Thiols and beyond
The science of Sauvignon Blanc
[This is a written version of a
presentation I made at Sauvignon 2016 in Blenheim, New Zealand]
It would be fair to consider Sauvignon
Blanc as a model system for understanding wine flavour. Over recent
years, the New Zealand wine industry has funded a significant
research project that has looked at all the whole circle of the
science of Sauvignon, from grape growing, to winemaking, to
perception of wine. It has revealed a lot about the connections
between terroir, viticulture, winemaking and wine flavour, and
there's no other grape variety that we understand so much about. And
one group of chemicals in particular has become newly trendy: thiols.
Five years ago, no one had ever heard of them. Now, people are
putting 'thiols' in their tasting notes. Does anyone really know
what is meant by this term?
The simple story: what makes Marlborough
Sauvignon so distinctive?
There's a simple story, and
this is a good place to start in understanding the success of Kiwi
Sauvignon. It's not a completely accurate story, and there are some
missing details, but this is a good place to start.
The two impact
compounds important in Marlborough Sauvignon are methoxypyrazines
and thiols. Marlborough Sauvignon is particularly high in these two
elements. Methoxypyazines are responsible for green flavours: think
green pepper, tomato leaf, grassiness. And thiols are responsible
for passionfruit, grapefruit and tropical fruit aromatics. So good
Marlborough Sauvignon is about greenness allied to thiol
aromatic interest. This is what sets it apart.
What is a thiol?
So what is a thiol? Also
known as mercaptans, thiols are sulfur-containing organic compounds
with a sulfur atom bound to a hydrogen atom, which creates the SH
group ('sulfhydryl' or 'thiol' group). Sulfur containing volatile
compounds are often very smelly, and there are a whole group of them
that, in wine, contribute to fruit aromas (e.g. blackcurrants,
grapefruit, passionfruit and guava), and they also play a role in
the aromas of other foods and drinks, such as roasted coffee,
popcorn, grilled meat and beer.
Sauvignon Blanc has three thiols that
are important in varietal aroma:
Perception threshold in
Concentration found in NZ
Factors affecting thiol levels
So which factors affect thiol
levels, and if they are desirable how can their concentrations be
increased? And why are they so high in Marlborough Sauvignon Blanc?
Thiols aren't present in the grapes.
Instead, they are formed from precursors present in grape must by
the action of yeasts. And only a portion of the precursor pool in
the grapes is transformed into thiols by the yeasts. It's not simply
a question of getting the right yeast strain: if it were that
simple, anyone could make high thiol wines. Yeasts do have some
effect, but it is limited.
There are many factors that affect the
levels of precursors in the grapes, and it isn't a simple story.
There's the unique high UV environment that New Zealand's vineyards
enjoy. There are site influences, and vintage influences – some
sites typically produce higher thiol Sauvignons, and sometimes a
site produces a high thiol Sauvignon one year and not another.
There's a well known phenomenon where the first crop off a site
often wins lots of medals because of high thiol levels. And there's
the interesting observation that machine-picked fruit typically has
around 10 times the level of thiols than handpicked fruit. What is
it about machine harvesting that has this effect? Could it be the
disruption of picking sets some enzymatic changes in place which
then result in the rise of thiol precursors in the must? Or, a more
novel idea: could machine harvesting be causing the release of
semiochemicals (volatile alarm signals) that then signal to nearby
vines upstream of the block being harvest, that then results in
physiological changes, raising the levels of thiol precursors.
Producing Sauvignon with elevated thiols
So how, practically, can you
make a Sauvignon with elevated thiol levels? We still don't know
exactly which environmental factors affect thiol precursor levels.
But the key is converting more of those precursors into the actual
thiols during fermentation. There seem to be two important ways of
doing this. First, it is important to protect the C6 green leaf
volatiles – precursors of thiols – by getting SO2 in early. The
second is to work with elevated levels of H2S (hydrogen sulfide)
very early in ferments. One way of achieving this is to start a
ferment that you deliberately allow to get stinky with lots of H2S,
such as by working with full solids, stressing the yeasts, having
low nitrogen levels and then piping this stinky H2S into the main
ferments at an early stage. This toying with reduction is really
So, once you have these
thiols, what can you do to preserve them? The key thing is
temperature of storage, because the very aromatic passion fruit
thiol 3MHA is transformed back to 3MH by acid hydrolysis. If
Sauvignon is stored at 10 C then after two years you will have lost
half your 3MHA. If you store it at 20 C then you lose half every two
months. So shipping temperatures are important, and you might want
to store wine cool and have several bottling runs.
This is the simple story. But
there's a more complex story, too. Humans are not measuring devices;
green is more than just methoxypyrazines; and thiols aren't quite as
important as we thought they were.
Wine flavour chemistry
First, we need to think a bit
about wine flavour chemistry. The wine is a whole, and we need to
consider the flavour molecules in wine together. It's possible to
split wine aroma chemicals into three main groups: global wine
aroma, contributory compounds and impact compounds.
Global wine aroma
This is the set of molecules
found in almost all wines which gives the base wine aroma. Wine
smells of wine!
Ethanol (don’t underestimate this one!)
Higher alcohols (e.g. butyric, isoamylic,
One present in grapes (β-damascenone);
rest produced by metabolism of yeasts
20 aromatic chemicals in all wines make
global wine odour
Acids (acetic, butyric, hexanoic,
Ethyl esters from fatty acids
These contributory compounds
are a group of 16 compounds present in most wines, but at low
levels. They frequently occur below the detection threshold, but
they have an effect that is synergistic with other aroma compounds.
Then we have the impact compounds. Not all
varieties have these, but many do. They are aroma molecules with a
strong imprint on the wine. These are some examples:
Polyfunctional thiols (mercaptans) -
4MMP, which has a box tree aroma (4.2 ng/litre detection threshold),
3MHA, which has a tropical fruit scent (60
ng/litre) and 3MH
Rotundone: gives pepperiness to Syrah,
at incredibly tiny concentrations
2-methoxy-3-isobutylpyrazine (MIBP) is the key
Monoterpenes, such as linalool, floral,
Rose-cis oxide: characteristic of
The non-volatile wine matrix
These are wine constituents
that don’t have any aromatic characteristic of their own influence
strongly the way that the various aromatic molecules present in wine
are perceived. Work by Vicente Ferreira has looked at this, with
surprising results. If you take white wine aromatics and add them to
the non-volatile wine matrix of a red wine, the wine will smell like
a red wine, and vice versa. The matrix is having a powerful effect
on the smell of the wine, even though it doesn't smell itself.
The significance? It's important to
consider the wine as a whole. While we sometimes need to break wine
down into its components to understand it, we need to be aware that
all these aroma compounds act together to create wine flavour. This
is where reconstitution experiments are interesting.
Work by Frank Benkwitz, Laura
Nicolau and colleagues has examined Sauvignon aroma by looking to
see which the important aroma compounds are, and then taking a
deodorized wine base and adding these compounds at their original
concentrations to create a model wine. The power of this approach is
you get to look at how aroma compounds interact in the context of
the wine. You can take out one compound at a time, or in groups, or
in any combination you want. Some of the results are very
The terpenes linalool and α-terpineol
had a huge impact when omitted, even though they are present at
relatively low concentrations. If esters are removed, there is a
small drop in intensity for most of the descriptors, and a large
decrease in ‘passion fruit skin stalk’ and ‘sweet sweaty passion
fruit’, both of which were previously thought to be associated with
thiols. What these results indicate is that the thiol story is just
a little bit too simple, and we need to think about interactions
among the different flavour molecules.
One further twist. Green is more than
just methoxypyrazine. Recent results indicate that the green pepper
character comes from the green-smelling C6 compounds such as hexanal,
as well as the methoxypyrazines. Also, the mix of thiols and other
compounds can also add to greenness. Asparagus aroma has an
influence from dimethyl sulfide.
What can we conclude? Thiols and
methoxypyrazines are elevated in Marlborough Sauvignon, and they are
important in helping produce such distinctive wines, with green
allied to lovely fruity aromatics. But there's more to Sauvignon
than just these two impact compounds. This is a list of some of the
important compounds in Sauvignon aroma.
wine is made
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