In recent years the craft beer market has been dominated by waves of interest in particular beer styles. We have seen the popularity of IPAs, Imperial Stouts, New England IPAs, Fruit IPAs and Kettle Sours ebb and flow, driven by tides of enthusiasm on social media, and by people’s desire to experience the next big thing. To the brewer this brings an extra layer of excitement, not just in tasting these innovative new styles but in learning how to brew them successfully.

In 2018 we have seen a surge in interest in the so called ‘Brut’ IPA, a style very much in its infancy.  Originally brewed in San Francisco by Kim Sturdavant of Social Brewing the style is defined by extremely low levels of unfermentable dextrin material at the end of fermentation. This leads to a crisp and extra dry, (but not drying) mouthfeel.

The style is perhaps the perfect contrast to the trend for New England IPAs which are characterized by their extreme levels of dry hop character and full mouthfeel borne of chloride heavy water.

In addition to the brut character these beers tend to exhibit very high levels of carbonation and effervescence as well as a moderate to high hop aroma leading to the popular comparison with the wines of Champagne. The production of the Brut IPA hinges on the use of a class of exogenous enzyme called an amyloglucosidase (AMG) in the brewing process. (Lallemand brand name Glucoamylase 400). 

Enzymes and just what is AMG?

Enzymes are complex proteins which act as catalysts to accelerate the speed of a reaction. Put simply, they breakdown biological molecules into their constituent parts.  There are several factors that can affect enzyme activity including temperature and pH. An increase in temperature increases the rate of reaction up to a point but above, and outside of the optimal range can reduce the efficacy and even denature the enzyme.

The use of enzymes in brewing is certainly nothing new but is now gaining wider interest and increasing application in the craft brewing sector to push boundaries and achieve specific sensory profiles. Indeed, AMG has typically been used in brewing for creating low-carb beers and reducing sweetness in higher ABV beers. Brut IPA is a good example of using an enzyme in a new and different way. 

In a standard beer, water and malted barley are mixed in the brew house to produce a mash. The main purpose of this is to create sugar for fermentation. During this process enzymes are released from the malted barley which begin to break down the starch present in the barley endosperm into sugars. The two most significant of these enzymes are alpha and beta amylase.

Starch, a polymer of glucose, is joined by glycosidic bonds and is present in two forms, an unbranched variety known as amylose joined by alpha 1,4 bonds, and a branched variety known as amylopectin joined by alpha 1,4 bonds with alpha 1,6 branch points. (See fig 1)

In the brewing process the glycosidic bonds in starch are broken down by enzymes in a process known as hydrolysis. (See fig 2) Alpha amylase acts as an endoenzyme hydrolyzing the 1,4 glycosidic bonds within the starch chain to produce shorter lengths, whilst beta amylase acts as an exoenzyme hydrolyzing the 1,4 glycosidic bond two glucose units away from the end of the chain releasing maltose (see fig 3).

Maltose is a disaccharide made up of two glucose molecules and is the main sugar utilized by yeast in a brewery fermentation. Beta amylase activity is critical to the fermentability of a normal wort. Crucially, neither alpha nor beta amylase will hydrolyse the alpha 1,6 glycosidic bond at  branch points in the amylopectin chain, and it is the consequence of this which leads to the presence of unfermentable dextrin at the end of fermentation in conventionally produced beers.

These unfermentable dextrins are in some part responsible for the mouthfeel of a beer and their presence, or absence can produce a beer that is deemed sweet or dry respectively.   

In a brut IPA the unfermentable dextrin material responsible for mouth feel and residual present gravity is absent. This is due to the addition of an exogenous amyloglucosidase enzyme to the mash or fermenter. Amyloglucosidase is produced from the fungus Aspergillus Niger and works by hydrolysing both alpha 1,6 and 1,4 bonds one glucose unit away from the end of the starch chain releasing free glucose into solution (See fig 3).

This glucose is then utilized by the yeast during fermentation producing alcohol and CO2. In this way all of the available starch from the malted barley is converted into sugar and fermented. The classic use of an enzyme such as AMG is to increase fermentability of worts leading to greater alcohol yields and a more efficient use of raw materials.        

How to produce a Brut IPA

When considering how best to produce a brut IPA one must consider the relative advantages and disadvantages of the different methods. There are three main options to choose from. An addition of AMG enzyme to the mash, an addition of AMG to the fermentation vessel or an addition to both.

An addition to the mash tun ensures that there is no active enzyme moving forward to the fermentation vessel as it is deactivated in the boil. The disadvantage is that the rate of addition needs to be high to ensure complete starch breakdown. Lallemand recommend an addition of 2-4 litres per tonne, although this ought to be optimized for individual processes.

The enzyme requires thorough distribution throughout the mash to ensure uniform breakdown of starch and so it is recommended that the enzyme is added incrementally throughout the mashing process.

An enzyme addition to the fermentation vessel has the advantage of a much lower dose rate and complete breakdown of any remaining dextrin material but has the disadvantage of the presence of an active enzyme in the final beer.

The consequence of this is that great care must be taken to ensure that fermentation has proceeded to dryness prior to packaging the final product. Failing to ensure starch breakdown and fermentation have completed to satisfactory levels will lead to increasing levels of sweetness in filtered or yeast free beer and pressure buildup and potential bottle bombs in bottle conditioned beers. The recommended dose rate is an addition of AMG to the fermentation vessel at the start of fermentation at a level of 10ml/hl.

The Lallemand recommendation is an addition of enzyme to both the mash and the fermenter. This ensures fermentation in a good timeframe, especially when using a Champagne strain, as well as complete starch breakdown.

The yeast strain chosen for fermentation of Brut IPA is choice for the brewer; however, the choice should be made in full awareness of the physiological properties of the yeast strain concerned. 

The two most popular options are a neutral ale strain such as Lallemand BRY97 or a Champagne strain such as the Lallemand CBC-1. A Champagne strain will only ferment simple sugars such as glucose, fructose and maltose, meaning that starch breakdown needs to be comprehensive to ensure a completely dry final product. 

A clean US style ale strain will ferment both maltotriose and maltose so, if starch breakdown is not complete due to less than ideal mashing conditions, or a lack of fermenter enzyme addition, it may be more forgiving to the brewer. Clearly the desired final sensory characteristics of the beer also need to be considered at this stage.

References: Michael J. Lewis, Tom W. Young, Brewing, 2002

Brut IPA Recipe

The grist for a Brut IPA should be predominantly pale with some wheat added for head retention if desired. Hop bitterness should be low, in the region of 15-25IBU. Alcohol content tends to be between 4.5% and 6.5%. Lallemand have created an example recipe, as well as best practice procedures which can be seen on the following pages. (See fig 4).

We have also put together a ‘best practices’ procedure (See fig 5).