Wood for food: a primer on pyrolysis

by Guillemette Barthouil


 Overview

Smoking is valued nowadays not only for preserving food, but also as a distinct set of flavours that can reflect the landscape of the region. Pyrolysis, the thermochemical decomposition of organic matter, breaks apart the three main components of wood: cellulose, hemicellulose and lignin. Each influences the colour, flavour, preservation and surface texture of the food – this information is summarized in Figure 4 below. Controlling pyrolysis is the key know-how involved in good smoking. Overall, for best results the wood moisture should be lower then 25% and its combustion temperature around 400°C.


“Smoking
is one of the oldest food preservation methods, probably having
arisen shortly after the development of cooking with fire.”
(Encyclopedia Brittanica)

Most
articles I have read for this research have started with the same
statement. Humanity has used this technique for so long, it seems
rather strange to try to explain it – but that is our job. All
chefs know one has to understand a product or technique in order to
make the most out of it. We use some scientific knowledge to enhance
the desired, latent characteristics of a product, with the goal of
better understanding how traditional knowledge could have been
cultivated in a geographic area. And smoking is a great example.

The
food history of the Nordic countries is particularly characterised by
a ‘storage economy’ – a concept applicable to most parts of the
world with seasons, here emphasised by the extreme climate. Most
processes that we study -– drying, salting, smoking, fermenting,
and others – were indispensable in these conditions. By increasing
heat and lowering humidity, smoking enabled food to dry up more
quickly, preserving it for long periods of time.

IMG_1891.JPG

In
the past decades, the taste for salted, dried and fermented food has
shifted toward fresher food, gravlax being a good
example. The taste for smoked products, however, has remained.
Smoking is now not only important for preserving food, it is a valued
flavor that can reflect the landscape of the region. Smoking in time
and place depends on many things: the seasonal fish available, the
smoking technique and temperature used – for cold smoking, the
temperature drops the further north you go – and most importantly,
the type of wood. Trees are part of the landscape before they become
an essential component of the product’s taste. Alder trees growing
in humid areas have been used along the west coastline of Norway and
on the island of Bornholm in the Baltic; birch is a typical flavor
from mountainous zones; dried manure is still used in the windy and
harsh Icelandic land since trees and shrubs are in short supply; this
list could go on. Understanding these traditions allows us to think
of other flavors that could emerge from a specific land as we come to
know it. Gaining a deeper understanding of wood pyrolysis and the
science of smoke will also help us in our choices.

Smoke
is not a simple gas. It is a mixture of three state of matter: an
aerosol of solid particles, liquid drops and vaporized chemicals.
These vaporized chemicals only count for 10% of the volume but do
more then 90% of the job. And as you might have guessed, the
composition of the smoke depends on the nature of the burning fuel
and the conditions of combustion.

Pyrolysis
is a thermochemical decomposition of organic matter brought about by
high temperatures. To produce quality smoke the wood should undergo
an incomplete combustion of organic materials in the presence of
limited oxygen and medium temperature.

Wood
consists of three primary materials. The wood’s cell wall is
composed of micro-fibrils of cellulose (40 to 50%) and hemicellulose
(15 to 25%) impregnated with lignin (15 to 30%).

 Figure 1 – Wood composition

Figure 1 – Wood composition

Cellulose is a carbohydrate polymer – a linear chain of D-glucose (between 15 to 15000) which forms the framework and the substance of all plant cell walls.

 Figure 2 – Cellulose

Figure 2 – Cellulose

Hemicellulose
is a matrix of polysaccharides present along cellulose is almost all
plant walls. It differs from cellulose in that hemicellulose is a
shorter chain branched polymer with low molecular weight. While
cellulose is composed only of glucose, hemicellulose can include
other sugars like xylose, mannose (both found predominantly in
hardwood trees), galactose (in softwood trees), rhamnose and
arabinose. Though both aggregates of different sugars, cellulose and
hemicellulose break down into similar molecules during the incomplete
combustion.

 Figure 3 – Hemicellulose

Figure 3 – Hemicellulose

Lignin
provides
compressional strength to the cell wall, unlike the flexible strength
conferred by cellulose. Without lignin, terrestrial plants probably
could not have reached the sizes they do today, as cellulose by
itself does not provide enough resistance to gravity.
It is made of intricately interlocked phenolic molecules –
essentially rings of carbon atoms with various additional chemical
group attached – and its one of the most complex known natural
substances. The higher the lignin content of wood, the harder it is
and the hotter it burns: its combustion releases 50% more heat than
cellulose.

The
sugar in cellulose and hemicellulose breaks apart into many of the
same molecules found in caramel. They are carbonyl molecules that
react with amino acids and sugars to create a Maillard-type reaction
that generates new flavours and yellow to dark brown color. During
pyrolysis, sweet maltitols, bread-like aroma from furans, nutty
lactones, and other volatile molecules are produced. All together
they soften the heavy phenolic compounds.

The
interlocked phenolic rings of lignin break apart from each other into
smaller, volatile phenols and other molecules which most often have
specific aromas. The most distinctive ones are isoeugenols, one of
the main flavour component of clove, creosols that bring peat notes,
vanillin that smells as it sounds, guaiacols contributing to the
general spiciness.

The
smoke has many other effects due to many different chemicals broken
apart during the pyrolysis. The following table tries to summarise
them.

 Figure 4 – Effects and mechanisms of pyrolysis

Figure 4 – Effects and mechanisms of pyrolysis As
you might have noticed in the ‘notes’ column, capturing the
desired smoking effects depends mainly on the wood humidity and the
smouldering temperature. Controlling the pyrolysis is the key
know-how of a good smoker. Freshly-cut
wood contains 40-60% moisture which is not suitable for smoking. A
good wood containing less than 25% moisture is preferred.
Professionals agree that the combustion temperature is best around
400°C. Higher than this, the flavour molecules are broken down into
simpler, harsh, or flavourless molecules. Lower pyrolysis – under
200°C – degrades cellulose and hemicellulose into acetic, formic
and other acids. These acids play an important preservative role but
they also make the food taste acrid at high levels. High lignin woods
require special attention since they burn too hot unless their
combustion is slowed by restricted airflow or higher moisture.

 Figure 5 – Smouldering temperatures

Figure 5 – Smouldering temperatures

Controlling
the smouldering temperature is not only important for flavours, it
influences the PAH concentration. Polycyclic Aromatic Hydrocarbons
(PAHs) are formed in incomplete combustion processes, which occur
whenever wood, coal or oil are burnt. Particular attention has been
paid to the highly carcinogenic
benzo[a]pyrene (BaP). A
number of studies on smoked foods reveals that the highest levels of
PAHs are found in products from traditional kilns that use
smouldering wood or sawdust. In such kilns the combustion temperature
is difficult to control and is usually very high. Toth
and Blaas (1972) found that there is a linear rise in the
concentrations of BaP and other PAHs in the smoke phase between smoke
production temperatures of 400 – 1000 °C. These traditional
techniques are often considered synonymous with ‘quality’ and
‘authenticity’…. so what should we do then? Eat
industrially-smoked products because they are ‘more healthy’? My
personal choice will always go toward taste but this does not mean we
cannot adapt traditions to make the best of both kinds of knowledge.
It has for example been proven that both cold-smoked and hot-smoked
fish by an external smoke generator had lower PAH content. Moreover a
Danish study reported that a cold-smoked fish has a lower BaP
concentration then hot-smoked ones. In
order to diminish the contamination of smoked products by PAH, the
clear recommendation is to use indirect smoking, preferably cold
smoke, and to maintain your smouldering temperature around 400°C.

These
conclusions stimulated me to experiment a little.

Hot-smoked
fishes are nowadays more popular in Scandinavia then cold-smoked
ones, mackerel being the most consumed. How could we achieve this
same taste without hot-smoking the fish and keeping the PAH low?
Hot-smoking occurs within the range of 52°C to 80°C.  At this
temperature food cooks while smoking. I therefore decided to cook the
fish first in our combi oven and then cold-smoke it.

IMG_1708.JPG

After
a few experiments trying to find the best cooking temperature and
humidity, here is the recipe:

Cold-smoked mackerel

For
very fresh mackerel of around 400 g.

Gut
them and brine them 8 h at 4°C in a solution of 20% salt of the total.

The
aim is to reach a salt content of 3% in the fish … like the salinity of
seawater in the Atlantic ocean…

Remove fish from the brine and place on a perforated tray. 

Cook
them in the oven at 70°C with 60% humidity.

Program
your combi oven so when the core temperature reach 59°C the oven
stops.

Take
them out of the oven and cool them down in the blast freezer until
they reach 4°C.

Cold
smoke them for about 24h. We used beech wood but many other options
are possible.

One
of the great thing about this technique is that you can accurately
control cooking temperature and humidity, things that are difficult
with a hot-smoker. This control ensures the smoke flavours stay light
and delicate, a taste certainly more suited to a contemporary palate.

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Bibliography

Duedahl-Olesen,
L.; White, S.; Binderup, M.L. Polycyclic Aromatic Hydrocarbons (PAH)
in Danish Smoked Fish and Meat Products, Polycyclic
Aromatic Compounds,

Vol. 26, 3, 2006, p. 163-184

Knockeart,
C., (1990), Le fumage du poisson, IFREMER

http://archimer.ifremer.fr/doc/00004/11490/8046.pdf

Mc
Gee, H. (2004), Food & Cooking: an encyclopedia of kitchen
science, history and culture, Hodder and Stoughton.

Myhrvold,
N. and al. (2011), The modernist cuisine: the art and science of
cooking, The cooking Lab, Vol 2, p 134 – 149.

Siesby Birgit, (1997), Scandinavian ways with fish, Fish: Food from Waters, Oxford Symposium, p 280 – 282

Toth,
L., Blaas, W., (1972). The effect of smoking technology on the
content of carcinogenic hydrocarbons in smoked meat products.
Fleischwirtschaft, 52, 1419-1422.

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