Brewing As A Science: What Is Fermentation?

For lovers of beer, it is an obvious fact that in order to make beer, you have to use yeast. Most people know that beer is a fermented beverage, just like the ever-popular kombucha drink that is hitting health stores everywhere, but not everyone knows what fermentation actually means.

This unique process is completely responsible for giving beer its alcohol, taking it from a funky tea to something that you can get drunk on at a football game. However, few people know how this process actually happens and what about yeast takes grain water and turns it into something that tastes delicious and has intoxicating effects. In this guide, we are going to give you a crash course on what it means to ferment beer and how it applies to your brewing process.

In a simple sense, yeast takes the glucose and turns it into alcohol and carbon dioxide gas. As most anyone knows, alcohol is the component of beer that is capable of getting you drunk. The carbon dioxide gas is responsible for lending beer the wonderful carbonation effect that it is so well known for.

These two products, born of the fermentation process, are signature aspects of beer that make it what it is known for. While other alcohol types also rely on fermentation, beer is generally known for its signature use of grain or barley as a base. Depending on what is used and how the beer itself is fermented, the alcohol and carbonation levels in the beer can change. You will probably have noticed that some beers have a higher alcohol content than others. You will likely also have noticed that some beer has more carbonation than other kinds. Each of these factors and how they play out in the brewing and fermenting process is determined by the approach that the individual making the beer makes.

The aspect of the beer that is involved in the fermentation process actually exists prior to it becoming beer. During the brewing process, a liquid called wort is made that is the result of boiling grains and their respective enzymes. This wort is then taken and mixed with a set amount of yeast in order to determine how the fermentation process will play out. The yeast will convert the glucose in the wort to the alcohol and carbon dioxide, and from this process, beer is made.

The fermentation process is not a universal constant. In reality, it changes from one type of beer to the next. Fermentation as a process is known to emit heat, and the temperature that the beer is kept at can determine what kind of beer it actually turns into. Some types of beer are maintained at higher temperatures while other types of beer are kept at lower temperatures. This, in combination with how long the fermentation process is carried out, can determine various factors about the beer. Some beers are fermented for a couple of weeks while others can be fermented for well over a month. It is all kept to the discretion of the creator.

When you make your beer, the fermentation process is considered one of the most relevant steps. Since it gives the beer some of the signature traits that we all know and love, it is easy to see why people stress this step so much. It is what takes an awkward brew and turns it into that delicious cold one at the end of a long day. As you pursue your brewing process, you will find that the various approaches to fermentation available can drastically impact the final product. Don’t hesitate to experiment with this step to see how you can alter your homemade brews.

Frequently Asked Questions (FAQ)

1) What exactly is fermentation in brewing?

• Fermentation is the metabolic process where yeast converts wort sugars (glucose, maltose, maltotriose) into ethanol and CO2, plus flavor-active compounds like esters and higher alcohols that shape beer aroma and mouthfeel.

2) How do ale and lager fermentations differ?

• Ale strains (Saccharomyces cerevisiae) ferment warm, typically 18–22°C, producing more esters/phenolics. Lager strains (S. pastorianus) ferment cool, typically 8–13°C, yielding cleaner profiles and requiring cold maturation (lagering).

3) What factors most influence flavor during fermentation?

• Pitch rate, wort oxygenation (generally 8–10 ppm for ales, 10–12 ppm for lagers), temperature profile, yeast strain genetics, nutrient availability (FAN, zinc), and pressure/spunding all influence ester/phenol balance and attenuation.

4) Why is oxygen bad after fermentation but needed before it?

• Yeast needs oxygen pre- or at pitch to synthesize sterols for healthy cell membranes. After active fermentation begins, oxygen exposure risks staling via oxidation (cardboard/sherry notes) and hop aroma loss.

5) What is spunding and why use it?

• Spunding is sealing the fermenter near terminal gravity to capture CO2, naturally carbonate, and suppress ester/fusel formation. It can improve foam stability and reduce oxygen ingress during transfer.

2025 Industry Trends in Fermentation

• Pressure fermentation normalization: 0.5–1.0 bar ale fermentations to speed turns and reduce ester formation without sacrificing drinkability.
• Yeast health analytics: affordable inline/at-line tools (viability, vitality, glycogen) inform repitch decisions and reduce off-flavors.
• Cold-side oxygen control: deaerated water (DAW), closed transfers, and DO targets ≤30 ppb at package become baseline for hop-forward beers.
• Bioengineered and hybrid yeasts: thiolized strains for tropical aromatics; diastaticus screening SOPs strengthen QA.
• Sustainability: CO2 recovery during fermentation and heat integration reduce utilities and footprint.

Fermentation Benchmarks (Ales vs. Lagers, 2024–2025)

Parameter	Ale (typical)	Lager (typical)	2025 Best Practice/Target	Notes / Sources
Pitch rate (million cells/mL/°P)	0.5–0.75	1.0–1.5	Match to OG and desired profile	ASBC/MBAA guidance
Wort oxygen (ppm)	8–10	10–12	Verify with DO meter	ASBC Methods
Fermentation temp (°C)	18–22	8–13	Tight control ±0.5°C	BA Quality Manual
Pressure during ferment	0–0.5 bar	0–1.0 bar	Spund near terminal	MBAA case studies
Attenuation (%)	72–80	74–82	Depends on strain/recipe	Vendor specs
Diacetyl (VDK) at package (ppb)	<50	<30	Forced VDK test pass	ASBC Beer-25
Packaged DO (ppb)	≤30	≤30	≤20 for draft	ASBC Beer-17
CO2 recovery capture (%)	30–70	30–70	≥60 with modern skids	BA sustainability

Authoritative sources:

• Brewers Association Quality/Sustainability: https://www.brewersassociation.org/
• ASBC Methods of Analysis (DO, VDK, micro): https://www.asbcnet.org/
• MBAA Technical Quarterly (fermentation control): https://www.mbaa.com/
• Yeast suppliers (strain tech sheets): Lallemand, Fermentis, White Labs

Latest Research Cases

Case Study 1: Pressure-Fermented Ale to Improve Turn Time (2025)
Background: A regional brewery sought higher throughput without adding tanks and wanted cleaner flavor for a core pale ale.
Solution: Implemented 0.7 bar fermentation pressure from mid-exponential phase, optimized pitch to 0.6 Mcells/mL/°P, oxygenated to 9 ppm, and tightened temp control to 19.5 ±0.3°C.
Results: Fermentation time −18 hours; ester reduction (isoamyl acetate −22%); natural carbonation to 2.3 vol; packaged DO ≤25 ppb via closed transfer; annual capacity +8% with unchanged cellar.

Case Study 2: Closed Transfers and DAW Cut Oxidation in Hoppy Lager (2024)
Background: Hop-forward lager exhibited early staling and diminished aroma by day 45 warm store.
Solution: Added deaerated water loop (<10 ppb O2), CO2-purged hoses/fittings, and closed FV→brite→filler transfers; spunding at 1.0°P from terminal for 2.4 vol CO2.
Results: Packaged DO dropped from 55–70 ppb to 18–25 ppb; sensory shelf-life +6–8 weeks at 4°C; complaint rate −35%; stable hop thiol intensity through day 60.

Expert Opinions

• Dr. Tom Shellhammer, Professor of Fermentation Science, Oregon State University
  “Control of oxygen and pressure during fermentation and transfer is central to preserving hop aroma and achieving consistent flavor stability.”
• Mary Pellettieri, Quality Consultant; author of “Quality Management for Craft Beer”
  “Validate the triumvirate of fermentation QA—pitch rate, oxygenation, and temperature. Pair that with a forced VDK test before crash to avoid diacetyl surprises.”
• Chris White, PhD, Founder, White Labs
  “Healthy yeast is the best insurance. Track viability, vitality, and glycogen; refresh nutrient and zinc protocols, and set objective cutoffs for repitching.”

Practical Tools/Resources

• Brewers Association Quality Manual (fermentation and packaging): https://www.brewersassociation.org/
• ASBC Methods (DO, VDK/forced diacetyl, microbiology): https://www.asbcnet.org/
• MBAA resources on pressure fermentation and spunding: https://www.mbaa.com/
• Yeast calculators and strain data:
• Lallemand Brewing: https://www.lallemandbrewing.com/
• Fermentis by Lesaffre: https://fermentis.com/
• White Labs: https://www.whitelabs.com/
• Oxygen control essentials: DO meters and DAW system primers via suppliers and BA sustainability hub

Last updated: 2025-08-29
Changelog: Added 5 FAQs on brewing fermentation fundamentals, 2025 trend table with fermentation benchmarks, two case studies on pressure fermentation and oxygen control, expert viewpoints, and practical resources.
Next review date & triggers: 2026-02-28 or earlier if ASBC/BA update DO or VDK methods, new yeast strain guidance impacts pitch/oxygen targets, or oxygen control technologies become standard at small scale.