Maillard Reactions 101: Theory
Charlie Scandrett, email@example.com
Posted to HBD 2390, 4/4/97
There are three basic non-enzymic browning reactions,
PYROLYSIS is simply scorching,(from the greek "pyro" - burning)
and involves the total loss of water from the sugar molecule and the breaking
of carbon-carbon linkages, i.e. the *destruction* of the sugar molecule.
This is what happened when your grandmother neglected the "candy" she was
making for toffee apples. The result was a burnt and inedible flavour.
CARAMELISATION is a heat induced *transformation* of reducing
sugars alone in a *concentrated* solution, through so called "anhydro sugars".
In this reaction the simpler sugars *lose water molecules from their structure*
through a process called "1:2 & 2:3-enolisation". This process is influenced
by pH and is a "steering" process for both Caramel and Maillard reactions.
Through many intermediates, and in the pH 2-7 range, D-fructose for example
can give rise to the Furans, Isomaltol and Maltol, well known bread crust
flavour/aromas. No compounds containing nitrogen result.
The commercial production of beer "caramel" is produced by boiling fermentable
*sugars* in the presence of ammonia, so it is really partly a Maillard
reaction ( a bitter one too). Ammonia is a source of nitrogen for this
reaction, because the pure Caramel reaction alone doesn't produce enough
*colour*. The solution is boiled until it thickens and the boiling point
reaches 130°C. Further thickening or a rise in temperature is then
avoided until the desired colour/flavour is reached. The pH is about 4-6
and it is called a "positive" caramel because that is the electric charge
of the resulting molecules. "Negative" caramels are produced at higher
temperatures and form different compounds and can affect clarity of drinks.
MAILLARD browning reactions involve simple sugars and amino acids
and simple peptides. They proceed during the kilning of malt, and during
wort boiling. They begin to occur at lower temperatures and at higher dilutions
than caramelisation. The rate can increase by 2-3 times for each 10°C
rise in temperature. However even long term storage of malt extract will
Maillard-brown at room temperature. Prize winning dark beers have been
coloured by this method as they had none of the harshness of some high
temperature Maillard reactions in roasted malts.
Maillard reactions have three basic phases.
The outcome will depend on which amino acids and sugars are available,
and what the pH and temperature aand concentration are.
The initial reaction is the condensation of an amino acid with a simple
sugar, which loses a molecule of water to form N-substituted aldosylamine.
This is unstable and undergoes the famous "Amadori rearrangement" to form
"1-amino-1-deoxy-2-ketoses" (known as "ketosamines") which can undergo
complex subsequent dehydration, fission and polymerization reactions.
But wait, I here you say! "A sugar loses a water molecule and undergoes
further dehydration?" Sounds like a Caramelisation reaction? *And it is!*
One of the reasons Caramel and Maillard reactions are confused in brewing
and food processing literature is that one of the Maillard paths is a simple
Caramel reaction, catalysed by amino acids. But now in Maillard, there
are a few guys called Schiff, Amadori and Strecker in your beer!
The ketosamine products of the Amadori rearrangement can then react three
ways in the second phase.
One is simply further dehydration (loss of two water molecules) into reductones
& dehydro reductones. These are essentially *caramel* products and
in their reduced state are powerful antioxidants. Dark beer has about 5
times the reducing potential of pale beer.
A second is the production of short chain hydrolyctic fission products
such as diacetyl, acetol, pyruvaldehyde etc. These then undergo the famous
"Strecker degradation" with amino acids to aldehydes and by condensation
to aldols. Negative aromas like 2- & 3-methyl-butanal and other aldehydes
are also formed. This process can produce in the third phase, the favourable
and important aroma of the Hetrocyclic compounds; Furnans, Furanones and
Pyrones like Isomaltol and Maltol.(mentioned in CARAMELISATION above) These
can be pleasant caramel/roasted/bread crust aroma/flavours or acrid/burnt
aroma/flavours. However negative Strecker aldehydes do not generally appear
in finished beer in concentrations above their threshold level. Vigorous
boiling and fermentation eliminate most of the more volatile Strecker aldehydes.
A third path is the Schiff's base/furfural path. This involves the loss
of 3 water molecules, then a reaction with amino acids and water. These
also undergo aldol condensation and polymerise further into true melanoids.
All these products react further with amino acids in the third phase to
form the brown pigments and flavour active compounds collectively called
"Melanoids". These can be off flavours (bitter, burnt), off aromas (burnt,
onion, solvent, rancid, sweaty, cabbage) or positive flavours (malty, bread
crust-like, caramel, coffee, roasted) and positive aromas. (bready, cracker,
Charlie (Brisbane, Australia)
Dextrose produces a favourable bready flavour at ~5% of sugars. It also
promotes the production of Melanoid reductones, which are oxygen accepting
Maillard products which act as anti-oxidants and improve shelf life.
High levels of amino acids also promote Maillard reactions even though
only a tiny fraction is consumed (a few %), particually Proline. (Really
an Imino Acid). This AA is produced at high levels in malt because it is
germinated in very wet conditions. It has led to over 120 specific compounds,
some with a bready aroma and some with a bitter taste. Proline does not
High concentration favours both Caramel and Maillard reactions, but dilution
eliminates caramel reactions.
Temperatures *over 100°C* favour the production of Pyrazines. In particular
the 2,*-dimethylpyrazines are the bready, nutty or caramel notes in dark
Doppelbock beer. (Narziss) However the same Pyrazines in different types
of beers produce significantly different aroma notes. (Thank God)
High levels of polyphenols favours Strecker degradation. Thus mash and
lauter technique will affect boil Maillard reactions.
pH. The ideal pH for wort is about 5.0 to 5.4. Higher readings will affect
the outcome of Maillard negatively. For small decoctions of first runnings,
it would be possible to adjust pH for different results.