Fermentation is where the magic happens—the point where yeast (or bacteria) turns sugar into alcohol, carbon dioxide, and a whole spectrum of flavours.
Fermentation isn’t just about bubbling spunding valves and watching krausen rise; it’s about controlling key factors to ensure a clean, consistent, and flavourful beer. Get it right, and you’ll not only produce better beer but prevent that long-term headache.
1. Happy Yeast, Happy Beer
Yeast is a living organism, and just like us brewers, it thrives when well-fed, well-rested, and stress-free. The health and viability of your yeast impacts everything from flavour development to attenuation. Under-pitching (too little yeast) can lead to sluggish fermentation and off-flavours, while over-pitching (too much yeast) can affect aroma and body as well as reducing final yield. Then there’s contamination with other yeast or bacteria – wild!
How to Maintain Yeast Health
- Use Fresh, Viable Yeast – Not all yeast is created equal, and just because you have a packet or slurry of yeast doesn’t mean it’s ready to go. Viability refers to how many of the yeast cells are still alive and capable of fermentation, while vitality is how healthy and active they are. Older yeast can be sluggish or even non-viable. For those re-pitching yeast, often check viability using microscopy and staining techniques such as Methylene Blue – stain that turns dead yeast cells blue while live cells remain clear. A count of the ratio between dead and live cells using a haemocytometer can help determine whether the yeast is fit for fermentation or if a fresh culture is needed.
- Store Yeast Correctly – Keep liquid yeast refrigerated (1–4°C) and dry yeast in a cool, dry place. Avoid freezing as it can rupture yeast cells. If using multiple strains, keep them separated to keep them pure as intended. If brewing both clean and wild beers, keep separate sets of equipment for each (including tubing, fermenters, and even vessels). Brettanomyces, Lactobacillus, and Pediococcus can be difficult to remove fully and infect future clean beers.
Preventing Infection and Cross-Contamination
- Wild yeast and bacteria are everywhere—on surfaces, in the air, our hands. While some microbes are desirable in mixed fermentations, unwanted contamination can create serious problems. Anything that touches cooled wort or beer must be thoroughly sanitised.
- Minimize Open-Air Exposure – Avoid leaving fermenters open longer than necessary. Wild yeast and bacteria can easily land in.
- Clean Up Spills Immediately – Spilled wort or beer can become a breeding ground for bacteria and mould. Keep the brewing space clean and dry!
A healthy yeast culture combined with good hygiene practices prevent surprises – though if you do feel there’s some unwanted features in there, speak to the Murphy’s technical teams. We’re here to advise – it’s what we do!
2. Temperature and pH: The Goldilocks Zone
Fermentation temperature is a big driver of flavour. Too warm, and your beer may develop excessive esters and fusel alcohols (ethyl acetate / nail polish — no thanks). Too cold, and your yeast might fall asleep before finishing the job.
- Ales (Saccharomyces cerevisiae) typically ferment best between 18–22°C, though some strains prefer higher.
- Lagers (Saccharomyces pastorianus) require lower temperatures, usually around 10–14°C, followed by cold conditioning.
- Mixed fermentations (think Brett, Lacto, and Pedio) have their own quirks, with some bacteria thriving in the 30°C+ range.
And it’s not just temperature – pH is important too. Just like in mashing that porridge, pH levels play a crucial role in fermentation. Most yeast strains prefer a pH range of 4.0–5.2 at the start of fermentation. Too high, and you risk bacterial contamination. Too low, and yeast can struggle to function properly. It’s about finding that “just right” balance.
3. Ensure a nutritional diet.
Fermentation also burns through nutrients. Yeast loves a good diet to help it power on through all that sugar. If your yeast struggles, fermentation can stall, off-flavours can develop, and attenuation may suffer. This is where yeast nutrients come in, providing essential compounds yeast require to do their job effectively.
- Zinc is crucial for enzyme function, cell growth, and healthy metabolism. Even in trace amounts, zinc helps prevent sluggish or stuck fermentations, particularly in high-gravity beers.
- FAN – Free Amino Nitrogen is essential for yeast protein synthesis and growth. Low nitrogen levels, especially in adjunct-heavy beers or with unmalted grains can lead to poor fermentation performance and sulphurous off-flavours. A dab of DAP (diammonium phosphate) is positive supplement to boost the nitrogen in the wort.
- Calcium – While better known for its role in mashing, calcium also helps yeast flocculation. It also important for cell wall health and improves overall fermentation efficiency.
Just like all good nutritionists, supplements come recommended. Yeast Vit, a powdered mix of vitamins, minerals and nitrogen can give that yeast metabolism a boost or if a liquid diet is preferred, Yeast Aid, offers a complete nutrient blend that can avoid sluggish fermentations and help prevent off-flavours.
4. Oxygen Before Fermentation, Not After
Oxygen is essential before fermentation starts. It helps yeast build healthy cell walls early on, but introducing oxygen post-fermentation can lead to oxidation, which results in stale, cardboard-like flavours.
To avoid oxidation limit splashing or aggressive transfers of beer once fermentation has started or finished. Purge tanks and containers with inert gases before filling. Want to monitor your gases? Speak to us about our partners in dissolved gas measurement or nitrogen generation.
5. Patience: Let the Yeast Finish the Job
Fermentation isn’t just about alcohol production—it’s also when yeast cleans up byproducts like diacetyl (butterscotch/popcorn) and acetaldehyde (green apple notes).
Diacetyl is produced during amino-acid synthesis during fermentation producing the compound α-acetolactate. α-acetolactate escapes the cell wall and spontaneously decarboxylates in to diacetyl. Yeast can absorb it and reduce the ketones groups that produce this off-flavour, but it takes time and temperature to do so. Generally, the customer isn’t a fan of buttery beers. A little bit in that ale might be ok but in that crisp lager – it just isn’t the one. Though time isn’t always on your side –the fermentation is slowing; the temperature is dropping, and that beer needs packaging. Enter Alpha Acetolactate Decarboxylase (ALDC for short.) A simple addition at the start of fermentation can prevent the precursors to diacetyl from forming and in turn reducing the chance of the common flavour complaint from occurring – perfect for brewing clean lagers and dry-hopped pales.
Don’t worry as much about those aldehydes – you covered that earlier with your nutritious wort and steady fermentation profile.
The Impact of Fermentation on Beer
Fermentation is where beer takes shape. It defines flavours, esters, phenols and alcohols. It’s more than crossing your fingers and hoping for the best—it’s where precision meets patience; transformation into something remarkable. Control the processes, respect your yeast, and you’ll be rewarded with a beer that’s balanced, refined, and truly worth the wait.