Publication from the Chocolate Review
by Samantha Madell
Chocolate flavour chemistry
What is flavour?
Flavour
is essentially caused by the combined effects of aroma, plus tastes
(tastes being: bitter, acidic, sweet, salty, and umami).
Scientists
have so far identified more than 600 volatile chemicals in cocoa and
chocolate [1], which collectively make up the classic aroma of
chocolate.
The tastes that occur naturally in cocoa are
primarily bitterness, and (especially after fermentation) acidity.
Sweetness is added to cocoa, usually in the form of sugar. And
sometimes, poor fermentation, drying, or roasting techniques might
result in the production of umami-type tastes, such as hamminess.
The
flavour of chocolate - while incredibly complex, and practically
impossible to synthesise - is not quite as mysterious as most people
seem to believe. Below, I've outlined what is known about chocolate
flavour development.
Chocolatey and nutty flavours
Classic
chocolate and nut flavours in cocoa are produced predominantly by
aldehydes and pyrazines, respectively. Aldehydes and pyrazines are
volatile chemicals which are produced in cocoa during roasting, as a
result of Maillard reactions.
To back-track one step: during
fermentation, sucrose in the cocoa bean is broken down by enzymes into
simple sugars, namely fructose and glucose. Simultaneously, a cocoa
protein is broken down by enzymes into amino acids and peptides. These
smaller molecules are known as "aroma precursors".
During
roasting, the aroma precursors react with the simple sugars under the
influence of heat to create new aroma chemicals, including a broad
array of aldehydes and pyrazines.
The aroma precursors and
aromas in Criollo and Forastero beans are essentially identical.
However, because Forastero beans are normally fermented for longer than
Criollos, they typically contain more aroma precursors prior to
roasting, and more aroma chemicals after roasting [2]. Which brings me
to my next point ...
Mild flavour (as traditionally associated with Criollo cocoa)
Many
experts (such as Beckett; Cook; Wood and Lass) have observed that
Criollo cocoa beans are characterised by a "mild" flavour. Wood and
Lass actually call it a "mild or weak chocolate flavour" [3]. Presilla
calls it a "neutral" flavour [4].
Criollo beans are
traditionally fermented for a much shorter time than Forastero beans.
As a result of short fermentation, Criollo beans are typically less
acidic than Forasteros (see notes below about acidity), and they end up
with fewer aroma precursors, and hence, fewer aromas after roasting.
Less acidity + fewer aroma chemicals = "mild", "weak", "neutral" flavour.
Low Acidity
As
one of the last steps in chocolate manufacturing, conching allows
acetic acid to evaporate out of chocolate. But there are other
important causes of low acidity that can prevent a chocolate
manufacturer from having to conch acidity out in the first place, such
as:
Gentle drying of beans after fermentation.
If the cocoa
beans are allowed to dry gently, either in the sun, or with
skillfully-managed forced-drying, then acetic acid will be able to
evaporate and escape through the shell during drying. (On the other
hand, rapid forced-drying is widely accepted to be the most significant
cause of excessive acidity, which I mention below).
Storage of harvested pods for up to 2 weeks before fermentation.
This
practice (which I don't personally advocate) causes a decrease in the
volume, as well as the water- and sugar-content of the pulp. The drier,
less sugary pulp reduces the microbial activity, meaning that less acid
is produced, and the resulting beans are therefore less acidic. The
same result has been achieved by spreading beans out to partially dry
prior to fermentation.
Excessive Acidity
During
fermentation, the cocoa cotyledons become more acidic (i.e. their pH
drops) as they are penetrated by acid (primarily acetic acid).
Acetic
acid is produced by bacteria belonging to the Acetobacteraceae family.
These bacteria dominate the second (aerobic) phase of fermentation.
Acetic acid production peaks about 72 hours after fermentation begins
[5], and Biehl has noted that diffusion of acetic acid through the
whole bean is slow [6].
However, it's quite common for Criollo
cocoa beans to be fermented for less than 72 hours - sometimes for only
48 hours. Hence, a short fermentation time typically produces less
acidic beans than a longer fermentation. (A short fermentation also has
major implications relating to taste and flavour development, as
mentioned under the "mild flavour" heading above).
The pH of
freshly-harvested beans is typically about 6.3. By the end of
fermentation and drying, the beans are more acidic, with a lower pH of
about 4.5 - 5.8.
Excessive acidity in dried cocoa beans is most
often caused by force-drying the beans too quickly, at too high a
temperature. This causes the shells to dry and harden very quickly,
thereby becoming impermeable to the acetic acid, which is then trapped
inside the bean [7]. (In practice, water is still able to permeate the
dried shell and escape ... I assume this is because water has a higher
vapor pressure than acetic acid).
Astringency
Polyphenols (such as tannins) are a major source of astringency in cocoa.
During
fermentation, polyphenols are converted (with the help of enzymes such
as polyphenol oxidase) into different chemicals within the bean, which
are much less astringent. Some of these reactions are essentially the
same as the aging process in red wine, whereby harsh-tasting
polyphenols polymerise into different chemicals which feel more
pleasant and "softer" on the tongue.
The fact that dried
(unroasted) cocoa beans are brown is proof of the presence of tannins
in the beans: the browning that takes place during fermentation is
caused by the enzymatic oxidation of tannin-based molecules. (Not to be
confused with the non-enzymatic browning that takes place during
roasting, courtesy of Maillard reactions).
Bitterness
Polyphenols are not only astringent (as mentioned above), they're also bitter.
Some bitterness can also be attributed to the alkaloids found in cocoa, namely theobromine and caffeine.
But
it's also worth noting that cocoa is naturally bitter. Contrary to what
some people believe, it's really perfectly normal for cocoa to be
somewhat bitter.
For instance, some people say that Criollo
cocoa doesn't have any bitterness at all, which is simply not true. For
example, see the study by Counet et al (2004): "Relationship between
procyanidin and flavor contents of cocoa liquors from different
origins". Counet's study is available in PDF format here:
http://www.uclouvain.be/cps/ucl/doc/inbr/documents/relation(1).pdf
Also,
if Criollo cocoa had no bitterness, then you wouldn't find premium
Criollo chocolate makers adding 30% sugar to their products. Thirty
percent is really a lot of sugar - enough to make any product sickly
sweet if, indeed, the base ingredient was "naturally sweet" to begin
with (which Criollo beans are not).
Caramel flavor
Roasting can produce caramel flavours - although not technically via caramelization, but rather via Maillard reactions.
Also,
conching has been found to substantially increase the levels of some
aroma chemicals, including furaneol, in chocolate. Furaneol has a
caramel aroma, and interestingly, it is also known as "strawberry
furanone", because it occurs naturally in strawberries, and is an
important contributor to the classic strawberry aroma.
Some
premium chocolates are said to have a "strawberries and cream"
character. Lengthy conching is likely to be a major contributor to this
particular characteristic.
Creaminess
A "creamy" character can be produced by long conching (see my comments above regarding the "strawberries and cream" hypothesis).
Floral notes
Roasting
can play a role in producing floral notes (for example, the aroma
compound phenylacetaldehyde is formed during roasting, and it has a
floral/honey aroma). But there are also undoubtedly other factors at
work in producing floral aromas in cocoa. Precisely what those factors
are remains something of a mystery.
For example, one of the most
common floral-smelling chemicals found in cocoa is linalool, a terpene
which occurs naturally in many flowers and fruits. Exactly where the
linalool in fermented cocoa beans comes from is uncertain, but it's
definitely there after fermentation, and prior to roasting [8].
Linalool
can be naturally synthesised by plants, and it can also be synthesised
by some yeasts, including Saccharomyces and Kluyveromyces, both of
which are associated with cocoa fermentation [9], [12]. This means that
the fermentation process itself is a possible source of linalool in
fermented cocoa beans.
Numerous different factors can affect the
synthesis of linalool, as well as other fragrant, floral-smelling
chemicals. These factors include:
• A plant's
symbiosis with soil-borne arbuscular mycorrhizal fungus (AMF). AMF
populations are adversely affected by fungicides, and low plant
biodiversity. Hence, a monocultural cocoa plantation which is routinely
treated with fungicides would be a hostile environment for these highly
beneficial mycorrhizae.
• Availability of nutrients
and sunlight. For example, scientists have found that high light levels
plus high nutrient levels can increase the amount of terpenes produced
by plants [10].
• The strains of yeast involved in
the fermentation, as mentioned above. For example, different yeast
populations are known to have an effect on the sensory character of
wines [11], and also on cocoa [12].
In 1963, in his book
"Chocolate Production and Use", L. Russell Cook wrote that the
distinctly floral aroma of Ecuador's Arriba cocoa beans had been
"deteriorating slowly for the past several decades" (p.63). Cook also
wrote that when Arriba trees were planted in countries other than
Ecuador, the trees produced "not a trace of the floral aroma for which
Arriba cocoa is sought".
If Cook was correct in his
observations, then it seems clear that Arriba's floral aroma is
strongly influenced by environmental factors. If terroir is partially
or wholly responsible for Arriba's distinct floral aroma, then one or
more of the factors listed above might be responsible.
An
alternate theory is that the Arriba flavour has disappeared because the
Arriba trees have been replaced with different varieties, most notably
CCN51. But Cook described the gradual diminution of the Arriba flavour
before CCN51 even existed. (The first CCN51 clone was produced in 1965,
two years after Cook's book was published).
To digress briefly
on the subject of soil nutrients: many chocolate lovers are probably
unaware that about 25kg of nitrogen per hectare of cocoa is removed
from plantations in the form of dried cocoa beans every year. That
nitrogen (and other nutrients) must be replaced somehow, or the
plantation will inevitably yield less and less over time. And it is
possible that a net-export of nutrients might have contributed to the
gradual loss of floral aromas in Ecuador's cocoa.
Fruity flavors
Fruitiness
in cocoa is strongly correlated with acidity, and it is well known that
fermentation increases the acidity (and therefore lowers the pH) of
cocoa beans (for reasons mentioned above).
Also, according to
Wood and Lass, cocoa beans naturally contain small amounts of citric
acid (0.5% after fermentation and drying), which may be enough to
provide a citrus zing.
If you have any doubt about the
relationship between acidity and fruitiness, try tasting some Dutched
(i.e. alkalized) cocoa powder, which is notoriously lacking in fruity
characteristics.
As Alice Medrich writes in her book, BitterSweet:
"The
best quality natural [i.e. nonalkalized] cocoa powder is complex and
fruity and wonderfully flavorful. [...]In reducing acidity, Dutching
strips some of the natural fruit flavors, high notes, and complexity
from cocoa." (pp.48,49)
Fruity (and floral) aromas can also come in the form of higher alcohols, produced by yeasts during fermentation.
Then
there's the effect (mentioned above) of conching, which can
substantially increase the amount of some fruity aroma chemicals, such
as strawberry furanone, in chocolate.
Clearly, the contributors to fruity flavours in cocoa are many and varied!
Hamminess
Hammy
flavours in cocoa are sometimes associated with smokiness. Ham is a
cured meat, which may - or may not - be smoked. Smoke contamination
does undoubtedly cause a smokey flavour in cocoa, but I'm not convinced
that "smokiness" and "hamminess" necessarily go hand in hand, or that
hamminess is caused by smoke.
Over-fermentation is known to cause "hammy" flavours ... but the important question is, why?
Conflicting
theories exist on this subject, so I'm not certain about the precise
cause (or causes) of "hamminess", but I do have a theory ...
Endoprotease
(a type of enzyme that breaks down the globulin storage protein during
cocoa fermentation) is highly stable, meaning that it is not
de-activated during fermentation. Hence, in beans left to ferment for
too long, endoprotease (and another enzyme, carboxypeptidase) will
eventually get around to breaking down the albumin storage protein, too.
When
cocoa albumin is broken down into its constituent parts by these
enzymes, it predominantly releases the following amino acids:
aspartic acid,
glutamic acid, and
asparagine.
Those
amino acids all have an umami (i.e. savoury) taste, and are associated
with foods like seaweed, asparagus, eggs, and meats.
Hence, my
theory is that the "hamminess" associated with over-fermentation comes
from the breakdown of the cocoa albumin into savoury-tasting amino
acids.
I also believe that deamidation of these amino acids
plays a role, producing ammonia, and yet more meaty flavours, but my
chemistry is on shaky ground here!
Heavy metal contamination
Cadmium in cocoa is likely to come from the soil that the tree was grown in - but what about lead?
Lead
contamination is a serious problem in chocolate products, but
above-ground contamination from leaded fuels and machinery seem to be
the major culprits.
Here's a relevant study that readers might
find useful and interesting: Lead Contamination in Cocoa and Cocoa
Products On this subject, does anyone know for sure what the source of
Dagoba's lead contamination turned out to be? I've read in several
places that the problem was traced to some lead solder used on one of
their grinding machines.
Smokey flavour
Using wood fires to
dry beans can cause a smokey flavour in the beans - but only if the
dryer is of a poor design (for example, it has no chimney), or is
poorly maintained (for example, the heat pipes can develop holes over
time). In other words, not all wood fire dried beans end up tasting
smokey.
Smoke contamination often happens because growers can't
afford to maintain or repair their dryers (not because they're lazy or
careless).
Sweetness
I
disagree with the popular notion that certain cocoa varieties are
"naturally sweet", although the issue here might come down to what an
individual's definition of sweetness is. For example, people quite
often describe the aroma of our 100%, unsweetened chocolate as "sweet"
(and then they get a surprise when they start munching on a mouthful of
it!)
There's nothing that I'm aware of in fermented cocoa that
would trigger the sweetness receptors in human taste buds. For example,
cocoa liquor contains 0% sugars.
But there seems to be a surprising amount of confusion surrounding the innocent little word "sweet".
I
define sweetness as a taste, which is detected by the taste buds.
Hence, by my definition, sweetness (as caused by sucrose, for example)
has no aroma, and is not inherently linked to aroma at all. But many
people expand the concept of sweetness to include the aroma of
"sweet-smelling" things like soap, perfume, flowers, and unsweetened
chocolate - all of which may taste anything but sweet!
There's
always a more accurate and illuminating term for any given aroma than
"sweet" - such as floral, honey, caramel, or chocolate. But sometimes
it can be really hard to figure out precisely what it is you're
smelling, and "sweet" seems to be a natural fall-back descriptor.
Finally,
as I mentioned earlier, if cocoa was sweet, there'd be no need to add
sugar to it to make it palatable. Some manufacturers do sell chocolate
with little or no sweetener, but the majority of people - even
dedicated chocolate connoisseurs - find such chocolate inedible. And
even Amedei's Porcelana bar was 30% sugar, last time I checked.
References:
1 Counet C.; Callemien, D.; Ouwerx C.; Collin S. (2002)
Use
of Gas Chromatography−Olfactometry To Identify Key Odorant
Compounds in Dark Chocolate. Comparison of Samples before and after
Conching, Journal of Agricultural and Food Chemistry, Vol. 50, pp.2385-2391
2 Counet C.; Ouwerx C.; Rosoux D.; Collin S. (2004)
Relationship between procyanidin and flavor contents of cocoa liquors from different origins, Journal of Agricultural and Food Chemistry, Vol. 52, pp.6243-9
3 Wood and Lass (1985)
Cocoa, 4th ed., Longman, London.
4 Maricel E. Presilla (2001)
The New Taste of Chocolate, Ten Speed Press, California.
5 R.F. Schwan (1998)
Cocoa Fermentations Conducted with a Defined Microbial Cocktail Inoculum, Applied and Environmental Microbiology, Vol.64 Issue 4, pp.1477-1483
6 B. Biehl, D. Passern, W. Sagemann (1981)
Effect of acetic acid on subcellular structures of cocoa bean cotyledons, Journal of the Science of Food and Agriculture, Vol. 33 Issue 11, pp.1101-1109
7 Haslam, E. (1998)
Practical Polyphenolics: From Structure to Molecular Recognition and and Physiological Action, Cambridge University Press
8 Ziegleder, G. (1990)
Linalool contents as characteristic of some flavor grade cocoas, Lebensmitteluntersuchung und -Forschung, Vol. 191, No. 4, pp.306-309
9 Carrau; Medina; Boido; Farina; Gaggero; Dellacassa; Versini; Henschke (2004)
De novo synthesis of monoterpenes by Saccharomyces cerevisiae wine yeasts, FEMS Microbiology Letters, Vol. 243 Issue 1, pp. 107-115
10 Lawler, I.R.; Foley, W.J.; Woodrow, I.E.; Cork, S.J. (1997)
The
effects of elevated CO2 atmospheres on the nutritional quality of
Eucalyptus foliage and its interaction with soil nutrient and light
availability, Oecologia, Vol. 109, pp.59-68
11 Vilanova, M.; Massneuf-Pomarede, I. (2005)
Characterization
of yeast strains from rías baixas (NW Spain) and their
contribution to the fermentation of albarino wine, Annals of microbiology, vol. 55, no. 1, pp.23-26
12 Leal, G.A; Gomes, L.H; Efraim, P; de Almeida T; Flavio C; Figueira, A. (2008)
Fermentation
of cacao (Theobroma cacao L.) seeds with a hybrid Kluyveromyces
marxianus strain improved product quality attributes, FEMS Yeast Research, Vol. 8, No. 5, August 2008 , pp.788-798
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