Polyphenols in wine
A review of this important group of wine
molecules, which includes tannins and anthocyanins
In the wine trade we talk a lot about polyphenols (also known as
phenolics), a group of compounds that include tannins and
anthocyanins. But I’m not sure most people really know what they are
referring to with any degree of certainty. So, for the benefit of
those who don’t have a solid grasp of organic chemistry, I’m going
to try to unpack this subject in a way that’s understandable to
normal, non-science-geek types, but without losing the important
details.
Polyphenols are a large group of compounds that use a chemical
structure called phenol as the basic building block. That’s where
the name comes from: ‘poly’ phenols are where more than one phenol
group is joined to another. They are probably the most important
group of flavour chemicals in red wines, but are of much less
importance in whites.
So what is phenol? It’s a chemical that consists of what is known as
a benzene ring (a structure consisting of 6 carbon atoms joined in a
ring with a hydrogen atom attached to each, formula C6H6)
with a hydroxyl group (-OH) substituted for one of the hydrogens,
and so its chemical formula is C6H5OH. Phenol
is an important industrial chemical, but we’re interested here in
phenolics that are naturally synthesized by plants, built up of one
or more phenol groups, modified in various ways and joined together.
Just to give you an idea of the complexity of
this subject, there are more than 8000 different phenolic
compounds produced by plants.
An important property of phenolic compounds is that they associate
spontaneously with a wide range of other chemical entities, such as
proteins and other phenolic compounds, by means of non-covalent
forces (chemical term alert!). A covalent bond is what we usually
think of when we talk about chemical reactions, when two atoms
combine by sharing electrons. A non-covalent bond is weaker and
doesn’t involve sharing electrons, but is a common way of joining
macromolecules together, for example through hydrogen bonding and
hydrophobic (avoiding water) effects.
So let’s look at some of the phenolic compounds that are important
in wine. Alas, some chemical names are inevitable here. First,
though, we need to consider the structure of the grape berry,
because different bits contain different compounds. So, let’s go
from the outside in. The skin has two types of cells, an outer layer
of clear epidermal cells, and then several layers (around 6 but it
depends on the variety) of hypodermal cells. Then there are three
different pulp tissues: the outer and inner mesocarp, and the
vascular tissue that separates them. Then we have the seed, which
has two layers of seed coat (testa) cells separated by a thin walled
parenchymal layer. The outer layer is a cuticle, and then between
this and the lignified inner testa cells there is layer of thin
walled parenchyma cells that contains almost all of the seed
phenolics. Interestingly, seed browning is now thought to be because
of the tannins present in this layer getting oxidised, and is not
associated with the process of lignification.
Non-flavonoid polyphenols: the pulp phenolics
We’ll begin in the pulp of the grape, with some polyphenols that are
found in both red and white grapes. The first distinction we will
make is between flavonoid and non-flavonoid polyphenols, and we’re
starting with the latter group. Generally speaking, these are found
in grapes and wines at low concentrations, with one exception:
hydroxycinnamic acids are the major phenolics in white wine, and are
also found in red wine. Another group is the benzoic acids, such as
gallic acid. The cinnamic and benzoic acids, also known as the ‘acid
phenols’, are small molecules and they’re often present in grapes in
a conjugated form (e.g. as esters or glycosides). These are easily
extracted from the pulp of grapes during pressing, and occur at
levels of 50–250 mg/litre. Typical levels in finished wines would be
130 mg/litre.
Other non-flavonoid phenolics found in wine include the hydrolysable
tannins (such as gallic and egallic acid) which come from oak
barrels, the volatile phenols (such as 4-ethyl phenol, produced by
Brettanomyces) and stilbenes (important in plant disease
resistance, including celebrity stilbene, the phytoalexin
resveratrol).
Flavonoid compounds – the skin and seed phenolics
These are the major phenolic compounds in grapes, and most of them
are found in grape skins, but they also come from seeds and stems.
This is where we bump into the two most famous types of the
polyphenols – anthocyanins and tannins. The flavonoid phenolic are
broken down into two groups: flavan-3-ols and flavonols.
Flavan-3-ols
First, we have flavan-3-ol monomers. These are responsible for
bitterness in wine, and they may also have an astringent taste. The
major ones are (+)-catechin, (–)-epicatechin, and (–)-epicatechin-3-O-gallate.
These are formed before veraison and change with ripening. They
mostly come from seeds.
Then we have polymers of flavan-3-ol subunits, which are generally
referred to as condensed tannins or proanthocyanidins (or
procyanidins). These are responsible for astringency in red wines
and come from the hypodermal layers of the skin and also the
parenchymal layer in the seed coat.
Most of the subunits that make up tannins are either catechin or
epicatechin, and these can be modified in various ways. The length
of this chain – the polymer length – can vary from two or three
subunits to over 30. The length of the chain is called the degree of
polymerization (DP), so a 10 DP tannins has 10 flavan-3-ol subunits
in it.
Skin tannins are usually much bigger than seed tannins, and they may
contain some epigallocatechin subunits. Seed tannins are smaller and
lack epigallocatechin subunits. They also have a higher proportion
of epicatechin gallate, which is rarely found in the skin tannins.
In the grape skins tannins will have a DP of up to 80, but the
average DP is around 30. In seeds, the average DP is 10.
Tannins in berries change during the ripening process. In the skin,
there’s little change in quantity of tannin from verasion to
harvest, but the DP changes significantly. From green to red berries
the average goes up from 7 to 11, and at harvest DP is around 30.
One of the reasons that tannins are important in wine is because of
the way that they affect the mouthfeel of the wine. The way we sense
them is by a mixture of taste and also touch. Tannins can have a
bitter taste, especially when they are smaller (with a low DP). But
the main way we sense them is by touch: they are astringent. They
bind to proteins in our saliva and then the tannin–protein complexes
precipitate, giving a drying sensation in the mouth.
It’s interesting to think about why tannins might be doing this in
our mouths. Plants form tannins as defence molecules, both for
defending against microbial attack and, because plants are extremely
vulnerable to being eaten, also acting as antifeedants (a chemical
that causes a pest, such as an insect, to stop eating). Plants are
literally rooted in place, and so they have had to go to great
lengths to make themselves unpalatable, acting as chemical factories
to produce a wide range of toxic defensive secondary metabolites, as
well as developing physical defences, such as thorns and stings.
One of the key roles of salivary proteins is to protect us from the
harmful effects of tannins by binding to them and precipitating them
before they reach the gut. This makes the plants more edible than
they otherwise would be, neutralizing one of their defences. If the
salivary proteins didn’t cause this precipitation, the tannins would
interact with digestive enzymes (which are also proteins) in our gut
and render them ineffective. This would reduce the palatability of
plant components by making them much less digestible.
The aversive taste of unripe fruits is in part due to high tannin
concentrations, with the plant using this as a way of keeping the
fruit from being consumed before the seeds are ready for dispersal,
along with colour changes and high acid/low sugar. We find the
bitter taste and astringent sensation of tannin aversive and, as
with such unpleasant oral sensations, the aversion can protect us
from harmful consumption. Thus the salivary proteins are potentially
filling two roles. They allow us to detect tannins in food and to
reject the food if the concentrations might be dangerous, and also
help neutralize any tannins present in food to be ingested.
One of the protein types found in our saliva is mucin, which is
involved in forming a lubricated, slippery protective layer over the
internal surface of the mouth. Tannins remove this lubrication,
causing a sense of dryness and puckering. This is what we describe
as “astringent.”
Related to astringency is the taste of bitterness. The majority of
tannins are chiefly sensed as astringent, but they can also be
tasted as ‘bitter’ when they are small enough to interact with
bitter receptors on the tongue. Tannins seem to reach their most
bitter taste at a DP of 4, and then decrease in bitterness and
increase in astringency, with this astringency peaking at a DP of 7
(according to some studies, at least – others suggest it carries on
increasing to DP 20), before becoming steadily less astringent as
they become larger.
The astringent nature of tannins can be moderated by the presence of
polysaccharides (sugars) or other wine components. It is also
modified by the chemical adornments that tannins can grab, and there
are many of these. In wine, tannins are continually changing their
length (DP) and adding things to their structure. So, structurally,
wine tannins can be incredibly complicated, and researchers are
still trying to correlate mouthfeel properties with structure.
Interestingly, tannins are more astringent with lower pH (that is,
wines with higher acidity taste more astringent, even with the same
tannin content) and less astringent with increasing alcohol.
However, the bitterness of tannins rises with alcohol level, and is
unchanged by pH changes.
Fining – the addition of proteins such as egg white or gelatin to a
wine – doesn’t actually remove much tannin. The popular idea is that
the proteins bind to tannins which then precipitate out. What
happens is that the proteins do bind to tannins but largely remain
in the wine, forming colloidal or soluble complexes that likely have
reduced astringency.
Anthocyanins
Anthocyanins are also falvan-3-ols, and are the main pigments in
wine, responsible for the colour of red wines. They are found along
with tannins in the hypodermal cells of the skin, except in
teinturier (red fleshed) grape varieties, where they are also found
in the pulp. (He and colleagues have published a detailed review on
anthocyanins for anyone wanting to dig deeper.)
Over 600 anthocyanins have been identified in nature, and these are
formed from six different basic anthocyanin structures, called
aglycones. These six are cyanidin (Cy), pelargonidin (Pg),
delphinidin (Dp), petunidin (Pt), malvidin (Mv) and peonidin (Pn).
These all differ in colour slightly with some more red, and some
more purple. Anthocyanins also differ in colour according to the pH,
with a more red colour at low pH and a more blue colour at high pH:
this can be seen when you rinse out a wine glass with a tiny bit of
red wine in it with tap water: the rise in pH causes the trace of
wine to go from red to blue-black.
These basic aglycones are not very stable, but may be modified by
chemical processes called glycosylation and acylation, which
improves their stability. The acylation can take place with the
addition of acetic, p-coumaric and caffeic acids. With these
sorts of modifications, there can be up to 20 different anthocyanins
in red grapes depending on the variety. It’s interesting to note
that Pinot Noir lacks acylated anthocyanins, which explains why
Pinots are usually paler in colour than most other red wines, and in
contrast Cabernet Sauvignon contains 18 of the 20 anthocyanins.
Indeed, the anthocyanin profile can be used as a fingerprint of the
grape variety, but this is only useful with fresh grapes: by the
time a wine has been made the anthocyanins will have been modified
enough for this no longer to work.
The anthocyanins can exist in several equilibrium forms in wine. One
of these forms is called the flavylium ion form and this is very
important because it is red coloured. Another form is the quinoidal
base, which has a blue colour. Only a small proportion of
anthocyanins are in these coloured forms in wine. It follows that if
more of the anthocyanins are in the flavylium or quinoidal form, the
wine will have more colour. When sulfur dioxide is added to wine it
has a bleaching effect because it binds some of the anthocyanin as a
colourless bisulfate adduct.
Anthocyanins are unstable in wine and aren’t that important for the
long-term colour of red wines. In addition to the anthocyanins there
are two major fermentation-derived colour groups. The first of these
is the pigmented polymers. These are formed by the chemical linkage
between tannins and anthocyanins. This is a covalent (strong)
linkage and is very important in forming stable colour in wines. The
evidence suggests that most of the pigmented polymer formation
occurs during fermentation, but according to some reports by the end
of fermentation about 25% of the anthocyanins are thought to be
complexed with tannins, and in barrel-aged reds this figure can rise
to 40% within a year.
Barrels help because they provide a bit of exposure to oxygen which
helps in forming these complexes, as well as supplying some extra
tannins. Acetaldehyde, the product the oxidation of alcohol, helps
to form these bonds. During fermentation, the formation of tannin–anthocyanin
complexes helps retain more of both tannins and anthocyanins in the
wine, as it makes them more soluble and stops them dropping out as a
deposit. These polymeric pigments also help with colour intensity:
around two-thirds of the anthocyanins are in a coloured form when
they are complexed with tannins, as opposed to around 20% of the
free anthocyanins that are in a coloured form.
Then there’s another group called the anthocyanin-derived pigments,
which arise from reactions between anthocyanins and other phenolics
and aldehydes. This is a massive, complicated class of pigments, and
is an area of intense current research, with new members are being
added all the time. These are referred to as pyroanthocyanins, and
include the vitisins, portisins and oxovitisins. They are stable and
are resistant to sulfur dioxide bleaching. Most of them have a
yellow/orange colour with the exception of portisins which are blue.
The phenomenon of copigmentation
needs a mention. The coloured anthocyanins (red flavilium or blue
quinoidal base) are planar structures and these can react with other
planar structures (in this case these will be referred to as
copigments) such that they form a molecular stack that excludes
water. This protects the anthocyanins from hydration, increases the
colour intensity and shifts the colour towards purple. The
copigments are usually other phenolic compounds, and in particular
the favonols, which we will discuss below. This is one of the
reasons that red grapes are sometimes co-fermented with a small
proportion of white grapes: the white grape skins provide copigments
so even though you’d expect that including white grapes would reduce
the colour of the wine, they don’t. In fact, the opposite occurs.
The classic combination is a small proportion of Viognier together
with Syrah/Shiraz. But flavonol concentration is also increased by
UV light exposure, so you might expect grapes that have experienced
higher sunlight exposure to show a deeper colour through increased
copigmentation effects.
Flavonols
The final group of phenolic compounds we will look at is the
flavonols. They are found in the skins of both red and white grapes
and act as sunscreens against UV-A and UV-B light wavelengths.
Flavonol levels increase in response to enhanced UV exposure. They
have a yellow colour which can contribute to the colour of white
wines, but which is masked in red wines. The most important of the
flavonols is quercitin, but kaempferol, myrcetin, laricitrin,
isorhamnetin and syringetin are also found. White grapes lack
myricetin, laricitrin and syringetin.
They have high antioxidant capability, but perhaps their most
important role is as acting as co-pigments with anthocyanins to
increase the colour of red wines.
See
also:
 Tannins
in wine: a review
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