The essence of esters

Mark Dredge gets fruity, with some magical brewing by-products


“I think they’re the most important aspect of beer,” says Sam Dickison, co-founder and head brewer at Boxcar Brewery. “You put all the ingredients in and the esters are the cookie cutter that cuts that shape out. The whole beer is defined by the esters.”  

Esters are volatile aromas created by the yeast during fermentation, and they give beer a breadth of sweet-smelling fruitiness – banana, apple, pear, honey, aniseed, peach – distinct from the fruitiness we get from hops. In some beers the esters will be obvious, like in a Hefeweizen with its banana ester aroma, while in others they can be subtle, like the faint fruitiness of a pale lager, but there are esters in every beer, and the yeast’s presence and character defines each style. As Ben Landsberry, brewer at The Kernel, says: “It would be weird if [esters] weren’t there. If you took them away you’d be, like, something’s not right.” 

Scientifically “an ester is a flavour-active chemical which is formed within a yeast cell by the reaction of an alcohol and a fatty acid,” says Andrew Paterson, Technical Sales Manager of yeast supplier Lallemand.

As yeast is creating alcohol (ethanol as well as higher alcohols) there are a lot of different compounds moving around within the yeast cell, including organic acids and different enzymes. Those enzymes catalyse reactions between alcohols and acids, and new chemicals – or esters – are formed. “There could be hundreds of different esters in your beer but maybe only a few are above their flavour threshold,” says Paterson.

The genetic makeup of the particular yeast strain, plus brewing processes (fermentation temperatures, alcohol strength, tank geometry), will determine the ester profile in the beer. Common esters include ethyl acetate (lightly fruity, or solvent-like, found in most beers), ethyl hexanoate (sweet apple, found in English ale) and isoamyl acetate (the banana aroma in Hefeweizen), while esters also “work synergistically meaning they can interact together and present as a new flavour,” says Paterson. A banana ester plus sweet apple together might smell something like creamy stone fruit – it’s like mixing two colours to create a new one.

It used to be that we only discussed esters in relation to Hefeweizen or Belgian ales, with their aromas all coming from the yeast, but that’s changing, and the popularity of New England-style IPAs means brewers are deliberately trying to get lots of fruity esters alongside the hops. 


“My primary mission is to get a really fruity yeast, really happy, and expressing esters as much as it can,” says Boxcar’s Dickison. “Playing with hops is a beer-by-beer ‘let’s see what these hops do’ kind of thing… The hops come second to esters.” 

Using characterful yeast in hoppy beers is a relatively recent shift for many brewers

Using characterful yeast in hoppy beers is a relatively recent shift for many brewers: “People didn’t focus on [esters] so much with West Coast IPAs as they’re really low level,” and the hops cover over the yeast aroma anyway, “but people are focusing on esters a lot more now, and realising that they’re arguably the main flavour of beer.”  

For his hazy modern IPAs, Dickison is trying to great an ester profile of “addictively delicious fruit”. Think Fruit Salad sweets, peach, apple and vanilla. To get that he uses a blend of different yeasts: “I like blends because I feel like a lot of yeasts have some aspects where I’d prefer less of one thing and more of the other stuff. It’s interesting to see if in a blend, flavours from another yeast can mask some of the flavours you’re not so keen on.” 

The yeasts in these New England-style IPAs are all related to old English ale strains. We might not associate a strong yeast aroma with English ales, certainly not in comparison to hazy IPAs, but “with subtle hops and background malt flavours, the yeast is singing more than anything else in [English ales],” says Dickison. In fact, for many traditional British breweries the yeast flavour is their trademark character and any similarities you might taste between one brewery’s beers is likely from the unique ester profile of their house yeast strain. Harvey’s is bruised apple, Fuller’s is marmalade, Timothy Taylor’s is stone fruit and pear, even Guinness has a strong berry ester flavour when you drink it fresh.

Popular New England yeasts include London Ale III and London Fog, and over multiple generations and mutations, with natural and scientific selection, and manipulations in the brewhouse, the old English ale yeasts transformed to give a much more expansive range of aromas and flavours, and that perfectly suits the shift towards extremely fruity New England-style IPAs.

“I fell in love with [London Ale III] and its slightly sweet vanilla notes,” says James Heffron, co-founder and head brewer at Verdant Brewing. Over time the yeast they were using took on new characteristics specific to Verdant and when they had the yeast analysed they discovered “it had genetically detached from London Ale III,” and had become a slightly different strain. 

Verdant’s IPAs are known for being very aromatic and having a full, smooth texture to them, and the esters are integral to that. “If you’re making a New England-style beer then you definitely want that yeast to be a part of the drinking experience,” says Heffron. “The yeast character we like is a lovely soft creaminess. It seems weird to describe an aroma as creamy, but it is… It’s a bit like vanilla custard,” he says. The suggestively sweet-smelling yeast mixes with the tropical, citrus and stone fruit from the hops, giving Verdant a “lovely melding of the yeast character and the hop character, and a sort of blurred line between the two.”

With the increasing popularity of this type of beer, and specifically one with that perceived creaminess, Verdant collaborated with Lallemand to have their yeast commercially available for brewers around the world. 

“It produces loads of apricot and a really nice vanilla ester,” says Jimmy Hatherley, founder of Unity Brewing, who is now using Lallemand’s Verdant IPA yeast (Hatherley also likes that the yeast produces lots of glycerol, a flavourless sugar alcohol, and that adds to the silky mouthfeel). It’s got “lovely, fluffy, soft flavours which compliment that juicy, hazy style so nicely.”

Yeast character has always been fundamental to Unity’s beers, but it wasn’t initially with IPAs in mind: “The reason why I started brewing New England-influenced IPAs in the first place was coming from enjoying brewing Belgian styles that focus on the yeast,” says Hatherley. 

In a Belgian-style beer, “you have to build the recipe thinking about the yeast,” says Hatherley. Most Belgian ales are defined by their ester profile, and aromas might include pear, mild alcohol, banana, citrus, dried fruit, stone fruit, aniseed and almond. Think about a Belgian Blonde, Saison, Tripel or Quadrupel and that aroma is all from the yeast; esters are the flavour of Belgian beer.

With Belgian-inspired wild or sour beers there’s even more yeast character. At The Kernel, their Bière de Saison uses a mixed culture fermentation. Saison ale yeast is active at the beginning of fermentation, and after a week the beer has a strong banana ester aroma, “but you wouldn’t say there’s any banana in the finished beer,” says brewer Ben Landsberry, and that’s because of the wild yeast brettanomyces. This starts working later in the fermentation, and it works for longer, and the brett is able to physically break down the isoamyl acetate (the banana ester) and turn it into something different, as well as transforming other esters into new compounds (“all those things are still in there, they’re just tasting different to us now; they are chemically a different compound,” says Landsberry), and giving some of its own new esters, notably pineapple and fermented tropical fruit. Add dry hops or fruit into the beer and there are even more compounds for the yeast to biotransform, creating more new aromas and flavours. 

Brett beers are sometimes described as having a ‘barnyard’ or ‘farmy’ character, and these aromas are also from the yeast, but they are not esters, they’re phenols. This is relevant because during fermentation yeast is producing a lot of flavour-active compounds beyond the esters, and most of those are considered to be off-flavours if in high volumes or inappropriate to style, including higher alcohol (solvent), phenols (clove), acetaldehyde (apple), diacetyl (buttery) and sulphur (eggy, vegetal). 

One thing that makes esters stand out is they generally taste nice, and they’re not normally a defect

“One thing that makes esters stand out is they generally taste nice, and they’re not normally a defect unless you have them in really high quantities,” says Lallemand’s Andrew Paterson, and while brewers can try to manipulate their fermentation to draw out more esters, they also have to manage good fermentation techniques to get good flavours and not bad ones. As Unity’s Hatherley puts it: “If you get a really nice healthy fermentation, your beer tastes good.”

Every beer contains esters, whether they are prominent or subtle. Where malt gives beer bakery flavours (bread, caramel, coffee), and hops give fruit and herbs, yeast gives it definition, it gives complexity and form, and it turns something flat into something three-dimensional. Esters are the essence of every beer.

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