Turn up the heat: Investigating the Maillard reaction

Turn up the heat: Investigating the Maillard reaction

Direct-fired stills are celebrated for their impact on flavour creation, largely due to increased levels of Maillard reactions — but what are these, and how do they impact a whisky’s flavour?


Image: The copper stills at Stauning Distillery, Denmark

Production | 24 Jun 2024 | Issue 198 | By Kristiane Sherry

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How do you like your steak cooked? (Or halloumi or aubergine, for the vegetarians and vegans out there.) From barely warm to thoroughly scorched, it is clear how caramelisation and burning in cooking can transform an ingredient’s taste. The way these flavours develop is down to sequences called Maillard reactions — and they are a common way to build a whisky’s signature profile, too.


In short, Maillard reactions happen when sugars react with amino acids. Heat speeds the reactions up, which is why rare and well-done steaks taste very different. Maillard reactions are pretty common in whisky making too, most notably occurring in direct-fired stills during distillation.


Back in the day, all pot stills were direct fired. Oil, coal, or even wood was burned underneath the still to heat the liquid. The advent of cheaper, more efficient technology has seen most whisky distilleries adopt steam as their heat source of choice. Steam is seen as an easier way to produce a consistent spirit in terms of energy efficiency, flavour, time, and cost. Those distillers that use direct-fired stills today are generally vocal about their methods. This heating process usually creates heat spots and inconsistencies which, when harnessed, can result in a spirit with deeper, more complex flavours. This character is often attributed to the increased level of Maillard reactions in the still, especially if solids left over from fermentation are involved.


“When we started in 2005, we didn’t know anything about whisky at all,” says Alex Munch, co-founder at Denmark-based Stauning Whisky. “We decided that if we did it, we had to do it from scratch, the way they did 200 years ago.” Stauning might look modern, but its malting floors and 24 direct-fired pot stills point to a more traditional philosophy. When investment and expansion came in 2016, it was critical to keep the character from the direct-fired stills that had become a Stauning signature. “We agreed that direct fire was a part of the DNA.”

The Stauning Distillery

While the team has never used steam-fired stills for its rye and malt spirit, Munch says “mistakes” have made the importance of direct fire clear. “What happens when it’s too hot, what happens when it’s not very hot, you see differences in the alcohol depending on temperature and residue of solids.” This is all down to those Maillard reactions.


The Stauning team is so proud of this character that, unusually, they let visitors taste the low wines — the 8–10% ABV liquid left after the first pot still run. “They say, ‘Wow! This is very thick, very full, very fruity.’” The inconsistent heating and how the solids slightly ‘catch’ create the character.


Over in Scotland, Speyside’s Glenfiddich Distillery has an interesting relationship to direct-fired stills and Maillard reactions. The producer expanded into Still House 3, a brand-new set-up, in 2020, with some indirect-fired stills moved over from the original Still House 1. Yet all the pots in Still House 2 have been direct fired since the early 1970s.


“Extensive trialling was done to ensure we maintained new make character and quality throughout the expansion, although it’s important to note we have always run a combination of direct and indirect fire,” explains Glenfiddich brand ambassador Struan Grant Ralph.

Glenfiddich brand ambassador Struan Grant Ralph

The Glenfiddich signature style is light and fruity, unlike the heavier spirit usually associated with direct firing — so how do its distillers harness the Maillard reactions accordingly? “We use a quarl underneath our direct-fire stills to refract the heat onto the copper so it isn’t concentrated into one place,” Ralph continues. “Imagine this like a snail shell of refractory bricks which spreads the heat evenly onto the base of the still, rather than one direct point of heat.”


Of course, the directness and intensity of heat is only one factor at play within the stills themselves. Maillard reactions’ flavour creation depends on much more. “There’s still shape, things like a bigger base, whether they are less tall,” says Rachel Barrie, master blender and senior leader of Scotch at Brown-Forman. In Scotland, the drink group’s distilleries include GlenDronach — which had direct-fired stills up until 2005 — as well as Benriach and Glenglassaugh. “It’s just that synchronicity, how to dial up certain flavours. You do have things to play with.”


Take it a step back upstream, and the type of grain used in production will have a sizeable influence on the nature of Maillard reactions in the still. For Stauning’s Munch, whether a production run is rye or barley is significant. Even the grain variety can have an impact, due to the thickness of the husk.


“Rye as a grain is so much more difficult to handle than the barley,” he details. “We don’t have the husk to help us kind of filtrate the solids away.” The result is something very sticky. Right now, Stauning operates at about 65 per cent rye and 35 per cent barley in its mash bills. “If we go just slightly higher, we risk burning, really heavy burning in the stills.”

The floor maltings at Stauning

The amount of burning, and subsequent Maillard reactions, can be managed by carefully monitoring the temperature of the wash when it enters the still. “It’s so important to get it right. We just want that slight scorch to the solids,” Munch adds.


For Barrie, the make-up of the grains matters, too. “Something like corn or wheat has very little protein. So, they have very low levels of amino acids.” Without amino acids, those tasty Maillard reactions do not happen.


It may seem like a lot of trial and error, and it is. “As is the nature of this reaction, there is an ability to overreduce the sugars,” explains Kelsey McKechnie, malt master at Balvenie. “In the same way that we can overcook sugar at home and create a burnt mixture, this process can happen in wash stills.”


Glenfiddich’s Ralph agrees. He says it is all about making sure the reactions are kept below the key caramelisation point, at about 100°C. “They can however drive the production of furfural and other compounds that  may be unpleasant or even harmful. I guess it all depends what kind of new make style you are aiming for. Certainly for Glenfiddich with a lighter fruity style they are less important than at other distilleries.”

GlenDronach Distillery

Thankfully, even what seem like serious mistakes can yield remarkable outcomes. Over at Stauning, an overabundance of solids resulted in an overwhelming flavour of burnt toast in one batch of new make. But with age, the profile gets “much more full, much more complex, and much sweeter than if we do not make these mistakes,” Munch explains.


“One of the biggest burnt batches from the old distillery actually ended up being one of the best whiskies we have ever made,” he smiles. “The first couple of years it was terrible. Tasted like burnt toast. Then slowly, slowly, the flavour profile changed. When it was four to five years old, suddenly it became very, very good. Sweet, very complex, very deep, these caramel, chocolate, coffee flavours really pulled through.” Casks from the batch were snapped up by iconic Copenhagen restaurant Noma. “It clearly shows the changes in character and profile when you do stuff like this.”


For him, direct-fired stills are well worth the risks and experimentation. “Yes, you always have hot spots in the stills, but it’s always in the same place.” Similarly, different grain harvests will have different thicknesses of husks and be different to work with.


While saying he finds the uncertainties “charming”, Munch acknowledges it is not an approach that will suit everyone. “It depends on what kind of distillery you are. I know a lot of distilleries talk about consistency being the main thing, having the same liquid every day, every time. But when I look at things like wine, I actually prefer that change from year to year. The harvest shows this is a natural product. It is not a machine. This is not a factory. It’s a distillery.”

Rachel Barrie, master blender and senior leader of Scotch at Brown-Forman

Every reaction, from grain field to within bottle, builds up another intricate layer of flavour in a whisky. Barrie notes that many of these, even outside of the stills, are down to Maillard reactions. She uses GlenDronach as a case study: its robust, nutty, meaty character could be down to Maillard reactions in the stills, but they have been steam fired for almost two decades now.


The character could come from mashing. “Some distilleries have clear wort, some cloudy, but normally it’s somewhere in the middle,” she says. “When you have a little bit of cloudiness in your wort, you don’t filter it all out, you’re allowing some of the proteins into fermentation. And then obviously, these carry through into the distillation. So, there is an element of mash tun influence.” She also argues that the toasting and charring of casks could be seen as a way of promoting Maillard reactions.


“Science, of course, can be very complex, but can also be very simple,” she says. For example, grains and oak have very similar build-ups — lignin, hemicellulose, proteins, and lactones are all there, just in different proportions. Depending on who you talk to, cask maturation will contribute around 70 per cent of a whisky’s flavour. It is often where we first look to understand what an expression in our glass might taste like. But as even this one factor shows, there is much to be gleaned from going deeper. 

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