Study in collaboration with
the University of Turin

L’impatto dei tannini enologici sul vino

THE IMPACT OF OENOLOGICAL TANNINS ON WINE

PHENOLIC, ANTIOXIDANT AND SENSORY CHARACTERISTICS

Oenological tannins are adjuvants that can be used at different stages of production with the aim of increasing the antioxidant capacity and promoting the color stability of must and wine.

Oenological tannins

A wide range of formulations, both pure and blended, based on hydrolysable tannins (gallotannins and ellagitannins), proanthocyanidins from grape skins and seeds (prodelfinidins and proanthocyanidins) and exotic wood (prorobinetinidins and profisetinidins) are available for winemaking. In grapes, the condensed tannins and their compound subunits, the flavan-3-ol monomers, are naturally present in the grape skins and seeds, from which they are extracted in the must-wine during maceration.

In addition to the tannins naturally present in grapes, other (i.e. exogenous) tannins from grapes or other botanical sources can be used in the winemaking process as processing aids and are referred to as 'oenological tannins'. According to the International Code of Oenological Practices, their use is authorized to improve the color stabilization of musts and wines and facilitate wine ageing by increasing antioxidant and antioxidase capacity. They are produced through a direct solid-liquid extraction of plant material and can be found both as single species and as botanical species in blends, such as grape, oak, chestnut, quebracho, acacia and tara.

The effectiveness of adding tannins in winemaking is related to the botanical origin of the formulation, the chemical characteristics, the polyphenolic content, and the phase and dose of addition.

The chemical characteristics of tannins

Considering their chemical characteristics, oenological tannins can be divided into two groups: condensed and hydrolysable tannins.

Condensed tannins, also called proanthocyanidins, consist of polymers that differ in their monomeric structural units, the flavan-3-ols and flavan-3-4-diols.

In grapes, polymers based on flavan-3-ols, i.e. procyanidins and prodelphinidins, are found. The procyanidins consist of (+)-catechin and (-)-epicatechin with a different degree of gallic units, while the prodelphinidins are also composed of (-)-epigallocatechin and (+)-gallocatechin.

Other types of condensed tannins with flavan-3,4-diol subunits have been characterised in exotic woods commonly used in the wine industry: profisetinidins from quebracho (Schinopsis spp.), which are mainly composed of fisetinidol monomeric units, and prorobinetinidins containing robinetidinol as subunits, which are the main components of the Mimosaceae family (e.g. acacia) along with prodelfinidins and profisetinidins in a smaller percentage.

The hydrolysable tannins, on the other hand, are usually classified into gallotannins and ellagitannins. Gallotannins are present in nuts of plant origin and are gallic acid and D-glucose, with a different degree of substitution with the gallic fraction.
In contrast, ellagitannins consist of D-glucose and ellagic, gallic acids. They are commonly extracted from chestnut and oak and the eight most common forms are monomers, i.e. castalagin, vescalagin, grandinin, and dimers, i.e. roburin A, B, C, D.

The study and news on the impact of tannins

The study on the impact of tannins was conducted by our R&D team in collaboration with the University of Turin pool. Seventeen pure and blended tannins were examined (see Table 1).

The tannins were characterised in terms of polyphenolic content (see Table 2) and antioxidant capacity in a model wine and in a red wine after one month of storage, as well as the sensory characteristics of aroma, astringency and bitterness in water and red wine. The color characteristics of red wine added after one month of storage were also analyzed.

To better estimate the antioxidant capacity of the different tannins, the antioxidant power (AP) of each sample was calculated as the antioxidant capacity over the total phenolic concentration measured by the Folin-Ciocalteu (FC) method (see Table 3). This standardisation can be useful to understand how some classes of tannins are more antioxidant than others.

Table 1
Oenological tannins under study.

Sample	Group	Type	Description
Sd1	Proc/prod	pure	Proanthocyanidins from grape seeds Vitis vinifera L.
Sd2	Proc/prod	pure	Proanthocyanidins from grape seeds Vitis vinifera L.
Sk1	Proc/prod	pure	Proanthocyanidins from white grape seeds Vitis vinifera L.
Q1	Prof/pror	pure	Proanthocyanidins from quebracho
Q2	Prof/pror	pure	Proanthocyanidins from quebracho
Ac	Prof/pror	pure	Proanthocyanidins from Mimosaceae
Et1	Hydro	pure	Ellagitannins
Et2	Hydro	pure	Ellagitannins from Quercus spp
Gt	Hydro	pure	Gallotannins from Robina pseudoacacia galls
Mx1	Mix	mix	Proanthocyanidins from grape skin, quebracho, ellagitannins from Quercus spp
Mx2	Mix	mix	Proanthocyanidins and ellagitannins
Mx3	Mix	mix	Proanthocyanidins from grape skins Vitis vinifera L.and quebracho
Mx4	Mix	mix	Proanthocyanidins from grape skins and seeds of Vitis vinifera L. and quebracho
Mx5	Mix	mix	Proanthocyanidins from grape skins and seeds of Vitis vinifera L. and quebracho
Mx6	Mix	mix	Ellagitannins, gallotannins, and proanthocyanidins
Mx7	Mix	mix	Ellagitannins and proanthocyanidins
Mx8	Mix	mix	Ellagitannins and proanthocyanidins

Proc/prod = procyanidins/prodelphinidins; Prof/Pror = profisetinidins/prorobinetinidins;
Hydro = hydrolizable tannins, and Mix = mixed formulation.

THUS, FOR THE FIRST TIME, THE MOST SUITABLE OENOLOGICAL TANNINS HAVE BEEN DEFINED FOR DIFFERENT OENOLOGICAL APPLICATIONS. THEIR CONTRIBUTION TO WINE QUALITY IS LINKED TO THEIR ROLE IN THE AROMA AND LONGEVITY OF WINE.

Table 2
Polyphenolic characterisation of the formulations under study.

Sample	Group	Type	IPT (gallic acid/100g)	FC (gallic acid/100g)	BS (cyanidin g/100g)	MTC (gallic acid/100g)
Sd1	Proc/prod	pure	48.7+-5.7	89.8+-4.2	116.4+-5.7	36.5+-1.7
Sd2	Proc/prod	pure	28.7+-3.8	53.3+-2.2	71.3+-3.8	16.7+-3.0
Sk1	Proc/prod	pure	21.9+-1.5	33.1+-2.2	33.9+-1.5	6.0+-0.2
Q1	Prof/pror	pure	41.0+-0.9	77.0+-5.5	26.9+-0.9	19.7+-13.5
Q2	Prof/pror	pure	36.3+-2.1	64.4+-4.9	18.6+-2.1	27.8+-0.9
Ac	Prof/pror	pure	32.9+-1.7	54.4+-3.5	26.2+-1.7	22.0+-3.4
Et1	Hydro	pure	44.8+-2.0	52.2+-2.6		21.0+-7.3
Et2	Hydro	pure	37.5+-1.1	52.7+-4.1		19.1+-2.7
Gt	Hydro	pure	128.2+-4.6	101.4+-4.1		110.4+-1.2
Mx1	Mix	mix	39.1+-0.9	62.8+-4.2	25.9+-0.9	29.1+-1.4
Mx2	Mix	mix	36.6+-1.7	58.8+-2.9	14.9+-1.7	25.8+-2.9
Mx3	Mix	mix	36.8+-0.5	59.6+-4.0	23.8+-0.5	23.4+-3.2
Mx4	Mix	mix	32.9+-1.8	51.4+-2.0	22.2+-1.8	15.4+-5.7
Mx5	Mix	mix	32.2+-0.6	51.4+-1.0	20.9+-0.6	16.5+-7.3
Mx6	Mix	mix	35.2+-1.5	52.9+-1.4	10.6+-1.5	29.2+-0.8
Mx7	Mix	mix	28.2+-2.9	55.8+-2.6	20.8+-2.9	25.7+-0.6
Mx8	Mix	mix	34.0+-2.0	51.1+-4.8	5.9+-2.0	26.0+-0.8

Data are expressed as average value ± standard deviation.
FC = Folin-Ciocalteu method; BS = Bate-Smith method, MTC = Methylcellulose method.

Table 3
Antioxidant potency (AP) calculated as antioxidant capacity over total phenolic concentration.

Sample	Group	Type	ABTS AP	±	SD	DPPH AP	±	SD	FRAP AP	±	SD	CUPRAC AP	±	SD
Sd1	Proc/prod	pure	5.04	±	0.23	3.43	±	0.20	5.33	±	0.23	8.66	±	0.61
Sd2	Proc/prod	pure	6.95	±	0.24	4.83	±	0.23	8.53	±	0.42	10.96	±	0.15
Sk1	Proc/prod	pure	6.11	±	0.29	4.19	±	0.13	6.21	±	0.17	8.59	±	0.59
Q1	Prof/pror	pure	5.22	±	0.34	3.60	±	0.32	4.39	±	0.31	9.41	±	0.66
Q2	Prof/pror	pure	5.23	±	0.38	3.63	±	0.26	5.56	±	0.42	8.64	±	0.99
Ac	Prof/pror	pure	7.41	±	0.45	5.17	±	0.39	6.87	±	0.29	11.20	±	0.93
Et1	Hydro	pure	9.51	±	0.50	7.73	±	0.41	10.58	±	0.57	13.59	±	0.85
Et2	Hydro	pure	8.39	±	0.71	6.89	±	0.54	9.04	±	0.55	11.28	±	0.86
Gt	Hydro	pure	7.49	±	0.30	7.51	±	0.30	4.69	±	0.17	11.94	±	0.45
Mx1	Mix	mix	6.02	±	0.42	4.31	±	0.31	6.66	±	0.49	9.83	±	0.42
Mx2	Mix	mix	5.68	±	0.24	4.22	±	0.26	6.11	±	0.39	6.11	±	0.30
Mx3	Mix	mix	5.63	±	0.38	4.09	±	0.29	6.14	±	0.45	9.43	±	0.76
Mx4	Mix	mix	6.83	±	0.35	4.98	±	0.15	7.54	±	0.44	10.14	±	0.23
Mx5	Mix	mix	6.91	±	0.15	5.02	±	0.08	7.62	±	0.21	9.91	±	1.51
Mx6	Mix	mix	6.15	±	0.14	4.67	±	0.12	5.80	±	0.07	10.24	±	0.60
Mx7	Mix	mix	6.14	±	0.26	4.40	±	0.20	5.58	±	0.12	9.88	±	0.57
Mx8	Mix	mix	7.28	±	0.76	5.57	±	0.51	7.53	±	0.61	11.09	±	1.08

Data are expressed as average value ± standard deviation.

THE RESULTS OF THE STUDY

The results showed great variability between the formulations studied in terms of phenolic content, which appears to be closely related to their antioxidant capacity. In terms of origin, hydrolysable tannins presented the highest antioxidant capacity, followed by exotic wood-based formulations. A strong positive correlation was found in the antioxidant capacity of oenological tannins in model wine and red wine after one month of storage, particularly for ellagitannins, which also confirmed their ability to increase pigment polymerisation.

From a taste point of view, the differences are manifold and are related to the characteristics of the matrix itself.

Quebracho tannins were generally found to be the most bitter and astringent when tasted in water (40 g/hL), although the mouthfeel and aromatic descriptors of the tannins did not correlate with those of the added red wine.

Therefore, the choice of the formulation and its optimal dose requires characterisation in terms of polyphenolic content and antioxidant capacity, as these properties correlated well with those of wines added in a short storage period, while the sensory impact at oenological doses depends mainly on the characteristics of the wine.

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RELATIVE IMPACT OF OENOLOGICAL TANNINS IN MODEL SOLUTIONS AND RED WINE

Study in collaboration withthe University of Turin