicatechin, one of the major procyanidin monomers, was the lowest in the 50 % botrytised wines. The addition of tannins did not prevent the damage of Botrytis laccase when the grapes were contaminated at 50 %. However, despite oak and quebracho tannins not preventing epicatechin degradation, at 20 % Botrytis contamination tannins from grape seed, grape skin and even nutgall had a protective effect on laccase for this compound (Figure 4B) and for B4 and B3 procyanidin dimers (data not shown).

3.2. Impact on colour

Regarding the spectrophotometric and CIELAB chromatic parameters, the ANOVAs showed that botrytisation level had a significant effect on all "colour" variables (Table 2). No "Tannin" effect or interaction effect was observed (except a "Tannin" effect for % purple), showing that 20 % and 50 % botrytised wines were nearly similar in colour respectively. However, the “Tannin” effect for % purple corresponded to the fact that, at the 20 % botrytisation rate, the Gsk wine and N wine were significantly different from each other at α = 0.1 for this variable. In Figure 5, CIELAB parameters showed that L* and b* values were higher in 50 % than in 20 % botrytised wines; the wines therefore became lighter and yellower as botrytisation rate increased. Moreover, the colour differences (ΔE) between 20 % and 50 % botrytised wines were also calculated from parameters L*, a* and b* (Figure 5). In general, values greater than or equal to 3 indicate that the differences between two wines are visually perceptible (García-Marino et al., 2010). 20 % and 50 % botrytised wines were thus visually different, with a ΔE ranging between 14 and 26. Interestingly, ΔE differed among the 50 % botrytised wines: the wine with added oak tannins was significantly different from all the others, with more oxidized wine colour characteristics (slightly higher L* and h_ab). At 20 % botrytisation, only the wine with grape skin tannins (ΔE = 4.4) was noticeably different from the control. The 20 % botrytised wines with quebracho and oak tannins were similar, although both were different from the three other wines with grape skin, grape seed and nutgall tannins (close colour with ΔE < 2). The added tannins thus produced differential effects: Gsk tannin (and even Gse and N tannins) improved the colour of the 20 % botrytised wine, whereas oak tannin addition caused further colour degradation in the 50 % botrytised wine, which had already been degraded by the pathogen.

Figure 5. CIELAB parameters (L*, a*, b*, delta E) of 3-month-old wines after bottling, with or without added tannins.

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Gse = grape seed tannins, Gsk = grape skin tannins, Q = quebracho tannins, O = oak tannins, N = nutgall tannins.

4.1. Wines without added tannins

The visual and smell/taste of the control wines obtained from 20 % and 50 % botrytised grapes were compared with the control wine made from healthy grapes (0 % Botrytis) without the addition of any tannin (Table 3). Significant differences in the visual features were observed: botrytised wines were more orange in colour, and were even brick/brown and less purplish at 50 % botrytisation. From an olfactory/taste point of view, differences were also significant (4 out of 18 judges did not perceive the differences between the 0 % and the 20 %, as opposed to 1 out of 18 between the 0 % and the 50 %). botrytised wines were described as being oxidized and jammy at 20 % botrytisation, while adjectives such as acescent, dusty, old and oxidized were used at 50 % botrytisation.

4.2. Effect of added tannins on visual quality of 20 % and 50 % botrytised wines

The 50 % botrytised wines with added tannins were compared visually to the 50 % botrytised control with no added tannins. The triangular tests (Table 3) showed that the wines supplemented with grape skin (Gsk 50 %) and oak (O 50 %) tannins were significantly different (α = 0.001) from the control. The Gsk-supplemented 50 % wine was described as being less orange in colour and more intense than the control. The O-supplemented 50 % wine appeared to be lighter and more orange. Ranking tests (Table 4) revealed that the addition of nutgall tannins to 50 % botrytised musts tended to intensify the colour slightly. However, the addition of grape seed, quebracho and oak tannins caused a significant opposite effect. These results with oak tannins were in agreement with the descriptors used by judges in the triangular tests.

Triangular tests also showed significant differences with the addition of grape skin (α = 0.05) and oak tannins (α = 0.001) in all wines made from 20 % botrytised grapes. The addition of grape skin tannins resulted in a darker wine than that of the control. This result confirmed those obtained for the 50 % botrytised wines and the ΔE results (Figure 5). Regarding overall visual quality (Table 4), the judges ranked the 20 % Botrytis wine with added grape skin tannins first. The 20 % Botrytis control wine was ranked last. Furthermore, the judges suggested that the addition of oak tannins may have a positive impact on colour in 20 % botrytised wines (triangular tests). This result was not confirmed by the ranking tests (Table 4).

4.3. Impact of added tannins on the aromatic and taste quality of 20 % and 50 % botrytised wines

In terms of smell/taste, the 50 % botrytised wine supplemented with oak tannins was perceived as being significantly (α = 0.001) different from the control; i.e., it was described more positively than the control by the judges (Table 3). Although significant, there were fewer differences between the control and the wines with i) added grape seed tannins (α = 0.01) - described as being fruitier, ii) added grape skin tannins (α = 0.05) - described as expressing more prune and fewer mushroom flavours, and iii) added quebracho tannins (α = 0.05) - perceived as being more oxidized. Regarding the overall aromatic and taste qualities, the ranking tests showed no significant differences (Table 4). For 20 % botrytised wines, the triangular tests showed that only the 20 % botrytised Gsk-supplemented wine was perceived as being significantly different from the corresponding control (Table 3). Although the differences between these two wines were not significant in terms of overall flavour and taste quality (Table 4), the Gsk-supplemented 20 % botrytised wine was ranked better than the control, as was the O-supplemented 20 % botrytised wine. Finally, the N-supplemented 20 % and 50 % botrytised wines were ranked last for gustative quality.