Knights of the Mashing Fork

Maillard Reactions 101

1. Pyrolysis
2. Caramelisation
3. Maillard reactions.

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.

• 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 produce Pyrazines.
• High concentration favours both Caramel and Maillard reactions, but dilution eliminates caramel reactions.
• Temperatures *over 100C* 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.

METHODS OF CONTROLLED BROWNING

Method 2/ George de Piro/Noonan's method above. Place a little wort in the kettle and thicken. But *stir continously* and do not continue rapid evaporation after the increasing extract concentration reaches a boiling point of 110-130C (test with thermometer). Add more water or reduce heat to accomplish this. Continue ~20 minutes. Maillard reactions and Caramel molecular dehydration will take place.

Method 3/ Place total wort in a 25 litre pressure cooker and bring to designed maximum temperature for ~3-5 minutes. Release steam, uncover and boil normally for 60 minutes. Rapid and complete hot break, very dark but smooth Maillard reactions, and very high bittering hop isomerisation are the result of this. Very little caramelisation as sugar concentration is much lower than 1 and 2.

I have used methods 1 and 3 with great success. I have never tried 2. Acidifying the mixture, or adjusting the sugar concentration will also change the result.

THERMODYNAMICS OF SCORCHING

Why does the temperature rise so rapidly? Basically heat flow from flame or element to wort meets different resistances. The thermal conductivity of the metal (expressed as "coeficient of heat *transmission*") in the kettle wall, and the transfer resistances (expressed as "coeficient of heat *transfer*) at the gas/metal and the metal/wort interfaces. Of these, the transfer resistances are about 90% of the barrier because metals have good transmission qualities. The gas/metal is the highest of these resistances.With *turbulent flow* conditions (i.e. no boundary layers) this is a rough heat distribution diagram.

        o   oo   oo o o  o  o o  oo  o o  o
         o oo  o  o o o oo oo oo o oo o oo o
        o oo ooo  o o WORT at 100C oo oo o 
        ___________________________________
        ///////// METAL at 110C //////////
        __________________________________
         () () () () () () () () () () ()
            HOT GAS at > 950C

However if the wort is not moving turbulently, it forms a boundary layer on the surface of the kettle. This is a problem because the *transmission* coeficient for water is lousy. The boundary layer then heats up and begins to form a viscous caramelly layer which has less heat transmission properties. This reaches PYROLYSIS temperatures and a classic scorched layer of high temp Maillard products and carbonised cement forms on the kettle. This has less heat transmission properties and very poor heat transfer properties and so on. The temperature distribution diagram looks like this.

        o   oo   oo o o  o  o o  oo  o o  o
         o oo  o  o o o oo oo oo o oo o oo o
        o oo ooo  o o WORT at 100C oo oo o 
        ------------------------------------
        ***Carbonnised cement at 250C******
        _____________________________________
        ///////// METAL at 350C //////////
        __________________________________
         () () () () () () () () () () ()
            HOT GAS at > 950C

Charlie (Brisbane, Australia)