Maillard Reactions 101: Theory

Charlie Scandrett,

Posted to HBD 2390, 4/4/97

There are three basic non-enzymic browning reactions,
  1. Pyrolysis
  2. Caramelisation
  3. Maillard 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 130C. 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 10C 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. 1/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, fine malt)

The outcome will depend on which amino acids and sugars are available, and what the pH and temperature aand concentration are.

Charlie (Brisbane, Australia)