Specific monographs for gallotannins

RESOLUTION OIV-OENO 675C-2022

SPECIFIC MONOGRAPHS FOR GALLOTANNINS

THE GENERAL ASSEMBLY,

IN VIEW OF Article 2, paragraph 2 iv of the Agreement of 3 April 2001 establishing the International Organisation of Vine and Wine,

CONSIDERING the work of the "Specifications of Oenological Products" Expert Group,

CONSIDERING the resolution OIV-OENO 624-2022 “Update to the monograph on oenological tannins” which describes a general monograph,

CONSIDERING the need to set up specific monographs for each family of tannins,

DECIDES, at the proposal of Commission II "Oenology", to add the monograph COEI-1-GALLOT to the International Oenological Codex:

OENOLOGICAL TANNINS

Specific monograph on preparations containing gallic tannins

Gallotannins, or gallic tannins, are a sub-class of hydrolysable tannins. Oak (and chestnut) galls and tara (Caesalpinia spinosa) pod tannins are included in this sub-class.

1. Method for the determination of sub-class affiliations

1.1. Characterisation by high-performance liquid chromatography (HPLC)

1.1.1. Principle

This method is designed to verify the presence of gallotannins and measure their total concentration.

1.1.2. Reagents, materials and apparatus

1.1.2.1. Reagents

Gallic acid (purity > 96%), CAS No. 149-91-7

Ultrafiltered water (resistivity: 18.3 MΩ·cm)

Water (HPLC grade)

Methanol (HPLC grade)

Formic acid (HPLC grade)

1.1.2.2. Materials

100-mL borosilicate-glass flask

Filters with 0.45 µm pore size diameter

Plastic 1-mL syringe

1.1.2.3. Apparatus

Technical balance with precision of ± 0.01 g

Analytical balance with precision of ± 0.1 mg

Class-A volumetric glassware

Mass chromatographic system with detection by spectrometry composed of:

Gradient pump for binary or quaternary mix
Injector fitted with a loop of 10 μL
Spectrophotometric detector at 280 nm fixe wavelength
Column Phenomenex Kinetex (for example): 150 x 3.0 mm, 2.6 μm particle size
ESI-SIM (Single Ion Monitoring mode via Electro Spray Ionisation) ionisation source
Mass spectrometer detector: triple quadrupole time of flight (Q-ToF)

1.1.3. Preparation of samples and standards

Samples: weigh approximately 0.5 g of oenological tannins on the analytical balance and make a note of the weight. Dissolve the oenological tannins in 100 mL of ultrafiltered water in a 100-mL borosilicate-glass flask and mix well.

Preparation of standard solutions: put 10 mg of gallic acid in solution into 50 mL of ultrafiltered water, corresponding to a 200 mg/L concentration. Then carry out dilutions in ultrafiltered water to obtain 1, 5, 10, 25, 50, 75 and 100 mg/L concentrations.

Solvent A: HPLC-quality water containing 0.1% of formic acid.

Solvent B: methanol containing 0.1% of formic acid.

1.1.4. Procedure

Sample solution and standard solutions are filtered on 0.45 µm (pore size diameter) cellulose filters and analyse by chromatography under the following conditions given by way of example:

Injected volume: 10 μL of sample solution or standard solution of gallic acid

Detection at 280 nm

Composition of elution gradient: (time, % of solvent A)

0min, 99.0%; 2 min, 98.0%; 5 min, 97.0%; 6 min, 96.5%; 7 min, 96.0%; 8 min, 95.5%; 10 min, 95.0%; 14 min, 90.0%; 17 min, 85.0%; 23 min, 00%; 25 min, 8.0%; 29 min, 5.0%; 34 min, 1.0%; 45 min, 99.0% and 10 min for equilibrium.

Flow rate: 0.4 mL/min

Quantification and detection of the components of the following gallic tannins: gallic, digallic and quinic acid; 3-, 4- and 5-galloylquinic acid; tri-, tetra-, penta-, hexa-, hepta-, octa-, nona- and decagalloyl-glucose according to the ESI-SIM scan and Q-ToF detection (for example).

Table 1: Example chemical formulas and m/z of the different gallotannins (or gallic tannins)

Compounds	Chemical formula	m/z
Gallic acid	C₇H₆O₅	170.0
Digallic acid	C₁₄H₁₀O₉	322.2
Quinic acid	C₇H₁₂O₆	192.2
3-galloylquinic acid	C₂₈H₂₄O₁₈	648.1
4-galloylquinic acid	C₃₅H₂₈O₂₂	800.1
5-galloylquinic acid	C₄₂H₃₂O₂₆	952.7
Trigalloyl glucose	C₂₇H₂₄O₁₈	636.5
Tetragalloyl glucose	C₂₄H₂₈O₂₂	788.6
Pentagalloyl glucose	C₄₁H₃₂O₂₆	940.6
Hexagalloyl glucose	C₄₈H₃₆O₃₀	1092.8
Heptagalloyl glucose	C₅₅H₄₀O₃₄	1244.9
Octagalloyl glucose	C₆₂H₄₄O₃₈	1396.9
Nonagalloyl glucose	C₆₉H₄₈O₄₂	1548.1
Decagalloyl glucose	C₇₆H₅₂O₄₆	1707.2

Figure 1: Example ESI-SIM scan of gallotannins (or gallic tannins)

1.2. Conclusion

An oenological tannin is recognised as a gallotannin (or gallic tannin) when:

its total phenol content is higher than 65% (gravimetric method in Annex 1 of the general monograph OIV-OENO 624-2022),
its gallotannin content as characterised by the HPLC method is higher than 190 mg equivalent of gallic acid per gram of oenological tannins.

2. Properties and functionalities

The following compliance methods and criteria are only applicable when the property/functionality is claimed on the preparation of tannins.

2.1. Antioxidant ability

2.1.1. Principle

Determination of gallotannins’ antioxidant ability to contribute to the protection of must and wine from oxidation.

2.1.2. Products

2.1.2.1. Antioxidant capacity

DPPH (2,2-diphenyl-1-picrylhydrazyle): MM = 394.32

Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid): MM = 250.29

Methanol at 99.9% volume

96-well microplates reader (FLUOstar Omega - BMG Labtech, for example)

2.1.2.2. Direct oxygen consumption (OCR)

Ethanol at 96% volume, CAS No. 64-17-5

Tartaric acid: MM = 150.09, CAS No. 87-69-4

Iron (III) chloride hexahydrate: MM = 270.30, CAS No. 7705-08-0

Copper (II) sulfate pentahydrate: MM = 249.68, CAS No. 7758-98-7

Clear glass bottles with inserted pills of 0.75-cL capacity

NomaSens oximeter, for example

2.1.3. Protocols

2.1.3.1. Antioxidant capacity (DPPH assay)

0.15 g/L oenological tannin solution: dissolve 37.5 mg of oenological tannins in 500 mL of model wine solution (distilled water, 12% vol. of ethanol, 4 g/L of tartaric acid and pH adjusted to 3.5). Dilution of oenological tannins solution could be needed if the measurement absorbance is higher than 1 unit (in this case the dilution should be included in the calculation).

1mM Trolox solution: dissolve 125 mg of Trolox in 500 mL of model wine solution (distilled water, 12% vol. of ethanol, 4 g/L of tartaric acid and pH adjusted to 3.5).

Calibration curve: dissolve in 1, 0.8, 0.6, 0.4, 0.2 and 0.1 mL of 1 mM Trolox solution into 0, 0.2, 0.4, 0.6, 0.8 and 0.9 mL of model wine solution. These quantities correspond to 1, 0.8, 0.6, 0.4, 0.2 and 0.1 mM final concentration of Trolox respectively.

M DPPH solution: dissolve 2.36 mg of DPPH in 100 mL of methanol. The solution should be freshly prepared.

2.1.3.2. Direct oxygen consumption (OCR)

1 g/L oenological tannin solution: dissolve 0.75 g of oenological tannin in 750 mL of model wine solution.

Model wine solution: dissolve 4 g of tartaric acid, 2.25 mg of Iron (III) chloride hexahydrate and 0.225 mg of Copper (II) sulfate pentahydrate in 90 mL of ethanol and 660 mL of distilled water. The pH should be adjusted at 3.5.

2.1.4. Tests

2.1.4.1. Antioxidant capacity

First a blank containing solely reagent (RB) is measure at 515 nm by placing 190 µL of DPPH solution (1.3.1) in all the wells of the plate. Then, add 10 µL of oenological tannin solution (samples), distilled water (blank) or Trolox curve solution (standards) into the wells and measure (MS) at 515 nm after 30 min.

See Figure 2 for an example of how to fill the plate.

The formula to be applied for the calculation of the antioxidant capacity is as follows:

2) antioxidant capacity