The Maillard reaction is the chemical process that turns raw malt into the brown and black colours and the roasted, nutty, sometimes coffee-like flavours that dominate dark beer, and it happens whenever amino acids and sugars are heated together. Kilning malt is the first and largest application of it in brewing, since a green malt is pale and grassy while a heavily kilned malt is brown and tastes of toast, and the same reaction runs a second time in your boil and again in the kettle or the brewing vessel, adding more colour and more of the bready, biscuity character underneath. It is also the reason colour and roast are two separate things in beer, because the reaction produces a whole family of compounds at once, and the ones that give a stout its tarry, coffee and cocoa depth form at different temperatures than the ones that give a brown ale its toffee sweetness.
What The Reaction Actually Is
A reducing sugar and an amino acid meet under heat, and instead of staying separate they begin a long chain of rearrangements that ends in a large brown polymer with a nitrogen group hanging off it. That polymer is what food scientists and brewers call a melanoidin, and the first, lighter members of the family are the ones that give bread crust and roasted malt their colour. The reaction is not a single step but a cascade, so it never finishes and it never resolves into one compound, which is why two malts kilned to the same colour by two different kilns can taste noticeably different. Duration, temperature, moisture and the ratio of protein to sugar all move the result, and a kilned malt held longer at a lower temperature goes further down the same path than one flashed quickly at a higher one.
Where It Happens In The Brew House
There are three places in a brewery where the reaction is doing work, and each one adds a different layer rather than repeating the same note. The first and largest is the kiln, where green malt is dried and then held at temperatures high enough to drive off water and to push the reaction forward, and the colour of that malt is set there and can never be added back later. The second is the mash, where the malts sit in hot water and enzymes are at work, and while the mash is not hot enough to roast anything, the extracted melanoidins and the caramelised sugars formed in the kiln dissolve into the wort, so the wort arriving in the kettle is already coloured before anything else happens. The third is the boil, where wort held near its boiling point drives more of the reaction forward and produces the Maillard compounds that read as bread crust, toffee, and dried fruit rather than as plain roast.
Specialty malts are simply the kiln pushed further along that path, and their names describe how far. A kilned amber malt is taken just past the pale stage into light biscuit and caramel territory, a crystal malt is taken much further and has a strong sweet toffee note that survives into the finished glass, and a roasted barley or chocolate malt is taken far enough that the reaction has produced something close to coffee and dark cocoa. The further a malt is pushed, the less of its own grain character remains, so the very darkest malts contribute colour and roast rather than a malt flavour, and a well built dark beer uses them as a background against a base of pale malt that still supplies the bread and biscuit underneath.
| Malt or step | How far the reaction has gone | What it contributes |
|---|---|---|
| Pale malt, kilned lightly | Early, pale golden to light amber | Bread crust, biscuit, a neutral base colour |
| Amber malt | Moderate, amber to copper | Toffee, caramel, a round mid-tone colour |
| Caramel or crystal malt | Advanced, deep amber to brown | Sweet toffee, dark fruit, real body |
| Roasted barley | Far advanced, brown to black | Coffee, toast, dark grain, no fermentable sugar |
| Chocolate or black malt | Nearly complete, black | Cocoa, char, bitterness, colour only |
| The boil, on any wort | A further increment in all of them | Bread crust, biscuit, dried fruit, stability |
Colour And Roast Are Not The Same Thing
This is where most people are caught, and it matters when you are reading a label or trying to brew at home. A beer can be black and taste almost nothing of roast, which is what happens when a light kilned or a highly modified pale malt is dissolved in a large volume and the colour is being carried by melanoidins formed in the kiln rather than by anything roasted. It can also be a deep chestnut brown and taste strongly of coffee and cocoa, which is what a small quantity of a heavily roasted malt will do. Colour is a measure of how much polymer has formed, while roast is a measure of which members of the family that polymer is made of, and the higher compounds that smell of coffee and char need a good deal more heat than the ones that only darken. Two beers can therefore match in colour and be nothing alike in the glass, and if you want a roasted character you have to ask about the roasted malts, not about the colour.
What It Does For Flavour And For Keeping
The same reaction that makes the colour is doing several useful jobs at once in the finished beer. The brown polymers are unreactive with oxygen compared with the colourless compounds that would otherwise oxidise, so a well kilned malt contributes to the stability of a beer and a bottle that has sat in a warm room for a summer will taste more like its younger self than an unmalted equivalent would. The high molecular weight products also carry a lot of flavour, and much of the apparent body and the drying, mouth-coating quality in a stout comes from dissolved melanoidin rather than from alcohol or from residual sugar. Alongside that, the reaction creates hundreds of volatile aroma compounds, and the ones that survive fermentation are the reason a beer built on caramel and roasted malt still reads as bread and toast rather than as sweetness alone.
Fermentation changes all of that somewhat, since yeast consumes the simplest sugars and leaves the heavier molecules alone, which is how the kilned character survives into the bottle. What it does not do is re-create the reaction, so nothing that was not present in the wort can appear in the finished beer. This is the reason a roasty port or a coffee stout that tastes of neither has almost always had its roast character stripped out somewhere upstream, either by a light kilned substitute malt, by a blander pale base that dilutes the roasted portion, or by simply being under-attenuated and thin.
Using It When You Brew At Home
If you want more colour and more of the kilned kind of flavour, the base malt and the specialty malt are two separate controls, and working them one at a time is how you find out which one you were actually short of. Start by swapping your base two-row for an amber or a Vienna and taste before adding anything dark, since that changes body and colour together without pushing the reaction as far. Then add a caramel or crystal malt for sweetness and body. Only once those are right should you introduce a roasted barley or a chocolate malt, and when you do, add it in small steps, because a roasted malt has no fermentable sugar of its own and it takes a percentage point or two to change the character while a larger addition will simply make the beer taste of ash.
- Treat colour as the thing that happens first and roast as the thing that happens last, since a heavy kiln can make a black malt that adds almost no roast flavour to the finished beer.
- Keep a light kilned pale or two-row malt as the majority of the grist in a dark beer, because that is where the bread, biscuit and body come from, and the roasted malts belong in the minority.
- Add caramel and crystal malts for sweetness and mouthfeel rather than for roast, since the reaction at that stage produces toffee and dark fruit rather than coffee.
- Watch the boil as the place where character can go wrong, because a long vigorous boil on a wort already heavy with melanoidin pushes it toward a cooked, gravy-like note that no one wants.
- Do not expect a boozy, strong beer to taste roasted on its own, since the roasted character comes from the kilned malts and alcohol adds warmth and nothing of the sort.
Match Your Malt To The Style
Once you know that one reaction produces the whole range from pale biscuit to black cocoa, the malt list of any dark beer becomes readable rather than a wall of unfamiliar names. An English brown ale is amber and crystal malts carrying the colour with very little roast, a porter leans on a moderate amount of roasted barley for a dry coffee note, and an Irish dry stout is built from a pale base with enough black malt to give colour and cocoa while staying clean enough to drink early in the evening. If the beer in front of you is not doing one of those things, the problem is nearly always in the grist rather than in the brewery, and you can read the answer for yourself on a bag of malt. Our explanation of what malt does in brewing goes further on the other things the grain contributes, and our look at dry stout shows what a low-roast dark beer actually tastes like next to the roasted version.
