This five-part series focuses on providing brewers with structures and tips to navigate the fundamental areas where the collective effect of these small decisions has the greatest impact on beer quality.

Our previous article focussed on yeast nutrition and its crucial role in providing clean and repeatable fermentations, the essential first step in gaining control of flavour development. It’s now time to look at other components that impact the success of your brew.

Part 4 – Fermentation and Flavour

Fermentation is an inspiring part of brewing, still carrying an air of mystery and magic despite the volume of research available.

As yeast is pitched, many flavours of the beer have already been determined through your ingredient choices and process decisions. Fermentation can either refine and enhance these flavours, or unfortunately, completely unbalance and overpower them.

Once yeast health is assured, you can focus attention on manipulating fermentation parameters to control the major components that contribute to beer flavour. It is also the time for dry hop additions to have their impact on the taste and aromatic of the beer.

Esters

Control of ester production is a logical place to begin as they have a strong impact on flavour, yet are relatively easy to influence.

Alcohols and organic acids of varying chain lengths combine to form a wide variety of esters that can strongly impact flavour and aroma.

Yeast strain selection has a critical role in ester production and should be matched to the beer style as part of recipe development. Whether the chosen strain be naturally high or low in its ester production, there will be times when you need to nurture and promote the production of these esters, and other times when you will be seeking to restrain and limit.

Promotion of increased ester production can be achieved by:

• Raising the fermentation temperature.
• Reduction in wort oxygen levels.
• Fermenting under top pressure (increased dissolved carbon dioxide).
• High gravity recipe formulation, brewing above target alcohol and subsequent dilution.
• Increasing Zinc and Nitrogen content in the wort.
• Limiting trub carry over to the fermenter.

Esters also influence beer flavour indirectly. Those that are below their individual taste threshold can combine and ‘work together’ to have an impact on flavour and aroma.

For example, the perception of hops in a pale ale can be lifted by the presence of underlying esters, that individually are below their own flavour thresholds thus not influencing flavour.

As hopping rates increase to dominate aromatics, it’s important to recognise that the ester balance beneath the hops still has a role to play.

All the A’s

Alcohol, aldehydes and acids all contribute flavour and can quickly throw off the balance of a beer.

The pathways that produce and remove these compounds are highly complex and connected, making them difficult to control. Whilst there are instances where you are looking to promote these flavours, the challenge more often lies in limiting their impact.

Alcohol is clearly an essential component of beer flavour, with ethanol a primary output of fermentation.

Longer chain alcohols result from the metabolism of amino acids in the wort, in a pathway that is most active during cell growth. They are collectively termed fusel alcohols, are more aromatic than ethyl alcohol and are generally associated with negative flavour descriptors like ‘hot’ and ‘solvent’.

Reducing fusel alcohols:

• Reduce fermentation temperature to lessen metabolic activity.
• Ferment under pressure to increase dissolved carbon dioxide and inhibit yeast activity.
• Limit wort dissolved oxygen to control cell growth and in turn, reduce metabolic action.

Aldehydes are an essential part of the fermentation process. The most common is acetaldehyde, with a flavour of tart green apples.

Aldehydes are an intermediate compound created by the yeast that is later turned to alcohol, their presence is therefore common during active fermentation. Whilst there will always be low levels in finished beer, their impact on flavour should be negligible.

Reducing aldehydes:

• Healthy yeast is essential to convert the intermediary aldehydes into alcohol.
• Ferment at lower temperatures with lower pitching rates.
• Reduce top pressure to allow aldehydes to be scrubbed by carbon dioxide.
• Secondary fermentation can be used to mop up residual aldehyde if the primary fermentation was not of sufficient health.

Organic acids (such as lactic and acetic) are formed in low levels as part of a healthy fermentation, but a particular dominant flavour is generally cause for concern (unless, of course, you are making a sour beer).

Reducing organic acids:

• Maintain a healthy yeast culture to avoid dead yeast cells excreting acids (autolysis).
• Check your hygiene practices as detectable flavour is generally a sign of an infection.

Dry Hops

Once only playing their part on brew day, hops now contribute significant flavour directly to the fermentation process for many beer styles.

It can be hard to find two brewers with the same technique, however their primary goal is usually the same: to efficiently impart aroma and flavour from the hops into the beer!

Fundamentally, brewers should consider the following factors when developing their own dry hopping technique:

– Addition method & agitation – addition directly via a manway is a simple and effective first solution, however flavour extraction will increase with agitation. Low speed recirculation of a fermenter via centrifugal pump or utilising a hop cannon for the addition and mixing will increase efficiency and save money in the long term.

– Stage of fermentation – the advice on the best time to add hops is constantly evolving with new research and new styles. There are unique advantages to each approach that brewers can elect to focus on individually or layer into a combined method.

• Additions during active fermentation are best to promote biotransformation (hop compounds interacting with yeast). Pellets dissolve easily at the warmer temperatures and presence of hops during the fermentation assists in alleviating hop creep. The evolution of carbon dioxide assists with agitation but can have a negative scrubbing effect on hop oils.
• Waiting until the end of fermentation before cooling results in less aroma lost to the carbon dioxide scrubbing effect. Dry hop creep can cause attenuation and VDK issues.
• Dry hops added during crash cooling benefit from mixing induced from convection currents. Initially warmer temperatures help dissolve pellets and oils with the quick cooling alleviating hop creep and ‘trapping’ flavours.
• Dry hopping during cold maturation is gaining traction as a preferred technique. Oil dissolution has been shown to occur equally fast, with added benefits of less polyphenol extraction, reduced risk of hop creep and improvement to head retention.

– Contact time – Be cautious not to leave hops in contact with the beer for too long. It is now understood that the majority of the target hop fractions are extracted within the first 24-48 hours. Longer contact increases polyphenol extraction, leading to heightened grassy and herbal flavours.

– Hop selection – There is more at play than total oil content when it comes to measuring the ability of a dry hop addition to impart flavour and aroma. Layering different varieties known to have high impact underneath your elected ‘feature’ dry hop can enhance its perception.

– Crop variation and pellet density – Hop pellets vary not only from year to year, but batch to batch. It is easy to fall into set weights for dry hop additions; however, this will likely lead to inconsistent beer. Here is a great visual of how much impact density can have. Conduct a sensory evaluation on your hops and review the certificate of analysis for batch variation.

There is a flood of innovative advanced hop products becoming available, evolving with each iteration to closer replicate the flavours consumers are accustomed to.

Adopting their use (even in part) improves consistency, yields and can save costs. At the least, you should consider having options on hand to utilise in the case your usual dry hopping program fails to deliver the desired flavour impact.

The Home Stretch

Fermentation pathways are highly complex with a strong connection to, and dependence on, each other. The result is a fine equilibrium where small disparities quickly result in the targeted flavour balance being thrown off, or new compounds quickly overpowering the palate.

Successfully controlling the brewing process from recipe through to the end of fermentation with a clean, balanced and delicious beer is a fine achievement.

There is, of course, a few critical tasks to be managed to get the beer out of the brewhouse and into the glass in top condition. The last article of this series will focus on ensuring that the hard work to date is seen through to the ultimate goal: customers enjoying your beer!

You can find the first 3 articles in this series here:

Brewing Brilliant Beer, Pt 1: Getting Your Recipe Right

Brewing Brilliant Beer, Pt 2: Brewers Make Wort

Brewing Brilliant Beer, Pt 3: Yeast Makes Beer

This series is brought to you in partnership with